Evidence that GABA in the nucleus dorsalis raphé induces stimulation of locomotor activity and eating behavior

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

The additive effect of allopregnanolone on ghrelin's orexigenic effect in rats

The progesterone metabolite, allopregnanolone (AlloP), is a GABA_A receptor modulating steroid and is known to have orexigenic and pro-obesity effects. The neurobiological mechanisms underpinning these effects are most likely due to enhanced GABAergic signaling in the lateral arcuate nucleus (ARC) and medial paraventricular nucleus (PVN) of the hypothalamus. Inspired by the finding that GABAergic signaling is also important for the orexigenic effects of the circulating hormone, ghrelin, we sought to determine the extent to which AlloP (one of the most potent endogenous GABA_A-receptor modulators) operates alongside ghrelin to enhance food intake. Male rats with ad libitum access to standard chow were injected intravenously with AlloP and/or ghrelin, alone or in combination. The intake of the standard chow was greater after AlloP 1 mg/kg together with ghrelin 30 μg/kg than with 30 μg/kg ghrelin alone. Food intake was also increased for the combined treatment of AlloP 0.5 mg/kg + ghrelin 10 μg/kg, AlloP 1 mg/kg + ghrelin 10 μg/kg, and AlloP 0.5 mg/kg + ghrelin 30 μg/kg. There was no significant difference in food intake between the two ghrelin doses or between the two doses of AlloP and the vehicle. In electrophysiological studies, physiologically relevant concentrations of AlloP prolonged the current decay time of spontaneous inhibitory post-synaptic current of dissociated cells of the ARC and PVN. We conclude that AlloP enhances the hyperphagic effect of ghrelin, findings of potential relevance for the hyperphagia associated with the luteal phase of the reproductive cycle.

Effects of Injection of Gamma-Aminobutyric Acid Agonists into the Nucleus Accumbens on Naloxone-Induced Morphine Withdrawal

Aims: This study was to investigate the effects of local administration of gamma-aminobutyric acid (GABA) agonists into the nucleus accumbens (NAc) on naloxone-induced morphine withdrawal symptoms. Methods: Bilateral guide cannulas were stereotaxically implanted in the shell or core regions of the NAc of Sprague-Dawley rats. After a recovery period, 3 morphine pellets, each consisting of 75 mg morphine base, were placed subcutaneously on the first and third days of the study with the rats under mild ether anaesthesia. The GABA agonists, baclofen hydrochloride or muscimol hydrobromide, were injected into the NAc, and morphine withdrawal was induced by naloxone on the fifth day. Results: Administration of baclofen to the shell or core regions of the NAc of Sprague-Dawley rats led to statistically significant decreases in both behavioural and locomotor activity parameters during the morphine withdrawal period, compared to the control group. However, there were no statistically significant changes in locomotor activity or withdrawal behavioural parameters, with the exception of wet dog shakes, between control and muscimol-treated groups. Conclusion: These findings show that GABAergic conduction in the NAc is effective on the morphine withdrawal symptoms, and that both the shell and core regions of the NAc are associated with this effect.

Neurocircuitry of drug reward

In recent years, neuroscientists have produced profound conceptual and mechanistic advances on the neurocircuitry of reward and substance use disorders. Here, we will provide a brief review of intracranial drug self-administration and optogenetic self-stimulation studies that identified brain regions and neurotransmitter systems involved in drug- and reward-related behaviors. Also discussed is a theoretical framework that helps to understand the functional properties of the circuitry involved in these behaviors. The circuitry appears to be homeostatically regulated and mediate anticipatory processes that regulate behavioral interaction with the environment in response to salient stimuli. That is, abused drugs or, at least, some may act on basic motivation and mood processes, regulating behavior-environment interaction. Optogenetics and related technologies have begun to uncover detailed circuit mechanisms linking key brain regions in which abused drugs act for rewarding effects. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Brain reward circuitry beyond the mesolimbic dopamine system: a neurobiological theory

Reductionist attempts to dissect complex mechanisms into simpler elements are necessary, but not sufficient for understanding how biological properties like reward emerge out of neuronal activity. Recent studies on intracranial self-administration of neurochemicals (drugs) found that rats learn to self-administer various drugs into the mesolimbic dopamine structures-the posterior ventral tegmental area, medial shell nucleus accumbens and medial olfactory tubercle. In addition, studies found roles of non-dopaminergic mechanisms of the supramammillary, rostromedial tegmental and midbrain raphe nuclei in reward. To explain intracranial self-administration and related effects of various drug manipulations, I outlined a neurobiological theory claiming that there is an intrinsic central process that coordinates various selective functions (including perceptual, visceral, and reinforcement processes) into a global function of approach. Further, this coordinating process for approach arises from interactions between brain structures including those structures mentioned above and their closely linked regions: the medial prefrontal cortex, septal area, ventral pallidum, bed nucleus of stria terminalis, preoptic area, lateral hypothalamic areas, lateral habenula, periaqueductal gray, laterodorsal tegmental nucleus and parabrachical area.

The GABAB receptor agonist baclofen administered into the median and dorsal raphe nuclei is rewarding as shown by intracranial self-administration and conditioned place preference in rats

RATIONALE

The midbrain raphe regions have long been implicated in affective processes and disorders. There is increasing evidence to suggest that the median (MR) and dorsal raphe nuclei (DR) tonically inhibit reward-related processes.

OBJECTIVES

Stimulation of GABAB receptors in the midbrain raphe nuclei is known to inhibit local neurons, especially serotonergic neurons. We sought to determine if injections of the GABAB receptor agonist baclofen into the MR or DR are rewarding, using intracranial self-administration and conditioned place preference.

RESULTS

Rats quickly learned to lever press for infusions of baclofen (0.1–2.5 mM) into the MR, but not the ventral tegmental area or central linear nucleus. Rats increased lever pressing associated with intra-DR baclofen infusions, but not readily. Baclofen self-administration into the MR or DR was attenuated by coadministration of the GABAB receptor antagonist SCH 50911 (1 mM) or systemic pretreatment with the dopamine receptor antagonist SCH 23390 (0.025 mg/kg, i.p.). In addition, intra-DR and intra-MR injections of baclofen induced conditioned place preference; injection into DR was more effective.

CONCLUSIONS

Baclofen injections into the midbrain raphe nuclei are rewarding. Baclofen was more readily self-administered into the MR than into the DR, while baclofen injections into the DR more readily induced conditioned place preference than those into the MR. These sites may be differentially involved in aspects of reward. These findings suggest that MR or DR neurons containing GABAB receptors are involved in tonic inhibitory control over reward processes.

Reconciling the role of central serotonin neurons in human and animal behavior

Animal research suggests that central serotonergic neurons are involved in behavioral suppression, particularly anxiety-related inhibition. The hypothesis linking decreased serotonin transmission to reduced anxiety as the mechanism in the anxiolytic activity of benzodiazepines conflicts with most clinical observations. Serotonin antagonists show no marked capacity to alleviate anxiety. On the other hand, clinical signs of reduced serotonergic transmission (low 5-HIAA levels in the cerebrospinal fluid) are frequently associated with aggressiveness, suicide attempts, and increased anxiety. The target article attempts to reconcile such human and animal findings by investigating whether anxiety reduction or increased impulsivity is more Likely to account for animal behavioral changes associated with decreased serotonergic transmission. The effects of manipulating central serotonin in experimental anxiety paradigms in animals (punishment, extinction, novelty) are reviewed and compared with the effects of antianxiety drugs. Anxiety seems neither necessary nor sufficient to induce control by serotonergic neurons on behavior. Further evidence suggests that behavioral effects of anxiolytics thought to be mediated by decreases in anxiety are not caused by the ability of these drugs to reduce serotonin transmission. Blockade of serotonin transmission, especially at the level of the substantia nigra, results in a shift of behavior toward facilitation of responding. This behavioral shift is particularly marked when there is competition between acting and restraining response tendencies and when obstacles prevent the immediate attainment of an anticipated reward. It is proposed that serotonergic neurons are involved not only in behavioral arousal but also in enabling the organism to arrange or tolerate delay before acting. Decreases in serotonin transmission seem to be associated with the increased performance of behaviors that are usually suppressed, though not necessarily because of the alleviation of anxiety, which might contribute to the suppression.

The midbrain raphe nuclei mediate primary reinforcement via GABA(A) receptors

Because rats learn to lever-press for brief electrical stimulation of the median and dorsal raphe nuclei (MRN and DRN, respectively), these brain sites have long been implicated in reward processes. However, it is not clear whether the MRN and DRN integrate reward-related signals or merely contain fibers of passage involved in reward processes. To shed light on this issue, the present study employed chemicals that selectively modulate neurotransmission, in particular the GABA(A) receptor agonist muscimol. Rats quickly learned to lever-press for muscimol infusions (50 and 100 microM) into the MRN or DRN. Muscimol was not self-administered when cannulae were placed just outside these nuclei. The reinforcing effects of muscimol appeared to be greater when the drug was administered into the MRN than into the DRN, as demonstrated by higher infusion rates and better response discrimination. These observations are consistent with the additional finding that muscimol administration into the MRN, but not the DRN, induced conditioned place preference. The reinforcing effects of muscimol administration into the MRN were blocked by coadministration of the GABA(A) antagonist picrotoxin (100 microM) and by pretreatment with the dopamine receptor antagonist SCH 23390 (0.025 mg/kg, i.p.). The present results suggest that median and dorsal raphe neurons presumably inhibited by muscimol via GABA(A) receptors are involved in integration of primary reinforcement, and that median raphe neurons exert tonic inhibition over dopamine-dependent reward circuitry. The midbrain raphe nuclei may be involved in a variety of reward-related phenomena including drug addiction.

Evidence That GABA Mediates Dopaminergic and Serotonergic Pathways Associated With Locomotor Activity in Juvenile Chinook Salmon (Oncorhynchus tshawytscha)

The authors examined the control of locomotor activity in juvenile salmon (Oncorhynchus tshawytscha) by manipulating 3 neurotransmitter systems--gamma-amino-n-butyric acid (GABA), dopamine, and serotonin--as well as the neuropeptide corticotropin releasing hormone (CRH). Intracerebroventricular (ICV) injections of CRH and the GABAA agonist muscimol stimulated locomotor activity. The effect of muscimol was attenuated by administration of a dopamine receptor antagonist, haloperidol. Conversely, the administration of a dopamine uptake inhibitor (4',4"-difluoro-3-alpha-[diphenylmethoxy] tropane hydrochloride [DUI]) potentiated the effect of muscimol. They found no evidence that CRH-induced hyperactivity is mediated by dopaminergic systems following concurrent injections of haloperidol or DUI with CRH. Administration of muscimol either had no effect or attenuated the locomotor response to concurrent injections of CRH and fluoxetine, whereas the GABAA antagonist bicuculline methiodide potentiated the effect of CRH and fluoxetine.

Alterations in food intake elicited by GABA and opioid agonists and antagonists administered into the ventral tegmental area region of rats

Food intake is significantly increased following administration of mu-selective opioid agonists into the ventral tegmental area (VTA) region acting through multiple local opioid receptor subtypes. Since GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-enkephalin (DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either GABA(A) (bicuculline) or GABA(B) (saclofen) antagonists, and further, whether pretreatment with either general opioid or selective GABA receptor antagonists decreased feeding elicited by GABA(A) (muscimol) or GABA(B) (baclofen) agonists in the VTA region. DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either GABA(A) or GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived seizures following bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast, GABA(B) receptors are involved in the full expression of mu-opioid agonist-induced feeding in this region since saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with naltrexone in the VTA region reduced intake elicited by baclofen, but not muscimol. Finally, baclofen-induced feeding was significantly reduced by saclofen, but not bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between GABA(B) and opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.

GABA(A) and GABA(B) receptors in the anterior piriform cortex modulate feeding in rats

The effects of GABA(A) and GABA(B) receptors in the anterior piriform cortex (APC) on intake of an amino acid imbalanced diet and a basal diet were evaluated in rats. Administration of muscimol (GABA(A) receptor agonist) to the APC immediately suppressed ingestion of both amino acid imbalanced and basal diets. Central administration of bicuculline (a GABA(A) receptor antagonist) stimulated feeding of the amino acid imbalanced diet but had no effect on intake of the basal diet. The GABA(B) receptor antagonist phaclofen decreased consumption of the basal diet but did not affect consumption of the amino acid imbalanced diet. These findings demonstrate that manipulation of GABA-sensitive cells in the APC can have a pronounced effect on feeding behavior that is not selective to aminoprivic feeding. However, these data suggest that GABA(A) and GABA(B) receptors may function as regulators that are activated by monoaminergic systems and neuropeptides in response to amino acid imbalanced diet intake. Inhibitory effects of GABA(A) and GABA(B) receptors may modulate the pyramidal cells, contributing to the reduced feeding response to the amino acid imbalanced diet. Also, transcription of mRNA for both GABA receptors and the GABA reuptake transporter was affected by a threonine deficient but not a corrected diet, compared to the basal diet. Taken together, these results support the involvement of GABA receptors in the APC in feeding in general and the responses to amino acid deprivation in vivo.

gamma-Aminobutyric acid receptor subtype antagonists differentially alter opioid-induced feeding in the shell region of the nucleus accumbens in rats

Food intake is significantly increased by administration of mu-selective opioid agonists into the nucleus accumbens, particularly its shell region. Pretreatment with either opioid (mu, delta(1), delta(2) or kappa(1)) or dopaminergic (D(1)) receptor antagonists in the nucleus accumbens shell reduce mu opioid agonist-induced feeding. Selective GABA(A) (muscimol) and GABA(B) (baclofen) agonists administered into the nucleus accumbens shell each stimulate feeding which is respectively and selectively blocked by GABA(A) (bicuculline) and GABA(B) (saclofen) antagonists. The present study investigated whether feeding elicited by the mu-selective opioid agonist, [D-Ala(2),NMe(4),Gly-ol(5)]-enkephalin in the nucleus accumbens shell was decreased by intra-accumbens pretreatment with an equimolar dose range of either GABA(A) or GABA(B) antagonists, and further, whether general opioid or selective GABA antagonists decreased feeding elicited by GABA(A) or GABA(B) agonists in the nucleus accumbens shell. Feeding elicited by the mu-selective opioid agonist was dose-dependently increased following intra-accumbens pretreatment with GABA(A) (bicuculline) antagonism; this enhancement was significantly blocked by pretreatment with general or mu-selective opioid antagonists. In contrast, mu opioid agonist-induced feeding elicited from the nucleus accumbens shell was dose-dependently decreased by GABA(B) (saclofen) antagonism. Neither bicuculline nor saclofen in the nucleus accumbens shell altered baseline food intake. Whereas muscimol-induced feeding elicited from the nucleus accumbens shell was reduced by bicuculline and naltrexone, but not saclofen pretreatment, baclofen-induced feeding elicited from the nucleus accumbens shell was reduced by saclofen, but not by bicuculline or naltrexone. These data indicate that GABA(A) and GABA(B) receptor subtype antagonists differentially affect feeding elicited by mu opioid receptor agonists within the nucleus accumbens shell in rats.

Studies on the behavioral activation produced by stimulation of GABAB receptors in the median raphe nucleus

Injections of the GABAB agonist baclofen into the median raphe nucleus (MR) resulted in marked hyperactivity and in increases in food and water intake by non-deprived animals. The locomotor effects of baclofen were stereospecific and could be antagonized by coinjection of the GABAB antagonist 2-hydroxysaclofen. Hyperactivity was produced by lower doses of baclofen, at shorter latencies, when the drug was injected into the MR than when it applied to the dorsal raphe nucleus (DR) or the ventral tegmental area (VTA). The locomotor response to intra-MR baclofen was unaltered in animals pretreated with the serotonin synthesis inhibitor p-chlorophenylalanine. Finally, intra-MR injections of baclofen produced a large increase in dopamine metabolism in the nucleus accumbens and striatum but failed to alter hippocampal or striatal serotonin metabolism. These findings suggest that baclofen may produce increases in activity and ingestive behavior as a result of an action on non-serotonergic cells in the MR.

Interaction of taurine and related compounds with GABAergic neurones in the nucleus raphe dorsalis

The effects of GABAA (muscimol) and GABAB (baclofen) receptor agonists on spontaneous motor activity and food consumption of rats were compared to those produced by taurine and related compounds (3-aminopropanesulphonic acid, 5-aminovaleric acid, and guanidinoethanesulphonic acid). Local application of muscimol into the nucleus raphe dorsalis caused a dose-dependent increase in spontaneous motor activity. Muscimol-stimulated motor activity was blocked by picrotoxin. High doses relative to muscimol of 3-aminopropanesulphonic acid, guanidinoethanesulphonic acid, and 5-aminovaleric acid also attenuated the action of muscimol. Taurine by itself was ineffective on locomotion but enhanced the effect of a small dose of muscimol. Baclofen also stimulated activity but to a lesser extent than muscimol. Baclofen's stimulatory action on motor activity was partially blocked by 5-aminovaleric acid, whereas 3-aminopropanesulphonic acid was without effect. Muscimol and baclofen both increased food consumption of rats. Picrotoxin blocked this effect of muscimol, whereas the action of baclofen was blocked by 5-aminovaleric acid. Muscimol, taurine, and guanidinoethanesulphonic acid all reduced 5-hydroxytryptamine (5-HT) concentration in the hypothalamus. In radioligand binding studies, guanidinoethanesulphonic acid at micromolar concentrations displaced [3H]GABA from GABAA receptors. It is concluded that taurine may have a slight direct effect on GABA receptors but is more likely to act as an indirect neuromodulator of GABAergic neurotransmission in the brain.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

Initiation and execution of locomotion elicited by diencephalic stimulation: regional differences in response to nembutal

At moderate levels of Nembutal, within the anesthetic range, locomotor stepping can be elicited by brain stimulation. We determined if Nembutal (7, 14 and 28 mg/kg) had different effects on locomotion elicited by stimulation at different brain regions. Two regions were compared: the medial forebrain bundle (MFB, 13 sites) and the areas medial and dorsal to it (MED/DORSAL, 20 sites). Locomotion was produced by electrical stimulation (50 microA, 0.5 msec pulses, 10 to 160 Hz) of unrestrained rats in a rotary runway. The latency to initiate locomotion and the time to complete 1 revolution of the rotary were measured. With no drug, MFB locomotion was initiated sooner but took longer to complete than MED/DORSAL locomotion. Nembutal at 7 mg/kg did not affect initiation of MFB or MED/DORSAL locomotion. Nembutal at 14 mg/kg shortened MFB initiations, but this dose prolonged MED/DORSAL initiations. Initiations with both types of sites were blocked with 28 mg/kg. The 7 and 14 mg/kg doses prolonged the locomotor completion times of the MFB sites but not of the MED/DORSAL sites. The results indicate that the response to Nembutal differs qualitatively for locomotion elicited by stimulation of the MFB and locomotion elicited by stimulation of the medial and dorsal hypothalamus. The mechanisms underlying the difference remain to be elucidated; they may relate to nonlocomotor behaviors also elicited by stimulation or to the motivational states reflected in those behaviors.

Comparative studies of locomotor behavior following microinjections of muscimol into various sites in the paramedian tegmentum

Microinjections of various doses of muscimol into the median raphe nucleus, the dorsal raphe nucleus or the caudal portion of the ventral tegmental area elicited dose-dependent increases in locomotor activity. In contrast, injections into the rostral portion of the ventral tegmental area or the midline pontine tegmentum caudal to the median raphe were ineffective. Lower doses of muscimol were required to produce hyperactivity after injections into the median raphe than after injections into any of the other sites. These findings suggest that the median raphe nucleus is the most sensitive site in the paramedian tegmentum for the elicitation of hyperactivity by muscimol.

Comparative studies of the ingestive behaviors produced by microinjections of muscimol into the midbrain raphe nuclei of the ventral tegmental area of the rat

Microinjection of the GABA-A agonist muscimol into the median (MR) or dorsal (DR) raphe nuclei or the ventral tegmental area (VTA) of non-deprived rats induced intense feeding and drinking in a dose-dependent and site-specific manner. Lower doses of muscimol were required to increase food intake, spillage and water intake with injections into the MR than with injections into the other two sites. These data demonstrate that the MR is a more sensitive site for the elicitation of ingestive behavior than either the DR or the VTA.

Evidence that muscimol acts in the forced swimming test by activating the rat dopaminergic system

Muscimol as well as catecholaminergic drugs reduce immobility time in the forced swimming test. In view of the fact that GABAergic drugs may facilitate some brain catecholaminergic functions, we investigated as to whether or not muscimol would reduce immobility time through activation of catecholaminergic mechanisms. The effect of muscimol (2 mg/Kg i.p.) on reduction of immobility time was prevented by intraperitoneal alpha-methyl-para-tyrosine (250 mg/Kg i.p.), which reduces brain catecholamine content, haloperidol (0.5 mg/Kg) and sulpiride (100 and 50 mg/Kg), antidopaminergic drugs, and meta-chlorphenyl-piperazine (0.6 and 1.25 mg/Kg), a serotonergic agonist, but not by clonidine (0.1 mg/Kg), an alpha2-adrenoceptor agonist, d, 1-propranolol (5 mg/Kg), an antagonist of beta-adrenergic receptors, or subcutaneous prazosin (3 mg/Kg), an alpha1-adrenolytic drug. Our findings indicate that a) muscimol reduces immobility time by stimulating dopaminergic neurons and b) activation of the serotonergic system antagonizes muscimol effect.

Behavioral effects of GABA agonists in relation to anxiety and benzodiazepine action

A considerable body of biochemical and neurophysiological evidence implicates GABA in anxiety and in benzodiazepine action. The present article surveys the behavioral effects of GABA agonists and their interactions with drugs acting at the benzodiazepine receptor in animal anxiety paradigms. Certain GABA agonists, notably valproate, simulate many behavioral actions of benzodiazepines. Moreover, several behavioral studies of the interaction of GABA agonists with benzodiazepines support the hypothesis of a benzodiazepine receptor complex with one or more GABA, benzodiazepine and probably other binding sites. However, there are also a number of anomalous findings of GABA agonist action alone and in combination with benzodiazepines. It is argued that these paradoxical results can better be accounted for in terms of the receptor complex and the distribution of the drugs, rather than by suggesting that the anxiolytic actions of benzodiazepines are not mediated by GABA systems. The potential clinical usefulness of GABA agonists in anxiety is commented upon.

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.

Neurotransmitters, anxiety and benzodiazepines: a behavioral review

The possible involvement of serotonin, GABA and opioid peptides in anxiety and in the mechanism of action of benzodiazepine tranquilizers have recently been the subjects of intensive biochemical, neurophysiological and behavioral research. The present review examines the behavioral evidence, viewing anxiety and benzodiazepine action as far as possible separately. Four behavioral paradigms of experimental anxiety or "conflict behaviors" are described and assessed for soundness with some practical considerations. The functional significance and pharmacology of benzodiazepine receptors are discussed, and the cases for a number of putative endogenous ligands are examined. Conflict behavior is attenuated by drugs which reduce functional serotonin activity and enhanced by serotonin agonists, but there is little evidence to implicate serotonin in benzodiazepine action. GABA antagonists both intensify conflict and reduce benzodiazepine effects, but evidence of the reverse effects with GABA agonists is more equivocal. The interpretation of behavioral effects of opiate agonists and antagonists and their interactions with benzodiazepines is hindered by their actions on motivational systems other than anxiety, and evidence for an important role of opioid peptides is only suggestive. Some promising lines for future research are indicated.

Hyperphagia caused by muscimol injection in the nucleus raphe dorsalis of rats: its control by 5-hydroxytryptamine in the nucleus accumbens

Muscimol injection in the nucleus raphe dorsalis caused intense eating by rats with access to food. A dose-related reduction of muscimol's effect was found after bilateral injections of 5-hydroxytryptamine (5-HT) in the nucleus accumbens (dose range 2.2-8.8 micrograms in 2 microliter) but no effect was observed when an even higher dose (17.6 micrograms) of 5-HT was injected in the caudate putamen. Eating by food-deprived rats was not changed by any dose of 5-HT injected either into the nucleus accumbens or the caudate putamen. (+)-Norfenfluramine, 20 micrograms, injected in the nucleus accumbens also reduced muscimol-induced eating but had no effect on the food intake of starved rats. The results suggest an important role of 5-HT in the nucleus accumbens in the control of certain types of hyperphagia in rats.

Are serotonergic neurons involved in the control of anxiety and in the anxiolytic activity of benzodiazepines?

Several studies have shown that, like benzodiazepines (BZP), treatments able to reduce or block the activity of CNS serotonergic (5-HT) neurons released punished behavior. Therefore, 5-HT mechanisms have been tentatively implicated in the anti-punishment (anxiolytic?) activity of BZP. Numerous data, however, are not in keeping with this hypothesis. Since not responding enables the animals to avoid punishment but also delays the receipt of food-reward, one of these factors could be an alteration of waiting capacities. Indeed, we have shown that diazepam released behavioral suppression in conflict schedules only when the duration of the punished periods exceeded 1 minute. Moreover, in rats allowed to choose in a T-maze between immediate-but-small vs. delayed-but-large reward, BZP significantly decreased the frequency with which the delayed reward was chosen, with 5-HT uptake blockers producing opposite effects. Therefore, one can hypothesize that BZP render the animals less prone than controls to tolerate delay of reward and that 5-HT mechanisms may be involved in this phenomenon. An altered tolerance to delay of reward should be taken into account when interpreting the BZP-induced release of behavioral inhibition in classical conflict procedures.

Selective involvement of dopamine in the nucleus accumbens in the feeding response elicited by muscimol injection in the nucleus raphe dorsalis of sated rats

Muscimol injection (100 ng) in the nucleus raphe dorsalis (NRD) caused intense eating in non-food-deprived rats. At a dose (10 micrograms) blocking dopamine mediated responses (examined by increased locomotion or stereotypy caused by systemically injected d-amphetamine), fluphenazine injected in the n. accumbens, but not in the striatum, significantly reduced the eating response elicited by muscimol in the NRD while food intake of deprived rats was not significantly modified by fluphenazine injected in either area. Fluphenazine (20 micrograms) in the striatum reduced eating in both conditions, but the animals showed marked sedation which obviously interfered with the feeding response. Dopamine release and synthesis, measured respectively by 3-methoxytyramine and accumulation of dihydroxyphenylalanine after aromatic amino acid decarboxylase inhibition, were significantly reduced in the n. accumbens, but not in the striatum, of muscimol treated animals. The metabolism of serotonin was reduced in both areas of muscimol treated rats. It is suggested that changes in dopamine receptor sensitivity, together with changes in serotonin function, might be involved in the feeding response caused by muscimol injection in the NRD.

Peripheral and central mechanisms of action of serotoninergic anorectic drugs

Several pieces of evidence indicate that brain serotoninergic systems play an inhibitory role in feeding, being specifically involved in regulating satiety and food selection. The anorectic drug fenfluramine has been shown to exert its effects by activating serotoninergic mechanisms. Since fenfluramine influences both central and peripheral serotonin stores, it is difficult to establish the relative contributions of the central and peripheral serotoninergic mechanisms in the regulation of feeding behaviour. In the present paper evidence is presented that changes in feeding come about In the present paper evidence is presented that changes in feeding come about after interventions in either the brain or the periphery. This evidence includes the observation that serotonin itself given subcutaneously causes a dose-related anorexia in rats trained to eat four hours a day, an effect antagonized not only by metergoline but also by xilamidine, a serotonin antagonist that does not cross the blood-brain barrier. Since serotonin given systemically cannot reach the brain, its effect is ascribed to the activation of peripheral mechanisms. Furthermore, as is the case with fenfluramine, subcutaneous administration of serotonin is able to completely counteract the overeating induced by the glucoprivic agents insulin and 2-deoxy-D-glucose (2-DG). It is concluded that activation of peripheral serotoninergic mechanisms is sufficient not only to reduce eating in rats trained to eat four hours a day, but also to control the hyperphagias brought about by insulin or 2-DG.

Elicitation of conspecific attack or defense in the male rat by intraventricular injection of a GABA agonist or antagonist

The involvement of central GABAergic mechanisms in the control over offensive and defensive behaviours in the rat was studied using intracerebroventricular injections (5 microliter) of a GABA agonist (THIP) or a GABA antagonist (bicuculline methiodide). Intracerebroventricular injections of THIP (1.25 and 2.5 micrograms) induced attacks and offensive sideways towards an untreated partner, in animals placed in a neutral area where no aggressive reactions occur in controls. Social approach behaviours (partner investigation, allogrooming) were also increased in both attacking and non-attacking animals, whereas individual behaviours (cage exploration, autogrooming, immobile posture) were decreased. Inversely, intracerebroventricular injections of bicuculline methiodide (62.5 and 125 ng) suppressed offensive items (attacks, offensive sideways, upright postures) in resident animals confronted with untreated intruders and increased occurrence of defensive sideways. This treatment also decreased reactions oriented towards the partner (investigation, allogrooming and crawl under/over), while increasing individual behaviours (cage exploration, immobile posture). These data demonstrate that activation of central GABA receptors elicits intraspecific offensive behaviours in the rat. On the contrary, blockage of these receptors induces defensive reactions and suppresses offensive behaviours. The involvement of these receptors in the neural control over aggressive behaviour in the rat is discussed.

Inhibitory influence of GABA on central serotonergic transmission. Involvement of the habenulo-raphé pathways in the GABAergic inhibition of ascending cerebral serotonergic neurons

In order to explore the anatomical nature of the inhibitory GABAergic control of cerebral serotonergic neurons exerted at the level of the anterior raphé cells in the rat, we have studied the effect of GABA agonist agents (given systemically or infused locally into the raphé dorsalis or medianus) on cerebral 5-hydroxytryptophan (5-HTP) accumulation after lesion or pharmacological manipulation of various raphé inputs. Destruction of noradrenergic pathways by local injection of 6-hydroxydopamine in the pedunculus cerebellaris superior or by systemic injection of DSP-4 (50 mg/kg i.p.), or alteration of central dopaminergic transmission (by systemic administration of apomorphine or haloperidol) failed to modify the ability of progabide (400 mg/kg i.p.) or dipropylacetamide (150 mg/kg i.p.) to diminish 5-HTP accumulation in the striatum, hippocampus and substantia nigra. In contrast, electrolytic lesion of the habenular nuclei blocked the ability of these compounds (given systemically) to reduce 5-HTP accumulation both in serotonergic nerve terminal and cell body (raphé dorsalis and medianus) areas. A similar blockade of the effects of GABA mimetics was seen after ibotenate-induced lesion of the habenula but not after electrolytic lesion of the stria medullaris (which conveys most of the afferents to the habenula). Acute cessation of impulse flow in the habenulo-raphé tract also prevented the depamide-induced diminution of cerebral 5-HTP accumulation. Finally, interruption of nerve transmission in the habenulo-raphé pathways (by means of electrolytic lesion of the habenula or fasciculus retroflexus) blocked the ability of GABA (100 micrograms) or muscimol (50 ng) injected into the raphé dorsalis or medianus to reduce 5-HTP accumulation in the corresponding serotonergic nerve terminal areas. It is concluded that the GABAergic inhibition of ascending serotonergic neurons exerted in the raphé dorsalis and medianus depends upon an ongoing neuronal activity in the habenulo-raphé pathways; GABA may exert its inhibitory control over serotonergic neurons by tuning down a facilitatory influence on these cells exerted by the habenula.

Inhibitory influence of GABA on central serotonergic transmission. Raphé nuclei as the neuroanatomical site of the GABAergic inhibition of cerebral serotonergic neurons

Acute injection of the gamma-aminobutyric acid (GABA) mimetics progabide, aminooxyacetic acid, gamma-acetylenic GABA and dipropylacetamide reduced 5-hydroxytryptophan (5-HTP) accumulation in serotonergic nerve terminal regions (prefrontal cortex, olfactory tubercle, septum, striatum, hypothalamus, hippocampus, substantia nigra, cerebellum and spinal cord) as well as in corresponding cell body areas (raphé dorsalis, medianus, pontis and magnus). This effect was antagonized by bicuculline. The inhibition of serotonin (5-HT) synthesis induced by a single progabide administration was accentuated on repeated treatment in the striatum, prefrontal cortex and cerebellum but was similar to that seen after acute treatment in the other areas. Local infusion of high concentrations of GABA or GABA mimetics into the striatum, septum or substantia nigra failed to modify 5-HTP accumulation in these areas. Cerebral hemitransection antagonized the ability of progabide (1200 mg/kg i.p.) to diminish 5-HTP accumulation in the striatum, hippocampus and prefrontal cortex. Intra-raphé dorsalis infusion of muscimol (0.1-100 ng) or GABA (1-100 micrograms) decreased 5-HT synthesis in the corresponding projection areas (e.g. striatum, substantia nigra, cortex) but not in the hippocampus or cerebellum. Conversely, intra-raphé medianus infusion of these drugs diminished 5-HTP accumulation in the corresponding projection areas (e.g. hippocampus, septum, cortex) but not in the striatum or cerebellum. Intra-raphé dorsalis or medianus injection of GABA antagonists (bicuculline, picrotoxinin, RU-5135) was without effect on cerebral 5-HT synthesis but antagonized the diminution of the amine synthesis observed in corresponding projection areas after intra-raphé dorsalis or medianus infusion of muscimol or GABA. These results suggest that GABA exerts an inhibitory (non-tonic) control over central serotonergic neurons which is mediated via GABA receptors located in the raphé nuclei.

Changes of serotonin and dopamine metabolism in various forebrain areas of rats injected with morphine either systemically or in the raphe nuclei dorsalis and medianus

The regional brain metabolism of serotonin (5-HT) and dopamine (DA) was studied in rats injected with morphine either systemically or in the nuclei raphe medianus (MR) or dorsalis (DR). A subcutaneous injection of 10 mg/kg morphine significantly raised the levels of 5-hydroxyindoleacetic acid (5-HIAA) in the diencephalon, striatum, nucleus accumbens and cortex with no effect in the hippocampus. Similar changes in 5-HT metabolism were found in animals injected with 5 micrograms/0.5 microliter in the DR whereas morphine injected in the MR raised 5-HIAA levels only in the nucleus accumbens. A subcutaneous or direct injection of morphine in the DR significantly raised the levels of homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) in the striatum and nucleus accumbens, but injection in the MR was ineffective. All the effects of morphine were blocked by naloxone, injected either intraperitoneally (1 mg/kg) or directly in the raphe nuclei (2 micrograms/0.5 microliter). Pretreatment with parachlorophenylalanine, an inhibitor of serotonin synthesis, significantly reduced the effect of morphine injected in the DR on dopamine metabolism in the striatum and nucleus accumbens. The data suggest that a major mechanism by which morphine increases 5-HT metabolism in the rat forebrain is activation of 5-HT cells in the nucleus raphe dorsalis, and this action may contribute to the increased DA metabolism found in the animal injected with morphine in this brain area.