Effects of intracerebral injections of 5,6-dihydroxytryptamine on central monoamine neurons: evidence for selective degeneration of central 5-hydroxytryptamine neurons

Impairment of the serotonergic neurons underlying reinforcement elicits extinction of the repeatedly reactivated context memory

We analyzed changes in the activity of individually identifiable neurons involved in the networks underlying feeding and withdrawal behaviors in snails before, during, and after aversive learning in vitro. Responses to food in the “reinforcing” serotonergic neurons involved in withdrawal changed significantly after training, implying that these serotonergic cells participate in the reactivation of memory and are involved in the reconsolidation process. In behavioral experiments it was shown that impairment of the functioning of the serotonergic system with the selective neurotoxin 5,7-DiHT did not change the memory, when tested once, but resulted in a complete extinction of the contextual memory after repeated reactivation of memory. Conversely, the cued memory to a specific type of food was significantly reduced but still present. Thus, we conclude that it is only for the context memory, that participation of the “reinforcing” serotonergic neurons in memory retrieval may be the gate condition for the choice between extinction/reconsolidation.

The effect of lowering the 5-hydroxytryptamine content of the rat spinal cord on analgesia produced by morphine

1. Injection of 5,6-dihydroxytryptamine (5,6-DHT, 50 mug) into a lateral cerebral ventricle of male rats lowered the 5-hydroxytryptamine (5-HT) content of the lumbar cord to 12.8% and reduced the analgesic effect of low doses of morphine (0.64-1.63 mg/kg), tested by exerting pressure on the foot; after doses of morphine of 1.33-1.63 mg/kg, the analgesic response was reduced or abolished in 33% of the rats, and after 0.64 mg/kg, 58% of the animals failed to respond normally.2. Two days after an I.P. injection of p-chlorophenylalanine (pCPA, 320 mg/kg), the loss of analgesic potency of morphine was more pronounced than after intraventricular 5,6-HDT. The 5-HT content was lowered to about 8% in the lumbar cord, and to 20% or less in pons and medulla.3. The experiments show that interference with the descending tryptaminergic axons innervating the cord is by itself sufficient to reduce analgesia due to morphine, but they do not exclude the possibility that other tryptaminergic neurones take part in the effect of pCPA. The contribution to the analgesic effect of morphine made by the interaction of tryptaminergic axons with the interneurones ;gating' the afferent impulses in the posterior columns is discussed.

Protective actions of the vesicular monoamine transporter 2 (VMAT2) in monoaminergic neurons

Vesicular monoamine transporters (VMATs) are responsible for the packaging of neurotransmitters such as dopamine, serotonin, norepinephrine, and epinephrine into synaptic vesicles. These proteins evolved from precursors in the major facilitator superfamily of transporters and are among the members of the toxin extruding antiporter family. While the primary function of VMATs is to sequester neurotransmitters within vesicles, they can also translocate toxicants away from cytosolic sites of action. In the case of dopamine, this dual role of VMAT2 is combined-dopamine is more readily oxidized in the cytosol where it can cause oxidative stress so packaging into vesicles serves two purposes: neurotransmission and neuroprotection. Furthermore, the deleterious effects of exogenous toxicants on dopamine neurons, such as MPTP, can be attenuated by VMAT2 activity. The active metabolite of MPTP can be kept within vesicles and prevented from disrupting mitochondrial function thereby sparing the dopamine neuron. The highly addictive drug methamphetamine is also neurotoxic to dopamine neurons by using dopamine itself to destroy the axon terminals. Methamphetamine interferes with vesicular sequestration and increases the production of dopamine, escalating the amount in the cytosol and leading to oxidative damage of terminal components. Vesicular transport seems to resist this process by sequestering much of the excess dopamine, which is illustrated by the enhanced methamphetamine neurotoxicity in VMAT2-deficient mice. It is increasingly evident that VMAT2 provides neuroprotection from both endogenous and exogenous toxicants and that while VMAT2 has been adapted by eukaryotes for synaptic transmission, it is derived from phylogenetically ancient proteins that originally evolved for the purpose of cellular protection.

From the Golgi–Cajal mapping to the transmitter-based characterization of the neuronal networks leading to two modes of brain communication: Wiring and volume transmission

After Golgi-Cajal mapped neural circuits, the discovery and mapping of the central monoamine neurons opened up for a new understanding of interneuronal communication by indicating that another form of communication exists. For instance, it was found that dopamine may be released as a prolactin inhibitory factor from the median eminence, indicating an alternative mode of dopamine communication in the brain. Subsequently, the analysis of the locus coeruleus noradrenaline neurons demonstrated a novel type of lower brainstem neuron that monosynaptically and globally innervated the entire CNS. Furthermore, the ascending raphe serotonin neuron systems were found to globally innervate the forebrain with few synapses, and where deficits in serotonergic function appeared to play a major role in depression. We propose that serotonin reuptake inhibitors may produce antidepressant effects through increasing serotonergic neurotrophism in serotonin nerve cells and their targets by transactivation of receptor tyrosine kinases (RTK), involving direct or indirect receptor/RTK interactions. Early chemical neuroanatomical work on the monoamine neurons, involving primitive nervous systems and analysis of peptide neurons, indicated the existence of alternative modes of communication apart from synaptic transmission. In 1986, Agnati and Fuxe introduced the theory of two main types of intercellular communication in the brain: wiring and volume transmission (WT and VT). Synchronization of phasic activity in the monoamine cell clusters through electrotonic coupling and synaptic transmission (WT) enables optimal VT of monoamines in the target regions. Experimental work suggests an integration of WT and VT signals via receptor-receptor interactions, and a new theory of receptor-connexin interactions in electrical and mixed synapses is introduced. Consequently, a new model of brain function must be built, in which communication includes both WT and VT and receptor-receptor interactions in the integration of signals. This will lead to the unified execution of information handling and trophism for optimal brain function and survival.

Depletion of serotonin selectively impairs short-term memory without affecting long-term memory in odor learning in the terrestrial slug Limax valentianus

The terrestrial slug Limax is able to acquire short-term and long-term memories during aversive odor-taste associative learning. We investigated the effect of the selective serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) on memory. Behavioral studies indicated that 5,7-DHT impaired short-term memory but not long-term memory. HPLC (high-performance liquid chromatography) analysis revealed that 5,7-DHT significantly reduced serotonin content in the central nervous system. The present study suggests that acquisition, retention, and/or retrieval of short-term memory involves serotonin, and neither acquisition nor retrieval of long-term memory requires serotonin at a level as high as that required for short-term memory.

Cellular mechanisms of behavioral plasticity in terrestrial snail

Functional organization of networks underlying withdrawal, feeding, and respiration in terrestrial gastropod snail Helix are described. Tracking the changes during non-associative and associative modifications of behavior, analysis of plasticity mechanisms in identified neurons involved in these networks allowed to formulate several conceptual principles which are not widely accepted. The review will present data underlying the following principles: 1. Command neuron concept can be applied only to all-or-none behavior. 2. Habituation is an active down-regulation process opposite to up-regulating sensitization. All long-term behavioral changes at least in part are associative. 3. Reinforcement is a motivational state mediated by neuromodulatory neurons and can be produced by activity of a single modulatory neuron. 4. Non-addressed ('soft-wired') neuromodulatory influences are necessary for acquisition of memory, while retention of memory depends mostly on 'hard-wired' local changes in synaptic connectivity. 5. Retrieval of declarative (sensory) and procedural (motor) memory involves different functional classes of neurons.

Interaction between serotonin and nitric oxide (NO) in the activation of the serotoninergic system in the common snail

The effects of serotonin and NO donors on serotoninergic neurons (more than 60) in the brain of the common snail Helix lucorum were studied. Serotonin and NO donors induced depolarization in all neurons. and increased spike activity and activated the synchronous synaptic input, including train-like input. resulting in the onset of synchronous train activity in all these neurons. The excitatory effect of serotonin was significantly decreased by 5,7-dihydroxytryptamine (5.7-DOT) and monomethylarginine--a blocker of endogenous NO synthesis. Both these substances blocked the serotonin activation of the synchronous train input. 5.7-DOT also blocked the activation of this input by NO donors. but had no effect on their excitatory actions. The effects of 5,7-DOT developed quickly, were reversible, and were comparable to the effects of serotonin receptor antagonists. The data obtained here provide evidence that serotonin and NO have similar regulatory effects on the serotoninergic system in the snail brain. Not only do they excite serotoninergic neurons, but they also coordinate their functioning by activating common synaptic inputs. which are apparently also serotoninergic. It is suggested that NO has the role of a second messenger during serotoninergic excitation and functions as a co-transmitter for the presynaptic input.

Effect of 5,7-dihydroxytryptamine on the food-aversive conditioning in the snail Helix lucorum L

The effects of 5,7-dihydroxytryptamine (5,7-DHT), a drug which selectively ablates serotonergic terminals, were examined on acquisition of food-aversive conditioned reflex in the snail Helix lucorum. The percent of feeding reactions decreased from 80 to 15% in the conditioned group of animals after 5-8 pairings of food and electric shock. The behavioral performance of 5,7-DHT-injected animals after the same training session coincided with the data received from the unpaired control group: the percent of feeding reactions remained the same as before the training. Conditioning was carried out on the semi-intact 'lip-CNS' preparations as well. Intracellular recordings from the neurons responding to the withdrawal reaction confirmed the results of the behavioral experiments. Elaboration of associative changes was effective on preparations made from normal snails, whereas no changes were noted in 5,7-DHT-treated and pseudoconditioned animals. In 5,7-DHT-treated animals some components of the feeding behavior and withdrawal reaction changed as well. The appetitive phase duration of feeding lengthened significantly, moreover the sensitization of the withdrawal reaction evoked by rhythmic tactile stimulation disappeared in preparations made from drug-treated snails.

Changes in brain serotonin levels affect leukocytes, lymphocytes, T-cell subpopulations and natural killer cell activity in rats

The intraventricular injection of 5,6-dihydroxytryptamine into desipramine-treated rats caused a marked reduction in serotonin levels 5 and 10 days after treatment without significantly affecting catecholamine levels. In these animals, total leukocytes were increased and lymphocytes decreased. The number of helper T-cells (W3/25+) was significantly decreased after 5 days, whereas the numbers of total T-cells (W3/13+) and suppressor T-cells (MRC OX8) were increased after 10 days. The percentages of total T-cells, helper T-cells and suppressor T-cells were significantly reduced after 5 days. Natural killer cell (NK) activity was markedly reduced after 5 days. This finding suggests that central serotonin-containing neurons might be involved in the regulation of certain parts of the peripheral immune system.

The influence of serotonergic inputs on peptide neurons in the rat suprachiasmatic nucleus: an immunocytochemical study

The influence of serotonin (5-hydroxytryptamine; 5-HT) innervation on peptide-containing neurons in the rat suprachiasmatic nucleus (SCN) was investigated by peroxidase-anti-peroxidase (PAP) immunocytochemistry. The 5-HT neuronal system was chemically severed by 5,6-dihydroxytryptamine (5,6-DHT) injection into the medial forebrain bundle bilaterally. After this treatment, a marked decrease of vasoactive intestinal peptide (VIP)-like immunoreactivity in neuronal perikarya occurred in the SCN corresponding to a decrease in number of 5-HT immunoreactive fibers and terminals. However, no alteration of arginine-vasopressin-like immunoreactivity was detected between 5,6-DHT-treated animals and the controls. It is speculated that VIP-like immunoreactive neurons play an important role in the SCN under the influence of strong 5-HT innervation.

Regeneration of serotonergic fibers in the rat hypothalamus following unilateral 5,7-dihydroxytryptamine injection

The time course of degeneration and regeneration of serotonin (5-HT) fibers in the rat hypothalamus was studied with 5-HT immunocytochemistry and [3H]5-HT uptake following unilateral injections of 5,7-dihydroxytryptamine (5,7-DHT) into the dorsolateral hypothalamus. Within 3 days of the lesion, 5-HT fibers in the ipsilateral hypothalamus were swollen and darkly stained for 5-HT. In the contralateral hypothalamus few swollen fibers were apparent and these were generally restricted to the area adjacent to the fornix. Swollen 5-HT fibers were evident in the ipsilateral hypothalamus 3-19 days post-lesion in the medial forebrain bundle (MFB) during which time there was a gradual decrease in their density. In the medial and periventricular areas of the ipsilateral hypothalamus there were essentially no 5-HT fibers 7-30 days post-lesion. Sprouting 5-HT fibers were observed 12-19 days post-lesion. Thirty days post-lesion the density of 5-HT fibers in the MFB appeared normal; however, medial and periventricular areas remained denervated. Fifty days post-lesion there was an apparent bilateral hyperinnervation in the lateral and dorsomedial hypothalamic areas of 5,7-DHT-injected animals as compared to sham-injected animals. The morphological data were paralleled by changes in [3H]5-HT uptake. Seven days post-lesion specific high affinity uptake was reduced to 27% of sham in the ipsilateral hypothalamus and to 53% of sham in the contralateral hypothalamus. By 50 days post-lesion, specific high affinity uptake of [3H]5-HT was 141% of sham in the ipsilateral hypothalamus and 96% of sham in the contralateral hypothalamus.

The effects of intracerebroventricular administration of 5,6-dihydroxytryptamine to neonatal rats

5,6-Dihydroxytryptamine was administered intracerebroventricularly to neonatal rats on days 3 and 6 (25, 50, 75 or 100 micrograms) or on day 12 (50, 75 or 100 micrograms) after birth. Administration on days 3 and 6 increased serotonin (5-HT) in the diencephalon, decreased it in the telencephalon and produced variable results in the brainstem. The dose of 100 micrograms eliminated the peak in locomotor activity that normally occurs at day 15. Administration on day 12 produced increased 5-HT levels in the diencephalon and the brainstem. The dose of 100 micrograms delayed the decrease in activity after the peak. Administration on days 3 and 6 or on day 12 produced non-specific damage throughout the brain, although dopamine levels were normal. The behavioral effects did not correspond with the neurochemical effects and probably were due to the pronounced non-specific effects of the treatment.

Stimulation of the rat dorsal raphe in vivo releases labeled serotonin from the parietal cortex

In vivo release of labeled serotonin ([3H]5-HT) from the parietal cortex was investigated by cortical cup technique and electrical stimulation of midbrain raphe in rats anesthetized with pentobarbital sodium. The spontaneous efflux of tritium from the parietal cortex preload with [3H]5-HT followed a multiphasic exponential course. After 120 min, the rate of efflux appeared to fit the single exponential function (slow phase). Imipramine (10(-6)-10(-3) M) produced a dose-dependent increase in the spontaneous release. When pargyline in concentrations ranging from 10(-4) to 10(-3) M were added to the medium in the cup, the unchanged [3H]5-HT significantly increased in a dose-dependent manner and the slow declining coefficient of tritium efflux significantly decreased in the presence of 10(-4) pargyline. Stimulation of the rostral two-thirds of the dorsal raphe and the lateral 5-HT bundle originating from the dorsal raphe significantly increased the release of [3H]5-HT and its metabolites while stimulation of the caudal one-third of the dorsal raphe did not produce a significant increase in the release of [3H]5-HT and its metabolites. Stimulation of the medium raphe produced no or only a slight increase in the release of [3H]5-HT and its metabolites. These findings are a direct demonstration of the in vivo release of [3H]5-HT from the parietal cortex with stimulation of the dorsal raphe, particularly the rostral two-thirds of the nucleus and provide the neurochemical evidence for the dorsal raphe-cortical 5-HT pathway via the lateral 5-HT bundle.

Effect of intracisternal 5,-7-dihydroxytryptamine on the acute antihypertensive action of propranolol in the sino-aortic denervated anaesthetized dog

1 The anti-hypertensive effects of intravenously and intracisternally administered (+/-)-propranolol were studied in anaesthetized dogs with acute neurogenic (sino-aortic denervation) hypertension. The animals were pretreated 7 days earlier with intracisternally administered 5,7-dihydroxytryptamine (5,7-DHT 200 microgram/kg plus desipramine 5 mg/kg i.v.). 2 5,7-DHT (plus desipramine) failed to decrease both basic blood pressure and heart rate measured before sino-aortic denervation. After 5,7-DHT (plus desipramine) pretreatment, acute sino-aortic denervation induced a rise in blood pressure and stimulated the heart rate, these effects being similar (in intensity and duration) to those observed in control (saline-pre-treated) debuffered dogs during the first hour following the deafferentation. 3 In debuffered dogs, (+/-)-propranolol given by intracisternal (50 microgram/kg) or intravenous (300 microgram/kg) routes decreased both blood pressure and heart rate. 4 5,-DHT (plus desipramine) pretreatment abolished the antihypertensive effect of intracisternal propranolol whereas the action of intravenous propranolol was only delayed. In contrast, this pretreatment failed to reduce and even sometimes enhanced the negative chronotropic response induced by propranolol. 5 These results suggest that central 5-hydroxytryptaminergic pathways play an important role in the acute hypotension elicited by intracisternal (+/-)-propranolol in debuffered hypertensive anaesthetized dogs, but little, if any in propranolol-induced bradycardia.

Biochemistry and ultrastructure of serotonergic nerve endings in the lobster: serotonin and octopamine are contained in different nerve endings

In this article we report that the distribution of serotonin in the lobster nervous system parallels the distribution of octopamine and that the same tissues that contain endogenous serotonin can synthesize it from tryptophan. Octopamine and serotonin are highly concentrated in a neurosecretory region of the second thoracic roots in association with a group of neurosecretory cells. The roots possess separate high-affinity uptake systems for both serotonin and tryptophan. Radioactive serotonin, accumulated in tissues during incubations with either tritiated serotonin or tritiated tryptophan, can be released, in a calcium-dependent manner, by depolarization with potassium. A detailed morphological examination of the second thoracic roots shows four distinct categories of nerve endings in the vicinity of the neurosecretory cells. Octopamine is synthesized in one of these types of endings and serotonin in another. The high-affinity uptake systems for serotonin and tryptophan are found only in association with the endings that make serotonin. These endings and all the biochemical parameters of serotonin metabolism in the roots are selectively destroyed by previous injection of animals with the neurotoxin 5,7-dihydroxytryptamine.

Evidence for a receptor supersensitivity following impairment of central serotoninergic activity in the rabbit

In order to investigate whether a chronic impairment of neuronal serotoninergic transmission in the CNS could result in a receptor supersensitivity, rabbits were pretreated either with 5,6-dihydroxytryptamine (5,6-DHT) or p-chlorophenylalanine (PCPA) and then tested for their hyperthermic response to serotoninergic agonists. A previous (10 days before) intracerebroventricular injection of 5,6-DHT (75 microgram into each cerebral ventricle) significantly potentiated the increase in body temperature induced either by quipazine (1 mg/kg i.v.) or by 5-hydroxytryptophan (5-HTP 2 mg/kg i.v.) in combination with a MAO inhibitor (phenylethylhydrazine 10 mg/kg i.v. 16 h before). Pretreatment with PCPA (100 mg/kg s.c. four times on alternate days, the last dose 48 h before the experiment) also enhanced the hyperthermic effect of quipazine, whereas it inhibited the hyperthermic response to 5-HTP plus MAO inhibitor. These results suggest the existence of a receptor supersensitivity following prolonged blockade of serotoninergic neuronal transmission in the CNS.

Facilitation of memory consolidation by vasopressin: mediation by terminals of the dorsal noradrenergic bundle?

Administration of arginine-vasopressin (AVP, 5 micrograms, s.c.) immediately after the learning trial results in a long-term facilitation of a one-trial learning passive avoidance response. This effect of AVP is absent in animals with prior destruction of the ascending dorsal noradrenergic bundle by bilateral microinjection of 6-hydroxydopamine (6-OHDA). Postlearning local microinjection of a minute amount of AVP via chronically implanted cannulae into the locus coeruleus did not influence passive avoidance behavior. Upon injection into the midbrain dorsal raphe nucleus, however AVP facilitated passive avoidance behavior. This effect, however, was absent in rats receiving previous microinjection of 5,6-dihydroxytryptamine (5,6-DHT) or of 6-OHDA into the dorsal raphe nucleus. Bilateral 6-OHDA-induced lesions of the nucleus accumbens or 5,6-DHT-induced destruction of the dorsal raphe nucleus did not prevent the effect of AVP administered subcutaneously. The data suggest that vasopressin facilitates memory consolidation processes by modulating noradrenergic neurotransmission in terminals of the dorsal noradrenergic bundle. The serotoninergic neuronal network originating from the dorsal raphe nucleus has a secondary--norepinephrine-mediated--influence upon these processes.

Radioautographic characterization of a serotonin-accumulating nerve cell group in adult rat hypothalamus

Intensely labeled nerve cell bodies were identified by radioautography within the pars ventralis of nucleus dorsomedialis hypothalami (hdv), following intraventricular perfusion with 10(-5) or 10(-4) M tritiated serotonin [3H]5-HT in adult rats pretreated with a monoamine oxidase inhibitor. This selective reaction, which involved approximately 1000 neurons on each side of the third ventricle, was unaltered by concomitant administration of 10(-3) M non-radioactive norepinephrine, and was absent after intraventricular injection of 10(-5) or 10(-4) M tritiated norepinephrine. The 3H-labeled 5-HT nerve cell bodies were loosely grouped within the inner and caudal half of the hdv, and appeared morphologically similar to the unreactive neurons among which they were interspersed. Within the same region, numerous labeled axonal varicosities were also detected, which were never found in synaptic contact with the reactive cells. If the 3H-labeled 5-HT neurons contain endogenous 5-HT, they might constitute an intrinsic source of 5-HT innervation in the adult rat hypothalamus.

Evaluation of the non-specific effects of catecholamine and serotonin neurotoxins by injection into the medial forebrain bundle of the rat

Low doses of 6-hydroxydopamine (6-OHDA), 5,6-dihydroxytryptamine (5,6-DHT) and 5,7-dihydroxytryptamine (5,7-DHT) that have previously been shown to produce behavioral change following intracerebral infusion were injected into the medial forebrain bundle of the rat. This site contains serotonin (5-HT), norepinephrine (NE), and dopamine (DA) fibers whose anatomical locations have been described. Damage to these fiber systems was quantified by measuring depletion of telencephalic 5-HT, NE and DA. The effects of infusions of 6-OHDA, 5,6-DHT and 5,7-DHT were compared to the effects of unequivocally non-specific electrolytic lesions and copper sulfate infusions. Survival time was varied to evaluate the amount of regeneration that could be expected over periods from 8 to 60 days. Amine levels were found to be stable over the time period examined. With the doses used, evidence was found to support the position that non-specific damage caused by general cytotoxic effects of 6-OHDA and 5,7-DHT is minimized sufficiently to permit the acquisition of useful data on the function of central catecholamine and indoleamine systems.

Bulbospinal tryptaminergic neurones. A search for the role of bulbospinal tryptaminergic neurones in the control of sympathetic activity

1. The possible role of bulbospinal tryptaminergic neurones in the control of sympathetic activity has been investigated in anaesthetised cats. 2. Depletion of spinal cord stores of 5-hydroxytryptamine was achieved by systemic administration of p-chlorophenylalanine or by intraspinal microinjections of 5,6-dihydroxytryptamine. 3. Blood pressure was little changed by these treatments, neither was the pattern of ongoing activity in sympathetic nerves (arterial pulse rhythmicity and respiratory modulation), the influence of pulmonary afferent nerves on this activity (determined by an airway occlusion technique), the sympatho-inhibitory influence of the carotid sinus baroreceptors, nor the sympatho-inhibitory or -excitatory influences of somatic afferent nerves. 4. Since both p-chlorophenylalanine and 5,6-dihydroxytryptamine treatment caused extensive disruption of the bulbospinal tryptaminergic neurones, it was concluded that these play no significant role in the mediation of the responses tested in anaesthetised cats in the present study.

Effect of central nervous system-acting drugs after selective destruction by neurotoxins of 5-hydroxytryptamine fibers in the brain

Some behavioral effects of CNS-acting drugs have been studied in two animal species after functional impairment of central serotonergic activity. In rabbits, pretreatment with p-CPA or with 5,6-DHT counteracted the hyperthermia induced by d-amphetamine or apomorphine; the same pretreatments were ineffective in modifying pyrogen-induced fever. These data indicate a modulatory role of the 5-HT system in the hyperthermic response to dopaminergic agonists, such as d-amphetamine and apomorphine, and rule out 5-HT involvement in the hyperpyrexia induced by pyrogen. A previous intraventricular injection of 5,6-DHT significantly potentiated the increase in body temperature induced by 5-HTP in combination with a MAO inhibitor. Pretreatment with p-CPA, instead, strongly inhibited the hyperthermic response to 5-HTP. In unanesthetized fowl, at the time of the most dramatic degenerative signs of indoleaminergic neurons induced by 5,6-DHT, as documented by histofluorescence pictures, intraventricular infusion of 5-HT induced more intense behavioral, electrocortical, and body temperature modifications than in control animals. These results suggest the existence of receptor supersensitivity after 5-HT nerve ending degeneration but not after brain 5-HT depletion. The hyperthermic effect of a presynaptically acting drug, fenfluramine, was counteracted in rabbits and in fowl pretreated with either p-CPA or 5,6-DHT. Since identical behavioral, electrocortical, and body temperature effects have been observed after 5-HT or 5,6-DHT infusion into the third ventricle of fowl, it may be concluded that 5,6-DHT behaves also as a central 5-HT receptor agonist.

Midbrain raphe lesion in the newborn rat I. Neurophysiological aspects of sleep

Electrolytic lesions of the anterior raphe nuclei in the 4-6 day puppy rat were performed stereotaxically. The subsequent development of the sleep parameters in these animals was followed up to the second month of life. No difference could be found between the lesioned and control rats, neither in the qualitative nor in the quantitative characteristics of slow wave sleep (SWS) and paradoxical sleep (PS). However, when performing identical lesions on 3-5 week-old animals, a relative insomnia was obtained, more severe for PS than for SWS. The correlation of the neurophysiological data with the biochemical analysis of monoamines in the forebrain revealed that the serotoninergic (5-HT) system of the anterior raphe in the early lesioned animals was almost totally destroyed, with no recovery up to 9 months of age. Our results indicate that the anterior raphe nuclei are not functional during the first week of age, in regard to sleep control, whereas they do play an important role in sleep regulation after 3 weeks of age. Furthermore, some compensatory mechanisms which develop in the early lesioned animal as a consequence of the lesion, could explain the exhibition of normal sleep ontogenesis. The possible nature of these mechanisms is discussed in light of the monoaminergic theory of sleep control.

Accumulation of 5-HT in non-terminal axons after p-chloro-N-methylamphetamine without degeneration of identified 5-HT nerve terminals

The effect of a single injection of d,1-p-chloro-N-methylamphetamine (PCMA) on 5-hydroxytryptamine (5-HT)- containing neurons in rat brain was investigated using fluorescence histochemical, electron microscopic and biochemical methods. PCMA caused in a dose-dependent manner (from 4.3 mg/kg), an increase of formaldehyde-induced indoleamine (IA) fluorescence in swollen non-terminal axons during the first 6 days and, in contrast, a diminution of IA fluorescence in nerve terminal regions for up to 42 days after treatment. These changes did not appear to be the result of destruction of 5-HT nerve terminals since at all time intervals investigated (12 h to 42 days), the fine structure and frequency of supra-ependymal 5-HT nerve terminals were unaffected. Moreover, no degenerating nerve terminals were observed in the suprachiasmatic nucleus. A marked transient decrease of IA fluorescence on day 2 in the 5-HT cell bodies B3-B9 was not followed by obvious morphological changes up to 42 days after PCMA. Therefore, the reduced 5-HT content of brain up to 42 days after treatment seems not to be due to a destruction of 5-HT neurons. Moreover, the damage to non-terminal 5-HT axons, as indicated by the 5-HT accumulation, seems not to be severe, at least not to those axons projecting to the cerebral ventricles and suprachiasmatic nucleus, since no degeneration of 5-HT nerve terminals was observed at any of the times investigated.

Serotonin axons in the supra- and subependymal plexuses and in the leptomeninges; their roles in local alterations of cerebrospinal fluid and vasomotor activity

Extensive plexuses of serotonin axons form a supra- and subependymal system in the walls of the ventricles, in the arachnoid sheath around major cerebral blood vessels, and in the pia over the spinal cord. These have been demonstrated by autoradiography after continuous intraventricular perfusions of exogenous [3H]5-HT in rats and monkeys. The axons accumulate 5,6-DHT rendering them electron opaque, but have no uptake systems for [3H]NE. After treatment with MAO inhibitors and [3H]5-HT, the axonal boutons contain large (70nm) variably dense synaptic vesicles, and small (35 nm) vesicles each equipped with a dense dot. The latter vesicles are not seen in untreated controls. Electrical stimulation in the raphe nuclei causes significant increases in axonal [3H]5-HT uptake indicating that the fibers originate in the raphe. Quantitatively, the supraependymal plexus is variable, profuse over the dorsal and ventral aqueductal surfaces, sparse over the lateral aspects. Individual raphe neurons have their specific uptake affinities for [3H]5-HT that are independent of tracer concentration or diffusion gradient. It is suggested that raphe neurons with low 5-HT uptake may utilize other neurotransmitters. Two new functional roles are proposed: (1) the serotonin ventricular and pial axons are probably important modifiers of local cerebrospinal fluid (CSF) composition so that regional CSF variations in 5-HT and its metabolites are highly probable; (2) the subarachnoid plexus around major cerebral vessels may contribute to local vasomotor action, thus affecting the cerebral blood flow. The possible significance of these serotonin systems for an understanding of certain neurological entities such as migraine and hemodynamic regulation in cerebral vascular disease is indicated.

Effect of harmine and brain lesions on apomorphine induced motor activity

Application of harmine (10 mg/kg IP) 30 min before apomorphine decreased the motoric effects of the latter. Following harmine an increase in 5-HT and a decrease in 5-HIAA in different brain regions have been found. Injection of 5,6-DHT into nucleus medianus raphe 7 days before the experiment caused a significant increase of the apomorphine effect. Harmine pretreatment reduced this escessive motility as well as additional lesion of the substantia nigra with 6-OH-DA. Lesion induced by 6-OH-DA alone was without significant effect on the hypermotility following apomorphine. Application of PCPA 3 days before testing elicited an increase of apomorphine-induced hypermotolity which could be abolished by preceding harmine application. The experiments demonstrate the inhibitory effect of the central serotoninergic system on the apomorphine syndrome as well as the serotoninergic-dopaminergic interaction in hypermotility.

Disappearance of supra-ependymal 5-HT axons in the rat forebrain after electrolytic and 5,6-DHT-induced lesions of the medial forebrain bundle

The present study was intended to demonstrate the origin of supra-ependymal 5-hydroxytryptamine axons in the rat forebrain. Electrolytic lesions and injections of 5,6-dihydroxytryptamine (10 mug in 4 mul) were carried out unilaterally in and close to the medial forebrain bundle in the posterior hypothalamus of rats. Ten to 14 days later, terminal axons and formaldehyde-induced indolealkylamine fluorescence had virtually disappeared supra-ependymally in the lateral ventricles and interventricular foramina ipsilateral to the lesion if the indolealkylamine axons passing through the medial part of the medial forebrain bundle had been destroyed. No changes were observed, electron microscopically or fluorescence histochemically, in ventricles contralateral to the lesion. It is concluded that the supra-ependymal serotonergic nerve terminals in the lateral ventricles and interventricular foramina originate, uncrossed, from non-terminal axons passing through the medial forebrain bundle in the posterior hypothalamus.

Impairment of thermoregulation, food and water intakes in the rat after hypothalamic injections of 5,6-dihydroxytryptamine

A bilateral microinjection into the anterior hypothalamus of 5,6-dihydroxytryptamin (5,6-DHT), a substance that lesions serotonin (5-HT)-containing neurons, caused a rise in the body temperature of the rat. The anatomical sites were the same as those at which 5-HT given in the same dose range (1.25-2.5 mug) evoked a similar hyperthermia. When exposed for one hour to a temperature of either 35 degrees C or 8 degrees C, the rats were not able to defend against the heat or cold, respectively. The magnitude of this thermoregulatory deficit depended upon the dose of 5,6-DHT given as well as the site of injection. A partial recovery from the warmth deficit was evident 13-17 days following the 5,6-DHT microinjection. Food and water intakes were also suppressed significantly and body weights declined concomitantly. These results provide additional evidence to support the view that a serotonergic mechanism in the hypothalamus is involved in both thermoregulation and the control of ingestive behavior.