The rostromedial tegmental nucleus RMTg is not a critical site for ethanol-induced motor activation in rats

Rationale Opioid drugs indirectly activate dopamine (DA) neurons in the ventral tegmental area (VTA) through a disinhibition mechanism mediated by mu opioid receptors (MORs) present both on the GABA projection neurons located in the medial tegmental nucleus/tail of the VTA (RMTg/tVTA) and on the VTA GABA interneurons. It is well demonstrated that ethanol, like opioid drugs, provokes VTA DA neuron disinhibition by interacting (through its secondary metabolite, salsolinol) with MORs present in VTA GABA interneurons, but it is not known whether ethanol could disinhibit VTA DA neurons through the MORs present in the RMTg/tVTA. Objectives The objective of the present study was to determine whether ethanol, directly microinjected into the tVTA/RMTg, is also able to induce VTA DA neurons disinhibition. Methods Disinhibition of VTA DA neurons was indirectly assessed through the analysis of the motor activity of rats. Cannulae were placed into the tVTA/RMTg to perform microinjections of DAMGO (0.13 nmol), ethanol (150 or 300 nmol) or acetaldehyde (250 nmol) in animals pre-treated with either aCSF or the irreversible antagonist of MORs, beta-funaltrexamine (beta-FNA; 2.5 nmol). After injections, spontaneous activity was monitored for 30 min. Results Neither ethanol nor acetaldehyde directly administered into the RMTg/tVTA were able to increase the locomotor activity of rats at doses that, in previous studies performed in the posterior VTA, were effective in increasing motor activities. However, microinjections of 0.13 nmol of DAMGO into the tVTA/RMTg significantly increased the locomotor activity of rats. These activating effects were reduced by local pre-treatment of rats with beta-FNA (2.5 nmol). Conclusions The tVTA/RMTg does not appear to be a key brain region for the disinhibiting action of ethanol on VTA DA neurons. The absence of dopamine in the tVTA/RMTg extracellular medium, the lack of local ethanol metabolism or both could explain the present results.