Giuliano Pillolla

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.

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.

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.

Oxytocin injected into the ventral tegmental area induces penile erection and increases extracellular dopamine in the nucleus accumbens and paraventricular nucleus of the hypothalamus of male rats.

The neuropeptide oxytocin (20–100 ng), induces penile erection when injected unilaterally into the caudal but not rostral mesencephalic ventral tegmental area (VTA) of male Sprague–Dawley rats. Such pro‐erectile effect started 30 min after treatment and was abolished by the prior injection of d(CH2)5Tyr(Me)2‐Orn8‐vasotocin (1 µg), an oxytocin receptor antagonist injected into the same caudal ventral tegmental area or of haloperidol (1 µg), a dopamine receptor antagonist, injected either into the nucleus accumbens shell (NAs) or into the paraventricular nucleus of the hypothalamus (PVN) ipsilateral to the injected ventral tegmental area. Penile erection was seen 15 min after the occurrence of, or concomitantly to, an increase in extracellular dopamine and its metabolite 3,4‐dihydroxyphenylacetic acid (DOPAC) in the dialysate obtained from the nucleus accumbens or the paraventricular nucleus, which was also abolished by d(CH2)5Tyr(Me)2‐Orn8‐vasotocin (1 µg), injected into the ventral tegmental area before oxytocin. In the caudal ventral tegmental area oxytocin‐containing axons/fibres (originating from the paraventricular nucleus) appeared to closely contact cell bodies of mesolimbic dopaminergic neurons retrogradely labelled with Fluorogold injected into the nucleus accumbens shell, suggesting that oxytocin effects are mediated by the activation of mesolimbic dopaminergic neurons, followed in turn by that of incerto‐hypothalamic dopaminergic neurons impinging on oxytocinergic neurons mediating penile erection. As the stimulation of paraventricular dopamine receptors not only induces penile erection, but also increases mesolimbic dopamine neurotransmission by activating oxytocinergic neurons, these results provide further support for the existence of a neural circuit in which dopamine and oxytocin influence both the consummatory and motivational/rewarding aspects of sexual behaviour.

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.

Cannabinoids modulate spontaneous neuronal activity and evoked inhibition of locus coeruleus noradrenergic neurons.

The noradrenergic pathway arising from the locus coeruleus (LC) is involved in the regulation of attention, arousal, cognitive processes and sleep. These physiological activities are affected by Cannabis exposure − both in humans and laboratory animals. In addition, exogenous cannabinoids, as well as pharmacological and genetic manipulation of the endocannabinoid system, are known to influence emotional states (e.g. anxiety) for which a contributory role of the LC‐noradrenergic system has long been postulated. However, whether cannabinoid administration would affect the LC neuronal activity in vivo is still unknown. To this end, single‐unit extracellular recordings were performed from LC noradrenergic cells in anaesthetized rats. Intravenous injection of both the synthetic cannabinoid agonist, WIN55212‐2, and the main psychoactive principle of Cannabis, Δ9‐tetrahydrocannabinol, dose‐dependently increased the firing rate of LC noradrenergic neurons, with WIN55212‐2 being the most efficacious. Similar results were obtained by the administration of these drugs into a lateral ventricle. Cannabinoid‐induced stimulation of LC noradrenergic neuronal activity was counteracted by SR141716A, a cannabinoid receptor antagonist/reverse agonist, which by itself slightly reduced LC discharge rate. Moreover, WIN55212‐2 suppressed the inhibition of noradrenergic cells produced by stimulation of the major γ‐aminobutyric acid (GABA)ergic afferent to the LC, the nucleus prepositus hypoglossi. Altogether, these findings suggest the involvement of noradrenergic pathways in some consequences of Cannabis intake (e.g. cognitive and attention deficits, anxiety reactions), as well as a role for cannabinoid receptors in basic brain activities sustaining arousal and emotional states.

Cannabinoids inhibit excitatory inputs to neurons in the shell of the nucleus accumbens: an in vivo electrophysiological study.

The nucleus accumbens (NAc) represents a critical site for the rewarding properties of diverse classes of drugs of abuse. Glutamatergic afferents to the NAc are involved in the actions of psychostimulants and opioids, while the potentiation of dopaminergic neurotransmission in the NAc is a common feature of abused drugs, including cannabinoids. Cannabinoid receptors (CB1) are densely expressed in regions that provide excitatory innervation to the NAc, such as the amygdala, the cortex and the hippocampus. Recent in vitro evidence suggests that indeed cannabinoids modulate glutamatergic synapses in the NAc. In this study we recorded extracellularly from neurons in the shell of the NAc which responded to the stimulation of the baso‐lateral amygdala (BLA) or the medial prefrontal cortex (PFC) in urethane anaesthetized rats. BLA or PFC stimulation induced generation of action potentials in NAc neurons. This excitatory effect was strongly inhibited by the synthetic cannabinoid agonists WIN 55212,2 (0.062–0.25 mg/kg, i.v.) and HU‐210 (0.125–0.25 mg/kg, i.v.) or the psychoactive principle of Cannabis delta(9)‐tetrahydrocannabinol (1.0 mg/kg, i.v.). Neither the D1 or D2 dopamine receptor antagonists (SCH23390 0.5–1.0 mg/kg, sulpiride 5–10 mg/kg, i.v.) or the opioid antagonist naloxone (1.0 mg/kg, i.v.) were able to reverse the action of cannabinoids, while the selective CB1 receptor antagonist/reverse agonist SR141716A (0.5 mg/kg, i.v.) fully suppressed the action of cannabinoid agonists, whereas per se had no significant effect. These results provide evidence that cannabinoids, in common with other drugs of abuse, in vivo strongly inhibit the excitability of neurons in the shell of the NAc.

Involvement of the endogenous cannabinoid system in the effects of alcohol in the mesolimbic reward circuit: electrophysiological evidence in vivo

RationaleSeveral lines of evidence indicate that the endogenous cannabinoid system is involved in the pharmacological and behavioural effects of alcohol. The mesolimbic dopaminergic (DA) system and the nucleus accumbens (NAc) process rewarding properties of drugs of abuse, including alcohol and cannabinoids, whereas endocannabinoids in these regions modulate synaptic function and mediate short- and long-term forms of synaptic plasticity.ObjectivesThe present study was designed to investigate the contribution of the endogenous cannabinoid system in alcohol electrophysiological effects in the mesolimbic reward circuit.MethodsWe utilized extracellular single cell recordings from ventral tegmental area (VTA) DA and NAc neurons in anesthetized rats. DA neurons were antidromically identified as projecting to the shell of NAc, whereas NAc putative medium spiny neurons were identified by their evoked responses to basolateral amygdala (BLA) stimulation. ResultsAlcohol stimulated firing rate of VTA DA neurons and inhibited BLA-evoked NAc neuron spiking responses. The cannabinoid type-1 receptor (CB1) antagonist rimonabant (SR141716A) fully antagonized alcohol effect in both regions. In the NAc, either inhibition of the major catabolic enzyme of the endocannabinoid anandamide, the fatty-acid amyd hydrolase, with URB597 or a pretreatment with the CB1 receptor agonist WIN55212-2 significantly depressed alcohol-induced effects in the NAc.ConclusionsThese results corroborate the notion of the involvement of endocannabinoids and their receptors in the actions of alcohol and highlight the endocannabinoid system as a valuable target in the therapy for alcoholism.

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.