Synaptic organization of serotonin-immunoreactive fibers in primary visual cortex of the macaque monkey

Serotoninergic afferents preferentially innervate distinct subclasses of peptidergic interneurons in the rat visual cortex

Although it is well documented that the non-pyramidal neurons of the cerebral cortex are under the influence of the vast serotoninergic input, the ultrastructural substrate for such functional interactions appears largely obscure. We sought to address this issue by dual immunoelectron microscopy, combining antibodies against serotonin (5-HT) and three neurochemical markers for peptidergic interneurons, namely somatostatin (SRIF), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP). The gold-substituted silver-peroxidase method was employed to intensify and differentiate the end-product of the peptide-immunoreaction from the non-intensified 5-HT fibers. Mainly the SRIF but also the NPY neurons were encountered among the postsynaptic targets of the 5-HT boutons. Recipients of synapses were perikarya and proximal dendrites of SRIF and NPY cells but also distal dendrites of the SRIF neurons. Neither synaptic relationships nor close appositions were ever identified between 5-HT boutons and VIP-immunoreactive elements. This remarkable synaptic preference/avoidance of 5-HT afferents for specific peptidergic subpopulations reveals a 'wired' component of cortical serotonin neurotransmission, which should be carefully interpreted within the frame of the available literature for extrasynaptic serotonin release.

Event-related potentials. Do they reflect central serotonergic neurotransmission and do they predict clinical response to serotonin agonists?

The increasing knowledge concerning anatomical structures and cellular processes underlying event-related potentials (ERP) as well as methodological advances in ERP data analysis (e.g. dipole source analysis) begin to bridge the gap between ERP and neurochemical aspects. Reliable indicators of the serotonin system are urgently needed because of its role in pathophysiology and as target of pharmacotherapeutic interventions in psychiatric disorders. Converging arguments from preclinical and clinical studies support the hypothesis that the loudness dependence of the auditory evoked N1/P2-response (LDAEP) is regulated by the level of central serotonergic neurotransmission. Dipole source analysis represents an important methodological advance in this context, because the two N1/P2-subcomponents, generated by the primary and secondary auditory cortex known to be differentially innervated by serotonergic fibers, can be separated. A pronounced LDAEP of primary auditory cortices is supposed to reflect low central serotonergic neurotransmission, and vice versa. LDAEP is a parameter with potential clinical value since subgroups of patients with a serotonergic dysfunction can be identified and can be treated more specifically. In depressed patients, a significant relationship between strong LDAEP, indicating low serotonergic function, and a favorable response to SSRI has been found. Additionally, there is evidence from several studies with patients with affective disorders implicating a strong LDAEP as a predictor of favorable response to a preventive lithium treatment.

Ultrastructural localization of the serotonin transporter in superficial and deep layers of the rat prelimbic prefrontal cortex and its spatial relationship to dopamine terminals

Dopamine levels within the prefrontal cortex (PFC) can be manipulated by selective inhibitors of the serotonin transporter (SERT). However, the cellular mechanisms underlying these effects are not clear. The present study sought to examine the distribution of immunogold-silver labeling for SERT (SERT-ir) in the rat prelimbic PFC and to describe its ultrastructural spatial relationship to dopamine axons labeled by immunoperoxidase staining for tyrosine hydroxylase (TH-ir). SERT was localized to axonal profiles that ranged in size from fine caliber fibers containing dense SERT-ir, primarily along the membrane, and rarely forming synapses to large, spherical varicosities exhibiting less dense staining, mainly within the cytoplasm, and more commonly forming synapses. Synaptic contacts of SERT profiles were typically asymmetric, axospinous, and more frequent in superficial (38%) than deep (19%) layers. For TH-ir profiles, only 24% were within the same 13.8 microm(2) microenvironment as SERT-ir profiles. Furthermore, TH-ir and SERT-ir profiles were rarely directly apposed to each other or convergent onto common dendritic structures. Instead, these two profiles were typically separated by an average distance of 1.30 microm in the coronal plane, a value that did not vary with the size of SERT-ir axons, the amount of SERT labeling, or the cortical layer examined. These results are consistent with two populations of SERT profiles within the rat prelimbic PFC that may arise from different raphe nuclei or that represent varicose and intervaricose portions of the same axons. Moreover, the functional interactions between cortical serotonin and dopamine systems that may contribute to the therapeutic efficacy of antidepressant drugs are likely to occur over distances greater than 1 microm.

Identifying psychiatric patients with serotonergic dysfunctions by event-related potentials

The increasing knowledge concerning anatomical structures and cellular processes underlying event-related potentials (ERP) as well as methodological advances in ERP data analysis (e.g. dipole source analysis) is beginning to bridge the gap between ERP and neurochemical aspects. Reliable indicators of the serotonin system are urgently needed because of its role in pathophysiology and as target of pharmacotherapeutic interventions in psychiatric disorders. Converging arguments from preclinical and clinical studies support the hypothesis that the loudness dependence of the auditory evoked N1/P2-response (LDAEP) is regulated by the level of central serotonergic neurotransmission. Dipole source analysis represents an important methodological advance in this context, because the two N1/P2-subcomponents, generated by the primary and secondary auditory cortex known to be differentially innervated by serotonergic fibres, can be separated. A pronounced LDAEP of primary auditory cortices is supposed to reflect low central serotonergic neurotransmission, and vice versa. LDAEP is a parameter with potential clinical value since subgroups of patients with a serotonergic dysfunction can be identified and can be treated more specifically. In depressed patients, a significant relationship between strong LDAEP, indicating low serotonergic function, and a favourable response to SSRI has been found. Additionally, there is evidence from several studies with patients with affective disorders that a strong LDAEP predicts favourable response to a preventive lithium treatment.

Impairment of depth perception in multiple sclerosis is improved by treatment with AC pulsed electromagnetic fields

Multiple sclerosis (MS) is associated with postural instability and an increased risk of falling which is facilitated by a variety of factors including diminished visual acuity, diplopia, ataxia, apraxia of gait, and peripheral neuropathy. Deficient binocular depth perception may also contribute to a higher incidence of postural instability and falling in these patients who, for example, find it an extremely difficult task to walk on uneven ground, over curbs, or up and down steps. I report a 51 year old woman with secondary progressive MS who experienced difficulties with binocular depth perception resulting in frequent falls and injuries. Deficient depth perception was demonstrated also on spontaneous drawing of a cube. Following a series of transcranial treatments with AC pulsed electromagnetic fields (EMFs) of 7,5 picotesla flux density, the patient experienced a major improvement in depth perception which was evident particularly on ascending and descending stairs. These clinical changes were associated with an improvement in spatial organization and depth perception on drawing a cube. These findings suggest that in MS impairment of depth perception, which is encoded in the primary visual cortex (area 17) and visual association cortex (areas 18 and 19), may be improved by administration of AC pulsed EMFs of picotesla flux density. The primary visual cortex is densely innervated by serotonergic neurons which modulate visual information processing. Cerebral serotonin concentrations are diminished in MS patients and at least some aspects of deficient depth perception in MS may be related to dysfunction of serotonergic transmission in the primary visual cortex. It is suggested that transcranial AC pulsed applications of EMFs improve depth perception partly by augmenting serotonergic transmission in the visual cortex.

Structural basis of the cholinergic and serotonergic modulation of GABAergic neurons in the hippocampus

Ascending subcortical pathways effectively modulate hippocampal information processing. Two components, the cholinergic and serotonergic pathways have been demonstrated to play an important role in the generation of behaviour-dependent hippocampal EEG patterns. Several findings suggest that the above projections influence the activity of hippocampal interneurons. Here we review the available data from physiological, pharmacological and receptor localization experiments, drawing attention to the crucial role of interneurons in the transfer and amplification of subcortical effects on cortical information processing. We hypothesize that, by exerting diverse actions on different subsets of interneurons, the cholinergic and serotonergic systems might change the balance of somatic and dendritic inhibition, and consequently change the integrative properties of hippocampal principal cells.

Medial septal and median raphe innervation of vasoactive intestinal polypeptide-containing interneurons in the hippocampus

Vasoactive intestinal polypeptide-immunoreactive interneurons are known to form three anatomically and neurochemically well-characterized neuron populations in the hippocampus. Two of these establish synaptic contacts selectively with other GABAergic cells (interneuron-selective cells), whereas the third type innervates pyramidal cell bodies and proximal dendrites like a conventional basket cell. Our aim was to examine which of the vasoactive intestinal polypeptide-containing interneuron populations are among the targets of GABAergic septohippocampal and serotonergic raphe-hippocampal pathways. Anterograde tracing with Phaseolus vulgaris leucoagglutinin combined with double immunocytochemistry for vasoactive intestinal polypeptide was used at the light and electron microscopic levels. Our results show that both interneuron-selective cells and vasoactive intestinal polypeptide-containing basket cells receive synaptic input from the medial septum and median raphe nucleus. The GABAergic component of the septohippocampal pathway establishes multiple contacts on both cell types. In the case of the raphe-hippocampal projection, single or double contacts were more frequent on vasoactive intestinal polypeptide-positive interneuron selective cells (76%), whereas multiple contacts predominated on basket cells (83%). The extrinsic GABAergic innervation of interneuron-selective cells in the hippocampus indicates a complex interaction among GABAergic systems, which might ensure the timing and rhythmic synchronization of inhibitory processes in the hippocampus. On the other hand, our results suggest that the serotonergic effect on perisomatic inhibition is exerted via vasoactive intestinal polypeptide-containing basket cells that are functionally distinct from their parvalbumin-positive relatives, which appear to escape control of serotonergic as well as local interneuron-selective cells.

The central 5-HT3 receptor in CNS disorders

Among the characterized 5-HT receptors of the central nervous system, the type 3 receptor subtype (5-HT3R) is the only one known to be a ligand-gated ion channel. Its early pharmacological characterization and mapping by radioligand binding autoradiography suggested that this receptor may, among other actions, regulate dopamine release in the nigro-striatal pathway and reduce alcohol consumption in experimental animals while antagonists of this receptor have been reported to treat anxiety disorders. Following the cloning of this receptor in 1991, direct cellular localization was made possible by in situ hybridization and immunohistochemical analysis. Here we summarize our recent efforts showing that 5-HT3R-expressing neurons are mainly GABA containing cells in the rat neocortex, olfactory cortex, hippocampus, and amygdala which also often contain cholecystokinin (CCK) immunoreactivity. These results provide a means to unify some of the initial pharmacological observations.

5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites

To identify the cortical sites where 5-hydroxytryptamine2A (5-HT2A) serotonin receptors respond to the action of hallucinogens and atypical antipsychotic drugs, we have examined the cellular and subcellular distribution of these receptors in the cerebral cortex of macaque monkeys (with a focus on prefrontal areas) by using light and electron microscopic immunocytochemical techniques. 5-HT2A receptor immunoreactivity was detected in all cortical layers, among which layers II and III and layers V and VI were intensely stained, and layer IV was weakly labeled. The majority of the receptor-labeled cells were pyramidal neurons and the most intense immunolabeling was consistently confined to their parallelly aligned proximal apical dendrites that formed two intensely stained bands above and below layer IV. In double-label experiments, 5-HT2A label was found in calbindin D28k-positive, nonphosphorylated-neurofilament-positive, and immuno-negative pyramidal cells, suggesting that probably all pyramidal cells express 5-HT2A receptors. 5-HT2A label was also found in large- and medium-size interneurons, some of which were immuno-positive for calbindin. 5-HT2A receptor label was also associated with axon terminals. These findings reconcile the data on the receptor's cortical physiology and localization by (i) establishing that 5-HT2A receptors are located postsynaptically and presynaptically, (ii) demonstrating that pyramidal neurons constitute the major 5-HT2A-receptor-expressing cells in the cortex, and (iii) supporting the view that the apical dendritic field proximal to the pyramidal cell soma is the "hot spot" for 5-HT2A-receptor-mediated physiological actions relevant to normal and "psychotic" functional states of the cerebral cortex.

Fast synaptic signaling by nicotinic acetylcholine and serotonin 5-HT3 receptors in developing visual cortex

Cholinergic and serotonergic fiber systems invade the developing visual cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupled receptors, because fast cholinergic or serotonergic neurotransmission has never been observed in neocortex. However, acetylcholine and serotonin also act on ligand-gated ion channels; the nicotinic acetylcholine receptor and the serotonin 5-HT3 receptor, respectively. Here, using whole-cell patch-clamp techniques in developing ferret visual cortex, we pharmacologically isolated fast, spontaneous, and evoked cholinergic and serotonergic synaptic events in pyramidal cells and interneurons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetylcholine and serotonin can mediate fast synaptic transmission in the visual cortex; the early onset of these mechanisms suggests a role during initial stages of circuit formation and during subsequent experience-dependent remodeling of cortical connections.

A proposed mechanism for memory and learning based upon very high frequency signals in the serotonergic neuronal system

Evidence that the serotonergic neuronal system is associated with memory and learning is discussed. It is proposed that the serotonergic neuronal system has the capacity to generate very high oscillatory frequencies which propagate signals along the microtubule cytoskeletal structure of this neuronal system. A mechanism whereby the very high frequencies couple in a co-operative and synchronous fashion with sensory and event related signals in frequency specific channels is described. The specific and discrete frequency channels involve the receptor areas of serotonergic activity in a model of information storage and retrieval.

Selective expression of m2 muscarinic receptor in the parvocellular channel of the primate visual cortex

Visual information in primates is relayed from the dorsal lateral geniculate nucleus to the cerebral cortex by three parallel neuronal channels designated the parvocellular, magnocellular, and interlaminar pathways. Here we report that m2 muscarinic acetylcholine receptor in the macaque monkey visual cortex is selectively associated with synaptic circuits subserving the function of only one of these channels. The m2 receptor protein is enriched both in layer IV axons originating from parvocellular layers of the dorsal lateral geniculate nucleus and in cytochrome oxidase poor interblob compartments in layers II and III, which are linked with the parvocellular pathway. In these compartments, m2 receptors appear to be heteroreceptors, i.e., they are associated predominantly with asymmetric, noncholinergic synapses, suggesting a selective role in the modulation of excitatory neurotransmission through the parvocellular visual channel.

Serotonergic axons in monkey prefrontal cerebral cortex synapse predominantly on interneurons as demonstrated by serial section electron microscopy

Anatomical approaches were used to describe the distribution, appearance, and synaptic interactions of serotonin (5-HT)-immunoreactive axons in monkey prefrontal cortex. A plexus of 5-HT axons was found throughout the gray matter, with an especially high density in layer I and a slight increase in layer IV. They were strikingly heterogeneous, with a gradient of morphologies ranging from fine and nonvaricose to highly varicose or thick and nonvaricose. Electron microscopy showed that both varicose and nonvaricose axons were typically filled with clear vesicles and less abundant dense core vesicles. A serial section analysis of 5-HT varicosities in layers I, III, and V showed consistent results across layers. Only about 23% of labeled varicosities formed identifiable synapses. These synapses were consistently asymmetric and were 2-5 serial sections (or 0.08-0.38 mu) in diameter. Targets of identified 5-HT synapses were dendritic shafts with the exception of one cell soma. Followed in serial sections, postsynaptic dendrites typically had morphological features of interneurons, i.e. they lacked spines, had a high density of synaptic inputs, and often had a varicose morphology. Only 8% of postsynaptic shafts were classified as pyramidal dendrites. This is in striking contrast to our previous study in this cortex of dopamine axons, which synapsed predominantly on pyramidal dendrites. These are the first results to indicate that interneurons are the major recipient of identifiable 5-HT synapses in the monkey prefrontal cortex.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Cholinergic synaptic circuitry in the macaque prefrontal cortex

Surprisingly little is known about the synaptic architecture of the cholinergic innervation in the primate cerebral cortex in spite of its acknowledged relevance to cognitive processing and Alzheimer's disease. To address this knowledge gap, we examined serially sectioned cholinergic axons in supra- and infragranular layers of the macaque prefrontal cortex by using an antibody against the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT). The tissue bound antibody was visualized with both immunoperoxidase and silver-enhanced diaminobenzidine sulfide (SEDS) techniques. Both methods revealed that cholinergic axons make synapses in all cortical layers and that these synapses are exclusively symmetric. Cholinergic axons formed synapses primarily on dendritic shafts (70.5%), dendritic spines (25%), and, to a lesser extent, cell bodies (4.5%). Both pyramidal neurons and cells exhibiting the morphological features of GABAergic cells were targets of the cholinergic innervation. Some spiny dendritic shafts received multiple, closely spaced synapses, suggesting that a subset of pyramidal neurons may be subject to a particularly strong cholinergic influence. Analysis of synaptic incidence of cholinergic profiles in the supragranular layers of the prefrontal cortex by the SEDS technique revealed that definitive synaptic junctions were formed by 44% of the cholinergic boutons. An unexpected finding was that cholinergic boutons were frequently apposed to spines and small dendrites without making any visible synaptic specializations. These same spines and dendrites often received asymmetric synapses, presumably of thalamocortical or corticocortical origin. Present ultrastructural findings suggest that acetylcholine may have a dual modulatory effect in the neocortex: one through classical synaptic junctions on dendritic shafts and spines, and the other through nonsynaptic appositions in close vicinity to asymmetric synapses. Further physiological studies are necessary to test the hypothesis of the nonsynaptic release of acetylcholine in the cortex.

5-hydroxydopamine-labeled dopaminergic axons: three-dimensional reconstructions of axons, synapses and postsynaptic targets in rat neostriatum

Previous studies employing 5-hydroxydopamine to identify nigrostriatal dopaminergic axons and their synapses found that labeled axons made few synapses or that asymmetric contacts predominated. In contrast, recent studies using tyrosine hydroxylase or dopamine antibody techniques indicate that presumed dopaminergic axons form small symmetric contacts. We re-examined 5-hydroxydopamine-labeled material from the rat neostriatum using serial three-dimensional reconstruction techniques to characterize the morphology of labeled axons, synapses and postsynaptic targets. This ultrastructural analysis revealed a class of heavily labeled axons that are small (0.06-1.5 microns in diameter) and lack large varicosities. These axons form small (0.011-0.09 microns 2), en passant, symmetric synapses, mainly onto dendritic spines and spiny dendritic shafts and, in some cases, onto aspiny dendritic segments near branch points. The sites of these synapses along the axon appeared unrelated to the locations of axonal enlargements, suggesting that counting varicosities may not be an accurate indication of the extent of dopaminergic innervation in the neostriatum. The characteristics of these 5-hydroxydopamine-labeled elements correspond in all respects to axons and synapses identified as dopaminergic by immunohistochemistry in previous studies. In tissue in which all labeled and unlabeled synapses were classified, approximately 9% of all synapses were identified as dopaminergic by this type of label. Three-dimensional reconstructions provided additional insight concerning the interaction of dopaminergic afferents with postsynaptic striatal targets and their relation to other afferents to these neurons. They reveal that a short, unbranched dopaminergic axonal segment can make multiple synapses onto dendritic spines, shafts and branch points of one or more dendrites. In addition, one dendrite can receive contacts from several labeled axons. Dopamine synapses onto spines are always associated with unlabeled, asymmetric synapses onto the same spine. Synapses of various morphologies with a distinctly different, lighter form of labeling were much rarer, and may represent other aminergic afferents to the neostriatum. The presence of this second form of label in earlier 5-hydroxydopamine studies may have contributed to the long-standing controversy over the appearance of dopaminergic synapses examined by different techniques. Our results help to resolve this controversy and confirm that the nigrostriatal projection makes small symmetric synapses with a variety of striatal targets.

The noradrenergic innervation density of the monkey paraventricular nucleus is not altered by early social deprivation

A series of neuroanatomic analyses have been undertaken to identify potential neuropathological changes seen in monkeys exposed to early social deprivation, which leads to psychopathology, inappropriate responses to stress and appetitive disorders. The animals used in this study were either socially reared or maternal- and peer-deprived. Within this framework, the distribution and density of noradrenergic (and adrenergic) varicosities was assessed in the hypothalamic paraventricular nucleus of rhesus monkeys using dopamine-beta-hydroxylase immunohistochemistry combined with laser scanning microscopy. Quantitative analysis of dopamine-beta-hydroxylase-immunoreactive varicosity density within magnocellular and parvicellular regions revealed no significant differences between rearing conditions, suggesting that this chemically identified afferent input to the paraventricular nucleus was not affected by the early environmental insult of social deprivation. The apparent lack of vulnerability of the paraventricular nucleus to differential rearing conditions contrasts with the neuropathological changes observed in several discrete brain regions.

Association of m1 and m2 muscarinic receptor proteins with asymmetric synapses in the primate cerebral cortex: morphological evidence for cholinergic modulation of excitatory neurotransmission

Muscarinic m1 receptors traditionally are considered to be postsynaptic to cholinergic fibers, while m2 receptors are largely presynaptic receptors associated with axons. We have examined the distribution of these receptor proteins in the monkey cerebral cortex and obtained results that are at odds with this expectation. Using immunohistochemistry with specific antibodies to recombinant m1 and m2 muscarinic receptor proteins, we have demonstrated that both m1 and m2 receptors are prominently associated with noncholinergic asymmetric synapses as well as with the symmetric synapses that characterize the cholinergic pathways in the neocortex. At asymmetric synapses, both m1 and m2 receptor immunoreactivity is observed postsynaptically within spines and dendrites; the m2 receptor is also found in presynaptic axon terminals which, in the visual cortex, resemble the parvicellular geniculocortical pathway. In addition, m2 labeling was also found in a subset of nonpyramidal neurons. These findings establish that the m2 receptor is located postsynaptically as well as presynaptically. The association of m1 and m2 receptors with asymmetric synapses in central pathways, which use excitatory amino acids as neurotransmitters, provides a morphological basis for cholinergic modulation of excitatory neurotransmission.

Intensity dependence of auditory evoked potentials as an indicator of central serotonergic neurotransmission: a new hypothesis

Because of the increasing importance of the central serotonergic neurotransmission for pathogenetic concepts and its role as a target of pharmacotherapeutic interventions in psychiatry, reliable indicators of this system are needed. It is proposed that the stimulus intensity dependence of auditory evoked N1/P2-component, which is probably modulated by cortical serotonergic innervation, may be a useful and noninvasive indicator of behaviorally relevant aspects of serotonergic activity. Converging evidence from our own studies as well as from the literature suggests that a pronounced intensity dependence of auditory evoked N1/P2-component reflects low central serotonergic neurotransmission. Recent findings concerning general functional aspects of the brain serotonin system reveal that this system is well qualified for adjusting individual levels of sensory processing ("set the tone"), especially in the primary auditory cortex in which the N1/P2-component is mainly generated. Dipole source analysis represents an important methodological advance in this context because it allows the separation of N1/P2-subcomponents generated in the primary auditory cortex from those generated in secondary auditory areas.

Calretinin is present in non-pyramidal cells of the rat hippocampus--III. Their inputs from the median raphe and medial septal nuclei

The subcortical innervation of a recently described subpopulation of non-pyramidal neurons, containing the calcium binding protein, calretinin, was investigated in the rat hippocampus using the anterograde tracer Phaseolus vulgaris-leucoagglutinin and double immunocytochemistry for calretinin and serotonin at the light and electron microscopic levels. Our results show that the GABAergic component of the septohippocampal pathway and the serotonergic raphe afferents establish multiple synaptic contacts with the calretinin-immunoreactive interneurons. The majority of the targets of both pathways were spine-free calretinin neurons known to innervate the dendritic region of the principal cells, but the GABAergic septal pathway was found to terminate also on the spiny neurons of stratum lucidum of the CA3 region and in the dentate hilus. The present results demonstrate that the serotonergic raphe-hippocampal and the GABAergic septohippocampal pathways are able to modulate dendritic inhibition of principal cells via calretinin-containing GABAergic interneurons.

Noradrenergic innervation of the hypothalamus of rhesus monkeys: distribution of dopamine-beta-hydroxylase immunoreactive fibers and quantitative analysis of varicosities in the paraventricular nucleus

The distribution of noradrenergic processes within the hypothalamus of rhesus monkeys (Macaca mulatta) was examined by immunohistochemistry with an antibody against dopamine-beta-hydroxylase. The results revealed that the pattern of dopamine-beta-hydroxylase immunoreactivity varied systematically throughout the rhesus monkey hypothalamus. Extremely high densities of dopamine-beta-hydroxylase-immunoreactive processes were observed in the paraventricular and supraoptic nuclei, while relatively lower levels were found in the arcuate and dorsomedial nuclei and in the medial preoptic, perifornical, and suprachiasmatic areas. Moderate levels of dopamine-beta-hydroxylase immunoreactivity were found throughout the lateral hypothalamic area and in the internal lamina of the median eminence. Very few immunoreactive processes were found in the ventromedial nucleus or in the mammillary complex. Other midline diencephalic structures were found to have high densities of dopamine-beta-hydroxylase immunoreactivity, including the paraventricular nucleus of the thalamus and a discrete subregion of nucleus reuniens, the magnocellular subfascicular nucleus. A moderate density of dopamine-beta-hydroxylase immunoreactive processes were found in the rhomboid nucleus and zona incerta whereas little dopamine-beta-hydroxylase immunoreactivity was found in the fields of Forel, nucleus reuniens, or subthalamic nucleus. The differential distribution of dopamine-beta-hydroxylase-immunoreactive processes may reflect a potential role of norepinephrine as a regulator of a variety of functions associated with the nuclei that are most heavily innervated, e.g., neuroendocrine release from the paraventricular and supraoptic nuclei, and gonadotropin release from the medial preoptic area and mediobasal hypothalamus. Additionally, quantitative analysis of dopamine-beta-hydroxylase-immunoreactive varicosities was performed on a laser scanning microscope in both magnocellular and parvicellular regions of the paraventricular nucleus of the hypothalamus. The methodology employed in this study allowed for the high resolution of immunoreactive profiles through the volume of tissue being analyzed, and was more accurate than conventional light microscopy in terms of varicosity quantification. Quantitatively, a significant difference in the density of dopamine-beta-hydroxylase-immunoreactive varicosities was found between magnocellular and parvicellular regions, suggesting that parvicellular neurons received a denser noradrenergic input. These differential patterns may reflect an important functional role for norepinephrine in the regulation of anterior pituitary secretion through the hypothalamic-pituitary-adrenal stress axis.

Developmental alteration of serotonin neurons in the raphe nucleus of rats with methylazoxymethanol-induced microcephaly

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM), an anti-mitotic agent, on day 15 of gestation induces severe microcephaly in the offspring. The present study first investigated a developmental alteration of serotonin (5HT) neurons immunohistochemically in the dorsal and median raphe nuclei in serial sections in both control and microcephalic rats (MAM-rats) at 35 days of age. 5HT-immunoreactive neurons in the MAM-rats were reduced in number and irregularly distributed in the dorsal and median raphe nuclei compared with those in the control. The dendrites of neurons in these nuclei in the MAM-rats were very short and twisted. A follow-up observation on the development of the cerebral cortex at 5, 9 and 28 days of age was performed using Nissl-stained preparations, which revealed a disorganized cell arrangement in the cerebral cortex of the MAM-rats at the very early postnatal period. Furthermore, the distribution of 5HT-immunoreactive fibers into the cerebral cortex was also examined using brains of 28 days of age. In MAM-rats of this age, abnormally tortuous 5HT-immunoreactive fibers were observed in the cerebral cortex. 5HT neurons in the raphe nuclei are known to project their ascending axons widely into the entire cortical area during the 1st postnatal week. Thus, the association of disorganized cortical cell arrangement and the hyperdense and tortuous distribution of 5HT-immunoreactive fibers in the cerebral cortex support the idea of target-dependent secondary degeneration of 5HT neurons in the dorsal and median raphe nuclei of the MAM-rats.

Simultaneous Demonstration of Serotonin-immunoreactive Terminals and GABAergic Neurons in the VPL Nucleus of the Cat Thalamus

Pre-embedding immunoperoxidase (for serotonin) and postembedding immunogold (for gamma-aminobutyric acid; GABA) labelling were combined at light and electron microscopic levels to demonstrate the neuronal targets of serotonin (5-HT) afferents in the ventral posterior lateral nucleus (VPL) of the cat thalamus. 5-HT-immunoreactive fibres and terminal varicosities were found in close proximity to GABA-immunoreactive interneurons and non-GABAergic relay neurons. Ultrastructurally, the vast majority of 5-HT terminals made close membrane contacts without overt membrane specializations with GABAergic axon terminals, GABAergic presynaptic dendrites and GABAergic somata. A very small number of 5-HT terminals formed typical asymmetrical synapses with GABAergic presynaptic dendrites and with dendritic shafts of relay cells. Some 5-HT terminals participated with the presynaptic dendrites in triadic synaptic arrangements. These findings suggest a dual innervation pattern by 5-HT afferents in VPL and the release of 5-HT in large part at sites not associated with morphologically detectable synapses.

The selective innervation by serotoninergic axons of calbindin-containing interneurons in the neocortex and hippocampus of the marmoset

The serotoninergic input to the mammalian cerebral cortex originates in the median and the dorsal raphe nuclei. Median raphe neurons have been previously shown to give rise to beaded varicose axons which form dense pericellular arrays (baskets) surrounding the soma and the proximal dendrites of certain cortical neurons. In the present study, we have searched for specific markers characterizing the neurons of the marmoset neocortex and hippocampus surrounded by these thick varicose serotonin-containing fibers. The non-pyramidal nature of these neurons, suggested by their dendritic arborization, was correlated, in immunocytochemical experiments with double-labelling to demonstrate their surrounding serotonin-containing basket and their content of glutamic acid decarboxylase (GAD) or of the calcium-binding protein calbindin. Another calcium-binding protein common in numerous non-pyramidal cortical neurons, parvalbumin, was never found in neurons surrounded by serotonin-containing baskets. This organization was found in all areas of the neocortex and of the hippocampus where serotonin-containing baskets were present. One of the serotoninergic cortical inputs which originates from the brainstem tegmentum, traditionally described as "diffuse," proves to be highly selective in that a subset of its axons terminates preferentially on a subpopulation of inhibitory interneurons of the cerebral cortex. It may be emphasized that this subset of cortical interneurons has now been shown to be characterized not only by its axonal and dendritic arborization and its neurotransmitter, but also by a specific type of input which can modulate cortical function in a unique manner.

Serotoninergic innervation of the ferret cerebral cortex. II. Postnatal development

We have investigated the serotoninergic innervation of the ferret cortex from the day of birth to adulthood with immunohistochemical techniques. Due to the premature birth of ferrets, this period spans the entire generation of cells located within the upper cortical layers and their subsequent migration to their final positions. Already at birth, serotoninergic fibers innervate the developing cortex. This innervation is most dense within the marginal zone, the subplate region, and the lower portion of the cortical plate. As long as cell migration continues, serotoninergic fibers enter the expanding portions of the cortex. Only the region just below the marginal zone where newly arriving cells are added to the cortical plate is not innervated by the ingrowing fibers. When the bulk of cell migration ceases, during the third postnatal week, this gap disappears and the fibers gradually form a continuous innervation from the pia to the ventricle. As the cortex matures, the serotoninergic fibers become successively confined to the upper layers, to generate the adult pattern. In the adult ferret cortex, the highest innervation density is found within layers 1, 2, and 3, with a much sparser innervation within the lower layers (Voigt and de Lima, J. Comp. Neurol. 314:403-414, 1991). The dense innervation in the deep cortical layers is only transient, virtually disappearing toward adulthood. These results suggest that serotoninergic axons innervate cortical layers as soon as newly arriving cells reach their final positions within the cortex. This early innervation lends support to the idea that serotonin may play a role during development of the cerebral cortex.

Serotoninergic innervation of the ferret cerebral cortex. I. Adult pattern

We have investigated the serotoninergic innervation of the adult ferret cerebral cortex with immunohistochemical techniques. Distribution pattern of serotoninergic fibers in the ferret neocortex is characterized by a decrease in the density of fibers as one moves from the pial surface towards the white matter. Throughout the entire cerebral cortex, the serotoninergic fibers are very dense within the supragranular layers, especially within layer 1. In contrast, granular and infragranular layers exhibit only a sparse innervation. Although this general pattern of innervation is roughly the same in all cortical areas, significant variations in the fiber density are apparent in different regions. Areas 17, 1, 6, and 8 (primary visual cortex, presumptive somatosensory cortex, presumptive motor cortex, and prefrontal cortex, respectively) are described in more detail to illustrate the diversity of the serotoninergic innervation patterns. The density of innervation is highest in areas 1 and 6, intermediate in area 8, and lowest in area 17. It is noteworthy that while areas 1, 6, and 8 show a marked decrease in fiber density at the boundary between layer 3 and 4, the less strongly innervated area 17 shows a change in density in the transition from layer 2 to layer 3. The types of fibers found within the ferret cortex are similar to those described in other mammalian species. The bulk of the innervation is made by very fine fusiform axons with small ovoid varicosities. In addition to this fiber type, axons with thick round varicosities and some smooth nonvaricose axons were found. The latter types occur in very small numbers within the supragranular layers and mostly in more anterior cortical regions. While the general innervation pattern and the fiber types are similar to those described in the cat cerebral cortex, the pericellular baskets found in the cat cortex (Mulligan and Törk, J Comp Neurol 270:86-110, 1988) are not seen in the ferret.

Comparison of serotonin 5-HT1 receptors and innervation in the visual cortex of normal and dark-reared cats

The visual cortical serotoninergic system was compared in normal and dark-reared cats to determine whether visual experience is necessary for its normal development. In vitro receptor binding of [3H]5-HT indicated an increase in 5-HT1 receptor number in dark-reared cats with no change in affinity. This elevation was specific to the visual cortex and no changes were found in the frontal cortex as a result of dark rearing. Autoradiographic histology revealed that in the normal cat visual cortex, 5-HT1 receptors were present in all cortical layers and were slightly more dense in supragranular and infragranular layers. In dark-reared cats, there was a marked elevation in receptor density in supragranular and infragranular layers and little change within layer IV. Immunohistochemical techniques (anti-5-HT) were used to compare serotoninergic innervation in the visual cortex of normal and dark-reared cats. In normal cat visual cortex, serotonin fibers were most dense in the superficial layers (I-III), least dense in layers IV and VI, and intermediate in layer V. No differences were found between normal and dark-reared cats in the laminar distribution or density of serotoninergic innervation. These results indicate that visual experience is necessary for the normal development of the visual cortical serotonin system. The findings that the effects of dark rearing were specific to the visual cortex and that within the visual cortex these effects were specific to supra- and infragranular layers are consistent with a possible role for serotonin in the prolonged physiological plasticity that occurs in the visual cortex of dark-reared cats.

Parvalbumin in the monkey striate cortex: a quantitative immunoelectron-microscopy study

Parvalbumin (PV) is present in a subpopulation of interneurons in the visual cortex, and also in thalamic afferents to the neocortex of primates. The object of this study is to confirm by immunoelectron-microscopy the presence of intrinsic and extrinsic connections containing parvalbumin in the monkey visual cortex, by the demonstration of parvalbumin-immunoreactivity in symmetric and asymmetric synapses. We analyzed the distribution of parvalbumin-immunoreactive profiles at the ultrastructural level in the primary visual cortex of old world monkeys (Macaca fascicularis). It has been shown by others that parvalbumin-immunoreactive cells resemble non-spiny stellate cells, double-bouquet cells, chandelier and basket cells. These neurons are known to be inhibitory and to form symmetric synapses. In fact, we observed that the vast majority of parvalbumin-immunoreactive synaptic contacts in the primary visual cortex of Macaca fascicularis are of the symmetric type (81.7%). Since parvalbumin-positive asymmetric contacts are also present (18.3%) and occur mostly in the thalamic recipient layers, 4C and 4A (9.9%), these afferents probably derive from parvalbumin-immunoreactive neurons located in the dorsal lateral geniculate nucleus of the thalamus.

A new method for quenching correction leads to revisions of data in receptor autoradiography

Differential quenching of beta-emission affects strongly the analysis of receptor distribution patterns in quantitative receptor autoradiography with tritiated ligands. Different methods for the quenching correction have been described in the past, but some of these are of limited value, if a detailed anatomical parcellation is necessary. Other methods correct exclusively local variations in lipid concentration, which is an important, but only one of several factors causing quenching. A new method for the measurement of quenching (or autoradiographic efficiency) is presented, which permits an anatomically detailed and direct determination of the total quenching without lipid extraction procedures. This method is based on the measurement of autoradiographic efficiency in cryostat sections homogeneously labeled with tritiated formaldehyde by an underlying gelatine section containing this labeled compound. Regional and layer specific measurements of autoradiographic efficiency in cortical and subcortical regions of the human and rat brain are reported. A significant correlation was found between the density of myelin and autoradiographic efficiency but other factors were also shown to influence differential quenching. The use of the here presented correction procedure leads to revisions of the laminar distribution patterns reported for different receptors in human and rat cortical areas. Our results show, that a complete quenching correction is necessary for the mapping of receptor distributions with tritiated ligands.

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.

Distribution of two morphologically distinct subsets of serotoninergic axons in the cerebral cortex of the marmoset

The serotoninergic innervation of the marmoset (New World monkey, Callithrix jacchus) cerebral cortex has been analyzed by using immunocytochemistry. The use of a sensitive monoclonal antibody against serotonin allowed the visualization of the fine morphology of individual axons. Two types of terminal axons were demonstrated: one has sparse, small, ovoid varicosities (dia. less than 1 micron), and the other has large, spheroidal varicosities (up to 5 microns in dia.), which are more densely clustered. The first type of axon is distributed through all cortical layers, with a characteristic laminar distribution that varies from area to area. The second type of axons was distributed sparsely in all regions but was markedly denser in the frontal and anterior parietal lobes, and in the hippocampal formation. Axons with large varicosities typically surrounded certain cell bodies and proximal dendrites, forming pericellular arrays, or baskets. These morphological specializations were most frequent in the frontal and anterior parietal cortex, where they were found around stellate and horizontal cells in layer I and around stellate and bipolar cells in layer II and III. Similar baskets were also found in the hippocampal formation, mainly along the border between the hilus and the granule cell layer of the dentate gyrus, across the CA4 field, and at each side of the pyramidal cell layer of the CA3 regions. The distribution and cellular morphology of the cell surrounded by the 5-HT basket fibres were suggestive of a subpopulation of interneurons, possibly GABAergic and/or peptidergic. In agreement with previous reports on the innervation of the cerebral cortex of other mammalian species, the marmoset cerebral cortex is innervated by two separate subsystems of serotoninergic axons. One of these may have a strong and specific influence on the cortical inhibitory circuitry, via relay through cortical interneurons.

Antibody labeling of functional subdivisions in visual cortex: Cat-301 immunoreactivity in striate and extrastriate cortex of the macaque monkey

We have examined the distribution of immunoreactivity for the monoclonal antibody Cat-301 in visual cortex of the macaque monkey. Remarkably, those portions of striate cortex (V1) and extrastriate cortex that are most immunoreactive for Cat-301 are anatomically interconnected and are dominated by inputs arising from the magnocellular layers of the LGN (which are themselves highly immunoreactive). In particular, we found that a band of Cat-301 labeled neurons known to exist in layer 4 of V1 is centered on the boundary between layers 4C alpha and 4B and thus includes portions of both the primary target of the magnocellular LGN and its subsequent relay through layer 4B. We also demonstrated consistently strong Cat-301 immunoreactivity in all three extrastriate targets of layer 4B: areas V3, MT, and the cytochrome-oxidase (CO) enriched thick stripes of V2. In V2, there was a close correlation between Cat-301 labeling and clusters of cells projecting to MT but not to V4. This was true even in regions where the CO pattern was equivocal or irregular, indicating that Cat-301 is a more reliable marker than CO for the thick-stripe subregions of V2. Finally, we found strong Cat-301 immunoreactivity in at least parts of areas V3A, the MST complex, and the posterior parietal complex, but not in area V4 or inferotemporal cortex. The molecular specificity revealed by this single marker thus correlates with functionally specific subdivisions at each hierarchical level over nearly the entire known extent of the visual pathway in macaques. This supports the notion that these subdivisions form an anatomically, physiologically, and now molecularly distinct pathway known as the M-stream.

Morphology of the cells within the inferior temporal gyrus that project to the prefrontal cortex in the macaque monkey

The primate neocortex possesses an extraordinary degree of regional specialization. Virtually all cortical functions are dependent upon a complex system of reciprocal connections between related cortical regions that allow for distributed information processing. Although some aspects of the organization of these corticocortical projections are understood, little is known about the morphology and afferents to the cells of origin of long corticocortical projections in primates. We combined intracellular injection of Lucifer Yellow (LY) in fixed tissue with in vivo retrograde transport of fast blue to study the dendritic morphology of neurons within the inferior temporal gyrus (ITG) and the superior temporal sulcus (STS) that furnish corticocortical projections to the prefrontal cortex. The fast blue retrogradely labeled cells formed two clearly defined bands within the inferior temporal cortex: a supragranular band that corresponded to layer III, and an infragranular band that corresponded to layers V and VI. After Lucifer Yellow intracellular filling, these retrogradely labeled cells projecting to the prefrontal cortex were found to be morphologically very heterogeneous. Although all filled cells had spiny dendrites, they presented a wide range of cell body sizes and dendritic tree morphologies. In layer III, the majority of cells were typical pyramids of various sizes. In layers V-VI, numerous typical pyramidal cells were present. In addition, significant numbers of modified pyramidal forms were found, including vertical and horizontal fusiform cells, asymmetrical pyramids and multipolar cells. The entire dendritic arbor of individual subtypes in layers III, V, and VI was restricted to a few cortical layers, but as a group these cells had dendrites spanning the whole cortical depth. We suggest that corticocortically projecting cells are distinct from subcortically projecting cells and consist of a defined set of morphological and functional subgroups, each of which is driven by a distinct set of afferents and likely possesses different response properties.

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.

Ultrastructural relationships of serotonin axon terminals in the cerebral cortex of the adult rat

PAP immunocytochemistry with an antiserum against serotonin (5-HT)-glutaraldehyde-protein conjugate (kindly donated by M. Geffard) was used to analyze the ultrastructural relationships of 5-HT axon terminals (varicosities) in the frontal (Fr1-Fr2), parietal (Par1), and occipital (Oc1M-Oc2) cortex of adult rats. One hundred-forty-five immunostained varicosities from Fr1-Fr2 (54 from layers I-II; 91 from layer VI) and 97 each from the upper layers (I-II) of Par1 and OcM1-Oc2 were examined in groups of serial thin sections (mean number of sections in series: 3.2 to 7.3). These terminals were of comparable shape and size in the 4 cortical sectors examined, and averaged 0.66 +/- 0.2 microns in mean diameter. The proportion of varicosities engaged in synaptic contact was evaluated by linear transformation of the relationship between the frequency of observed synaptic junctions and the number of thin sections available for examination. Reliability of the sampling was evidenced by a high coefficient of correlation (r greater than 0.95) in each cortical sector. The synaptic incidence extrapolated for whole varicosities ranged from 28% (layer VI of Fr1-Fr2) to 46% (Par1), without statistically significant differences between the 4 sectors examined. The interregional mean could thus be evaluated at 38%. The synaptic 5-HT terminals always made asymmetrical junctions, which were exclusively found on dendritic spines and shafts, and appeared more frequent on spines than shafts in the deep frontal and the upper occipital cortex. In all 4 sectors, dendritic shafts and spines and other axonal varicosities were frequently encountered in the immediate microenvironment of the immunostained varicosities. It is concluded that the cortical 5-HT innervation is predominantly nonjunctional throughout the neocortex of the adult rat, which reinforces earlier views of a highly divergent afferent system with particular functional properties and perhaps capable of widespread, global and/or sustained influences in this part of the brain.

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.

Schizophrenia: a disease of interhemispheric processes at forebrain and brainstem levels?

The evidence is convincing that each human cerebral hemisphere is capable of human mental activity. This being so, every normal human thought and action demands either a consensus between the two hemispheres, or a dominance of one over the other, in any event integrated into a unity of conscious mentation. How this is achieved remains wholly mysterious, but anatomical and behavioral data suggest that the two hemispheres, and their respective bilateral, anatomical-functional components, maintain a dynamic equilibrium through neural competition. While the forebrain commissures must contribute substantially to this competitive process, it is emphasized in this review that the serotonergic raphé nuclei of pons and mesencephalon are also participants in interhemispheric events. Each side of the raphé projects heavily to both sides of the forebrain, and each is in receipt of bilateral input from the forebrain and the habenulo-interpeduncular system. A multifarious loop thus exists between the two hemispheres, comprised of both forebrain commissural and brainstem paths. There are many reasons for believing that perturbation of this loop, by a variety of pathogenic agents or processes, probably including severe mental stress in susceptible individuals, underlies the extraordinarily diverse symptomatology of schizophrenia. Abnormality of features reflecting interhemispheric processes is common in schizophrenic patients; and the 'first rank' symptoms of delusions or hallucinations are prototypical of what might be expected were the two hemispheres unable to integrate their potentially independent thoughts. Furthermore, additional evidence suggests that the disorder lies within, or is focused primarily through, the raphé serotonergic system, that plays such a fundamental role in consciousness, in dreaming, in response to psychotomimetic drugs, and probably in movement, and even the trophic state of the neocortex. This system is also well situated to control the dopaminergic neurons of the ventral tegmental area, thus relating to the prominence of dopaminergic features in schizophrenia; and the lipofuscin loading and intimate relation with blood vessels and ependyma may make neurons of the raphé uniquely vulnerable to deleterious agents.

Ultrastructural analysis of somatostatin-immunoreactive neurons and synapses in the temporal and occipital cortex of the macaque monkey

Somatostatin-containing neurons and terminals have been analyzed in monkey temporal and occipital cortex by using light and electron microscopic immunohistochemistry. An antibody against Somatostatin-28, that was shown previously preferentially to label fibers (Morrison et al.: Brain Research 262:344-351, 1983), was utilized. As expected, few cell bodies were labeled. At the electron microscopic level, labeled cells presented a characteristic asymmetric position of the nucleus and very few symmetric or asymmetric synapses on the somatic surface. In all areas examined, somatostatin fibers formed a dense plexus in the most superficial layers (I-upper III). The density of labeled fibers in intermediate (deep III-IV) and deep layers (V-VI) varied considerably among areas. The synaptic relationships of the immunoreactive fibers were analyzed and postsynaptic targets quantified in V1, V2, and the superior and inferior temporal gyrus (STG and ITG, respectively). The synapses formed by somatostatin-labeled boutons were of the symmetric type (type II) and the primary postsynaptic targets were dendritic shafts. No regional differences were found in the distribution of the postsynaptic targets in layers I-upper III. The pattern of synapses in the deep layers was examined in STG. The frequency and distribution of postsynaptic targets was similar to the superficial layers of STG and the other temporal and occipital regions. In intermediate layers of the temporal cortex areas there was an increase in the proportion of synapses on dendritic spines. In a correlated light and electron microscopic analysis we examined synapses made by radial fibers in these regions and found that although the main targets are distal dendritic shafts, almost 40% of synapses were on dendritic spines. We suggest that the radial fibers may originate from a specialized interneuron, previously described as the double bouquet cell, and that this particular subset of somatostatin-containing double bouquet cells is likely to exhibit a very high degree of regional heterogeneity with a preference for association cortices with extensive corticocortical convergence.