Progesterone withdrawal II: Insensitivity to the sedative effects of a benzodiazepine

Previous results from this lab have demonstrated that the GABA- modulatory steroid 3α-OH-5α-pregnan-20-one (3α,5α-THP) exhibits withdrawal properties, increasing anxiety [M.A. Gallo, S.S. Smith, Progesterone withdrawal decreases latency to and increases duration of electrified prod burial: a possible rat model of PMS anxiety, Pharmacol. Biochem. 46 (1993) 897-904.] and seizure susceptibility [S.S. Smith, Q.H. Gong, F.-C. Hsu, R.S. Markowitz, J.M.H. ffrench-Mullen, X. Li, GABA(A) receptor α4 subunit suppression prevents withdrawal properties of an endogenous steroid, Nature 392 (1998) 926-930.] upon abrupt discontinuation after chronic administration of its parent compound, progesterone (P), in a manner similar to other GABA-modulatory drugs. Further, we have demonstrated that withdrawal from P produces insensitivity to the potentiating effects of the benzodiazepine (BDZ) lorazepam (LZM) on GABA-gated Cl^- current [A.-M.N. Costa, K.T. Spence, S.S. Smith, J.M.H. ffrench-Mullen, Withdrawal from the endogenous steroid progesterone results in GABA(A) currents insensitive to BDZ modulation in rats CA1 hippocampus, J. Neurophysiology 74 (1995) 464- 469; S.S. Smith, Q.H. Gong, F.-C. Hsu, R.S. Markowitz, J.M.H. ffrench- Mullen, X. Li, GABA(A) receptor α4 subunit suppression prevents withdrawal properties of an endogenous steroid, Nature 392 (1998) 926-930.], assessed using whole cell patch clamp procedures on pyramidal neurons acutely dissociated from CA1 hippocampus. The purpose of the present study was to examine the withdrawal effects of P on the sedative potency of LZM, tested behaviorally as the ability to maintain position on a variable speed treadmill following LZM administration (0.75 mg/kg). Both continuous (continuous release P capsule, single withdrawal) as well as discontinuous (multiple P injection, multiple withdrawal) paradigms were tested. Longer continuous release paradigms were more effective in abolishing the sedative effects of LZM, without producing a change in baseline response. The LZM insensitivity observed following the multiple withdrawal paradigm was prevented by prior intraventricular administration of antisense oligonucleotide against the α4 subunit of the GABA(A) receptor. These results support the hypothesis that withdrawal from P decreases the behavioral response to LZM as a direct result of increases in the α4 subunit of the GABA(A) receptor. Withdrawal from P occurs endogenously during pre- menstrual and post-partum periods, when decreased response to BDZ has been reported.