Similarity of luteinizing hormone surges induced by medial preoptic stimulation in female rats blocked with pentobarbital, morphine, chlorpromazine, or atropine.

Adult proestrous rats were subjected to either electrochemical or electrical stimulation of the medial preoptic area after ovulation-blocking dosage with either pentobarbital (PTBL), morphine, chlorpromazine, or atropine. For electrochemical stimulation (ECS), 233 microA anodal DC for 30 sec (7000 mu coulombs) was delivered through a unipolar stainless steel electrode. For electrical stimulation (ES), 750 microA biphasic pulse pairs at 30 Hz, on and off each 15 sec, were delivered through a coaxial platinum electrode, in four 5-min bursts equally spaced during 60 min. In PTBL-blocked rats this stimulus produced LH surges equivalent to those after continuous stimulation for 60 min. Blood was collected 60 and 90 min after ECS or after the start of ES. Mean serum LH concentrations (RIA) maximal at 90 min after ECS, were similar under the four blocking agents (1270-1486 ng/ml serum in terms of NIAMDD LH RP-1). Like-wise, after ES there were no significant differences (P greater than 0.05) among the mean LH levels at 60 min (310-571 ng/ml serum). The 90-min values showed a downward, but not significant, trend in the case of PTBL, chlorpromazine, and atropine plus PTBL. Under morphine an apparent upward trend was due largely to an animal having an especially large increase. With rare exception, full ovulation was evident at terminal laparotomy on the morning after stimulation. Thus, whichever of the several drugs was used, medical preoptic area stimulation with given parameters induced section of comparable amounts of LH. None of the drugs appears to have primary suppressive action on the preoptic-tuberal system, on the availability of LHRH or its release into the portal vessels, on the ability of the pituitary to release LH, or on the ovarian response.