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

The Dopaminergic Cells in the Median Raphe Region Regulate Social Behavior in Male Mice

According to previous studies, the median raphe region (MRR) is known to contribute significantly to social behavior. Besides serotonin, there have also been reports of a small population of dopaminergic neurons in this region. Dopamine is linked to reward and locomotion, but very little is known about its role in the MRR. To address that, we first confirmed the presence of dopaminergic cells in the MRR of mice (immunohistochemistry, RT-PCR), and then also in humans (RT-PCR) using healthy donor samples to prove translational relevance. Next, we used chemogenetic technology in mice containing the Cre enzyme under the promoter of the dopamine transporter. With the help of an adeno-associated virus, designer receptors exclusively activated by designer drugs (DREADDs) were expressed in the dopaminergic cells of the MRR to manipulate their activity. Four weeks later, we performed an extensive behavioral characterization 30 min after the injection of the artificial ligand (Clozapine-N-Oxide). Stimulation of the dopaminergic cells in the MRR decreased social interest without influencing aggression and with an increase in social discrimination. Additionally, inhibition of the same cells increased the friendly social behavior during social interaction test. No behavioral changes were detected in anxiety, memory or locomotion. All in all, dopaminergic cells were present in both the mouse and human samples from the MRR, and the manipulation of the dopaminergic neurons in the MRR elicited a specific social response.

Chemogenetic inhibition of cells in the paramedian midbrain tegmentum increases locomotor activity in rats

Pronounced hyperactivity can be produced by lesions or pharmacological inhibition of cells in the median raphe nucleus (MR) located in the paramedian midbrain tegmentum. In the current study we examined whether a similar effect can be seen after chemogenetic inhibition of cells in this region using the DREADD (Designer Receptors Exclusively Activated by Designer Drugs) approach. We found that the DREADD ligand clozapine-N-oxide (CNO) increased locomotor activity in animals expressing the inhibitory DREADD hM4Di, but not those injected with a control virus in the MR. The effect was of rapid onset and short duration and persisted for at least four months after virus injections. Histological examination of the brains indicated that labeled fibers followed the known projection patterns of the MR to a variety of forebrain and midbrain structures. These findings confirm the role of the MR region in the control of locomotion and suggest that the DREADD technique may be a useful approach to the study of the functional architecture of this complex area. Methodological and interpretive aspects of DREADD studies are discussed.

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.

The mesopontine rostromedial tegmental nucleus: A structure targeted by the lateral habenula that projects to the ventral tegmental area of Tsai and substantia nigra compacta

Prior studies revealed that aversive stimuli and psychostimulant drugs elicit Fos expression in neurons clustered above and behind the interpeduncular nucleus that project strongly to the ventral tegmental area (VTA) and substantia nigra (SN) compacta (C). Other reports suggest that these neurons modulate responses to aversive stimuli. We now designate the region containing them as the "mesopontine rostromedial tegmental nucleus" (RMTg) and report herein on its neuroanatomy. Dense micro-opioid receptor and somatostatin immunoreactivity characterize the RMTg, as do neurons projecting to the VTA/SNC that are enriched in GAD67 mRNA. Strong inputs to the RMTg arise in the lateral habenula (LHb) and, to a lesser extent, the SN. Other inputs come from the frontal cortex, ventral striatopallidum, extended amygdala, septum, preoptic region, lateral, paraventricular and posterior hypothalamus, zona incerta, periaqueductal gray, intermediate layers of the contralateral superior colliculus, dorsal raphe, mesencephalic, pontine and medullary reticular formation, and the following nuclei: parafascicular, supramammillary, mammillary, ventral lateral geniculate, deep mesencephalic, red, pedunculopontine and laterodorsal tegmental, cuneiform, parabrachial, and deep cerebellar. The RMTg has meager outputs to the forebrain, mainly to the ventral pallidum, preoptic-lateral hypothalamic continuum, and midline-intralaminar thalamus, but much heavier outputs to the brainstem, including, most prominently, the VTA/SNC, as noted above, and to medial tegmentum, pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe, and locus ceruleus and subceruleus. The RMTg may integrate multiple forebrain and brainstem inputs in relation to a dominant LHb input. Its outputs to neuromodulatory projection systems likely converge with direct LHb projections to those structures.

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.

GABAergic control of the ascending input from the median raphe nucleus to the limbic system

The median raphe nucleus (MRN) is the primary source of serotonergic afferents to the limbic system that are generally considered to suppress hippocampal theta oscillations. GABA receptors are expressed in the MRN by serotonergic and nonserotonergic cells, including GABAergic and glutamatergic neurons. This study investigated the mechanisms by which the fluctuating GABA tone in the MRN leads to induction or suppression of hippocampal theta rhythm. We found that MRN application of the GABA(A) agonist muscimol (0.05-1.0 mM) or GABA(B) agonist baclofen (0.2 mM) by reverse microdialysis had strong theta promoting effects. The GABA(A) antagonist bicuculline infused in low concentrations (0.1, 0.2 mM) eliminated theta rhythm. A short period of theta activity of higher than normal frequency preceded hippocampal desynchronization in 46% of rats. Bicuculline in larger concentrations (0.5, 1.0, 2.0 mM) resulted in a biphasic response of an initial short (<10 min) hippocampal desynchronization followed by stable theta rhythm that lasted as long as the infusion continued. The frequency and amplitude of theta waves were higher than in control recordings and the oscillations showed a conspicuous intermittent character. Hippocampal theta rhythm elicited by MRN administration of bicuculline could be completely (0.5 mM bicuculline) or partially (1.0 mM bicuculline) blocked by simultaneous infusion of the GABA(B) antagonist CGP35348. Our findings suggest that the GABAergic network may have two opposing functions in the MRN: relieving the theta-generators from serotonergic inhibition and regulating the activity of a theta-promoting circuitry by the fluctuating GABA tone. The two mechanisms may be involved in different functions.

Neural mechanisms of freezing and passive aversive behaviors

Cues that predict aversive outcomes often produce marked inhibitions of behavior known as freezing, but it is unknown exactly what neural pathways cause this inhibition. The amygdala and bed nucleus of the stria terminalis, along with their projections to the periaqueductal gray, are strongly implicated in freezing, but it is not known how these structures inhibit motor output. The median raphe nucleus (MRN), which contains a major population of serotonin neurons, has also been implicated in freezing, but the serotonin neurons themselves do not seem to be involved, leaving it uncertain which neurons in this area promote freezing. Our recent work suggests that GABAergic neurons just lateral to the MRN, but not within the MRN, regulate freezing via projections to midbrain dopamine neurons. Because freezing pathways may control a variety of other passive aversive behaviors, their elucidation may help understand the mechanisms of addictions and compulsions, which involve a failure of aversive outcomes to inhibit behavior.

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.

GABA(B) receptors in the median raphe nucleus: distribution and role in the serotonergic control of hippocampal activity

Previous studies have shown that serotonergic neurons of the median raphe nucleus have a suppressive effect on theta synchronization in the hippocampus. Median raphe lesion, suppression of 5-HT neuronal activity by administration of GABA(A) receptor antagonist or by glutamate blockade or depletion produced long-lasting non-interrupted hippocampal theta in freely behaving rats independent of behavior and in rats anesthetized with urethane. Serotonergic neurons show a characteristic sleep-wake pattern of activity and there is evidence that GABAergic mechanisms play an important role in their regulation. In this study we analyzed the distribution and subcellular localization of GABA(B) receptors in the midbrain raphe complex using combined 5-HT/GABA(B) receptor immunohistochemistry at the light and electron microscopic levels and studied the effects of their pharmacological manipulation on hippocampal electroencephalographic activity in urethane-anesthetized rats. We found that sustained infusion of the GABA(B) receptor agonist baclofen into the median raphe nucleus, using the microdialysis technique, elicited lasting theta activity in the hippocampus. The effect was antagonized by selective GABA(B) receptor antagonists. The predominant localization of GABA(B) receptors in the median, as well as in dorsal raphe was found on serotonergic neurons which strongly indicates that the increase in theta occurrence after baclofen injection resulted from suppression of the serotonergic output originating from the median raphe. On the electron microscopic level, we found GABA(B) receptors located extrasynaptically indicating that these receptors are preferentially activated by strong inputs, i.e. when GABA released from the synaptic terminals is sufficient to spill over from the synaptic cleft. Such conditions might be satisfied during rapid eye movement sleep when GABAergic neurons in the raphe are firing at their highest rate and in rhythmic synchronized bursts. Our data indicate that midbrain raphe GABA(B) mechanisms play an important role in behavioral state control and in hippocampal activity, in particular.

Developmental lead exposure in rats: is a behavioral sequel extended at F2 generation?

Lead toxicity was studied in rats exposed from conception until weaning and assessed by monitoring offspring behavior in both the open field and elevated plus maze and by determining tissue lead in an assessment schedule extended to first (F1) and second (F2) generations. Dams utilized for the F1 generation were submitted to 750 ppm of lead (acetate) in drinking water during pregnancy and lactation. For F1 pups, behavioral alterations were not detected in the elevated plus maze, while in the open field, spontaneous locomotor activity as well as time of both grooming and rearing increased, while freezing time decreased in 30- and 90-day-old rats. Lead content was higher in tissues of 1- and 30-day-old pups. However, in 90-day-old rats, lead was detected only in the femur. F2 generation was lead-free but still presented alterations in both locomotor activity and grooming in 30- and 90-day-old pups. It appears that developmental lead exposure may cause behavioral effects during the developmental stage of the F1 generation, which remains throughout the animal's adult life as a sequel, regardless of lead accumulation, and is extended to the F2 generation of rats.

Dissociation of hippocampal serotonin release and locomotor activity following pharmacological manipulations of the median raphe nucleus

In vivo microdialysis was used to investigate the role of serotonin in the locomotor hyperactivity produced by injections of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), muscimol and baclofen into the median raphe nucleus (MR) of unanesthetized rats. Intra-MR injections of the GABA(A) agonist muscimol (25 ng) resulted in a pronounced increase in locomotor activity which was accompanied by a 42% decrease in hippocampal serotonin release during the first hour following injection. Intra-MR injections of the GABA(B) agonist baclofen (125 ng) induced hyperactivity of a similar magnitude, but failed to affect hippocampal serotonin release. In contrast, the serotonin (5-HT1A) agonist 8-OHDPAT (5 microg) produced only a small effect on locomotor activity but reduced hippocampal serotonin output by 51%. These findings demonstrate that it is possible to dissociate the effects of intra-MR drug injections on locomotor activity and hippocampal 5-HT release and strongly support the view that nonserotonergic neurons in the paramedian tegmentum are importantly involved in the control of behavioral arousal.

Differential regulation of 5-hydroxytryptamine release by GABAA and GABAB receptors in midbrain raphe nuclei and forebrain of rats

1. Extracellular 5-hydroxytryptamine (5-HT) was determined in dorsal raphe nucleus (DRN), median raphe nucleus (MRN) and nucleus accumbens by use of microdialysis in unanaesthetized rats. 2. Infusion of the gamma-aminobutyric acid (GABA)A receptor agonist muscimol into DRN and MRN resulted in decreased 5-HT in DRN and MRN, respectively. Muscimol infusion into nucleus accumbens had no effect on 5-HT. 3. Infusion of the GABAA receptor antagonist bicuculline into DRN resulted in increased DRN and nucleus accumbens 5-HT. Bicuculline infusion into MRN had no effect on 5-HT. This suggests that endogenous GABA had a tonic, GABAA receptor-mediated inhibitory effect on 5-HT in DRN, but not in MRN. 4. Infusion of the GABAB receptor agonist baclofen into DRN produced a decrease in DRN 5-HT. Baclofen infusion into nucleus accumbens resulted in decreased nucleus accumbens 5-HT. This suggests that GABAB receptors are present in the area of cell bodies and terminals of 5-hydroxytryptaminergic neurones. 5. Infusion of the GABAB receptor antagonists phaclofen and 2-hydroxysaclofen had no effect on midbrain raphe and forebrain 5-HT. This suggests that GABAB receptors did not contribute to tonic inhibition of 5-HT release. 6. In conclusion, 5-HT release is physiologically regulated by distinct subtypes of GABA receptors in presynaptic and postsynaptic sites.

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.

Median and dorsal raphe injections of the 5-HT1A agonist, 8-OH-DPAT, and the GABAA agonist, muscimol, increase voluntary ethanol intake in Wistar rats

Low doses of the selective 5-HT1A agonist 8-OH-DPAT increase ethanol intake in a limited access paradigm following peripheral injection. This may be due to a reduction in 5-HT neurotransmission following activation of raphe somatodendritic autoreceptors. In order to test this hypothesis, and to determine the effects of selective reductions in raphe 5-HT activity, experiments examined the effects of injecting 8-OH-DPAT into the dorsal raphe (0, 0.02, 0.1, 1 and 2.5 micrograms) or the median raphe (0, 0.1, 1 and 5 micrograms) in rats trained to drink 12% ethanol for 40 min each day. The effects of the GABAA agonist, muscimol, on ethanol intake were also examined. Ethanol intake was increased at the highest dose of 8-OH-DPAT following injection into either site, with no change in water intake. Thus, the effects of 8-OH-DPAT are selective for ethanol. The selective 5-HT1A antagonist, (+)-WAY100135 (0.3, 1 and 3 mg/kg), blocked the effect of 8-OH-DPAT, showing that activation of 5-HT1A receptors underlies the ethanol drinking induced by 8-OH-DPAT. These results are consistent with the idea that reduced 5-HT function increases ethanol intake. Several behavioral mechanisms for this effect are discussed. Muscimol (50-100 ng) also increased ethanol drinking. Following injection into the median raphe, muscimol also stimulated water intake. These effects are probably due to non-specific behavioural activation induced by this treatment. However, the effect of muscimol in the dorsal raphe was specific for ethanol since water intake was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

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.

Nonserotonergic control of nucleus accumbens dopamine metabolism by the median raphe nucleus

Injections of the GABA agonist muscimol into the median raphe nucleus (MR) have been shown to result in an acceleration of dopamine metabolism within the nucleus accumbens. To examine whether serotonergic mechanisms play a role in this effect, muscimol or its vehicle was injected into the MR of either control subjects or of rats that had received prior injections of the serotonin-depleting agent p-chlorophenylalanine (PCPA). Although PCPA treatments produced massive depletions of forebrain serotonin, they failed to alter the effect of muscimol infusions on dopamine metabolism. This finding suggests that the effects of intra-MR injections of muscimol on accumbens dopamine turnover do not result entirely from an interaction between serotonergic and dopaminergic systems.

Dissociation of haloperidol-induced "anhedonia" and catalepsy by lesions of the dorsal raphe nucleus

Electrolytic lesions of the dorsal raphe nucleus were found to attenuate haloperidol-induced catalepsy, but did not alter haloperidol's ability to suppress the intake of a highly palatable saccharin/glucose mixture by nondeprived rats. These results suggest that neuroleptic-induced suppression of the drinking of palatable fluids is not secondary to the types of motor deficits that result in catalepsy.

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.

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.