Interactions between serotonergic and enkephalinergic neurons in rat striatum and hypothalamus

Serotonergic regulation of the GABAergic transmission in the rat basal ganglia

The GABAergic neurons represent a major neuronal population in the basal ganglia. Although alterations in serotonin (5-HT) transmission are associated with neurodegenerative diseases involving these regions, the influence exerted by 5-HT afferents on GABAergic populations remains poorly understood. Here, we examined the consequences of 5,7-dihydroxytryptamine-induced lesion of 5-HT neurons on glutamic acid decarboxylase (GAD) activity, mRNA expression of the two isoforms of the enzyme, GAD65 and GAD67, GABA uptake, and extracellular GABA levels in the striatum. The 5-HT depletion produced an increase in GAD activity without modifying GAD65 and GAD67 mRNA levels, suggesting that 5-HT acts at the posttranscriptional level to regulate striatal GABA synthesis. No change in GAD activity was measured in the main striatal target structures, the globus pallidus and substantia nigra. Striatal GABA uptake and extracellular levels of GABA measured under basal conditions in freely moving rats were maintained in a normal range following 5-HT deprivation. By contrast, depolarization-induced increases in extracellular levels of GABA were larger in the striatum of 5-HT-deprived rats than in controls, which may be accounted for by an increase in a releasable pool of GABA due to increased synthesis rate. Together, these results suggest that 5-HT afferents may exert a phasic inhibitory control on striatal GABA transmission. Therefore, a decrease in striatal 5-HT transmission in disease states, such as Parkinson's disease, may contribute to pathological changes in striatal GABA neuron activity by increasing their reactivity to depolarizing stimuli.

Enkephalin contributes to the locomotor stimulating effects of 3,4-methylenedioxy-N-methylamphetamine

3,4-methylenedioxy-N-methylamphetamine (MDMA, 'Ecstasy') is a potent inhibitor of serotonin uptake, which induces both an increase in locomotion and a decrease in exploratory activity in rodents. Serotonin 5-HT1B receptors, located on the terminals of striatal efferent neurons, have been suggested to mediate these motor effects of MDMA. Striatal neurons projecting to the globus pallidus contain met-enkephalin, whilst those projecting to the substantia nigra contain substance P. We therefore analysed the levels of both peptides using radioimmunocytochemistry after MDMA administration (10 mg/kg, 3 h) in wild-type and 5-HT1B receptor knockout mice. Our results demonstrate that MDMA induces a decrease in pallidal met-enkephalin immunolabelling in wild-type, but not in 5-HT1B receptor knockout mice. Similar results were obtained following treatment with the 5-HT1A/1B agonist RU24969 (5 mg/kg, 3 h), suggesting that activation of 5-HT1B receptors leads to a reduction in met-enkephalin levels in the globus pallidus. In contrast, MDMA had no effect on the nigral substance P levels. We have previously shown that both MDMA and RU24969 fail to stimulate locomotor activity in 5-HT1B receptor knockout mice. Our present data indicate that the opioid antagonist naloxone suppressed the locomotor effects of MDMA. This study is the first to demonstrate that Enk contributes to MDMA-induced increases in locomotor activity. Such an effect may be related to the 5-HT control of pallidal met-enkephalin levels via the 5-HT1B receptors.

Alterations in behavior and opioid gene expression induced by the novel tropane analog WF-31

The effects of the acute administration of the serotonin-selective tropane analog, [2beta-propanoyl-3beta-(4-isopropylphenyl)-tropane, WF-31, on spontaneous locomotor activity were measured and compared to those of the highly selective serotonin uptake inhibitor, fluoxetine and cocaine, a non-selective re-uptake inhibitor of dopamine and serotonin. WF-31 (1, 10 and 30 mg/kg)-elicited increases in locomotor behaviors when compared to vehicle-treated rats. This increased activity was blocked by pre-treatment with the dopaminergic antagonist, flupenthixol, suggesting that these effects may be mediated by dopaminergic mechanisms. Cocaine, but not fluoxetine, also elicited increases in behaviors. In addition, the effects of these three compounds on opioid peptide gene expression were also assessed using in situ hybridization histochemistry in the same animals. The acute administration of both WF-31 and cocaine increased the expression of preprodynorphin mRNA in the dorsal striatum whereas fluoxetine had no effect. Expression of striatal preproenkephalin mRNA was augmented by all three compounds. Within the nucleus accumbens, PPD mRNA levels were affected only by treatment with WF-31, an effect that was blocked by pre-treatment with flupenthixol. In contrast, the acute administration of both WF-31 and fluoxetine, but not cocaine, increased the expression of preproenkephalin mRNA. These increases, however, were not reversed by pre-treatment with flupenthixol. Despite its profile in vitro as a relatively selective serotonin re-uptake inhibitor, some of the in vivo actions of WF-31 appear to be mediated by dopaminergic mechanisms. These data further suggest that the mechanisms underlying expression of the opioid peptides in the nucleus accumbens may vary from those in the dorsal striatum.

Selective effects of partial and severe lesions of the serotonergic systems on Met-enkephalin and substance P neurons in rat basal ganglia

The effects of partial (80%) vs. severe (> 95%) depletion of serotonin (5-HT) on peptide expression in basal ganglia were examined using immunocytochemical and in situ hybridization histochemical approaches. Topographical analysis of the changes in Met-enkephalin (Met-enk) and substance P (SP) levels were performed on the rat striatum, globus pallidus and substantia nigra 3 weeks after injecting 3 microl (partial lesion) or 6 microl (severe lesion) 5,7-dihydroxytryptamine (6.6 microg/microl) into the anterior raphe nuclei. Both kinds of lesion led to significant increases (39-42%) in Met-enk immunoreactivity in the striatum; a corresponding increase (21%) was detected in the globus pallidus only after severe 5-HT depletion. Only the severe lesion increased the SP immunoreactivity in the striatum (32%) and substantia nigra (26%). Neither striatal preproenkephalin nor preprotachykinin levels showed significant differences with the control values. These results suggest that the neuronal accumulation of Met-enk or SP may be attributable to post-transcriptional events, such as a blockade of the peptide release, and that 5-HT may, thus, exert a facilitatory influence on the striatal output neurons. The results obtained after partial lesion indicate a preferential sensitivity of striatal Met-enk vs. SP containing terminals to the 5-HT denervation. These differences are illustrated in selective regional changes in peptide labeling. These data point to some balance exerted by the serotonergic and dopaminergic inputs on these neuronal populations.

Preprotachykinin and preproenkephalin mRNA expression within striatal subregions in response to altered serotonin transmission

The effects of lowered serotonin (5-hydroxytryptamine; 5-HT) neurotransmission on preprotachykinin (PPT) and preproenkephalin (PPE) mRNA levels were examined in subregions of the striatum. Adult male rats were treated systemically with para-chlorophenylalanine (pCPA; 350 mg/kg single i.p. injection) which reduced forebrain 5-HT amounts to approximately 20% of saline-injected controls at 24 and 48 h. As measured by Northern analysis, PPT and PPE mRNA levels were elevated 50% and 160% respectively in the anterior ventromedial striatum (region included nucleus accumbens). PPT mRNA levels were raised 90% in posterior striatum (at the level of the globus pallidus) by 48 h post-pCPA injection. To determine if increased PPT and PPE mRNA levels represented a transient response to brief 5-HT inhibition, additional experiments were performed to provide continual suppression of 5-HT within the striatum. First, rats received daily intraperitoneal injections of saline or the 5-HT1A receptor agonist, 8-OH-DPAT (1 mg/kg), for 7 days to reduce 5-HT release from raphestriatal terminals. In a parallel experiment, the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT, 5 micrograms), was stereotaxically injected into the striatum as a means to permanently remove 5-HT terminals. Although levels of each mRNA species were differentially sensitive to 5,7-DHT or 8-OH-DPAT, PPT and PPE mRNAs were lowered between 30-55% within the anterior dorsolateral and ventromedial striatum. Although these results support previous studies suggesting an overall positive regulatory role of serotonin on striatal tachykinin biosynthesis, PPT and PPE gene regulation in certain striatal subregions may by differentially sensitive to lowered 5-HT neurotransmission. This suggestion is supported by observations that acute systemic stimulation of 5-HT2A/C receptors with DOI (7 mg/kg single i.p. injection) raised PPT and PPE mRNA levels within anterior dorsolateral (30-60%) and posterior (100-200%) striata, but not within the anterior ventromedial striatum.

Further studies to examine the nature of dexfenfluramine-induced suppression of heroin self-administration

The present series of experiments sought to investigate further the mechanism by which dexfenfluramine, a selective 5-HT releaser/reuptake inhibitor, reduces heroin self-administration by male Wistar rats. In experiment 1, the effect of combined intravenous heroin and intraperitoneal dexfenfluramine injections on operant responding for food was examined. In experiment 2, the maintenance of dexfenfluramine suppression of heroin self-administration following chronic (7 day) treatment was evaluated. Finally, in experiment 3, the ability of various 5-HT antagonists to block the dexfenfluramine suppression was examined. The results from experiment 1 suggest that sensorimotor deficits/malaise potentially associated with heroin/dexfenfluramine combinations are unlikely to account for the reductions in heroin self-administration. Experiment 2 suggested that the suppressant effect of dexfenfluramine on heroin responding may diminish rapidly following chronic treatment. Finally, central 5-HT1 and/or 5-HT2, but not 5-HT3, receptors may underlie the suppressant effects of dexfenfluramine on heroin self-administration.

Regional changes in neuropeptide levels after 5,7-dihydroxytryptamine-induced serotonin depletion in the rat brain

The levels of five neuropeptides (substance-P, somatostatin, cholecystokinin octapeptide, methionine-enkephalin and dynorphin) were examined in the brain and the spinal cord of rats 2 weeks after intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT). 5,7-DHT injection caused a significant reduction of the serotonin level in all regions of the brain. The level of each neuropeptide except dynorphin significantly increased in specific regions of the brain after 5,7-DHT treatment without any decrease in their levels in any region. Since, coexistence and interaction between classical neurotransmitters and neuropeptides in the same neurons have been reported, both are indispensable for evaluating pathophysiological state of the brain function associated with abnormal neural transmission. The present findings together with previous reports suggest that neuropeptides act as neurotransmitters and compensate for the impaired function of the serotonergic systems.

Opiate antagonists inhibit feeding induced by 8-OH-DPAT: possible mediation in the nucleus accumbens

Previous work has shown that the 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) elicits a variety of behaviours, including feeding in rats. These effects are accompanied by reduced 5-HT neurotransmission resulting from activation of somatodendritic 5-HT receptors located in the midbrain raphe nuclei. Dopamine antagonists injected either peripherally or into the nucleus accumbens reverse 8-OH-DPAT-induced feeding. Thus a facilitation of dopamine activity, secondary to reduced 5-HT activity, may be involved in mediating 8-OH-DPAT-induced feeding. Opiate antagonists have been shown previously to reduce several dopamine-dependent behaviours including feeding induced by dopaminergic drugs, tail pinch and electrical brain stimulation. Therefore experiments were conducted to assess the effects of opiate antagonists on feeding induced by peripheral, and raphe injection of 8-OH-DPAT in free-feeding rats. Following SC injection naloxone (0.1-10 mg/kg) dose-dependently reduced the feeding response induced by 100 micrograms/kg 8-OH-DPAT (SC). The lowest effective dose of naloxone was 1 mg/kg. This dose of naloxone also suppressed feeding induced by 8-OH-DPAT injected into either the dorsal (1 microgram) or median (0.5 micrograms) raphe. Microinjecting 2 micrograms naloxone together with 8-OH-DPAT into either of these sites failed to prevent the increased feeding. These results indicate that the effects of naloxone are mediated at sites distal to the raphe nuclei. One possible site may be the nucleus accumbens, since methyl-naltrexone (0.3, 1 or 3 micrograms) injected into this site blocked the feeding responses to intra-raphe 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of reserpine on catecholamine contents and met-enkephalin and beta-endorphin levels in the hypothalamus and the pituitary

1. The effects of reserpine treatment on the contents of catecholamines and opioid peptides have been studied in the rat hypothalamus and pituitary. 2. Hypothalamic and pituitary catecholamines were drastically depleted following acute reserpine treatment. 3. Reserpine treatment also resulted in a significant decrease in immunoactive met-enkephalin content in both the hypothalamus (25%) and the anterior lobe (50%), but not in the neurointermediate lobe of the pituitary. 4. No changes were observed in immunoactive beta-endorphin levels. 5. These findings suggest that the met-enkephalin contents in the hypothalamus and the anterior pituitary may be under catecholaminergic control. 6. The lack of effect of acute reserpine treatment on immunoactive beta-endorphin contents might be due to the opposing effects of adrenergic and dopaminergic mechanisms.

Opioid involvement in the adaptive change of 5-HT1 receptors induced by chronic restraint

Rats immobilized for 2 h daily for 7 days showed an increased behavioral response (forepaw treading and hind-limb abduction) to 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) 24 h after the last stress session. An injection of naloxone before each stress session fully antagonized the increased behavioral reactivity to 5-MeODMT. Treatment with morphine or beta-endorphin associated with each immobilization session for 3 days produced a response to 5-MeODMT higher than that of animals subjected to immobilization only. Chronic immobilization for 7 days did not affect the shaking behavior induced by 5-hydroxytryptophan (5-HTP) 24 h after the last restraint session. These findings suggest that chronic stress may induce a selective adaptive change of the 5-HT1 site and activate an opioid mechanism that is most likely to be involved in the development of this adaptive change.

Effects of opiate agonists and an antagonist on food intake and brain neurotransmitters in normophagic and obese "cafeteria" rats

The relationship between the effects of opiates on food intake and on central monoamines in various brain areas was investigated in normophagic and obese "cafeteria" rats. Three agonists, beta-endorphin, dynorphin, and D-Ser2-Leu-Enk-Thr6 (DSLET) and an antagonist, naltrexone, were used. The three agonists enhanced feeling in normophagic rats but had different dopaminergic effects. Serotonergic metabolism increased concomitantly with the enhancement of feeding by the agonists, whereas it decreased following treatment with the antagonist naltrexone. In the cafeteria rats, although the feeding effects of dynorphin and DSLET occurred earlier, there was a complete lack of monoaminergic effects. beta-Endorphin was completely devoid of effects in this model. There would, thus, appear to be a positive correlation between the behavioural effects of these opiates and serotonergic metabolism in normophagic rats, while stimulated feeding situations ("cafeteria" rats) the disruption of a monoaminergic modulation does not prohibit a direct effect on feeding.

Pharmacology of neuronal gene expression

Pharmacological treatments were used to estimate trans-synaptic regulation of opioid peptide gene expression occuring at specific neurotransmitter receptors. In vitro and in vivo studies have shown that different signal-transduction mechanisms regulate the transcription of proenkephalin, proopiomelanocortin and nerve growth factor mRNA. The activation of receptors coupled to adenylate cyclase elicited the increase of proenkephalin and nerve growth factor gene expression. Therefore, a cAMP-dependent mechanism was suggested to be involved in such regulation. However, the temporal delay between the elevation of the intracellular cAMP content and the increase in nerve growth factor and proenkephalin mRNAs prompted us to investigate whether additional mechanisms associated with the second messenger were operative in the regulation of the expression of these two genes. We report evidence that a protein(s), probably functioning as a trans-acting factor, might be involved in the regulation of nerve growth factor gene transcription. The characterization and isolation of these DNA regulatory proteins will provide the pharmacologist with valuable information for the development of new compounds in the therapy of mental disorders.

The proposed role of neurotransmitter receptors in the pathophysiology of human myoclonic disorders

The hypothesis that central neurotransmitter receptor abnormalities are the basis of human myoclonic disorders is novel. Receptor abnormalities in any of several different neurotransmitter systems implicated in myoclonus may be genetic or the consequence of various brain injuries. These abnormalities might define pharmacologic subgroups of possible relevance to the clinical, neurophysiologic, and pathologic heterogeneity of myoclonus. Receptor abnormalities may be the primary pathophysiologic defect, involving the surface recognition site or effector-transducer mechanisms in the defect, involving the surface recognition site or effector-transducer mechanisms in the post-synaptic cell. Alternatively, changes in receptor density or affinity may be adaptive (recovery of function) or maladaptive (supersensitivity, subsensitivity, dysregulation). Drug treatments, then, could be targeted at the receptor changes, reversing abnormalities and enhancing compensatory mechanisms. Current therapy may inadvertently have such an effect. Polytherapy may be justified to target more than one component of the "pharmacologic receptor". Identification of receptor abnormalities in human post-mortem brain may have diagnostic and therapeutic significance. New advances in the pharmacologic selectivity of receptor agonists and antagonists and in the measurement of receptors should be applied to the problem of myoclonus.

Effects of acute and chronic administration of Leu-enkephalin on cultured serotonergic neurons: evidence for opioids as inhibitory neuronal growth factors

Leu-enkephalin, at concentrations between 18 microM and 1.8 pM, was administered in a single or daily dose to dissociated mesencephalic raphe cell cultures maintained for 3 or 5 days. Daily administration of Leu-enkephalin produced an inhibition of high affinity uptake of [3H]5-HT, a measure of serotonergic process outgrowth in cultures of fetal neurons. This inhibition was maximal at a dose of 18 nM in both 3 (59%, P less than 0.05)- and 5 (38%, P less than 0.05)-day cultures. The expression of uptake was consistently lower in 5-day cultures than in 3-day cultures at all concentrations tested. In marked contrast, a single dose of Leu-enkephalin at the time of plating stimulated uptake in 3- and 5-day cultures. Maximal stimulation was observed at 180 nM for both 3 (191%, P less than 0.05)- and 5 (140%, P less than 0.05)-day cultures. The results obtained after a single dose of the opioid may reflect a paradoxical stimulation probably due to a rebound mechanism of receptors since co-administration of bacitracin (0.5 mg/ml), an aminopeptidase inhibitor, resulted in inhibition of the uptake expression. Together these results indicate that Leu-enkephalin can function as an inhibitory regulatory growth factor for neuronal cultures when constant exposure to this opioid is maintained over time.

Effect of neurotransmitters on the system that transports Tyr-MIF-1 and the enkephalins across the blood-brain barrier: a dominant role for serotonin

Neurotransmitters and neuropeptides interact in several ways. We studied a new type of interaction: the effect of neurotransmitters on the saturable system that transports Tyr-MIF-1 and the enkephalins out of the central nervous system (CNS). The neurotransmitters were introduced into the lateral ventricle of the brain with radioiodinated peptide, using an established method previously shown to accurately quantify the amount of peptide being transported from the CNS to the blood. Serotonin inhibited transport, histamine stimulated transport, and dopamine, acetylcholine, epinephrine, GABA, kainic acid, cAMP and cGMP were without effect. Cyproheptadine, a serotonin antagonist, stimulated transport. Of several psychotropic agents tested, only tranylcypromine had a statistically significant effect and stimulated transport. Of the serotonin receptor specific agents tested, those with 5HT1 activity most consistently affected transport. We conclude that serotonin, and perhaps histamine, are important modulators of the system that transports Tyr-MIF-1 and the enkephalins out of the CNS.

Effect of antiepileptic and antimyoclonic drugs on serotonin receptors in vitro

To determine if the mechanism of action of clinical and investigational antiepileptic and antimyoclonic drugs or neuropeptides involves direct actions at serotonin (5-HT) receptors, the activity of various compounds in vitro at 5-HT1 (with subtypes) and 5-HT2 sites was measured in adult rat brainstem, spinal cord, and neocortex. Adrenocorticotropic hormone (ACTH1-39) noncompetitively inhibited specific binding at 5-HT1, 5-HT1A, and 5-HT2 sites in brainstem and neocortex [concentrations required to displace 50% of ligand binding (IC50S) 4-8 X 10(-5) M]. ACTH1-24, ACTH1-17, and ACTH4-10 were sequentially less active, and ACTH34-39 and corticosterone were inactive. D-Ala2, Leu5-enkephalinamide, but not D-Ala2, Met5-enkephalinamide, also displaced spinal and neocortical 5-HT2 sites (IC50 6 X 10(-5) M). Piracetam, glycine, and the clinical antiepileptics valproate, phenacemide, phenytoin, carbamazepine, phenobarbital, diazepam, clonazepam, nitrazepam, and ethosuximide did not displace serotonergic radioligands, but melacimide showed some activity at 5-HT1 sites (IC50 7-9 X 10(-5) M). Anticonvulsant inactivity at 5-HT receptors in vitro correlates with the lack of antimyoclonic activity in 5-HT lesion myoclonic models but not with antimyoclonic efficacy in humans. These data indicate that acute effects of these anticonvulsants cannot be attributed to direct action at the 5-HT receptor recognition site in the rat. In contrast, ACTH showed mild in vitro displacement and regional specificity but only at micromolar concentrations.

Effect of aspirin on serotonin and met-enkephalin in brain: correlation with the antinociceptive activity of the drug

Intravenous administration of acetyl salicylate of lysine, a soluble salt of aspirin, reduced in rats the firing discharge of thalamic neurones, evoked by noxious stimuli. Concomitantly, concentrations of 5-hydroxyindole acetic acid increased, while those of met-enkephalin-like immuno-reactive derivatives were decreased in several areas of the brain. Similar electrophysiological and biochemical responses were obtained by administering tryptophan or 5-hydroxytryptophan plus carbidopa. The effect of aspirin on the evoked firing of the thalamic neurones was counteracted by pretreating the animals with metergoline. On the other hand, naloxone did not antagonize the inhibitory effect of aspirin and 5-hydroxytryptophan on pain-induced neuronal excitation. These data indicate that a serotonin-, but not a naloxone-sensitive opiate mechanism, may be relevant for aspirin-mediated antinociception.

Neurochemical perspectives of the narcoleptic syndrome

Narcolepsy has been defined as a disorder of excessive sleep often associated with cataplexy, sleep paralysis and hypnagogic hallucinations. Although the pathophysiology of the narcoleptic syndrome is not well understood, derangement in the functions of CNS catecholamines and serotonin (5-HT) have been implicated. In the present paper we summarize evidence to suggest a role for the endogenous opioids in the regulation of normal sleep and in the pathophysiology of the narcoleptic syndrome.

Increase in paradoxical sleep after destruction of serotoninergic innervation in the nucleus tractus solitarius of the rat

The evolution of paradoxical sleep, slow-wave sleep and arterial pressure was studied following microinjection of 5,7-dihydroxytryptamine in the nucleus tractus solitarius in rats. The extent of the lesions was assessed using immunohistochemistry for serotonin. Global lesions of serotoninergic nerve terminals of the intermediate and commissural regions of the nucleus produced an important and long-lasting increase in paradoxical sleep (+50-70%), a decrease in slow-wave sleep (-20%) and a moderate increase of arterial pressure during all states of the sleep-wake cycle. In addition, more discrete lesions indicated that only the lesion of the area near the obex produced the longer term increase of paradoxical sleep whereas only the lesion of the commissural region of the nucleus produced the long-term decrease of slow-wave sleep. These data demonstrate that serotoninergic projections to the nucleus tractus solitarius exert a regulatory influence upon the specific mechanisms responsible for paradoxical sleep and slow-wave sleep in rats. Furthermore, they suggest that serotonin within the nucleus tractus solitarius plays an important role in the homeostatic cardiovascular and sleep-wake-cycle regulation in rats.

Saturable transport of peptides across the blood-brain barrier

Peptides can be transported across the blood-brain barrier by saturable transport systems. One system, characterized with radioactively labeled Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide), is specific for some of the small peptides with an N-terminal tyrosine, including Tyr-MIF-1, the enkephalins, beta-casomorphin, and dynorphin (1-8). Another separate system transports vasopressin-like peptides. The choroid plexus has at least one system distinguishable from those above that is capable of uptake and possibly transport of opiate-like peptides. The possibility of saturable transport of other peptides has been investigated to a varying degree. Specificity, stereo-specificity, saturability, allosteric regulation, modulation by physiologic and pharmacologic manipulations, and noncompetitive inhibition have been demonstrated to occur in peptide transport systems and suggest a role for them in physiology and disease.

Reduction of elevated CSF beta-endorphin by fenfluramine in infantile autism

Fenfluramine therapy has been reported to improve behavior in infantile autism and has been associated with a decrease in abnormally increased blood serotonin content. The primary central effect has not been proved to be serotonergic. Beta-endorphin is involved in the anorexic effect of fenfluramine and may play a role in autism. Nine children with infantile autism were treated with fenfluramine in double-blind, placebo-crossover design. Transient anorexia was the only adverse effect. Autistic behavior was reported to improve in three patients, but objective psychometric tests were unchanged. Beta-endorphin-like immunoreactivity was determined in lumbar cerebrospinal fluid of patients during and before or after treatment with fenfluramine and then was compared to normal controls. Beta-endorphin was elevated significantly in the baseline autistic group (p less than .005) and was reduced toward control values during fenfluramine treatment. The results are consistent with a role for beta-endorphin in infantile autism and in the mechanism of fenfluramine treatment.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

Involvement of endogenous opioid peptides in fenfluramine anorexia

The effect of chronic fenfluramine (20 mg/kg, once daily) injections on the brain and peripheral immunoreactive (ir) dynorphin (DYN), alpha-neoendorphin (ANEO) and beta-endorphin (BE) was studied in rats. Fenfluramine injected repeatedly for 5 and 9 days induced anorexia. In the same animals there were no significant changes in the ir-DYN and ir-ANEO contents in the brain and pituitary. However, the ir-DYN and ir-ANEO contents in the gastrointestinal tract (duodenum) were markedly decreased after 5 and 9 days of fenfluramine injection. In contrast to ir-DYN and ir-ANEO, there was an increase in the hypothalamic and a decrease in the anterior lobe of pituitary ir-BE content. There was no significant change in the neurointermediate (NI) lobe of the pituitary. The results of our study suggest that part of the fenfluramine anorexia may be mediated by the peripheral prodynorphin and central beta-endorphin systems.

The endogenous opioid system in Gilles de la Tourette syndrome

The various neurotransmitter systems postulated to be involved in the pathogenesis of Gilles de la Tourette syndrome(GTS) are described with special reference to the endogenous opioid system(EOS). Malfunction of the opioid system is proposed as the underlying disturbance in this disease causing secondary dysfunction of the other systems. Furthermore, the various symptoms of the illness are examined also in terms of dysfunction of the endogenous opioid system.

Distribution of enkephalin-immunoreactive cell bodies in relation to serotonin-containing neurons in the raphe nuclei of the cat: immunohistochemical evidence for the coexistence of enkephalins and serotonin in certain cells

In this study we have examined the distribution of enkephalin-like immunoreactive (ELI) cell bodies in the cat raphe nuclei pallidus (NRP), obscurus (NRO), magnus (NRM), pontis and dorsalis (NRD) after intraventricular administration of colchicine. All the raphe nuclei examined were observed to contain ELI cell bodies along their whole caudorostral extent. By comparing consecutive sections treated separately with anti-5-HT and enkephalin-antiserum it was observed that certain 5-HT cells in each raphe nucleus contain ELI material. A quantitative estimation was attempted. In NRP and NRO approximately half of the total immunoreactive neuronal population appeared to be immunoreactive for both 5-HT and the enkephalins. In NRM the proportion would be one-third, whereas it seemed almost negligible in NRD. Among the 5-HT cells, approximately two-thirds might be ELI in NRP and NRO, and one-half in NRM.

Neurochemical mechanism of action of drugs which modify feeding via the serotoninergic system

The neurochemical mechanisms by which drugs acting on central serotoninergic system modify feeding were reviewed. Fenfluramine, a clinically effective appetite suppressant, releases serotonin from nerve terminals and inhibits its reuptake, and considerable evidence suggests that these effects mediate its anorectic activity. The D isomer of fenfluramine is particularly specific in affecting serotonin mechanisms and causing anorexia. Transmitters other than serotonin such as acetylcholine, catecholamines and GABA are also affected by systemic administration of fenfluramine, but some of these effects are secondary to fenfluramine's action on serotoninergic mechanisms. Moreover, there is no evidence that these brain substances are involved in fenfluramine's ability to cause anorexia. Several studies with drugs affecting different serotonin mechanisms such as release and uptake or mimicking the action of serotonin at post-synaptic receptors suggest that increase serotonin release and direct stimulation of postsynaptic receptors are the most effective mechanisms for causing depression of food intake, although inhibition of serotonin uptake may also contribute in appropriate conditions. Development of serotonin receptor hyposensitivity and, in some instances, decreased serotonin levels may lead to tolerance to the anorectic activity of drugs enhancing serotonin transmission, the degree of this depending critically on the type of effect on serotonin mechanisms and intensity and duration of serotonin receptor activation. Recent evidence suggests that a decrease in serotonin function causes stimulation of feeding. This may lead to development of new strategies for the treatment of clinical anorexias.

Differential regulation of the brain and gut immunoreactive dynorphin by the serotonin system

Administration of drugs such as fenfluramine, 20-40 mg/kg, and m-chlorophenylpiperazine (m-CPP), 2.5-5 mg/kg, which release serotonin or activate postsynaptic serotonin receptors, respectively, induced a dramatic decrease in the duodenal content of immunoreactive dynorphin (ir-DYN). The effect was antagonized by cyproheptadine, 1 mg/kg. Similarly, acute administration of the specific serotonin reuptake blockers fluvoxamine, 15 mg/kg, or femoxetine, 10 mg/kg, and 5-hydroxytryptophan (5-HTP), 40-160 mg/kg, evoked a marked decrease in the duodenal content of ir-DYN. A combined administration of fluvoxamine or femoxetine and 5-HTP failed to potentiate the effect of individual treatment. Only a higher dose of fenfluramine, 40 mg/kg, increased the ir-DYN content in the hypothalamus. These results suggest that the brain and gut ir-DYN is independently regulated by the serotonin system and that a serotonin mechanism might stimulate release of the gut dynorphin content.

Hypothalamic neurotransmitter function in experimentally induced hyperprolactinemia

It is known that animals or patients bearing a prolactin (PRL)-secreting tumor (PST) do not suppress PRL levels after administration of indirectly acting dopamine agonists, namely nomifensine (Nom), and are not responsive to the PRL releasing effect of antidopaminergic drugs and opioid peptides. Since the action of these drugs is mediated through the tuberoinfundibular dopaminergic (TIDA) system, these findings have been taken to indicate that animals and humans bearing prolactinomas have a defective TIDA function. Alternatively, PRL unresponsiveness to these drugs could be due to hyperfunction of TIDA system for the feedback action of high PRL levels. To clarify whether hypo- or hyperfunction of the TIDA system was responsible for such behaviour, we tested the effect of a synthetic opioid peptide (FK 33-824), a DA receptor antagonist, domperidone (Dom), and of Nom on PRL secretion in two experimental models of non-tumoral hyperprolactinemia, i.e. rats bearing ectopic pituitaries since 3 days (TP rats), or treated with ovine PRL (oPRL 250 micrograms, twice daily for 3 days), in which existence of an increased TIDA function has been demonstrated. FK 33-824 (0.5 mg/kg i.p.) increased significantly plasma PRL levels in control rats but failed to do so in TP rats and it elicited a significantly lower PRL response than in controls in rats treated with oPRL. In both experimental models, a PRL secretagogue, e.g. 5-hydroxytryptophan (50 mg/kg i.p.), elicited the same response as in controls, indicating that the pituitary PRL pool was preserved.(ABSTRACT TRUNCATED AT 250 WORDS)

A reduction of the tone of 5-hydroxytryptamine neurons decreases utilization rates of striatal and hypothalamic enkephalins

The administration of drugs such as fenfluramine, p-chlorophenylalanine or 5,7-dihydroxytryptamine, which lower brain 5-hydroxytryptamine content, increases the enkephalin content of hypothalamus and striatum. None of these treatments changes proenkephalin mRNA content. On the basis of these results, we propose that 5-hydroxytryptamine does not increase the biosynthesis of enkephalin, but might increase the content of the neuropeptide by reducing peptide utilization.

Basic and regulatory mechanisms of in vitro release of Met-enkephalin from the dorsal zone of the rat spinal cord

Under control conditions, superfused slices of the dorsal half of the lumbar enlargement from adult rats released Met-enkephalin-like material (MELM) that behaved as authentic Met-enkephalin under two different chromatographic procedures (Bio-gel filtration, HPLC). MELM release increased markedly on exposure of slices to batrachotoxin (0.5 microM) or to an excess of K+ (28 and 56 mM instead of 5.6 mM). The K+-evoked release was totally dependent on the presence of Ca2+ in the superfusing fluid whereas the spontaneous efflux of MELM was only partially Ca2+-dependent. Further experiments performed with tissues of polyarthritic rats indicated that the increase in their MELM levels was associated with a lower fractional rate constant of MELM release, therefore suggesting that spinal Met-enkephalin turnover might be reduced in chronically suffering animals. Examination of the possible modulation of MELM release by various neuroactive compounds present within the dorsal horn revealed that cholecystokinin (10 microM), but not its desulphated derivative, substance P-sulphoxide (10 microM), and to a lesser extent substance P, enhanced the K+-evoked MELM release. In contrast, gamma-aminobutyric acid (10 microM) and (-)-baclofen (1 microM) partially prevented the stimulatory effect of K+ on MELM release. Other compounds such as serotonin, somatostatin, and neurotensin altered neither the spontaneous nor the K+-evoked release of MELM.

Effects of beta-endorphin fragment 6-31 on morphine- and beta-endorphin-induced growth hormone and prolactin release

A fragment of human beta-endorphin (beta h-EP-(6-31] has been proposed as an endogenous inhibitor of beta-endorphin. We have evaluated the effects of the fragment on beta-endorphin- and morphine-induced prolactin and growth hormone release. It inhibited both morphine- and beta-endorphin-induced prolactin release, while it was ineffective on the release of growth hormone.

Presence of opiate receptors on striatal serotoninergic nerve terminals

After degeneration of serotoninergic neurons induced by either transection of the ascending neuronal pathways originating from the nucleus raphe dorsalis or intraventricular 5,6-dihydroxytryptamine administration, the number of binding sites for [3H]D-Ala2, Met5-enkephalinamide was significantly reduced. This decrease in binding sites does not seem to be related to the opiate receptors present on dopaminergic terminals, nor is it due to a simple decrease in serotoninergic neuronal tone, since after p-chlorophenylalanine (100 mg/kg X 4 days) the number of striatal binding sites for the opiate ligand was not diminished. On the other hand, shortly after mechanical interruption of the raphe-striatal serotoninergic fibers, at a time when the metabolic processes are still functioning in the lesioned neurons, morphine still increased the striatal content of 5-hydroxyindoleacetic acid. These results suggest the presence of opiate receptors on striatal serotoninergic terminals, where they may modulate the presynaptic activity of these neurons.