Miriam Melis

Endocannabinoids Mediate Presynaptic Inhibition of Glutamatergic Transmission in Rat Ventral Tegmental Area Dopamine Neurons through Activation of CB1 Receptors

The endogenous cannabinoid system has been shown to play a crucial role in controlling neuronal excitability and synaptic transmission. In this study we investigated the effects of a cannabinoid receptor (CB-R) agonist WIN 55,212-2 (WIN) on excitatory synaptic transmission in the rat ventral tegmental area (VTA). Whole-cell patch clamp recordings were performed from VTA dopamine (DA) neurons in an in vitro slice preparation. WIN reduced both NMDA and AMPA EPSCs, as well as miniature EPSCs (mEPSCs), and increased the paired-pulse ratio, indicating a presynaptic locus of its action. We also found that WIN-induced effects were dose-dependent and mimicked by the CB1-R agonist HU210. Furthermore, two CB1-R antagonists, AM281 and SR141716A, blocked WIN-induced effects, suggesting that WIN modulates excitatory synaptic transmission via activation of CB1-Rs. Our additional finding that both AM281 and SR141716A per se increased NMDA EPSCs suggests that endogenous cannabinoids, released from depolarized postsynaptic neurons, might act retrogradely on presynaptic CB1-Rs to suppress glutamate release. Hence, we report that a type of synaptic modulation, previously termed depolarization-induced suppression of excitation (DSE), is present also in the VTA as a calcium-dependent phenomenon, blocked by both AM281 and SR141716A, and occluded by WIN. Importantly, DSE was partially blocked by the D2DA antagonist eticlopride and enhanced by the D2DA agonist quinpirole without changing the presynaptic cannabinoid sensitivity. These results indicate that the two pathways work in a cooperative manner to release endocannabinoids in the VTA, where they play a role as retrograde messengers for DSE via CB1-Rs.

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.

Prefrontal Cortex Stimulation Induces 2-Arachidonoyl-Glycerol-Mediated Suppression of Excitation in Dopamine Neurons

Endocannabinoids form a novel class of retrograde messengers that modulate short- and long-term synaptic plasticity. Depolarization-induced suppression of excitation (DSE) and inhibition (DSI) are the best characterized transient forms of endocannabinoid-mediated synaptic modulation. Stimulation protocols consisting of long-lasting voltage steps to the postsynaptic cell are routinely used to evoke DSE-DSI. Little is known, however, about more physiological conditions under which these molecules are released in vitro. Moreover, the occurrence in vivo of such forms of endocannabinoid-mediated modulation is still controversial. Here we show that physiologically relevant patterns of synaptic activity induce a transient suppression of excitatory transmission onto dopamine neurons in vitro. Accordingly, in vivo endocannabinoids depress the increase in firing and bursting activity evoked in dopamine neurons by prefrontal cortex stimulation. This phenomenon is selectively mediated by the endocannabinoid 2-arachidonoyl-glycerol (2-AG), which activates presynaptic cannabinoid type 1 receptors. 2-AG synthesis involves activation of metabotropic glutamate receptors and Ca2+ mobilization from intracellular stores. These findings indicate that dopamine neurons release 2-AG to shape afferent activity and ultimately their own firing pattern. This novel endocannabinoid-mediated self-regulatory role of dopamine neurons may bear relevance in the pathogenesis of neuropsychiatric disorders such as schizophrenia and addiction.

Oxytocin-induced penile erection and yawning: Site of action in the brain

The effect of NG-nitro-L-arginine methyl ester (NAME), a potent inhibitor of nitric oxide (NO) synthase, injected into different brain areas on penile erection and yawning induced by apomorphine or oxytocin was studied in male rats. The compound was found to be able to prevent the above behavioral responses dose dependently when injected into the paraventricular nucleus of the hypothalamus (PVN), but not in the caudate nucleus, medial septum, preoptic area, and the CA1 field of the hippocampus. When injected in the PVN, 5 micrograms of NAME induced a 30% reduction of apomorphine and oxytocin responses, while 20 micrograms induced an almost complete reduction. The effect of NAME seems to be related to the inhibition of guanylate cyclase secondary to the prevention of NO formation, because a dose-dependent reduction of apomorphine and oxytocin responses was obtained also with the inhibitor of guanylate cyclase methylene blue injected intracerebroventricularly (100-400 micrograms ICV), but not into the PVN. The results provide further support for a neurotransmitter role of central NO in the control of penile erection and yawning.

Paraventricular nucleus lesion prevents yawning and penile erection induced by apomorphine and oxytocin but not by ACTH in rats.

The effect of electrolytic lesion of the paraventricular nucleus of the hypothalamus (PVN) on yawning and penile erection induced by apomorphine, oxytocin and adrenocorticotropic hormone (ACTH1-24) was studied in male rats. In sham-operated rats, apomorphine (50 micrograms/kg s.c.), oxytocin (30 ng i.c.v.), and ACTH1-24 (10 micrograms i.c.v.) significantly increased the number of yawning and penile erection episodes. In PVN-lesioned rats, apomorphine- and oxytocin-, but not ACTH-induced responses were strongly reduced. These results confirm our previous observations showing that the PVN has a crucial role in the expression of yawning and penile erection induced by dopamino-mimetic drugs and oxytocin, and suggest that ACTH-derived peptides induce the above responses by a mechanism not involving PVN hypothalamic dopamine or oxytocin.

Adolescent exposure to cannabinoids induces long-Lasting changes in the response to drugs of abuse of rat midbrain dopamine neurons

BACKGROUND Recent studies have raised concerns about subtle long-lasting neurobiological changes that might be triggered by exposure to Cannabis derivatives, especially in a critical phase of brain maturation, such as puberty. The mesolimbic dopamine (DA) system, involved in the processing of drug-induced reward, is a locus of action of cannabinoids and endocannabinoids. Thus, we compared the effects of repeated cannabinoid administration in adolescent and adult rats on DA neuronal functions and responses to drugs of abuse. METHODS Single-unit extracellular recordings from antidromically identified mesoaccumbens DA neurons and from their target cells in the nucleus accumbens were carried out in urethane-anesthetized rats. Animals were pretreated during adolescence or adulthood, for 3 days, with the cannabinoid agonist WIN55212.2 (WIN) or vehicle and allowed a 2-week interval. RESULTS In cannabinoid-administered rats, DA neurons were significantly less responsive to the stimulating action of WIN, regardless of the age of pretreatment; however, in the adolescent group, but not in the adult, long-lasting cross-tolerance developed to morphine, cocaine, and amphetamine. CONCLUSIONS Our study suggests that an enduring form of neuronal adaptation occurs in DA neurons after subchronic cannabinoid intake at a young age, affecting subsequent responses to drugs of abuse.

Endogenous fatty acid ethanolamides suppress nicotine-induced activation of mesolimbic dopamine neurons through nuclear receptors

Nicotine stimulates the activity of mesolimbic dopamine neurons, which is believed to mediate the rewarding and addictive properties of tobacco use. Accumulating evidence suggests that the endocannabinoid system might play a major role in neuronal mechanisms underlying the rewarding properties of drugs of abuse, including nicotine. Here, we investigated the modulation of nicotine effects by the endocannabinoid system on dopamine neurons in the ventral tegmental area with electrophysiological techniques in vivo and in vitro. We discovered that pharmacological inhibition of fatty acid amide hydrolase (FAAH), the enzyme that catabolizes fatty acid ethanolamides, among which the endocannabinoid anandamide (AEA) is the best known, suppressed nicotine-induced excitation of dopamine cells. Importantly, this effect was mimicked by the administration of the FAAH substrates oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), but not methanandamide, the hydrolysis resistant analog of AEA. OEA and PEA are naturally occurring lipid signaling molecules structurally related to AEA, but devoid of affinity for cannabinoid receptors. They blocked the effects of nicotine by activation of the peroxisome proliferator-activated receptor-α (PPAR-α), a nuclear receptor transcription factor involved in several aspects of lipid metabolism and energy balance. Activation of PPAR-α triggered a nongenomic stimulation of tyrosine kinases, which might lead to phosphorylation and negative regulation of neuronal nicotinic acetylcholine receptors. These data indicate for the first time that the anorexic lipids OEA and PEA possess neuromodulatory properties as endogenous ligands of PPAR-α in the brain and provide a potential new target for the treatment of nicotine addiction.

Increase in meso-prefrontal dopaminergic activity after stimulation of CB1 receptors by cannabinoids.

The intravenous administration of the psychoactive constituent of marijuana, delta9-tetrahydrocannabinol (delta9-THC) (62.5-1000 microg/kg), and the synthetic cannabinoid agonist WIN 55212,2 (WIN) (62.5-500 microg/kg), produced a dose-related increase in the firing rate and burst firing in the majority of antidromically identified meso-prefrontal dopaminergic neurons. In a restricted number of neurons (n=4), WIN administration did not increase firing rate but produced an increment of bursting activity. These effects of the cannabinoids were reversed by the intravenous administration of SR 141716 A, a selective cannabinoid antagonist (1 mg/kg), per se ineffective to modify the electrical activity of dopaminergic neurons. The results indicate that stimulation of cannabinoid CB1 receptors produces an activation of meso-prefrontal dopaminergic transmission. Considering that supranormal stimulation of D1 dopamine receptors in the prefrontal cortex has been shown to impair working memory, the present results suggest that the negative effects of cannabinoids on cognitive processes might be related to the activation of dopaminergic transmission in the prefrontal cortex.

Cannabinoids modulate neuronal firing in the rat basolateral amygdala: evidence for CB1- and non-CB1-mediated actions.

Recent evidence indicates that the basolateral amygdala (BLA) may be involved in behavioural effects induced by cannabinoids. High levels of CB1 cannabinoid receptors have been shown in this region, where they modulate excitatory and inhibitory synaptic transmission. However, the neurophysiological effects of these opposing synaptic actions have not been investigated in vivo. To this purpose, single-unit extracellular recordings were performed in urethane anaesthetized rats in order to determine whether exogenously applied cannabinoids influenced the spontaneous or evoked electrical activity of neurons in the BLA. The effects of cannabinoids were found to be dependent on the characteristics of the neurons examined and on the properties of the agents used. We tested and compared two structurally different synthetic cannabinoid receptor agonists, the highly potent HU-210 (0.125-1.0 mg/kg, i.v.) and WIN55212-2 (WIN, 0.125-1.0 mg/kg, i.v.). With a CB1 cannabinoid receptor-dependent mechanism, HU-210 potently inhibited the firing rate of BLA interneurons whereas WIN modulated the discharge rate in a biphasic manner. By contrast, BLA projection neurons, antidromically identified from the shell of the nucleus accumbens, were significantly inhibited by WIN at all doses tested, while HU-210 administration led to less consistent effects, since only 1.0 mg/kg inhibited firing rate in the majority of recorded neurons. Additionally, WIN, but not HU-210, significantly attenuated short-latency spiking activity in BLA projection neurons evoked by electrical stimulation of the medial prefrontal cortex. In these neurons, WIN-induced effects were antagonised by the non-selective cannabinoid receptor antagonist SR141716A and by the vanilloid receptor antagonist capsazepine, but not by the selective CB1 antagonist AM-251. Taken together, our findings indicate that the overall excitability of efferent neurons in the BLA is strongly reduced by WIN in a non-CB1-dependent manner. In this effect, the contribution of a novel cannabinoid-vanilloid-sensitive putative non-CB1 receptors, the existence of which was postulated in recent reports, might play a role.

Different mechanisms for dopaminergic excitation induced by opiates and cannabinoids in the rat midbrain.

1. The mechanism underlying morphine and cannabinoid-induced excitation of meso-accumbens and nigro-striatal dopaminergic neurons was investigated by extracellular single unit recording techniques coupled with antidromic activation from the nucleus accumbens and striatum respectively, in unanesthetized rats. 2. The intravenous administration of cumulative doses (1-4 mg/kg) of morphine, dose-dependently increased the firing rate of dopaminergic neurons projecting to the nucleus accumbens and neostriatum, while the same doses inhibited the activity of pars reticulata neurons of the substantia nigra. Both effects were antagonized by naloxone (0.1 mg/kg i.v.) but not by the selective CB1 receptor antagonist SR 141716A (1 mg/kg i.v.). 3. The intravenous administration of cumulative doses (0.125-0.5 mg/kg) of delta9-tetrahydrocannabinol (delta9-THC) also increased the firing rate of meso-accumbens and nigro-striatal dopaminergic neurons; this effect was antagonized by SR 141716A (1 mg/kg i.v.), but not by naloxone. 4. Furthermore, nor delta9-THC up to a dose of 1 mg/kg, maximally effective in stimulating dopamine neurons, neither SR 141716A (1 mg/kg i.v.) at a dose able to reverse the stimulatory effect of delta9, THC on dopamine cells, did alter the activity of SNr neurons. 5. The results indicate that morphine and delta9-THC activate dopaminergic neurons through distinct receptor-mediated mechanisms; morphine may act by removing the inhibitory input from substantia nigra pars reticulata neurons (an effect mediated by mu-opioid receptors). Alternatively, the delta9-THC-induced excitation of dopaminergic neurons seems to be mediated by CB1 cannabinoid receptors, while neither mu-opioid receptors nor substantia nigra pars reticulata neurons are involved.