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

Morphological details of primate axons and dendrites revealed by extracellular injection of biocytin: an economic and reliable alternative to PHA-L

The objective of this study was to determine if biocytin would reliably label details of distant axons and dendrites when injected extracellularly in primates. Biocytin (2.5-5%) was injected iontophoretically or by pressure into several areas of the visual and somatosensory systems of macaque monkeys, squirrel monkeys, tree shrews and galagos. After survival times that ranged from 9 h to 2 weeks, fine details of anterogradely filled axons and/or retrogradely filled dendrites were reliably revealed with an avidin-biotin-HRP complex (ABC solution) that was enhanced with heavy metals. Biocytin labeling was successfully combined with choline acetyltransferase (ChAT) or cytochrome oxidase (CO) histochemistry to reveal double-labeled cells. Our results show that biocytin is a versatile, easy-to-use label that completely fills cell processes both anterogradely and retrogradely in several primate species.

Membrane properties and synaptic responses of rat striatal neurones in vitro

1. A tissue slice containing a section of striatum was cut obliquely from rat brain so as to preserve adjacent cortex and pallidum. Intracellular recordings were made from 368 neurones, using either conventional or tight-seal configurations. 2. Two types of neurone were distinguished electrophysiologically. Principal cells (96%) had very negative resting potentials (-89 mV) and a low input resistance at the resting membrane potential (39 M omega): membrane conductance (10 nS at -65 mV) increased within tens of milliseconds after the onset of hyperpolarization (99 nS at -120 mV). Secondary cells (4%) had less negative resting potentials (-60 mV) and a higher input resistance (117 m omega at the resting potential): hyperpolarization caused an inward current to develop over hundreds of milliseconds that had the properties of H-current. 3. Most principal cells were activated antidromically by electrical stimulation of the globus pallidus or internal capsule. Intracellular labelling with biocytin showed that principal cells had a medium sized soma (10-18 microns), extensive dendritic trees densely studded with spines and, in some cases, a main axon which extended towards the globus pallidus. 4. Electrical stimulation of the corpus callosum or external capsule evoked a depolarizing postsynaptic potential. This synaptic potential was reversibly blocked by a combination of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and DL-2-amino-5-phosphonovaleric acid (APV, 30 microM), but was unaffected by bicuculline (30 microM) and picrotoxin (100 microM). The underlying synaptic current had a fast component (time to peak about 4 ms), the amplitude of which was linearly related to membrane potential and which was blocked by CNQX; in CNQX the synaptic current had a slower component (time to peak about 10 ms) which showed voltage dependence typical of N-methyl-D-aspartate (NMDA) receptors. Both currents reversed at -5 mV. 5. Focal electrical stimulation within the striatum (100-300 microns from the site of intracellular recording) evoked a synaptic potential that was partially blocked (45-95%) by CNQX and APV: the remaining synaptic potential was blocked by bicuculline (30 microM). The bicuculline-sensitive synaptic current reversed at the chloride equilibrium potential. 6. The findings confirm that the majority of neostriatal neurones (principal cells, medium spiny neurones) project to the pallidum and receive synaptic inputs from cerebral cortex mediated by an excitatory amino acid acting through NMDA and non-NMDA receptors. These cells also receive synaptic inputs from intrinsic striatal neurones mediated by GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending pathways connecting the male-specific visual system of flies to the neck and flight motor

During sexual pursuit, male flies Sarcophaga bullata, stabilize the image of a pursued target on the dorso-frontal acute zone of their compound eyes. By retinotopic projection, this region is represented in the upper frontal part of the lobula where it is sampled by ensembles of male-specific motion- and flicker-sensitive interneurons. Intracellular recordings of descending neurons, followed by biocytin injection, demonstrate that male-specific neurons are dye-coupled to specific descending neurons and that the response characteristics of these descending neurons closely resemble those of male-specific lobula neurons. Such descending neurons are biocytin-coupled in the thoracic ganglia, revealing their connections with ipsilateral frontal nerve motor neurons supplying muscles that move the head and with contralateral basalar muscle motor neurons that control wing beat amplitude. Recordings from neck muscle motor neurons demonstrate that although they respond to movement of panoramic motion, they also selectively respond to movement of small targets presented to the male-specific acute zone. The present results are discussed with respect to anatomical and physiological studies of sex-specific interneurons and with respect to sex-specific visual behavior. The present study, and those of the two preceding papers, provide a revision of Land and Collett's hypothetical circuit underlying target localization and motor control in males pursuing females.

The anterograde and retrograde transport of neurobiotin in the central nervous system of the rat: comparison with biocytin

In order to test whether neurobiotin, an analogue of biotin, is transported by neurones in the central nervous system, injections of varying volumes of a 5% solution were made into different regions of the rat brain. Following perfusion-fixation, the sites of injection and possible sites of transport were sectioned and incubated with an avidin-biotin-peroxidase complex and then subjected to a peroxidase reaction. All injection parameters and sites were directly compared to equivalent injections of the closely related substance, biocytin, which is effectively transported in an anterograde fashion. The sites of injection of neurobiotin were characterised by large areas of labelling that were more extensive than those produced by equivalent injections of biocytin but with less intense labelling of individual neurones. Each of the injections of neurobiotin gave rise to marked anterograde labelling, the characteristics of which were similar to those produced by biocytin, but due to the larger injection sites was heavier than that produced by biocytin. Each of the injections of biocytin or neurobiotin also gave rise to retrograde labelling; the degree of labelling was far greater with neurobiotin but varied between pathways. Retrograde labelling only occurred to a minor degree in some pathways but was particularly marked in others, e.g., the striatonigral pathway. As with biocytin, tract-tracing with neurobiotin can be applied to electron microscopy and can readily be combined with immunocytochemistry for endogenous substances. It is concluded that neurobiotin is an effective anterograde and retrograde marker that may be of use in studies in the central nervous system, particularly in large homogeneous structures such as cortical fields and the striatum.

Physiologic properties of human dentate granule cells in slices prepared from epileptic patients

The neurophysiological properties of human dentate granule cells were studied in hippocampal slices prepared from patients undergoing surgical treatment for medically intractable temporal lobe epilepsy. In 24 neurons which were morphologically identified as dentate granule cells by intracellular staining with biocytin, there were 2 types of synaptic responses to perforant path stimulation: one showed an EPSP-IPSP sequence (n = 10) and the other showed prolonged EPSPs without accompanying hyperpolarizing IPSPs (n = 14). The prolonged EPSPs were markedly retarded by the application of an NMDA receptor antagonist, APV. Membrane properties of neurons showing the different classes of synaptic responses were similar in resting membrane potential (pooled average: -56.2 mV +/- 0.94 SEM) and spike amplitude (pooled average: 65.2 mV +/- 1.69 SEM). However, membrane resistance tended to be lower in neurons with prolonged EPSPs (31.8 M omega +/- 2.63 SEM) than in neurons that showed EPSP-IPSP responses (40.2 +/- 4.33) (P less than 0.05, Fisher). No spontaneous and/or evoked burst firing was observed. These data provide fuller information on the neurophysiological properties of human dentate granule cells in surgically resected epileptogenic hippocampus, implicating a role of NMDA receptor activation in human temporal lobe epilepsy.

The coding of signals in the electric communication of the gymnotiform fish Eigenmannia: from electroreceptors to neurons in the torus semicircularis of the midbrain

In the context of aggression and courtship, Eigenmannia repeatedly interrupts its electric organ discharges (EODs) These interruptions contain low-frequency components as well as high-frequency transients and, therefore, stimulate ampullary and tuberous electroreceptors, respectively. Information provided by these two classes of receptors is relayed along separate pathways, via the electrosensory lateral line lobe (ELL) of the hindbrain, to the dorsal torus semicircularis (TSd) of the midbrain. Some neurons of the torus receive inputs from both types of receptors and some respond predominantly to EOD interruptions while being rather insensitive to other forms of signal modulations. This high selectivity appears to result from convergence and gating of inputs from individually less selective neurons.

Age-dependent alterations in the operations of hippocampal neural networks

Results from numerous studies suggest that the functioning of rat hippocampal neural networks during the second postnatal week of life differs distinctly from that in the mature brain. During this critical period, network behavior might be considered hyperexcitable. Spontaneous network-driven bursts of synaptic potentials, which have not been reported in mature hippocampus, are commonly observed. While these events could be attributable to a late onset of GABAergic synaptic transmission, results suggest that this is not the case. In the immature hippocampus orthodromic stimulation leads to prolonged depolarizations and often repetitive synchronized discharging of the entire CA3 population. These events are in many ways reproduced by application of drugs that suppress GABAergic synaptic transmission. The synchronized discharging of the CA3 population is blocked by excitatory amino acid antagonists. This finding, coupled with our growing understanding of the role that recurrent excitation plays in CA3 network functioning, has led to the hypothesis that the differences in network behavior early in life may be largely attributable to an overabundance of local-circuit recurrent excitatory synapses. With maturation, axon collaterals and attendant synapses would regress to achieve an adult complement. Results from dual intracellular recordings as well as anatomical studies of individual CA3 pyramidal cells support this hypothesis. Unique properties of the NMDA receptor at these recurrent excitatory synapses early in life may also promote network excitability. The participation of extracellular Ca2+ in the voltage dependency of the NMDA receptor-linked iontophore could also contribute to synapse consolidation during maturation and thus in the establishment of network connectivity.

Structure and function of neurons in the complex of the nucleus electrosensorius of the gymnotiform fish Eigenmannia: detection and processing of electric signals in social communication

The complex of the diencephalic nucleus electrosensorius (nE) provides an interface between the electrosensory processing performed by the torus semicircularis and the control of specific behavioral responses. The rostral portion of the nE comprises two subdivisions that differ in the response properties and projection patterns of their neurons. First, the nEb, which contains neurons that are driven almost exclusively by beat patterns generated by the interference of electric organ discharges (EODs) of similar frequencies. Second, the area medial to the nEb, comprising the lateral pretectum (PT) and the nE-acusticolateralis region (nEar, 1 B-D), which contains neurons excited predominantly by EOD interruptions, signals associated with aggression and courtship. Neurons in the second area commonly receive convergent inputs originating from ampullary and tuberous electroreceptors, which respond to the low-frequency and high-frequency components of EOD interruptions, respectively. Projections of these neurons to hypothalamic areas linked to the pituitary may mediate modulations of a fish's endocrine state that are caused by exposure to EOD interruptions of its mate.

Contribution of NMDA receptors to postsynaptic potentials and paired-pulse facilitation in identified neurons of the rat nucleus accumbens in vitro

The principal aim of this study was to characterize the transmitter mechanisms mediating fast postsynaptic potentials in identified neurons of the rat nucleus accumbens. Using the biocytin-avidin labeling technique, impaled neurons were identified as medium spiny neurons. The basic membrane characteristics of these neurons were determined. Local electrical stimulation or stimulation of the corpus callosum elicited a depolarizing postsynaptic potential consisting of an EPSP often followed by an IPSP. The quisqualate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (4 microM) abolished most of the depolarizing postsynaptic potential. The N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid depressed a small part of the decay phase of the depolarizing postsynaptic potential. Paired-pulse facilitation of postsynaptic potentials was found using interstimulus-intervals between 10 and 150 ms. N-methyl-D-aspartate receptors were found to contribute only slightly to the facilitation of the decay phase of the depolarizing postsynaptic potential, but not to its rising phase. This contribution was particularly clear under conditions of reduced GABAA receptor mediated inhibition. The present study indicates that postsynaptic responses of medium spiny neurons in the nucleus accumbens to local stimulation or stimulation of neocortical afferents are primarily mediated by quisqualate/kainate receptors. The contribution of NMDA receptors is normally limited to a portion of the decay phase of these responses, but is enlarged in the absence of GABAergic inhibition and following paired-pulse stimulation.

Comparison of three intracellular markers for combined electrophysiological, morphological and immunohistochemical analyses

Hypothalamic paraventricular and supraoptic neurons were recorded intracellularly in coronal slices and injected with Lucifer yellow, ethidium bromide or biocytin. Electrical properties, morphological staining and neurophysin immunohistochemistry were compared among the 3 markers. Lucifer yellow electrodes had a high resistance and frequently blocked during experiments. Neurons recorded with Lucifer yellow electrodes had low input resistances and low-amplitude, broad spikes. Lucifer yellow labeling in whole mount was highly fluorescent, revealing distal dendrites and axons. Of cells injected with Lucifer yellow, 64% were recovered but were faint after immunohistochemical processing. Recordings with ethidium bromide electrodes were similar to controls, although electrode blockage sometimes occurred. Only somata and proximal dendrites of ethidium bromide-filled neurons were visible in whole-mount. Forty percent of cells injected with ethidium bromide were recovered after immunohistochemical processing; these were invariably faint. Recordings with biocytin-filled electrodes were similar to control recordings. Biocytin-filled, HRP-labeled cells showed distal dendrites and often dendritic spines and axons in 50-75-microns sections. Seventy percent of biocytin-injected cells labeled with fluorescent markers were recovered and remained strongly labeled after immunohistochemical processing. Biocytin had the best electrical and staining properties for combined electrophysiological and anatomical studies.

Synaptic relationships between GABA-immunoreactive neurons and an identified uniglomerular projection neuron in the antennal lobe of Periplaneta americana: a double-labeling electron microscopic study

Two types of central neurons in the antennal lobe of the American cockroach Periplaneta americana were labeled with a combination of two specific markers. Their synaptic contacts were characterized and their distribution on the neurons examined. A uniglomerular pheromone-sensitive projection neuron with dendritic arbor in the male-specific macroglomerulus (attractant neuron) was characterized physiologically by intracellular recording and then filled with biocytin, which was converted to a marker for this individual neuron by a preembedding procedure. In a postembedding procedure local, multiglomerular interneurons were marked by immunogold labeling of GABA. Two kinds of synaptic contacts were found on the attractant neuron. (i) Input synapses from GABA-immunoreactive profiles. There were many of these, which (together with results of previous studies) suggests that local interneurons mediate polysynaptic transmission from antennal receptor fibers to the projection neuron. (ii) Output synapses onto GABA-immunoreactive profiles and onto non-identified neurons. These contacts indicate that signals generated by the projection neurons in a given glomerulus are passed back to multiglomerular interneurons and hence are also transmitted to other glomeruli.

The rod circuit in the rabbit retina

Mammalian retinae have a well-defined neuronal pathway that serves rod vision. In rabbit retina, the different populations of interneurons in the rod pathway can be selectively labeled, either separately or in combination. The rod bipolar cells show protein kinase C immunoreactivity; the rod (AII) amacrine cells can be distinguished in nuclear-yellow labeled retina; the rod reciprocal (S1 & S2) amacrine cells accumulate serotonin; and the dopaminergic amacrine cells show tyrosine-hydroxylase immunoreactivity. Furthermore, intracellular dye injection of the microscopically identified interneurons enables whole-population and single-cell studies to be combined in the same tissue. Using this approach, we have been able to analyze systematically the neuronal architecture of the rod circuit across the rabbit retina and compare its organization with that of the rod circuit in central cat retina. In rabbit retina, the rod interneurons are not organized in a uniform neuronal module that is simply scaled up from central to peripheral retina. Moreover, peripheral fields in superior and inferior retina that have equivalent densities of each neuronal type show markedly different rod bipolar to AII amacrine convergence ratios, with the result that many more rod photoreceptors converge on an AII amacrine cell in superior retina. In rabbit retina, much of the convergence in the rod circuit occurs in the outer retina whereas, in central cat retina, it is more evenly distributed between the inner and outer retina.

Reidentification of larval interneurons in the pupal stage of the tobacco hornworm, Manduca sexta

The abdominal prolegs are the primary locomotory appendages of Manduca sexta larvae. After the prolegs are lost at pupation, some of the proleg motoneurons die while the survivors are respecified to carry out different functions in the adult moth. As a first step toward investigating the process of functional respecification at the synaptic level, we searched for larval interneurons that affected the activity of proleg motoneurons, and followed these interneurons into the pupal stage. Interneurons were judged to be individually identifiable based on their effects on proleg motoneuron activity and their anatomical features. Seven larval interneurons were identified and placed in five physiological classes based on their effects on proleg motoneurons: ipsilateral excitors, contralateral excitors, ipsilateral inhibitors, contralateral inhibitors, and bilateral inhibitor-excitors. Four of the larval interneurons produced apparently monosynaptic postsynaptic potentials in proleg motoneuron. Of the five larval interneurons that were reidentified in the early pupal stage, two showed minor but consistent structural modifications from the larval stage. Interneurons that produced unitary postsynaptic potentials in larval motoneurons continued to do so in pupal motoneurons. These studies demonstrate that individually identified interneurons can be followed through the larval-pupal transformation, during the initial stages of motoneuron respecification.

Iontophoretic application of NMDA produces different types of excitatory responses in developing human cortical and caudate neurons

The effects of iontophoretically applied N-methyl-D-aspartate (NMDA) were assessed in human neocortical and caudate neurons. NMDA depolarized cell membranes, decreased input conductances and induced firing. The discharge patterns differed in the two areas studied. In neocortex, NMDA produced repetitive spikes or bursts. In caudate, it induced slow, rhythmic plateau depolarizations accompanied by an initial burst of action potentials, followed by low amplitude, long duration spikes. After hyperpolarizations were seen after each depolarization in the caudate. These variations in patterns of excitation may relate to differences in local circuits intrinsic to each region, and/or to membrane conductances specific to each type of cell.

Intracellular study of rat entopeduncular nucleus neurons in an in vitro slice preparation: response to subthalamic stimulation

Responses of rat entopeduncular nucleus (EP) neurons after stimulation of the subthalamic nucleus (STh) and the morphology of the EP neurons were studied using brain slice preparations. EP neurons were classified into two types based on their electrophysiological properties as reported previously. Of 87 EP neurons, 72 were Type I and the rest were Type II. Synaptic responses to STh stimulation were different in these two cell types. STh stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by strong inhibitory postsynaptic potentials (IPSPs) in Type I neurons and EPSPs without strong IPSPs in Type II neurons. The EPSPs were considered to be monosynaptic because no large change in the latency (1.7 +/- 0.5 ms) resulted by alteration of stimulus intensity. The EPSPs were reversibly suppressed by kynurenic acid in a dose-dependent manner. Bath application of (+)-tubocurarine (10-50 microM) had no effect on EPSPs or IPSPs. Bath application of bicuculline methiodide (50-100 microM) markedly suppressed IPSPs evoked by STh stimulation and at the same time increased the amplitude and duration of EPSPs without affecting the latency. In the presence of bicuculline methiodide, EPSPs could induce plateau potentials and slow action potentials. Some type I and Type II neurons were intracellularly labeled by biocytin. Type I neurons were located throughout the EP but Type II neurons were located mainly in the dorsal portion of the EP. Medium sized somata of both Type I and Type II neurons were spine-free and fusiform or round in shape. They had 3-4 thick primary dendrites with diameters of 2-5 micron that branched into thin secondary dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical differentiation of electrophysiologically defined neuronal populations in the region of the rat hypothalamic paraventricular nucleus

Intracellular recording and labeling were combined with neurophysin immunohistochemistry to study neurons in the paraventricular nucleus region of the rat hypothalamus. Neuronal membrane properties were examined in hypothalamic slices, and cells were labeled by injecting biocytin or Lucifer yellow. Slices were then embedded, sectioned, and immunohistochemically processed for neurophysin. Immunoreactivity patterns, and in some cases counterstaining, enabled determinations of the cytoarchitectonic positions of recorded cells to be made. Recorded cells were divided into three types according to their electrophysiological characteristics. The first type lacked low-threshold Ca2+ spikes and displayed linear current-voltage relations, a short time constant, and evidence for an A current. These were relatively large cells that were typically immunoreactive for neurophysin and were situated near other neurophysin-positive neurons. The second type had relatively small low-threshold potentials that did not generate bursts of Na+ spikes. These cells had heterogeneous current-voltage relations and intermediate time constants. They did not label for neurophysin, and most were located in the parvicellular subregion of the paraventricular nucleus. The third type had large low-threshold Ca2- spikes that generated bursts of Na+ spikes, and these cells had nonlinear current-voltage relations and long time constants. These neurons were dorsal or dorsolateral to the paraventricular nucleus and were not immunoreactive for neurophysin. These results indicate that paraventricular magnocellular neurons lack low-threshold potentials, whereas paraventricular parvicellular neurons display low-threshold potentials that generate one or two action potentials. Neurons that fire spike bursts from low-threshold potentials are adjacent to the paraventricular nucleus, confirming earlier reports.

Central origin of the efferent neurons projecting to the eyes of Limulus polyphemus

Circadian rhythms affect the anatomy, physiology, and biochemistry of the visual cells in the eyes of the horseshoe crab (Limulus polyphemus). These rhythms are mediated by the activity of efferent neurons that project from the central nervous system to all of the eyes. In this study, the optic nerves of Limulus were backfilled with Neurobiotin revealing the location of efferent cell bodies and their projections through the central nervous system. We propose that this efferent system mediates the circadian changes in visual functions in Limulus. Whether these cells are the circadian pacemaker neurons is unknown. The cell bodies of the efferent neurons are ovoid and have a diameter of 40-80 microns. They lie within the cheliceral ganglion of the tritocerebrum, just posterior to the protocerebrum. This ganglion is on the lateral edge of the circumesophageal ring, near the middle of the dorsal-ventral axis of the ring. Each optic nerve contains axons from both ipsilateral and contralateral efferent cells, and some, possibly all, of them project bilaterally and to more than one type of optic nerve. The efferent axons form a tract that projects anteriorly from the cell bodies to the protocerebrum, and bifurcates just lateral to the protocerebral bridge. One branch crosses the midline and projects anteriorly to the optic tract and medulla on the side contralateral to the cell of origin; the other branch follows a symmetric pathway on the ipsilateral side. Small branches arising from the major efferent axons in the optic tract project through the ocellar ganglia to the median optic nerves. The efferent axons branch again in the medulla, and some of these branches innervate the ventral optic nerves. The major branches of the efferent axons continue through the lamina and enter the lateral optic nerve.

Electrophysiology of dopaminergic and non-dopaminergic neurones of the guinea-pig substantia nigra pars compacta in vitro

1. The membrane properties of substantia nigra pars compacta neurones were studied using an in vitro slice preparation of guinea-pig midbrain. 2. Neurones were divided into two classes based on their electrophysiological properties: bursting neurones displayed a burst of several rapidly accommodating action potentials in response to relaxation of hyperpolarizing current injected through the microelectrode, while non-bursting neurones produced regularly spaced action potentials. These neuronal types were found to be electrophysiologically distinct from those recorded in the substantia nigra pars reticulata and the subthalamic nucleus. 3. Non-bursting neurones, which comprised ca 85% of the sampled cells, were characterized by a slow, pacemaker pattern of firing at rest, broad action potentials, a pronounced spike after-hyperpolarization, long membrane time constants, and strong transient outward and time-dependent inward rectification. 4. Bursting neurones, comprising ca 15% of the sample, displayed rapid firing rates at rest, fast action potentials, a shallow spike after-hyperpolarization and briefer membrane time constants. All of these parameters were significantly different from those of the non-bursting type. Bursting neurones lacked transient outward or time-dependent inward rectification. 5. Both types of cells were capable of generating pronounced calcium-dependent, low-threshold spikes in the presence of tetrodotoxin (TTX). However, only the non-bursting type displayed calcium-dependent rhythmic oscillations in membrane potential near resting potential in the presence of TTX. The firing rate, action potential shape and after-hyperpolarization of non-bursting neurones were strongly influenced by calcium-dependent currents. 6. The majority of cells were injected with biocytin, which allowed morphological reconstruction of the neurones and confirmation of their location within the pars compacta. Non-bursting neurones had variable soma shapes and their dendrites were mostly directed in a medio-lateral direction. Many cells extended some of their dendrites into the pars reticulata. Bursting neurones were mainly fusiform in shape with their dendrites oriented in a medio-lateral direction; a few had dendrites extending into the pars reticulata. 7. Thirty-six neurones were also double labelled using a combination of biocytin or Lucifer Yellow injection with tyrosine hydroxylase (TH) immunohistochemistry. Non-bursting neurones all displayed TH immunofluorescence, while none of the bursting neurones were TH positive.(ABSTRACT TRUNCATED AT 400 WORDS)

Confocal microscopy and three-dimensional reconstruction of electrophysiologically identified neurons in thick brain slices

Three-dimensional morphology and electrophysiology were correlated from individual neurons in a thick brain slice preparation. The hippocampal formation from immature and adult rats was cut transverse to the longitudinal axis into 500 microns-thick slices which were maintained under physiologic conditions. Individual neurons were impaled and physiologically characterized using microelectrodes. Recordings were made from the soma and in some cases from a dendrite. The impaled neurons were filled through the microelectrode with the fluorescent dye lucifer yellow and imaged by confocal scanning laser microscopy using an analog preprocessor. As many as 180 optical sections were recorded as a function of depth through the slices. Images are presented as a series of optical sections, stereo pairs, or three-dimensional reconstructions. Both stereo contouring and volume rendering methods were employed, and the reconstructions were viewed from any arbitrary perspective. Dendritic and axonal fields were separated from each other and displayed separately or as different pseudocolors. The three-dimensional reconstructions provided perspectives that were difficult or impossible to appreciate by viewing the optical sections or conventionally formed stereo pairs.

Many diverse types of retinal neurons show tracer coupling when injected with biocytin or Neurobiotin

This study demonstrates that the junctional connections between rod-signal interneurons in mammalian retina can be visualized by tracer coupling, following intracellular injection of the biotinylated compounds, biocytin and Neurobiotin. In addition, many other types of retinal neurons -including B-type horizontal cells and several types of retinal ganglion cells-show specific patterns of tracer coupling, usually to cells of the same neuronal type but occasionally to cells of other neuronal classes. These findings suggest that electronic transmission occurs commonly throughout the retina and, consequently, diverse types of retinal neurons may form functional networks of coupled cells.

Double-labelling with rhodamine beads and biocytin: a technique for studying corticospinal and other projection neurons in vitro

Corticospinal neurons retrogradely labelled with rhodamine-labelled latex microspheres (RLMs) in vivo were studied intracellularly in a slice preparation up to 13 months later with electrodes containing biocytin. The physiological properties of these double-labelled corticospinal neurons were indistinguishable from those of comparable neurons which were impaled with biocytin-containing electrodes without prior RLM-labelling, and neurons studied with potassium acetate-filled electrodes in similar areas. Thus, neither labelling with RLMs nor injection of biocytin affected neuronal properties. This important advantage of RLMs makes them suitable for prelabelling projection neurons in vivo for subsequent studies that take advantage of the versatility of a brain slice preparation. In addition to its lack of effects on neuronal properties, intracellular labelling with biocytin also provides high-quality morphological details ideal for anatomical analysis. The compatibility of retrograde labelling with RLMs and intracellular staining with biocytin make this a useful combined technique for tracking electrophysiological and anatomical changes in identified projection neurons over time.

Inventory and distribution of synapses of identified uniglomerular projection neurons in the antennal lobe of Periplaneta americana

Uniglomerular projection neurons in the antennal lobe of Periplaneta americana, the axons of which connect the lobe to the protocerebrum, were labeled by intracellular injection of Lucifer Yellow or biocytin. The fine structure of individual neurons within the antennal lobe was examined after the injected substances had been converted (by immunohistochemical or histochemical treatment) to electron microscopically visible reaction products. Seven projection neurons were investigated, including attractant neurons, with dendritic arbors in the macroglomerulus, and projection neurons of normal-sized glomeruli. From reconstructions of thin serial sections and examination of additional processes present at various places in the arborization regions, the distribution of synapses within the glomeruli was inferred. Although the projection neurons differ from one another in their glomerular arborization patterns, they are very similar in the spatial segregation of their input and output synapses within the arborization. Output synapses are found on the thick part of the fiber near its site of entry into the glomerulus, as well as in regions within the glomerulus where the neuron has begun to ramify into thinner fibers. In the latter regions, the many output synapses are accompanied by occasional input synapses; hence these are regarded as transitional regions. At the terminal arbors only input synapses were found. This suggests that neurons with dense terminal arborizations receive particularly numerous inputs in these regions. The large number of input synapses reflects the high degree of convergence of afferents onto projection neurons previously demonstrated physiologically. However, the presence of numerous output synapses indicates that projection neurons not only transport sensory information into the protocerebrum but are also a major component of the neuronal circuitry within the antennal lobe.

Post-inhibitory excitation and inhibition in layer V pyramidal neurones from cat sensorimotor cortex

1. The effect of conditioning pre-pulses on repetitive firing evoked by intracellular current injection was studied in layer V pyramidal neurones in a brain slice preparation of cat sensorimotor cortex. Most cells displayed spike frequency adaptation (monotonic decline of firing rate to a tonic value) for several hundred milliseconds when depolarized from resting potential, but the cells differed in their response when pre-pulses to other potentials were employed. In one group of cells, the initial firing rate increased as the pre-pulse potential was made more negative (post-hyperpolarization excitation). Adaptation was abolished by depolarizing prepulses. In a second group, the initial firing rate decreased as the pre-pulse potential was made more negative (post-hyperpolarization inhibition). Hyperpolarizing pre-pulses caused the initial firing to fall below and accelerate to the tonic rate over a period of several seconds. A third group displayed a mixture of these two responses: the first three to seven interspike intervals became progressively shorter and subsequent intervals became progressively longer as the conditioning pre-pulse was made more negative (post-hyperpolarization mixed response). 2. Cells were filled with horseradish peroxidase or biocytin after the effect of pre-pulses was determined. All cells whose firing patterns were altered by pre-pulses were large layer V pyramidal neurones. Cells showing post-hyperpolarization excitation or a mixed response had tap root dendrites, fewer spines on the apical dendrite and larger soma diameters than cells showing post-hyperpolarization inhibition. 3. Other electrophysiological parameters varied systematically with the response to conditioning pre-pulses. Both the mean action potential duration and the input resistance of cells showing post-hyperpolarization excitation were about half the values measured in cells showing post-hyperpolarization inhibition. Values were intermediate in cells showing a post-hyperpolarization mixed response. The after-hyperpolarization following a single evoked action potential was 20% briefer in cells showing post-hyperpolarization excitation compared to those showing inhibition. 4. Membrane current measured during voltage clamp suggested that two ionic mechanisms accounted for the three response patterns. Post-hyperpolarization excitation was caused by deactivation of the inward rectifier current (Ih). Selective reduction of Ih with extracellular caesium diminished post-hyperpolarization excitation, whereas blockade of calcium influx had no effect. Post-hyperpolarization inhibition was caused by enhanced activation of a slowly inactivating potassium current. Selective reduction of this current with 4-aminopyridine diminished the post-hyperpolarization inhibition. 5. Chord conductances underlying both Ih and the slow-transient potassium current were measured and divided by leakage conductance to control for differences in cell size.(ABSTRACT TRUNCATED AT 400 WORDS)

A note on the use of biocytin in anterograde tracing studies in the central nervous system: application at both light and electron microscopic level

A new neuroanatomical method based on the anterograde transport of biocytin has recently been reported. The validity of this method is examined closely on a number of well established pathways in the central nervous system, as well as the experimental parameters necessary for its effective use. iontophoretic application of this amino acid-biotin complex allows the placement of very discrete injections. At the injection sites neurones appear to be completely filled, whereas fibres of passage are left unlabelled. From the injection site axons can be traced at the light microscopic level to their terminal fields, where their pattern of termination and morphology can be clearly visualised. These anterogradely labelled fibres can be examined further by electron microscopy to identify synaptic specializations. The uptake and transport of biocytin, following iontophoretic injection, appears to be similar to that observed using techniques employing radiolabelled amino acids or Phaseolus vulgaris-leucoagglutinin (PHA-L), in that it is transported predominantly in the anterograde direction. However, biocytin offers some advantages over these techniques in that its detection is relatively easy at electron microscopic levels. Furthermore, biocytin appears to be transported rapidly making it possible to study short pathways under acute experimental conditions.

Electrophysiological characteristics of cells within mesencephalon suspension grafts

Both spontaneous and evoked extracellular electrophysiological activity of neurons within fetal mesencephalon suspension grafts to the dopamine-depleted striatum of rats were examined. In some cases, extracellular recording was combined with intracellular labeling to identify recorded neurons. Grafted rats displaying a complete cessation of ipsilateral rotations following amphetamine administration were examined at post-implantation time intervals of two, four, five, eight and nine months. Four separate classes of neurons were distinguished within the transplanted striatum based on electrophysiological properties. The first of these groups, the type I cells, appeared to be non-grafted striatal neurons. When spontaneously active, these striatal-like cells fired bursts of action potentials separated by periods of decreased activity. Evoked responses in these cells were characteristic of striatal cells. Type I cells which were intracellularly labeled were found outside the grafts and displayed the characteristic morphology of the medium spiny neuron of the neostriatum. The other three cell classes displayed electrophysiological properties similar to neurons recorded in situ within the reticular formation, substantia nigra pars compacta and substantia nigra pars reticulata. Neurons from these three groups which were labeled with an intracellular marker were found to lie within the suspension grafts. The spontaneous activity of the pars compacta dopaminergic-like neurons was predominantly irregular, with some cells also firing in a regular or pacemaker-like pattern. Infrequently, irregular firing dopaminergic-like neurons displayed episodes of doublet bursting. Many of the grafted neurons responded to electrical stimulation of prefrontal cortex and striatum, indicating that the graft was receiving functional inputs from host neurons. Comparison of the firing rate and pattern of grafted neurons to in situ mesencephalic neurons as a function of time following grafting suggested that the grafted neurons and/or the neuronal circuitry is slowly developing within the host environment. A prolonged time-course for the maturation of the graft may be reflected in the time required to achieve improvements in some behavioral deficits following transplantation. However, the relatively rapid recovery of drug-induced rotational asymmetry following grafting suggests that this form of recovery may not require mature functioning of the grafted neurons.

Convergence of synaptic inputs from the striatum and the globus pallidus onto identified nigrocollicular cells in the rat: a double anterograde labelling study

Two major sources of afferent synaptic inputs to projection neurons in the rat substantia nigra reticulata are the striatum and the globus pallidus. In order to understand better the functional relationships between these two afferents in the control of the activity of nigrofugal neurons, experiments have been performed to test the possibility that single nigrofugal cells receive convergent synaptic inputs from the striatum and the globus pallidus. To address this question we have used two different approaches. First, we have developed a double anterograde labelling technique suitable for both light and electron microscopy and combined this procedure with the retrograde transport of lectin-conjugated horseradish peroxidase in order to retrogradely label the nigrocollicular cells. Second, we have combined the anterograde transport of Phaseolus vulgaris-leucoagglutinin from the globus pallidus and immunocytochemistry for DARPP-32 as a marker for the striatal terminals, with the retrograde transport of lectin-conjugated horseradish peroxidase from the superior colliculus. In the double anterograde labelling experiment, biocytin was injected in the striatum, Phaseolus vulgaris-leucoagglutinin in the globus pallidus and lectin-conjugated horseradish peroxidase in the superior colliculus. Following these injections, rich plexuses of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals were found in the ventral two-thirds of the substantia nigra. The biocytin-positive terminals (striatonigral) were generally small and formed rich plexuses without any apparent neuronal association whereas the Phaseolus vulgaris-leucoagglutinin-labelled terminals (pallidonigral) were much larger and formed baskets around the perikarya of retrogradely and non retrogradely labelled cells in the substantia nigra reticulata. In areas of the substantia nigra reticulata where the fields of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals overlapped, the perikarya and the proximal dendrites of retrogradely and non retrogradely labelled cells were found to be apposed by numerous Phaseolus vulgaris-leucoagglutinin-immunoreactive pallidonigral terminals and a few biocytin-labelled striatonigral terminals. In the sections prepared for electron microscopy, the biocytin was localized using 3,3'-diaminobenzidine tetrahydrochloride whereas Phaseolus vulgaris-leucoagglutinin was localized using benzidine dihydrochloride. It was thus possible to distinguish the biocytin- from the Phaseolus vulgaris-leucoagglutinin-labelled terminals in the electron microscope by the texture of the reaction product associated with them.4+ Examination of 231 biocytin-labelled (striatonigral) terminals and 105 Phaseolus vulgaris-leucoagglutinin-immunoreactive (pallidronigral) terminals revealed that the striatonigral terminals were generally small, contained few mitochondria and formed symmetric synapses predominantly with the distal dendrites (77%) and far less frequently with the perikarya (3%) of substantia nigra reticulata cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of pedunculopontine neurons and their postsynaptic responses following stimulation of substantia nigra reticulata

Membrane properties and postsynaptic responses to stimulation of the substantia nigra reticulata (SNr) of the neurons in rat pedunculopontine nucleus (PPN) were studied in an in vitro parasagittal slice preparation using intracellular recording techniques. Based on electrical membrane properties, PPN neurons were classified into 3 types (types I, II and II). The unique feature of the type I neuron was the low threshold calcium spike while the type II neuron had various inward and outward rectifications. The type III neuron showed no such features as those observed in type I or II neurons. Some recorded neurons were intracellularly labeled with biocytin to study their morphology, and their transmitter phenotype was investigated by immunocytochemistry for choline acetyltransferase (ChAT). The type I and III neurons were found to be non-cholinergic, but 50% of the labeled type II neurons were immunopositive for ChAT. Morphological features of type II neurons were also different from type I or III neurons. The soma of the type II neuron was almost always more than twice as large as that of type I and III neurons. Inhibitory postsynaptic potentials (IPSPs) were induced in all 3 types of PPN neurons following stimulation of SNr. SNr-induced IPSPs were usually followed by a slow depolarizing potential from which rebound spikes were triggered. These rebound excitations were found only in type I and II neurons. These data indicate that heterogeneous groups of neurons exist in the PPN in terms of morphology, transmitter phenotypes and electrical membrane properties.

Parameters affecting imaging of the horseradish-peroxidase-diaminobenzidine reaction product in the confocal scanning laser microscope

Neurons intracellularly filled with biocytin and labelled with nickel-intensified diaminobenzidine (DAB/Ni) can be imaged on the confocal scanning laser microscope in order to obtain three-dimensional and optical section images of neurons. Intensification of the DAB reaction product with nickel was found to be crucial for obtaining a workable signal level. On the other hand, the high absorption of light by the reaction product severely attenuated the detection of structures lying directly underneath, and the intensity of the unattenuated laser used for imaging faded or damaged DAB/Ni reaction product. We have determined that reduction of the laser intensity combined with the use of proper objective lenses and non-laser-based imaging for preliminary adjustments of the specimen all work to reduce or eliminate damage, and also improve the image. These items must be kept in mind when imaging and analysing DAB-labelled structures in the laser-based confocal microscope.

Intracellular analysis of effects of phencyclidine on rat neostriatal neurons recorded in vitro

The effects of phencyclidine (PCP) on the synaptic and direct membrane properties of neostriatal neurons were assessed using intracellular recordings from 62 neurons in rat neostriatal slices. Electrophysiological measurements were obtained before, during and after bath application of PCP. At all concentrations (1-300 microM) PCP raised current thresholds for evoking synaptic potentials and reduced excitability (the firing rate of directly evoked action potentials). In a smaller number of cells PCP (50-300 microM) increased input resistance. These effects were long-lasting and neurons did not show reversal with washes as long as 75 min. Some neurons were injected with biocytin for identification. All were medium-sized spiny type I neurons typical of rodent neostriatum.

Somatodendritic morphology of on- and off-cells in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) contains two classes of physiologically defined neurons, on-cells and off-cells, that are implicated in nociceptive modulation. In a continuing effort to detail the neural circuitry that underlies the activity of these two distinct neuronal types, the somatodendritic morphology of on- and off-cells was studied in the cat, rat, and ferret. In lightly anesthetized animals, on-cells increased and off-cells decreased their discharge rate during a withdrawal reflex evoked by noxious stimuli. Following their physiological characterization by using intracellular recording, on- and off-cells were injected with either horseradish peroxidase or biocytin and their somatodendritic arborizations were examined. Labeled on- and off-cells included fusiform and stellate cells of all sizes as well as large multipolar neurons. Although the somatic shape of both on- and off-cells in RVM was heterogeneous, off-cells tended to be fusiform neurons whose long axis was oriented mediolaterally. The dendritic domains of both on- and off-cells extended bilaterally past the lateral edge of the trapezoid body or pyramid and ventrally to, and sometimes including, the trapezoid body or pyramid. In contrast to their extensive mediolateral spread, the dendritic domains of both cell types were limited to the ventral half of the reticular formation and were compressed along the rostrocaudal axis. The dendritic arbor of individual on- and off-cells extended well beyond the cytoarchitectonic boundaries of any single nuclear region, within the domain delineated as the RVM. The spatial domains of the dendritic arbors of on- and off-cells are further evidence that the on- and off-cells throughout the RVM constitute an integrated unit in the modulation of nociceptive transmission.

Serotonin enhances excitability in neostriatal neurons by reducing voltage-dependent potassium currents

The physiological effects of serotonin (5-HT) on rat neostriatal neurons were investigated using current-clamp techniques in neostriatal slices and voltage-clamp techniques in acutely dissociated adult neostriatal neurons. In most neurons (35/51), bath-applied 5-HT (10-60 microM) decreased the first spike latency and increased the evoked firing frequency. Membrane input resistance was also increased in most neurons (33/35) but could not explain the enhanced responsiveness. Tetrodotoxin, at concentrations sufficient to block spike production, did not block the ability of 5-HT to enhance the slow ramp-like voltage trajectory produced by depolarizing current injection. The role of potassium currents in the 5-HT effect was examined using whole cell voltage-clamp; in 6 of 9 neurons, 5-HT reversibly decreased inactivating potassium currents activated by depolarization. These experiments suggest that 5-HT's effect on the ramp trajectory may be mediated by a reduction of potassium currents activated by sub-threshold depolarization.

Identification of neurons producing long-term potentiation in the cat motor cortex: intracellular recordings and labeling

Intracellular, in vivo recordings were used to identify and subsequently to label neurons in the cat motor cortex in which long-term potentiation (LTP) was induced. Thirty-nine motor cortical neurons that produced excitatory postsynaptic potentials (EPSPs) in response to microstimulation in areas 1-2 (SI) or in area 5a (SIII) were studied. Amplitudes of EPSPs produced in response to test stimulation (1 Hz) were recorded before and after tetanic stimulation (200 Hz, 20 seconds). In 25/39 cells (64%), EPSP amplitudes were significantly increased following the tetanic stimulation (65 +/- 51% average increase), and remained at the potentiated level as long as stable recordings could be maintained (20 +/- 18 minutes, maximum = 90 minutes). LTP was induced exclusively in cells that produced monosynaptic EPSPs in response to area 1-2 or area 5a stimulation. Of the 39 analyzed cells, 13 were labeled by intracellular injections of 5% biocytin. Neurons in which LTP was induced included both pyramidal and nonpyramidal cells and were located exclusively in layers II or III of the motor cortex; cells in deeper cortical layers were not potentiated. These findings indicate that various corticocortical inputs can increase the efficacy of synaptic transmission in a subset of motor cortical neurons. We propose that this plasticity in synaptic transmission constitutes one of the bases of motor learning and memory.

Opioid actions on single nucleus raphe magnus neurons from rat and guinea-pig in vitro

1. Intracellular recordings were made from neurons of the nucleus raphe magnus (NRM) from rat (n = 128) and guinea-pig (n = 115). Two types of cells were found in each, primary (103 in rat, 27 in guinea-pig) and secondary cells (25 in rat, 88 in guinea-pig). 2. Primary cells had input resistances of 186 +/- 9 M omega (n = 9) in rat and 255 +/- 50 M omega (n = 11) in guinea-pig. The action potential in each was about 1.5 ms in duration. Synaptic potentials were evoked by focal electrical stimulation and consisted of both gamma-aminobutyric acid (GABA) and excitatory amino acid components. 3. Morphine, [Met5]enkephalin (ME) and [D-Ala2,N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) depressed the amplitude of the GABA-mediated synaptic potential by a maximum of 50-65% and had little effect on the excitatory amino acid-mediated synaptic potential. There was no effect of these opioids on the resting membrane potential or input resistance of primary cells in rat or guinea-pig. 4. Secondary cells had short duration action potentials (less than 1 ms) and an input resistance of 354 +/- 47 M omega in rat (n = 6) and 290 +/- 40 M omega in guinea-pig (n = 15). The synaptic potential observed in the cells of this group was mediated by activation of only excitatory amino acid receptors. 5. ME hyperpolarized and/or abolished the spontaneous firing in sixteen out of twenty-four neurons in the secondary group from rat and eight out of eighty-four neurons from guinea-pig. ME induced an outward current at -60 mV that reversed polarity at potentials more negative than -92 +/- 3 mV in rat (n = 6) and -98 +/- 2 mV in guinea-pig (n = 18). The reversal potential of the opioid current was shifted to less negative potentials when the external potassium concentration was increased, as predicted by the Nernst equation. 6. The morphology of the two types of cells were distinguishable in that primary cells were oval (29 x 18 microns in rat; 36 x 19 microns in guinea-pig) with two to four thick tapering dendrites that branched within 50 microns of the cell body. Secondary cells were generally round or oval (about 24 x 13 microns in rat; 27 x 17 microns in guinea-pig) with two to five thin non-tapering dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential activation of glutamate receptors by spontaneously released transmitter in slices of neocortex

Whole-cell recordings were made from neurons in neocortical brain slices in order to characterize excitatory synaptic currents mediated by glutamate receptors. Glutamate receptor antagonists, D-aminophosphonovalerate (D-APV) and CNQX, selectively attenuated distinct components in evoked synaptic currents, and were used to differentiate spontaneous synaptic currents mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Spontaneous excitatory synaptic currents were independent of action potentials, varied linearly with voltage, and were blocked by the non-NMDA receptor antagonist CNQX. An NMDA receptor-mediated component was not apparent in these spontaneous synaptic currents, however, when magnesium was omitted from the recording medium, fluctuations in current and sustained inward current became apparent, and these were blocked by the NMDA receptor antagonist D-APV. Based on these findings, we conclude that NMDA and non-NMDA receptors are activated differentially by transmitter released independently of action potentials.

Confocal scanning laser microscope images of hippocampal neurons intracellularly labeled with biocytin

We have assessed the properties and usefulness of confocal scanning laser microscopy in the reflection mode for the study of neuronal morphology. In this mode, the confocal microscope detects the light reflected off the specimen as opposed to the light emitted by a fluorescent label. Neurons in slices of rat hippocampus were filled with biocytin and reacted sequentially with avidin-horseradish peroxidase and nickel-intensified diaminobenzidine (DAB/Ni). In all parts of the neuron the DAB/Ni reaction product produced a strong reflection signal in the confocal microscope. The stereo images revealed aspects of three-dimensional hippocampal cell morphology such as the conical shape of the dendritic fields and a characteristic branching pattern of the axon. Labelling neurons intracellularly is an established technique for identifying physiologically-characterized neurons. Recently, confocal microscopy has become a powerful method for examining the three-dimensional morphology of biological specimens. The resulting images in this paper show that reflection-mode confocal microscopy provides an excellent representation of the filled neurons in three dimensions and presents an opportunity for correlative electrophysiological and morphological studies and extension to the electron-microscopic level.

Inward Rectification (I) in Immunocytochemically-ldentified Vasopressin and Oxytocin Neurons of Guinea-Pig Supraoptic Nucleus

Intracellular recordings of magnocellular neurons from the supraoptic nucleus of guinea-pigs were made with KCI/K citrate- and biocytin-filled electrodes. Fifty of 99 cells exhibited a time-dependent inward rectification (TDR). The TDR was activated during hyperpolarizing current pulses to membrane potentials more hyperpolarized than -75 mV. In voltage-clamp recordings, an inward current appeared at voltage steps more hyperpolarized than -75 mV, with properties similar to the slow inward rectifier (I(h)) described in other tissues. The I(h) was blocked by 2 mM CsCI. BaCI(2) (100 to 500 muM) did not block the I(h). Immunocytochemical identification of the recorded cells revealed that both vasopressin (AVP)- and oxytocin (OT)- containing neurons exhibited an I(h).

Double- and triple-labeling of functionally characterized central neurons projecting to peripheral targets studied in vitro

The use of in vitro preparations such as brain slices poses difficulties in determining the correct identity of cells under study. To circumvent this problem, we first used a fluorescence pre-labeling technique (rhodamine-dextran-lysine) to identify cranial motoneurons projecting to the tongue (hypoglossal motoneurons) in the guinea-pig. Following preparation of slices, cells were recorded intracellularly and their electrophysiological properties determined. The cells were then intracellularly stained with both a fluorescence label (Lucifer Yellow) and with the stable, non-fading label biocytin. Under fluorescent illumination, the great majority of recorded cells within the hypoglossal nucleus were double-labeled (rhodamine and Lucifer Yellow) suggesting that most are indeed motoneurons. Biocytin injected into the same motoneurons provided permanent and detailed images of their morphology. Intracellularly stained cells surrounding the hypoglossal nucleus were not labeled with rhodamine and had distinct electro-physiological properties. The use of the retrogradely transported marker rhodamine-dextran-lysine allows the unambiguous identification of motoneurons in a brainstem slice. The combined intracellular injection of Lucifer Yellow and biocytin provides a simple means of melding the advantages of a fluorescent label (compatible with other fluorescence labels and with immunocytochemistry) with the benefits of a stable, non-fading, electron-dense marker. Application of this technique should prove useful in establishing morphological and functional correlates in other areas of the CNS.

Tuberal supraoptic neurons--I. Morphological and electrophysiological characteristics observed with intracellular recording and biocytin filling in vitro

Previous studies of the tuberal, or retrochiasmatic, portion of the supraoptic nucleus suggest its functional similarity to the more densely populated anterior supraoptic nucleus, but the basic electrophysiological and morphological features of tuberal supraoptic nucleus neurons have not been described. Using the hypothalamo-neurohypophysial explant preparation in the rat, intracellular recordings and biocytin injections were made in tuberal supraoptic nucleus neurons and the results indicate that the two parts of the nucleus are similar. The generally oval-shaped somata of tuberal supraoptic nucleus neurons exhibited short, irregularly shaped appendages, and possessed 2-5 varicose, sparsely branching dendrites oriented in the horizontal plane. Many tuberal supraoptic nucleus neurons could be antidromically stimulated (mean latency = 6.4 ms). Filled neurons had varicose axons which were traced to the median eminence and even as far as the neural stalk, but which did not bifurcate. Both axons and dendrites were sparsely invested with short, hair-like appendages. The input resistance of the recorded neurons (mean = 177.7 M omega) was positively correlated with the membrane time constant (mean = 13.1 ms; r = 0.83). Tuberal supraoptic nucleus neurons displayed a prominent afterhyperpolarization following individual spikes or bursts of spikes, as well as firing frequency adaptation in response to positive current pulses. Although numbering far fewer than those of the anterior supraoptic nucleus, tuberal supraoptic nucleus neurons have axons which are more often intact in this preparation, and a dendritic tree which radiates within the plane of the explant. Thus these neurons should provide a useful model for further study of the electrophysiological and morphological characteristics of mammalian neurosecretory neurons.

Subthreshold Na+-dependent theta-like rhythmicity in stellate cells of entorhinal cortex layer II

The oscillation of membrane potential in mammalian central neurons is of interest because it relates to the role of oscillations in brain function. It has been proposed that the entorhinal cortex (EC), particularly the stellate cells of layer II (ECIIscs), plays an important part in the genesis of the theta rhythm. These neurons occupy a key position in the neocortex-hippocampus-neocortex circuit, a crucial crossroad in memory functions. Neuronal oscillations typically rely on the activation of voltage-dependent Ca2+ conductances and the Ca2+ -dependent K+ conductance that usually follows, as seen in other limbic subcortical structures generating theta rhythmicity. Here we report, however, that similar oscillations are generated in ECIIscs by a Na+ conductance. The finding of a subthreshold, voltage-gated, Na+ -dependent rhythmic membrane oscillation in mammalian neurons indicates that rhythmicity in heterogeneous neuronal networks may be supported by different sets of intrinsic ionic mechanisms in each of the neuronal elements involved.

Distinct mechanisms of modulation in a neuronal oscillator generate different social signals in the electric fish Hypopomus

The medullary pacemaker nucleus of the gymnotiform electric fish, Hypopomus, is a relatively simple neuronal oscillator which contains pacemaker cells and relay cells. The pacemaker cells generate a regular discharge cycle and drive the relay cells which trigger pulse-like electric organ discharges (EODs). The diencephalic prepacemaker nucleus (PPN) projects to the pacemaker nucleus and modulates its activity to generate a variety of specific discharge patterns which serve as communicatory signals (Figs. 2 and 3). While inducing such signals by microiontophoresis of L-glutamate to the region of the PPN (Fig. 4) of curarized animals, we monitored the activity of neurons in the pacemaker nucleus intracellularly. We found that pacemaker cells and relay cells were affected differently in a manner specific to the type of EOD modulation (Figs. 5-10). The normal sequence of pacemaker cell and relay cell firing was maintained during gradual rises and falls in discharge rate. Both types of cells ceased to fire during interruptions following a decline in discharge rate. During sudden interruptions, however, relay cells were steadily depolarized, while pacemaker cells continued to fire regularly. Short and rapid barrages of EODs, called "chirps", were generated through direct and synchronous activation of the relay cells whose action potentials invaded pacemaker cells antidromically and interfered with their otherwise regular firing pattern.

Biocytin: a versatile anterograde neuroanatomical tract-tracing alternative

Biocytin, a naturally occurring low molecular weight analog of biotin, was evaluated as a neuroanatomical tract-tracing marker in the adult rat brain. Since it retains high-affinity binding to avidin, biocytin can be labelled with avidinylated visualization reagents. Iontophoretic or pressure injections resulted in filling of cell bodies and dendrites around the injection site and their efferent axonal processes and boutons. Retrogradely labelled neurons were occasionally observed at a distance but only with large injections. Anterograde tracing with biocytin is successful even in animals that are quite old, in contrast to lectins and HRP conjugates, and offers advantages in delivery, tissue processing, selection of light and/or electron microscopic labels, time to obtain results, and cost over many conventional tracers.