Intra-raphe muscimol induced hyperactivity depends on ascending serotonin projections

Ingestive and locomotor behaviours induced by pharmacological manipulation of <Alpha>-adrenoceptors into the median raphe nucleus

The present study evaluated the involvement of α-adrenoceptors of the median raphe nucleus (MRN) in satiated rats, in food and water intake and motor behaviour. Control groups were treated with saline (SAL) or adrenaline (ADR), injected into the MRN seven minutes after injection of the vehicle used to solubilize the antagonists, propylene glycol (PLG) or SAL. Experimental groups were treated with an α-adrenoceptor antagonist, prazosin (α1, 20 or 40 nmol) or yohimbine (α2, 20 or 40 nmol) or phentolamine (non-selective α, 20 or 40 nmol), followed (later) by injection of ADR or SAL. Behaviour was recorded for 30 min. The injection of ADR and the blockade of α1 receptors resulted in hyperphagia whereas blocking α2 or α1 and α2 simultaneously did not change feeding behaviour. Pre-treatment with prazosin, followed by injection of ADR was not able to cause an increase in the amount of food ingested, while the higher dose of the α1 antagonist reduced the latency to start feeding. Pre-treatment with prazosin also caused hyperactivity. However, pre-treatment with phentolamine or yohimbine was able to block ADR-induced feeding. The present study supports the hypothesis that there is a tonic activation of α1-adrenoceptors in the MRN in satiated rats, which activates an inhibitory influence in areas that control food intake. Injection of ADR seems to activate α2 receptors, resulting in a decrease in the availability of endogenous catecholamines, which reduces the release of the signal that inhibits food intake, leading to hyperphagia.

Dopamine is differentially involved in the locomotor hyperactivity produced by manipulations of opioid, GABA and glutamate receptors in the median raphe nucleus

The median raphe nucleus (MR) has been shown to exert a powerful influence on behavioral arousal and marked locomotor hyperactivity can be produced by intra-MR injections of a variety of drugs including GABAA and GABAB agonists, excitatory amino acid antagonists, and μ- and δ-opioid agonists. Other studies have indicated that the MR exerts an inhibitory influence on ascending dopamine systems, suggesting that MR induced alterations in activity may be mediated through changes in dopaminergic transmission. In the present study, we explored this possibility by examining whether systemic administration of the preferential D2 dopamine antagonist haloperidol is able to antagonize the hyperactivity produced by intra-MR injections of various drugs. We found that haloperidol completely blocked the locomotor response to intra-MR injections of the μ-opioid receptor agonist DAMGO and the δ-opioid receptor agonist DPDPE. In marked contrast, at doses which abolished the locomotor response to systemic amphetamine, haloperidol had no effect on the hyperactivity induced by intra-MR injections of GABAA agonist muscimol, the GABAB agonist baclofen, or the kainate/quisqualate antagonist pBB-PZDA, even though it suppressed baseline activity in these same animals. These results indicate that there must be at least two mechanisms capable of influencing behavioral arousal within the MR region, one of which is dependent on D2 dopamine receptors and the other is not.

Inactivation of the dorsal raphé nucleus reduces the anxiogenic response of rats running an alley for intravenous cocaine

Rats traversing a straight alley once a day for delivery of a single i.v. injection of cocaine develop over trials an ambivalence about entering the goal box. This ambivalence is characterized by the increasing occurrence of "retreat behaviors" where animals leave the start box and run quickly to the goal box, but then stop at the entry point and "retreat" back toward the start box. This unique pattern of retreat behavior has been shown to reflect a form of "approach-avoidance conflict" that stems from the animals' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Cocaine blocks reuptake of the serotonergic (5-HT) transporter and serotonin has been implicated in the modulation of anxiety. It was therefore of interest to determine whether inactivation of the serotonergic cell bodies residing in the dorsal raphé nucleus (DRN) and projecting to brain areas critical for the modulation of anxiety, would alter the anxiogenic state exhibited by rats running an alley for single daily i.v. injections of 1.0mg/kg cocaine. Reversible inactivation of the DRN was accomplished by intracranial application of a mixed solution of the GABA agonists baclofen and muscimol. While DRN inactivation had no impact on the subjects' motivation to initiate responding (i.e., latencies to leave the start box were unaffected) it reliably reduced the frequency of approach-avoidance retreat behaviors (conflict behavior). These data suggest that inactivation of the dorsal raphé reduces the conflict/anxiety otherwise present in experienced cocaine-seeking animals.

The supramammillo-septal-hippocampal pathway mediates sensorimotor gating impairment and hyperlocomotion induced by MK-801 and ketamine in rats

RATIONALE

Ketamine or MK-801 induced sensorimotor gating deficit, but the underlying neural mechanisms are not completely known. We have previously demonstrated that the medial septum (MS) mediated the phencyclidine-induced deficit in prepulse inhibition of the acoustic startle (PPI) in rats.

OBJECTIVES

We investigated the involvement of the supramammillary area (SUM) to MS pathway in PPI impairment and behavioral hyperlocomotion induced by MK-801 or ketamine in rats and correlated the behavioral deficits with hippocampal gamma wave increase.

MATERIALS AND METHODS

Ketamine (6 mg/kg, s.c.) or MK-801 (0.5 mg/kg, i.p.) was administered after infusion of saline or the GABA(A) receptor agonist, muscimol (0.25 microg), into the MS or SUM. Locomotion, PPI, and hippocampal electroencephalogram (EEG) were recorded.

RESULTS

MK-801 or ketamine induced PPI impairment and behavioral hyperlocomotion, accompanied by an increase in hippocampal gamma waves (30-100 Hz). The changes in behavior and gamma waves induced by ketamine or MK-801 were antagonized by pre-infusion of muscimol, but not saline, into the SUM or MS. Infusion of muscimol into the SUM alone did not significantly affect PPI, but it suppressed spontaneous locomotor behavior and hippocampal EEG. Infusion of ionotropic glutamate receptor antagonists into the MS did not affect the PPI deficit or the gamma wave increase after MK-801.

CONCLUSIONS

A non-glutamatergic component of the supramammillo-septal pathway mediates the hyperlocomotion and the deficits in PPI induced by MK-801 or ketamine. Inactivation of the MS or SUM normalized both the hippocampal gamma waves and the behavioral deficits (PPI impairment and hyperlocomotion).

Limbic system participates in mediating the effects of general anesthetics

In a previous study, we reported that inactivation of the medial septum or the hippocampus by muscimol, a GABA(A) receptor agonist, potentiated the effects of a general anesthetic. In this study, we further investigated whether other structures that are connected to the septohippocampal system are involved in mediating general anesthesia. In freely behaving rats, muscimol (0.25 microg) or saline was infused intracerebrally into one of four areas-the supramammillary area (SUM), nucleus accumbens (NAC), ventral pallidum (VP), and ventral tegmental area (VTA)-and righting, pain, and EEG responses were recorded following either halothane or sodium pentobarbital, representing inhalational and injectable general anesthetic, respectively. The effect of halothane (2%) or pentobarbital (20 mg/kg i.p.) in abolishing the righting, pain response, or low-voltage neocortical activity was enhanced, and the initial behavioral hyperactivity (delirium) was reduced, after muscimol as compared to after saline infusion in SUM, NAC, VP, and VTA. EEGs in the hippocampus and the sensorimotor cortex following halothane or pentobarbital showed increased delta, and decreased hippocampal theta and gamma waves after muscimol infusion as compared to saline infusion in SUM, NAC, VP, and VTA. By contrast, infusion of muscimol in the median raphe increased locomotion and did not significantly alter the behavioral or EEG effects of halothane or pentobarbital. It is suggested that structures that activate the limbic cortices (MS, SUM, and VTA but not the median raphe) or mediate the output of the hippocampus (NAC and VP) normally participate in maintaining consciousness and inactivation of these structures potentiates the response to a general anesthetic.

Effects of injections of 8-hydroxy-2-(di-n-propylamino)tetralin or muscimol in the median raphe nucleus on c-fos mRNA in the rat brain

Inhibition of the median raphe nucleus (MRN) by the local injection of 5-HT(1A) or GABA(A) receptor agonists produces strong activational effects on feeding, drinking and locomotor activity. Using an animal model of relapse, we have shown that intra-MRN injection of the 5-HT(1A) autoreceptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reinstates alcohol seeking in rats. The circuitry underlying the behavioral effects of intra-MRN injection of these drugs is not known. In order to identify the brain areas that may be involved, we measured levels of mRNA of the immediate early gene c-fos in discrete nuclei of the rat brain following intra-MRN infusions of these drugs. Male Wistar rats received intra-MRN infusions of 8-OH-DPAT (1 mug), muscimol (25 ng) or saline vehicle immediately prior to placement in locomotor activity chambers. Thirty minutes later, they were decapitated, and their brains processed for in situ hybridization of c-fos mRNA. In agreement with previous reports, injections of 8-OH-DPAT or muscimol into the MRN resulted in large increases in locomotor activity. Intra-MRN injections of these drugs increased c-fos in a number of brain nuclei previously shown to be involved in the rewarding effects of drugs of abuse in a regionally specific manner. Both drugs significantly increased the expression of c-fos mRNA in the medial frontal cortex, nucleus accumbens, lateral septum, dorsal bed nucleus of the stria terminalis and ventral tegmental area. In the ventral hippocampus, only 8-OH-DPAT increased c-fos, while in the basolateral nucleus of the amygdala and locus coeruleus, it was increased only by muscimol. These results are discussed in terms of the projections of the MRN and the pathways involved in relapse to alcohol and drug seeking.

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.

Collateral projections from the median raphe nucleus to the medial septum and hippocampus

It has previously been shown that the median raphe nucleus (MR) is a source of pronounced projections to the septum and hippocampus. The present study examined collateral projections from MR to the medial septum (MS) and to various regions of the hippocampus. The fluorescent retrograde tracers, Fluororuby and Fluorogold, were injected into the septum and hippocampus, respectively, and the median raphe nucleus was examined for the presence of single- and double-labeled neurons. The dorsal raphe nucleus (DR) was also examined for the presence of single- and double-labeled cells and comparisons were made with the MR. The main findings were: (1) pronounced numbers of retrogradely labeled cells (approximately 50 cells/section) were present in MR with injections in the MS or in various regions of the hippocampus; (2) approximately 8-12% of MR cells were double-labeled following paired injections in the MS-CA1, MS-CA3, and MS-dentate gyrus of the dorsal hippocampus, the lateral MS-dentate gyrus, and the MS-ventral hippocampus; (3) single- and double-labeled cells were intermingled throughout MR and present in greater numbers in the rostral than caudal MR; and (4) significantly more single- and double-labeled cells were present in MR than in DR with all combinations of injections. These findings demonstrate that MR projects strongly to the MS and hippocampus, and that a significant population of MR neurons (8-12%) sends collateral projections to both sites. It is well established that the MR nucleus serves a direct role in the desynchronization of the electroencephalographic (EEG) activity of the hippocampus-or the blockade of the hippocampal theta rhythm. The MR neurons that we have identified with collateral projections to the septum and hippocampus may be critically involved in the modulation/control of the hippocampal EEG. A role for the MR in memory associated functions of the hippocampus is discussed.

Priming pattern determines the correlation between hippocampal theta activity and locomotor stepping elicited by stimulation in anesthetized rats

The after-effects of locomotor stimulation are a transient facilitation of locomotor initiation (the priming effect), and a transient increase in hippocampal rhythmic slow activity in the 3-6 Hz band of the theta range. The similar time course of the two effects suggests that hippocampal 3-6 Hz activity may be linked to the excitability of locomotor initiation. This study tested the hypothesis that power in the 3-6 Hz band that is present prior to stimulation would predict the magnitude of elicited stepping. Stimulation electrodes were implanted in 15 locomotor sites of 10 anesthetized rats (urethane, 800 mg/kg). Hindlimb stepping was elicited by a single control train of electrical stimulation presented once every 62 s. On test trials, a test train at the same intensity followed the control train at varying control/test intervals (15-36 s) to assess the priming effect on stepping. The priming pattern determined whether hippocampal 3-6 Hz power predicted the amount of stepping to be elicited by a stimulation train. Positive correlation (0.47>r>0.22) was found for seven out of eight sites showing positive priming effects. Correlation was absent for three other sites that showed non-significant priming effects and were mixed for four sites that showed negative effects. Sites with positive priming patterns, compared to sites with inconsistent or negative priming patterns, had similar trends in post-stimulation 3-6 Hz power, smaller increases in 6-8 Hz power during the control train and lower 1-3 Hz power during the periods immediately before the control stimulation. For six of 15 sites, regardless of the priming pattern, 1-3 Hz power was inversely related to subsequent stepping, and in three cases provided an independent predictor of stepping. Stimulation at two sites produced discrete episodes of post-stimulation stepping. In one of these cases, a 0.5-Hz increase in peak frequency of hippocampal activity preceded stepping. The results show that the association between hippocampal 3-6 Hz activity and the excitability of locomotor initiation is sufficiently specific to allow prediction of the magnitude of stepping by the prior levels of 3-6 Hz power. However, the occurrence of negative priming effects during prominent 3-6 Hz activity indicates that other factors determine the actual stepping and they can suppress the correlation between theta activity and subsequent locomotion.

Hippocampal theta activity and facilitated locomotor stepping produced by GABA injections in the midbrain raphe region

Inactivation of neurons in the midbrain raphe region produces increases in locomotor activity, and it appears that they function to suppress locomotion. Inactivation of neurons there also produces hippocampal slow wave (theta) activity and it appears that they also function to inhibit rhythmic activity in the hippocampus. We determined whether the degree of association between the two effects was consistent with the operation of a single mechanism. Stimulation electrodes were implanted into locomotor sites of the hypothalamus of 34 urethane-anesthetized rats. Hindlimb stepping was elicited by 5.12-s trains of perifornical electrical stimulation presented once per minute. Hippocampal theta activity was recorded across the CA1 layer of the dorsal hippocampus. GABA injections were used to locate raphe sites at which neuronal inactivation influenced stepping and hippocampal activity. A glass pipette (80-microm tip) was inserted into the midbrain, and injections of GABA (50-100 mg/0.1-0.2 microl) were made in 70 sites in the midbrain. Injections at 34 sites facilitated stimulation-elicited stepping, and at 17 sites, they also produced intertrial stepping. Facilitating injections, but not ineffective or suppressive injections, increased the mean peak frequency of hippocampal activity, and increased power in the 4-5 Hz band during the period that preceded the stimulation trains, but did not change the 5-6 Hz activity produced during the stimulation trains. Priming locomotor stimulation which also facilitated stepping produced generally similar increases in pre-stimulation peak frequency and 4-Hz power. The magnitudes of the increases in stepping and 4-Hz power were uncorrelated. The increase in 4-Hz power appeared earlier than the increase in stepping in 18 of 34 cases, and later in 11 cases; no increases in 4-Hz power were apparent in five cases. The results indicate that pre-locomotor 4-Hz hippocampal activity in the urethane-anesthetized rat is loosely coupled with facilitated locomotor initiation. Neurons in the midbrain raphe region appear to suppress both processes, but the low degree of association between the magnitudes and onset times of increases in stepping and hippocampal 4-Hz power indicate the operation of multiple mechanisms.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Injections of muscimol into the median raphe nucleus produce hippocampal theta rhythm in the urethane anesthetized rat

It has previously been shown that serotonergic [5-hydroxytryptamine (5-HT)] neurons of the median raphe nucleus (MR) are critically involved in the control of the hippocampal electroencephalogram (EEG). Activation of MR 5-HT neurons desynchronizes the hippocampal EEG, whereas inhibition of MR 5-HT activity produces hippocampal theta rhythm. The MR contains an intrinsic population of gamma-aminobutyric acid (GABA) containing neurons that synapse on 5-HT cells of the MR. The present study examined the effects on the hippocampal EEG of injections of the GABAA agonist muscimol hydrobromide into the MR. Low doses of muscimol (0.5 microgram) produced hippocampal theta rhythm at a mean latency of 6.81 min and for a mean duration of 23.6 min. Higher doses (1.0 microgram and 3.0 micrograms, respectively) produced theta at mean latencies of 2.24 min and 3.2 min and for mean durations of 31.84 min and 24.88 min. Injections of muscimol into regions adjacent to the MR generated theta at significantly longer latencies or were without effect. The present results indicate that MR injections of muscimol produce theta by inhibiting the activity of MR 5-HT neurons. It is concluded that MR GABAergic systems, via their influence on MR 5-HT cells, serve an important role in the control of the hippocampal EEG.

Injections of excitatory amino acid antagonists into the median raphe nucleus produce hippocampal theta rhythm in the urethane-anesthetized rat

The median raphe nucleus (MR) exerts a pronounced desynchronizing influence on the hippocampal EEG. MR stimulation disrupts theta, while MR lesions produce constant uninterrupted theta. The MR receives pronounced excitatory amino acid (EAA)-containing afferents that have been implicated in several MR-mediated behaviors. The present study examined the effects on the hippocampal EEG of MR injections of the following EAA antagonists in the urethane-anesthetized rat: 2-amino-7-phosphonoheptanoate (AP-7), dizocilpine maleate (MK-801), and gamma-glutamyl-aminomethylsulfonic acid (GAMS). MR injections of the competitive (AP-7) and non-competitive (MK-801) N-methyl-D-aspartic acid (NMDA) receptor antagonists produced theta at short latencies (2.86 min; 4.02 min, respectively) and for long durations (116.1 min; 66.8 min, respectively). It was further shown that the theta-eliciting effects of AP-7 injections could be reliably and temporarily reversed with MR injections of NMDA. MR injections of the kainate/quisqualate receptor antagonist (GAMS) also produced theta at relatively short latencies (6.5 min) and for long durations (60.5 min) indicating that EAA effects on the MR are not NMDA receptor specific. Injections of each of the foregoing EAA antagonists into regions of the brainstem adjacent to the MR including the dorsal raphe nucleus and the medullary or pontine reticular formation generated theta at very long latencies or were without effect. The present findings indicate EAA afferents to the MR normally exert an excitatory influence on the MR in its desynchronization of the hippocampal EEG, whereas the removal of EAA inputs to MR produces the opposite: a reduction of MR activity and hence the elicitation of theta.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic structure and connectivity of serotonin terminals in the ventral tegmental area: potential sites for modulation of mesolimbic dopamine neurons

Microinfusion of serotonin (5-hydroxytryptamine; 5-HT) into the ventral tegmental area enhances the release of dopamine in the nucleus accumbens, a major target of midbrain dopamine neurons. We examined the synaptic basis for 5-HT modulation of neurons in the ventral tegmental area which either (i) project to the nucleus accumbens or (ii) contain the catecholamine synthesizing enzyme tyrosine hydroxylase, a marker of dopamine neurons in this brain region. In the first study, immunoperoxidase labeling of 5-HT in the ventral tegmental area was combined with retrograde transport of gold particles following unilateral injections of the tracer into the nucleus accumbens of adult rats. The gold particles had been previously coupled to wheat germ agglutinin conjugated to inactive horseradish peroxidase. Gold particles were enlarged for visualization using a silver enhancement procedure. By brightfield microscopy, retrogradely labeled neurons contained black punctate granules within their perikarya and proximal processes. The labeled cells were scattered ipsilateral to the injection within the paranigral and parabrachial subdivisions of the ventral tegmental area. Both regions also contained 5-HT immunoreactive varicosities. By electron microscopy, irrespective of the ventral tegmental subdivision, 5-HT labeling was seen primarily in unmyelinated axons and axon terminals. The terminals contained small, clear and large dense core vesicles and ranged from 0.3 micron to 1.4 microns in cross-sectional diameter. 22% (n = 250) of the axon terminals containing 5-HT immunoreactivity formed synaptic contacts with neurons containing the retrograde label. Of these 5-HT terminals, 16% formed asymmetric type contacts and 6% formed symmetric junctions on the retrogradely labeled neurons. The remaining 5-HT terminals were either apposed to (but lacked recognized synapses on) perikarya and large dendrites containing the retrogradely transported protein-gold tracer or contacted unlabeled neurons. In the second set of experiments combining immunoperoxidase of 5-HT and immunogold silver for tyrosine hydroxylase, 32% (n = 250) of the 5-HT-labeled terminals formed synaptic junctions with perikarya or dendrites containing tyrosine hydroxylase immunoreactivity. Of these 5-HT terminals, 23% formed asymmetric type junctions. The remainder were either symmetric or lacked recognized membrane densities. The prominence of asymmetric junctions formed by 5-HT-labeled terminals on neurons projecting to the nucleus accumbens and those containing tyrosine hydroxylase in the ventral tegmental area suggests a cellular basis for serotonergic excitation of mesoaccumbens dopamine neurons. Additionally, the multiplicity of junctions formed by 5-HT terminals on targets with or without retrograde labeling or tyrosine hydroxylase immunoreactivity is consistent with known diverse physiological actions of 5-HT in the tegmental area.

Pharmacological suppression of the median raphe nucleus with serotonin1A agonists, 8-OH-DPAT and buspirone, produces hippocampal theta rhythm in the rat

The effects on the hippocampal electroencephalogram of microinjections of procaine hydrochloride and the serotonin1A agonists, 8-OH-DPAT and buspirone, into the median raphe nucleus were examined in the urethane anesthetized rat. Injections of procaine, 8-OH-DPAT or buspirone into the median raphe nucleus produced a change in the hippocampal electroencephalogram from a spontaneous desynchronized pattern to a synchronized pattern (theta rhythm) within short latencies and for long durations post-injection. Procaine was shown to elicit theta at a mean latency of 52 s and for a mean duration of 21.75 min; buspirone at a mean latency of 2 min and for a mean duration of 34.5 min. A dose dependent relationship was observed between 8-OH-DPAT injections and latencies but not durations. Small doses (0.5 micrograms) of 8-OH-DPAT produced theta at a mean latency of 1.33 min and large doses (3.0 micrograms) at a mean latency of 1.17 min. 8-OH-DPAT injections generated theta for a mean duration of 62 min. Injections of each of these substances into structures dorsal, lateral or rostrocaudal to the median raphe (dorsal raphe nucleus, pontine reticular formation, caudal linear nucleus or raphe pontis, respectively) failed to generate theta or in a few cases produced theta at very long latencies (> 24 min). Saline injections in the median raphe nucleus or control structures were without effect. The demonstration that agents injected into the median raphe nucleus that inhibit its activity (procaine and serotonin1A agonists) produce theta indicate that serotonin-containing median raphe neurons normally suppress theta or are involved in the control of hippocampal desynchronization. The present findings are consistent with previous work showing that median raphe nucleus stimulation desynchronizes the hippocampal electroencephalogram and that median raphe nucleus lesions produce constant theta, but are at odds with the proposal that serotonergic mechanisms may play a role in the generation of the theta rhythm.

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.

Regulation of nucleus accumbens dopamine release by the dorsal raphe nucleus in the rat

The effects of microinfusing L-glutamate, serotonin (5-HT), (+-)-8-hydroxy-2-(di-N-propylamino) tetralin (8-OH DPAT; a 5-HT1A agonist), and muscimol (a GABAA agonist) into the dorsal raphe nucleus on the extracellular levels of 5-HT, dopamine (DA) and their metabolites in the nucleus accumbens were studied in unanesthetized, freely moving, adult male Wistar rats, using the technique of microdialysis coupled with small-bore HPLC. Administration of 0.75 micrograms L-glutamate produced a 25-50% increase (P less than 0.05) in the extracellular levels of both 5-HT and DA. On the other hand, infusion of 8-OH DPAT and, to a lesser extent, 5-HT produced a significant (P less than 0.05) decrease in the extracellular levels of both 5-HT and DA. Muscimol (0.25 or 0.50 microgram) had little effect on the extracellular concentrations of 5-HT or DA following its administration. In general, the extracellular levels of the major metabolites of 5-HT and DA in the nucleus accumbens were not altered by microinfusion of any of the agents. The data indicate that (a) the 5-HT neurons projecting to the nucleus accumbens from the dorsal raphe nucleus can be activated by excitatory amino acid receptors and inhibited by stimulation of 5-HT1A autoreceptors, and (b) the dorsal raphe nucleus 5-HT neuronal system may regulate the ventral tegmental area DA projection to the nucleus accumbens.

Muscimol injections into the median raphe nucleus increase serum ACTH and corticosterone concentrations via a nonserotonergic mechanism

Midbrain raphe serotonin (5-HT) neurons can influence the pituitary-adrenal axis. The midbrain raphe nuclei also contain a number of non-5-HT neurons, including gamma-aminobutyric acid (GABA) interneurons which can modulate 5-HT neuronal activity. We investigated the effects of intraraphe injections of the GABAA agonist, muscimol, on serum adrenocorticotropin hormone (ACTH) and corticosterone concentrations. Rats were infused with muscimol (0, 25, 50, and 100 ng in 0.5 microliters saline) into the median raphe nucleus (MR). The animals were killed 30 min later, and trunk blood was collected for measurement of serum concentrations of ACTH and corticosterone by radioimmunoassay. Muscimol dose dependently increased plasma concentrations of these two pituitary-adrenal hormones. In order to determine the role of MR 5-HT neurons in these effects, separate groups of implanted animals were infused with either the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) or ascorbic acid vehicle into the MR. Two weeks later, the animals were infused with muscimol (100 ng in 0.5 microliters) and sacrificed as above. Treatment with 5,7-DHT, which markedly reduced hippocampal concentrations of 5-HT (-83%) and 5-HIAA (-73%), did not block intra-MR muscimol-induced elevations in ACTH and corticosterone. Thus, 5-HT neurons within the MR apparently do not mediate the increased activity of the pituitary-adrenal axis produced by stimulation of MR GABAA receptors.

Differential behavioural activation following intra-raphe infusion of 5-HT1A receptor agonists

Microinfusion of the selective 5-HT1A receptor agonist, 8-hydroxy-(di-N-propylamino)tetralin (8-OHDPAT), into the dorsal raphe nucleus (DRN) produced a marked behavioural hypoactivity and flat body posture. Injections of similar doses into the median raphe nucleus (MRN) elicited hyperactivity but no postural change. Reductions in rearing and grooming were also observed after DRN and MRN infusions of 8-OHDPAT. The behavioural profiles of other 5-HT1A selective compounds, gepirone and BMY7378 were found to be similar to 8-OHDPAT. The contrasting behavioural profiles of the 5-HT1A agents observed after DRN or MRN microinfusions are probably related to the differential innervation of forebrain structures by each raphe nucleus. Thus, the present data confirms and extends previous results illustrating the influence of 5-HT systems on motor behaviour in the rat and identifies unique behavioural profiles following activation of the DRN and MRN.

Intra-midbrain raphe injections of the neurokinin-3 agonist senktide inhibit food and water intake in the rat

Microinjection and lesion studies have implicated the midbrain dorsal (DR) and median raphe (MR) nuclei in behavioral arousal. This behavioral state is manifested as locomotor hyperactivity, hyperphagia, hyperdipsia and increases in plasma corticosteroid release. Intra-midbrain raphe injections of the GABAA agonist muscimol elicit this behavioral activation. We have demonstrated that similar infusions of tachykinins produce locomotor hyperactivity through activation of neurokinin-3 (NK-3) receptors located on serotonin cell bodies. The purpose of the present study was to determine the effects of intra-MR and DR infusions of senktide, an NK-3 agonist, on food and water consumption in nondeprived rats. Male Sprague-Dawley rats were implanted with indwelling intra-MR or intra-DR cannula. Infusions of muscimol (25 ng/0.5 microliters) into the MR increased water intake, while MR and DR infusions increased food consumption. In contrast, intra-MR injections of senktide decreased water intake and intra-MR and DR injections decreased food intake. The results suggest that the behavioral states induced by muscimol and neurokinin infusions into the raphe are distinct and that raphe/neurokinin pathways are involved in consummatory mechanisms.

Effects of 5,7-dihydroxytryptamine injections in the fornix-fimbria on locomotor activity in photocell cages and the open field

Microinjections of 5,7-dihydroxytryptamine (5,7-DHT) into the fornix-fimbria (FF) reduced dorsal hippocampal [3]5-HT uptake to 40% of control levels. The FF 5,7-DHT lesions increased nocturnal activity in photocell cages, but reduced central ambulation in diurnal open field tests. The lesions also disrupted both habituation of rearing across days in the open field and alternation in a Y-maze. Hence FF-derived hippocampal 5-HT terminals participate in controlling activity, but their role depends on the test apparatus and procedures.

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.

Control of food intake by kainate/quisqualate receptors in the median raphe nucleus

In previous studies we have reported that increases in food and water intake can be produced by microinjections of both NMDA selective and broad spectrum excitatory amino acid antagonists into the median raphe nucleus (MR). In the current experiments we examined the influence of kainate/quisqualate receptors in the MR on ingestive behavior. The consumption of food and water by deprived rats could be suppressed by intra-MR microinjections of the excitatory amino acid agonists kainic acid (5-10 ng in 0.5 microliters vehicle) and quisqualic acid (125-500 ng). Conversely, intra-raphe injections of the kainate/quisqualate receptor antagonists pBB-PZDA (1.25-2.5 micrograms) and GAMS (10-20 micrograms) elicited feeding in nondeprived animals. pBB-PZDA was more potent in eliciting ingestive behavior than was the selective NMDA antagonist 2-amino-6-phosphonohexanoic acid, suggesting that the effects of pBB-PZDA were not mediated through the NMDA receptor. The current findings suggest that ingestive behaviors are tonically inhibited by excitatory amino acids acting at kainate/quisqualate receptors in the vicinity of the MR.

The search for the hypnogenic center

1. Electrophysiological and lesion studies have suggested that a number of specific sites in the brainstem and basal forebrain may be involved in the regulation of sleep and waking. In contrast, a study of glucose consumption as measured by the 2-deoxyglucose technique reported a generalized decrease in nonREM sleep compared to waking. The rate of protein synthesis was relatively unchanged in nonREM sleep. 2. Another approach to understanding sleep regulation is to study the mechanism by which hypnotic drugs affect the nervous system. This may be done at both a molecular and neuroanatomic level. Studies with B-carbolines, inverse agonists of benzodiazepines (BZs), indicate that sleep induction by BZs is mediated by binding at the BZ recognition site of the BZ receptor complex. Binding at this site by a long-acting B-carboline parallels the time course of its arousing effects. A study with an enantiomeric BZ indicates that the effects on sleep are stereospecific. It is conceivable that some inverse agonists or enantiomeric benzodiazepines might be developed for clinical use as analeptics. 3. Microinjection of a BZ into the dorsal raphe nucleus acutely increases wakefulness, while administration into the medial preoptic area of the hypothalamus enhances sleep maintenance.

Serotonin microinfusion into the ventral tegmental area increases accumbens dopamine release

The effects of microinfusion of serotonin (5-HT) agents as well as glutamate and muscimol into the ventral tegmental area (VTA) on dopamine (DA) release in the ipsilateral nucleus accumbens (ACC) were investigated in freely moving rats, using a push-pull perfusion procedure. The baseline values for DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were approximately 0.24, 8.4, 2.1 and 2.7 pmol/15 min, respectively, in the push-pull perfusate of the ACC. When microinfused into the VTA, glutamate (0.74 microgram) significantly (p less than 0.05) increased the contents of DOPAC (110%) and HVA (90%) over baseline levels in the perfusate. On the other hand, 0.5 microgram muscimol (a gamma-amino-n-butyric acid, GABA, agonist) significantly, (p less than 0.05) decreased both DA (40%) and DOPAC (20%) levels relative to baseline values. Administration of 2 micrograms 5-HT into the VTA caused a significant (p less than 0.05) elevation in the perfusate levels of DOPAC (80%) and HVA (70%) over baseline values. A similar effect was obtained with a nonselective 5-HT1 agonist but not with a selective 5-HT1A agonist. The results suggest that 5-HT innervations in the VTA may have an excitatory action possibly via 5-HT1B rather than 5-HT1A receptors on the mesolimbic DA system projecting to the ACC and that this DA system may also be regulated by glutamatergic and GABAergic (via GABAA receptors) inputs.

Elicitation of feeding, drinking, and gnawing following microinjections of muscimol into the median raphe nucleus of rats

Previous studies have demonstrated that injections of muscimol into the median raphe nucleus (MR) result in large increases in locomotor activity and food intake. The current experiment extends these results by showing that intra-MR muscimol injections in nondeprived rats also elicit nonprandial drinking and gnawing of wooden blocks. These findings indicate that stimulation of GABA receptors within the MR is able to energize a wide range of oral behaviors and is compatible with the view that the MR may be part of a "nonspecific" behavioral activation system.

Behavioral and biochemical evidence for a functional role of excitatory amino acids in the median raphe nucleus

Recent experiments have suggested the existence of excitatory amino acid (EAA)-containing afferents to the median raphe nucleus. In the present study we investigated the functional significance of EAAs in the median raphe (MR) by examining the behavioral and biochemical effects of intra-raphe injections of EAA antagonists. Injections of kynurenic acid, gamma-glutamylglycine, 2-amino-5-phosphonovaleric acid (2-APV) and 2-amino-7-phosphonoheptanoic acid (2-APH) into the median raphe resulted in marked hyperactivity. In contrast to the effect of 2-APV and 2-APH, intra-raphe injections of the homologues of these compounds with 4, 6 or 8 carbon atoms, which have a lower affinity for excitatory amino acid receptors, were without significant effects on activity. Additionally, the effects of 2-APV were shown to be stereospecific to the active D-isomer further suggesting receptor mediation of the effect. Injections of EAA antagonists into the dorsal raphe nucleus or the ventral tegmental area were much less effective in increasing activity than were injections into the MR, suggesting anatomical specificity of the effect. Injections of 2-APV into the median raphe were also shown to result in a reduction of serotonin metabolism within the hippocampus and an increase in dopamine metabolism within the nucleus accumbens and the magnitude of both of these effects was positively correlated with the behavioral responses to the injections. These findings suggest that cells within the median raphe may be subject to a tonic excitation exerted through EAA receptors.

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.

Is dopamine involved in the hyperactivity produced by injections of muscimol into the median raphe nucleus?

Many studies have shown that experimental manipulations of the median raphe nucleus are able to produce profound effects on locomotor activity. Other data indicate that the raphe nuclei may exert an inhibitory influence over dopamine systems projecting to the forebrain. These results raise the possibility that the median raphe's influence over locomotion may be mediated through alterations in forebrain dopamine release. We examined this possibility in the current report by studying the role of dopamine in the hyperactivity produced by microinjections of the GABA agonist muscimol into the median raphe. Muscimol injections resulted in pronounced hyperactivity which was accompanied by a decrease in serotonin metabolism within the hippocampus and an increase in dopamine metabolism within the nucleus accumbens. Systemic injections of high doses of the dopamine antagonist haloperidol, however, were not able to attenuate muscimol's effect on activity. These results suggest that dopaminergic mechanisms do not play an essential role in mediating the effects of intraraphe muscimol on locomotor activity.

Stimulation of ingestive behaviors following injections of excitatory amino acid antagonists into the median raphe nucleus

Anatomical and pharmacological evidence suggests that excitatory amino acids (EAA's) may function as neurotransmitters within the median raphe nucleus (MR). Previous studies have shown that injections of EAA antagonists into the MR result in marked hyperactivity. The current report extends these findings by demonstrating that intra-raphe injections of two EAA antagonists, kynurenic acid and 2-amino-5-phosphonovaleric acid, result in dose-dependent increases in food and water intake in nondeprived rats. These results suggest that EAA's within the MR may play a role in the control of appetitively motivated behaviors.

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 against serotonin involvement in the hyperactivity produced by injections of muscimol into the median raphe nucleus

Microinjections of muscimol into the median raphe nucleus were found to result in pronounced hyperactivity which could not be attenuated by the serotonin depletion produced either by systemic treatment with p-chlorophenylalanine or by intra-raphe injections of 5,7-dihydroxytryptamine. Furthermore, hyperactivity could not be produced by intra-median raphe injections of serotonin or of fenfluramine, compounds which would be expected to inhibit serotonergic raphe cells. These results argue strongly against an essential involvement of serotonin in mediating the effects of intra-median raphe muscimol injections. Muscimol failed to produce hyperactivity, however, when injected into rats who had previously received an electrolytic median raphe lesion. This finding suggests that muscimol injected into the median raphe produces hyperactivity as a result of an action on local cell bodies, rather than by diffusion to a distant site. The simplest explanation of the current results is that muscimol injected into the median raphe produces hyperactivity as a result of an inhibition of nonserotonergic cells within the median raphe nucleus.

Suppression of locomotor activity produced by acute injections of kainic acid into the median raphe nucleus

Several lines of evidence suggest that cells within the immediate vicinity of the median raphe nucleus may exert an inhibitory influence on locomotor activity. If this theory were correct, one would expect that excitation of neurons within the median raphe would have a suppressive effect on behavior. This possibility was examined in the present study where low doses of the glutamate analogue kainic acid were injected into the median raphe nucleus of methylphenidate-pretreated rats. Our results indicate that these injections were able to antagonize, in a dose-dependent manner, both the horizontal locomotor activity and the nose poking induced by methylphenidate. These results provide further support for the existence of inhibitory median raphe influences on locomotion and suggest the possibility that endogenous excitatory amino acids may play a role in raphe functioning.

Midbrain stimulation in the anesthetized rat: direct locomotor effects and modulation of locomotion produced by hypothalamic stimulation

The midbrain contains circuits that modulate locomotion. To delineate some of the involved regions, low-level stimulation (25 microA, 10 s train of 0.5 ms pulses at 50 Hz) was applied to the midbrain during locomotor stepping. Stepping was elicited in the anesthetized (pentobarbital, 40 mg/kg) rat by stimulating the hypothalamus with 0.5 ms pulses at 40 Hz at various currents. The rat was held in a stereotaxic apparatus such that locomotor stepping movements turned a wheel. Facilitation of locomotion was produced by stimulation in the anterior ventromedial midbrain and in the posterodorsal midbrain. When presented alone, such stimulation produced locomotion. Inhibition of locomotion was produced by stimulation of the superior colliculus (ventral layers) and the ventromedial midbrain. Additional inhibitory sites were found in the central gray and the lateral tegmentum. Inhibitory collicular stimulation, when presented alone, was characterized by the absence of any hindlimb response. Inhibitory ventromedial stimulation, when presented alone, frequently produced poststimulation locomotion and when presented with hypothalamic stimulation was characterized by postinhibitory increases in locomotion. These results indicate that: (1) the locomotor effects of stimulation in midbrain and hypothalamic sites can summate: (2) multiple locomotor suppressive systems are present in the midbrain and among them are a collicular system and a ventromedial system.

Chlordiazepoxide facilitates erections and inhibits seminal emission in rats

Two experiments have been conducted to clarify the effects of the benzodiazepine chlordiazepoxide (CDP) on the different components of male rat sexual behavior. In the first experiment the effects on penile erectile reflexes (PR) of two doses (10 and 30 mg/kg, IP) of CDP were compared with those of vehicle, no-treatment and baseline pre-test in a repeated measures design. In the second experiment the different components of male sexual behavior - PR, mating behavior and spontaneous seminal emission SSE) - were extensively studied after vehicle, 3 and 30 mg/kg CDP treatments. CDP was found to increase significantly the percentage of rats showing penile reflexes, enhance the number of erections per test and shorten the latency to onset of reflexes. It lowered the percentage of animals displaying seminal emission during the PR test. In the mating behavior test, CDP abolished copulatory (and other) behavior at 30 mg/kg. It decreased the number of animals achieving ejaculation at the subataxic 3 mg/kg dose level. In the SSE 3-day test, CDP significantly reduced the weight and the number of plugs of seminal material emitted by the CDP-injected animals. The possible involvement of the serotonergic system in determining the dose-dependent increase in erections and decrease in seminal emission following CDP treatments is discussed. Clinical implications are also briefly considered.

Neuroanatomical site of the inhibitory influence of anxiolytic drugs on central serotonergic transmission

The neuroanatomical site of the inhibitory influence of anxiolytics on central serotonergic transmission has been investigated in the rat by studying the effect of systemic or intracerebral administration of these drugs on cerebral serotonin (5-HT) synthesis. Systemic administration of diazepam (3 mg/kg s.c.) or flunitrazepam (1 mg/kg, s.c.) caused a reduction of 5-HT synthesis (as measured by the accumulation of 5-hydroxytryptophan after inhibition of aromatic amino acid decarboxylase) in the hippocampus but not in the cerebral cortex, striatum, cerebellum or spinal cord of the rat. Zopiclone (22 mg/kg, s.c.) decreased the amine synthesis in hippocampus, striatum and prefrontal cortex. The decrease of hippocampal 5-HT synthesis induced by diazepam (5 mg/kg, s.c.) was antagonized by the benzodiazepine antagonist Ro 15-1788 (2 X 30 mg/kg, s.c.) but not by bicuculline (2 X 1 mg/kg, s.c.). Acute cerebral hemitransection or electrolytic lesion of the fasciculus retroflexus did not prevent the ability of diazepam (5 mg/kg, i.p.) to diminish hippocampal 5-HT synthesis. Local infusion of diazepam (15 micrograms) of flurazepam (1.5 micrograms) into the hippocampus of conscious rats (via indwelling cannulae) markedly reduced 5-HT synthesis in this brain area whereas infusion of these drugs into the raphé medianus (origin of the serotonergic afferents to the hippocampus) failed to affect hippocampal 5-HT synthesis. In contrast, local injection of muscimol (25-150 ng) into the raphé medianus reduced 5-HT synthesis in the hippocampus. This effect of muscimol was potentiated by a systemic administration of diazepam or an intra-raphé medianus infusion of flurazepam (at doses or concentrations which exhibited no intrinsic activity). It is concluded from these data that anxiolytic drugs exert an inhibitory influence on hippocampal serotonergic neurons which is mediated primarily via GABA-independent benzodiazepine receptors located in the vicinity of serotonergic nerve terminals.

Behavioral and biochemical studies of the substrates of median raphe lesion induced hyperactivity

Many authors have demonstrated that electrolytic lesions of the median raphe nucleus lead to dramatic hyperactivity, but little is known as to the neural substrates of this effect. In the current series of experiments we investigated this question by examining locomotor activity and forebrain serotonin levels after the placement of wire knife cuts in various locations around the median raphe. Activity was measured in a five minute open field test and a one hour tilt cage test. Knife cuts designed to transect the major ascending serotonergic projections of the median raphe led to a pronounced depletion of forebrain serotonin, but had no effect on locomotor activity in either testing situation. Knife cuts located antero-ventral to the median raphe, designed to interrupt raphe connections with the ventral tegmental area and interpeduncular nucleus, increased activity in the tilt cage but not in the open field test. These cuts produced only small effects on forebrain serotonin levels. Knife cuts caudal to the median raphe failed to influence forebrain serotonin levels, but produced a significant increase in both open field and tilt cage activity. The effects of the posterior and the anteroventral cuts on tilt cage locomotion were additive, suggesting that different fiber systems were damaged by the two cuts. These results demonstrate that it is possible to double dissociate changes in forebrain serotonin levels and locomotor activity with lesions in the vicinity of the median raphe and further show that ascending projections are unlikely to be the only pathways involved in the effects of median raphe lesions on locomotor behavior.

Differential effects of ethyl (R,S)-nipecotate on the behaviors of highly and minimally aggressive female golden hamsters

The GABA uptake inhibitor ethyl (R,S)-nipecotate produces a dose-dependent suppression of aggression in highly aggressive hamsters but not in minimally aggressive ones. This suppression occurs at doses below those producing peripheral cholinergic effects; at the highest dose used it persists after these effects have dissipated. Doses sufficient to suppress aggression have no significant effect on grooming, locomotor activity and other behaviors but do affect sunflower seed acceptance. The differential effects of the drug on highly and minimally aggressive animals may indicate that their differences in aggression are due to differences in endogenous GABAergic activity. These results, together with previous evidence for parallel circadian variation in GABA uptake and aggressive behavior, suggest that GABA uptake may be an important endogenous regulator of aggression.

GABA and the behavioral effects of anxiolytic drugs

Much recent research has shown that benzodiazepine binding sites in the central nervous system are associated with GABA receptors. It is therefore possible that the pharmacological and therapeutic effects of benzodiazepines and drugs with similar profiles are mediated through GABAergic mechanisms. In this paper the evidence is considered for a possible involvement of GABA in the behavioral effects of anxiolytic drugs. There are a number of reports that the behavioral actions of anxiolytics can be antagonised by GABA antagonists such as bicuculline or picrotoxin but there are many contradictory findings and these drugs are difficult to use effectively in behavioral studies. In general, GABA agonists do not exert anxiolytic-like behavioral effects after systemic injection but intracerebral administration of muscimol has been shown to produce benzodiazepine-like actions. Although a number of questions remain unanswered, current evidence does not provide strong support for a role for GABA in the behavioral effects of anxiolytic drugs.

Spontaneous and delayed spatial alternation following damage to specific neuronal elements within the nucleus medianus raphe

Previous studies have shown that rats with electrolytic lesions of the nucleus medianus raphe (MR) show alterations in spontaneous alternation and in the acquisition of a delayed spatial alternation task. The current study was designed to investigate whether these changes are secondary to forebrain serotonin depletion or if they are due to the destruction of MR cells or fibers of passage within the region of the MR. To this end, rats were prepared with either an electrolytic lesion of the nucleus, or were given an intra-MR injection of either the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or the excitotoxin ibotenate. Rats were then tested for the presence of spontaneous alternation in a T-maze, and were subsequently also trained on a contingently-reinforced delayed alternation task. Only the group with electrolytic lesions showed significant response perseveration in the spontaneous alternation task, although both the electrolytic and ibotenate groups were impaired in acquiring the delayed alternation task. Rats with 5,7-DHT injections performed comparable to controls in both tasks despite the fact that forebrain serotonin levels in this group were reduced at least as much as in the other two lesioned groups. These results suggest that these behavioral effects of MR lesions are due to the destruction of non-serotonergic fibers and/or cells within the region of the nucleus.