Effects of injections of 8-hydroxy-2-(di-n-propylamino)tetralin or muscimol in the median raphe nucleus on c-fos mRNA in the rat brain.

Inhibition of the median raphe nucleus (MRN) by the local injection of 5-HT(1A) or GABA(A) receptor agonists produces strong activational effects on feeding, drinking and locomotor activity. Using an animal model of relapse, we have shown that intra-MRN injection of the 5-HT(1A) autoreceptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reinstates alcohol seeking in rats. The circuitry underlying the behavioral effects of intra-MRN injection of these drugs is not known. In order to identify the brain areas that may be involved, we measured levels of mRNA of the immediate early gene c-fos in discrete nuclei of the rat brain following intra-MRN infusions of these drugs. Male Wistar rats received intra-MRN infusions of 8-OH-DPAT (1 mug), muscimol (25 ng) or saline vehicle immediately prior to placement in locomotor activity chambers. Thirty minutes later, they were decapitated, and their brains processed for in situ hybridization of c-fos mRNA. In agreement with previous reports, injections of 8-OH-DPAT or muscimol into the MRN resulted in large increases in locomotor activity. Intra-MRN injections of these drugs increased c-fos in a number of brain nuclei previously shown to be involved in the rewarding effects of drugs of abuse in a regionally specific manner. Both drugs significantly increased the expression of c-fos mRNA in the medial frontal cortex, nucleus accumbens, lateral septum, dorsal bed nucleus of the stria terminalis and ventral tegmental area. In the ventral hippocampus, only 8-OH-DPAT increased c-fos, while in the basolateral nucleus of the amygdala and locus coeruleus, it was increased only by muscimol. These results are discussed in terms of the projections of the MRN and the pathways involved in relapse to alcohol and drug seeking.