Moloecular Pharmacology Journal Club- September 1/98 Paper

DECREASED PRESYNAPTIC SENSITIVITY TO ADENOSINE AFTER COCAINE WITHDRAWAL

Olivier Manzoni, Didier Pujaltl, John Williams and Joel Bockaert

Journal of Neuroscience 18(19), 7996-8002 (1998)

Presenter: L.Niles

Presenter's Summary

Background:

Several studies have implicated the mesolimbic dopaminergic system which projects from the ventral tegmental area (VTA), and its targets especially the nucleus accumbens (NAc), in the reinforcing properties of addictive drugs including cocaine. Psychostimulants such as cocaine and amphetamine have been found to depress excitatory glutamatergic transmission in the NAc via D1-like dopamine receptors. Possible mechanisms are: (1) a direct effect involving activation of presynaptic D1-dopamine receptors; (2) postsynaptic activation of D1 receptors which interact with NMDA receptors to cause release of adenosine, with subsequent activation of presynaptic A1-adenosine receptors and inhibition of glutamatergic transmission. Presynaptic metabotropic glutamate receptors may also contribute to the inhibition of glutamate release in the NAc.

There is evidence that an antagonistic interaction occurs between the neuromodulator, adenosine and the neurotransmitter dopamine in the CNS. A role for adenosine in the long-term changes in the VTA, induced by chronic cocaine treatment, has been reported. In addition, presynaptic A1 adenosine receptors mediate a potent inhibition of glutamate release from cortical afferents. Therefore, in examining the neurochemical changes in the NAc following withdrawal from chronic cocaine, the authors focused on adenosine-mediated presynaptic regulation of excitatory (glutamatergic) transmission.

Present Work: Initial field EPSP (fEPSP) recordings in the core of the NAc indicated that synaptic responses, evoked by stimulating prefrontal cortical fibres, were strongly inhibited by adenosine. This inhibitory effect was completely blocked by A1 antagonists, and was localized presynaptically, as indicated by whole-cell patch-clamp experiments.

In drug treatment experiments, rats were injected daily with saline or cocaine (20 mg/kg, ip) for 2 weeks, and tests conducted 1 day or 8 days after the final cocaine injection. Adenosine (30 uM, 5 min) depressed evoked glutamate release by ~50% in the NAc of controls, but was ineffective in rats withdrawn from cocaine for 1 day or 8 days. Dose-response curves for adenosine were shifted to the right in cocaine-withdrawn animals. EC₅₀ values for treatment groups were:- Control (32 ą 4 uM, n=9) 1 Day withdrawn (70 ą 9 uM, n=9), 8 Day withdrawn (166 ą 3 uM, n=8). In contrast to the effect observed with adenosine, the dose-response curve for the selective A1 agonist, N6CPA, was similar in control and treated slices. This indicates that the effects observed with adenosine did not involve desensitization of presynaptic A1 receptors in the NAc of cocaine-withdrawn rats.

Other presynaptic inhibitors, such as dopamine or metabotropic glutamate receptor agonists, (L-AP-4, L-CCG1), did not differentiate between control or treated slices, which discounts a general decrease in presynaptic inhibition as the mechanism underlying the effect of cocaine withdrawal. Moreover, the effect of drug treatment was specific to the NAc, since adenosine was equally effective in suppressing glutamate release in hippocampal slices from either controls or cocaine-withdrawn rats.

Previous studies have indicated an overall upregulation of the cAMP-dependent cascade in the NAc of rats subjected to chronic cocaine treatment. Therefore, the authors examined the effects of forskolin, a direct activator of the adenylate cyclase-cAMP pathway. Forskolin caused a similar potentiation of fEPSPs in both control and chronic withdrawn rats. This potentiation was enhanced in the presence of the A1 antagonist, DPCPX, but there was again no difference between groups, indicating that chronic cocaine did not alter presynaptic adenylate cyclase activity in the NAc.

In addition to metabolic pathways, uptake systems are important in regulating the extracellular levels of adenosine in the brain. In keeping with the foregoing, a combination of the adenosine uptake inhibitors, D1PY and NBTI, caused a strong inhibition of excitatory synaptic transmission that was completely blocked by the A1 antagonist, DPCPX. However, the high levels of adenosine available, in the presence of uptake inhibitors, suppressed the fEPSP to a level where accurate determination of the effects of added adenosine was not possible. To resolve this problem, endogenous levels of adenosine were lowered by preincubation of slices with the phosphodiesterase inhibitor, R020-1724 (200 uM), which blocks adenosine production from cAMP metabolism. Under these conditions, an examination of the effect of adenosine in the presence of uptake blockade, revealed that the inhibitory effect of adenosine on glutamatergic transmission was identical in slices from naive and cocaine-withdrawn rats.

Conclusions: The authors conclude that withdrawal from cocaine caused an upregulation of adenosine transporters, since blockade of adenosine reuptake restored full sensitivity to the inhibitory effect of this neuromodulator on glutamate release in the NAc.

Why is this paper important?

The acute effects of cocaine are known to involve blockade of monoamine transporters with consequent elevation of extracellular dopamine and activation of its receptors. Thus, numerous studies have focused on dopaminergic mechanisms in attempting to clarify the neurochemical changes associated with cocaine addiction. The present findings suggest that other neuromodulator or neurotransmitter systems, unrelated to the primary targets of cocaine, are important in the long-term synaptic changes underlying addiction to this psychostimulant. This information will be useful in the development of novel therapeutic approaches to drug addiction.

Authors' Abstract: The nucleus accumbens (NAc) is a site mediating the rewarding properties of drugs of abuse, such as cocaine, amphetamine, opiates, nicotine, and alcohol (Wise and Bozarth, 1987; Koob, 1992; Samson and Haris, 1992; Woolverton and Johnson, 1992; Self and Nestler, 1995; Pontieri et al., 1996). Acute cocaine has been shown to decrease excitatory synaptic transmission mediated by the cortical afferents to the NAc (Nicola et al., 1996), but the effects of long-term cocaine treatment and withdrawal have not been explored. Here, we report that long-term (1 week) withdrawal from chronic cocaine reduced the potency of adenosine to presynapticaly inhibit glutamate (Glu) release by activating adenosine. A1 receptors. Adenosine A1 receptors were not desensitized, because the potency of the metabolically stable adenosine analog N⁶-cyclopentyladenosine was unchanged after chronic cocaine withdrawal. When adenosine transporters were blocked, the potency of adenosine to inhibit Glu release from naive and cocaine-withdrawn NAc slices was similar. These results suggest that one of the long-term consequences of cocaine withdrawal is an augmented uptake of adenosine. This long-lasting change expressed at the presynaptic excitatory inputs to the medium spiny output neurons in the NAc may help identify new therapeutic targets for the treatment of drug abuse.

McPharmNet - Molecular Pharmacology at McMaster / mcpharm@fhs.csu.mcmaster.ca /
Last revised October 6, 1998.