Annalisa Muntoni

Cannabinoids activate mesolimbic dopamine neurons by an action on cannabinoid CB1 receptors

The present study was designed to determine if cannabinoids share with other drugs of abuse the ability to stimulate mesolimbic dopaminergic neurons and if this effect is mediated by cannabinoid receptors. To this end, the effects of the prototypical cannabinoid, delta9 tetrahydrocannabinol ¿(-)-trans-(6aR,10aR)-6a,7,8,10a-tetrahydro-6,6,9-trimethyl- 3-pentyl-6H-dibenzo[b,d]pyran-1-ol¿, and the two highly potent synthetic cannabinoids, ¿(R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)-methyl]pyrrolo[1,2,3-d e]-1,4-benzoxazin-6-yl, +(1-naphtalenyl)methanone¿ WIN 55,212-2 and ¿(-)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl )-cicloexan-1-ol¿ CP 55,940, on the spontaneous discharge rate of meso-accumbens dopamine (A10 dopamine) neurons were studied in rats. The intravenous administration of delta9-tetrahydrocannabinol, WIN 55,212-2 and CP 55,940 (0.0625-1.0 mg/kg) produced a dose-dependent increase in the spontaneous firing of A10 dopamine neurons both in non-anesthetized and anesthetized rats, with a maximal percent increase of 120, 187 and 155 in non-anesthetized and 33, 102 and 52, respectively, in anesthetized rats. The stimulant response to cannabinoids was suppressed by the specific cannabinoid receptor antagonist ¿N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1H-pyrazole-3-carboxamide¿ SR 141716A, indicating a cannabinoid receptor-mediated effect. These findings support the contention that cannabinoids regulate mesolimbic dopamine transmission and may help to explain the addictive properties of marijuana.

Enduring Effects of Chronic Ethanol in the CNS: Basis for Alcoholism

This symposium focused on functional alterations in the mesolimbic dopamine system during the abstinence phase after chronic alcohol intake. Mark Brodie first described his recordings from midbrain slices prepared after chronic alcohol treatment in vivo by daily injection in C57BL/6J mice. No changes were found in the baseline firing frequency of dopaminergic neurones in the VTA (ventral tegmental area), but the excitation produced in these neurones by an acute ethanol challenge was significantly increased in neurons from ethanol-treated mice compared with those from the saline-treated controls. There was also a significant decrease in the inhibitory response to GABA by the dopamine neurones following the chronic ethanol treatment. These data suggest that the timing pattern and mode of ethanol administration may determine the types of changes observed in dopaminergic reward area neurons. Annalisa Muntoni lectured on the relationship between electrophysiological and biochemical in vivo evidence supporting a reduction in tonic activity of dopamine neurons projecting to the nucleus accumbens at various times after suspension of chronic ethanol treatment and morphological changes affecting dopamine neurons in rat VTA. Hilary J. Little then described changes in dopaminergic neurone function in the VTA during the abstinence phase. Decreases in baseline firing were seen at 6 days after withdrawal of mice from chronic ethanol treatment but were not apparent after 2 months abstinence. Increases in the affinity of D1 receptors in the striatum, but not in the cerebral cortex, were seen however up to 2 months after withdrawal. Scott Steffensen then described his studies recording in vivo from GABA containing neurones in the VTA in freely moving rats. Chronic ethanol administration enhanced the baseline activity of these neurones and resulted in tolerance to the inhibition by ethanol of these neurones. His results demonstrated selective adaptive circuit responses within the VTA or in extrategmental structures that regulate VTA-GABA neurone activity.

Marked decrease of A10 dopamine neuronal firing during ethanol withdrawal syndrome in rats

The electrophysiological activity of mesoaccumbens dopaminergic neurons was monitored during the ethanol-withdrawal syndrome in ethanol-dependent and in control rats. Spontaneous firing was reduced by about half in ethanol-dependent rats as compared to controls. Likewise, the number of spikes/burst was also reduced in ethanol-dependent rats. These results are consistent with the reduction in dopamine release observed during ethanol-withdrawal syndrome and may provide the basis for the aversive effects of the ethanol-withdrawal syndrome.

Effects of acute, chronic ethanol and withdrawal on dorsal raphe neurons: electrophysiological studies

The effect of a single intravenous administration of ethanol (0.25-1.0 g/kg) on the spontaneous activity of putative serotonin neurons of the dorsal raphe nucleus was studied in unanesthetized rats. Ethanol produced a slight but progressive decline in neuronal activity in 67% (six of nine) of all neurons tested. The remaining 33% (three of nine) were unresponsive. Upon withdrawal of chronic ethanol treatment (1-5 g/kg every 6 h for six consecutive days, 12 h from last ethanol administration), the mean firine rate of dorsal raphe neurons was found to be significantly reduced, by about 30% (n=71), as compared with the control group (n=83), whereas the cells/track index was unaltered. Under these conditions, ethanol administration further reduced firing rate in 67% (four of six) of all the neurons tested. In the remaining 33% (two of six), no response was observed. At 72 h after the last ethanol administration, the mean firing rate of dorsal raphe neurons was found to be within control values (n=90). Further, to evaluate the functional status of the autoreceptors under control conditions and after withdrawal from chronic ethanol, the selective serotonin-1A receptor agonist 8-hydroxy-(2-di-n-propylamino)tetralin was administered intravenously in cumulative doses (1-16 microg/kg) and dose-response curves were generated for both groups. Autoreceptor sensitivity of dorsal raphe neurons was found to be not statistically different in control and ethanol withdrawn rats (n=6 for both groups) as indexed by a similar potency displayed by 8-hydroxy-(2-di-n-propylamino)tetralin in reducing the spontaneous activity of dorsal raphe neurons. The results indicate that, in spite of the widespread use of serotonin transmission potentiating agents in the treatment of alcoholism, neither acute nor withdrawal from chronic ethanol administration produces drastic effects on dorsal raphe neurons. However, the inhibition of dorsal raphe neuronal activity after acute ethanol may be due to the reported ability of ethanol to increase serotonin release from terminal areas. This increased serotonin tone could, at the level of recurrent axon collaterals in the dorsal raphe nucleus, reduce the spontaneous activity of the cells. On the other hand, a similar reduction in spontaneous activity after withdrawal from ethanol correlates well with the reduction in serotonin levels observed under these conditions in microdialysis studies.

γ-Hydroxybutyric acid (GHB) and the mesoaccumbens reward circuit: Evidence for GABAB receptor-mediated effects

γ-Hydroxybutyric acid (GHB) is a short-chain fatty acid naturally occurring in the mammalian brain, which recently emerged as a major recreational drug of abuse. GHB has multiple neuronal mechanisms including activation of both the GABAB receptor, and a distinct GHB-specific receptor. This complex GHB-GABAB receptor interaction is probably responsible for the multifaceted pharmacological, behavioral and toxicological profile of GHB. Drugs of abuse exert remarkably similar effects upon reward-related circuits, in particular the mesolimbic dopaminergic system and the nucleus accumbens (NAc). We used single unit recordings in vivo from urethane-anesthetized rats to characterize the effects of GHB on evoked firing in NAc “shell” neurons and on spontaneous activity of antidromically identified dopamine (DA) cells located in the ventral tegmental area. GHB was studied in comparison with the GABAB receptor agonist baclofen and antagonist (2S)(+)-5,5-dimethyl-2-morpholineacetic acid (SCH50911). Additionally, we utilized a GHB analog, γ-(p-methoxybenzil)-γ-hydroxybutyric acid (NCS-435), devoid of GABAB binding properties, but with high affinity for specific GHB binding sites. In common with other drugs of abuse, GHB depressed firing in NAc neurons evoked by the stimulation of the basolateral amygdala. On DA neurons, GHB exerted heterogeneous effects, which were correlated to the baseline firing rate of the cells but led to a moderate stimulation of the DA system. All GHB actions were mediated by GABAB receptors, since they were blocked by SCH50911 and were not mimicked by NCS-435. Our study indicates that the electrophysiological profile of GHB is close to typical drugs of abuse: both inhibition of NAc neurons and moderate to strong stimulation of DA transmission are distinctive features of diverse classes of abused drugs. Moreover, it is concluded that addictive and rewarding properties of GHB do not necessarily involve a putative high affinity GHB receptor.

Heterogeneous responses of substantia nigra pars reticulata neurons to γ-hydroxybutyric acid administration

The effect of intravenous administration of gamma-hydroxybutyric acid (GHB) (50-400 mg/kg) on the firing rate of substantia nigra pars reticulata (SN-PR) neurons was studied by making single cell extracellular recordings in unanesthetized rats. For comparison, the effect of intravenous muscimol (0.5-2 mg/kg) and ethanol (0.5-2 g/kg) was also studied. GHB produced variable effects: dose-related inhibition in 7 out of 18 (38.8%) neurons and no significant change in 11 out of 18 (61.2%) neurons tested. In contrast, muscimol and ethanol produced a dose-related inhibition of the SN-PR firing rate. The results indicate that GHB, unlike muscimol and ethanol, has no profound effect on the activity of SN-PR neurons, and thus disinhibition of dopaminergic units, through inhibition of SN-PR neurons, is probably not the mechanism by which GHB stimulates the firing rate of dopaminergic neurons.