The action of baclofen on neurons of the substantia nigra and of the ventral tegmental area

A Review of the Potential Mechanisms of Action of Baclofen in Alcohol Use Disorder

Baclofen, a GABA-B receptor agonist, is a promising treatment for alcohol use disorder (AUD). Its mechanism of action in this condition is unknown. GABA-B receptors interact with many biological systems potentially involved in AUD, including transduction pathways and neurotransmitter systems. Preclinical studies have shown that GABA-B receptors are involved in memory storage and retrieval, reward, motivation, mood and anxiety; neuroimaging studies in humans show that baclofen produces region-specific alterations in cerebral activity; GABA-B receptor activation may have neuroprotective effects; baclofen also has anti-inflammatory properties that may be of interest in the context of addiction. However, none of these biological effects fully explain the mechanism of action of baclofen in AUD. Data from clinical studies have provided a certain number of elements which may be useful for the comprehension of its mechanism of action: baclofen typically induces a state of indifference toward alcohol; the effective dose of baclofen in AUD is extremely variable from one patient to another; higher treatment doses correlate with the severity of the addiction; many of the side effects of baclofen resemble those of alcohol, raising the possibility that baclofen acts as a substitution drug; usually, however, there is no tolerance to the effects of baclofen during long-term AUD treatment. In the present article, the biological effects of baclofen are reviewed in the light of its clinical effects in AUD, assuming that, in many instances, clinical effects can be reliable indicators of underlying biological processes. In conclusion, it is proposed that baclofen may suppress the Pavlovian association between cues and rewards through an action in a critical part of the dopaminergic network (the amygdala), thereby normalizing the functional connectivity in the reward network. It is also proposed that this action of baclofen is made possible by the fact that baclofen and alcohol act on similar brain systems in certain regions of the brain.

Psychosis Following an Increase in Intrathecal Baclofen

Baclofen is a commonly used medication to treat spasticity in neurologic disorders. In the traumatic brain injury (TBI) population, the intrathecal administration of baclofen is often preferred over oral administration due to cognitive side effects. Here we report on a case of a psychotic episode following an increase in intrathecal baclofen in a young man with a history of a TBI. Although intrathecal baclofen is commonly used and is generally well tolerated, this case highlights an important potential effect of intrathecal baclofen that has rarely been reported in the literature.

LEVEL OF EVIDENCE

V.

The neurobiology of opiate motivation

Opiates are a highly addictive class of drugs that have been reported to possess both dopamine-dependent and dopamine-independent rewarding properties. The search for how, if at all, these distinct mechanisms of motivation are related is of great interest in drug addiction research. Recent electrophysiological, molecular, and behavioral work has greatly improved our understanding of this process. In particular, the signaling properties of GABA(A) receptors located on GABA neurons in the ventral tegmental area (VTA) appear to be crucial to understanding the interplay between dopamine-dependent and dopamine-independent mechanisms of opiate motivation.

GABAB receptor activation attenuates the stimulant but not mesolimbic dopamine response to ethanol in FAST mice

Neural processes influenced by γ-aminobutyric acid B (GABA(B)) receptors appear to contribute to acute ethanol sensitivity, including the difference between lines of mice bred for extreme sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effect of ethanol. One goal of the current study was to determine whether selection of the FAST and SLOW lines resulted in changes in GABA(B) receptor function, since the lines differ in sensitivity to the GABA(B) receptor agonist baclofen and baclofen attenuates the stimulant response to ethanol in FAST mice. A second goal was to determine whether the baclofen-induced reduction in ethanol stimulation in FAST mice is associated with an attenuation of the mesolimbic dopamine response to ethanol. In Experiment 1, the FAST and SLOW lines were found to not differ in GABA(B) receptor function (measured by baclofen-stimulated [(35)S]GTPγS binding) in whole brain or in several regional preparations, except in the striatum in one of the two replicate sets of selected lines. In Experiment 2, baclofen-induced attenuation of the locomotor stimulant response to ethanol in FAST mice was not accompanied by a reduction in dopamine levels in the nucleus accumbens, as measured by microdialysis. These data suggest that, overall, GABA(B) receptor function does not play an integral role in the genetic difference in ethanol sensitivity between the FAST and SLOW lines. Further, although GABA(B) receptors do modulate the locomotor stimulant response to ethanol in FAST mice, this effect does not appear to be due to a reduction in tonic dopamine signaling in the nucleus accumbens.

Rapid dopamine signaling differentially modulates distinct microcircuits within the nucleus accumbens during sucrose-directed behavior

The mesolimbic dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) is critical in mediating reward-related behaviors, but the precise role of dopamine in this process remains unknown. We completed a series of studies to examine whether coincident changes occur in NAc cell firing and rapid dopamine release during goal-directed behaviors for sucrose and if so, to determine whether the two are causally linked. We show that distinct populations of NAc neurons differentially encode sucrose-directed behaviors, and using a combined electrophysiology/electrochemistry technique, further show that it is at those locations that rapid dopamine signaling is observed. To determine causality, NAc cell firing was recorded during selective pharmacological inactivation of dopamine burst firing using the NMDA receptor antagonist, AP-5. We show that phasic dopamine selectively modulates excitatory but not inhibitory responses of NAc neurons during sucrose-seeking behavior. Thus, rapid dopamine signaling does not exert global actions in the NAc but selectively modulates discrete NAc microcircuits that ultimately influence goal-directed actions.

The effect of baclofen on alterations in the sleep patterns induced by different stressors in rats

We have previously reported that sleep patterns are significantly affected by both physical and psychological stress induced by a communication box; however, the mechanism by which stress alters sleep patterns was not established. In the present study, we investigated the role of gamma-aminobutyric acid (GABA), acting through the GABA(B) receptor, on stress-induced changes in sleep patterns. Our results show that physical stress increased the total wakefulness time by increasing sleep latency and inhibiting both rapid eye movement (REM) and non rapid eye movement (NREM) sleep during a 6 h sleep-recording period. The GABA(B) agonist baclofen (20 pmol/2 microl) attenuated the effects of physical stress on sleep latency, total wakefulness, and NREM sleep, but not total REM sleep. In contrast, psychological stress enhanced total REM sleep and shortened REM sleep latency without altering other sleep patterns. The effect of psychological stress on total REM sleep was also reversed by baclofen. These results suggest that GABA via GABA(B) receptors may play a role in the regulation of specific sleep patterns by both physical and psychological stress.

Psychological stress-induced changes in sleep patterns and their generation mechanism

Recent findings have increasingly shown that sleep patterns are significantly influenced by psychological stress, such as social defeat, novelty stress, contextual fear stress, and psychological stress induced by the communication box. However, the exact association between psychological stress and sleep is still poorly understood. Therefore, in the present paper we will review related work based on our recent investigations. We have previously reported that total rapid eye movement (REM) sleep, but not non-rapid eye movement (NREM) sleep that is enhanced by psychological stress induced by the communication box in rats (Cui et al., 2007). In past decades strong evidence showed that neurotransmitters play a key role in the variations of the sleep patterns, such as acetylcholine, GABA and others. In addition to neurotransmitters, the hypothalamic-pituitary-adrenal (HPA) axis is another important factor which influences sleep patterns. Therefore, this review will focus on the involvement of the neurotransmitters and the HPA axis in the changes of sleep patterns in response to psychological stress.

Baclofen blocks expression and sensitization of anxiety-like behavior in an animal model of repeated stress and ethanol withdrawal

BACKGROUND

Repeated exposures to forced ethanol diets (EDs) or restraint stress sensitize anxiety-like behavior during a future ethanol withdrawal. The present investigation assessed whether pretreatment of rats with agents targeting receptor systems thought to be important in treating relapse in alcoholic patients would prevent sensitization of anxiety-like behavior.

METHODS

Groups of rats were exposed to either (1) three 5-day cycles of ED with 2 days of withdrawal between cycles, (2) continuous ED, or (3) 5 days of ED in a single cycle preceded by 2 episodes of restraint stress 6 days apart. Drugs [baclofen, acamprosate, naloxone, lamotrigine, ifenprodil, dizocilpine (MK-801), CGS19755, diazepam, flumazenil, or 6-methyl-2-(phenylethynyl)pyridine] were given prophylactically during the first and second withdrawal periods only or, in separate baclofen experiments, acutely during the third withdrawal or during withdrawal from continuous ED. Baclofen administration preceded each stress session in the stress-withdrawal protocols. Anxiety-like behavior was assessed in the social interaction (SI) test 5 hours after the ethanol was removed or after 3 days of abstinence.

RESULTS

Baclofen (1.25, 2.5, and 5 mg/kg), flumazenil (5 mg/kg), and diazepam (1 mg/kg) blocked the reduction in SI induced by ethanol withdrawal. Among the drugs that alter glutamate function, only acamprosate (300 mg/kg) was effective. In the stress protocols, baclofen (5 mg/kg) given before each of the 2 restraint stress sessions before ethanol exposure or before stress during abstinence also attenuated SI deficits.

CONCLUSIONS

These findings suggest that GABAB and GABAA, but not glutamate or opioid mechanisms, are involved in adaptive changes associated with anxiety-like behavior induced by these repeated ethanol-withdrawal and stress-withdrawal paradigms. The lack of action of agents attenuating different aspects of glutamate function suggests that acamprosate's action is related to some other, as yet undetermined, mechanism.

Differential involvement of ventral tegmental GABA(A) and GABA(B) receptors in the regulation of the nucleus accumbens dopamine response to stress

Evidence indicates that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated by glutamate (GLUT) projections from medial prefrontal cortex (PFC) to NAcc and the ventral tegmental area (VTA). Local NMDA receptor blockade in NAcc has previously been shown to enhance the DA stress response in this region as well as in the VTA. This raises the possibility that the NAcc DA stress response is regulated by GLUT acting at NMDA receptors located on NAcc GABA output neurons that project to the VTA where GABA is known to regulate DA cell activity. Thus, in the present study, we used voltammetry to examine the effects of intra-VTA administration of GABA(A) and GABA(B) agonists and antagonists on restraint stress-induced increases in NAcc DA. The results show that local VTA GABA(B) receptor activation with baclofen (0.01, 0.1 and 1.0 nmol) dose-dependently inhibited the NAcc DA stress response whereas GABA(B) receptor blockade with phaclofen had the opposite effect, resulting in a dose-dependent potentiation of the stress response. A similar potentiation of the NAcc DA stress response was observed following VTA GABA(A) receptor blockade with bicuculline, but only at the highest dose (1.0 nmol). Interestingly, intra-VTA injection of the GABA(A) receptor agonist, muscimol, at the lowest dose (0.01 nmol) but not at the higher doses (0.1 or 1.0 nmol) also potentiated the NAcc DA stress response, suggesting an action mediated primarily at GABA(A) receptors located on non-DA neurons. These results indicate that the NAcc DA stress response is regulated by GABA afferents to VTA DA cells and that this action is differentially mediated by GABA(A) and GABA(B) receptors. The data suggest that the relevant GABA(B) receptors are located on DA neurons whereas the GABA(A) receptors are located on GABA interneurons and perhaps also on DA cells. The present findings are also consistent with the idea that the corticofugal GLUT input to NAcc indirectly regulates stress-induced DA release in this region through the GABA feedback pathway to VTA.

GABA(B) receptor-positive modulation decreases selective molecular and behavioral effects of cocaine

Exposure to cocaine induces selective behavioral and molecular adaptations. In rodents, acute cocaine induces increased locomotor activity, whereas prolonged drug exposure results in behavioral locomotor sensitization, which is thought to be a consequence of drug-induced neuroadaptive changes. Recent attention has been given to compounds activating GABA(B) receptors as potential antiaddictive therapies. In particular, the principle of allosteric positive GABA(B) receptor modulators is very promising in this respect, as positive modulators lack the sedative and muscle relaxant properties of full GABA(B) receptor agonists such as baclofen. Here, we investigated the effects of systemic application of the GABA(B) receptor-positive modulator GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4, 6-diamine) in animals treated with acute and chronic cocaine administration. Both GS39783 and baclofen dose dependently attenuated acute cocaine-induced hyperlocomotion. Furthermore, both compounds also efficiently blocked cocaine-induced Fos induction in the striatal complex. In chronic studies, GS39783 induced a modest attenuation of cocaine-induced locomotor sensitization. Chronic cocaine induces the accumulation of the transcription factor deltaFosB and upregulates cAMP-response-element-binding protein (CREB) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). GS39783 blocked the induction/activation of DARPP-32 and CREB in the nucleus accumbens and dorsal striatum and partially inhibited deltaFosB accumulation in the dorsal striatum. In summary, our data provide evidence that GS39783 attenuates the acute behavioral effects of cocaine exposure in rodents and in addition prevents the induction of selective long-term adaptive changes in dopaminergic signaling pathways. Further investigation of GABA(B) receptor-positive modulation as a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse is therefore warranted.

GABAB receptor stimulation accentuates the locomotor effects of morphine in mice bred for extreme sensitivity to the stimulant effects of ethanol

Mice selectively bred for divergent sensitivity to the locomotor stimulant effects of ethanol (FAST and SLOW) also differ in their locomotor response to morphine. The GABA(B) receptor has been implicated in the mediation of locomotor stimulation to both ethanol and morphine, and a reduction in ethanol-induced stimulation has been found with the GABA(B) receptor agonist baclofen in FAST mice. We hypothesized that GABA(B) receptor activation would also attenuate the locomotor stimulant responses to morphine in these mice. In order to test this hypothesis, baclofen was administered to FAST-1 and FAST-2 mice 15 min prior to morphine, and activity was recorded for 30 min. Baclofen attenuated stimulation to 32 mg/kg morphine in FAST-1 mice, but only at a dose that also reduced saline activity. There was no stimulant response to 32 mg/kg morphine in FAST-2 mice, or to 16 mg/kg or 48 mg/kg morphine in FAST-1 mice, but the combination of baclofen with these morphine doses accentuated locomotor activity. Therefore, it appears that GABA(B) receptor activation is not a common mechanism for the locomotor stimulant responses to ethanol and morphine in FAST mice; however, these data suggest that GABA(B) receptor activation may instead enhance some of the behavioral effects of morphine.

Repeated administration of the GABAB receptor agonist CGP44532 decreased nicotine self-administration, and acute administration decreased cue-induced reinstatement of nicotine-seeking in rats

Acute administration of gamma-aminobutyric acid B (GABAB) receptor agonists decreased nicotine, cocaine, ethanol, and heroin self-administration. GABAB receptor agonists also decreased cue-induced cocaine craving or seeking in humans and animals, respectively. The present study investigated the effects of repeated subcutaneous administration of the GABAB receptor agonist CGP44532 on nicotine- and food-maintained responding under a fixed ratio 5 schedule of reinforcement. The second part of the study determined whether contingent presentation of previously nicotine-associated cues reinstated extinguished nicotine-seeking behavior, and whether acute subcutaneous CGP44532 administration affected cue-induced reinstatement of extinguished nicotine-seeking behavior. The results indicated that repeated administration of 0.25 mg/kg CGP44532 selectively decreased nicotine self-administration compared to food-maintained responding during the first 7 days of treatment. Repeated administration of 0.5 mg/kg/day CGP44532 nonselectively decreased both nicotine- and food-maintained responding. Contingent presentation of previously nicotine-associated cues reinstated extinguished nicotine-seeking behavior. Further, acute CGP44532 administration (0.125 and 0.25 mg/kg) decreased cue-induced reinstatement of nicotine-seeking behavior. In summary, the present results indicated that 0.25 mg/kg/day CGP44532 selectively decreased nicotine self-administration compared to food-maintained responding, and acute administration of CGP44532 (0.125 and 0.25 mg/kg) dose-dependently decreased cue-induced reinstatement of nicotine-seeking behavior.

Reduced inhibitory action of a GABAB receptor agonist on [3H]-dopamine release from rat ventral tegmental area in vitro after chronic nicotine administration

Background

The activation of GABA_B receptors in the ventral tegmental area (VTA) has been suggested to attenuate the rewarding properties of psychostimulants, including nicotine. However, the neurochemical mechanism that underlie this effect remains unknown. Since GABA_B receptors modulate the release of several neurotransmitters in the mammalian brain, we have characterised the effect of the GABA_B receptor agonist baclofen on the release of [³H]-dopamine ([³H]-DA) from VTA slices of naïve rats and of rats pre-treated with nicotine.

Results

In naïve rats, baclofen concentration-dependently inhibited the electrically evoked release of [³H]-DA from the isolated VTA (EC₅₀ = 0.103 μM, 95% CI = 0.043–0.249), without affecting the basal [³H]-monoamine overflow. This effect was mediated by activation of GABA_B receptors as it was blocked by the selective receptor antagonist CGP55845A. Chronic administration of nicotine (0.4 mg kg^-1, s.c., for 14 days) affected neither the basal nor the electrically evoked release of [³H]-DA from VTA slices. However, the inhibitory effect of baclofen (10 μM) on the stimulated [³H]-monoamine overflow was abolished in rats pre-treated with nicotine as compared to saline-injected controls.

Conclusions

Our results demonstrate that GABA_B receptor activation reduces the release of DA from the rat VTA. In addition, a reduced sensitivity of VTA GABA_B receptors appears to develop after chronic exposure to nicotine. The resulting disinhibition of VTA DA neurones might therefore contribute to the sensitised dopaminergic responses observed in the rat mesocorticolimbic system following repeated administration of nicotine.

Opiate state controls bi-directional reward signaling via GABAA receptors in the ventral tegmental area

The neural mechanisms that mediate the transition from a drug-naive state to a state of drug dependence and addiction are not yet known. Here we show that a discrete population of GABA(A) receptors in the mammalian ventral tegmental area (VTA) serves as a potential addiction switching mechanism by gating reward transmission through one of two neural motivational systems: either a dopamine-independent (opiate-naive) or a dopaminergic (opiate-dependent or opiate-withdrawn) system. Bi-directional transmission of reward signals through this GABA(A) receptor substrate is dynamically controlled by the opiate state of the organism and involves a molecular alteration of the GABA(A) receptor. After opiate exposure and subsequent withdrawal, the functional conductance properties of the rat VTA GABA(A) receptor switch from an inhibitory to an excitatory signaling mode.

GABA(A) receptors in the ventral tegmental area control bidirectional reward signalling between dopaminergic and non-dopaminergic neural motivational systems

In the midbrain ventral tegmental area (VTA), both dopaminergic and nondopaminergic neural substrates mediate various behavioural reward phenomena. VTA GABAergic neurons are anatomically positioned to influence the activity of both the mesolimbic dopamine system and nondopamine efferents from the VTA. In order to examine the possible functional role of VTA GABA(A) receptors in neural reward processes, we performed discrete, bilateral microinjections of the GABA(A) receptor agonist, muscimol, or the GABA(A) receptor antagonist, bicuculline, into the VTA. Using a fully counterbalanced, unbiased conditioned place-preference paradigm, we demonstrate that activation of VTA GABA(A) receptors, with the GABA(A) receptor agonist muscimol (5--50 ng/microL), or inhibition of VTA GABA(A) receptors, with the GABA(A) receptor antagonist bicuculline (5--50 ng/microL), both produce robust rewarding effects. Furthermore, these rewarding effects can be pharmacologically dissociated: blockade of dopamine receptors with a dopamine receptor antagonist, alpha-flupenthixol (0.8 mg/kg; i.p.), or concurrent activation of VTA GABA(B) receptors with a GABA(B) receptor agonist, baclofen (70 ng/microL), blocked the rewarding properties of the GABA(A) receptor agonist, but had no effect on the rewarding properties of the GABA(A) receptor antagonist. These results suggest that, within the VTA, a single GABA(A) receptor substrate controls bidirectional reward signalling between dopaminergic and nondopaminergic brain reward systems.

Effect of intracerebral administration of NMDA and AMPA on dopamine and glutamate release in the ventral pallidum and on motor behavior

The present study investigates the modulation of the ventral tegmental area (VTA)-ventral pallidum (VP) dopaminergic system by glutamate agonists in rats. The glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were infused via reversed microdialysis into the VTA, and dopamine (DA), glutamate, and aspartate levels in the VTA and ipsilateral VP were monitored together with motor behavior screened in an open field. NMDA (750 microM) infusion, as well as AMPA (50 microM) infusion, induced an increase of DA and glutamate levels in the VTA, followed by an increase of DA levels in the ipsilateral VP and by enhanced locomotor activity. The increase of DA in the VP was similar after administration of these two glutamate agonists, although motor activity was more pronounced and showed an earlier onset after NMDA infusion. Glutamate levels in the VP were not increased by the stimulation of DA release. It is concluded that DA is released from mesencephalic DA neurons projecting to the VP and that these neurons are controlled by glutamatergic systems, via NMDA and AMPA receptors. Thus, DA in the VP has to be considered as a substantial modulator. Dysregulation of the mesopallidal DA neurons, as well as their glutamatergic control, may play an additional or distinct role in disorders like schizophrenia and drug addiction.

Baclofen-induced antinociception and nicotinic receptor mechanism(s)

In this study, the influences of nicotinic receptor agents on baclofen-induced antinociception in the tail-flick test have been studied. Intraperitoneal administration of baclofen (2.5, 5 and 10 mg/kg) to mice induced a dose-dependent antinociception in the tail-flick test. Subcutaneous injection of nicotine (0.5-2.5 mg/kg) also caused a dose-dependent antinociceptive response. Intracerebral (10 and 20 microg/mouse) but not intraperitoneal administration of hexamethonium (5 and 10 mg/kg) to mice decreased the response of both nicotine and baclofen. However, administration of the GABA(B) antagonist CGP 35348 (100 and 200 mg/kg) decreased the response induced by baclofen but not by nicotine. It is concluded that at least part of the baclofen-induced antinociception may be mediated through a nicotinic mechanism.

Baclofen alters ethanol-stimulated activity but not conditioned place preference or taste aversion in mice

The present experiments examined the effects of the GABA(B) receptor agonist, baclofen, on the acquisition of ethanol-induced conditioned place preference (CPP) and conditioned taste aversion (CTA) in male DBA/2J mice. Mice in the CPP experiment received four pairings of ethanol (2g/kg) with a distinctive floor stimulus for a 5-min conditioning session (CS+ sessions). On intervening days (CS- sessions), mice received saline injections paired with a different floor type. On CS+ days, mice also received one of four doses of baclofen (0.0. 2.5, 5.0, or 7.5 mg/kg) 15 min before an injection of ethanol. For the preference test, all mice received saline injections, and were placed on a half-grid and half-hole floor for a 60-min session. Baclofen dose dependently reduced ethanol-stimulated activity, but did not alter the magnitude of ethanol-induced CPP at any dose. For the CTA experiment, mice were adapted to a 2-h per day water restriction regimen followed by five conditioning trials every 48 h. During conditioning trials, subjects received an injection of saline or baclofen (2.0 and 6.0 mg/kg) 15 min before injection of 2 g/kg ethanol or saline following 1-h access to a saccharin solution. Baclofen did not alter the magnitude of ethanol-induced CTA at any dose. In addition, baclofen alone did not produce a CTA. Overall, these studies show that activation of GABA(B) receptors with baclofen reduces ethanol-induced locomotor activation, but does not alter ethanol's rewarding or aversive effects in the CPP and CTA paradigms in DBA/2J mice.

Dopamine depletion reorganizes projections from the nucleus accumbens and ventral pallidum that mediate opioid-induced motor activity

Motor activity elicited pharmacologically from the nucleus accumbens by the mu-opioid receptor agonist D-Ala-Tyr-Gly-NMePhe-Gly-OH (DAMGO) is augmented in rats sustaining dopamine depletions. GABAergic projections from the nucleus accumbens to ventral pallidum and ventral tegmental area (VTA) are involved because stimulation of GABAB receptors in the VTA (by baclofen) or GABAA receptors in the ventral pallidum (by muscimol) inhibit the motor response induced by the microinjection of DAMGO into the nucleus accumbens. The present study was done to determine which of these projections is mediating the augmented DAMGO-induced motor activity that follows 6-hydroxydopamine lesions of the nucleus accumbens. The inhibition of DAMGO-induced activation by pallidal injections of muscimol was markedly attenuated in lesioned animals, whereas the inhibition by VTA injections with baclofen was greatly enhanced. A similar switch in emphasis from pallidal to mesencephalic efferents was not observed for dopamine-induced motor activity, because muscimol microinjections inhibited the response elicited by dopamine microinjection into the nucleus accumbens in all subjects. The stimulation of mu-opioid receptors in the ventral pallidum also elicits motor activation, and this is blocked by baclofen microinjection into the VTA. However, after dopamine depletion in the nucleus accumbens, baclofen in the VTA was ineffective in blocking the motor response by DAMGO in the ventral pallidum. These data reveal that dopamine depletion in the nucleus accumbens produces a lesion-induced plasticity that alters the effect of mu-opioid receptor stimulation on efferent projections from the nucleus accumbens and ventral pallidum.

Prolonged severe withdrawal symptoms after acute-on-chronic baclofen overdose

INTRODUCTION

Baclofen is frequently used to treat muscle spasticity due to spinal cord injury and multiple sclerosis. Baclofen overdose can lead to coma, respiratory depression, hyporeflexia, and flaccidity. An abrupt decrease in the dose of baclofen due to surgery or a rapid tapering program may result in severe baclofen withdrawal syndrome manifesting hallucinations, delirium, seizures, and high fever. Severe baclofen withdrawal syndrome secondary to intentional overdose, however, has not received mention.

CASE REPORT

A 42-year-old male receiving chronic baclofen therapy, 20 mg/d, attempted suicide by ingesting at least 800 mg of baclofen. He was found in coma 2 hours postingestion with depressed respirations, areflexia, hypotonia, bradycardia, and hypotension. Treatment with intravenous fluids, atropine, dopamine, and hemodialysis was associated with restoration of consciousness within 2 days but disorientation, hallucinations, fever, delirium, hypotension, bradycardia, and coma developed during the following week. Baclofen withdrawal syndrome was not diagnosed until hospital day 9, when reinstitution of baclofen rapidly stabilized his condition. Oral overdosage of baclofen causes severe neurological and cardiovascular manifestations due to its GABA and dominant cholinergic effects. Severe baclofen withdrawal syndrome is manifest by neuropsychiatric manifestations and hemodynamic instability. Caution should be exercised after a baclofen overdose in patients receiving chronic baclofen therapy.

Nucleus accumbens dopamine release modulation by mesolimbic GABAA receptors-an in vivo electrochemical study

The role of GABA receptors in regulating the mesolimbic dopamine (DA) system and drug reinforced behaviors has not been well characterized. Using fast-cyclic voltammetry, the effects of specific GABA receptor modulation on DA release in the nucleus accumbens (NAcc) and heroin self-administration (SA) behavior was investigated. The GABAA agonist muscimol, administered either intravenously or directly into the ventral tegmental area (VTA), significantly increased DA release in the NAcc in 7 of the 10 rats tested. DA release decreased in the remaining three rats; both effects were blocked by pretreatment with the GABAA receptor antagonist bicuculline. In contrast, the GABAB agonist baclofen decreased, while 2-OH-saclofen (a GABAB antagonist) increased DA release in the NAcc. However, when VTA GABAB receptors were previously activated or inactivated by microinjections of baclofen or 2-OH-saclofen, systemic injections of muscimol caused an inhibition of NAcc DA release. These results suggest that GABAA receptors may be co-localized on both DA neurons and non-DA (GABAergic) interneurons in the VTA, with the effects of GABAA determined by the net effect of both direct inhibition and indirect disinhibition of DA neurons. Finally, although a DA releaser, muscimol was neither self-administered in drug naive rats, nor did it substitute for heroin in rats previously trained to self-administer heroin, suggesting that GABAA receptors appear to play a complex role in mediating drug reinforcement, depending upon the dynamic functional state of GABAA receptors on both tegmental DA and non-DA neurons.

Effects of pharmacologic increases in brain GABA levels on cocaine-induced changes in extracellular dopamine

Cocaine-induced increases in extracellular dopamine (DA) concentrations were measured using in vivo microdialysis techniques in the nucleus accumbens (NACC) of freely moving rats. In control animals, cocaine increased extracellular DA concentrations approximately 482% 60 min following administration, returning to baseline values 200 min later. When administered 2 h following an acute dose of gamma-vinyl-GABA (GVG, Vigabatrin), cocaine-induced increases in extracellular DA were reduced to approximately 365% of baseline values. Chronic GVG administration further dose-dependently attenuated the effects of cocaine but did not alter the rate of increase or the rate of return to baseline values. These results indicate that GVG, a drug that increases brain GABA concentrations, is effective in attenuating the effect of cocaine on NACC DA. Taken with our earlier findings, these results support the targeting of brain GABAergic systems as a potentially effective pharmacologic treatment strategy for cocaine addiction.

Characteristics of GABAB receptor binding sites on rat whole brain synaptic membranes. 1983

Saturable binding of (±)-[³H]-baclofen and [³H]-γ-aminobutyric acid ([³H]-GABA) to rat brain crude synaptic membranes has been examined by means of a centrifugation assay.

The binding of [³H]-baclofen could be detected in fresh or previously frozen tissue and was dependent on the presence of physiological concentrations of Ca²⁺ or Mg²⁺ although a lower affinity Na⁺-dependent component could also be observed. Both components probably reflect binding to receptor recognition sites.

The saturable portion of bound [³H]-baclofen formed 20.3 ± 6.9% of total bound ligand. This could be displaced by GABA (IC₅₀ = 0.04 μm), (–)-baclofen (0.04 μm) and to a much lesser extent by (+)-baclofen (33 μm). Isoguvacine, piperidine-4-sulphonic acid and bicuculline methobromide were inactive (up to 100 μm) and muscimol was only weakly active (IC₅₀ = 12.3 μm).

Saturable binding of [³H]-GABA increased on adding CaCl₂ or MgSO₄ (up to 2.5 Mm and 5.0 Mm respectively) to the Tris-HCl incubation solution. This binding (GABA_B site binding) was additional to the bicuculline-sensitive binding of GABA (GABA_A site binding) and could be completely displaced by (–)-baclofen (IC₅₀ = 0.13 μm).

Increasing the Ca²⁺ concentration (0 to 2.5 Mm) increased the binding capacity of the membranes without changing their affinity for the ligand.

The binding of [³H]-GABA to GABA_B sites could be demonstrated in fresh as well as previously frozen membranes with a doubling of the affinity being produced by freezing. Further incubation with the non-ionic detergent Triton-X-100 (0.05% v/v) reduced the binding capacity by 50%.

The pharmacological profile of displacers of [³H]-GABA from GABA_B sites correlated well with that for [³H]-baclofen displacement. A correlation with data previously obtained in isolated preparations of rat atria and mouse vas deferens was also apparent.

It is concluded that [³H]-baclofen or [³H]-GABA are both ligands for the same bicuculline-insensitive, divalent cation-dependent binding sites in the rat brain.

Activation of ventral tegmental GABAB receptors inhibits morphine-induced place preference in rats

The effect of microinjection of a GABAB receptor agonist, baclofen, into the ventral tegmental area on the rewarding effect of morphine was investigated using the conditioned place preference paradigm in rats. Morphine (1-8 mg/kg, s.c.) caused a dose-related place preference for the drug-associated place. In contrast, microinjection of baclofen (0.1-1 nmol/side) into the ventral tegmental area did not produce a significant preference for either compartment of the test box. Pretreatment with baclofen (0.1-1 nmol/side) into the ventral tegmental area dose dependently suppressed the morphine (8 mg/kg, s.c)-induced place preference. This suppression of the morphine (8 mg/kg, s.c.)-induced place preference by baclofen (1 nmol/side), but not with the GABAA receptor antagonist bicuculline (1 nmol/side). The present results suggest that a decrease in GABAB neurotransmission in the ventral tegmental area, which may be produced via inhibition of a tonic GABAergic input by morphine, may be involved in the expression of the rewarding effect of morphine.

Local pressure application of cannabinoid agonists increases spontaneous activity of rat substantia nigra pars reticulata neurons without affecting response to iontophoretically-applied GABA

This study tested the hypothesis that cannabinoid agonists, applied locally into the pars reticulata of substantia nigra (SNpr), could modulate striatonigral transmission, without affecting the response of SNpr neurons to iontophoretically-applied GABA. Multibarreled glass capillary electrode assemblies were used for extracellular recording of the spontaneous electrical activity of single SNpr cells in anesthetized rats. Local pressure ejection of the cannabinoid agonists Win 55212-2 (WIN2) and CP 55940 increased SNpr spontaneous firing rate by 13-46%, similar to the effects of systemic injections. Neither WIN2 nor CP 55940 had an effect on the slowing of SNpr neuron activity in response to iontophoretic GABA. Local pressure application of Win 55212-3 (the much less active enantiomer of WIN2) produced an insignificant decrease in SNpr firing rate. Similarly, locally applied vehicle (45% 2-hydroxypropyl-beta-cyclodextrin) produced insignificant decreases in SNpr firing. A second application of cannabinoid agonist produced a much smaller effect, suggesting desensitization. Increasing the interval between CP 55940 applications to 45 min showed recovery of sensitivity to the agonist. Local application of the cannabinoid antagonist, SR 141716A, significantly decreased spontaneous cell firing by 34%. CP 55940, when given immediately following or concurrently with the antagonist application failed to produce the expected increase in discharge rate over baseline. A second application of CP 55940 45 min later produced a 26% increase in firing rate. Bicuculline methiodide (BMI) was applied locally causing a significant increase in SNpr cell firing. CP 55940, when locally administered concurrently with bicuculline methiodide, had no further effect on the firing rate of the cell. Based on the reported presynaptic localization of cannabinoid receptors in SNpr, these findings suggest that cannabinoids act within the SNpr to modulate striatonigral neurotransmission presynaptically. The effect of SR 141716A suggests that an endogenous cannabinoid may mediate striato-nigral transmission.

The role of mesoaccumbens--pallidal circuitry in novelty-induced behavioral activation

When exposed to an environment for the first time, rats express greater behavioral activation than rats which were previously habituated to that environment. The circuit containing the ventral tegmental area, nucleus accumbens and ventral pallidum is required for the expression of locomotor activity elicited by amphetamine-like psychostimulants. It was hypothesized that this circuit is necessary for the expression of novelty-induced motor activity. Dopamine is a neurotransmitter in the projection from the ventral tegmental area to the nucleus accumbens, while GABA is contained in the projections from the nucleus accumbens to the ventral pallidum and from the ventral pallidum back to the ventral tegmental area. Prior to exposing rats to a novel or habituated environment, they received a microinjection of either saline vehicle or one of the following drugs: fluphenazine (dopamine antagonist) into the nucleus accumbens, muscimol (GABAA agonist) into the ventral pallidum, or baclofen GABAB agonist) into the ventral tegmental area. Each of these pretreatments prevented novelty-induced motor activation without suppressing the activity of habituated animals. In contrast, when these microinjections were made into adjacent motor nuclei of the basal ganglia, including fluphenazine into the striatum, muscimol into the globus pallidus and baclofen into the substantia nigra, they were ineffective in blocking novelty-induced motor activity. These data indicate that the integrity of the circuit that contains the ventral tegmental area, nucleus accumbens and ventral pallidum is required for the manifestation of novelty-induced motor activity.

GABAA receptor-mediated inhibition of N-methyl-D-aspartate-evoked [3H]dopamine release from mesencephalic cell cultures

Direct activations of both GABAA and GABAB receptors are known to hyperpolarize dopaminergic neurons. However systemic or intra-ventral tegmental administration of a GABAA receptor agonist produces paradoxical depolarization of mesencephalic dopaminergic neurons and increases dopamine release. Thus indirect excitation appears to preclude observation of inhibitory GABAA effects on dopamine release in intact tissue. The present study used cultures of isolated cells from rat ventral mesencephalon to characterize effects of GABAA and GABAB receptor activation on evoked dopamine release. The GABAA receptor agonist, muscimol, produced a potent and complete inhibition of N-methyl-D-aspartate (NMDA)-evoked [3H]dopamine release. This effect was blocked by the GABAA receptor antagonist, picrotoxin, and enhanced by flunitrazepam. Omission of Mg2+ greatly reduced the inhibitory effect of muscimol on NMDA-evoked [3H]dopamine release. Muscimol had little or no effect on [3H]dopamine release evoked by the non-NMDA receptor agonists, quisqualate and kainate. The GABAB receptor agonist, baclofen, slightly inhibited NMDA-evoked [3H]dopamine release and had no effect on release evoked by quisqualate or kainate. Endogenous GABA released by the mesencephalic cells also appeared to inhibit NMDA-evoked [3H]dopamine release mainly via a GABAA receptor-mediated mechanism. This is suggested by the observations that NMDA-evoked [3H]dopamine release was potentiated by picrotoxin but not by the GABAB receptor antagonist, phaclofen, and that blockade of extracellular GABA removal, with amino-oxyacetic acid and beta-alanine, inhibited NMDA-evoked [3H]dopamine release in a picrotoxin-sensitive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory synaptic potentials in guinea-pig substantia nigra dopamine neurones in vitro

1. The properties of stimulus-evoked and spontaneous inhibitory synaptic potentials were examined in guinea-pig substantia nigra dopamine neurones in sagittal and coronal midbrain slices in the presence of glutamate receptor antagonists. 2. Focal electrical stimulation within the substantia nigra, cerebral peduncle, internal capsule or the striatum evoked a biphasic IPSP consisting of a fast and a slow component, with peak latencies of about 30 and 250 ms, respectively. The fast component was sensitive to chloride injection, reversed polarity at -79.4 +/- 1.1 mV and was blocked by the GABAA receptor antagonists picrotoxin and bicuculline. The slow IPSP reversed at -99.3 +/- 5.4 mV and was blocked by the GABAB receptor antagonists 2-hydroxysaclofen and CGP 35348. 3. Spontaneous IPSPs were observed in many neurones. These events reversed polarity at -77.5 +/- 2.6 mV and were completely blocked by bicuculline and/or picrotoxin. In the presence of TTX, small spontaneous events remained which probably represent miniature IPSPs. In coronal slices, application of 4-aminopyridine raised the frequency of spontaneous IPSPs, presumably by activating nigral interneurones, but failed to reveal spontaneous biphasic IPSPs or spontaneous pure slow IPSPs. 4. The amplitude of the fast IPSPs fluctuated from trial to trial. Amplitude histograms of minimal fast IPSPs displayed evenly spaced peaks, suggesting that synaptic transmission is quantal at these synapses. The measured peak spacing depended on the driving force for Cl-. 5. The fast IPSP showed little or no paired-pulse depression, and in the presence of 2-hydroxysaclofen (400-600 microM) showed paired-pulse facilitation. The GABAB agonist baclofen inhibited the fast IPSP via a presynaptic mechanism. The pharmacologically isolated slow IPSP showed marked paired-pulse facilitation. 6. It is concluded that synaptic inhibition in the substantia nigra is mediated by GABA, is relatively resistant to frequency-dependent depression and is regulated by presynaptic GABAB autoreceptors. Striatonigral and pallidonigral fibres activate both GABAA and GABAB receptors, while intranigral pathways appear to activate predominantly GABAA receptors.

GABA and enkephalin projection from the nucleus accumbens and ventral pallidum to the ventral tegmental area

GABAergic and enkephalinergic afferents to the ventral tegmental area were investigated in the rat using retrograde tracing techniques combined with in situ hybridization. Following iontophoretic deposit of Fluoro-Gold in the ventral tegmental area labeling in the forebrain was most dense in the shell of the nucleus accumbens, rostral ventromedial ventral pallidum and diagonal band of Broca. A smaller density was also observed in the lateral septum. In these forebrain regions, the portion of retrogradely labeled cells that contained mRNA for glutamate decarboxylase ranged from 25% to 50%, whereas only 5% to 15% were double-labeled for preproenkephalin mRNA. Cells double-labeled with either glutamate decarboxylase or preproenkephalin mRNA were most numerous in the lateral septum, shell of the nucleus accumbens, rostral ventral pallidum and diagonal band of Broca. Large Fluoro-Gold deposits which invaded the medial substantia nigra resulted in a significant number of retrogradely labeled cells in the core of the nucleus accumbens, and a portion of these neurons also contained mRNA for glutamate decarboxylase or preproenkephalin. These data demonstrate the presence of GABAergic and enkephalinergic neurons projecting from the nucleus accumbens, ventral pallidum and diagonal band of Broca to the ventral tegmental area.

Neurological manifestations of baclofen withdrawal

Baclofen is a central nervous system agent that is commonly used for the treatment of muscle spasticity in spinal cord injury patients. Acute withdrawal of this medication can induce the development of neurological symptoms, including seizure disorder, psychosis, hallucinations and visual disturbances. We report 3 cases of acute central nervous system symptoms that developed in spinal cord injury patients. Each patient had been chronically maintained on a baclofen regimen to control muscle spasticity. Symptoms developed shortly after baclofen therapy was interrupted following genitourinary surgery. It is important that urologists become familiar with the symptomatology of baclofen withdrawal, the methods of its prevention and the appropriate therapy should the syndrome develop.

Mesolimbic dopamine neurotransmission is increased by administration of mu-opioid receptor antagonists

Microdialysis and high pressure liquid chromatography were used to assess the effects of ventral tegmental area microinjections of the mu-opioid receptor antagonists D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and beta-funaltrexamine (beta-FNA) on extracellular ventral striatal dopamine and metabolite concentrations. While CTOP is known to antagonize the increases in extracellular ventral striatal dopamine and dopamine metabolite concentrations induced by ventral tegmental area microinjections of a mu-opioid receptor agonist, it produced dose-orderly increases in ventral striatal dopamine and dopamine metabolite concentrations when administered by itself. beta-FNA also elevated dopamine and metabolite concentrations. These mu-agonist-like effects of the mu-opioid receptor antagonists were unexpected and suggest that a complex local circuitry mediates opioid-dopamine interactions in the ventral tegmental area. Since mu-opioids are known to act on ventral tegmental neurons that contain gamma-aminobutyric acid (GABA), a model of interactions between GABAergic afferents to the ventral tegmental area and ventral tegmental GABAergic interneurons is proposed.

Topographic heterogeneity of substantia nigra neurons: diversity in intrinsic membrane properties and synaptic inputs

The passive and active membrane properties of substantia nigra neurons were recorded in vitro at various locations throughout its anterior-posterior extent and their responses to extracellular electrical stimulation within the pars reticulata were analysed. One class of nigral pars compacta cell showed the well-established electrophysiological characteristics of mesencephalic dopaminergic neurons, i.e. spontaneous discharge in a very rhythmic, pacemaker fashion without bursting activity and with broad action potentials. However, these neurons could be subdivided further according to differences in electrophysiological properties which correlated with their position within the substantia nigra. Thus, neurons recorded from the anterior part of the substantia nigra, at the level of the mammilary bodies displayed a significantly higher firing rate and shorter action potential than those located in posterior slices at the level of the accessory optic tract. The location of the cell was also a critical factor in its response to stimulation of the pars reticulata: in anterior slices only 45.5% of the cells responded with inhibitory postsynaptic potentials to stimulation, while in posterior slices inhibitory postsynaptic potentials occurred in 85.7% of the neurons (n = 44). In addition, anteriorly located neurons were more sensitive to direct electrical stimulation than posteriorly located cells and they also exhibited excitatory postsynaptic potentials (33%) on pars reticulata stimulation. However, the actual properties of inhibitory postsynaptic potentials were essentially the same in these neurons irrespective of whether they were located either in the anterior or posterior part of the nigra: reversal potentials of inhibitory postsynaptic potentials were found at two distinct potentials indicating involvement of both GABAA and GABAB receptors. This deduction is also supported by additional pharmacological findings: application of the GABAA antagonist, bicuculline methiodide and/or GABAB antagonist, 2-hydroxysaclofen blocked both the inhibitory postsynaptic potentials and the cessation of spontaneous firing activity of the cells to stimulation of the pars reticulata. The other type of pars compacta neuron recorded discharges phasically and was located exclusively in the anterior pole of the substantia nigra. These cells showed a wide range of spontaneous firing activity, a non-rhythmic, irregular pattern of firing, a shorter action potential width and the presence of a low-threshold calcium conductance. These "phasic" neurons also differed greatly from other compacta neurons in their response to pars reticulata stimulation: spontaneous activity of these cells was not inhibited nor did they show inhibitory postsynaptic potentials. Instead, the majority was preferentially activated by direct stimulation of the dendrites, although excitatory postsynaptic potentials could also be evoked.(ABSTRACT TRUNCATED AT 400 WORDS)

Baclofen-induced dyskinesia

Baclofen (Lioresal) is a derivative of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). It is used to treat spasticity particularly for the relief of flexor spasms, pain, clonus, and muscular rigidity. There have been many rare neurologic side effects reported with its use. These side effects, in particular, hallucinations and seizures, have been observed predominantly following precipitous withdrawal of the drug. We present a case demonstrating a muscular dyskinetic side effect when baclofen treatment was first initiated. The mechanism by which baclofen affects spasticity and how the resulting side effect of dyskinesia developed in our patient is not known. They are, however, most probably related to dopamine receptor hypersensitivity and the resulting imbalance of the dopaminergic/cholinergic systems. Clinicians should be aware of this additional adverse effect of muscular dyskinesia, with the use of baclofen, and its reversibility when baclofen is discontinued.

Pertussis Toxin in the A10 Region Increases Dopamine Synthesis and Metabolism

Inhibitory regulation of dopamine neurons is mediated by dopamine autoreceptor and gamma-aminobutyric acidB receptor opening of potassium channels. Increased potassium conductance by either receptor is G protein dependent. To evaluate the role of G proteins in vivo, pertussis toxin (PTX) was microinjected into the A10 dopamine region and changes in dopamine metabolism and synthesis measured. PTX produced an elevation in dopamine metabolism and synthesis in the A10 region and nucleus accumbens for up to 4 days after injection. By day 7 the levels of the dopamine precursor and metabolites had returned to normal. A less consistent increase was also measured in the A9 dopamine region and the prefrontal cortex. Although dopamine synthesis and metabolism had returned to normal by day 7, the in vitro ADP-ribosylation of G proteins in the A10 region by PTX remained depressed by approximately 50% from day 1 to day 14 after administration, returning to normal by day 30. The data suggest that in vivo ribosylation of G proteins may lead to a short-term attenuation of the tonic inhibitory control of dopamine neurons, which can be compensated for by PTX-insensitive mechanisms.

Dopamine transmission in the initiation and expression of drug- and stress-induced sensitization of motor activity

Progress has been made over the last 10 years in determining the neural mechanisms of sensitization induced by amphetamine-like psychostimulants, opioids and stressors. Changes in dopamine transmission in axon terminal fields such as the nucleus accumbens appear to underlie the expression of sensitization, but the actions of drugs and stressors in the somatodendritic regions of the A10/A9 dopamine neurons seem critical for the initiation of sensitization. Manipulations that increase somatodendritic dopamine release and permit the stimulation of D1 dopamine receptors in this region induce changes in the dopamine system that lead to the development of long-term sensitization. However, it is not known exactly how the changes in the A10/A9 region are encoded to permit augmented dopamine transmission in the terminal field. One possibility is that the dopamine neurons of sensitized animals have become increasingly sensitive to excitatory pharmacological and environmental stimuli or desensitized to inhibitory regulation. Alternatively, changes in cellular activity or protein synthesis may result in a change in the presynaptic regulation of axon terminal dopamine release.

Effects of baclofen and bicuculline on inhibition in the fascia dentata and hippocampus regio superior

The effects of microiontophoretically applied baclofen, bicuculline and phaclofen were studied on evoked field responses, paired-pulse (PP) plasticity and single-unit activity of dentate granule cells (DGCs) and CA1 pyramidal cells (PCs) in anesthetized rats. The GABAB agonist, baclofen, increased population spike (PS) amplitudes in the dentate evoked by perforant path stimulation but decreased PS amplitudes in CA1 evoked by Schaffer collateral stimulation, whereas the GABAA antagonist, bicuculline, increased PS amplitudes in both regions. Neither baclofen nor bicuculline had significant effects on dendritically recorded population excitatory postsynaptic potentials (EPSPs) in the dentate or CA1 evoked by stimulation of their respective afferents. Control PP curves in the dentate revealed a triphasic response of inhibition/potentiation/inhibition, whereas control PP curves in CA1 manifested a biphasic response of inhibition/potentiation of test/conditioned PS amplitudes. Baclofen and bicuculline reversed the early and late phases of PP inhibition in the dentate and the early phase of PP inhibition in CA1. The GABAB antagonist, phaclofen, selectively reversed the effects of baclofen on PP inhibition in both the dentate and CA1. Whereas baclofen had no effect, bicuculline incre sed and phaclofen decreased DGC single-unit spontaneous firing rate, while baclofen decreased and bicuculline and phaclofen increased PC firing rate. These results support and extend studies suggesting that GABAergic feedback inhibition of DGCs and PCs is mediated by postsynaptic GABAA receptors and feedback inhibition of PCs is mediated by postsynaptic GABAB receptors. Our results also provide significant new evidence suggesting that postsynaptic inhibition in the dentate is not regulated by GABAB receptors and that feedback and feedforward inhibition of DGCs and PCs is regulated by presynaptic GABAB receptors located on GABAergic interneurons.

GABA receptors: are cellular differences reflected in function?

The putative involvement of GABAA and GABAB receptors in various behavioral and physiological effects is summarized in Table III. A division of function among the two types of GABA receptors appears to exist. GABAA receptors mediate feeding, cardiovascular regulation, anxiolytic effects, and anticonvulsive activity. GABAB receptors, on the other hand, are involved in analgesia, cardiovascular regulation, and depression. Although there is some overlap and shared functions among the receptor types, it is evident that GABAA and GABAB receptors have different behavioral and physiological profiles. Feeding, anticonvulsive activity and anxiety, for example, primarily involve GABAA receptors. Analgesia and depression, on the other hand, are GABAB effects. In those cases where GABAA and GABAB receptors mediate similar functions (e.g. cardiovascular regulation), they do so by affecting different transmitter systems and cellular mechanisms. It is proposed, therefore, that GABAA and GABAB receptors differ not only at the cellular level, but that they also have different functions in the mammalian central nervous system. The association of different subtypes of a receptor with different functions and mechanisms of action is not unique to the GABA system. D1 and D2 receptors in the dopamine system, for example, also exhibit some separation of function as do the mu, delta and kappa types of opiate receptors. Different subtypes of neurotransmitter receptors, therefore, appear to be a general organizing principle used by the brain to transduce chemical signals into different functional responses. A better understanding of the exact processes through which cellular signals are transformed into functional responses is a goal of future research.

Intracellular recording from putative dopamine-containing neurons in the ventral tegmental area of Tsai in a brain slice preparation

Coronal slices of rat mesencephalon containing the ventral tegmental area of Tsai (VTA) and the substantia nigra were prepared. Stable intracellular recordings were obtained from presumed dopamine (DA)-containing neurons in the VTA. Both silent and spontaneously active cells were encountered; spontaneously active neurons fired in an extremely regular pacemaker-like fashion. These neurons had resting membrane potentials ranging from -45 to -75 mV and input resistances ranging from 80-400 M omega. DA-containing neurons in the VTA demonstrated marked anomalous rectification in response to hyperpolarizing current pulses. Application of DA or the GABAB agonist, baclofen, to the bathing medium produced suppression of spontaneous firing, sometimes accompanied by membrane hyperpolarization. Neuronal input resistance was not changed consistently by DA and was generally reduced by baclofen.

Role of cholinergic and GABAergic neuronal systems in cycloheximide-induced amnesia in mice

The role of cholinergic and GABAergic neuronal systems on the cycloheximide (CXM)-induced amnesia was investigated using the step-down-type passive avoidance task in mice. CXM (7.5-120 mg/kg, SC) given just after the training caused amnesia (indicated by short latency to step down from the platform on the grid floor) in the retention test conducted 24 hr later in a dose-dependent fashion. In the CXM (60 mg/kg)-treated mice, a choline esterase inhibitor, physostigmine (PHY; 0.125 and 0.25 mg/kg, IP), or GABA agonists, muscimol (1 and 2 mg/kg, IP) and baclofen (6 and 12 mg/kg, IP), given just after training markedly prolonged step down latency (SDL), indicating reversal of amnesia. The antiamnesic action of PHY (0.125 mg/kg) was almost completely antagonized by a central acetylcholine antagonist, scopolamine (3 mg/kg, SC), but not by a peripheral acetylcholine antagonist, butylscopolamine (3 mg/kg, SC). Furthermore, the antiamnesic action of muscimol (2 mg/kg) was reversed by GABA antagonists, picrotoxin (0.5 mg/kg, SC) and bicuculline (0.5 mg/kg, SC), while the effect of baclofen (12 mg/kg) was reversed by picrotoxin (0.5 mg/kg), but not by bicuculline (0.5 mg/kg). These results suggest that the dysfunction of cholinergic and GABAergic neuronal systems play an important role in the CXM-induced memory impairment on the passive avoidance task.

On the potassium conductance increase activated by GABAB and dopamine D2 receptors in rat substantia nigra neurones

1. Intracellular recordings were made from 193 substantia nigra zona compacta neurones in slices of rat mesencephalon. All cells were hyperpolarized by baclofen; this was accompanied by a fall in input resistance. Cells voltage clamped at -60 mV showed an outward current associated with a conductance increase in response to baclofen. The baclofen effects were concentration dependent (effective range 0.3-30 microM); the concentration producing half the maximal effect was 1.5 microM. (-)-Baclofen was 300-700 times more potent than (+)-baclofen. 2. The potential change or membrane current caused by baclofen reversed polarity at -108.8 +/- 1.1 mV (n = 10) when the potassium ion concentration was 2.5 mM, -96.0 +/- 2.8 mV (n = 3) in 4.5 mM-potassium and -76.6 +/- 1.7 mV (n = 5) in 10.5 mM-potassium. The relationship between reversal potential and potassium concentration conformed to the Nernst equation. 3. Dopamine was also applied to 119 of these neurones; all exhibited either a hyperpolarization or an outward current. 4. Baclofen and dopamine outward currents were reduced reversibly by barium (100-300 microM) and tetraethylammonium (10 mM). Superfusion for 5-10 min with solutions presumed to block calcium currents reduced, but did not abolish, responses to baclofen. The effect of baclofen persisted in tetrodotoxin (1 microM). 5. Superfusion of gamma-aminobutyric acid (GABA, 0.3-3 mM) caused either membrane depolarization or hyperpolarization, accompanied by a fall in input resistance. The depolarization was mimicked by muscimol (10 microM) and blocked by bicuculline methiodide (10-100 microM); the hyperpolarization was resistant to bicuculline. Nipecotic acid (500 microM) enhanced the effect of GABA, but was without effect upon the actions of muscimol and baclofen. 6. The effect of dopamine was enhanced by cocaine (10 microM) and antagonized by (-)-sulpiride (0.1-1 microM), whereas the actions of baclofen were unaffected by cocaine or (-)-sulpiride. The maximum outward current produced by dopamine was approximately half that produced by baclofen. 7. Outward currents produced by dopamine were reversibly occluded by maximal outward currents caused by baclofen. 8. Baclofen and dopamine hyperpolarizations were unaffected by intracerebroventricular injection of animals with pertussis toxin. 9. Cells impaled with electrodes containing guanosine 5'-O-(3-thiotriphosphate) (1 mM) were hyperpolarized by both baclofen and dopamine, but the membrane potential did not fully return to its original level when agonist application was discontinued. 10. It is concluded that activation of both dopamine D2 and GABAB receptors may increase the same potassium conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Baclofen increases the potassium conductance of rat locus coeruleus neurons recorded in brain slices

Baclofen causes a concentration-dependent inhibition of spontaneous firing, hyperpolarization and resistance decrease in locus coeruleus (LC) neurons recorded intracellularly in a brain slice preparation. The (-) isomer is active while the (+) isomer has little or no activity which indicates that the baclofen effect is stereoselective. Baclofen action on LC neurons is a direct postsynaptic effect since it remains in low Ca2+, high Mg2+ media. Baclofen actions on LC neurons are resistant to the GABAA antagonist bicuculline. The baclofen-induced hyperpolarization reverses at the K+ equilibrium potential, as estimated by the reversal potential of the post-stimulus hyperpolarization which follows an evoked train of action potentials. When the K+ concentration in the superfusion media is increased, the reversal potential for the baclofen-induced hyperpolarization shifts linearly with a slope of 61 mV per 10-fold change as predicted by the Nernst equation for a pure K+ conductance. The baclofen-induced K+ conductance increase is prevented by addition of the K+-channel blocker Ba2+ to the external media. Taken together, these data suggest that baclofen directly hyperpolarizes LC neurons by activation of GABAB receptors which leads to an increase in K+ conductance.

Responsiveness to 'autoreceptor' doses of apomorphine inversely correlated with the firing rate of dopaminergic A9 neurons: action of baclofen

Supersensitivity of dopamine (DA) autoreceptors develops soon after acute treatments with baclofen or gamma-butyrolactone, two drugs which suppress DA neuronal firing. We have hypothesized that this effect might reflect a condition associated with the low firing rate of DA cells induced by such treatments rather than a long-term adaptive modification of DA receptor sensitivity. In this study we show that the degree of the intravenous apomorphine-induced inhibition of A9-DA neurons is inversely correlated to the basal firing rate of these neurons. When administered after baclofen, apomorphine further reduced DA cell activities in a manner proportional to the predrug firing rate, as in control rats. Since the minute doses of apomorphine which were used are thought to selectively activate DA autoreceptors, our finding may indicate that DA autoreceptor stimulation is more effective when the activity of the DA neurons is low, or it is decreased by drugs such as baclofen.

Pre-and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones

Using intracellular recording techniques, the effects of beta-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 microM ( +/- ) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a 'leak' or resting potassium conductance (gK), was added to the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Baclofen attenuates hyperpolarizing not depolarizing responses of caudate neurons in cat

The effects of baclofen, a gamma-aminobutyric acid (GABA) analogue, were studied on the intracellular responses of caudate neurons to cortical and thalamic stimulation. Systemic or intracaudate injections of baclofen did not reduce the initial excitatory postsynaptic potential (EPSP) to these stimuli; however, it did completely block evoked hyperpolarizations. These results suggest that the GABA-b receptor (one possible site of baclofen action) is not found on the corticostriate synaptic terminals. Furthermore, our results clearly indicate that evoked hyperpolarizations recorded in caudate neurons are not the result of cortical disfacilitation.

Stereoselectivity of L-baclofen in hippocampal slices of the rat

Extra- and intracellular recording from hippocampal slices of the rat revealed the following effects when baclofen (BF) (0.1-10 microM) was added to the perfusion fluid: a block of synaptic potentials evoked by stimulation of stratum radiatum; a direct hyperpolarization and a conductance increase (for potassium ions) of CA1 pyramidal cells. All this activity was found in the L- none in the D-enantiomer. D-BF did not antagonize the action of L-BF.

Hyperpolarizing action of baclofen on neurons in the rat substantia nigra slice

GABA, muscimol and L-baclofen inhibited zona compacta neurons in the rat substantia nigra slice, D-baclofen was inactive, L-baclofen produced a potent hyperpolarization of the membrane potential. The actions of muscimol but not GABA and L-baclofen were reversed by bicuculline. The actions of muscimol and baclofen are likely to be mediated by interactions with two different receptors.

Dopamine receptor-mediated regulation of incertohypothalamic dopaminergic neurons in the male rat

The incertohypothalamic dopaminergic (DA) neuronal system has been divided into a rostral component of neurons originating in the rostral periventricular nucleus and projecting to the preopticosuprachiasmatic and medial preoptic nuclei and a caudal component originating in the medial zona incerta and projecting to the dorsomedial and anterior hypothalamic nuclei. The purpose of the present study was to determine if the activity of these intrahypothalmic DA neurons is regulated by DA receptor-mediated mechanisms, as are those in the major ascending nigrostriatal and mesolimbic neurons, or if they resemble another group of intrahypothalamic DA neurons, those that comprise the tuberoinfundibular system, which are not responsive to the acute actions of DA agonists or antagonists. The rate of DA turnover (decline after alpha-methyltyrosine) in micropunched regions of the striatum (ST), nucleus accumbens (NA) and hypothalamic regions which contain cell bodies or terminals of incertohypothalamic DA neurons was increased after administration of a DA antagonist (haloperidol) and decreased after administration of a DA agonist (bromocriptine). gamma-Butyrolactone increased DA concentrations in the ST, NA and hypothalamic brain regions containing incertohypothalamic DA neurons, and this effect was blocked by the DA agonist apomorphine. In contrast, none of these treatments affected the concentration or rate of turnover of DA in the median eminence (terminal region of tuberoinfundibular neurons). Injections of either gamma-hydroxybutyric acid or baclofen into the substantia nigra/ventral tegmental region of the midbrain increased DA concentrations in the NA and/or ST but failed to alter DA concentrations in any hypothalamic region. These results suggest that the incertohypothalamic DA system is composed of neurons whose activity can be rapidly modulated by DA receptor-mediated mechanisms and thus resemble the DA neurons in the major ascending nigrostriatal and mesolimbic systems rather than the hypothalamic neurons which comprise the tuberoinfundibular DA system.