Immunohistochemical characterisation of Fos-positive cells in brainstem monoaminergic nuclei following intracranial self-stimulation of the medial forebrain bundle in the rat

Fos immunostaining was used as a marker of neuronal activity following intr acranial self-stimulation (ICSS) of the medial forebrain bundle (MFB) in th e rat, and was combined with immunostaining for tyrosine hydroxylase (TH), serotonin (5-HT), gamma -aminobutyric acid (GABA), or NR1 (one of the gluta mate N-methyl-D-aspartate receptor subunits) for purposes of neurochemical identification. ICSS induced a significant but different degree of increase in the number of Fos-immunopositive (Fos+) cells in the six brainstem mono aminergic nuclei examined, which included the ventral tegmental area (VTA), substantia nigra pars compacta (SNc), dorsal raphe nucleus (DR), median ra phe nucleus (MR), locus coeruleus (LC), and A7 noradrenaline cells. Densely labelled Fos+ cells were observed in the LC following ICSS, and many of th ese Fos+ cells were colocalized with TH. Similarly, many of Fos+ cells in t he A7 and DR/MR were colocalized with TH and 5-HT, respectively. By contras t, a smaller number of Fos+ cells was detected in the VTA and SNc following the ICSS, and in these regions the majority of Fos+ cells were not colocal ized with TH. Although results among regions quantitatively differed, the I CSS induced a significant increase in the number of double-labelled cells ( GABA+/Fos+ or NR1+/Fos+) in all of the VTA, DR, and LC, in which the ICSS p roduced an ipsilaterally weighted increase in Fos-like immunoreactivity. Th ese results suggest that ICSS of the MFB induces differential Fos expressio n within monoaminergic and GABAergic neurons in brainstem monoaminergic nuc lei under modulation by glutamatergic afferents.