Resumo

The recently described rostromedial tegmental nucleus (RMTg) is a structure that contains many GABAergic neurons and extends from the caudal pole of the ventral tegmental area (VTA) into the mesopontine tegmentum. The RMTg, which is activated by aversive stimuli and inhibited by rewards, receives a substantial glutamatergic input from the lateral habenula (LHb) and sends massive GABAergic projections to the entire VTA-nigra complex. Since LHb stimulation exerts a powerful inhibitory influence onto dopamine (DA) cells in the VTA, the RMTg has been implicated as an important source for signaling aversive events to DA neurons in the VTA. To further characterize these two important anatomical hallmarks of the RMTg, we compared by anterograde and retrograde tracing techniques the exact topographic position of retrogradely labeled neurons in the RMTg resulting from VTA injections with that of anterogradely labeled axons emerging from the LHb. Furthermore, we mapped the distribution of important AMPA and NMDA-type glutamate (Glu) receptor subunits in the RMtg. One group of adult male Wistar rats (n = 10) received single injections of the retrograde tracer cholera toxin subunit b (CTb) centered in the rostral VTA or RMTg. In another group of rats (n = 12), the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected in the LHb and CTb was injected in the VTA in the same animals. Both tracers were delivered iontophoretically and revealed by immunoperoxidase or double immunofluorescence techniques. The distribution of the most common AMPA (GluR1, GluR2, GluR2/3, GluR4) and NMDA (NMDAR1) type glutamate (Glu) receptor subunits, as well as the GABAergic marker GAD67 in the RMTg was investigated by standard immunoperoxidase or immunofluorescence techniques in naive animals and in rats which received prior injections of CTb in the VTA. LHb projections to the mesopontine tegmentum were found to be organized in a topographic manner, with projections to the RMTg mainly arising from the lateral segment of the LHb (LHbL). Most of the LHbL efferents in the mesopontine tegmentum were directed to the RMTg and a dense terminal field was found in the same topographic position in the RMTg than GAD67+ neurons and retrogradely labeled neurons projecting to the VTA. In contrast, very few PHA-L+ terminals were present in the VTA proper. Among the Glu receptor subunits studied, GluR2 and NMDAR1 were the most widely distributed in the RMTg, whereas GluR1 and GluR4 + neurons were rarely found. Double labeling experiments showed PHA-L+ fibers frequently apposed to CTb+ neurons in the RMTg and revealed that all VTA projecting neurons in the RMTg express the GluR2/3 subunits. Overall, our findings specify that LHb efferents mainly arise from the lateral segment of the LHb and reinforce the notion that the uniform inhibitory influence of the LHb onto DA cells in the VTA is intermediated by the RMTg with a pivotal role for the GluR2/3 subunits.