Light and electron microscopic immunocytochemistry of β-endorphin/β-LPH-like immunoreactive neurons in the arcuate nucleus and surrounding areas of the rat hypothalamus

β-Endorphin/β-LPH-like immunoreactive neurons in the hypothalamic arcuate nucleus and its surrounding areas were visualized by light and electron microscopic immunocytochemistry. Immunoreactive processes were found in the vicinity of the pia mater, in the lateral part of the external layer of the median eminence and near the lateral wall of the third ventricle. Neuronal perikarya contained immunoreactive dense granules as well as developed cell organellae. They received neuronal inputs from other neurons through axoplasmic and axodendritic synapses. Immunoreactive neuronal processes containing dense granules and mitochondria were found as preterminal elements on non-immunoreactive neuronal soma and dendrites. Immunoreactive processes also make intimate contact with capillaries in the arcuate nucleus near the median eminence.     

Vasoactive intestinal peptide immunoreactive neurons in the rat suprachiasmatic nucleus demonstrate diurnal variation

The suprachiasmatic nucleus (SCN) of mammals is considered to be a circadian oscillator and it also demonstrates circadian rhythmicity of its multiple unit activity. A number of neuropeptides have been found in the SCN. Vasoactive intestinal peptide (VIP)- and vasopressin-containing neurons comprise large populations of these cells and have a distinct distribution within the nucleus. Therefore we attempted to examine whether the VIP neurons show a diurnal alteration of their immunoreactivity by combined immunocytochemistry and color image analysis. Our results demonstrate that VIP-like immunoreactive neurons show a diurnal change in the amount of immunoreactivity. Immunoreactivity was most intense in the sections from rats maintained in the cyclic photoperiod and sacrificed at 02.00 h and weakest in the SCN from animals sacrificed at 14.00 h. We considered that VIP-like immunoreactive neurons showed diurnal variation of VIP synthesis depending strongly on the light from the retina.     

Commissural afferents to the rat hippocampus terminate on vasoactive intestinal polypeptide-like immunoreactive non-pyramidal neurons. An EM immunocytochemical degeneration study

In the rat hippocampus, bipolar non-pyramidal neurons in stratum radiatum and stratum oriens and multipolar neurons in stratum lacunosum-moleculare react for vasoactive intestinal polypeptide (VIP) immunostaining, but pyramidal cells do not. Such bipolar VIP-like immunoreactive neurons in strata radiatum and oriens of regio superior were studied by electron microscopy for synaptic contacts with commissural afferents. The commissural fibers were identified by their anterograde degeneration induced by contralateral fimbria transections 2 days before sacrifice. Electron-dense degenerated boutons of commissural origin were found in synaptic contact with the cell bodies and dendrites of the VIP-like immunoreactive non-pyramidal cells.     

The fine structures of the suprachiasmatic nucleus of the golden hamster

The fine structure of the suprachiasmatic nucleus of the golden hamster was studied with special reference to the synaptic endings. The somata of SCN neurons contained well developed cytoplasmic organelles including the Golgi complex, mitochondria and polysomes. The nuclei had deeply invaginated nuclear membrane. Some neurons were characterized by the presence of a large number of granulated vesicles and an abundance of cytoplasmic organelles. Several kinds of synapses and gap junctions were also observed. Axo-somatic synapses were identified and could be differentiated based upon possessing two types of presynaptic elements. The first contained clear round synaptic vesicles (40-60 nm in diameter), the second contained clear round vesicles (40-60 nm in diameter) and dense cored vesicles (70-120 nm in diameter). Asymmetrical synaptic membrane thickening were observed on both the pre- and postsynaptic sides of most of the axosomatic synapses. Axo-dendritic synapses could also be divided into two sub-types according to their membrane specializations. In Type 1, the axon terminals contained both clear round vesicles and dense cored vesicles and formed asymmetrical synapses. Terminals in the second group (type 2) were characterized by symmetrical synapses that contained clear, round vesicles as well as dense cored vesicles. The type 1 terminals were evenly distributed throughout the SCN, but the type 2 terminals were encountered more frequently in the ventral SCN. These observations indicate that the ventral and dorsal components of the SCN may possess different functional roles.     

Anatomy of the gustatory system in the hamster: synaptology of facial afferent terminals in the solitary nucleus

The solitary nucleus is the first level of the central nervous system where processing of taste information can occur. A structural basis for that processing was investigated. Facial taste afferent axons were labelled by application of horseradish peroxidase to either the chorda tympani or the geniculate ganglion. The labelled afferent fibers in the rostral solitary nucleus were studied with light and electron microscopy. Preterminal facial taste afferent axons enter the nucleus from the solitary tract with a pronounced lateral to medial trajectory. The axons bear numerous preterminal and terminal swellings that, with the electron microscope, were identified as synaptic endings located in glomeruli. The endings are ovoid or scalloped, indented by structures that surround them. The primary afferent endings contain large, round vesicles and synapse, by means of slightly asymmetrical junctional complexes, on small dendrites and spines. Two types of unlabelled endings, surrounding the labelled ones, contact the dendrites receiving taste afferent input or contact the endings of taste afferent axons themselves. One type is variable in size and contains scattered large round vesicles. It resembles a presynaptic dendrite. The other is a small axonal ending packed with small, pleomorphic vesicles, that engages in symmetrical junctions. The synaptic milieu of the taste endings allows for the possibility of modulation of taste-elicited activity in afferent endings or second-order neurons by other, possibly interneuronal, inputs.     

Fine structural studies of cholecystokinin-8-like immunoreactive neurons and axon terminals in the nucleus of tractus solitarius of the rat

Cholecystokinin (CCK)-8-like immunoreactive structures in the nucleus of tractus solitarius (NTS) were studied by using the peroxidase-antiperoxidase (PAP) immunohistochemical method. Immunoreactivity was localized in cell bodies and nerve fibers. The perikarya were oval or fusiform (average length 13 micron) and were mostly located in the dorsal half of the medial subnucleus of the NTS at the level of the area postrema (AP). One to three straight immunoreactive dendritelike processes emerged from the perikarya. Neurons that had first been identified under light microscopy were also studied by electron microscopy. Each neuron had a moderate amount of cytoplasm and an oval or elongated nucleus that was eccentrically located in the soma. A few synaptic inputs were found on the CCK immunoreactive perikarya, while a moderate number were seen on both proximal and distal dendrites. These neurons received both asymmetrical and symmetrical synaptic inputs. The immunoreactive dendrites were most frequently in asymmetrical synaptic contact with nonreactive boutons (max. 2.7 micron in diameter) containing fairly densely packed, small round vesicles. CCK immunoreactive boutons located in the NTS at the level of the AP were analyzed using electron microscopy; these boutons formed asymmetrical synaptic contact with other neuronal elements. Their postsynaptic targets were immunoreactive and nonreactive perikarya and dendrites. These data suggest that CCK-containing afferents might affect the neurotransmission of heterogenous types of solitary neurons.     

Immuno-electronmicroscopic studies on the gamma-aminobutyric acid and glycine receptor in the intermediolateral nucleus of the thoracic spinal cord of rats and guinea pigs.

Gamma-aminobutyric acid (GABA) and glycine are known as major inhibitory neurotransmitters in the intermediolateral nucleus of the spinal cord. Distribution and density of GABA immunoreactive axon varicosities and glycine receptor immunoreactive dendrites and somata in the intermediolateral nucleus were examined by immuno-electronmicroscopy. GABA immunoreaction was observed in the axon varicosities of axo-dendritic and axo-somatic synapses. Glycine receptor immunoreaction was seen in association with the postsynaptic membrane of dendrites and soma. GABA immunoreactive axon varicosities were larger (1.01 +/- 0.49 x 1.20 +/- 0.38 microns) than axon varicosities presynaptic to glycine receptors (0.72 +/- 0.22 x 0.98 +/- 0.33 microns). The density of GABA immunoreactive axon varicosities was 3.65/100 microns 2 and that of glycine receptor immunoreactive synapses was 4.78/100 microns 2. A subpopulation of GABA immunoreactive axons (42%) made synaptic contact with glycine receptor immunoreactive dendrites or soma, indicating the coexistence of GABA and glycine.     

Ultrastructure of cholinergic neurons in the laterodorsal tegmental nucleus of the rat: Interaction with catecholamine fibers

The ultrastructure of choline acetyltransferase (ChAT)-immunoreactive neurons in the laterodorsal tegmental nucleus (TLD) of the rat was investigated by immunohistochemical techniques. The immunoreactive neurons were medium to large in size, with a few elongated dendrites, contained well-developed cytoplasm, and a nucleus with deep infoldings. They received many nonimmunoreactive, mostly asymmetric synaptic inputs on their soma and dendrites. ChAT-immunoreactive, usually myelinated, axons were occasionally seen in TLD. Only one immunoreactive axon terminal was observed within TLD, and it made synaptic contact with a nonimmunoreactive neuronal perikaryon. The synaptic interactions between ChAT-immunoreactive neurons and tyrosine hydroxylase (TH)-immunoreactive fibers in the TLD were investigated with a double immunohistochemical staining method. ChAT-immunoreactivity detected with a beta-galactosidase method was light blue-green in the light microscope and formed dot-like electron dense particles at the electron microscopic level. TH-immunoreactivity, visualized with a nickel-enhanced immunoperoxidase method, was dark blue-black in the light microscope and diffusely opaque in the electron microscope. Therefore, the difference between these two kinds of immunoreactivity could be quite easily distinguished at both light and electron microscopic levels. In the light microscope, TH-positive fibers were often closely apposed to ChAT-immunoreactive cell bodies and dendrites in TLD. In the electron microscope, the cell soma and proximal dendrites of ChAT-immunoreactive neurons received synaptic contacts from TH-immunoreactive axon terminals. These results provide a morphological basis for catecholaminergic regulation of the cholinergic reticular system.     

Nucleus of the accessory optic tract. Light and electron microscopic study in normal monkeys and after eye enucleation

The fine morphology and synaptic organization of the accessory optic tract nucleus was investigated in monkeys (Macaca mulatta) by light and electron microscopic methods. Nissl stains delineated the wedge-shaped nucleus between the brachium of the inferior colliculus and the medial lemniscus. Most of the neurons are of medium size with dark coarse Nissl bodies. Smaller and paler cells are also present. Golgi material revealed the two classic type I and II neurons, the former with early bifurcating dendrites exhibiting numerous spines. Electron microscopy showed mostly medium size neurons with relatively frequent occurrence of somatic spines. The dendritic profiles have a thick smooth initial protion followed by the appearance of increasing number of spines. The dendrites bifurcate close to the soma and finally end in brushlike fashion. Axon terminals make synaptic contacts with the soma, dendrites (spines and trunk) and dendritic endings. The latter articulations appear as “glomeruli”. At this level, profiles of ambiguous nature, possibly dendrites of Golgi type II cells, containing synaptic vesicles are frequently seen. Following eye enucleation, both the contralateral and ipsilateral nuclei showed terminal boutons in different stages of degeneration depending on the survival time. The optic fibers terminate on the soma as well as on the proximal spineless part of the dendrites and at the dendritic bifurcations. There are no optic terminals participating in the “glomeruli”. This nucleus receives direct retinal fibers and the system is both crossed and uncrossed. The presence of abundant axosomatic contacts of optic terminals as well as the absence of such endings in the glomeruli suggest a very different functional mechanism than that of the geniculate pathway. The nucleus receives many more afferents of unknown origin.     

Distribution of the VIP-like immunoreactive neurons in the cat central nervous system

Immunohistochemical topographic localization of the vasoactive intestinal polypeptide (VIP)-like immunoreactive neurons in the cat brain was investigated using a peroxidase anti-peroxidase technique. VIP-like immunoreactive neurons were mainly localized in the cerebral cortex, limbic cortex, hypothalamic nuclei; suprachiasmatic nucleus, supraoptic nucleus, paraventricular nucleus, periventricular nucleus and arcuate nucleus, and in the midbrain; such as the central grey and the raphe nucleus. It was demonstrated that VIP-like immunoreactive neurons were widely distributed in the cat brain, particularly in the hypothalamus, compared with those of the rat and mouse; though whether these differences were species-related or due to differences in the physiological conditions remains to be determined. This is the first report of VIP neuronal perikarya in the arcuate nucleus of mammalian species, although these cells are present in the arcuate nucleus of birds.     

Hippocampal and midline thalamic fibers and terminals in relation to the choline acetyltransferase-immunoreactive neurons in nucleus accumbens of the rat: a light and electron microscopic study

The synaptic interactions between terminals of allocorticostriatal and thalamostriatal fibers and the cholinergic neurons in the nucleus accumbens were investigated using degeneration and dual labelling immunocytochemistry in Wistar rats. The presumptive cholinergic neurons were labelled with antibodies directed against choline acetyltransferase and the afferent fibers were labelled anterogradely with Phaseolus vulgaris-leucoagglutinin. Fibers from the subiculum of the hippocampal formation and from the midline and intralaminar thalamus project densely into the medial nucleus accumbens where they overlap a relatively dense population of choline acetyltransferase-immunoreactive neurons. Varicosities containing Phaseolus vulgaris-leucoagglutinin juxtapose the immunoreactive neurons. To study the possibility that the cholinergic neurons could be the synaptic targets of these incoming fibers, the subiculum, the fornix, and the midline/intralaminar thalamus were lesioned in separate animals and brain sections were immunoprocessed for choline acetyltransferase and studied with the electron microscope. In addition, dual-labelling electron microscopic immunocytochemistry was employed. In total, 164 synaptic terminals from the subiculum/hippocampus and 130 from the midline/intralaminar thalamus were examined; all formed asymmetrical synaptic specializations. No hippocampal endings were seen to contact the somata or primary dendrites of the choline acetyltransferase-immunoreactive neurons; however, three were found in synaptic contact with distal, immunolabelled dendritic shafts. Most hippocampal terminals established contacts with unlabelled spines. Fifteen percent of the thalamic endings were found to synapse on the somata and the primary and distal dendrites of the choline acetyltransferase-immunoreactive neurons. The remaining thalamic terminals established synaptic junctions with small unlabelled dendrites or spines. These findings have important implications not only for our understanding of the synaptic organization of the hippocampal and thalamic projections to the nucleus accumbens, but also for the contribution of the cholinergic neurons to the circuitry of this nucleus.     

Light and electron microscopic immunocytochemistry of GRF-like immunoreactive neurons and terminals in the rat hypothalamic arcuate nuclues and median eminence

Growth hormone-releasing factor (GRF) synthesizing neuronal perikarya and terminals were investigated by light and electron microscopic immunocytochemistry using rat hypothalamus. Immunoreactive neuronal perikarya were located mainly in the ventrolateral part of the arcuate nucleus. They contained well developed cell organella such as mitochondria and rough surfaced endoplasmic reticulum with some expansion. They also contained immunoreactive dense granules (80-120 nm in diameter). On the surface of the immunoreactive neuronal perikarya were frequently found non-immunoreactive axo-somatic synapses. Therefore, the GRF-like immunoreactive neurons were assumed to receive neuronal inputs from other neurons on their neuronal soma. In the external layer of the median eminence large numbers of immunoreactive terminals were distributed particularly around the capillaries of the portal vessel. Electron microscopic immunocytochemistry revealed large numbers of immunoreactive terminals containing immunoreactive dense granules, synaptic vesicles and mitochondria in the vicinity of the basement membrane of the pericapillary space of the portal vessel. Therefore, we concluded that GRF-like immunoreactive substances are released into the portal capillaries from the nerve terminals, which originate from the neuronal perikarya in the ventrolateral part of the arcuate nucleus, and act on growth hormone release in the anterior pituitary. We also suggest that GRF-like immunoreactive neurons have abundant terminal arborization in the external layer of the median eminence.     

Electron microscopic observations of axon collateral synaptic endings of cat oculomotor motoneurons stained by intracellular injection of horseradish peroxidase

Motoneurons in the cat oculomotor nucleus have been identified electrophysiologically and stained by intracellular injection of horseradish peroxidase. Axon collateral arborizations with preterminal and terminal boutons identified by light microscopy corresponded to synaptic endings observed by electron microscopy. Despite variations in size and shape, synaptic endings showed similar ultrastructural features and established asymmetrical predominantly axodendritic synaptic contacts usually characterized by the presence of subjunctional dense bodies underlying the postsynaptic membrane densification.     

Synaptic organization of globular bushy cells in the ventral cochlear nucleus of the cat: a quantitative study.

The synaptic organization of globular bushy cells of the anteroventral cochlear nucleus was quantitatively analyzed in order to understand better their functional attributes. A method was devised to estimate the concentrations and relative proportions of synapses on the entire postsynaptic surface of Golgi-impregnated neurons, by sampling with limited series of sections for electron microscopy. This provided a characteristic synaptic profile which was homogeneous for the population measured. The total concentration of synaptic endings decreases with distance from the soma. The cochlear, presumably glutamatergic and excitatory, endings with large spherical vesicles (LS) account for most of this decrease. Of the noncochlear inputs, the putative glycinergic endings with flattened vesicles (FL) decrease slightly, and the presumed GABAergic terminals with pleomorphic vesicles (PL) maintain a relatively constant concentration, while endings with small spherical vesicles (SS) increase on the distal dendrites. LS endings have the largest proportion of synapses near the soma, while FL synapses maintain a constant proportion in all cell regions, and PL and SS proportions increase on higher-order dendrites. Excitatory and inhibitory synapses have significant inputs to the axon hillock and initial segment, as well as to the distal dendrites, where dual synapses may provide a way to sample the activity of surrounding neurons. These features must be considered in explanations of physiological properties, such as the synaptic security, level of spontaneous activity, and well-timed, rapid onset responses, as well as their potential for normalizing and synchronizing an important inhibitory pathway involved in binaural signal processing. Synaptic profile analysis should be useful for experimental studies and for developing realistic computational models.     

The fine structural localization of glutamate decarboxylase in synaptic terminals of rodent cerebellum

Glutamic acid decar?ylase (GAD), the enzyme that synthesizes the putative neurotransmitter γ-aminobutyric acid (GABA), has been localized by peroxidase labeling antibody techniques at the light and electron microscopic levels in rodent cerebellum. Specific anti-GAD peroxidase product was highly localized in certain synaptic terminals in close association with the membranes of synaptic vesicles and mitochondria but not within these organelles. GAD-positive terminals were seen on the somata and proximal dendrites of neurons in the deep cerebellar nuclei. Other positive terminals were presumed Golgi type II endings of synaptic glomeruli in the granular layer. Positive terminals were also seen in the molecular layer, including presumed basket cell endings which contained product on smooth membrane cisternae in the preterminal axon as well as around synaptic vesicles and mitochondria.These observations correlate well with a large body of evidence that certain synaptic connections in the cerebellum are inhibitory and that many, if not all, of the presynaptic components of these connections may use GABA as their neurotransmitter.     

Differential distribution of synaptic endings containing glutamate, glycine, and GABA in the rat dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) integrates the synaptic information depending on the organization of the excitatory and inhibitory connections. This study provides, qualitatively and quantitatively, analyses of the organization and distribution of excitatory and inhibitory input on projection neurons (fusiform cells), and inhibitory interneurons (vertical and cartwheel cells) in the DCN, using a combination of high-resolution ultrastructural techniques together with postembedding immunogold labeling. The combination of ultrastructural morphometry together with immunogold labeling enables the identification and quantification of four major synaptic inputs according to their neurotransmitter content. Only one category of synaptic ending was immunoreactive for glutamate and three for glycine and/or gamma-aminobutyric-acid (GABA). Among those, nine subtypes of synaptic endings were identified. These differed in their ultrastructural characteristics and distribution in the nucleus and on three cell types analyzed. Four of the subtypes were immunoreactive for glutamate and contained round synaptic vesicles, whereas five were immunoreactive for glycine and/or GABA and contained flattened or pleomorphic synaptic vesicles. The analysis of the distribution of the nine synaptic endings on the cell types revealed that eight distributed on fusiform cells, six on vertical cells and five on cartwheel cells. In addition, postembedding immunogold labeling of the glycine receptor alpha1 subunit showed that it was present at postsynaptic membranes in apposition to synaptic endings containing flattened or pleomorphic synaptic vesicles and immunoreactive for glycine and/or GABA on the three cells analyzed. This information is valuable to our understanding of the response properties of DCN neurons.     

A light and electron microscopic study of the dorsal motor nucleus of the vagus nerve in the cat

The morphology of the dorsal motor nucleus of the vagus nerve (DMV) in the cat was studied with the aid of light and electron microscopy. In frozen sections stained by the Kluver-Barrera method or stained to show retrograde labeling in the DMV following injections of horseradish peroxidase (HRP) in the cervical vagus nerve and the stomach wall a range of sizes of DMV neurons was observed but it was not possible to distinguish separate types. In contrast, two distinct types of neurons, one medium-sized and the other small, were identified with the light microscope in Golgi-Cox and 1-μm Epon sections and with the electron microscope in ultrathin sections. The medium-sized neurons had a range of sizes but generally measured 18 × 25 μm and possessed three to four proximal dendrites which branched two or three times. Spines were observed occasionally on the soma and on dendrites. These neurons contained a well-developed cytoplasm and a noninvaginated round to oval nucleus. The small neurons generally measured 9 × 14 μm and were round or slightly elongated in shape. Their dendritic processes were fewer and thinner than those of the medium-sized neurons and extended for shorter lengths. Their soma contained scanty cytoplasm and an invaginated nucleus. The medium-sized neurons outnumbered the small neurons by more than three to one but both neuronal types were distributed evenly throughout the nucleus. The medium-sized neurons seemed to correspond in size to the parasympathetic efferent neurons of the viscera as indicated by the HRP studies. Axosomatic synapses on both types of neurons and axodendritic synapses were observed in the DMV. Terminals containing mainly small clear round vesicles and making asymmetrical contact with the postsynaptic membrane were involved in the majority of synapses on both the soma and dendrites. Terminals containing predominantly pleomorphic vesicles and making symmetrical contact with the postsynaptic membrane were also common, comprising up to one-third of all synapses observed. Serial sections revealed that most synaptic terminals contained varying numbers of large (75–110 nm) dense-cored vesicles. Smaller dense-cored vesicles (45–55 nm) were sometimes observed, often close to the area of synaptic contact. Terminals 1–2 μm in diameter which contacted dendrites 1–3 μm in diameter formed the most common synaptic combination throughout the rostral to caudal extent of the DMV. No distinct regional differences were observed with respect to distribution of synaptic types.     

GABA-immunoreactive synaptic boutons in the rat basal forebrain: comparison of neurons that project to the neocortex with pallidosubthalamic neurons

Although the basal forebrain, including the globus pallidus, contains a high concentration of gamma-aminobutyric acid (GABA), it is not known whether all types of neuron in the globus pallidus receive GABAergic synaptic input. We have studied two types of neuron: typical pallidal neurons that project to the subthalamic nucleus and magnocellular neurons which are found in the medial and ventral borders of the globus and project to the sensorimotor cortex. The postembedding immunogold staining of endogenous GABA revealed many preterminal axons and synaptic boutons that contained GABA immunoreactivity. Neurons that projected to the neocortex were postsynaptic to some of the GABA-immunoreactive boutons, the majority of which formed symmetrical membrane specializations. From a series of random electron micrographs through the perikarya and proximal dendrites of such retrogradely labelled neurons the density of GABA-containing afferent synaptic boutons was estimated to be 0.58 GABA-containing boutons per 100 micron of neuronal membrane. The GABA-containing boutons accounted for 72% of the total afferent input in the proximal regions of the pallidocortical neurons examined. The pallidosubthalamic neurons received many more afferent boutons than did the cortically projecting neurons, a high proportion (80.4%) of which were immunoreactive for GABA. The density of GABA-containing boutons in contact with pallidosubthalamic neurons was 8.9 boutons per 100 micron. It is concluded that cortically projecting basal forebrain neurons, that are probably cholinergic, are innervated by GABA-containing afferent boutons. However, pallidosubthalamic neurons in the same part of the basal forebrain are much more densely innervated by GABA-containing boutons.     

Distribution, origin, and fine structures of cholecystokinin-8-like immunoreactive terminals in the nucleus ventromedialis hypothalami of the rat

The distribution, origin, and fine structures of cholecystokinin-8-like immunoreactive ( CCKI ) fibers in the nucleus ventromedialis hypothalami (vm) of the rat were examined using immunohistochemistry. CCKI fibers were moderately concentrated in the ventrolateral part of the vm, decreasing in number dorsomedially. However, no CCKI cells were seen in the vm even in colchicine-treated rats. In addition, the destruction of the lateral part of the nucleus parabrachialis dorsalis (pbd) where numerous CCKI cells were seen resulted in a marked decrease in CCKI fibers in the vm on the operated side. Electron microscopic observations revealed that CCKI terminals make synaptic contact with soma, spine, and the proximal segment of the dendrite. The neurons making synaptic contact with the CCKI fibers had moderately electron-lucent cytoplasm. These findings suggest that the CCKI fibers in the vm originate from CCKI cells in the lateral part of the pbd and directly influence the vm neurons.     

Subcellular distribution of 5-HT(1B) and 5-HT(7) receptors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), a circadian oscillator, receives glutamatergic afferents from the retina and serotonergic (5-HT) afferents from the median raphe. 5-HT(1B) and 5-HT(7) receptor agonists inhibit the effects of light on SCN circadian activity. Electron microscopic (EM) immunocytochemical procedures were used to determine the subcellular localization of 5-HT(1B) and 5-HT(7) receptors in the SCN. 5-HT(1B) receptor immunostaining was associated with the plasma membrane of thin unmyelinated axons, preterminal axons, and terminals of optic and nonoptic origin. 5-HT(1B) receptor immunostaining in terminals was almost never observed at the synaptic active zone. To a much lesser extent, 5-HT(1B) immunoreaction product was noted in dendrites and somata of SCN neurons. 5-HT(7) receptor immunoreactivity in gamma-aminobutyric acid (GABA), vasoactive intestinal polypeptide (VIP), and vasopressin (VP) neuronal elements in the SCN was examined by using double-label procedures. 5-HT(7) receptor immunoreaction product was often observed in GABA-, VIP-, and VP-immunoreactive dendrites as postsynaptic receptors and in axonal terminals as presynaptic receptors. 5-HT(7) receptor immunoreactivity in terminals and dendrites was often associated with the plasma membrane but very seldom at the active zone. In GABA-, VIP-, and VP-immunoreactive perikarya, 5-HT(7) receptor immunoreaction product was distributed throughout the cytoplasm often in association with the endoplasmic reticulum and the Golgi complex. The distribution of 5-HT(1B) receptors in presynaptic afferent terminals and postsynaptic SCN processes, as well as the distribution of 5-HT(7) receptors in both pre- and postsynaptic GABA, VIP, and VP SCN processes, suggests that serotonin plays a significant role in the regulation of circadian rhythms by modulating SCN synaptic activity.