Motor stimulant effects of ethanol and acetaldehyde injected into the posterior ventral tegmental area of rats: role of opioid receptors

A recently published study has shown that microinjections of ethanol, or its metabolite, acetaldehyde into the substantia nigra pars reticulata, are able to produce behavioral activation in rats. Another brain site that could participate in such effects is the ventral tegmental area (VTA). We have investigated the locomotor-activating effects of local microinjections of ethanol and acetaldehyde into the posterior VTA of rats and the role of opioid receptors in such effects. Cannulae were placed into the posterior VTA to perform microinjections of ethanol (75 or 150 nmol) or acetaldehyde (25 or 250 nmol) in animals not previously microinjected or microinjected with either the nonselective opioid antagonist naltrexone (13.2 nmol) or the irreversible antagonist of the A mu-opioid receptors beta-funaltrexamine (beta-FNA; 2.5 nmol). After injections, spontaneous activity was monitored for 60 min. Injections of ethanol or acetaldehyde into the VTA increased the locomotor activity of rats with maximal effects at doses of 150 nmol for ethanol and 250 nmol for acetaldehyde. These locomotor-activating effects were reduced by previously administering naltrexone (13.2 nmol) or beta-FNA (2.5 nmol) into the VTA. The posterior VTA is another brain region involved in the locomotor activation after the intracerebroventricular administration of ethanol or acetaldehyde. Our data indicate that opioid receptors, particularly the A mu-opioid receptors, could be the target of the actions of these compounds in the VTA. These results are consistent with the hypothesis that acetaldehyde could be a mediator of some ethanol effects.