Antidromically identified serotonergic neurons in the rat midbrain raphe: evidence for collateral inhibition

In vivo electrochemical detection of extraneuronal 5-hydroxyindole acetic acid and norepinephrine in the dorsal raphe nucleus of urethane-anesthetized rats

In vivo electrochemical detection of endogenous neurotransmitters was done in the dorsal raphe nucleus of urethane-anesthetized male Sprague-Dawley rats. Stereotaxically implanted carbon paste electrodes were scanned using a cyclic voltammetry amplifier with semiderivative signal processing over the potential range--0.2 to +0.5 V at the rate of 10 mV/s. Two distinct peaks were observed at +0.15 V (peak 1) and +0.25 V (peak 2), respectively. Peak identification was assessed by comparing the oxidation potential observed in vivo with those observed in in vitro experiments using pure catecholamines, indoleamines and their metabolites as well as ascorbic acid. Further characterization of in vivo peaks was done by observing changes in electrochemical peaks as well as tissue neurotransmitter concentrations after pharmacological manipulations. p-Chlorophenylalanine, m-hydroxybenzylhydrazine (NSD-1015), pargyline, alpha-methyl-p-tyrosine and fusaric acid were administered in an effort to block catecholamine or serotonin synthesis or degradation. Results of these experiments revealed that peak 1 primarily represents extracellular norepinephrine, while peak 2 is primarily produced by extracellular 5-hydroxyindoleacetic acid (5-HIAA).

Effects of estrogen on the excitability of neurons projecting from the noradrenergic A1 region to the preoptic and anterior hypothalamic area

Estradiol benzoate administered to ovariectomized female rats significantly elevated the mean spontaneous firing rate and frequency of successful antidromic propagation into the somatodendritic spike and significantly reduced the strength of post-stimulus inhibition in presumed A1 noradrenergic neurons projecting directly to the preoptic and anterior hypothalamic area. The occurrence of both antidromic spikes and post-stimulus inhibition of the majority of these neurons was completely abolished by the injection of 6-hydroxydopamine but not by 5, 7-dihydroxytryptamine directly into the medial forebrain bundle.

Differences in the responses of raphe nuclei to repetitive somatosensory stimulation

The responses of single units in raphe (R.) nuclei dorsalis, magnus, pallidus, and obscurus to repetitive stimulation of peripheral nerves were studied in the chloralose-anesthetized cat. Low-intensity electrical stimuli (1.5T) which activated the large-diameter fibers were applied to the common peroneal and lateral gastrocnemius nerves at 0.25, 0.5, 1 and 2 Hz. Responses evoked at each frequently were compared with the control response evoked at 0.1 Hz. All units isolated demonstrated response decrements during periods of stimulation greater than or equal to 0.5 Hz. The long-term effects of repetitive stimulation, however, varied among the four nuclei. After 150 stimulus presentations at 0.5, 1, or 2 Hz, sensory responsiveness decreased in R. dorsalis but was enhanced in caudal R. obscurus units. Changes in the responsiveness of the other two nuclei were not significantly different from control. Responses to twin pulses indicated that R. neurons are not well suited to relay rapid, repetitive stimuli. The functional significance of these observations has implications for the role of the raphe in habituation and in the modulation of sensory traffic to higher centers. Raphe responses are also contrasted to the known responses of reticular formation neurons to repetitive stimulation.

Dorsal raphe neurons: self-inhibition by an amphetamine-induced release of endogenous serotonin

A direct infusion of amphetamine into the dorsal raphe nucleus of the rat inhibited the activity of serotonergic neurons in this site. An intravenous injection of 5-methoxy-N,N-dimethyltryptamine, a serotonin autoreceptor agonist, mimicked this effect. The amphetamine-induced depression of firing rate was blocked by a subsequent injection of methiothepin, a putative serotonin autoreceptor antagonist, but not by pretreatment with a-methyl-p-tyrosine which depletes brain catecholamines. Amphetamine infusions into the surrounding periaqueductal gray or brainstem reticular formation failed to change dorsal raphe activity. The results of these studies indicate that endogenous serotonin, which can be released by a direct infusion of amphetamine, suppresses neuronal activity in the dorsal raphe nucleus by a process of self-inhibition.

Activity of serotonin-containing neurons in the nucleus raphe pallidus of freely moving cats

Serotonergic neurons within nucleus raphe pallidus (NRP) of freely moving cats initially were distinguished by their slow (less than 8 Hz), regular discharge and long duration (mean = 2.3 ms) action potentials. The activity of serotonergic NRP neurons was highest during active waking (mean = 4.85 +/- 0.37 spikes/s) and gradually slowed, with little change in firing pattern, during the transition from waking through slow wave sleep (middle of SWS: mean = 3.76 +/- 0.36 spikes/s). In REM sleep there was a precipitous decrease in firing rate (mean = 0.92 +/- 0.23 spikes/s) and loss of discharge regularity. Although there was no significant difference in firing rate between active and quiet waking, discharge rates were significantly increased during transient elevations of the EMG, but these rate increases usually were associated with specific motor behaviors only. The activity of serotonergic NRP neurons during SWS was not related to the occurrence of either sleep spindles in the cortical EEG or PGO waves recorded from the lateral geniculate nucleus. These neurons also were relatively unresponsive to phasic auditory or visual stimuli, with most of the neurons examined showing weak excitatory responses. Activity of all serotonergic NRP neurons tested was suppressed (mean = -81.3 +/- 4.3%) by the serotonergic agonist 5-methoxy-N,N-dimethyltryptamine (250 micrograms/kg, i.m.). The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus of freely moving cats and the issue of homogeneity in central serotonergic systems is discussed.

Raphe - cerebellum interactions. I. Effects of cerebellar stimulation and harmaline administration on single unit activity of midbrain raphe neurons in the rat

The firing patterns of single raphe units at the posterior midbrain level were examined in chloralosed rats to assess the effects of cerebellar stimulation and/or harmaline administration. Raphe cells were grouped according to their spontaneous firing rate and other characteristics into two categories. From a total sample of 160 cells, 106 (66%) presenting a slow regular discharge pattern were classified as serotonergic (5-HT cells), whereas 35 (22%), having a faster firing rate, were considered non serotonergic (NS cells). Moreover, 19 (12%) raphe units were non categorized. Cerebellar juxtafastigial (JF) stimulation modified the discharge pattern of 56 (35%) raphe units. The remaining 65% were unaffected by the stimulation. Of the 41 5-HT cells affected by JF stimulation, 28 neurons (68%) showed a systematic increase of their firing rate, whereas of the 12 NS cells affected 8 neurons (66%) were inhibited. It thus appears that cerebellar stimulation has an opposite effect on raphe units according to the cell types. Harmaline administration suppressed the activity of 5-HT cells and increased the discharge rate of NS cells. Moreover, we noticed in the latter units a phase modulation of the firing pattern by pauses occurring with a fixed periodicity of 2.5 to 10 s. Considered in the context of previous studies, these results strongly suggest an inhibitory influence of the raphe system on the olivo-cerebellar circuitry.

Serotonergic excitation from dorsal raphe stimulation recorded intracellularly from rat caudate-putamen

Similar to other afferents to rat caudate-putamen, stimulation of the dorsal raphe nucleus evokes a series of 3 responses which can be recorded intracellularly. An initial depolarization is followed by a long-lasting inhibition which is, in turn, terminated by another period of depolarization. Pharmacological manipulations demonstrate that the initial depolarizing potential is serotonergic. Depletion of serotonin by means of prior treatment with para-chlorophenylalanine leads to a reduction in the amplitude of the depolarization which can be evoked by maximal stimulation of dorsal raphe. Neither the long-lasting hyperpolarization nor the late excitation which follow the initial depolarization is affected. Replacement of serotonin in levels by injection of 5-hydroxytryptophan results in a restoration of the amplitude of the depolarizing response. The latency of the initial depolarization is, however, unchanged in serotonin-depleted animals. This together with the observation in some cells of a component of the initial depolarization resistant to para-chlorophenylalanine treatment, suggests that there is a non-serotonergic excitation which precedes that mediated by serotonin.

Amine pathways and sleep regulation

The development of histofluorescence anatomical techniques has allowed a wide variety of direct new approaches to be applied to studies of the functions of the reticular formation. Many reticular nuclei and pathways, previously poorly understood, have now taken on a specific chemical identity. Thus, it has been possible to directly visualize constituent biogenic amine systems in the core of the brainstem and manipulate these by pharmacological, lesioning and stimulation procedures. Biochemical neuroanatomy has, therefore, provided the essential chemical dissections of neuronal systems as a basis for examining the individual roles of neurotransmitters in behavior. In sleep biology transmitter morphology has directly led to monoamine interaction theories of the entire vigilance continuum and has identified the substrates critical for pharmacological approaches to unravelling the roles of individual amine systems in different aspects of the sleep-waking states. Various of these studies as applied to sleep biology are reviewed in this paper.

Morphology and intracellular responses of an identified dorsal raphe projection neuron

Stimulation of ventral medial tegmentum elicits an antidromic action potential and an inhibitory postsynaptic potential in dorsal raphe projection neurons. The inhibitory postsynaptic potential is not wholly due to activation of recurrent inhibitory circuits as it is not monosynaptic and its onset precedes the antidromic action potential. Light microscopic examination of horseradish peroxidase-filled projection neurons reveals a neuron type with radiating, poorly branched dendrites and terminal dendritic thickets. The axon of the exemplary neuron presented is seen to leave the nucleus but gives off a single collateral while still within the parent cell's dendritic domain.

Intracellular recordings from serotonergic dorsal raphe neurons: pacemaker potentials and the effect of LSD

Intracellular recordings in vivo from serotonergic dorsal raphe neurons of the rat brain reveal that these cells undergo a pronounced postspike hyperpolarization followed by a gradual interspike depolarization leading to the succeeding spike. Such repetitive cycles of interspike hyperpolarization and depolarization, which can be termed "pacemaker potentials', can account for the automaticity of these cells. When serotonergic neuronal firing is inhibited by LSD, such pacemaker potentials no longer occur and the cells remain in an hyperpolarized state.

Effect of 5-HT depletion of the hippocampus on neuronal transmission from perforant path through dentate gyrus

Stimulation of the perforant pathway (pp) elicits a characteristic evoked action potential (EAP) in the granule cell layer of the dentate gyrus. The EAP was recorded in rats depleted of hippocampal serotonin (5-HT) by prior injection of p-chloroamphetamine (PCA) or 5,7-dihydroxy-tryptamine (5,7-DHT) as well as in untreated animals during two behavioral states, slow-wave sleep (SWS) and still-alert behavior (SAL). As reported previously, in untreated rats the amplitude of the EAP response was significantly greater during SWS than SAL. Stimulation of the median raphe nucleus (MR) prior to stimulating the pp (prestimulation) augmented the EAP response, but only during SWS. In contrast, in animals injected with PCA or 5,7-DHT there was no difference of the amplitude of the EAP during SWS and SAL. However, the augmentation of the EAP during SWS produced by prestimulation of the median raphe was still present. It is concluded that 5-HT innervation of the dentate gyrus may be involved in the behavioral modulation of the EAP response. Modulation of the EAP following prestimulation of the MR appears to be effected by a non-serotonergic input to the dentate gyrus originating in, or coursing through, the median raphe.

In vivo release of serotonin in two raphe nuclei (raphe dorsalis and magnus) of the cat

The release of 3H-serotonin (3H-5-HT) endogenously synthesized from 3H-tryptophan was estimated in both dorsalis and magnus (MRN) raphe nuclei of anaesthetized "encéphale isolé" cats, by using push-pull cannulae. Resting steady state in the release of 3H-5-HT was observed 30 min after the beginning of superfusion with L-3H-tryptophan. The amounts of 3H-5-HT released in the DRN and the MRN are much greater than those measured simultaneously in the caudate nucleus. A marked increase either in the 5-HT release was seen in the presence of fluoxetine, a potent reuptake blocker of 5-HT, or during local depolarization with potassium chloride. The spontaneous release was diminished by removing Ca++ and by adding cobalt to the medium. Tetrodotoxin (TTX) decreased the 5-HT release in the DRN and, based on previously established, blocked the stimulating effect of batrachotoxin. According to the pharmacological characteristics of the monoamine dendritic release determined for dopamine in the substantia nigra [17], our results suggest that 5-HT release processes in the DRN correspond to a release from nerve endings, not from dendrites. The purpose of this study was to determine if the 5-HT released in the DRN is released from either axon terminals or dendrites. Morphological studies performed on the DRN do not consistently demonstrate the high density of serotoninergic varicosities in the DRN. In addition, two types of 5-HT axonal varicosities, characterized by their synaptic or non-synaptic junctions, are present in the brain. The concept that the quantities of 5-HT released could vary from one type compared to the other is discussed.

Raphe unit activity in freely moving cats: effects of phasic auditory and visual stimuli

The effects of phasic auditory or visual stimuli upon the single unit activity of serotonergic neurons within the dorsal raphe nucleus (DRN) were studied in freely moving cats. The predominant response to auditory stimulation (86% of the cells) was excitation, with a mean latency of 40 +/- 3 ms (S.E.M.) and a mean duration of 64 +/- 4 ms. This was typically followed by a longer period (206 +/- 32 ms) with unit activity below the baseline level. This did not appear to be a stimulus-induced inhibition of unit activity, however, since its duration closely corresponded to the normal interspike interval for that particular neuron. The response to repetitive auditory stimulation showed no evidence of habituation and was even present during sleep. A similar response, although generally of lesser magnitude, was evoked by a phasic visual stimulation in 64% of the cells tested. The mean latency for the response to visual stimulation was 53 +/- 4 ms, the mean duration of excitation was 76 +/- 7 ms, and the mean duration of the subsequent suppressed period was 239 +/- 37 ms. The response to the visual stimulus also showed no evidence of habituation. These data indicate that serotonergic neurons of the DRN are driven, with similar temporal characteristics, by stimuli in two different sensory modalities. We hypothesize that these similar effects are attributable to a common excitatory input.

The identification of serotonergic neurons in the nucleus raphe magnus by conduction velocity

Histochemical studies have described serotonergic neurons in the nucleus raphe magnus, and since their axons are small, they would be expected to have low conduction velocities. However, previous studies have reported few slow-conducting units in the nucleus. In these studies, raphe-spinal neurons, detected by antidromic activation, were found to exhibit a wide range of conduction velocities, including numbers of slow-conducting units. In a second set of experiments, the number of raphe-spinal units found in control rats and those treated with 5,7-dihydroxytryptamine, a serotonin neurotoxin, were compared. Two groups of slow-conducting units were reduced in treated animals and those units, conducting between 0.7-1.0 m/sec and 3.1-6.0 m/sec, were presumed to be serotonergic. These neurons comprised about 40% of the total found in the nucleus.

Aminergic neurons: state control and plasticity in three model systems

Aminergic neurons have particular functions in many systems, and in this review their role is discussed and compared in three systems: those parts of the central nervous system controlling sleep and waking in the cut; the superior cervical ganglion: and the isolated nervous system of Aplysia. In the cat the aminergic neurons are most important in a waking state during which time external information is received, processed, and can be retrieved, and during which time habituation and sensitization occur. Aminergic neurons appear to have similar roles in state control in plasticity in both the Aplysia nervous system and the superior cervical ganglion. The striking similarities in the role of aminergic neurons in these three systems support the speculation that aminergic neurons have uniquely important roles in regulation of the plastic properties of neurons.

The effects of tryptophan and manipulations of serotonergic receptors on tonic immobility in chickens

The effects of serotonergic manipulations on tonic immobility (TI) were examined. Systemic injections of tryptophan enhanced TI duration. This effect was reversed by quipazine, a 5-HT receptor agonist, and p-chloroamphetamine, a 5-HT releaser. Separately, these drugs caused marked reductions in TI duration. Fenfluramine, which promotes 5-HT release, also reduced TI duration. The quipazine attenuation of TI was prevented by pretreatment with the 5-HT receptor blocker cinanserin. The results are discussed in terms of 5-HT receptor mechanisms and the raphe model of tonic immobility.

Noradrenergic innervation of serotonergic neurons in the dorsal raphe: demonstration by electron microscopic autoradiography

In this study, noradrenergic (NE) terminals in the dorsal raphe were identified by [3H]NE electron microscopic (EM) autoradiography. Lesioning of NE terminals by treatment with the selective catecholamine neurotoxin, 6-hydroxydopamine produced a marked decrease in NE-labelled terminals. [3H]5-HT EM autoradiography of the dorsal raphe produced labelling of cell bodies, dendrites and axons but labelled terminals with synaptic junctions were not observed. Serotonergic (5-HT) neurons were identified at an early stage of degeneration following treatment with the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). When both [3H]NE autoradiography and 5,7-DHT lesioning were combined, a majority of NE-labelled terminals, which formed synaptic specializations, innervated degenerating dendrites. These findings suggest that NE terminals directly innervate 5-HT cells in the dorsal raphe.

Suppression of serotonergic neuronal firing by alpha-adrenoceptor antagonists: evidence against GABA mediation

Recent pharmacological studies have shown that administration of alpha-adrenoceptor antagonists, either systemically or locally in the vicinity of 5-HT cells of the dorsal raphe, suppresses their firing activity. In light of the prominent NE innervation of the dorsal raphe nucleus, these findings suggest that blockade of NE transmission in the dorsal raphe by these drugs underlies the suppression produced. The finding that systemic administration of picrotoxin, a GABA antagonist, partially reverses the suppression of 5-HT cells produced by systemic application of alpha-adrenoceptor antagonists led to the proposal that GABA interneurons located within the dorsal raphe mediate this suppression of 5-HT cell firing. This proposal has been tested, in this study, by examining the ability of two GABA antagonists, picrotoxin and bicuculline methiodide, when applied iontophoretically to reverse the suppression produced by two alpha-adrenoceptor antagonists, WB-4101 and phentolamine. First, evidence is presented that WB-4101 and phentolamine suppress 5-HT cell firing specifically by their blockade of alpha-adrenoreceptors. Second, the inability of both GABA antagonists tested to interfere with the suppression produced by these alpha-adrenoceptor antagonists is reported. These findings provide evidence against the proposal that GABA mediates the suppression of 5-HT cells by alpha-adrenoceptor antagonists.

Neurochemical modulation of sensory-motor reactivity: acoustic and tactile startle reflexes

The present review argues that the startle reflex is particularly well suited as a model system to analyze how drugs alter stimulus reactivity and reflex excitability. It then reviews all the literature to date on how drugs or lesions that are thought to alter neurochemical transmitter systems affect acoustic and/or tactile startle. Hypotheses are presented to account for how serotonin, dopamine, norepinephrine, acetylcholine, and opiates modulate startle. Effects on startle plasticity such as habituation, sensitization, and potentiation resulting from prior associative learning are also included.

Differential effects of noxious and non-noxious input on neurones according to location in ventral periaqueductal grey or dorsal raphe nucleus

Nociceptive and non-nociceptive input to the dorsal raphe nucleus (DR) and to the surrounding periaqueductal grey (PAG) was studied in chloralose-anaesthetized rats. Single units in the midbrain responding to electrical stimulation of a coccygeal nerve were recorded with glass micropipettes. A fluorescence histochemical technique was applied to identify recording sites in the DR and PAG. 109 DR-units, 141 PAG-units and 95 units from surrounding structures were tested for responsiveness to electrical nerve stimulation. In 53% of the DR-units, but in only 20% of the PAG- and SN-units, ongoing activity was inhibited by electrical stimulation (I-units) while 42% of the PAG- and SN-units but only 24% of the DR-units were electrically excited (E-units). 40 E-units and 24 I-units were tested with repeated noxious radiant heat stimuli applied to the tail or hindpaws. 70% of the E-units were excited by heating, and in 54% of the I-units ongoing activity was inhibited by heating. The majority of the former units were located in the PAG, and most of the latter were proven to be DR-neurones. In 75% of the E-units and in 12.5% of the I-units the heat effect was in the opposite direction. The findings are discussed in terms of the now well-established role of the PAG-region in the descending control of pain. The properties of the PAG-E-units suggest that this system is involved in a negative feedback circuit by which pain transmission to the CNS limits itself. DR-I-units may be involved via an additional small loop with the PAG to disinhibit the activation of the PAG pain control system.

Serotonergic intraventricular axons in the habenular region. Phagocytosis after induced degeneration

Both intracerebroventricular injection of 5,7-dihydroxytryptamine and electrolytical midbrain-raphe lesions in rats induce degeneration of supraependymal axons (SEAs) normally occurring in large numbers upon the ependyma of the medial habenular nucleus and habenular commissure. It is concluded that the intraventricular axon plexus in the epithalamic region is comprised of serotonergic (5-HT) fibers originating in the dorsal and/or median raphe nuclei. Besides the elimination of SEAs, conspicious features were a marked reduction in the number of cilia, degenerative signs in the habenular ependyma, and the emergence of large numbers of supraependymal macrophages, being most probably involved in phagocytosis of the axonal debris. It is suggested that the nucleus habenulae medialis is influenced serotonergically by the midbrain raphe via (1) a direct projection upon its neurons and (2) an indirect projection by way of the intraventricular axon plexus. The origin of intraventricular macrophages is discussed in relation to recent data in the literature.

Denervation supersensitivity to serotonin in rat forebrain: single cell studies

To investigate the development of denervation supersensitivity to serotonin (5-hydroxytryptamine, 5-HT) in the amygdala (AMYG) and the ventral lateral geniculate nucleus (vLGN), single cell recordings, microiontophoretic, histochemical and biochemical techniques were used in the present study. 5-HT projections to the vLGN and the AMYG were destroyed by 5,7-dihydroxytryptamine (5,7-DHT, a relatively selective toxin for 5-HT neurons) injected directly into the lateral ventricle or the ascending 5-HT pathway in the ventromedial tegmentum area. Enhanced responsiveness of cells to the inhibitory effect of microiontophoretically applied 5-HT (ionto-5-HT) began to develop within 24 h and approached a maximum 7 days after 5,7-DHT pretreatment. In general, the time courses for the reduction in both the density of 5-HT fluorescent varicosities and synaptosomal 5-HT uptake activity paralleled the time course for the development of denervation supersensitivity to 5-HT. During the first 2 days after 5,7-DHT, the enhanced sensitivity was selective for 5-HT; responses to D-lysergic acid diethylamide (LSD), norepinephrine (NE) and gamma-aminobutyric acid (GABA) were unchanged. Seven or more days after 5,7-DHT there was a marked increase of the responsiveness of neurons in the vLGN and the AMYG to both 5-HT and LSD (a 5-HT agonist which is not a substrate for the high affinity 5-HT uptake system). At these later times, the responsiveness of cells in the AMYG to NE and to a lesser extent GABA was also increased. In contrast to the marked supersensitivity seen after 5,7-DHT induced denervation, chronic administration of parachlorophenylalanine, a 5-HT synthesis inhibitor, failed to induce 5-HT supersensitivity.

Rapid changes in enkephalin levels in rat striatum and hypothalamus induced by diazepam

The acute treatment of rats with diazepam induces pronounced changes in brain enkephalin concentrations, as was estimated for methionine(met)-enkephalin and in some representative experiments for leucine(leu)-enkephalin, employing highly specific radioimmunoassays. Diazepam selectively increased the enkephalin concentrations in the hypothalamus by about 35%, and lowered it in the corpus striatum by roughly 25%; no changes could be detected in the medulla oblongata/pons or midbrain. The drug-induced changes displayed a rapid onset. Peak effects were reached by 2 to 5 min after injection. Changes observed in the hypothalamus were only short lasting and were apparently parallelled by diazepam concentrations in the brain, whereas the decrease in the striatum was of markedly longer duration. Presently, the mechanism underlying all these changes is unknown. Whereas an increase in enkephalin concentrations in the hypothalamus may be discussed in terms of the anti-stress effect of benzodiazepines, the observed drop in striatal enkephalin is not obviously to be correlated to behavioural changes induced by these drugs.

[Effect of cooling and electrical stimulation of nuclei of raphe system on states of alertness in cat]

In cats prepared in a 'semi-chronic' manner (spinal cord transected, brachial plexus sectioned bilaterally) all states of alertness are present; their quantitative evolution is characterized by an immediate postoperative period with continuous wakefulness and a secondary period of recovery of the states of sleep, which are then maintained. In such a preparation, localized moderate cooling (+10 degrees C) of the nucleus raphe dorsalis induces slow wave sleep and paradoxical sleep; the same type of cooling of the nucleus raphe magnus induces wakefulness; however, the electrical stimulation of these two nuclei always induces wakefulness. Cooling of the nuclei raphe centralis or pontis suppresses only the 'ponto-geniculo-occipital' (PGO) waves. Colling of the nuclei raphe obscurus and pallidus induces wakedfulness, but stage I of slow wave sleep may occur. Treatment with p-chlorophenylalanine (PCPA) before making the semi-chronic preparation produces the classical syndrome characterized by cortical activation and continuous discharge of PGO waves; in this case cooling of the nucleus raphe dorsalis no longer induces slow wave sleep but only paradoxical sleep; after injection of DL-5-HTP, slow wave sleep is obtained again through cooling; this effect diminishes progressively.

The raphe nuclei of the rabbit brain stem

The raphe nuclei of the rabbit brain stem were found in the midline and adjacent reticular formation of the medulla, pons, and mesencephalon. Nuclei raphe obscurus, pallidus, and magnus were located in the medulla. Nucleus raphe pontis and the caudal portion of nuclei raphe dorsalis and centralis superior were present in the pons. The rostral portion of nuclei raphe dorsalis and centralis superior, and nuclei linearis caudalis and intermedius were present in the msencephalon. Wings of neurons extended from the midline clusters of raphe neurons into the adjacent reticular formation. These wings of neurons contained serotonergic perikarya which were cytoarchitecturally indistinguishable from the midline neurons. A detailed localization of these nuclei is presented in atlas form. These raphe nuclei contained heterogeneous populations of neurons which varied in the size, shape and density of the cell bodies. In addition, the dendritic branching, specific orientation of dendrites, and appearance of spines were distinct for each of the raphe nuclei. Individual raphe nuclei often contained several subpopulations of neurons characterized by unique spatial configuration and orientation. The main morphological similarities of the raphe nuclei are location in or adjacent to the midline, the presence of serotonergic cell bodies in all raphe nuclei except the linear nuclei, and heterogeneous cell populations.

Description of an indolaminergic cell component in the cat locus coeruleus: a fluorescence histochemical and radioautographic study

Using Falck-Hillarp fluorescence histochemical and radioautographic techniques, it has been found that, in addition to the well-known catecholaminergic cells, the locus coeruleus (LC) of the cat contains a sizeable component of indolaminergic neurons. Indolaminergic cell bodies occur in all subdivisions of the LC complex. They are most numerous in the LC proper and subcoeruleus area, but are also present in the medial and lateral parabrachial, and Kölliker-Fuse nuclei. In all, the indolaminergic cells are estimated to make up 7-10% of the monoaminergic neuronal population of the LC complex. With the exception of the Kölliker-Fuse nucleus, where somewhat larger cells occur, the indolaminergic cell bodies in different parts of the LC complex share a common fluorescence histochemical appearance. They display round to fusiform shapes and measure 30 x 18 micron on the average, which makes them cytoarchitectonically similar to the small type of noradrenergic cells in the LC. The formaldehyde-induced fluorescence of the indolaminergic cells in the LC complex was analyzed microspectrofluorometrically and the recorded excitation and emission spectra (maxima at 370 and 530 nm, respectively) were found to be identical with those recorded from midline raphe neurons. No evidence of noradrenaline content was found in the indolaminergic cells of the LC. Radioautographic experiments after intratissular injections of tritiated serotonin showed that the indolaminergic cells of the LC complex possess uptake mechanisms for serotonin. Taken together these results provide strong evidence for serotonin being the transmitter of the indolaminergic neurons discovered in the LC of the cat.

Sites of action of amphetamine intrinsic to catecholaminergic nuclei: catecholaminergic presynaptic dendrites and axons

1. It is now well established that the behavioral actions of amphetamine, especially locomotor and stereotyped behaviors, are dependent in part upon the release of catecholamines in the central nervous system. 2. A variety of empirical evidence has established that the release of catecholamines by amphetamine leads to changes in neuronal activity in regions postsynaptic to catecholaminergic nerve terminals. In addition, release of catecholamines is accompanied by a marked inhibition of neuronal firing in catecholaminergic neurons. 3. At least two conceptions have been advanced to account for this marked decrease in catecholaminergic neuronal activity. (a) In one of these, release of catecholamines from presynaptic terminals is thought to lead to a compensatory decrease in neuronal firing rate by means of a postsynaptic neuronal feedback loop from regions innervated by catecholaminergic neurons. (b) In the other, decreased neuronal activity is hypothesized to result from local release of catecholamines onto or near catecholaminergic neuronal dendrites and somata, a phenomenon that has been characterized as self-inhibition. 4. For dopaminergic neurons, recent biochemical, neuropharmacological and neuroanatomical evidence suggests that the latter process could be subserved by dopamine released from dopaminergic neuronal dendrites, i.e., "presynaptic" dendrites.

A raphe dendrite bundle in the rabbit medulla

A Golgi-Cox, histofluorescence, and electron microscopic examination of the serotonergic raphe nuclei of the rabbit medulla has revealed a large, vertically-oriented midline dendrite bundle extending from the floor of the fourth ventricle to the ventral boundary of nucleus raphe pallidus. The bundle was confined to the medulla, and averaged 150-200 micrometer in width in the adult. This dendrite bundle received contributions from four major sources: (1) Dendrites of midline and paramedian neurons of nucleus raphe obscurus; (2) Dendrites of midline and paramedian neurons of nucleus raphe pallidus; (3) Shafts from tanycytes located on the midline floor of the fourth ventricle; and (4) Dendrites from neurons of the medullary reticular formation. Perikarya and dendrites of serotonergic raphe neurons frequently abutted tanycyte shafts, midline bhood vessels, and perikarya and dendrites of other raphe neurons. The tanycyte shafts extended from the floor of the fourth ventricle into the bundle, and often ran the entire length of the bundle, where they intertwined themselves among neurons and dendrites of the medullary raphe nuclei. This study suggests that neurons of the medullary raphe may be influenced by communication channels including dendro-dendritic contacts within the midline bundle, fourth ventricular cerebrospinal fluid-borne influences through tanycyte shafts, blood-borne influences through the direct neuronal-vascular relationship in the raphe, and traditionally described axonal contacts impinging upon raphe neurons. We suggest that the raphe neurons might act as both neurons and endocrine-neural transducer cells.