In hamsters,
progesterone (P) in the hypothalamus and ventral tegmental area (VTA) is necessary for receptivity; in rats, hypothalamic P induces receptivity and midbrain P further enhances it. How P exerts its effects in the VTA on
lordosis is of interest because few
estrogen-induced P receptors (PRs) have been identified there. Sexual receptivity of rats and hamsters is enhanced when P's actions in the VTA are restricted to the membrane and when the
gamma-aminobutyric acid (
GABA)A agonist,
muscimol, is infused into the VTA, but attenuated with infusions of the
GABA(A) antagonist,
bicuculline. The
dopamine (DA) agonist.
SKF38393, rapidly enhances receptivity when infused intravenously; this effect can be blocked by both DA receptor (DR) and PR antagonists. This study investigated the importance of PRs,
glutamic acid decarboxylase (GAD), the
enzyme responsible for
GABA production,
GABA(A) receptors (
GBRs), and DRs in the VTA of cycling rats and hamsters for the expression of
lordosis. Proestrous and diestrous animals implanted with bilateral VTA
cannulae were pre-tested for receptivity, infused with either an antagonist (
RU38486 (20 microg),
bicuculline (100 ng),
SCH23390 (100 ng)),
anti-sense oligonucleotide (against PR (250 ng), GAD (500 ng), D1 (500 ng), D5 (250 ng)), or control infusions to each
cannulae and re-tested. Vehicle and scrambled
oligonucleotides were infused as controls and elicited similar effects. Antagonists of
GBRs and DRs significantly reduced
lordosis on post-tests compared to the PR antagonist and control conditions in rats and hamsters.
Lordosis was significantly reduced, compared to controls, only by
anti-sense oligonucleotides for GAD and D1- and D5-DR subtypes. These data suggest that in the VTA GABAergic and dopaminergic neurons may be more important in the mediation of sexual receptivity than neurons containing intracellular PRs.