Tryptophan hydroxylase in the rat brain

Orexin-A inputs onto visuomotor cell groups in the monkey brainstem

Orexin-A, synthesized by neurons of the lateral hypothalamus helps to maintain wakefulness through excitatory projections to nuclei involved in arousal. Obvious changes in eye movements, eyelid position and pupil reactions seen in the transition to sleep led to the investigation of orexin-A projections to visuomotor cell groups to determine whether direct pathways exist that may modify visuomotor behaviors during the sleep-wake cycle. Histological markers were used to define these specific visuomotor cell groups in monkey brainstem sections and combined with orexin-A immunostaining. The dense supply by orexin-A boutons around adjacent neurons in the dorsal raphe nucleus served as a control standard for a strong orexin-A input. The quantitative analysis assessing various functional cell groups of the oculomotor system revealed that almost no input from orexin-A terminals reached motoneurons supplying the singly-innervated muscle fibers of the extraocular muscles in the oculomotor nucleus, the omnipause neurons in the nucleus raphe interpositus and the premotor neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus. In contrast, the motoneurons supplying the multiply-innervated muscle fibers of the extraocular muscles, the motoneurons of the levator palpebrae muscle in the central caudal nucleus, and especially the preganglionic neurons supplying the ciliary ganglion received a strong orexin input. We interpret these results as evidence that orexin-A does modulate pupil size, lid position, and possibly convergence and eye alignment via the motoneurons of multiply-innervated muscle fibres. However orexin-A does not directly modulate premotor pathways for saccades or the singly-innervated muscle fibre motoneurons.

Changes in steady-state levels of tryptophan hydroxylase protein in adult rat brain after neonatal 6-hydroxydopamine lesion

A recently developed technique of immunoautoradiography on nitrocellulose transfers of serial frozen sections was used to determine tryptophan hydroxylase concentration in selected areas of the adult rat brain following neonatal 6-hydroxydopamine destruction of nigrostriatal dopamine neurons. Particular attention was paid to the neostriatum, known to be serotonin-hyperinnervated under these conditions, and to the nucleus raphe dorsalis, containing the cell bodies of origin for these nerve terminals. The hippocampus was also investigated as a territory of structurally intact serotonin innervation arising primarily from the nucleus raphe medianus. Tryptophan hydroxylase protein was measured at successive transverse levels across the entire caudorostral extent of all these regions. Similar measurements of tyrosine hydroxylase protein across the substantia nigra and the neostriatum verified the disappearance of the nigrostriatal dopamine neurons. The average tryptophan hydroxylase tissue concentration in the dorsal third of the serotonin-hyperinnervated neostriatum was up by 36% above control, i.e. significantly less than the number of its serotonin axon terminals or varicosities. This was therefore indicative of a lowering of the tryptophan hydroxylase protein content per serotonin ending. Interestingly, a tight correlation between the respective level-by-level concentrations of tryptophan hydroxylase and tyrosine hydroxylase protein in the control neostriatum allowed the prediction the tryptophan hydroxylase concentration after dopamine denervation with a serotonin hyperinnervation. Tryptophan hydroxylase concentration was also significantly reduced in both the nucleus raphe dorsalis and nucleus raphe medianus, notably at those raphe dorsalis levels known to give rise to the serotonin hyperinnervation of neostriatum. It is hypothesized that the lower steady-state level of tryptophan hydroxylase inside the terminals and cell bodies of hyperinnervating serotonin neurons was the result of a feedback inhibition of the synthesis of the enzyme by its end-product, presumably because of the increased amount of serotonin in these terminals.

An immunocytochemical study using the PAP method for tyrosine hydroxylase and serotonin in alternate sections, and in situ hybridization to detect tryptophan hydroxylase mRNA in the rat's locus ceruleus

In situ hybridization (LARRSON and HOUGAARD 1990), using the APAAP complex, shows that many small and medium-sized neurons with signals of tryptophan hydroxylase mRNA are uniformly scattered throughout the rat's locus ceruleus, and that a few extrinsic neurons just ventro-medial to it also shows the hybridization signals. The specificity of this technique has been established by Northern blot hybridization. Synthesis of serotonin in intrinsic neurons is indicated not only by the results obtained from in situ hybridization, but also by the fact that, in distribution, masked serotonin cells, immunocytochemically revealed after pargyline and 5-hydroxytryptophan loading, correspond to the neurons showing mRNA hybridization signals. Identification of the same neurons in adjacent cryostat sections, immunostained alternately for serotonin or tyrosine hydroxylase after loading, provides evidence for the coexistence of serotonin and noradrenaline in a single neuron of this center. A few extrinsic, non-specific indoleamine cells located just ventro-medial to the center may be related to the lateralization of the raphe's neurons. The expression of tryptophan hydroxylase in CNS appears to be restricted to the specific regions such as the locus ceruleus and raphe's nuclei.

The response of suprachiasmatic neurons of the rat hypothalamus to photic and serotonergic stimulation

Single-unit activity of the rat suprachiasmatic nucleus and surrounding regions was recorded in vivo. Photically responsive neurons were typically inhibited by low intravenous doses of the serotonin (5-HT) agonist quipazine. The 5-HT antagonist, metergoline, administered in a low intravenous dose, reversed the effects of quipazine. The subsequent response to photic stimulation was essentially identical to the initial photic response. Thus the photic response of neurons in the suprachiasmatic nucleus (SCN) and adjacent regions is relatively uninfluenced by 5-HT blockade.

The distribution of serotonin immunoreactivity in the rat locus ceruleus after intraventricular injections of either 5,6- or 5,7-dihydroxytryptamine with special reference to serotonin synthesis

The localization of serotonin-immunoreactivity (5-HT-IR) in the locus ceruleus (LC) of rats was studied by the peroxidase-anti-peroxidase method using a purified antibody obtained from a rabbit. Antibody production was performed according to the method of Grota and Brown (1974). The antibody was applied to serial cryostat sections with alternate counterstaining by cresyl violet, after intraventricular injections of 5,6-dihydroxytryptamine or 5,7-dihydroxytryptamine prior to treatment with pargyline and a precursor of 5-HT. The majority of LC neurons were immunopositive, and more than half of all LC neurons clearly showed 5-HT-IR. Although core cells were the most predominant, all types of neurons were immunopositive, and randomly scattered throughout the LC. The uptake inhibitor, Lilly 110140, administered in sufficient amounts prior to an injection of pargyline, did not reduce 5-HT-IR within the LC. The results suggest that LC neurons receive 5-hydroxytryptophan (5-HTP) through an afferent vascular-neuronal channel and/or by diffusion from blood capillaries much more than 5-HT itself. We consider from these results that all types of LC neurons throughout the nucleus are masked 5-HT cells, and that the majority of LC neurons utilize blood-borne 5-HTP as an immediate precursor for intraneuronal 5-HT synthesis.

Quantitative morphometric analysis of two types of serotonin-immunoreactive nerve fibres differentially responding to p-chlorophenylalanine treatment in the rat brain

Two types of serotonin neurons differentially responding to treatment with p-chlorophenylalanine, a potent inhibitor of serotonin synthesis, have been demonstrated in the rat brain by immunohistochemistry using a serotonin monoclonal antibody in combination with a quantitative morphometric analysis. In control rats, the numerical density of serotonin-immunoreactive varicosities and terminals varied considerably from region to region. In p-chlorophenylalanine-treated rats, the numerical density was reduced in every region, but the degree of reduction in each region differed markedly. No correlation was seen in values of the numerical density between the control and p-chlorophenylalanine-treated rats, indicating that serotonin-positive axons in p-chlorophenylalanine-treated rats were present independent of the degrees of the axon density seen in the controls. It is postulated, therefore, that there are at least two types of positive axons; one highly sensitive to p-chlorophenylalanine treatment and another relatively resistant to it. Since the numerical density in any region was always smaller in p-chlorophenylalanine-treated rats than in the controls, p-chlorophenylalanine-sensitive axons appeared to be more or less distributed in every region where positive axons were detectable under normal conditions. However, a number of regions were innervated purely by p-chlorophenylalanine-sensitive axons. By contrast, p-chlorophenylalanine-insensitive axons were observed in several definite regions. These regions appeared to be related to the limbic system, or cranial motor and parasympathetic nuclei. In the latter nuclei, p-chlorophenylalanine-insensitive axons seemed to possess such common morphological features as having relatively large varicosities and being situated adjacent to neural somata, suggesting their inhibitory roles in neurotransmission. The present result indicates that the two types of serotonin axons may play important roles in the behavioural events observed in rats following p-chlorophenylalanine treatment.

Responsiveness of suprachiasmatic and ventral lateral geniculate neurons to serotonin and imipramine: a microiontophoretic study in normal and imipramine-treated rats

The suprachiasmatic nuclei (SCN) are a major pacemaker of circadian rhythms in mammals. The SCN receive a direct retinal projection and a second optic input via the ventral lateral geniculate nucleus (vLGN). Both visual pathways mediate the entrainment of circadian rhythms, whereas both the SCN and the vLGN receive serotonergic afferents from the raphe nuclei. We investigated the effects of microiontophoretically applied serotonin (5HT) on SCN and vLGN cells in normal rats and rats chronically treated with the 5HT reuptake blocker imipramine (IMI). In the SCN of both groups over 40% of all recorded cells (N = 80) responded to 5HT with a dose-dependent suppression of their spontaneous or glutamate-evoked discharge, while twenty percent were tonically light-responsive. Except for one cell with an inconsistent 5HT response, none of the visual SCN neurons were 5HT-sensitive. In the vLGN of normal and IMI-treated rats about 60% of the cells recorded (N = 42) were inhibited by 5HT. In IMI-treated rats a few cases of excitation by 5HT were encountered in the vLGN. Visual as well as non-visual vLGN cells were responsive to 5HT. Microiontophoretic application of IMI resulted in suppression of electrical activity in both brain regions and enhanced the response induced by 5HT. Chronic IMI-treatment produced a significant increase in the sensitivity of cells in the SCN and vLGN to iontophoresed 5HT, without affecting the relative magnitude of the inhibition. The recovery from 5HT-induced inhibition was slow in these animals. Interestingly, the spontaneous discharge rate of both 5HT-sensitive and 5HT-insensitive SCN and vLGN cells was significantly lower in the imipramine-treated group.(ABSTRACT TRUNCATED AT 250 WORDS)

Central neurotoxic effects of guanethidine: altered serotonin and enkephalin neurons within the area postrema

This study reveals that the 5-HT and enkephalin cell body population of the area postrema (AP) is dramatically reduced via exposure to the peripherally circulating neurotoxin guanethidine. Efforts to restore cell body numbers to control levels, with colchicine and monoamine oxidase inhibitors, subsequent to guanethidine treatment are ineffective. However, 5-HT and enkephalin immunostaining of surrounding bulbar nuclei appears normal, implying that the neurotoxic effect of guanethidine is restricted to the AP. In addition, gamma-aminobutyric acid and neurotensin immunostaining within the AP appears normal, subsequent to guanethidine treatment, suggesting that the neurotoxic effect is restricted to specific AP cell populations.

B-adrenergic regulation of N-acetylserotonin (NAS) synthesis in the rat cerebellum

The synthesis of N-acetylserotonin (NAS) in the pineal gland is dependent upon the activity of the enzymes tryptophan-hydroxylase, 1-aromatic amino acid decarboxylase and N-acetyltransferase. Pineal N-acetyltransferase activity is regulated by the level of B-adrenergic activation. N-acetylserotonin (NAS) has also been identified in extra-pineal brain tissue. In order to investigate whether extra-pineal brain NAS levels are regulated by tryptophan hydroxylase and B-adrenergic activity, the effects of tryptophan hydroxylase inhibitors (parachlorophenylalanine and 6-fluoro-tryptophan) and adrenergic drugs (l-isoproterenol and propranolol) were examined. NAS was evaluated in the cerebellum of the rat using quantitative NAS-immunohistochemistry. A significant decrease in NAS-immunofluorescence was observed after tryptophan hydroxylase inhibition. Treatment with l-isoproterenol, a B-adrenergic agonist, resulted in a significant increase in NAS-immunofluorescence intensity. This effect was blocked by propranolol, a B-adrenergic blocking agent. These data indicate that the synthesis of NAS, in the cerebellum utilizes the established serotonin pathway and that NAS synthesis in the cerebellum is regulated by a B-adrenergic mechanism similar to that in the pineal gland.

Isthmic origin of neurons of the rat substantia nigra

This study provides new data on the time of origin, the generation site and the migration route of the young neurons of the substantia nigra of the rat during embryogenesis. The neurons of the substantia nigra are generated on day 12, 13, 14 and 15 of gestation. They settle following a light spatiotemporal rostrocaudal gradient from day 12 to 15. The neurons of the substantia nigra are generated at two different points of the basal plate at the level of the fovea isthmi (meso-isthmic junction) and migrate in radial pattern as two definite streams toward the ventral mesencephalon. From this point they move rostralwards along the surface towards their final site. The main findings of this work are the disclosure that the neurons of the substantia nigra are generated in the region of the isthmus rhombencephali and that its cells do not migrate between existing cells of the mesencephalic tegmentum but first migrate ventralwards in a radial pattern and then rostrally towards their definite site. Numerous neurons of the basal mesencephalon and of the midline structures of the caudal mesencephalon are apparently derived from the region of the fovea isthmi.

Distribution of neuropeptides in the limbic system of the rat: the bed nucleus of the stria terminalis, septum and preoptic area

The distribution of the neuropeptides vasoactive intestinal polypeptide, cholecystokinin octapeptide, substance P, neurotensin, methionine-enkephalin and somatostatin has been mapped immunocytochemically in the bed nucleus of the stria terminalis, one of the major sites of termination for efferent projections from the amygdala. Immunoreactive fibres and terminals were distributed more or less topographically and largely in accordance with the previously described localization of peptide-containing cell bodies in the amygdala and the amygdaloid projection fields in the bed nucleus as described by neuroanatomical techniques. Thus, vasoactive intestinal polypeptide, which was found in some of the lateral amygdaloid nuclei, had a substantial projection to the lateral bed nucleus. The lateral bed nucleus also contained cholecystokinin-octapeptide, substance P, neurotensin and methionine-enkephalin immunoreactivity which probably derived from the central amygdaloid nucleus, whilst cholecystokinin-octapeptide, and especially substance P-containing fibres, were found in the medial bed nucleus and probably arise from cells in the medial amygdala. Reciprocal amygdalopetal projections were suggested by the presence of substance P- and somatostatin-containing cell bodies in the mediodorsal bed nucleus and vasoactive intestinal polypeptide cells in the lateral bed nucleus, but somatostatin otherwise had a widespread distribution. Numerous local peptidergic connections were also noted both within the bed nucleus and between it and adjacent structures, especially the preoptic area, hypothalamus and the basal ganglia. These data provide further evidence that neuropeptides play a major role in the connectivity of the limbic system and show that the bed nucleus of the stria terminalis is an important relay station, particularly between amygdaloid efferents and other forebrain areas.

Brain serotonergic activity and plasma amino acid levels in genetically obese Zucker rats

In order to test the hypothesis that serotonergic activity is abnormal in the brains of genetically obese Zucker rats, levels of serotonin (5-HT); its amino acid precursor, tryptophan (Trp), and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were measured in eight brain regions in groups of obese and non-obese male rats. Plasma albumin levels as well as levels of amino acids and related compounds in plasma and in a cortical sample were also determined in the same animals. While Trp was lower in several brain regions of the obese animals, the only region showing a depressed level of 5-HT in the obese group was the mesencephalon. Obese animals also had a lower amount of 5-HIAA in the diencephalon, but no other differences were significant. Both elevations and depressions were observed in cortical amino acid levels in obese animals. The level of plasma albumin was increased in the obese group. Free Trp was decreased in the plasma of obese rats while levels of other amino acids (methionine, leucine, isoleucine, valine and phenylalanine) which compete with Trp for transport across the blood-brain barrier were elevated. Thus the combination of lower plasma free Trp and increased levels of competitive amino acids appears to contribute to decreased levels of Trp in the brain of genetically obese rats.

A microassay for simultaneous measurement of in vivo rates of tryptophan hydroxylation and levels of serotonin in discrete brain nuclei

A sensitive procedure for simultaneous determination of in vivo rates of tryptophan hydroxylation and levels of serotonin (5-HT) in discrete rat brain nuclei is described. Rates of tryptophan hydroxylation are estimated by 5-hydroxytryptophan (5-HTP) accumulation following l-aromatic amino acid decarboxylase inhibition by R04-4602/1. 5-HTP is separated from 5-HT by liquid cation exchange after which both 5-HTP and 5-HT are measured by a sensitive radioenzymatic assay. Following decarboxylase inhibition, 5-HTP accumulates over 30 min in 6 brain nuclei examined, with negligible levels of 5-HTP being measured in the absence of decarboxylase inhibition. 5-HT levels do not change significantly up to 45 min after decarboxylase inhibition. Comparison of rates of tryptophan hydroxylation determined in 12 different microdissected rat brain areas reveals a greater rate of 5-HT biosynthetic activity in raphe nuclei containing 5-HT cell bodies than in nuclei containing 5-HT terminals. Pretreatment with para-chlorophenylalanine markedly reduces both 5-HTP accumulation and 5-HT levels in the nucleus raphe dorsalis. With this procedure quantities as little as 10 pg of both 5-HTP and 5-HT can be measured, allowing estimation of in vivo serotonin biosynthesis in microgram quantities of brain tissue.

Identification of serotonin and non-serotonin-containing neurons of the mid-brain raphe projecting to the entorhinal area and the hippocampal formation. A combined immunohistochemical and fluorescent retrograde tracing study in the rat brain

We have studied the localization of serotonin- and non-serotonin-containing cell bodies in the midbrain raphe nuclei that project to the entorhinal area and the hippocampal formation in the rat brain, using the technique of combined retrograde fluorescent tracing and immunohistochemistry on the same tissue section. The branching properties of these neurons were studied by retrograde double labelling using two fluorochromes which emit fluorescence with different spectral characteristics. After injections of granular blue or propidium iodide into the medial entorhinal area, retrogradely-labelled cells were found situated bilaterally in the caudal half of the dorsal raphe nucleus, the medial part of the median raphe and throughout the rostrocaudal extension of the nucleus reticularis tegmentipontis. Injections placed successively more laterally in the entorhinal area labelled progressively less cells contralaterally in the dorsal raphe and the reticular tegmental nucleus of the pons. After fluorochrome injections into the dorsal part of the hippocampal formation, retrogradely-labelled cells were found in the caudal part of the dorsal raphe, in the peripheral part of the median raphe and to a minor extent in the medial part of this nucleus, but not in the nucleus reticularis tegmentipontis. The experiments with double retrograde fluorescent tracing showed that the raphe nuclei do not send bilateral projections to the entorhinal area in spite of the fact that many of these cells are located contralateral to the injected hemisphere in single labelling experiments. Injections of the fluorochromes into the entorhinal area and hippocampal formation showed that at least 10% of the raphe cells project to both areas simultaneously. Analysis of sections incubated with antiserum to serotonin showed that a majority of the retrogradely-labelled versus serotonin-immunoreactive cells was found to vary within different parts of the individual raphe nuclei: the ventromedial part of the dorsal, the medial part of the median and the nucleus reticularis tegmentipontis being the highest. The findings indicate that both serotonin- and non-serotonin-containing neurons in the raphe innervate the hippocampal region, that these projections may be crossed but not bilateral, and that the same neuron in the raphe may influence the neural activity in the entorhinal area and the hippocampus simultaneously.

Alterations in tyrosine hydroxylase activity elicited by raphe nuclei lesions in the rat locus coeruleus: evidence for the involvement of serotonin afferents

The time course of the variations in tyrosine hydroxylase (TH) activity was measured in the rat locus coeruleus (LC) after lesions of the nucleus raphe dorsalis (NRD), nucleus raphe centralis superior (NRCS) and nucleus raphe pontis (NRP). A certain number of lesions were performed in the raphe magnus (RM), the caudal and rostral NRP and the caudal and rostral NRCS, lateral to raphe nuclei and in adrenalectomized animals. The serotonin (5-HT) content in the LC was also determined after these lesions. Only raphe nuclei producing significant decreases in the 5-HT content in the LC are successful in provoking increases in the TH activity in the LC, thus these results suggest that the noradrenaline (NA) synthesis in the LC may be regulated by 5-HT afferents. Moreover, intraventricular injections of 5,6-dihydroxytryptamine (5,6-DHT) and administration of parachlorophenylalanine (PCPA) also produce significant increases in TH in the LC. After immunotitrations of TH in the LC it was shown that, with exception of a high dose of 5,6-DHT (75 micrograms), all these treatments provoke an increase in the concentration of the enzyme. It therefore seems that one of the functional roles of 5-HT in the LC could be the regulation of the concentration of TH.

Serotonergic innervation of the lateral hypothalamus: evidence from synaptosomal uptake studies

Uptake of tritiated serotonin by synaptosomes prepared from rat lateral hypothalamus was examined. Uptake of serotonin into lateral hypothalamic synaptosomes occurred by both saturable and non-saturable processes. The saturable process was a high affinity transport with kinetic parameters that agree closely with those previously reported for serotonin uptake into synaptosomes prepared from other brain regions. Fluoxetine, a selective inhibitor of uptake into serotonergic neurons, was a potent inhibitor of serotonin uptake into lateral hypothalamic synaptosomes. Desipramine and benztropine, noradrenergic and dopaminergic uptake inhibitors respectively, were much less effective. Damage to the ascending serotonergic system, by either electrolytic lesion of the dorsal or median raphe nucleus, or by 5,7 dihydroxytryptamine injections into the midbrain serotonergic pathways, significantly reduced the uptake of serotonin by lateral hypothalamic synaptosomes. Taken together, these data provide further evidence for a serotonergic terminal field within the lateral hypothalamus.

The distribution and orientation of serotonin fibers in the entorhinal and other retrohippocampal areas. An immunohistochemical study with anti-serotonin antibodies in the rats brain

The serotonin (5-hydroxytryptamine; 5-HT) innervation of the retrohippocampal region (subiculum, pre- and parasubiculum, area 29e, medial and lateral entorhinal area) in the rat brain has been examined with antibodies against 5-HT used in combination with fluorescence histochemistry. Analysis of consecutive sections cut in the coronal, sagittal, and horizontal planes revealed a widespread distribution of 5-HT immunoreactive fibers throughout the retrohippocampal region. This innervation was with regard to the morphological characteristics of the 5-HT fibers, their density and their spatial orientation. On the basis of morphological criteria, four different types of 5-HT positive processes were distinguished: (a) fine, convoluted fibers with small (approximately 0.5-0.8 micron), round and evenly spaced varicosities: (b) fine fibers with small elongated, irregularly distributed varicosities; (c) thick, possibly myelinated fibers, and (d) a terminal plexus with large (5-10 micron), irregularly spaced varicosities. Analysis of the laminar distribution of the 5-HT fibers showed that whereas all layers contain 5-HT positive fibers, the molecular layer was the most densely innervated. The 5-HT fibers were found to be oriented both parallel and transverse to the longitudinal axis of medial and lateral entorhinal area. This grid-like arrangement was less pronounced in the presubiculum. Although the 5-HT innervation of the retrohippocampal region was found to be dominated by a widespread and apparently diffuse pattern, several areas contained dense clusters of preterminal 5-HT processes: area 29e, dorsal presubiculum (layer II), lateral entorhinal area (layer III and ventral layer II) and the transitional zone of the ventral entorhinal area. The 5-HT fibers were found to enter the retrohippocampal region primarily by three different routes; from the ventral and dorsal aspects and from the piriform and lateral neocortex (via the perirhinal area). Most of the fibers enter the region by the ventral route and these were found to ascend in all layers but predominantly in layer I. The location of the 5-HT cells giving rise to the innervation of the entorhinal area was studied by combining retrograde transport of fluorescent tracers with immunohistochemistry on the same tissue section. Both ipsi- and contralaterally located cells in the dorsal and median raphe nuclei were found to project to the entorhinal area. Most, but not all, of these retrogradely labeled cell bodies also contain 5-HT immunoreactivity.

Characterization and radioautography of [3H]LSD binding by rat brain slices in vitro: the effect of 5-hydroxytryptamine

Binding of D-[3H]lysergic acid diethylamide (LSD) to rat coronal brain slices and its blockade by 5-hydroxytryptamine (5-HT) had characteristics similar to those of brain homogenates in respect of KD, kinetics and reversibility of binding. Radioautography was done on slices that had been incubated in 6 nM [3H] LSD and on adjacent slices incubated in the same concentration of tritiated LSD plus 10(-5) M of 5-HT. Choroid plexus showed densest labeling of [3H] LSD. In neuropil, dense labeling occurred within parts of the hippocampal formation except for fields CA2 and CA3 which were sparsely labeled. All layers of the cortex except the posterior cingulate gyrus were labeled by LSD. 5-HT blocked labeling of choroid plexus, hippocampal formation, septum, pons, medulla and parts of cortex but only reduced labeling of most other structures. LSD binding sites may relate to some of its pharmacological effects.

Immunohistochemical localization of serotonin nerve terminals in the lateral entorhinal cortex of the rat: demonstration of two separate patterns of innervation from the midbrain raphe

The distribution of serotonin (5-hydroxytryptamine, 5HT) containing nerve terminals in the lateral entorhinal cortex (LEC) has been studied using antibodies against 5-HT in combination with fluorescence histochemistry. Thin, varicose, branching fibers were found to be distributed in a relatively even, diffuse pattern throughout all layers of the LEC. The largest amount of this type of 5HT innervation was in Layer I. This diffuse pattern of 5-HT terminals was supplemented by a dense network of 5HT terminals restricted to Layer III of a small (approximately 1 mm) strip of the LEC. The fibers in this layer were thicker and more convoluted and contained larger varicosities than fibers in any other layer. The existence of a distinct innervation by 5-HT terminals of only a small portion of the LEC demonstrates a hitherto unrecognized and important principle of heterogeneity in 5-HT innervation of cortex. It suggests that 5-5HT neurons in the raphe can selectively influence specific, narrow regions of the lateral perforant path system, which, in turn, affect the intrinsic hippocampal circuits.

Genetic studies of daily variations of first-step enzymes of monoamines metabolism in the brain of inbred strains of mice and hybrids. I. Daily variations of tryptophan hydroxylase activity in the nuclei raphe dorsalis, raphe centralis and in the striatum

A daily variation of tryptophan hydroxylase (TrH) activity was observed in the raphe dorsalis (RD), raphe centralis (RC) and striatum (St) of 3 inbred strains of mice (BALB/c, C57BL/6, C57BR) and of the reciprocal hybrids obtained from Balb/c and C57BL6. Significant differences of the characteristics of these rhythms have been found in the same strain between different structures and for the same structure between different strains. In RD and RC hybridization led to less defined daily variations which, in the striatum, remained well synchronized and could be controlled by a dominant genetic mechanism part. These results help to discuss evidence for selective control mechanisms of regulation responsible for daily variation of TrH in the 5-HT cell bodies and terminals and their relative independence.

Afferent projections to the ventral tegmental area of Tsai and interfascicular nucleus: a horseradish peroxidase study in the rat

Using the retrograde transport of horseradish peroxidase (HRP), a study has been made of projections to the ventral tegmental area of Tsai (VTA) and related dopaminergic cell groups (A 10). In order to minimise the possibility of damage to fibres of passage, a technique was evolved for the microiontophoresis of HRP such that minimal current strengths and durations were applied. In addition to a sham injection, control injections were also made to the medial lemnisuc, red nucleus, deep tegmental decussations, mesencephalic reticular formation and brachium conjunctivum. Following HRP injections confined to the areas of the VTA containing the dopamine cell groups, labelled neurons appeared in prefrontal cortex, dorsal bank of rhinal sulcus, nucleus accumbens, bed nucleus of stria terminalis, amygdala, diagonal band of Broca, substantis innominata, magnocellular preoptic area, medial and lateral preoptic areas, anterior, lateral and postero-dorsal hypothalamus, lateral habenular, nucleus parafascicular nucleus of thalamus, superior colliculus, nucleus raphe dorsalis, nucleus raphe nagnus and pontis, dorsal and ventral parabrachial nuclei, locus coeruleus and deep cerebellar nuclei. Regions containing catecholamine groups A 1, A 5, A 6, A 7, A 9, A 13 and the serotonin group B 7 corresponded to the topography of labeled cell groups. Injections of HRP to the interfascicular nucleus resulted in labeling predominantly confined to the medial habenular and median raphe nuclei. The results are discussed in relation to the known connections of these regions. Other regions of the brain labelled by VTA injections are assessed in relation to control injections and the limitations of the HRP technique. A review of the organisation of some of these afferents in relation to the known cortical-subcortical-mesencephalic projection systems, suggests that the VTA is in a position to recieve information from a massively convergent system derived ultimately from the entire archi-, paleo-, and neo-cerebral cortices. In addition A 10 dopaminergic neurons are known to project to restricted regions of both pre-frontal and entorhinal cortices, which themselves also recieve massively convergent association cortico-cortical connections. It would appear reasonable to propose that these neurons perform a correspondingly important integrative function.

Investigation of possible interactions between substance P and transmitter mechanisms in the substantia nigra and corpus striatum of the rat

The effect of substnace P (SP) on the uptake and release of radiolabelled dopamine (3H-DA), 5-hydroxytryptamine (3H-5-HT) and y-aminobutyric acid (3H-GABA) was studied in slices of rat substantia nigra and corpus triatum. SP, 10(-9) to 10(-5) m, failed to modify the uptakes of these compounds during incubations (10-90 min) with slices of either brain region. SP, 10(-6)M, had no apparent effect on the spontaneous output of any of these compounds in either substantia nigra or corpus striatum. In the corpus striatum, SP seemed to potentiate the potassium-stimulated outflow of 3H-DA and 3H-5-HT, but not 3H-GABA, while the realeases from substantia nigra were unaffected. Morphine (10(-3)M), but not met-enkephalin (5 X 10(-6)M), weakly antagonized K+- EVOKED RELEASE OF 3/-DA in the corpus striatum. These results are discussed with reference to the possible interaction of SP with transmitter mechanisms at presynaptic sites in the central nervous system.

Evidence concerning the anatomical location of the dopamine stimulated adenylate cyclase in the substantia nigra

The dopamine (DA)-sensitive adenylate cyclase in the substantia nigra was assayed in rats which had been subjected to 3 different kinds of brain lesion: (1) unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle; (2) unilateral lesions of the descending strio-nigral and pallido-nigral projections; (3) total lesions of the serotoninergic raphe-nigral pathway. Lesions of the medial forebrain bundle causing 97% depletion of striatal DA, 72% depletion of nigral tyrosine hydroxylase, and no change in nigral glutamate decarboxylase (GAD), resulted in no change in basal or DA-stimulated cyclic AMP production ipsilateral to the injection. Lesions of the globus pallidus, causing 70% and 79% reductions in GAD and substance P respectively in the ipsilateral nigra, produced a reduction in basal cyclic AMP production and abolished the normal increase in cyclic AMP produced by DA on the side of the lesion. Lesions to the dorsal and median raphe nuclei did not affect the normal DA-sensitive adenylate cyclase response in the nigra. The results suggest that one of the neurotransmitter functions of DA in this brain region may be to modulate the release of psi-aminobutyric acid (GABA) or substance P from synaptic terminals afferent to the nigra.

Daily variations of various parameters of serotonin metabolism in the rat brain. I. Circadian variations of tryptophan-5-hydroxylase in the raphe nuclei and the striatum

Daily changes in tryptophan-5-hydroxylase (TrH) activity have been studied in six different 5-HT-containing cell groups (B3, B4, B5, B6, B7, B8,) in the brain stem of rats maintained in a regular cycle of 12 h of light and 12 h of darkness. Significant circadian alternations of TrH activity were observed in most of the raphe nuclei tested, although the changes were not identical from one structure to another. The striatum showed daily variations in TrH activity in opposite phase to the nucleus raphe dorsalis which projects specific terminals into this area. Monoamine oxidase (MAO) activity was simultaneously estimated in these nuclei but did not exhibit and significant rhythm, suggesting a specific regulation of TrH. Total protein levels were also subject to daily changes.

Inhibiton of neurons in the amygdala by dorsal raphe stimulation: mediation through a direct serotonergic pathway

This study presents data showing that the dorsal raphe nucleus (DRN) has a marked inhibitory influence upon neurons in the amygdala and that this inhibitory effect is mediated by a direct DRN-amygdala serotonergic pathway. The evidence may be briefly summarized as follows:(1) on the same amygdaloid cells, both iontophoresis of serotonin (5-HT) and electrical stimulation of the DRN markedly inhibited spontaneous single unit activities; (2) the latency of DRN-induced inhibition was relatively short and is compatible with the conduction velocities (which were determined by antidromic activation of the 5-HT pathway) of unmyelinated 5-HT fibers; (3) destruction of 5-HT projections by 5,7-dihydroxytryptamine (5,7-DHT) or pharmacological depletion of 5-HT by parachlorophenylalanine (PCPA) prevented the inhibitory responsed to DRN stimulation in the great majority of cells studied; (4) in PCPA-pretreated animals, injection of 5-hydroxytryptophan (5-HTP) reversed the PCPA effect, restoring the responses of amygdaloid cells to DRN stimulation. In the amygdala, the presumptive 5-HT antagonists which we tested did not block the inhibitory effects of 5-HT except that intravenously administered LSD blocked the inhibitory responses produced by submaximal DRN stimulation. The implications of these results for the possible functions of 5-HT in the amygdala is discussed.

Evidence for the existence of a raphe projection to the substantia nigra in rat

The effect of electrical stimulation of the median raphe nucleus on the activity of spontaneously firing single neurones in the substantia nigra and mesencephalic reticular formation (MRF) has been investigated in urethane anaesthetized rats. Depression of activity was the predominant effect observed although this was sometimes accompanied by periods of excitation. Some neurones were only excited. The latency of inhibition of substantia nigra neurones was constant whereas that of MRF neurones was more variable. Microiontophoretically applied 5-hydroxy-tryptamine (5-HT) and dopamine (DA) produced mainly inhibition of neuronal activity, but excitation and biphasic effects were also seen. There was a good correlation between the direction of neuronal responses in the substantia nigra to median raphe stimulation and to the effects of 5-HT but not DA. Discrete electrolytic lesions of the median raphe nucleus were followed by a decrease in 5-HT but not GABA concentrations in the substantia nigra. In addition striatal 5-HT, GABA and NA concentrations were unchanged whereas striatal DA was increased. These observations strongly suggest that the substantia nigra receives a direct inhibitory input from the median raphe nucleus and this pathway uses a 5-HT-like neurotransmitter. This pathway probably contributes to the regulation of nigrostriatal dopaminergic transmission.