A versatile means of intracellular labeling: injection of biocytin and its detection with avidin conjugates

Subthreshold rectification in neostriatal spiny projection neurons

Intracellular recordings from slice preparations were used to assess the subthreshold electrophysiological behavior of rat neostriatal projection neurons. Both current steps and ramp currents were used to estimate the current-voltage relationship (I-V plot). Inward rectification in the subthreshold range was a characteristic of most neurons. The amount of rectification varied greatly, and it was complex: membrane voltage trajectories in response to ramps were made up by almost piece-wise changes in the rate of voltage rise, suggesting that multiple conductances contribute to the subthreshold range. Inward current blockers such as tetrodotoxin (TTX) or Cd2+ decreased inward rectification, whereas outward current blockers such as tetraethylammonium (TEA) or 4-aminopyridine (4-AP) increased inward rectification. However, most inward rectification was due to TEA- and Cs(+)-sensitive conductances and not to TTX- or Cd(2+)-sensitive conductances. Cs(+)-sensitive conductances predominated at more negative membrane potentials, whereas 4-AP-sensitive conductances predominated at just +/- 10 mV below the firing threshold. In spite of a very slow activation, there was evidence for transient outward currents modulating the response, i.e., 4-AP-sensitivity, and voltage-sensitivity for firing frequency and threshold. TEA-sensitive conductances also contributed toward fixing the firing threshold. These results imply the contribution of various ion conductances on the shaping of the characteristic physiological firing recorded in vivo. Modulation of these responses by transmitters or peptides may help to understand neural processing in the neostriatum.

Homogeneity of intracellular electrophysiological properties in different neuronal subtypes in medial preoptic slices containing the sexually dimorphic nucleus of the rat

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is larger in male than in female rats, the male phenotype requiring the presence of circulating androgens perinatally. These experiments investigated the intracellular electrophysiology and morphology of SDN-POA neurons and compared these properties with those of other medial preoptic area (MPOA) neurons. Biocytin-injected cells in the SDN-POA either had one or two primary dendrites, or they had multipolar dendritic arrays; dendrites were aspiny or sparsely spiny and displayed limited branching. Neurons in other parts of the MPOA were similar morphologically. Regardless of morphology, neurons situated in either the SDN-POA or surrounding MPOA had low-threshold potentials and linear or nearly linear current-voltage relations. In most (73%) cells, stimulation of the dorsal preoptic region evoked a fast excitatory postsynaptic potential followed by a fast inhibitory postsynaptic potential (IPSP). Bicuculline blocked the fast IPSPs, which reversed near the Cl2 equilibrium potential (-71 +/- 5 mV), indicating their mediation by gamma-aminobutyric acid (GABA)A receptors. Neurons in the SDN-POA have electrophysiological properties similar to those of other medial preoptic cells. When compared with the hypothalamic paraventricular nucleus, the MPOA appears relatively homogeneous electrophysiologically. This is despite the morphological variability within this population of neurons and heterogeneities that are also apparent at other levels of analysis. Finally, GABA-mediated, inhibitory synaptic contacts are widespread among medial preoptic neurons, consistent with indications from earlier reports that GABA provides a link in the feedback actions of gonadal steroids on the release of gonadotropic hormones.

Long-range horizontal connections between supragranular pyramidal cells in the extrastriate visual cortex of the rat

In this study, we examined the morphological structure and synaptic physiology of long-range axon projections among supragranular pyramidal cells in the extrastriate visual cortex of the rat. Intra- and extracellular recordings from layer II/III pyramidal cells were performed in brain slices of area 18a following extracellular stimulation of either the underlying white matter or within layer II/III. Neurons were injected with biocytin for two-dimensional reconstruction of their axon arborizations. The conduction velocity of afferent fibers (0.58 m/s) was twice as high as that of intracortical tangential fibers (0.28 m/s). Layer II/III cells were mainly di- or polysynaptically driven by afferent activation, but predominantly monosynaptically driven from intracortical stimulation sites. The afferent as well as intracortically evoked postsynaptic potentials showed a very similar time course and shape. From both stimulation sites, suprathreshold action potentials could be elicited. The current threshold for a postsynaptic response and the slope and width of excitatory postsynaptic potentials (EPSPs) increased with the distance of lateral stimulation. The morphological properties of layer II/III pyramidal cell axon collaterals closely corresponded to the electrophysiological results. Long-range intraareal axon collaterals could be followed up to 1 mm within the supragranular layers. Their length-distance distribution showed an inverse relationship to the threshold currents of EPSPs. Pyramidal cells exhibited regularly spaced patches of horizontal axon collaterals with an interpatch distance of about 250 microns. We concluded that the supragranular horizontal network in the extrastriate visual cortex of the rat is qualitatively very similar to that of cats and monkeys. However, quantitative differences exist in its spatial extent and physiological characteristics.

Norepinephrine modulates excitatory amino acid-induced responses in developing human and adult rat cerebral cortex

These experiments were designed to assess the ability of norepinephrine and its beta-receptor agonist, isoproterenol, to modulate responses induced by activation of excitatory amino acid receptors in brain slices obtained from developing human cortex or adult rat cortex. Human cortical slices were obtained from children undergoing surgery for intractable epilepsy (9 months to 10 yr of age). For comparison, slices were also obtained from rats (2-3 months of age). Iontophoretic application of glutamate, N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) produced excitatory responses consisting of membrane depolarizations accompanied by action potentials. Iontophoretic or bath application of norepinephrine or isoproterenol enhanced responses evoked by glutamate or N-methyl-D-aspartate. Depolarizations occurred with shorter latencies and their amplitudes increased. Action potential frequency was also increased and responses were of longer duration. In contrast, norepinephrine or isoproterenol had no effect on responses induced by AMPA. The enhancement of responses induced by N-methyl-D-aspartate or glutamate was antagonized by the beta-adrenergic receptor antagonist propranolol. Similar findings were obtained from neurons in humans or rats. These results suggest that norepinephrine, possibly via beta-receptors, potentiates responses mediated by glutamate and N-methyl-D-aspartate receptors without affecting those mediated by AMPA receptors. These effects were observed at all ages studied, indicating that the ability of norepinephrine to modulate excitatory neuronal transmission is well developed in human cortex by 9 months of age.

Analysis of synaptic inputs and targets of physiologically characterized neurons in rat frontal cortex: combined in vivo intracellular recording and immunolabeling

Ultrastructural immunocytochemical identification of transmitters in afferent terminals and targets of individual physiologically characterized neurons is essential for understanding the complex circuitry within the mammalian neocortex. For this type of analysis, we examined the utility of combining in vivo intracellular recording and biocytin injections with silver intensified 1 nm immunogold labeling of GABA and the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). These transmitters are found to local neurons and afferents known to prominently modulate the activity of pyramidal neurons in the neocortex. Individual neurons were physiologically characterized and filled with biocytin in the frontal cortex of anesthetized rats. The brains were then preserved by vascular perfusion with aldehydes. Single vibratome sections through the recording site were reacted (1) for immunoperoxidase detection of biocytin and (2) for immunogold labeling of GABA or TH. Dually labeled sections were processed for light microscopy or embedded in plastic for electron microscopy. The dense peroxidase product for biocytin was detected in pyramidal neurons. These were located in superficial as well as deep cortical laminae, and were readily distinguished from immunogold silver labeling. GABA labeled terminals formed symmetric synapses with larger biocytin filled dendrites, whereas the TH labeled terminals contacted distal dendrites and spines. Peroxidase labeling for biocytin also was seen in a few axon terminals forming synapses with unlabeled and with GABA immunoreactive dendrites. These results suggest that single pyramidal neurons of the rat frontal cortex receive dual input from both GABA and catecholamine terminals. Additionally, this study demonstrates the usefulness of silver enhancement of 1 nm colloidal gold prior to plastic embedding for electron microscopic detection of neurotransmitters within afferents and targets of neurons physiologically characterized in vivo.

Anatomical organization of the limb premotor network in the turtle (Chrysemys picta) revealed by in vitro transport of biocytin and neurobiotin

The in vitro turtle brainstem-cerebellum preparation has been a valuable tool in the study of central motor programs. In the present study, we investigate the anatomical organization of the turtle rubrocerebellar limb premotor network and its sensory connections in vitro by combining the rapid anterograde and retrograde transport of neurobiotin and biocytin with the extended viability of the isolated turtle brainstem-cerebellum. These compounds retrogradely labeled soma, dendrites, and axons, and orthogradely labeled axons and, to a lesser extent, terminals. The chelonian red nucleus receives a dense input from the contralateral lateral cerebellar nucleus and projects heavily to the contralateral spinal cord. Rubral axons sparsely innervate the lateral cerebellar nucleus and project heavily to the lateral reticular nucleus. Lateral reticular axons heavily innervate the lateral cerebellar nucleus before terminating in the pars lateralis of the cerebellar cortex as mossy fibers. These prominent, recurrent loops among the lateral cerebellar nucleus, red nucleus, and lateral reticular nucleus constitute the turtle rubrocerebellar limb premotor network. Sensory inputs to the red nucleus originate in the contralateral dorsal column nuclei, the principal trigeminal nucleus, and the spinothalamic system. These sites project bilaterally to the lateral reticular nucleus. The lateral cerebellar nucleus receives a contralateral input from the dorsal column nuclei. The red nucleus projects sparsely to the dorsal column nuclei. The red nucleus also receives an ipsilateral descending projection from the suprapeduncular nucleus, located in the diencephalon, and an ascending input from the rostral rhombencephalic reticular formation. An ipsilateral descending pathway originating in the red nucleus is likely to be the rubro-olivary tract.

Synaptic innervation of neurones in the internal pallidal segment by the subthalamic nucleus and the external pallidum in monkeys

In order to better understand the way by which the subthalamic nucleus interacts with the globus pallidus to control the output of the basal ganglia, we carried out a series of experiments to investigate the pattern of synaptic innervation of the pallidal neurones by the subthalamic terminals in the squirrel monkey. To address this problem we used the anterograde transport of biocytin. Following injections of biocytin in the subthalamic nucleus, rich plexuses of labelled fibres and varicosities formed bands that lay along the medullary lamina in both segments of the ipsilateral pallidum. At the electron microscopic level, two populations of biocytin-containing terminals were identified in the internal pallidum (GPi). A first group of small to medium-sized terminals (type 1; mean cross-sectional area +/- S.D. = 0.41 +/- 0.04 microns 2) contained round vesicles and formed asymmetric synapses with dendritic shafts (95%) of mixed sizes (maximum diameter ranging from 0.3 to 4.0 microns) and spine-like structures (5%). The second group of terminals (type 2) contained pleiomorphic vesicles, had a larger cross-sectional area (mean +/- S.D. = 0.9 +/- 0.4 micron 2) and formed symmetric synapses predominantly with perikarya (41%) and large dendrites (57%). In some cases, the two types of terminals converged at the level of single GPi neurones. Postembedding immunogold method revealed that the type 2 terminals displayed gamma-aminobutyric acid (GABA) immunoreactivity, whereas the type 1 terminals did not. In the external pallidum (GPe), injections in the subthalamic nucleus labelled both type 1 or type 2 terminals. However, the labelled type 2 boutons were much less abundant in GPe than in GPi. The presence of biocytin-labelled perikarya in GPe and the fact that the type 2 terminals displayed GABA immunoreactivity led us to suspect that these terminals were derived from axons of GPe neurones. In agreement with this hypothesis, injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in GPe labelled terminals in GPi that displayed the morphological features and a pattern of synaptic organization similar to the type 2 terminals. In conclusion, the results of our study demonstrate that the subthalamopallidal terminals form asymmetric synapses that are distributed along the dendritic tree of GPe and GPi neurones. In contrast, the GPe projection to GPi gives rise to large GABA-containing terminals that form symmetric synapses predominantly with the proximal region of pallidal neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution and coverage of A- and B-type horizontal cells stained with Neurobiotin in the rabbit retina

Both A- and B-type horizontal cells in the rabbit retina were labeled by brief in vitro incubations of the isolated retina in the blue fluorescent dye 4,6-diamino-2-phenylindole. Intracellular injection of Lucifer Yellow into the somata revealed the morphology of the individual cells. Dye-coupling with Lucifer Yellow was seen only between A-type horizontal cells. By contrast, injection of the tracer Neurobiotin showed dye-coupling between both A- and B-type horizontal cells. There also appeared to be coupling between the axon terminals of B-type horizontal cells. The extensive dye-coupling seen following injection of Neurobiotin into a single horizontal cell soma can be used to obtain population counts of each cell type. Staining of large numbers of each cell type across the retina showed that each type increased in number and declined in dendritic diameter as the visual streak was approached, such that relatively constant coverage across the retina was maintained. In the visual streak, A-type horizontal cells numbered 555 cells/mm2 and averaged 120 microns in diameter, compared to 1375 cells/mm2 and 100 microns for B-type horizontal cells. In the periphery, the A- and B-types numbered 250 cells/mm2 and 400 cells/mm2, respectively. The average diameters of the dendritic trees at these locations were 225 microns for the A-type and 175 microns for the B-type. Coverage across the retina averaged almost six for A-type horizontal cells and 8-10 for the B-type. A-type horizontal cells in the visual streak whose elliptical dendritic fields were shown by Bloomfield (1992) to correlate physiologically with orientation bias were shown to be dye-coupled to cells with symmetrical dendritic fields.

The network properties of bipolar-bipolar cell coupling in the retina of teleost fishes

Retinal bipolar cells exhibit a center-surround antagonistic receptive field to a light stimulus (Werblin & Dowling, 1969; Kaneko, 1970), and thus constitute an early stage of spatial information processing. We injected Lucifer Yellow and a small biotinylated tracer, biocytin, into bipolar cells of the teleost retina to examine electrical coupling in these cells. Lucifer-Yellow coupling was observed in one of 55 stained bipolar cells; the coupling pattern was one injected bipolar cell and three surrounding cells. Biocytin coupling was observed in 16 of 55 stained bipolar cells, six of which were ON center and ten OFF center. Although biocytin usually coupled to three to six bipolar cells, some OFF-center bipolar cells showed strong coupling to more than 20 cells. The biocytin-coupled bipolar cells were morphologically homologous. Membrane appositions resembling gap junctions were found between dendrites and between axon terminals of neighboring bipolar cells. In the strongest biocytin-coupled bipolar cells, the contacts between bipolar cells and cone photoreceptor cells were examined after reconstruction of the dendritic trees of five well-stained, serially sectioned OFF-center bipolar cells. Each of these bipolar cells was in contact with different numbers of cones: 11 to 20 for twin cones and two to four for single cones. This implies that, although these bipolar cells belong to the same category, the signal inputs differ among bipolar cells. Numerical simulation conducted on a hexagonal array network model demonstrated that the electrical coupling of bipolar cells can decrease the difference in input (approximately 80%) without causing significant loss of spatial resolution. Our results suggest that electrical coupling of bipolar cells has the advantage of decreasing the dispersion of input signals from cones, and permits bipolar cells of the same class to respond to light with similar properties.

Electrophysiological and morphological classification of myenteric neurons in the proximal colon of the guinea-pig

Intracellular recordings were made from myenteric neurons in the proximal colon of the guinea-pig. The electrical behaviour of the neurons in response to intracellular depolarizing current pulses, and to internodal strand stimulation, was recorded. The intracellular electrode contained the intracellular marker biocytin which was injected into impaled neurons for subsequent histochemistry. Proximal colon myenteric neurons displayed electrophysiological properties similar to myenteric neurons in the small intestine, and were classified as either AH- or S-neurons. AH-neurons were characterized by the presence of a slow afterhyperpolarization following an action potential. Internodal strand stimulation evoked slow excitatory synaptic potentials in five out of six AH-neurons tested, but did not evoke fast excitatory synaptic potentials in 26 AH-neurons tested. S-neurons lacked a slow afterhyperpolarization, but internodal strand stimulation evoked fast excitatory synaptic potentials in all 113 neurons and slow excitatory synaptic potentials in seven out of 17 tested. A subpopulation of AH-neurons displayed a rhythmic oscillation in membrane potential which could be triggered by an action potential. S-neurons could be subdivided into those that fired tonically and those that fired phasically in response to long depolarizing current pulses. About 80% of the AH-neurons were immunoreactive for calbindin, as were 10% of S-neurons. A further 17% of S-neurons, but no AH neurons, were calretinin immunoreactive. Morphological analysis of filled neurons revealed eight distinct classes. Neurons electrophysiologically classified as AH typically had a large, oval soma and several long tapering processes. Processes of AH-neurons branched into many adjacent ganglia. Almost all S-neurons were uniaxonal and many axons ended in an expansion bulb in the myenteric plexus. S-neurons typically had broad, lamellar processes, or short, spiny processes. Roughly equal proportions of S-neurons had oral or anal projection. However, almost all S-neurons that were immunoreactive for calbindin or calretinin projected orally. The results indicate that myenteric neurons in the proximal colon of the guinea-pig are electrophysiologically similar to myenteric neurons in the small intestine, but there are a greater number of morphological and chemical categories.

Intrinsic cortical connections in macaque visual area V2: evidence for interaction between different functional streams

Area V2 of macaque visual cortex represents an important but poorly understood stage in visual processing. To provide a better understanding of the region, we studied the organization of its intrinsic cortical connections by making focal (200-300 microns) iontophoretic microinjections of the tracer biocytin. Alternate tissue sections were tested for biocytin, cytochrome oxidase (CO), or Cat-301 immunoreactivity to localize biocytin label relative to the three stripelike compartments that characterize this area. Biocytin-labeled pyramidal neurons of layers 2/3, and, to a lesser extent, layer 5, provided laterally spreading axon projections that terminated in discrete patches (250-300 microns diameter), primarily in layers 1-3. Any injected locus in V2 projected to 10-15 similarly sized patches, up to 4 mm from the injection site, and distributed in an elongated field orthogonal to the stripe compartments. We noted prominent patchy connections within, as well as between, individual compartments, perhaps reflecting functional substructures within stripes. Each stripe compartment projected to all three compartments but with different relative frequencies; CO-rich compartments projected mainly to other CO-rich compartments (75%), whereas CO-poor compartments projected equally to CO-rich and CO-poor compartments. We therefore emphasize the existence of substantial interconnections among all three V2 compartments. As further evidence for crosstalk between visual channels, we also noted an input to the V2 "thick" CO stripes from V1 cells in layer 4A as a distinct population in addition to the neurons of layer 4B. Thus, the CO stripe architecture may not be a marker for strictly segregated parallel visual pathways through V2.

Golgi-like labeling of a single neuron recorded extracellularly

We describe a novel and powerful extracellular method for the staining of a single neuron identified by electrophysiological criteria. This single-unit technique involves the use of glass micro-electrodes (tip diameter: 1.5-2.5 microns) filled with a saline solution (NaCl; 0.5 M) containing 1.5% of biocytin or Neurobiotin. Once a neuron is recorded, isolated and identified, the tracer is delivered by anodal current pulses of a few nA. The cell must remain well isolated and alive during the ejection procedure to ensure optimal staining. Evidence is provided that the labeled neuron is actually the one that was recorded. Our simple method is reliable with a success rate exceeding 85%. The advantages and pitfalls are discussed. This single-unit labeling technique could further be combined with any other procedures ranging from biological to behavioral studies.

Whole-cell patch-clamp recordings from visualized bulbospinal neurons in the brainstem slices

The purpose of this study was to develop a method for electrophysiological characterization of retrogradely labeled bulbospinal neurons in the specific cytoarchitectonic regions in the brainstem slices. Several days after the spinal cord was injected with the carbocyanine dye, DiI, retrogradely labeled bulbospinal neurons were visualized by epifluorescence optics in the brainstem slices with the aid of a silicon intensifier tube (SIT) camera. Labeled somata were routinely seen in the caudal raphe nuclei, rostroventral medial and lateral portions of the medulla, the A5 group and in other medullary sites known to project to the spinal cord. Electrophysiological properties of the DiI-labeled neurons were assessed by whole-cell recordings using micropipettes filled with biocytin. The slices were subsequently processed for dual visualization of biocytin and serotonin or a marker for noradrenergic neurons, tyrosine hydroxylase (TH). The electrophysiological properties of bulbospinal neurons were correlated with their morphology and neurochemical content. This technique may be useful in other areas of CNS for studying morphology, neurochemical content and physiology of retrogradely labeled neurons.

A comparison of synapses onto the somata of intrinsically bursting and regular spiking neurons in layer V of rat SmI cortex

Regular spiking (RS) and intrinsically bursting (IB) neurons show distinct differences in their inhibitory responses. Under various conditions, the synaptic responses of RS cells display marked inhibitory postsynaptic potentials (IPSPs), whereas the responses of most IB cells do not (Silva et al: Soc Neurosci Abstr 14:883, 1988; Chagnac-Amitai and Connors: J Neurophysiol 61:747, 62:1149, 1989; Connors and Gutnick: TINS 13:99, 1990). This investigation is designed to determine if differences in the inhibitory responses of RS versus IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified by using intracellular recording and labeling, examined with the light microscope, and then serial thin-sectioned prior to examination with the electron microscope. Axonal terminals presynaptic to their somata and proximal dendrites were identified and classified according to criteria described by Peters and coworkers (Peters et al: J Neurocytol 19:584, 1990; Peters and Harriman: J Neurocytol 19:154, 1990; 21:679, 1992). The locations of these boutons were displayed on the surfaces of 3-D reconstructions of the somata and proximal dendrites. The reconstructions were produced directly from the serial thin sections by using a novel, electron microscopic, image-processing computer resource. Our analysis showed no significant difference in the types and concentration of boutons presynaptic to the cell bodies and proximal dendrites of intrinsically bursting versus regular spiking neurons. We conclude that the differences observed in the inhibitory responses of intrinsically bursting versus regular spiking neurons cannot be explained by differences in the concentrations of synapses onto their somata.

Electrophysiological characterization of CA2 pyramidal cells from epileptic humans

The CA2 region of the hippocampus is more resistant to the principal cell loss seen in CA1 and CA3 in both animal models of temporal lobe epilepsy and in medial temporal lobe sclerosis (MTS), a common neuropathological finding in human temporal lobe epilepsy. There is extensive synaptic reorganization in the MTS hippocampi that is not seen in the hippocampi of patients with tumor-associated temporal lobe epilepsy (TTLE). The authors examined the electrophysiological properties of CA2 pyramidal cells from these two types of human hippocampi. The two main findings are that most MTS cells do not have clear evidence for inhibition yet do not fire synaptically evoked bursts; and that mossy fiber stimulation could evoke excitatory postsynaptic potentials (EPSPs) in the MTS tissue, but not the TTLE cells. These data suggest that in MTS, CA2 cells are resistant to firing epileptiform bursts which may account for their survival. Moreover, the granule cell-CA2 cell connection represents a novel form of synaptic plasticity in this disease.

Biotin staining in the giant fiber systems of the lobster

The avidin-biotin-complex method is a popular immunocytochemical technique. This method labels consistently a group of neurons in the lobster ventral nerve cord in the absence of primary antibodies. The specific staining is due to a relatively high level of endogenous biotin (or biocytin) in these neurons. These biotin-positive neurons are located in the supraesophageal, thoracic, and abdominal ganglia. Intraaxonal injection of Lucifer yellow followed by Texas red-conjugated streptavidin staining reveals that the neurons are members of the medial giant (MG) and lateral giant (LG) systems, which are important in mediating rapid tail flipping during escape maneuvers. In neuronal somata, staining is restricted to the cytoplasm. Within MG axons, staining appears as punctate, subaxolemmal structures. Preincubating nerve cords in biocytin or direct intraaxonal injection of biocytin enhances staining of these punctate organelles. In LG axons, staining is localized to fragments of braided filamentous structures that also appear to be associated with the axolemma. Preincubation of ventral nerve cords in various concentrations of biocytin results in the appearance of additional groups of stained neurons, suggesting that there are subsets of neurons with specific biocytin-uptake or -retention mechanisms. In the crayfish, biotin-positive staining is confined to the MG neurons; the LG neurons are not stained. In the earthworm, no staining is observed in the MG and LG axon escape systems. In the goldfish, no biotin-staining is seen in the Mauthner neurons and their axons. The significance of specific localization of biotin or biocytin to subsets of neurons is unclear. It may reflect the presence of high levels of biocytin moieties on biotin-dependent enzymes. Biotin is an important cofactor in the catalytic functions of several decarboxylases crucial in energy production and lipogenesis. Axons of the giant fiber systems in lobsters and crayfish may have high energy and fatty acid synthesis requirements. Increased levels of biotin accumulation may also be related to other functions of the giant axon systems, such as the formation of electrical synapses among themselves and with phasic motoneurons.

Morphological study of long axonal projections of ventral medullary inspiratory neurons in the rat

The aim of this study was to examine medullary and spinal axonal projections of inspiratory bulbospinal neurons of the rostral ventral respiratory group (VRG) in the rat. A direct visualization of long (9.8-33 mm) axonal branches, including those projecting to the contralateral side of the medulla oblongata and the spinal cord, was possible due to intracellular labeling with neurobiotin and long survival times (up to 22 h) after injections. Seven of the nine labeled neurons had bilateral descending axons, which were located in discrete regions of the spinal white matter; ipsilateral axons in the lateral and dorsolateral funiculus, contralateral in the ventral and ventromedial funiculus. The collaterals issued by these axons at the mid-cervical level formed close appositions with dendrites of phrenic motoneurons, which had also been labeled with neurobiotin. None of these collaterals crossed the midline. The significance of this finding is discussed in relation to the crossed-phrenic phenomenon. Additional spinal collaterals were identified in the C1 and T1 segments. Within the medulla, collaterals with multiple varicosities were identified in the lateral tegmental field and in the dorsomedial medulla (in the hypoglossal nucleus and in the nucleus of the solitary tract). These results demonstrate that inspiratory VRG neurons in the rat have some features which have not been previously described in the cat, including frequent bilateral spinal projection and projection to the nucleus of the solitary tract. In addition, this study shows that intracellular labeling with neurobiotin offers an effective way of tracing long axonal projections, supplementing results previously obtainable only with antidromic mapping, and providing morphological details which could not be observed in previous studies using labeling with horseradish peroxidase.

Double-staining of horizontal and amacrine cells by intracellular injection with lucifer yellow and biocytin in carp retina

Horizontal and amacrine cells in the isolated carp retina were impaled with micropipette electrode, identified by their characteristic light responses, and injected iontophoretically with markers for morphological study. Both Lucifer Yellow CH and biocytin were injected simultaneously. Lucifer Yellow was seen by its own fluorescence while biocytin was visualized by binding with Texas Red-linked or horseradish peroxidase-conjugated avidin. For cone-connected horizontal cells, biocytin-coupled cells were found to be approximately five-times more numerous than Lucifer Yellow-coupled cells. Coupling for both tracers was consistently hampered by intravitreally applied dopamine. In untreated retinas, the injected Lucifer Yellow was restricted within one rod-connected horizontal cell, while biocytin revealed several coupled neighbors. Amacrine cells, labeled by the tracers, were morphologically grouped into eight types, based on our earlier classification. Among them, amacrine cells, belonging to three types (Fnd, Pmb or Pma), were confirmed to be Lucifer Yellow-coupled, and the number of biocytin-coupled cells was more numerous (about 2.5 times) than that of Lucifer Yellow-coupled cells. Most amacrine cells (i.e. Pwd, Fnb and Fna) showed biocytin-coupling with no Lucifer Yellow-coupling. A few classified (i.e. Pwb and Fwa) and unclassified cells did not show any coupling. Since the tracer coupling takes place via gap junctions, the majority of amacrine cells, belonging to certain homologous types, appear to be functionally coupled with each other in the inner plexiform layer. However, dopamine did not influence the range of tracer coupling between amacrine cells in the carp retina under the present experimental conditions.

Neurophysiological, pharmacological and morphological properties of human caudate neurons recorded in vitro

Tissue samples from the caudate nucleus were obtained from eight children (eight to 172 months of age) who underwent hemispherectomies for the relief of intractable seizures. Neurophysiological, pharmacological and morphological properties of caudate neurons were characterized by intracellular recordings in an in vitro slice preparation. These properties were compared with those of tissue obtained from animal studies. Electrophysiological properties of human caudate neurons that were similar to those of cat caudate and rat neostriatal cells included resting membrane potential, input resistance, action potential rise time, fall time, duration and action potential afterhyperpolarization amplitude, as well as the general characteristics of locally evoked synaptic responses. Properties that were different included action potential amplitudes and time-constants. Human caudate neurons also displayed responses similar to those of cat caudate or rat neostriatal cells to manipulation of excitatory amino acid receptor systems and to dopamine application. Kynurenic acid, a broad-spectrum excitatory amino acid receptor antagonist, decreased the amplitude of evoked synaptic responses, indicating that they were partially mediated by excitatory amino acids. In Mg2+ free Ringer's solution, the amplitudes and durations of postsynaptic responses were increased and bursts of action potentials were induced. These effects were mediated by activation of N-methyl-D-aspartate receptors since they were blocked by 2-amino-5-phosphonovalerate, a specific N-methyl-D-aspartate-receptor antagonist. Iontophoretic application of N-methyl-D-aspartate also induced membrane oscillations and bursts in almost all caudate neurons. Dopamine decreased the amplitude of postsynaptic responses, an effect antagonized by domperidone, a selective D2 dopamine receptor antagonist. Developmentally, the greatest change was an increase in action potential amplitude, although input resistance decreased and action potential afterhyperpolarization amplitude increased. Postsynaptic responses were similar across age. All but one of the caudate neurons identified by intracellular injection of biocytin or Lucifer Yellow were medium-sized spiny cells. These experiments show that human caudate neurons display a number of electrophysiological properties similar to rat neostriatal or cat caudate neurons recorded in brain slices. Furthermore, few electrophysiological parameters changed significantly over the age period examined suggesting that the human caudate at eight months displays many of the neuronal functions of the more mature caudate nucleus.

Light- and electron-microscopic study of electrophysiologically characterized neurons in the mediolateral part of the lateral spetum of the guinea-pig

In slices of guinea-pig brains, 36 neurons located in the mediolateral part of the lateral septum were stained intracellularly with horseradish peroxidase (n = 28) or biocytin (n = 8) after electrophysiological characterization. These neurons belonged to class A neurons (n = 23), which generated pronounced Ca(++)-dependent high-threshold spikes in control medium, or to class C neurons (n = 9), which were recognized by the occurrence of small-amplitude sodic spikes followed by slower larger calcic spikes. The present results demonstrate that, despite the variety of individual cell types, the major morphological population (30/36 cells) was composed of a homogeneous class of large-sized neurons that displayed thick primary dendrites and abundant dendritic appendages. The remaining 6 cells were small-sized, poorly-spiny neurons. Somatic spines were observed on 5 out of the 30 large cells and on one out of the six smaller cells. Labeled axons were mainly oriented to the anterior commissure. The axons of nine cells richly collateralized near the perikaryon. Ultrastructural examination of 3 horseradish peroxidase-injected cells showed indented nuclei, classic organelles and somatic spines. Terminal boutons established symmetric synapses with the injected cells. These results describe the morphological features of electrophysiologically identified neurons and indicate that class A and class C neurons are distributed among morphological populations differing in perikaryal size. This suggests that the different electrical properties of class A and class C neurons reflect recordings from different parts of the neuron rather than from neurons of different types. Furthermore, the present findings demonstrate that, in the guinea-pig, electrical and morphological characteristics of somatospiny neurons are comparable with those of non-somatospiny neurons. Somatospiny neurons have a recognized integrative role in the hippocampo-septo-hypothalamic complex.

Electrophysiological characteristics of immunochemically identified rat oxytocin and vasopressin neurones in vitro

1. Intracellular recordings were made from supraoptic neurones in vitro from hypothalamic explants prepared from adult male rats. Neurones were injected with biotinylated markers, and of thirty-nine labelled neurones, nineteen were identified immunocytochemically as containing oxytocin-neurophysin and twenty as containing vasopressin-neurophysin. 2. Vasopressin and oxytocin neurones did not differ in their resting membrane potential, input resistance, membrane time constant, action potential height from threshold, action potential width at half-amplitude, and spike hyperpolarizing after-potential amplitude. Both cell types exhibited spike broadening during brief, evoked spike trains (6-8 spikes), but the degree of broadening was slightly greater for vasopressin neurones. When hyperpolarized below -75 mV, all but one neurone exhibited a transient outward rectification to depolarizing pulses, which delayed the occurrence of the first spike. 3. Both cell types exhibited a long after-hyperpolarizing potential (AHP) following brief spike trains evoked either with a square wave pulse or using 5 ms pulses in a train. There were no significant differences between cell types in the size of the AHP evoked with nine spikes, or in the time constant of its decay. The maximal AHP evoked by a 180 ms pulse was elicited by an average of twelve to thirteen spikes, and neither the size of this maximal AHP nor its time constant of decay were different for the two cell types. 4. In most oxytocin and vasopressin neurones the AHP, and concomitantly spike frequency adaptation, were markedly reduced by the bee venom apamin and by d-tubocurarine, known blockers of a Ca(2+)-mediated K+ conductance. However, a minority of neurones, of both cell types, were relatively resistant to both agents. 5. In untreated neurones, 55% of vasopressin neurones and 32% of oxytocin neurones exhibited a depolarizing after-potential (DAP) after individual spikes or, more commonly, after brief trains of spikes evoked with current pulses. For each neurone with a DAP, bursts of spikes could be evoked if the membrane potential was sufficiently depolarized such that the DAP reached spike threshold. In four out of five vasopressin neurones a DAP became evident only after pharmacological blockade of the AHP, whereas in six oxytocin neurones tested no such masking was found. 6. The firing patterns of neurones were examined at rest and after varying the membrane potential with continuous current injection. No identifying pattern was strictly associated with either cell type, and a substantial number of neurones were silent at rest.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological and morphological differentiation of chandelier and basket cells in the rat hippocampal formation: a study combining intracellular recording and intracellular staining with biocytin

Using standard intracellular recording techniques 38 nonpyramidal cells or interneurons have been sampled in hippocampal slices of the rat. Among 38 physiologically identified interneurons, all 27 cells labeled with biocytin were morphologically demonstrated to be nonpyramidal and nongranule cells. The vast majority of these cells showed typical fast spiking discharges, i.e., a shorter duration action potential followed by a brief but prominent after hyperpolarisation potential without frequency adaptation in response to prolonged depolarizing current injection. However, 4 cells clearly exhibited frequency adaptation. Based on their axonal arborizations, the former group included basket interneurons innervating the principle cell body layers and axodendritic interneurons projecting to the molecular layer of the dentate gyrus; whereas the latter 4 cells belonged to chandelier interneurons selectively terminating the axon initial segments of principle cells. These results support the notion that interneurons in the hippocampal formation are heterogeneous with respect to their morphology and electrophysiological characteristics, suggesting that the electrophysiological behavior of hippocampal interneurons may be associated with their functional activities.

Murine embryonal carcinoma-derived neurons survive and mature following transplantation into adult rat striatum

P19 embryonal carcinoma cells are pluripotent and can be efficiently induced to differentiate in culture into neurons and astroglia by brief treatment with retinoic acid. Retinoic acid-treated P19 cells survive after grafting into the adult rat striatum and differentiate into neurons and glia within the transplantation site. No tumours develop from the grafted cells which continue to express foreign genes that had been transfected into the parental P19 cells. The neurons in these grafts express a variety of neurotransmitters similar to those formed in retinoic acid-treated P19 cell cultures and they mature to acquire the electrophysiological properties expected of fully developed neurons. These results suggest that P19 cells may be used for studies related to neuronal cell development and maturation and that P19 cells may be considered for cell replacement strategies in neurodegenerative disorders of the central nervous system.

The African wave-type electric fish, Gymnarchus niloticus, lacks corollary discharge mechanisms for electrosensory gating

Gymnarchus niloticus, a wave-type African electric fish, performs its jamming avoidance response by relying solely upon afferent signals and does not use corollary discharges from the pacemaker nucleus in the medulla which generates the rhythmicity of electric organ discharges. This is in sharp contrast to the mode of sensory processing found in closely related African pulse-type electric fishes where afferent signals are gated by corollary discharges from the pacemaker for the distinction of exafferent and reafferent stimuli. Does Gymnarchus still possess a corollary discharge mechanism for other behavioral tasks but does not use it for the jamming avoidance response? In this study, I recorded from and labeled medullary neuronal structures that either generate or convey the pacemaker signal for electric organ discharges to examine whether this information is also sent directly to any sensory areas. The pacemaker nucleus was identified as the site of generation of the pacemaking signal. The pacemaker neurons project exclusively to the lateral relay nucleus which, in turn projects exclusively to the medial relay nucleus. Neurons in the medial relay nucleus send unbranched axons to the spinal electromotoneurons. These neurons are entirely devoted to drive the electric organ discharges, and no axon collaterals from these neurons were found to project to any sensory areas. This indicates that Gymnarchus does not possess the neuronal hardware for a corollary discharge mechanism.

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular labeling of cat spinal neurons using a tetramethylrhodamine-dextran amine conjugate

Tetramethylrhodamine-dextran is a highly fluorescent neuroanatomical tracer that, in its 10,000 MW form, has seen widespread use as a sensitive anterograde tract-tracing label. We report here the use of a lower molecular weight tetramethylrhodamine-dextran (3000 MW; Molecular Probes, OR) as an in vivo intracellular marker of locomotor-related spinal neurons. In the paralyzed, decerebrate cat preparation, fictive locomotion was evoked by electrical stimulation of the mesencephalic locomotor region. Extracellular and intracellular potentials of rhythmically active spinal neurons were recorded using microelectrodes filled with 2% tetramethylrhodamine-dextran (3000 MW) in 0.9% saline (impedance 5-20 Mohm). Following impalement and electrophysiological characterization, neurons were iontophoretically injected for 2-30 min with 3-10 nA of pulsed positive current. Animals were then perfused 30 min to 7 h postinjection with a variety of paraformaldehyde- and glutaraldehyde-containing fixatives. After tissue sectioning, more than 90% of the injected neurons were recovered. Choline acetyltransferase-immunoreactivity could be demonstrated in a subpopulation of tetramethylrhodamine-dextran-labeled neurons. This technique, in addition to producing high-quality electrodes, has the advantages of rapid yet extensive filling of neuronal processes, no tissue processing prior to visualization, and compatibility with immunohistochemistry.

Effects of target depletion on adult mammalian central neurons: morphological correlates

The morphological sequelae induced by target removal were studied on adult cat abducens internuclear neurons at both the somata and terminal axon arborization levels. The neuronal target--the medial rectus motoneurons of the oculomotor nucleus--was selectively destroyed by the injection of toxic ricin into the medial rectus muscle. Retrograde labeling with horseradish peroxidase demonstrated the survival of the entire population of abducens internuclear neurons up to one year after target removal. However, soma size was reduced by about 20% three months postlesion and maintained for one year. At the ultrastructural level, a considerable deafferentation of abducens internuclear neurons was observed at short intervals (i.e. 10 days after lesion). Large regions of the plasmalemma appeared devoid of presynaptic boutons but were covered instead by glial processes. The detachment of synaptic endings was selective on abducens internuclear neurons since nearby motoneurons always showed a normal synaptic coverage. By one month, abducens internuclear neurons recovered a normal density of receiving axosomatic synapses. Anterogradely biocytin-labeled axon terminals of abducens internuclear neurons remained in place after the lesion of medial rectus motoneurons, although with a progressive decrease in density. Ultrastructural examination of the oculomotor nucleus 10 days after the lesion revealed numerous empty spaces left by the dead motoneurons. Targetless boutons were observed surrounded by large extracellular gaps, still apposed to remnants of the postsynaptic membrane or, finally, ensheathed by glial processes. At longer intervals (> one month), the ultrastructure of the oculomotor nucleus was re-established and labeled boutons were observed contacting either unidentified dendrites within the neuropil or the soma and proximal dendrites of the oculomotor internuclear neurons, that project to the abducens nucleus. Labeled boutons were never found contacting with the oculomotor internuclear neurons either in control tissue or at short periods after ricin injection. These results indicate that the availability of undamaged neurons close to the lost target motoneurons might support the long-term survival of abducens internuclear neurons. Specifically, the oculomotor internuclear neurons, which likely suffer a partial deafferentation after medial rectus motoneuron loss, constitute a potential new target for the abducens internuclear neurons. The reinnervation of a new target might explain the recovery of synaptic and firing properties of abducens internuclear neurons after medial rectus motoneuron lesion, which occurred with a similar time course, as described in the accompanying paper [de la Cruz R. R. et al. (1994) Neuroscience 58, 81-97.].

Spontaneous activity and responses to sensory stimulation in ventrobasal thalamic neurons in the rat: an in vivo intracellular recording and staining study

Spontaneous activity and responses to sensory stimulation in ventrobasal (VB) thalamic neurons were studied in barbiturate-anesthetized rats through intracellular recordings. The recordings were carried out with micropipettes filled with K acetate KCl plus horseradish peroxidase (HRP), our KCl plus biocytin. Two types of spontaneous depolarizing events were observed: fast potentials (FPs), characterized by a low amplitude (5.3 +/- 1.8 mV [mean and standard deviation]), a fast rising slope (1.15 +/- 0.19 msec), and a short duration (8.47 +/- 0.89 msec); and slow potentials (SPs), characterized by a larger and more variable amplitude (9.1 +/- 5.6 mV) and a longer duration (62.5 +/- 27.2 msec), with a slower rising slope (26.2 +/- 6.4 msec). The potential changes elicited by sensory stimuli delivered manually were similar to those elicited by electronically gated short air jets to the receptive fields. FPs were evoked by sensory stimulation in 62.7% of the recorded neurons, and SPs in the remaining 37.3%. Both types of events could occur spontaneously in the same neuron, but only one of them was triggered by stimulation of the receptive field. Five neurons that were successfully stained with either HRP or biocytin were studied in detail. All were medium-sized stellate cells, with spine-like appendages sparsely distributed along slender radiating dendrites. The axons took a rostrolateral course across the VB, and all but one left one or two thin collaterals in the reticular thalamic nucleus. No overt morphological differences were observed between VB neurons that responded with FPS or SPs to sensory stimulation.

Topography of pyramidal neuron intrinsic connections in macaque monkey prefrontal cortex (areas 9 and 46)

An understanding of the normal organization of prefrontal cortex is essential to the recognition of pathology underlying human behavioral disorders believed to depend on this region. We have therefore studied the pattern of intrinsic intra- and interlaminar pyramidal neuron connectivity in prefrontal areas 9 and 46 (of Walker) in macaque monkey cerebral cortex (anterior to the arcuate sulcus between the principal sulcus and midline). We made focal (200-400 microns) injections of biocytin and mapped the pattern of orthogradely transported label. Injections made into the superficial layers label wide-ranging lateral projections within the same areas of prefrontal cortex. Projections local to such small injections form a narrow band of terminals in layers 1-3 (200-400 microns wide, 2-4 mm long) centered on the injection site. Collateral fibers spread orthogonal to this terminal band, making frequent bifurcations, to establish a series of parallel bands of terminals with uninnervated bands between, spaced regularly across the cortex (center to center 500-600 microns). The entire pattern of terminal label is stripe-like, with occasional narrower interbands and crosslinks between the bands, and can extend over 7-8 mm across the cortex. These projections arise from pyramidal neurons in layers 2, 3, and 5 and terminate in layers 1-3. The stripe-like pattern contrasts with patch-like patterns in other cortical regions (V1, V2, V4, motor, somatosensory) and is smaller in scale than stripe-like zones of corticocortical afferent terminals to this region, reported to be 300-750 microns wide and spaced 1.0-1.5 mm center to center.

Calcium spike underlying rhythmic firing in dopaminergic neurons of the rat substantia nigra

In order to study a possible mechanism for rhythmic firing of dopaminergic (DA) neurons, intracellular recordings were obtained from 56 rhythmically firing DA neurons in the rat substantia nigra compacta (SNc), using in vitro slice preparations. In the presence of TTX, spontaneous oscillation of the membrane potential was induced in SNc DA neurons when the membrane potential was depolarized more positive from -60 to -40 mV. Each oscillation wave was characterized by a pacemaker-like slow depolarization (PLSD) followed by a relatively prompt repolarization. As the DC depolarization was increased from -60 to -40 mV, the oscillation frequency increased from 0.5 to 5 Hz, but the amplitude of the wave decreased. Of 17 neurons tested in the presence of TTX, the maximum amplitudes of the oscillation varied from 10-15 mV in 8 neurons and were less than 5 mV in 9 neurons. In those 9 neurons, an application of TEA greatly enhanced (up to 15 mV) the amplitude of oscillation. The oscillation ceased when the membrane was hyperpolarized more negative than -60 mV. At the membrane potential more negative than -60 mV in the presence of TTX an injection of a depolarizing current pulse could evoke PLSD which was an all-or-nothing regenerative spike potential. The rate of rise of the PLSD changed depending on the intensity of injected current pulses but their amplitude remained constant. Its time-to-peak was slow (up to 1400 ms), while the decay time was relatively brief (< 500 ms). The threshold membrane potential for evoking PLSD was -53.7 +/- 3.2 mV (n = 10). This was higher than the previously reputed threshold for low threshold Ca2+ spike (LTS) (< -60 mV) and lower than that for high threshold Ca2+ spike (HTS) (> -35 mV) in SNc DA neurons. Even at a holding potential of -45 mV, a depolarizing current pulse could trigger PLSD while LTS was completely inactivated. Cd2+ (0.4 mM) abolished the oscillation and PLSD without marked effects on the LTS (n = 6). A low Ca2+ and high Mg2+ Ringer's solution also abolished the oscillation and PLSD (n = 4). An intracellular injection of EGTA markedly prolonged the decay time course of PLSD characterized by a slow and a relatively fast falling phase (n = 5). This would suggest an involvement of Ca(2+)-dependent K+ conductance and/or Ca2+ dependent inactivation of Ca2+ conductance during repolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantal components of unitary EPSCs at the mossy fibre synapse on CA3 pyramidal cells of rat hippocampus

1. Excitatory postsynaptic currents (EPSCs) were recorded in CA3 pyramidal cells of hippocampal slices of 15- to 24-day-old rats (22 degrees C) using the whole-cell configuration of the patch clamp technique. 2. Composite EPSCs were evoked by extracellular stimulation of the mossy fibre tract. Using the selective blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonopentanoic acid (APV), a major alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor-mediated component and a minor NMDA receptor-mediated component with slower time course were distinguished. For the AMPA/kainate receptor-mediated component, the peak current-voltage (I-V) relation was linear, with a reversal potential close to 0 mV. The half-maximal blocking concentration of CNQX was 353 nM. 3. Unitary EPSCs of the mossy fibre terminal (MF)-CA3 pyramidal cell synapse were evoked at membrane potentials of -70 to -90 mV by low-intensity extracellular stimulation of granule cell somata using fine-tipped pipettes. The EPSC peak amplitude as a function of stimulus intensity showed all-or-none behaviour. The region of low threshold was restricted to a few micrometres. This suggests that extracellular stimulation was focal, and that the stimulus-evoked EPSCs were unitary. 4. Latency and rise time histograms of EPSCs evoked by granule cell stimulation showed narrow unimodal distributions within each experiment. The mean latency was 4.2 +/- 1.0 ms, and the mean 20-80% rise time was 0.6 +/- 0.1 ms (23 cells). When fitted within the range 0.7 ms to 20 ms after the peak, the decay of the EPSCs with the fastest rise (rise time 0.5 ms or less) could be described by a single exponential function; the mean time constant was in the range 3.0-6.6 ms with a mean of 4.8 ms (8 cells). 5. Peak amplitudes of the EPSCs evoked by suprathreshold granule cell stimulation fluctuated between trials. The apparent EPSC peak conductance in normal extracellular solution (2 mM Ca2+, 1 mM Mg2+), excluding failures, was 1 nS. Reducing the Ca2+ concentration and increasing the Mg2+ concentration reduced the mean peak amplitude in a concentration-dependent manner. 6. Peaks in EPSC peak amplitude distributions were apparent in low Ca2+ and high Mg2+. Using the criteria of equidistance and the presence of peaks and dips in the autocorrelation function, five of nine EPSC peak amplitude distributions were judged to be quantal.(ABSTRACT TRUNCATED AT 400 WORDS)

The behavior of mossy cells of the rat dentate gyrus during theta oscillations in vivo

Intracellular current clamp recordings were obtained from mossy cells (n = 6, identified by intracellular injection of biocytin) of the dorsal dentate gyrus from rats under ketamine-xylazine anesthesia. During electroencephalographic theta rhythm (4-6 Hz), recorded with a macroelectrode placed in the contralateral dorsal hippocampus near the fissure, mossy cells displayed intracellular membrane potential oscillations at low frequencies (4-6 Hz) which appeared to be phase locked to the electroencephalographic theta rhythm. The frequency of the intracellular theta rhythm was independent of the membrane potential. However, the phase difference between the intracellular and the electroencephalographic theta rhythms as well as the amplitude of the intracellular theta oscillations were voltage-dependent. These findings are consistent with the hypothesis that rhythmic GABAA receptor-mediated inhibitory postsynaptic potentials contribute to the genesis of the intracellular theta rhythm. Indeed, mossy cells displayed an early, fast inhibitory postsynaptic potential in response to electrical stimulation of the entorhinal cortex, which most likely represents a GABAA receptor-mediated event, indicating that mossy cells possess functional GABAA receptors. At the resting membrane potential, mossy cells did not fire at each cycle of the electroencephalographic theta rhythm but fired only rarely (< 1 Hz). However, when they did fire they did so preferentially in phase with the peak positivity of the electroencephalographic theta rhythm. Reconstruction of two mossy cells with axonal projections to the inner molecular layer showed that the spatial extent of the influence such weakly discharging mossy cells may have on other dentate gyrus neurons during theta oscillations can be several millimeters in the septotemporal direction. In conclusion, these findings show that mossy cells of the rat hilus during ketamine-xylazine anesthesia participate in theta oscillations of the hippocampal formation, during which their low-frequency firing may contribute to the phase-locking of a large number of spatially distributed postsynaptic neurons with postsynaptic sites in the inner molecular layer of the dentate gyrus.

Antennular projections to the midbrain of the spiny lobster. III. Central arborizations of motoneurons

The central organization of antennular motoneurons in the brain of the spiny lobster, Panulirus argus, was analyzed by combining biocytin backfills with serial reconstructions of the antennular nerves and the brain. Eighty-nine to 99 antennular motoneurons occur in each hemibrain. The somata of the motoneurons are distributed in a consistent pattern in two complex soma clusters, the ventral paired mediolateral cluster of the deutocerebrum and the dorsal unpaired median cluster of the tritocerebrum. The motoneurons arborize ipsilaterally in the lateral and median antennular neuropils and the tegumentary neuropil. The backfills indicate a minimum of five morphological types of motoneurons with different arborization patterns. The innervation pattern of the motoneurons, together with previously reported innervation patterns of antennular sensory afferents, suggest that the lateral antennular neuropil is a lower motor center driving local antennular reflexes in response to chemical and mechanical stimulation of the antennule, whereas the median antennular neuropil is a lower motor center for equilibrium responses.

Modulation by divalent cations of the current generated by vasopressin in facial motoneurons

Vasopressin generates a voltage-gated, sodium-dependent current in facial motoneurons in brainstem slices. Reducing the extracellular calcium concentration from 2 to 0.01 mM caused a 30 to 120% increase in the amplitude of this current. Lowering extracellular magnesium also enhanced it, but less efficiently. In the physiological solution, the response of facial neurons to vasopressin is thus partially blocked. Increasing extracellular calcium was without effect. Current-voltage curves indicate that the vasopressin current reversed at around 0 mV and suggest that the low-calcium-induced potentiation was due to an attenuation of the region of negative slope conductance.

Monosynaptic innervation of trigeminal motor neurones involved in mastication by neurones of the parvicellular reticular formation

In order to determine whether neurones in the parvicellular reticular formation are in direct synaptic contact with motor neurones innervating masticatory muscles, a combined retrograde and anterograde transport study was carried out in the rat at both light and electron microscopic levels. The animals received injections of the retrograde tracers wheat germ agglutinin conjugated to horseradish peroxidase or cholera toxin B conjugated to horseradish peroxidase into the masticatory muscles and of the anterograde tracer biocytin into the ipsilateral parvicellular reticular formation. The trigeminal motor nucleus was then examined for both anterograde and retrograde labelling in the light and electron microscopes. Retrogradely labelled motor neurones were identified in the trigeminal motor nucleus. They were large and their locations within the nucleus depended on the muscle injected. In addition, terminals anterogradely labelled with the biocytin that was injected in the parvicellular reticular formation were identified throughout the motor nucleus. At the electron microscopic level, the retrogradely labelled cells were found to receive input both from distinct types of unlabelled terminals and from terminals that were anterogradely labelled from the parvicellular reticular formation. The labelled terminals comprised one of the four classes of afferent terminals, being 1-2 microns in diameter and densely packed with spherical vesicles. They formed mostly asymmetrical but also symmetrical synapses with the labelled perikarya and dendrites. Anterogradely labelled terminals were also observed to form both symmetrical and asymmetrical synaptic contacts with unlabelled structures in the motor nucleus. It is concluded that neurones in the parvicellular reticular formation form direct synaptic contact with motor neurones of masticatory muscles. This pathway may represent the anatomical substrate by which the reticular formation exerts at least part of its influence on mastication. Since the parvicellular reticular formation receives input from the substantia nigra pars reticulata, it is possible that this pathway represents a system whereby the basal ganglia directly influence orofacial movement.

Expiratory neurons of the Bötzinger Complex in the rat: a morphological study following intracellular labeling with biocytin

The term "Bötzinger Complex" (BOT) refers to a distinct group of neurons, located near the rostral portion of the nucleus ambiguus, which are known to play an important role in the control of respiratory movements. Previous studies conducted in cats have demonstrated that most of these neurons are active during expiration, exerting a monosynaptic inhibitory action on several subpopulations of inspiratory neurons in the medulla and spinal cord. The aim of this study was to examine morphological properties and possible synaptic targets of BOT neurons in the rat. Forty-one expiratory neurons were labeled intracellularly with biocytin; 12 were interneurons (BOT neurons) and 29 were motoneurons. The latter could not be antidromically activated following stimulation of the superior laryngeal or vagal nerves. BOT neurons showed extensive axonal arborisations in the ipsilateral medulla, with some projections to the contralateral side. Bouton-like axon varicosities mainly clustered in two areas: near the parent cell bodies, and in the area corresponding to the rostral part of the ventral respiratory group (VRG). In five pairs of labeled neurons, each consisting of one BOT neuron and one inspiratory neuron in the rostral VRG, no appositions were identified at the light microscopic level between axons of BOT neurons and dendrites or cell bodies of inspiratory neurons. These results demonstrate that some features of BOT expiratory neurons in the rat are similar to those previously described in cats. The differences include their more ventral location in relation to the compact formation of nucleus ambiguus (retrofacial nucleus), and the relative paucity in the rat of neurons displaying an augmenting pattern of activity and of neurons with spinally projecting axons. In addition, we were unable to find morphological evidence for contacts between labeled BOT neurons and ipsilateral inspiratory neurons near the obex level, a finding not consistent with previous electrophysiological studies in the cat in which such synaptic connections have been identified.

Heterogeneity of rat corticospinal neurons

In order to examine the degree of diversity within a population of cortical projection neurons, rat corticospinal cells were retrogradely labeled in vivo by injecting rhodamine-tagged microspheres into the cervical spinal cord, and subsequently studied electrophysiologically and anatomically in neocortical slices maintained in vitro, by use of standard current clamp techniques and a double-labeling protocol (Tseng et al., J. Neurosci. Meth. 37:121-131, 1991). Three different subgroups were distinguished on the basis of their spiking behavior: (1) Adapting cells had a marked fast (50 ms) and slow phase (200 ms) of spike frequency adaptation; (2) regular spiking (RS) cells had only a period of fast adaptation; (3) some regular spiking neurons had prominent depolarizing afterpotentials (DAPs) and could generate bursts of spikes, often in repetitive fashion (RSDAP cells). Subgroups of RSDAP cells had different patterns of burst responses to depolarizing current pulses, suggesting differences in the types and/or sites of underlying ionic conductances. Adapting cells had a slightly higher membrane input resistance and more prominent slow hyperpolarizing afterpotentials than RS and RSDAP neurons; however, the activation of presumed anomalous rectifier current by intracellular hyperpolarizations was less prominent in adapting neurons. Orthodromic stimulation in layer I evoked presumed excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs)in all three types of cells, but prominent short-latency IPSPs were found in a higher percentage of adapting neurons. The morphology of electrophysiologically characterized corticospinal neurons was studied following intracellular injection of biocytin. All three spiking types were typical layer V pyramids with apical dendrites reaching layer 1, basal dendrites in infragranular layers, and deep-directed axons that had a moderate density of local collaterals in lower cortical layers. The profuseness of dendrites, examined by Sholl's analysis of two-dimensional, camera lucida-reconstructed neurons was comparable in the three neuronal subgroups, although a smaller somatic area and more slender apical dendritic trunk were found in adapting neurons. Our results suggest that corticospinal cells in rats are a heterogeneous population of projection neurons with respect to their spiking behavior, membrane properties, synaptic connections, and, to a lesser extent, their morphology. This diversity revealed in vitro adds new complexity to the classification of corticospinal neurons.

Dye-coupling among neurons of the rat locus coeruleus during postnatal development

Simultaneous recordings from pairs of locus coeruleus neurons in neonatal rat brain slices previously demonstrated synchronous, subthreshold oscillations of membrane potential (rats < 24 days old) and electronic-coupling between 40% of pairs of neurons from rats less than 10 days old. In the present study, slices from 1-21 day-old rats were stained with avidin-HRP-diaminobenzidine only if a single neuron per slice was impaled for longer than 10 min using an electrode containing biocytin. In slices from rats less than one week old, multiple stained neurons (3.8 +/- 0.6 neurons/slice) were observed in 10 of 11 slices. Apparent contacts between stained neurons were observed at varying distances along dendrites. In rats older than one week significantly fewer multiple stained neurons were observed (three of 20 slices). The proportion of neurons displaying spontaneous subthreshold oscillations of membrane potential decreased with age, and the frequencies of subthreshold oscillations of membrane potential and entrained action potentials increased with age. The presence of multiple stained neurons was not correlated with the occurrence of subthreshold oscillations, cell input resistance, or the number of coupled neurons predicted from the shape of electronic potentials. In recordings from neurons displaying subthreshold oscillations, input resistance was lower and the number of coupled neurons predicted from electrotonic potentials was greater than in those without oscillations. These results suggest that low resistance pathways are common between locus coeruleus neurons in brain slices from rats younger than about one week old, consistent with previous electrotonic-coupling studies.(ABSTRACT TRUNCATED AT 250 WORDS)

The ontogeny of repetitive firing and its modulation by norepinephrine in rat neocortical neurons

The postnatal ontogeny of electrical properties was studied in rat sensorimotor cortical neurons (P6 to adult) using intracellular recording in an in vitro slice preparation. Many action potential properties and input resistance changed during the first 4 postnatal weeks. Repetitive firing behavior also changed during the first postnatal month. Spike-frequency adaptation was much stronger in immature neurons. At 1 week postnatal, the majority of cortical neurons would only fire for less than 200 ms regardless of the intensity of long depolarizing current injections. These cells were normal in other parameters and could fire throughout a depolarizing current injection in the presence of inorganic calcium channel blockers or norepinephrine (NE), suggesting that the inability to fire was not due to injury. The frequency with which we encountered cells with this extreme adaptation decreased with age. Spike-frequency adaptation in immature neurons appears to be primarily controlled by Ca-dependent K+ conductances as in mature neurons. In mature and immature neurons, three afterhyperpolarizations (AHPs) could be distinguished by their rate of decline. The fast AHP followed repolarization of a single spike and was only partially Ca- and K-dependent. The medium duration AHP was Ca-dependent and apamin-sensitive and the slow AHP was partially Ca-dependent and not blocked by apamin. NE decreased the slow Ca-dependent AHP via beta-adrenergic receptors. This effect of NE on AHPs appeared qualitatively similar throughout postnatal development. NE had a proportionately greater effect in younger neurons, however, due to their relatively larger slow AHP. The quantitative differences of NE's action on the slow AHP (sAHP) led to a qualitative difference in NE's effect on firing behavior. The effects of NE on firing behavior may therefore be greater during times critical for cortical maturation.

The electrical geometry, electrical properties and synaptic connections onto rat V motoneurones in vitro

1. We have developed a tissue slice preparation which allows the study of the actions of single presynaptic neurones onto single trigeminal motoneurones in the immature rat. Our aim in this first stage of the work has been to assess the validity of this preparation as a model for responses obtained in vivo from trigeminal motoneurones in adult rats. We have quantified the integrative properties of the motoneurones and also the variability in transmission at synapses of single presynaptic neurones onto the motoneurones. This data has then been compared to similar published data obtained from adult (rat) trigeminal motoneurones in vivo. 2. Quantitative reconstructions were made of the morphology of three motoneurones which had been labelled with biocytin by intracellular injection. The neurones gave off six to nine dendrites, of mean length 522 microns (S.D. = 160; n = 22), which branched on average 10.5 times to produce 11.45 end-terminations per dendrite (S.D. = 8.57; n = 22). The mean surface area of the dendrites was 0.92 x 10(4) microns2 (S.D. = 0.67; n = 22), and, for individual cells, the ratio of the combined dendritic surface area to the total neuronal surface area ranged from 98.3 to 99.2% (n = 3). At dendritic branch points the ratio of the summed diameters of the daughter dendrites to the 3/2 power against the parent dendrite to the 3/2 power was 1.09 (S.D. = 0.21; n = 217), allowing branch points to be collapsed into a single cylinder. The equivalent cylinder diameter of the combined dendritic tree remained approximately constant over the proximal 25-40% of the equivalent electrical length of the dendritic tree and then showed tapering. The tapering could be ascribed to termination of dendrites at different electrical distances from the soma. 3. Electrical properties were determined for a total of eighty-seven motoneurones, all with membrane potentials more negative than 60 mV (mean = 66.0 mV; S.D. = 5.2) and spikes which overshot zero (mean spike amplitude = 77 mV; S.D. = 10.5; n = 87). The spikes were followed by after-hyperpolarizations (AHPs) of mean amplitude 2.2 mV (S.D. = 1.7; n = 47), and mean duration 54.1 ms (S.D. = 9.5; n = 47). The mean input resistance of the neurones was 7.5 M omega (S.D. = 2.5; n = 69), the mean membrane time constant was 3.5 ms (S.D. = 2.2; n = 35), and the mean rheobase was 1.6 nA (S.D. = 1.1; n = 56).(ABSTRACT TRUNCATED AT 400 WORDS)

A high degree of spatial selectivity in the axonal and dendritic domains of physiologically identified local-circuit neurons in the dentate gyrus of the rat hippocampus

The axonal and dendritic domains of neurons with extensive, locally arborizing axons were delineated in the dentate gyrus of the rat hippocampus. In horizontally cut slice preparations neurons were briefly recorded and subsequently filled with biocytin when one or several of the following physiological properties were observed: (i) high-amplitude short-latency spike afterhyperpolarization; (ii) lack of spike frequency adaptation; (iii) high firing rate in response to depolarizing current. In a sample of 14 neurons, sufficient dendritic and/or axonal detail was recovered to identify them as non-principal cells, i.e. non-granule, non-mossy cells. Five distinct types of cells were recognized, based on the spatial distribution of dendrites, presumably reflecting the availability of afferents, and on the basis of the highly selective distribution of their axon terminals, indicating synaptic target selectivity. They are: (1) the hilar cell forming a dense axonal plexus in the commissural and association pathway terminal field (HICAP cell; horizontal axon extent 1.6 mm) in the inner one-third of the molecular layer, and having dendrites extending from the hilus to the top of the molecular layer; (2) the hilar cell with its axon ramifying in the perforant path terminal field (HIPP cell, horizontal axon extent 2.0 mm) in the outer two-thirds of the molecular layer, whereas its spiny dendrites were restricted to the hilus; (3) the molecular layer cell with its dendritic and axonal domains confined to the perforant path terminal zone (MOPP cell, horizontal extent of axon 2.0 mm); (4) the dentate basket cell (horizontal axon extent 0.9 mm) had most of its axon concentrated in the granule cell layer, the remainder being localized in the inner molecular layer and hilus; (5) the hilar chandelier cell, or axo-axonic cell (horizontal axon extent 1.1 mm), densely innervating the granule cell layer with fascicles of radially oriented terminal rows, and also forming an extensive plexus in the hilus. The three cell types having their somata in the hilus projected to granule cells at the same septo-temporal level where their cell bodies were located. The results demonstrate that there is a spatially selective innervation of the granule cells by at least five distinct types of dentate neurons, which terminate in several instances in mutually exclusive domains. Their dendrites may have access to all (HICAP cell) or only a few (e.g. HIPP and MOPP cell) of the hippocampal afferents. This arrangement provides a framework for independent interaction between the output of local circuit neurons and subsets of excitatory afferents providing input to principal cells.

Pharmacological and immunohistochemical evidence for serotonergic modulation of cholinergic nucleus basalis neurons

Identified electrophysiologically by low threshold bursts and transient outward rectification, cholinergic nucleus basalis neurons were recorded and labelled intracellularly in guinea-pig basal forebrain slices. By means of a triple labelling immunofluorescent technique, serotonin-immunoreactive fibres were visualized in close proximity to the soma and dendrites of the biocytin-labelled, choline acetyl transferase (ChAT)-immunoreactive cells. By bath application, 5-hydroxytryptamine (5-HT) produced a direct hyperpolarization of the identified cells which was mimicked by 5-HT1A receptor agonists, suggesting that it may inhibit the tonic firing but also modulate the low threshold bursting of the cholinergic nucleus basalis neurons.

Biocytin pellets: an alternative technique for massive anterograde labeling of neuronal pathways in vivo and in vitro

The present study demonstrates that biocytin suspended in pellets of either coagulated chicken plasma (plasmaclot) or gelatine, produces intense anterograde axonal and terminal labeling and dense retrograde Golgi like labeling of neurons when injected into the brain parenchyma of young adult rats. The technique worked perfectly on hippocampal pathways like the mossy fiber system, the hilodentate associational and commissural fiber systems, CA3 Schaffer collaterals, the entorhinal perforant path to fascia dentata and hippocampus, as well as frontal motor cortical efferent and afferent fiber tracts. This pellet tracer delivery technique also proved very useful when applied on hippocampal slice cultures, where small pellets of plasmaclot embedded biocytin resulted in very discrete uptake sites with dense labeling of small groups of neurons and their projections.

Intracellular filling in fixed brain slices using Miniruby, a fluorescent biocytin compound

Biocytin is useful for intracellular filling in living slices because it is soluble, has high electrophoretic mobility and a high affinity for avidin. In fixed slices, however, membrane potential cannot be used to signify that a cell is impaled. Thus, it is necessary to inject a fluorescent molecule so that impalement and filling can be visually monitored. As biocytin does not fluoresce, it cannot be used by itself in fixed slices. Here, we report that a biocytin-dextran (MW 10 kDa and 40 kDa) compound, Miniruby (MR), is a useful intracellular marker for injecting neurons in fixed slices. Fixed slices (200-400 microns) of adult rat olfactory bulb, piriform cortex, midbrain periaqueductal gray and locus coeruleus were used. Slices were stained by 0.001% ethidium bromide so that cell bodies could be visualized. The slices were imaged and filled using a specially designed hinged, epi-fluorescent microscope. A cell was impaled with a pipette containing 1-5% MR; positive pulsed constant current (1-5 nA; 300-400 ms on and 600-700 ms off; approximately 10 min) was applied until the fine dendrites were brightly fluorescent. Slices were post-fixed for 6-12 h, then reacted by a conventional ABC-DAB protocol. Miniruby has several advantages: (1) it is easy to visualize the electrode in relation to the cell bodies; (2) the staining procedure is very sensitive, does not require immunohistochemistry, and the reaction product is light stable; (3) injected neurons, dendrites and initial part of axons are well visualized by bright-field microscopy. It should be possible to analyze MR filled cells at the EM level.

Histamine enhances the depolarizing afterpotential of immunohistochemically identified vasopressin neurons in the rat supraoptic nucleus via H1-receptor activation

Previous studies have demonstrated that histamine primarily excites unidentified neurons in the rat supraoptic nucleus. We investigated the neuromodulatory effects of histamine on immunohistochemically identified vasopressin neurons in the rat supraoptic nucleus using intracellular recording techniques from the hypothalamo-neurohypophysial explant. Exogenous application of histamine (0.1-100 microM) to vasopressinergic neurons produced a small membrane depolarization accompanied by an increase of up to 100% in the amplitude of the depolarizing afterpotential that follows current-evoked trains of action potentials. The enhancement of the depolarizing afterpotential by histamine did not depend upon the depolarization. Further, histamine enhanced the amplitude of the depolarizing afterpotential when blocking the afterhyperpolarizing potential with d-tubocurarine or apamin, and in the presence of tetrodotoxin and d-tubocurarine or apamin, indicating a postsynaptic action of histamine on the depolarizing afterpotential that is not simply a reflection of a decrease in the afterhyperpolarizing potential. These toxins also had no effect on the histamine-induced depolarization. The enhancement of the depolarizing afterpotential by histamine was mimicked by the histamine H1-receptor agonist 2-thiazolylethylamine and was reduced or blocked by the H1-receptor antagonist promethazine, but was not blocked or reduced in the presence of the histamine H2-receptor antagonist, cimetidine. In summary, these results show that the excitatory effect of histamine on immunohistochemically identified vasopressin neurons in the supraoptic nucleus is due in part to the H1-receptor-mediated enhancement of the depolarizing afterpotential independent of any change in the afterhyperpolarizing potential or membrane potential.

Connections between the deutocerebrum and the protocerebrum, and neuroanatomy of several classes of deutocerebral projection neurons in the brain of male Periplaneta americana

The topography and neuroanatomy of fibers connecting the deutocerebrum to the protocerebrum in the brain of the American cockroach Periplaneta americana were investigated by staining single or multiple deutocerebral neurons with cobalt, Lucifer Yellow, or biocytin. Five tracts are distinguished on the basis of their routes from origins in the antennal lobe to the protocerebral neuropil: the inner antenno-cerebral tract (IACT); antenno-cerebral tracts II, III, and IV (ACT II, III, IV), and the outer antenno-cerebral tract (OACT). These tracts are largely composed of the axons of four classes of deutocerebral projection neurons, which have been identified morphologically; the neuronal arborizations in the glomeruli of the antennal lobe and in the protocerebral projection regions have been examined. Projection neurons with processes in the inner antenno-cerebral tract and in the antenno-cerebral tract II each innervate a single glomerulus in the antennal lobe, and both types have terminals in the calyces of the mushroom bodies and in the lateral lobe of the protocerebrum. The axons of pheromone-sensitive projection neurons with dendritic trees in the male-specific macroglomerulus seem to run exclusively in the inner antenno-cerebral tract. Subgroups of these pheromone sensitive neurons differ in relative sensitivity to the two female attractant components as well as in the arborization pattern of their dendrites in the macroglomerulus. The projection neurons of two other classes each innervate many glomeruli in the antennal lobe, those of one class sending their axons into the protocerebrum in the antenno-cerebral tract IV and the other, in the outer antenno-cerebral tract. The neurons of antenno-cerebral tract IV innervate not only the mushroom body calyces and the lateral lobe but also neuropil regions not previously described in the cockroach. Neurons with axons in the outer antenno-cerebral tract have no terminals in the calyces but innervate the lateral lobe and the neuropil surrounding the tract. The morphological findings presented here show that, in addition to the tracts previously documented in the cockroach brain, there are other, presumably olfactory, connections between the deutocerebrum and the protocerebrum.