Topographical Fos induction within the ventral midbrain and projection sites following self-stimulation of the posterior mesencephalon.

Rats will readily perform an operant response to self-administer electrical stimulation to the posterior mesencephalon (PM). Previous results show that axons that support self-stimulation travel between the PM and the ventral tegmental area (VTA) and that their activation increases firing of VTA neurons. The present work sought to extend these findings by describing the distribution of ventral midbrain neurons affected by PM self-stimulation. In Experiment 1, ventral midbrain Fos-immunoreactivity (IR) was assessed in three groups of rats implanted with a monopolar electrode; two groups were trained to self-administer stimulation, but only one was allowed to self-stimulate on the test day, whereas the third was never trained or tested. Self-stimulation induced prominent Fos-IR that was differentially distributed within the VTA and substantia nigra (SN). Control rats showed only sparse labeling. In Experiment 2, ventral midbrain Fos-IR was assessed with three additional groups trained to self-administer PM stimulation and tested as follows: Group-1 was allowed to self-stimulate, Group-2 received stimulation at parameters that failed to support self-stimulation (deemed non-rewarding) "yoked" to the rate of responding of Group-1, and Group-3 received no stimulation. PM self-stimulation induced Fos-IR throughout the rostral-caudal VTA and within the SN reticulata. Non-rewarding stimulation induced sparse Fos-IR, comparable to no stimulation. Fos-IR specific to PM self-stimulation was also observed within the bed nucleus of the stria terminalis (BNST) and nucleus accumbens (NAS)-shell, but not within NAS-core, caudate putamen, medial prefrontal or orbital cortices. These findings are consistent with evidence that reward or positive reinforcement can be triggered by chemical and electrical stimulation over a large rostral-caudal extent of the VTA. They suggest that among ventral midbrain projection sites, the BNST and NAS-shell constitute important components of the circuitry implicated in reward. They provide additional support for the functional link between neurons that support PM and VTA self-stimulation, and offer topographical guidance to future attempts at their identification.