5HT Innervation Research Articles

Chronic-Stress-Induced Behavioral Changes Associated with Subregion-Selective Serotonin Cell Death in the Dorsal Raphe.

The current study examined the neurochemical mechanisms and neuroanatomical changes underlying coexisting behavioral effects associated with chronic-stress-induced alterations in serotonin (5HT) neurons. Chronic unpredictable stress (CUS) to adult male rats produced depression-like changes with cognitive dysfunction and selective cell death in the interfascicular nucleus of the dorsal raphe (DRif), resulting in decreased 5HTergic innervation of medial prefrontal cortex (mPFC). Twenty-one days of CUS decreased basal plasma levels of corticosterone and produced a shorter latency to immobility and longer durations of immobility in the force-swim test that persisted for 1 month after CUS. Deficits in acquisition, recall, perseveration, and reversal learning were evident 1 month after CUS. MK801 treatment during CUS blocked the changes in the forced-swim test and deficits in memory recall. These behavioral changes were associated with terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive soma and the eventual loss of 5HT neurons in the DRif and its projections to the mPFC as evidenced by fewer labeled cells in the DRif after retrograde tracer injections into the mPFC of stressed rats. Similar to the effects of MK801 on behavior, MK801 pretreatment during stress blocked the CUS-induced decreases in 5HT soma within the DRif and its projections to the mPFC. Finally, the depression-like behaviors were blocked by acute injection of the 5HT2A/C agonist (-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride into the mPFC before forced-swim testing. These results identify a cause and mechanism of 5HTergic dysfunction of the mPFC and associated mood and cognitive behaviors.SIGNIFICANCE STATEMENT Chronic stress causes persistent mood and cognitive changes typically associated with dysregulated serotonin (5HT) transmission in the medial prefrontal cortex (mPFC), but the cause of this dysregulation is unknown. Prior studies have focused on 5HTergic terminals in this region, but this study shows that chronic stress causes NMDA-receptor-dependent and subregion-specific cell death of 5HT neurons in the dorsal raphe. The consequent decreased 5HT innervation of the mPFC was associated with mood and cognitive changes that persisted long after the termination of stress. These findings identify a mechanism of subregion-selective death of 5HT neurons in the dorsal raphe, a defined neuroanatomical pathway, and a behavioral phenotype that mirror stress-associated diseases such as major depressive disorder.

The neuronal basis of copper induced modulation of anxiety state in rat

Recently, studies have provided strong evidence indicating the involvement of trace elements in the physiopathology of psychiatric disorders, particularly anxiety. We aimed, through the present study, to describe the effect of acute exposure to Cu (10mg/kg BW) on anxiety state together with the serotoninergic and dopaminergic systems in rat by means of neurobehavioral tests (elevated plus maze, dark light box) and immunohistochemistry using anti-serotonin (5HT) and anti-tyrosine hydroxylase (TH). Our data report that Cu enhanced 5HT innervation in the dorsal raphe nucleus (DRN) together with a loss of TH expression within the ventral tegmental area (VTA), Substantia nigra compacta (SNc) and their subsequent outputs including the medial forebrain bundle (MFB) and striatum. In the elevated plus maze Cu significantly increased the time and the number of entries into the open arms, and raised the time spent in the Dark Box indicating a clear reduced anxiety state induced by Cu. The present data show for the first time a powerful neuro-modulatory potential of Cu in rat which involves primarily a dysfunction of 5HT and DA neurotransmissions.

Serotonin modulates anxiety-like behaviors during withdrawal from adolescent anabolic–androgenic steroid exposure in Syrian hamsters

From the U.S. to Europe and Australia anabolic steroid abuse remains high in the adolescent population. This is concerning given that anabolic steroid use is associated with a higher incidence of pathological anxiety that often appears during withdrawal from use. This study uses pubertal Syrian hamsters (Mesocricetus auratus) to investigate the hypothesis that adolescent anabolic/androgenic steroid (AAS) exposure predisposes hamsters to heightened levels of anxiety during AAS withdrawal that is modulated by serotonin (5HT) neural signaling. In the first two sets of experiments, adolescent AAS-treated hamsters were tested for anxiety 21days after the cessation of AAS administration (i.e., during AAS withdrawal) using the elevated plus maze (EPM), dark/light (DL), and seed finding (SF) tests and then examined for differences in 5HT afferent innervation to select areas of the brain important for anxiety. In the EPM and DL tests, adolescent AAS exposure leads to significant increases in anxiety-like response during AAS withdrawal. AAS-treated hamsters showed long-term reductions in 5HT innervation within several areas of the hamster brain implicated in anxiety, most notably the anterior hypothalamus and the central and medial amygdala. However, no differences in 5HT were found in other anxiety areas, e.g., frontal cortex and lateral septum. In the last experiment, adolescent AAS-treated hamsters were scored for anxiety on the 21st day of AAS withdrawal following the systemic administration of saline or one of three doses of fluoxetine, a selective serotonin reuptake inhibitor. Saline-treated hamsters showed high levels of AAS withdrawal-induced anxiety, while treatment with fluoxetine reduced AAS withdrawal-induced anxiety. These findings indicate that early AAS exposure has potent anxiogenic effects during AAS withdrawal that are modulated, in part, by 5HT signaling.

The rat SCO responsiveness to prolonged water deprivation: Implication of Reissner's fiber and serotonin system

The osmotic stress is a potent stimulus that can trigger several peripheral as well as central impairments. The brain is a vulnerable target of the osmotic stress and particularly circumventricular organs (CVOs) regarding their strategic localization as sensory organs of biochemical changes in the blood and cerebrospinal fluid circulations. The subcommissural organ (SCO) is a CVO which releases doubly in the CSF and blood circulation a glycoprotein called Reissner's fiber (RF) that has been associated to several functions including electrolyte and water balances. The present work was aimed on the assessment of the secretory activity of the SCO and its serotoninergic innervation following 2weeks of total water restriction in Wistar rat. Using the immunohistochemistry of RF and serotonin (5HT), our data showed a significant overall reduction of RF immunoreactivity within both ependymal and hypendymal cells of the SCO of dehydrated rats compared to their corresponding controls, this decrease was concomitant with an enhancement of fibers 5HT immunoreactivity in the SCO as well as in the classical ependyma and in the dorsal raphe nucleus (DRN), constituting the origin of this innervation. The present findings support the possible involvement of the SCO in the response to prolonged water deprivation by decreasing its secretory materials which may result from either a direct peripheral hormonal control and/or the consequence of the enhanced 5HT innervation of the SCO.

MDMA administration decreases serotonin but not N-acetylaspartate in the rat brain

In animals, repeated administration of 3,4-methylenedioxymethamphetamine (MDMA) reduces markers of serotonergic activity and studies show similar serotonergic deficits in human MDMA users. Using proton-magnetic resonance spectroscopy ( 1H-MRS) at 11.7 Tesla, we measured the metabolic neurochemical profile in intact, discrete tissue punches taken from prefrontal cortex, anterior striatum, and hippocampus of rats administered MDMA (5 mg/kg IP, 4× q 2 h) or saline and euthanized 7 days after the last injection. Monoamine content was measured with HPLC in contralateral punches from striatum and hippocampus to compare the MDMA-induced loss of 5HT innervation with constituents in the 1H-MRS profile. When assessed 7 days after the last MDMA injection, levels of hippocampal and striatal serotonin (5HT) were significantly reduced, consistent with published animal studies. N-Acetylaspartate (NAA) levels were significantly increased in prefrontal cortex and not affected in anterior striatum or hippocampus; myo-inositol (INS) levels were increased in prefrontal cortex and hippocampus but not anterior striatum. Glutamate levels were increased in prefrontal cortex and decreased in hippocampus, while GABA levels were decreased only in hippocampus. The data suggest that NAA may not reliably reflect MDMA-induced 5HT neurotoxicity. However, the collective pattern of changes in 5HT, INS, glutamate and GABA is consistent with persistent hippocampal neuroadaptations caused by MDMA.

Immunohistochemical evaluation of the effect of lead exposure on subcommissural organ innervation and secretion in Shaw's Jird ( Meriones shawi)

The secretory activity of subcommissural organ cells is controlled by various extrinsic and intrinsic factors. Lead has been recognised as a neurotoxic heavy metal, since it induces morphological and functional abnormalities in the brain. In this work, we examined the effect of lead exposure on the subcommissural organ (SCO), a brain gland known by its secretion of Reissner's fiber (RF) in cerebro-spinal fluid. Glycoprotein secretion and serotonin (5HT) innervation of the SCO was examined after acute and chronic lead exposures in the sub-desert rodent Meriones shawi. Lead exposures were achieved by, respectively, intra-peritoneal injection of 25 mg/kg body weight of lead acetate for 3 days and 0.5% of lead acetate in the drinking water over 4 months until adult age. 5HT and RF immunolabeling in the SCO revealed several serotoninergic fibers reaching the SCO and abundant secretory material. An increase in both 5HT innervation and secretory material of the SCO was recorded after both acute and chronic lead exposure. These results show that lead exposure affects the serotonergic innervation of the SCO. Moreover, the enhancement of SCO secretion suggests a role of this gland in neuroprotection and lead detoxification of the brain in Meriones shawi.

Fetal and adolescent nicotine administration: effects on CNS serotonergic systems

Nicotine is a neuroteratogen that targets synaptic function during critical developmental stages and recent studies indicate that CNS vulnerability extends into adolescence, the time that smoking typically commences. We administered nicotine to pregnant or adolescent rats via continuous minipump infusions, using dose rates that replicate the plasma nicotine levels found in smokers. Fetal nicotine exposure (gestational days 4–21) decreased the cerebrocortical binding of paroxetine (PXT), a marker for the serotonin (5HT) transporter, likely indicative of a decrease in nerve terminals in that region; the effect lasted into adulthood. There was a corresponding increase in PXT binding in the midbrain/brainstem, the region containing the 5HT cell bodies that project to the cerebral cortex, a pattern typical of reactive sprouting in response to nerve terminal damage. After adolescent nicotine treatment (postnatal days 30–47), PXT binding was reduced in the hippocampus and striatum instead of the cerebral cortex, again accompanied by increased binding in the midbrain and brainstem; the patterns of effects within each region were gender-selective, although both males and females displayed abnormalities. Superimposed on this overall effect, there were transient increases in PXT binding, likely due to acute stimulant effects of nicotine. We also assessed 5HT presynaptic activity (5HIAA/5HT ratio). Withdrawal from adolescent nicotine treatment led to suppression of activity in the cerebral cortex and activation in the midbrain. These results indicate that both fetal and adolescent nicotine exposure elicit apparent damage to 5HT projections with reactive increases in regions containing 5HT cell bodies. Long-term changes in 5HT innervation and/or synaptic activity may play a role in the subsequent development of depression in the offspring of women who smoke during pregnancy or in adolescent smokers.

Biogenic amine systems in the fruit fly Drosophila melanogaster.

Biogenic amines are important neuroactive molecules of the central nervous system (CNS) of several insect species. Serotonin (5HT), dopamine (DA), histamine (HA), and octopamine (OA) are the amines which have been extensively studied in Drosophila melanogaster. Each one of the four aminergic neuronal systems exhibits a stereotypic pattern of a small number of neurons that are widely distributed in the fly CNS. In this review, histochemical and immunocytochemical data on the distribution of the amine neurons in the larval and adult nervous system, are summarized. The majority of DA and 5HT neurons are interneurons, most of which are found in bilateral clusters. 5HT innervation is found in the feeding apparatus as well as in the endocrine organ of the larva, the ring gland. The octopaminergic neuronal population consists of both interneurons and efferent neurons. In the larval CNS all OA immunoreactive somata are localized in the midline of the ventral ganglion while in the adult CNS both unpaired neurons and bilateral clusters of immunoreactive cells are observed. One target of OA innervation is the abdominal muscles of the larval body wall where OA immunoreactivity is associated with the type II boutons in the axonal terminals. Histamine is mainly found in all photoreceptor cells where it is considered to be the major neurotransmitter molecule, and in specific mechanosensory neurons of the peripheral nervous system. Similarities between specific aminergic neurons and innervation sites in Drosophila and in other insect species are discussed. In addition, studies on the development and differentiation of 5HT and DA neurons are reviewed and data on the localization of 5HT, DA, and OA receptors are included as well. Finally, an overview on the isolation of the genes and the mutations in the amine biosynthetic pathways is presented and the implications of the molecular genetic approach in Drosophila are discussed.

Neonatal raphe lesions increase dopamine fibers in prefrontal cortex of adult rats.

In order to characterize how the dopamine (DA) and serotonin (5HT) systems may be interacting in medial prefrontal cortex (mPFC) during postnatal development, the specific toxin 5,7-dihydroxytryptamine (5,7-DHT) was used to induce lesions of the nucleus raphe dorsalis (NRD) in neonatal rats and the density of tyrosine hydroxylase-immunoreactive varicosities (TH-IRv) was assessed. During the early adult period, lesioned rats showed a significant increase in the density of the TH-IR fibers in layers V and VI when compared with sham-treated animals. These results suggest that postnatal development in medial prefrontal cortex may be associated with a competitive interaction between cortical monoaminergic systems, such that an early disturbance in the development of the 5HT innervation can potentially induce a hyperinnervation of DA fibres.

Neural and astroglial effects of a chronic parachlorophenylalanine-induced serotonin synthesis inhibition.

Serotonin (5HT) is one of the classical neurotransmitters expressed earlier in the embryonic rat brain, and it was proposed as a developmental signal in the central nervous system. In the adult brain, 5HT seems to be involved in neuronal plasticity. It was postulated that S-100 protein, a glial neurotrophic factor, could be modulated by 5HT probably through the glial 5HT1A receptors. In a model of chronic inhibition of endogenous 5HT synthesis produced by the daily administration of parachlorophenylalanine (PCPA) for 2 wk, we have studied by immunohistochemical methods and digital morphometric analysis the expression of two proteins present in rat brain astrocytes: glial fibrillary acidic protein (GFAP) and S-100 protein. The effectiveness of the PCPA treatment was tested by the use of specific anti-5HT antibodies that showed absence of 5HT fibers in 5HT innervation areas like frontal cortex and hippocampus. Different effects of PCPA treatment on serotoninergic raphe nuclei were observed: dorsal raphe nucleus (DRN) seemed to be more sensitive to the PCPA's action than ventral raphe nucleus (VRN). In DRN and in the two 5HT innervation areas studied, glial cells responded to the 5HT depletion induced by PCPA showing astrocytes with large and tortuous processes. Astrocytes from 5HT-depleted regions showed higher immunostaining for S-100 protein than controls. There was not any modification in optical density of S-100 protein immunostaining in VRN, the area less sensitive to PCPA treatment. These observations indicated that astrocytes are sensitive to the 5HT level, and in presence of low 5HT concentration in the intercellular space, astrocytes could react by synthesizing glial proteins like GFAP and S-100 protein.

Host Serotonin Axons Innervate Intrastriatal Ventral Mesencephalic Grafts After Implantation in Newborn Rats

This study investigated the potential of immature and adult serotonin (5-HT) neurons for axonal growth into intrastriatal grafts of ventral mesencephalic tissue. Implantation of dissociated fetal (embryonic days 14-15) ventral mesencephalic tissue was carried out in immature [postnatal days (P) 5-14] and adult rat neostriatum. The brains were processed 2-6 months later for dopamine and 5-HT immunocytochemistry. A few grafts implanted into adult and P7 recipients contained small numbers of cotransplanted 5-HT cell bodies. These also displayed a rich network of 5-HT axons, even in adult rats prelesioned with 5,7-dihydroxytryptamine, indicating the graft origin of these axons. All other grafts were totally devoid of 5-HT cell bodies. After implantation in adults, such grafts contained rare 5-HT axons. In contrast, in P5-P7 recipients, they displayed many 5-HT fibres, which were uniformly distributed. Such was no longer the case after implantation in P14 recipients, which showed minimal 5-HT innervation, as in adult recipients. Processing of naïve rat brain at different ages for 5-HT immunocytochemistry showed that 5-HT axons were still clearly less numerous in the neostriatum at P21 than in adults, whereas in the substantia nigra the 5-HT innervation developed more rapidly and was comparable, at P21, to that of adults. It was concluded that 5-HT axons are able to grow into ventral mesencephalic grafts, but mainly at the fetal stage and with decreasing capacity after birth.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmitter expression and morphological development of embryonic medullary and mesencephalic raphé neurones after transplantation to the adult rat central nervous system

Suspensions of cells taken from the mesencephalic or medullary raphé regions of the 13-14 day old embryonic rat brain were injected into the hippocampus of adult rats which had previously been denervated of its serotoninergic input by 5,7-dihydroxytryptamine. At periods of up to 14 months after implantation, the brains were taken for immunohistochemical analysis of 5-hydroxytryptamine (5HT)-, substance P (SP)- and thyrotropin releasing hormone (TRH)-like immunoreactivity. Surviving grafts were found in all animals. The implants derived from mesencephalic raphé contained neurones immunoreactive to 5HT, SP or both substances together. On average 19% of the potential number of mesencephalic 5HT neurones were found in the grafts. Outgrowth of 5HT-immunoreactive fibres was extensive, and displayed the typical pattern of 5HT innervation in the normal hippocampus-the densest plexuses were found in the dentate gyrus, with sparser networks in the CA1 and CA2 regions. SP-positive processes were principally found only in the graft itself. Transplants of medullary raphé cells contained 5HT-immunoreactive neurones, some of which also contained SP-and/or TRH-like immunoreactivity, thereby mirroring the situation found in the caudal raphé complex in situ. An average of 18% of the total available medullary serotoninergic neurones were found at each graft site. A rich outgrowth of 5HT-immunoreactive varicose processes was evident, with the same pattern as the 5HT innervation by the mesencephalic raphé grafts, and as in the normal hippocampus. SP- and TRH-positive fibres were essentially detectable only in the graft, but not in the host hippocampus. The present studies indicate that the milieu of the hippocampus does not preferentially attenuate the survival of the serotoninergic cells which do not normally project to it. Nor is the fibre outgrowth of these medullary raphé 5HT neurones significantly different from that of the mesencephalic raphé. However, the hippocampal environment may be responsible for the appearance of SP-LI in the otherwise apparently solely 5HT-containing mesencephalic raphé neurones, and for repressing the outgrowth of fibres containing TRH-, or SP-like immunoreactivity, regardless of their origin.

Transmitter expression and morphological development of embryonic medullary and mesencephalic raphé neurones after transplantation to the adult rat central nervous system

Implants have been made of dissociated embryonic mesencephalic or medullary raphé cells into the adult rat striatum, previously depleted of its 5HT innervation. The transmitter complement and fibre outgrowth of the grafted neurones were analysed immunocytochemically. Serotonin-containing cells were found in both types of transplant, and the proportionate survival of the potential number of implanted 5HT cells was similar for each type of graft. However, these proportions were both greater than that described previously in transplants of mesencephalic raphé cells to the spinal cord. In addition, the proportionate survival of medullary substance P neurones grafted to the striatum was greater than that of medullary 5HT cells implanted in the same region. The transmitter complement of the medullary neurones was largely unaltered after transplantation. However, the mesencephalic grafts contained neurones storing 5HT- and/or substance P-, or TRH-like immunoreactivity. The 5HT/substance P and TRH neurones have so far not been encountered in the mesencephalon in situ using the present immunohistochemical methodology. Invasion of the host striatum by 5HT processes from the transplanted mesencephalic cells was extensive. Fibres from medullary raphé neurones, however, were restricted principally to within the graft itself. It is concluded that there may exist in the adult rat striatum a set of trophic factors for 5HT and substance P neurones different from those found in other regions of the central nervous system, such as spinal cord. Moreover, trophic agents in the host striatum appear to operate differentially on mesencephalic and medullary raphé 5HT neurones to regulate their axonal outgrowth. Lastly, the neurotransmitter phenotypic expression of the embryonic mesencephalic raphé cells may be susceptible to influences from the host environment.

Chapter 5 Spinal dorsal horn circuitry involved in the brain stem control of nociception

Publisher Summary This chapter discusses the spinal dorsal horn circuitry involved in the brain stem control of nociception. The role of several neurochemicals in the brain stem projections to dorsal horn sensory systems has been examined. The double immunofluorescence method provides information on the colocalization of neurochemicals within single neurons or axon terminals. The descending 5-hydroxytryptamine (5HT) modulation of dorsal horn neuronal activity exhibits a predominantly postsynaptic mechanism that occurs at a variety of sites. 5HT contacts are found on several different populations of long distance projection neurons and interneurons. There is no strong morphological support for 5HT presynaptic effects on primary afferent axons. The spinomesencephalic, spinothalamic and dorsal column postsynaptic (DCPS) neurons receive 5HT contacts, thus, 5HT innervation of projection pathways is a common feature of monoaminergic modulation of dorsal horn neural circuits. The different classes of projection neurons receiving 5HT inputs suggest that 5HT plays a role in modulating output systems having a diversity of function.

The serotonin-norepinephrine link hypothesis of affective disorders: receptor-receptor interactions in brain.

The morphological organization of central monoamine systems has suggested a functional linkage of noradrenergic and serotonergic neurons ever since Dahlstrom and Fuxe (1964) demonstrated, by means of a sensitive fluorescence method, the neuronal localization of norepinephrine (NE) and serotonin (5HT) and mapped NE and 5HT containing cell bodies and terminals in the central nervous system. These and more recent studies show that NE and 5HT neurons form monosynaptic pathways between the lower brain stem and the cerebral cortex (Dahlstrom and Fuxe, 1964; Anden et al., 1966; Moore and Bloom, 1979; Levitt and Moore, 1978; Morrison et al., 1982; Consolazione and Cuello, 1982). With the introduction of immunohistochemical techniques, it became evident that NE and 5HT containing neurons project to the entire neuraxis and, within the cerebral cortex, to all six cortical layers, though some topographical and species differences exist (Lindvall and Bjorklund, 1984; Levitt et al., 1984). Early work also indicated a high degree of catecholamine innervation of 5HT cell bodies in the raphe nuclei (Dahlstrom and Fuxe, 1964) and the existence of a 5HT innervation of NE cell bodies in the locus coeruleus (Pickel et al., 1975). The details of the monoaminergic pathways in the central nervous system have been authoritatively reviewed (Jacobowitz, 1978; Moore and Bloom, 1979; Consolazione and Cuello, 1982). As pointed out by Fuxe et al. (1978), there is little doubt that the 5HT and NE neurons are linked together neuroanatomically and that they influence one another at various points on the neural axis.

Serotonin turnover rate in raphe and cortex of mice infected with Venezuelan equine encephalomyelitis virus.

The turnover of serotonin (5HT) was determined in the raphe area and cortex of mice infected with Pixuna, a strain of intermediate virulence of Venezuelan equine encephalomyelitis virus (VEEV). NMRI‐mice, 24 days old, were inoculated intracerebrally (ic) with 300 LDS0 of the virus. The animals were sacrificed 4, 7, 15, 21, 30, and 60 days postinoculation. 5HT and 5‐hydrodryindolacetic acid (SHIAA) in raphe and cortex were determined by high performance liquid chromatography (HPLC) with electrochemical detection. Turnover rate of 5HT was determined by the administration of pargyline, p‐chlorophenylalanine, and probenecid. The content of 5HT or SHIAA and 5HT/5HIAA ratios were not significantly different in infected compared with control mice. However, a decrease of 5HT turnover rate, determined after pargyline treatment, was observed in the raphe and not in the cortex of infected mice at 4 and 7 days after the inoculation. The turnover rate/(5HT)0 in raphe is decreased in infected mice with signs of illness, suggesting a lower density of 5HT innervation in this brain area. The administration of p‐chlorophenylalanine and probenecid showed that the cortex is also affected, but the synthesis is less modified than metabolism or elimination. Cell bodies of 5HT neurons seem to be more susceptible than projections to infection by Pixuna strain of VEEV.

Postnatal development of serotonin nerve fibers in the somatosensory cortex of mice studied by immunohistochemistry.

Postnatal serotonin (5HT) innervation in the cerebral cortex of mice has been studied by 5HT immunohistochemistry. 5HT-like immunoreactive (5HT-LI) nerve fibers and terminals appeared to increase transiently, particularly in the somatosensory (Sm) cortex during early postnatal days. As pups grow, 5HT afferent inputs decreased rapidly to reach a similar pattern of distribution to that in adult animals. Since the transient increase was seen at a critical period (seventh postnatal day) for the differentiation of layer IV, it is suggested that increased 5HT concentrations might have an effect on thalamocortical inputs and/or cortical lamination of the developing brains.

Serotoninergic system and circadian rhythms of ACTH and corticosterone in rats.

The circadian rhythms of plasma ACTH and corticosterone and of locomotor activity were explored in chronically cannulated female rats, after elimination of serotoninergic (5HT) innervation of the SCN (suprachiasmatic nuclei) either by stereotaxic lesion of the median and dorsal midbrain raphe nuclei (RX) or by local injection of SCN with the neurotoxin 5,7-dihydroxytryptamine (5,7DHT). Completeness of 5HT denervation was checked on serial sections of the hypothalamus either by the Falk-Hillarp technique or by radioautography. Neither lesion eliminated the intrahypothalamic 5HT system, which, however does not take part in the 5HT innervation of the SCN. In both experimental series, the circadian rhythms of the three parameters investigated were maintained in unchanged phase relationships compared to the sham-lesioned controls, and with respect to the photoperiod (12 light-12 dark). However, the estimated amplitudes of the ACTH rhythms dropped by 43% (RX) to 47% (5,7DHT) and their mean levels by 44% (RX) to 60% (5,7DHT), whereas the corticosterone rhythm displayed normal amplitude and its mean level rose by 24% (RX) or 38% (5,7DHT). In regard to locomotor activity rhythm, the most noticeable alteration was a 25-55% increase in the light-phase activity of both experimental groups with a correlative increase in the L/D activity ratio. The essential role of 5HT innervation of the SCN therefore seems to be to facilitate circadian control of the ACTH rhythm.

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.