Dopamine receptor-mediated regulation of expression of Fos and its related antigens (FRA) in somatostatin neurons in the hypothalamic periventricular nucleus

Somatostatin (SS)-containing perikarya located within the hypothalamic periventricular nucleus (PeVN) comprise a heterogenous population of neurons with both local intrahypothalamic and distant extrahypothalamic axonal projection sites. The close proximity of SS perikarya and their dendrites to dopaminergic (DA) neuronal processes in the PeVN suggests that these peptidergic neurons may be regulated by DA receptor-mediated mechanisms. To test this, the effects of the D₁ agonist SKF 38393 and D_2/3 agonist quinelorane were examined on expression of the immediate early gene products Fos and its related antigens (FRA) in SS-immunoreactive (IR) neurons in the PeVN. SS-IR neurons were located in the most medial portion of the PeVN bordered medially by the third ventricle and laterally by tyrosine hydroxylase (TH)-IR neurons. In control rats, 10–15% of all SS-IR neurons contained FRA-IR. Activation of D₁ receptors with SKF 38393 had no effect on either the total number of SS-IR neurons or the number of SS-IR neurons containing FRA-IR. In contrast, activation of D_2/3 receptors with quinelorane decreased the number of SS-IR neurons containing FRA-IR, without affecting the total number of SS-IR neurons. The D_2/3 antagonist raclopride had no effect per se, but prevented the quinelorane-induced decrease in the number of SS neurons expressing FRA-IR. These results reveal that activation of D_2/3 (but not D₁) receptors inhibits expression of the immediate early gene products FRA in SS-containing neurons in the PeVN, but expression of FRA in SS neurons is not tonically inhibited by dopamine acting on D_2/3 receptors.