The monoamine innervation of rat cerebral cortex: synaptic and nonsynaptic axon terminals

Light and electron microscopical localisation of 5-HT-immunoreactive boutons in the rat trigeminal motor nucleus

We have used pre-embedding EM immunohistochemical methods to obtain quantitative data on the frequency and post-synaptic targets of 5-hydroxytryptamine-immunoreactive (5-HT-IR) boutons within the rat V motor nucleus. Thirteen percent (69/531) of all synaptic contacts in the motor nucleus involved 5-HT-IR boutons. Seventy-four percent of 5-HT-IR boutons made axo-dendritic contacts, 20% axo-somatic contacts, and 6% axo-axonic contacts. We conclude that a significant fraction of boutons in the motor nucleus are 5-HT-IR and most contribute to postsynaptic rather than presynaptic effects on trigeminal motoneurones.

Distribution of the pro-opiomelanocortin-immunoreactive axons in relation to the serotoninergic neurons in the dorsal raphe nucleus of the rat

The anatomical relationships between pro-opiomelanocortin-containing axons and serotonin neurons in the nucleus raphe dorsalis (NRD) of the rat were examined at the light microscope level with antibodies against CLIP (corticotropin-like intermediate lobe peptide), alpha-MSH (alpha-melanocyte-stimulating hormone) and serotonin. Sequential double labeling was performed with either immunofluorescence or peroxidase-antiperoxidase techniques. It was observed that the network of POMC-immunoreactive axons displayed a gradient of decreasing density from rostral to caudal levels and from dorsal to ventral parts or the NRD. The examples of close proximity between immunoreactive axons and serotonin cell bodies or dendrites were rather scarce. On the whole, the immunoreactive fibers seemed to run quasi-independently of the serotonin neurons.

Is the histaminergic neuron system a regulatory center for whole-brain activity?

Recent immunocytochemical studies have demonstrated the existence of histaminergic neurons in the brain, which are concentrated in the tuberomammillary nucleus of the posterior hypothalamus, and which project efferent fibers to almost all parts of the brain. Three subtypes of histamine receptors are widely distributed in the brain, not only on neurons but also on astrocytes and blood vessels. Consistent with its wide-ranging output, the histaminergic neuron system regulates various activities of the brain, such as the arousal state, brain energy metabolism, locomotor activity, neuroendocrine, autonomic and vestibular functions, feeding, drinking, sexual behavior, and analgesia--this regulation is possibly achieved by the histaminergic system as a whole.

Phrenic motoneuron morphology in the neonatal rat

The morphology of neonatal rat phrenic motoneurons was studied following retrograde labeling with horseradish peroxidase, which resulted in Golgi-like fills of phrenic motoneuron somata and dendrites. At birth, these neurons have well-developed dendritic trees with many characteristics described for phrenic motoneurons in the adult rat. The dendrites form tightly fasciculated bundles that emerge from the phrenic nucleus primarily along four axes: ventromedial, ventrolateral, dorsolateral, and rostral/caudal, with smaller and more variable projections directly lateral and ventral. Although sparse, some dendritic appendages were also present, and in a few animals, somata clustering was apparent. The most significant difference between adult and neonatal rat phrenic motoneurons is in the extent to which medially and laterally projecting dendrites extend beyond the borders of the ipsilateral gray matter. In the neonate, unlike the adult, these dendrites project extensively past the gray/white border to the edge of the hemicord. Ventromedial dendrites occasionally cross to the contralateral ventral horn in the ventral white commissure and laterally projecting dendrites could be seen reaching the edge of the cord, turning and traveling rostrally or caudally for up to 100 microns. Phrenic motoneurons are not unique in having long dendrites at birth. A brief comparative study showed that neonatal cervical, thoracic, and lumbar motoneurons also have long dendrites that project to the medial and lateral borders of the hemicord.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: III. Effects of adrenoceptor agonists and antagonists

Stimulation of the ventral noradrenergic ascending bundle (VNAB) at low frequencies (0.5/5 Hz) excited the majority (37/46, 80%) of single paraventricular nucleus (PVN) tuberoinfundibular neurones, with high frequency (50 Hz) trains of stimuli reversing the direction of the response to inhibition for 7/16 (44%) of these excited cells. Iontophoretic application of noradrenaline, or the alpha 1-adrenoceptor agonist 1-phenylephrine, increased the spontaneous electrical activity of most of the cells tested (94% and 72%), whilst application of the alpha 1-antagonist, ergotamine reduced the spontaneous activity of 44% of the cells tested and prevented the excitation following VNAB stimulation for 84% of the cells examined. Application of the beta-adrenoceptor antagonist, propranolol, increased the spontaneous activity of 77% of cells and prevented the inhibitory PVN neuronal responses following high frequency VNAB stimulation of 94% of the cells, often reversing the response to excitation similar to that observed following low frequency VNAB stimulation. The alpha 2-adrenoceptor antagonist, tolazoline, was found to evoke mixed responses from the cells examined but a trend towards a suppression of spontaneous activity and potentiation of VNAB stimulation-evoked responses was observed. The alpha 2-adrenoceptor agonist, clonidine, elicited an initial excitation from the majority of cells tested, with most of the cells then exhibiting an inhibition, either with or without continued application. Excitatory responses following stimulation of the sciatic nerve were recorded from the majority of cells (82.5%) and ergotamine was able to suppress this response for all four cells so tested.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA-like immunoreactivity in the tuberomammillary nucleus: an electron microscopic study in the rat

The organization of GABAergic elements in the histaminergic tuberomammillary nucleus has been examined by using antibodies against gamma-aminobutyric acid (GABA) and light and electron microscopy. Most neuronal perikarya of the ventral subgroup of the tuberomammillary nucleus were GABA immunoreactive (GABA-i). The morphology of the GABA-i perikarya was similar to the morphology of histaminergic perikarya described by Hayashi et al. ('84: J. Comp. Neurol. 229: 223-241) and Wouterlood et al. ('86: J. Comp. Neurol. 252:227-243). The GABA-i perikarya were contacted by relatively few terminals. The mean bouton covering ratio of GABA-i perikarya was 6.1%, whereas the mean bouton covering ratio for GABA-i dendrites in the tuberomammillary nucleus was 31%. Some of the presynaptic terminals were GABA-i. In addition, GABA-i perikarya and dendrites formed close contacts that never presented synaptic specializations. These results suggest that neurons of the histaminergic tuberomammillary nucleus contain the neurotransmitter GABA. Furthermore, GABA may act as a modulator of cellular processes within the tuberomammillary nucleus.

A new 5-hydroxy-indole derivative with preferential affinity for 5-HT1B binding sites

The affinities of several 5-hydroxy-indole derivatives for serotonin-1 (5-HT1) binding site subtypes, labeled with 2 nM [3H]5-HT, were assessed by quantitative autoradiography on rat brain sections. The results obtained with known ligands, namely 5-hydroxytryptamine (5-HT), 5-methoxytryptamine (5-Me-OT), 5-methoxy-N,N- dimethyl-tryptamine (5-Me-ODMT), 5-hydroxy-N,N-dimethyl-tryptamine (bufotenine) and 8-hydroxy-2-[di-N-propylamino]tetralin (8-OH-DPAT) demonstrate the reliability and the advantages of this technique for pharmacological studies. Novel serotonin derivatives were synthesized by carboxymethylation of the hydroxyl group. One of those new ligands, serotonin-O-carboxy-methyl- glycyl-tyrosinamide (S-CM-GTNH2), inhibited 2 nM [3H]5-HT binding to the substantia nigra with an IC50 of 22.4 nM, a value which is 22 times lower than that found in the dentate gyrus and choroid plexus. This demonstrates the preferential affinity of S-CM-GTNH2 for 5-HT1B versus 5-HT1A and 5-HT1C binding sites. S-CM-GTNH2 contains a tyrosine residue, which may be useful for the synthesis of a radioactive iodinated molecule and for the preparation of 'long-lasting ligands' linked through peptide bonds with a protein. These derivatives could be of great interest for ultrastructural and behavioral studies relevant to 5-HT1B sites.

Evidence for functional separation of alpha-1 and alpha-2 noradrenaline receptors by pre-synaptic terminal re-uptake mechanisms

Information transfer within the central nervous system is predominantly chemical in nature, and occurs both through synaptic specialisations and non-specific diffuse release. The localisation and description of receptors for these two types of neurotransmission is currently a contentious issue. In the present study, the noradrenaline reuptake inhibitor cocaine has been shown to overcome the inhibitory effects of idazoxan, a selective alpha-2 receptor antagonist, but not phentolamine, a non-selective alpha receptor antagonist, on eating following injection of noradrenaline into the rat hypothalamic paraventricular nucleus. Similarly, lesion by 6-hydroxydopamine of noradrenaline terminals in the paraventricular nucleus also reduced the efficacy of idazoxan in blocking eating induced by noradrenaline. These data confirm that postsynaptic alpha-2 receptors are involved in the feeding response to exogenous noradrenaline, but in addition, when taken in conjunction with previously published data, are used to suggest a differential distribution of NA receptors within the hypothalamic paraventricular nucleus. It is proposed that the alpha-2 subtype may be extrasynaptic, the alpha-1 subtype intrasynaptic.

Discharge of noradrenergic locus coeruleus neurons in behaving rats and monkeys suggests a role in vigilance

Recordings from noradrenergic locus coeruleus (LC) neurons in behaving rats and monkeys revealed that these cells decrease tonic discharge during sleep and also during certain high arousal behaviors (grooming and consumption) when attention (vigilance) was low. Sensory stimuli of many modalities phasically activated LC neurons. Response magnitudes varied with vigilance, similar to results for tonic activity. The most effective and reliable stimuli for eliciting LC responses were those that disrupted behavior and evoked orienting responses. Similar results were observed in behaving monkeys except that more intense stimuli were required for LC responses. Our more recent studies have examined LC activity in monkeys performing an "oddball" visual discrimination task. Monkeys were trained to release a lever after a target cue light that occurred randomly on 10% of trials; animals had to withhold responding during non-target cues. LC neurons selectively responded to the target cues during this task. During reversal training, LC neurons lost their response to the previous target cue and began responding to the new target light in parallel with behavioral reversal. Cortical event-related potentials were elicited in this task selectively by the same stimuli that evoked LC responses. Injections of lidocaine, GABA, or a synaptic decoupling solution into the nucleus paragigantocellularis in the rostral ventrolateral medulla, the major afferent to LC, eliminated responses of LC neurons to sciatic nerve stimulation or foot- or tail-pinch. This indicates that certain sensory information is relayed to LC through the excitatory amino acid (EAA) input from the ventrolateral medulla. The effect of prefrontal cortex (PFC) activation on LC neurons was examined in anesthetized rats. Single pulse PFC stimulation had no pronounced effect on LC neurons, consistent with our findings that this area does not innervate the LC nucleus. However, trains of PFC stimulation substantially activated most LC neurons. Thus, projections from the PFC may activate LC indirectly or through distal dendrites, suggesting a circuit whereby complex stimuli may influence LC neurons. The above results, in view of previous findings for postsynaptic effects of norepinephrine, are interpreted to reveal a role for the LC system in regulating attentional state or vigilance. The roles of major inputs to LC from the ventrolateral and dorsomedial medulla in sympathetic control and behavioral orienting responses, respectively, are integrated into this view of the LC system. It is proposed that the LC provides the cognitive complement to sympathetic function.

Ultrastructural aspects of the coeruleo-spinal projection

Few studies have focussed on the ultrastructure of the coeruleo-spinal projection. In rat the projections from the area of the locus coeruleus (LC) and subcoeruleus (SC) to lumbar motoneuronal cell groups exhibited two different types of terminals: E-type terminals, containing many very small vesicles and S-type terminals, containing many spherical vesicles and an occasional dense-cored vesicle. These findings are in agreement with data indicating the existence of a noradrenergic (NA) and a non-NA projection from the area of the LC and SC to the spinal cord. A study on dopamine-beta-hydroxylase (D beta H)-immunoreactive terminals in lumbar motoneuronal cell groups showed that they contained several granular vesicles, which were not found in the E- and S-type terminals. Only a few immunoreactive terminals exhibited a synaptic specialization in a single, thin section. A low incidence of synaptic junctions was also found for the E-type terminals, but not for the S-type. Based on this and other data, it is suggested that the E-type terminal is NA, while the S-type may contain a non-NA transmitter, possibly acetylcholine. A low incidence of synaptic junctions in single, thin sections may indicate the presence of non-synaptic NA terminals, but direct evidence from serial-section analysis is not available. In the superficial dorsal horn, terminals derived from the area of the LC and SC were identified at the ultrastructural level in two studies, one using the anterograde degeneration technique in opossum, the other (presented in this chapter) using WGA-HRP anterograde tracing in rat. It was found in both studies that most of the labeled structures were small axons (mostly unmyelinated), while few terminals were labeled. They contained mostly spherical vesicles and, according to the degeneration study, a variable number of dense-cored vesicles. The labeled terminals appeared to make regular synaptic contacts mostly with small dendrites and occasionally with spines. They were not present in glomeruli or engaged in presynaptic arrangements. A study on NA terminals showed similar results, although large granular vesicles were not observed and fewer synapses were seen. On the few data available at present it is concluded that in the spinal superficial dorsal horn, most terminals derived from the area of the LC and SC are NA and establish conventional synapses. However, a non-NA component cannot be excluded.

Iontophoresis of cortisol inhibits responses of identified paraventricular nucleus neurones to sciatic nerve stimulation

Responses of paraventricular nucleus (PVN) neurones were examined following stimulation of the sciatic nerve, and concomitant with iontophoretic application of cortisol. Sciatic nerve stimulation excited the majority of cells (22/24, 92%) and iontophoretic application of cortisol reduced the spontaneous activity of 16 of the cells tested (67%). Cortisol prevented neuronal responses to sciatic nerve stimulation in 11 cases (50%) but some of the cells inhibited by the steroid still responded to the stimulation, whilst some cells unaffected by cortisol alone were found not to respond during exposure to the stimulus. These results indicate an inhibitory role for glucocorticoids in the regulation of PVN neuronal activity and responses to afferent neural stimuli.

Serotonin reinnervation of the rat organum vasculosum laminae terminalis (OVLT) after 5,7-dihydroxytryptamine deafferentation

The time course and pattern of serotonin (5-HT) reinnervation in the rat organum vasculosum laminae terminalis (OVLT) following intracerebroventricular administration of 5,7-dihydroxytryptamine were examined by means of [3H]5-HT uptake radioautography. 5-HT axonal varicosities reappeared in the OVLT within 3 months post-lesion. Six months later, they were found to have preferentially reinvested the neurohemal contact area (juxtavascular zone) of the organ. Regenerated terminals further increased in number. At the end of a 16-month survival time, they displayed normal-looking distributional patterns, similar to those of age-matched control animals, and formed new synaptic junctions in the juxtaventricular zone. The cellular mechanisms possibly responsible for 5-HT recovery in the OVLT are discussed.

Ultrastructural Double-Labelling Study of Dopamine Terminals and GABA-Containing Neurons in Rat Anteromedial Cerebral Cortex

The aim of this study was to identify, at the ultrastructural level, the neuronal targets of dopamine afferents to the medial prefrontal and the anterior cingulate cortex of the adult rat. Since, in addition to pyramidal neurons, the cortical neuronal population mainly consists of GABAergic nonpyramidal intrinsic neurons, the simultaneous visualization of both dopamine- and GABA-containing neurons should leave the pyramidal neurons as the only unlabelled dopamine postsynaptic target. In this context, we used a double labelling immunocytochemical procedure: a pre-embedding PAP immunostaining to visualize monoclonal conjugated-dopamine (DA) antibody, followed by postembedding immunogold staining with a polyclonal conjugated-GABA antibody. In a single section sampling of 369 DA-immunoreactive (DA-IR) varicosities observed and the GABA-containing elements, 75% of the DA-IR terminals showed no indication of any contact with a GABA neuron. Twenty-five per cent were found in nonsynaptic contiguity with a GABA-immunoreactive neuronal element: axon, dendrite or cell body. When a DA varicosity was in nonsynaptic contiguity with a neuronal perikaryon (5% of cases), this cell was GABA positive. Ten per cent of the DA varicosities were contiguous to a GABA axon, but axoaxonic synapses in either direction were never observed. A symmetrical synapse between a DA varicosity and a GABA-containing dendrite was observed only once. The other 13 DA-IR terminals exhibiting a clear synaptic junction were apposed to nonGABA-containing dendrites, spines and shafts. Triads were observed in which a DA varicosity, forming or not a symmetrical synapse, was apposed to an unlabelled dendrite already receiving a symmetrical junction from another unlabelled axon. These data confirm and extend previous results designating the pyramidal cell dendritic tree as the main synaptic target of DA cortical afferents in rat and primate cerebral cortex. However, a direct effect of dopamine on a subpopulation of intrinsic GABA neurons cannot be excluded.

In the rat medial nucleus accumbens, hippocampal and catecholaminergic terminals converge on spiny neurons and are in apposition to each other

The nucleus accumbens septi (Acb) represents an interface between limbic and motor systems and a site for modulation of these integrative functions by ascending catecholaminergic, principally dopaminergic, axons. This modulatory regulation is most likely attributed to pre- or postsynaptic associations between limbic telencephalic and brainstem afferents. In the present investigation, we examined the ultrastructure and synaptic associations of hippocampal afferents, as well as their relation to catecholaminergic terminals, in the medial Acb of adult rats. Hippocampal afferents were identified by anterograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injected in the ventral subiculum, and by anterograde degeneration seen 2-3 days following lesion of the fimbria. Specific comparisons between these methods were made (1) to determine whether similar populations of terminals were labeled and (2) to assess the feasibility of combining degeneration with immunoperoxidase labeling for the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). Hippocampal afferents labeled with HRP were finely myelinated or unmyelinated and gave rise to small terminals (mean diameter 0.58 micron) containing mostly clear, round vesicles. Of the HRP-labeled terminals which made recognizable junctions, 85% (104/122) formed asymmetric synapses with the heads of dendritic spines. The remainder either formed asymmetric axodendritic synapses or symmetric junctions. Degenerating terminals were significantly smaller (mean diameter 0.35 micron) than terminals labeled with HRP. However, these also formed principally asymmetric axospinous synapses (89/102, 87%). Whether identified by HRP transport or anterograde degeneration, the hippocampal afferents comprised approximately 10% of all terminals and 30% of all asymmetric axospinous synapses in the medial Acb. In contrast to hippocampal afferents, TH-labeled terminals formed primarily symmetric contacts with dendritic shafts and the heads and necks of spines. Quantitative analysis of sections containing both anterograde degeneration and TH-immunoreactivity showed that 25% (26/104) of associations formed by degenerating hippocampal terminals involved convergent inputs with TH-labeled terminals on the same postsynaptic structure. These included dual input either to the same spine head or to different parts of the same dendrite. In addition, the plasma membranes of hippocampal and TH-labeled terminals were often directly apposed to each other (10/58, 17% of axo-axonal associations formed by degenerating terminals), without recognizable synaptic specializations.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of noradrenergic terminals and their synapses in the rat spinal dorsal horn: an immunohistochemical study

Noradrenergic fibers in the spinal dorsal horn originate from neurons in the A5-7 cell groups, and may participate in the modulation of pain. Here we studied the fine structure of noradrenergic terminals in the rat by immunohistochemistry using antiserum against dopamine-beta-hydroxylase (DBH). We also investigated the relationship between such terminals and primary afferent terminals. DBH-like immunoreactive terminals were found in lamina I and the outer layer of lamina II of the dorsal horn and they contained many clear round vesicles and some large granular vesicles. More than half of these terminals made synaptic contact with other neuronal elements with membrane specialization. Most of the postsynaptic structures of these terminals were small dendrites (69%); 28% were spines, and no synaptic contact was made with primary afferent terminals. These findings suggest that noradrenaline acts on the spinal dorsal horn neurons postsynaptically mainly via a direct synaptic mechanism.

Distribution of serotonin in the caudal neurosecretory complex. A light and electron microscopic study

The caudal neurosecretory complex (CNc) of poecilids has previously been shown to receive serotonergic inputs. In the present study, immunohistochemical techniques were applied at the light and electron microscopic levels to characterize serotonergic terminals in the neuroendocrine nucleus. A dense plexus of varicose fibers observed in the rostral CNc neuropil was absent in the spinal cords of deafferented fish, indicating that the origin of this input was extranuclear. Ultrastructural study revealed no direct contacts between labeled structures and neuroendocrine cells. Non-synaptic terminals (varicosities) were the predominantly labeled structures in the neuropil. Synaptic terminals were observed on cellular and axonal targets in the CNc. Small cells containing 70 nm dense-core vesicles received serotonergic input on their perikarya. Labeled synapses were also found on unlabeled axon terminals which made axo-axonal synapses on neuroendocrine processes. Non-synaptic terminals may be responsible for a variety of serotonin-mediated effects in the CNc. Synaptic interactions with local catecholaminergic and afferent cholinergic inputs to the CNc are likely.

Monoamine synaptic structure and localization in the central nervous system

The monoamines dopamine, noradrenaline, adrenaline, and serotonin as well as the diamine histamine have a widespread distribution in the central nervous system within synaptic terminals and nonsynaptic varicosities. In certain regions of the central nervous system the monoamines are contained in varicosities that have no synaptic specialization associated with them, suggesting a possible neuromodulatory role for some of the monoamines. The majority of monoamine labelled structures are synaptic terminals which are characterized by the presence of small, clear vesicles (40-60 nm) and large, granular vesicles (70-120 nm) within the terminal. A third population of vesicles--small, granular vesicles--which are visible only after histochemical staining, are probably the equivalent of the small, clear vesicles present after either autoradiographic or immunohistochemical labelling. Most monoamine containing terminals contact dendrites and dendritic spines and, less frequently, neuronal somata and other axons. Both asymmetrical and symmetrical membrane specializations are associated with monoaminergic terminals; however, asymmetrical contacts are the most frequent type found. These ultrastructural results indicate that monoamine containing terminals and varicosities in general share many common morphological features, but still have diverse functions.

Distribution and synaptic organization of serotoninergic and noradrenergic axons in the lateral geniculate nucleus of the rat

Antisera raised against the monoamines serotonin (5-HT) and noradrenaline (NA) were employed in a study designed to provide a detailed description of the distribution, morphology, and synaptic organization of the serotoninergic and noradrenergic afferents in the lateral geniculate nucleus (LGN) of the rat. The distribution patterns of the two types of immunoreactive fibers were distinct and largely complementary to each other. NA axons were particularly concentrated in the dorsal lateral geniculate nucleus (LGd), with the ventral lateral geniculate nucleus (LGv) and the intergeniculate leaflet (IGL) receiving substantially fewer fibers. In contrast, 5-HT axons, although present throughout the LGN, were preferentially concentrated in the LGv and IGL. 5-HT and NA axon terminals and axonal varicosities, examined in single and serial ultrathin sections, formed conventional synapses in the extraglomerular neuropil. The types of synapses and the nature of the postsynaptic targets were different for the two monoamines. 5-HT afferents formed asymmetrical synapses on dendritic spines and shafts of both presumptive relay cells and interneurons but established symmetrical synapses on cell bodies. However, NA afferents formed almost exclusively symmetrical synapses on dendritic spines and shafts and made no contacts with cell bodies. The present findings suggest that the 5-HT and NA afferents of the rat LGN, which are likely to influence certain stages of visual processing, exhibit distinct organizational principles and act at restricted sites as do other classical neurotransmitter systems.

5-hydroxytryptamine immunoreactive varicosities in the lamprey spinal cord have no synaptic specializations--an ultrastructural study

The distribution and fine structure of 5-hydroxytryptamine (5-HT) immunoreactive cell bodies and axonal varicosities have been studied in the lamprey spinal cord, using the peroxidase-antiperoxidase (PAP) immunohistochemical technique and subsequent analysis of ultrathin serial sections. Immunostained cell bodies were found in the ventral spinal cord close to the central canal. Immunostained varicosities were found throughout the spinal cord with the highest density in the ventromedial plexus and the dorsal horn. Only large granular vesicles could be clearly distinguished in immunostained cell bodies and varicosities, but it was concluded based on a comparison with unstained normal tissue that these boutons also contained small, pleomorphic agranular vesicles. Immunoreactive varicosities were studied in the ventromedial plexus, the dorsal horn, the dorsal column, the dorsolateral and ventrolateral funiculi and the grey matter. No morphological differences could be observed between varicosities in the different loci. The varicosities were in no case seen to make synaptic contact with surrounding neuronal elements, even when followed through serial sections. Consequently, 5-HT released from boutons in all parts of the spinal cord could be expected to act on 5-HT receptors located on nearby as well as distant receptors.

Serotonergic innervation of the lateral cervical nucleus: an immunohistochemical study in cats and monkeys (Aotus trivirgatus)

A serotonergic input to the lateral cervical nucleus of cats and monkeys (Aotus trivirgatus) was demonstrated with immunohistochemical methods. In both species, the lateral cervical nucleus was found to contain a network of serotonin-immunoreactive fibers. However, the density of labeled fibers was greater in the monkeys than in the cats. Most labeled fibers were thin and had irregularly spaced varicosities. Electron microscopic examination showed that labeled varicosities were in apposition with dendrites, neuronal somata and unlabeled terminals, but synapses were rare. The results demonstrate that the lateral cervical nucleus receives a serotonergic innervation, as is the case with other somatosensory relay structures such as the spinal dorsal horn and the dorsal column nuclei. The presence of a serotonergic innervation suggests that the transmission of somatosensory information through the lateral cervical nucleus is modulated by a descending pathway. However, its effect on the response properties of neurons in the lateral cervical nucleus is unknown.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphometrical and microdensitometrical studies on peptide- and tyrosine hydroxylase-like immunoreactivities in the forebrain of rats prenatally exposed to methylazoxymethanol acetate

Methylazoxymethanol acetate (MAM Ac) injected into pregnant rats at a dose of 25 mg/kg at gestational day 15 causes microcephaly due to an atrophy of various telencephalic areas, mainly neocortex, hippocampus and basal ganglia. Previous studies demonstrated alterations in various neurochemical markers of classical transmitter systems in these regions. The present paper deals with changes in peptide and tyrosine hydroxylase (TH)-containing neurons in MAM Ac-induced microcephaly using immunocytochemistry coupled with computer-assisted morphometry and microdensitometry. No change in the number of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons in the neocortex and neuropeptide Y (NPY)-immunoreactive neurons in the nucleus caudatus-putamen was found whereas cholecystokinin (CCK)-and NPY-immunoreactive neurons in the neocortex and CCK- and VIP-immunoreactive neurons in the hippocampus were decreased. The reduction of the latter peptide containing neuronal populations led to a maintained density of cells in MAM Ac-exposed rats, due to the parallel reduction of the overall mass of these regions. TH immunoreactivity was found to be unchanged in the basal ganglia, and increased in the cerebral cortex in agreement with previous reports on noradrenaline cortical system after MAM Ac exposure. The present results show a heterogenous vulnerability of different peptide immunoreactive neuronal populations to MAM Ac exposure. The sparing of VIP- and NPY-immunoreactive neurons may be due to their late development in the neocortex and striatum, respectively. The hypothesis is introduced that cortical VIP interneurons can develop independent of marked alterations in the intrinsic circuitry of the cortical region.

Ultrastructural localization of [125I]neurotensin binding sites to cholinergic neurons of the rat nucleus basalis magnocellularis

The distribution of specifically-labeled neurotensin binding sites was examined in relation to that of cholinergic neurons in the rat nucleus basalis magnocellularis at both light and electron microscopic levels. Lightly prefixed forebrain slices were either labeled with [125I](Tyr3) neurotensin alone or processed for combined [125I]neurotensin radioautography and acetylcholinesterase histochemistry. In light microscopic radioautographs from 1-microns-thick sections taken from the surface of single-labeled slices, silver grains were found to be preferentially localized over perikarya and proximal processes of nucleus basalis cells. The label was distributed both throughout the cytoplasm and along the plasma membrane of magnocellular neurons all of which were found to be cholinesterase-positive in a double-labeled material. Probability circle analysis of silver grain distribution in electron microscopic radioautographs confirmed that the major fraction (80-89%) of specifically-labeled binding sites associated with cholinesterase-reactive cell bodies and dendrites was intraneuronal. These intraneuronal sites were mainly dispersed throughout the cytoplasm and are thus likely to represent receptors undergoing synthesis, transport and/or recycling. A proportion of the specific label was also localized over the nucleus, suggesting that neurotensin could modulate the expression of acetylcholine-related enzymes in the nucleus basalis. The remainder of the grains (11-20%) were classified as shared, i.e. overlied the plasma membrane of acetylcholinesterase-positive neuronal perikarya and dendrites. Extrapolation from light microscopic data, combined with the observation that shared grains were detected at several contact points along the plasma membrane of cells which also exhibited exclusive grains, made it possible to ascribe these membrane-associated receptors to the cholinergic neurons themselves rather than to abutting cellular profiles. Comparison of grain distribution with the frequency of occurrence of elements directly abutting the plasma membrane of neurotensin-labeled/cholinesterase-positive perikarya indicated that labeled cell surface receptors were more or less evenly distributed along the membrane as opposed to being concentrated opposite abutting axon terminals endowed or not with a visible junctional specialization. The low incidence of labeled binding sites found in close association with abutting axons makes it unlikely that only this sub-population of sites corresponds to functional receptors. On the contrary, the dispersion of labeled receptors seen here along the plasma membrane of cholinergic neurons suggests that neurotensin acts primarily in a paracrine mode to influence the magnocellular cholinergic system in the nucleus basalis.

The role of serotonin in the control of cerebral activity: studies with intracerebral 5,7-dihydroxytryptamine

Intact rats treated with centrally acting antimuscarinic (atropinic) drugs display large amplitude irregular slow waves in both the neocortex and hippocampus during behavioral immobility and some stereotyped automatic behaviors (Type 2 behavior). However, rhythmical slow activity in the hippocampus and low voltage fast activity in the neocortex occur in close correlation with spontaneous changes in posture, head movement, walking, rearing, swimming or struggling when held (Type 1 behavior). It has previously been proposed that these waveforms, jointly referred to as atropine-resistant cerebral activation (ARCA) are dependent on ascending serotonergic projections. As a further test of this hypothesis, we have studied rats in which forebrain levels of serotonin and 5-hydroxyindoleacetic acid were reduced to 3-10% of control levels as a result of multiple intrabrainstem injections of 5,7-dihydroxytryptamine. This treatment strongly reduced or abolished ARCA in most cases but did not reduce atropine-sensitive cerebral activation which appears to be dependent on ascending cholinergic projections from the basal forebrain to the cerebral cortex. Therefore, ARCA appears to be dependent on ascending serotonergic inputs to the forebrain.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: I. Effects of stimulation of A1, A2, A6 and C2 cell groups

Extracellular electrical activity was recorded from 203 paraventricular nucleus (PVN) neurones antidromically identified as projecting to the median eminence. Spontaneous activity and the effects of stimulation of the A1, A2, A6 and C2 catecholaminergic cell groups upon the PVN neurones were examined. Cells were located at a mean height 2.29 +/- 0.03 mm above the base of the brain, corresponding with the corticotropin-releasing factor (CRF) rich component of the nucleus. The mean firing rate was 3.2 +/- 0.3 Hz and antidromic invasion latency was 9.9 +/- 0.3 msec. Seventy-six % of cells tested were activated by painful somatosensory stimuli. Electrical stimulation of the A1 or A2 region evoked excitatory responses from the majority of cells tested (76% and 85%, respectively), whilst stimulation of the A6 and C2 regions evoked more inhibitory responses (43% and 59%, respectively). Most responses (56%), whether excitatory or inhibitory, were not clearly defined in terms of latency, and were only observed following delivery of 5-10 single shocks at 0.5 Hz. Excitation recorded following A1 and A2 stimulation suggests a facilitatory role for noradrenaline in the regulation of PVN activity. Inhibitory responses following C2 stimulation indicate that adrenaline may serve to inhibit such activity, whilst the more mixed responses following A6 stimulation suggest that the projections of this region differ in some way from those of the A1 and A2 cells. Response reversals were observed, after delivery of higher frequency stimulation, for a substantial proportion (20%) of the cells tested.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: II. Effects of stimulation of the ventral noradrenergic ascending bundle: evidence for cotransmission

In order to further elucidate the neural mechanisms underlying the control of adrenocortical secretion, responses of paraventricular nucleus (PVN) tuberoinfundibular neurones were examined following stimulation of the ventral noradrenergic ascending bundle (VNAB). Stimulation at low frequencies (0.5/5 Hz) excited the majority (52/64, 81%) of cells but only 15 showed a clear-cut, stimulus-locked, activation with onset latency of 44.5 +/- 10.0 msec and offset at 71.9 +/- 11.3 msec: the remaining 37 excited cells showed overall increases in firing after delivery of 5-10 stimuli. High frequency (50 Hz) trains of stimuli reversed the direction of response to inhibition for 14/52 of the excited cells. Inhibition of (nor)adrenaline synthesis by alpha-methylparatyrosine was without effect upon the firing of cells examined or the distribution and latencies of their responses following low frequency stimulation; high frequency trains reversed the response direction of only 4/35 cells, (p less than 0.05 vs. control rats; chi 2-test). Intracerebroventricular administration of 6-hydroxydopamine, a catecholaminergic neurotoxin, reduced the proportion of cells excited by the stimulation (10/47; p less than 0.005; chi 2-test). Unit responses to painful somatosensory stimuli were recorded from the majority of the cells tested (74%), except following 6-hydroxydopamine treatment, when only 38% were excited (p less than 0.005; chi 2-test). The results demonstrate that the VNAB provides an excitatory input to the PVN and that noradrenaline is probably responsible for this effect but a cotransmitter (neuropeptide Y?) may also be responsible for the observed excitatory responses. Inhibitory responses following high frequency stimulation were probably also mediated by (nor)adrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of histamine H1-receptors on astrocytes in primary culture: [3H]mepyramine binding studies

The characteristics of histamine H1-receptors on astrocytes from the cerebral cortex of newborn rats in primary culture were analyzed with a [3H]mepyramine binding assay, and compared with those in the cerebral cortex. The apparent dissociation constant (KD) of [3H]mepyramine binding, the apparent inhibition constants (Ki) of various H1-ligands for [3H]mepyramine binding and the stereoselectivity of d- and l-chlorpheniramine for the inhibition of [3H]mepyramine binding to receptors on cultured astrocytes and to receptors in the brain tissue were very similar, indicating that these receptors are identical. The apparent density of H1-receptors (Bmax) on astrocytes was 262 +/- 60 fmol/mg protein, which was comparable to that in the brain tissue (194 +/- 24 fmol/mg protein). The development of H1-receptors on cultured astrocytes resembled the postnatal development of the receptors in the rat brain. These results suggest that astrocytes could be one of the main targets of the central histaminergic system.

Serotonin-containing axon terminals in the hypoglossal nucleus of the rat. An immuno-electronmicroscopic study

The morphology and distribution of serotonin-containing axon terminals in the rat hypoglossal nucleus (XII) was investigated immunocytochemically at the electron microscopic level. Serotonin-positive profiles were found throughout all regions of XII and included unmyelinated axons, varicosities and axon terminals. Most labeled profiles (68.1%) were nonsynaptic unmyelinated axons and varicosities, while synaptic profiles, ending on dendrites and somata, were seen less frequently (28.7%). The majority of labeled axon terminals (76.9%) ended on small-to-medium-sized dendrites. Most axodendritic terminals contained small, round agranular vesicles (20-55 microns), several large (60-100 microns) dense core vesicles, and were associated with a pronounced asymmetric postsynaptic specialization. By contrast, labeled axosomatic terminals were seen less often than those ending on dendrites (23.0%). Axosomatic terminals typically contained small, round, agranular and large dense core vesicles and were associated with a symmetric or no postsynaptic specialization. These results provide the structural substrates for elucidating the functional role of serotonin in tongue control.

The monoaminergic innervation of the cerebral cortex is not diffuse and nonspecific

It is generally thought that monoamines play a nonspecific role in modulating cortical activity. However, several lines of evidence now indicate that cortical monoaminergic afferents show remarkable anatomical specificity. In particular, there is unequivocal evidence for regional, laminar and intracortical specificity, and for action of monoamines through conventional synapses.

Cytoplasmic loop of beta-adrenergic receptors: synaptic and intracellular localization and relation to catecholaminergic neurons in the nuclei of the solitary tracts

Pharmacological studies suggest that beta-adrenergic receptors (beta AR) in the medial nuclei of the solitary tracts (m-NTS) facilitate presynaptic release of catecholamines and also function at postsynaptic sites. We have localized the antigenic sites for a monoclonal antibody against a peptide corresponding to amino acids 226-239 of beta AR in the m-NTS of rat brain. By light microscopy, immunoperoxidase labeling for this antibody was detected in somata and proximal processes of many small cells that were distributed throughout the rostrocaudal extent of the m-NTS. Electron microscopy confirmed the cytoplasmic localization of beta AR in perikarya and proximal dendrites of neurons. Immunoreactivity occurred as discrete patches associated with cytoplasmic surfaces of plasma membrane and with irregularly-shaped saccules with clear lumen in the immediate vicinity. Select regions of nuclear envelopes, mitochondrial membranes, and rough endoplasmic reticulum were also immunoreactive along their cytoplasmic surfaces. In contrast, the Golgi apparatus was labeled, but infrequently. Immunoreactivity was also detected at numerous post- and occasional presynaptic membrane specializations of select axodendritic junctions. Dual labeling for the beta AR-antibody by the immunoperoxidase method and for a rabbit antiserum against the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), by the immunoautoradiographic method within the same sections, further established the precise cellular relations between beta AR and catecholaminergic neurons. Immunoreactivity for beta AR was detected in numerous perikarya and proximal dendrites that did not show detectable levels of TH. However, a few cells were dually labeled for both antigens, as seen by both light and electron microscopy. The TH-labeled terminals formed synapses at junctions both with and without beta AR-like immunoreactivity. These results from the single and dual labeling studies: (1) confirm biochemical predictions that amino acids 226-239 of beta AR protein reside intracellularly; (2) provide the first ultrastructural evidence for beta AR localization within both pre- and postsynaptic membrane specializations of a subset of catecholaminergic synapses; and (3) suggest select intracellular sites that may be involved with synthesis and/or internalization and degradation of the receptor protein.

Developmental and comparative aspects of nonsynaptic release by the egg-laying controlling caudodorsal cells of basommatophoran snails

In an immunoelectron microscope study the postembryonic development of the cerebral caudodorsal cells (CDC) in the freshwater snail Lymnaea stagnalis was studied as well as the development of similar neurons in other basommatophoran families. The CDC of adult L. stagnalis control egg-laying and associated behaviors by releasing various peptides, including the ovulation hormone CDCH. The CDC release peptides from neurohemal axon terminals and from nonsynaptic release sites of axon collaterals. During postembryonic development the collateral system develops synchronously with the neurohemal area. The first collaterals appear in the cerebral commissure of juvenile snails (10 mm shell height; S = 10). Up to S = 30 they gradually increase in size and length and eventually run through the entire inner compartment. Secretory granules in both collaterals and neurohemal axon terminals increase in size as well. Immunoelectron microscopy combined with the TARI-method for the demonstration of exocytosis indicates that CDCH-release from collaterals and neurohemal terminals occurs already in S = 10; exocytosis of immunoreactive granule contents takes place from nonsynaptic release sites, unspecialized areas of the axolemma of the collaterals. Release activity in the collaterals gradually increases up to S greater than or equal to 20. Neurohemal release activity shows a similar picture except for a steep increase in adult snails. A distinct glial sheath, separating the neurohemal area from the collateral system, appears around S = 15. Representatives of three families of Basommatophora, viz. the lymnaeid L. ovata, the planorbid Planorbis planorbis, and the bulinid Bulinus truncatus possess a well-developed collateral system showing many signs of exocytosis. A glial sheath separates the collaterals from the neurohemal area. Secretory granules of the CDC in L. ovata stain weakly positive with the anti-CDCH antiserum. Since the other Basommatophora did not show immunoreactivity, the chemical structure of egg laying peptides in Basommatophora seems to be genus specific. Apparently the secretory activity of both the neurohemal area and the collateral system is not only important in the sexually mature animal, being involved in the control of egg laying and egg-laying behavior, but also in the juvenile snail. The finding of a collateral system in representatives of three basommatophoran families strongly indicates the importance of the system for the control of reproduction in basommatophoran snails in general.

Light and electron microscopic immunocytochemical analysis of the dopamine innervation of the rat visual cortex

The dopaminergic innervation of the rat primary (area 17) and secondary (areas 18 and 18a) visual cortical areas was examined immunocytochemically using an antiserum directed against dopamine. This innervation was characterized by the differential density of the respective afferents within individual visual areas. Area 18, especially its rostral part, was observed to receive a considerable amount of dopaminergic axons, whereas areas 17 and 18a were sparsely innervated. The innervation of all layers of area 18 seemed to consist to a considerable extent of axonal branches of radial fibres ascending from layer VI to layer I. At the ultrastructural level, dopamine profiles were found to display similar characteristics in all visual areas. Dopamine labelled axon-terminals and axonal varicosities, examined in single and serial ultrathin sections, were seen to form primarily asymmetrical synaptic contacts with dendritic profiles. These observations suggest a 'specific' innervation of cytoarchitectonically distinct cortical regions by dopaminergic axons.

Ultrastructural localization of tyrosine hydroxylase-like immunoreactivity in the rat hippocampal formation

The light and electron microscopic localization of antigenic sites for a polyclonal antiserum directed against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), was examined in the hippocampal formation of the rat brain with a double-bridged peroxidase-antiperoxidase method. By light microscopy, the majority of varicose processes with intense TH-like immunoreactivity (LI) were contained in the hilus of the dentate gyrus (DG) and strata radiatum and lacunosum-moleculare of the CA3 region of the hippocampus. Only a few immunoreactive fibers were observed in the molecular and granule cell layers of the DG, in strata oriens and pyramidale of CA3, and in all layers of CA1. Electron microscopy confirmed that these labeled processes were primarily axons and axon terminals. Terminals with TH-LI were 0.4-1.1 micron in diameter and contained many small clear vesicles and from 0 to 3 larger dense-core vesicles. The number and types of associations formed by terminals with TH-LI were remarkably similar in the DG and hippocampus proper despite known differences in intrinsic cells and function. In both regions, the majority of terminals with TH-LI formed junctions on small (distal dendrites (52% of 112 in the DG; 67% of 116 in CA3) and dendritic spines (30% in the DG; 18% in CA3) that were both asymmetric and symmetric. In the DG, axosomatic junctions (2% of 112) were symmetric and occurred exclusively on the perikarya of granule cells, whereas junctions on large (proximal) dendrites were more numerous (16%), exhibited symmetric as well as asymmetric membrane specializations, and were of both granule (molecular layer) and nongranule (hilus) cell origin. In CA3, synaptic contacts on perikarya (5% of 116) and large (proximal) dendrites (10%) of both pyramidal cell and nonpyramidal cell origin were few and all symmetric. The distribution and types of synaptic associations formed by terminals with TH-LI in the CA1 region paralleled that seen in the CA3 region. In both the dentate and hippocampus proper, 10% of the terminals with TH-LI were observed closely apposed to unlabeled terminals that formed asymmetric synapses with dendrites and dendritic spines. In rare instances, TH-immunoreactive terminals were found in close association with the basement membrane of blood vessels, astrocytic processes, or with other unlabeled terminals not forming recognizable junctions. In addition TH-LI was occasionally detected within the cytoplasm of a minority of astrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Some aspects of the communicational and computational organization of the brain

Some features of the morphofunctional organization of the CNS have been analysed. Different types of hierarchical organization have been recognized, each of which could deeply affect the circulation (communicational aspect) and elaboration (computational aspect) of information. These two aspects have been discussed on the basis of the existence of two types of electrochemical transmission in the CNS: wiring and volume transmission. By evaluating the CNS operations at different levels of analysis a 'computational hierarchical organization' has been delineated. This concept is very relevant to the understanding of the 'computational power' of the brain (Agnati & Fuxe 1984, Conrad 1985a). In fact, it leads to the distinction between horizontal and vertical elaboration of information. The hypothesis is introduced that in the vertical elaboration of information a central role may be played by the neuronal membrane. In fact, this structure can not only be influenced by the extra- and intracellular signals, but also effectively interconvert the electrical coding into the chemical coding of information. These aspects are discussed in the frame of the possible organization of the membrane into 'domains', each domain being a patch of membrane in which pre-selected molecular movements are possible, resulting in molecular interactions. The movement of a molecule outside its domain is considered energetically unfavourable. The possible formal treatment of this hypothesis is mentioned in Conrad's work (1985b).

Multiple types of neuropeptide Y-containing neurons in primate neocortex

The avidin-biotin-peroxidase method was used at the light and electron microscopic levels to analyze neuropeptide Y (NPY)-containing neurons in the neocortex of six adult macaque monkeys. Regions studied included various sensory, motor, limbic, and association areas, designated as 17, 18, 7, 22, 3, 4, 6, 24, and 9 by Brodmann (Beiträge zur Histologischen Lokalisation der Grosshirnrinde. Leipzig: Barth, '06). Several types of NPY-containing neurons can be distinguished by their laminar location, by the size of their perikarya, and by the size, shape, and pattern of ramification of their processes: 1) layer I small local circuit neurons; 2) layer II granule cells; 3) aspiny stellate cells located in layers II-III and V-VI, with long, slender dendrites; 4) sparsely spiny stellate cells; 5) aspiny stellate cells with long, horizontally oriented dendrites, whose cell body is situated in layer VI; 6) Martinotti cells in areas 9, 7, and 24; and 7) multipolar neurons situated in the white matter subjacent to the cortical gray. The possibility of additional neuronal types containing NPY is suggested by labeled densely spinous dendrites in area 6 and recurving axons and axonal loops in the supragranular layers in areas 7 and 9. No NPY-containing neurons were found in layer IV of any area, except layers IVA and B of the visual cortex. Likewise, nonneuronal elements were not labeled. The regional differences in the distribution of some NPY-containing neuron types may reflect adaptations of local neuronal circuits for specialized functions.

Light and electron microscopic immunocytochemical analysis of the noradrenaline innervation of the rat visual cortex

Immunocytochemistry with an antiserum against noradrenaline was used to examine the organization and morphology of noradrenergic axons in the rat visual cortex. Observations with the light microscope confirmed earlier reports concerning the distribution pattern of noradrenergic fibres, and provided some further clues about their intracortical organization. Particularly striking was the finding of fibres which followed an oscillating course within the boundaries of layers II-IV as they ran in the mediolateral direction. Examination of the morphological characteristics of noradrenaline-containing axon terminals in serial ultrathin sections has provided further evidence that the vast majority (87.6%) form conventional synapses in the visual and frontoparietal cortex, and has given clues about the postsynaptic elements involved in these synaptic contacts; they are, in decreasing frequency, spines, dendritic shafts of various diameters, and pyramidal and non-pyramidal somata. In addition, a few labelled terminals were visualized in close association with intracerebral capillaries.

Neuropeptide Y-like immunoreactivity in neurons of the solitary tract nuclei: vesicular localization and synaptic input from GABAergic terminals

The ultrastructural localization of neuropeptide Y-like immunoreactivity (NPY-LI) was examined in the medial nuclei of the solitary tracts (mNTS) of adult rat brain. Peroxidase-antiperoxidase (PAP) reaction product was localized extensively to the central lumen of large (100-150 nm), dense-core vesicles. The labeled vesicles were seen in axon terminals of untreated, control animals and in perikarya and dendrites of rats receiving intraventricular injections of colchicine 24 h prior to sacrifice. The labeled terminals were of two types. The first type contained numerous small, clear vesicles that were rimmed with peroxidase product and 1-6 large, dense-core vesicles that were labeled throughout their central lumen. The second type contained a more homogeneous population of labeled large, dense-core vesicles. Axon terminals showing NPY-LI formed either asymmetric synapses with unlabeled dendrites or were without recognized junctions. Within labeled terminals, as well as within perikarya and dendrites, the majority of the dense-core vesicles were located near non-synaptic portions of the plasmalemma that were heavily ensheathed with glial processes. Only a few unlabeled terminals penetrated the glial investments to form synaptic contacts on soma or dendrites containing NPY-LI. These synaptic contacts were of both symmetric and asymmetric types. Combined immunoperoxidase labeling for glutamic acid decarboxylase and immunogold labeling for NPY further established that at least some of the terminals forming symmetric junctions on the NPY-immunoreactive dendrites were GABAergic. These results provide ultrastructural evidence that in the mNTS, NPY-LI is localized principally to large dense-vesicles within neurons whose output is partially regulated by GABA. The preferential distribution of the labeled vesicles along non-synaptic, glial-invested portions of the plasmalemma suggests that neuronal NPY may modulate the activity of nearby astrocytes. Additionally, the localization of NPY-LI in terminals containing a mixed population of synaptic vesicles and forming asymmetric axodendritic junctions suggests that NPY and/or coexisting transmitter may also exert certain known hypotensive effects by excitation of local intrinsic or projection neurons in this brain region.

Neuroneuronal interconnections in the rat superior cervical ganglion; possible anatomical bases for modulatory interactions revealed by intracellular horseradish peroxidase labelling

Electrophysiologically identified neurons of rat superior cervical ganglion were intracellularly injected with horseradish peroxidase and processed for light and electron microscopic observation. At light microscope level, neurons could be classified according to their dendritic arborization pattern in the vicinity of the soma into radiate, tufted and intermediate types. Upon electrical stimulation of the internal and external carotid nerves it was observed that radiate and intermediate neurons sent their axons into one or the other of these nerve trunks, whereas a majority of tufted neurons gave no response to stimulation of either of these postganglionic nerves. Electron microscopic exploration of horseradish peroxidase-labelled neurons revealed a surprisingly high prevalence of interconnectivity between ganglionic neurons. These contacts were both dendrosomatic and dendrodendritic, and were a universal feature of the labelled neurons explored. Twenty-two of the 23 labelled cells were found to receive direct dendritic appositions on their somata, and 13 of these 23 cells were seen each to send their dendrites into contact with at least one unlabelled neuronal soma. Dendrodendritic contacts were observed for 87% of the labelled neurons, and most of the cells (80%) were seen to form triadic contacts which included two dendrites and a preganglionic nerve ending. All these figures represent minimum incidences. None of the dendrosomatic or dendrodendritic appositions observed was overtly synaptic although several morphological features indicated the possibility of somatic and or dendritic release and uptake at sites of apposition. It is suggested that the observed appositions provide anatomical substrates for modulatory interactions between the ganglionic neurons, possibly involving slow potentials or the switching of metabolic pathways.

Ultrastructural Characterization of Adrenocorticotrope Hormone (ACTH) Immunoreactive Fibres in the Mesencephalic Central Grey Substance of the Rat

The fine structural localization of fibres immunoreactive for the adrenocorticotrope hormone (ACTH) was studied in the mesencephalic central grey substance (MCG) of the male Wistar rat. Light microscopically, varicose ACTH-immunoreactive fibres were found throughout the MCG in a dorsal, lateral and ventral, periventricular position. Electron microscopically, the immunoreactivity was most prominent in the direct vicinity of electron-dense secretory granules in axonal varicosities, and, although to a lower degree, around other cytoplasmic organelles such as electron-lucent synaptic vesicles, mitochondria and microtubules. With serial section analysis two types of ACTH-immunoreactive varicosity were discerned. The first type is large, contains many, small electron-lucent synaptic vesicles, that are located in the vicinity of a morphologically well-defined synaptic contact. In this type of varicosity, large dense-core secretory granules are scarce. Immunoreactivity is low or absent, particularly near the active zone. The second type is strongly immunoreactive. It always contains many large, dense-core secretory granules; electron-lucent vesicles are rare. The smaller varicosities of this type never make synaptic contacts, but a few of the larger varicosities have synaptic contacts with dendrites of MCG cells.

Transient changes in cortical alpha 1 adrenoceptors and seizure threshold following electroconvulsive seizures in rats

Alpha 1 adrenoceptor density (Bmax) is consistently decreased in actively spiking human cortical epileptic foci. Interpretation of these unique human data is limited because all surgical excisions are completed shortly after a period of active seizure discharge. To determine the temporal profile of seizure-induced changes in cortical alpha 1 adrenoceptors we examined rats primed by 15 daily electroconvulsive seizures (ECS). Since the noradrenergic system has an inhibitory effect on epileptic activity, we also measured the postictal rise in minimal ECS seizure threshold. Animals were sacrificed immediately before or at intervals after the last scheduled seizure. Cortical membranes were assayed using [3H]prazosin as specific radioligand. Repeated ECS produced an increase in the number of cortical alpha 1 sites from 4 to 24 h postictally, but following the last seizure there was a transient 'normalization' of alpha 1 receptor density which persisted for 3 h. The postictal ECS seizure threshold also remained elevated for a 2 h period. Both these transient postictal changes may in part result from activation of the central NA system. Decreased alpha 1 adrenoceptors in surgical specimens of spiking cerebral cortex may also be a secondary response to focal seizure activity.

Modulation of the electrically evoked release of 5-[3H]hydroxytryptamine from rat cerebral cortex: effects of alpidem, CL 218872, and diazepam

The effect of omega (benzodiazepine)-receptor agonists, antagonists, and inverse agonists on the electrically evoked release of 5-[3H]hydroxytryptamine ([3H]5-HT) was studied in superfused slices of the rat frontal cerebral cortex. The electrically evoked release of [3H]5-HT was enhanced by nanomolar concentrations of diazepam and the selective omega 1-receptor agonists alpidem and CL 218872. The omega 1/omega 2- and omega 1-receptor antagonists flumazenil and CGS 8216, respectively, did not modify the electrically evoked release of [3H]5-HT. The omega 3-receptor agonist Ro 5-4864 and the omega 1-receptor inverse agonist ethyl-beta-carboline-3-carboxylate on their own did not affect the electrically evoked release of [3H]5-HT. On the other hand, the inverse agonist 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester (DMCM), at micromolar concentrations, inhibited both the spontaneous and the evoked release of [3H]5-HT. The facilitation of the electrically evoked release of [3H]5-HT by diazepam, alpidem, or CL 218872 was potentiated by gamma-aminobutyric acid (GABA). Exposure to flumazenil and CGS 8216 antagonized the facilitation by diazepam, alpidem, or CL 218872 of [3H]5-HT release. The inhibition of the release of [3H]5-HT by DMCM was not modified by exposure to either flumazenil, CGS 8216, or GABA. The inhibitory effect of DMCM was not observed when monoamine oxidase activity was inhibited by pargyline.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of spinal cord grafts with and without cografts of locus coeruleus in oculo

The ultrastructure of spinal cord and spinal cord-locus coeruleus double grafts transplanted to the anterior chamber of the eye of adult rats was studied. The present results show that single spinal cord grafts express several morphologically organotypical characteristics of normal spinal cord at the ultrastructural level. Thus, the parenchyma was divided into a cell-rich layer corresponding to the normal gray matter and an axonal layer with a large amount of myelinated fibers and immature oligodendrocytes. In the cellular layer, a variety of cell types of different sizes were observed. The neurons had different patterns of cytoplasmic organelles, including Nissl bodies, Golgi apparatus, mitochondria, and polysomes. The Nissl substance was variable and some neurons appeared to be immature. Although the spinal cord grafts are in a state of relative gliosis, surrounded by a glial barrier, cografted fetal locus coeruleus catecholamine neurons are able to innervate the spinal cord grafts and form anatomically relevant synapses with the spinal cord neuronal elements as revealed by TH-immunoelectron microscopy. In conclusion, several organotypical features of normal spinal cord are found. Examples also were found, however, of a disturbed and delayed development that have to be considered when evaluating the functional potential of grafted cells.