FOS-LIKE IMMUNOREACTIVITY IN THE MAMILLARY BODY AND THALAMUS FOLLOWING INJECTIONS OF MUSCIMOL INTO THE VENTRAL TEGMENTAL NUCLEUS OF GUDDEN IN THE RAT

In many neurons, increased rates of firing are accompanied by expressi on of the proto-oncoprotein Fos. The current study examined Fos-like i mmunoreactivity in the mamillary body and the anterior thalamus follow ing unilateral injections of the inhibitory GABA-A agonist muscimol in to the ventral tegmental nucleus of Gudden (VTN). These injections res ulted in a marked increase in Fos-like immunoreactivity ipsilaterally in both the medial mamillary nucleus and in its principle thalamic pro jection targets, the anteroventral and anteromedial thalamic nuclei. S ince the projection from the VTN to the mamillary body has been shown to contain a substantial GABAergic component, these results are likely to reflect a disinhibition of mamillothalamic circuitry resulting fro m suppression of tonic inhibitory inputs arising in the VTN.