The effect of pentobarbital on the electrical activity of LHRH pulse generator in the ovariectomized rat with or without estrogen priming.

Effects of pentobarbital sodium (PB) on the electrical activity of LHRH pulse generator were investigated in the ovariectomized (OVX) rat fitted with chronically implanted electrode arrays in the arcuate-median eminence region of the hypothalamus. PB was injected intraperitoneally (i.p.) at a dose of 32 mg/kg bw, which has blocked the surge of LH secretion when injected at 13.45 on the day of proestrus, presumably by blocking the activity of the LHRH surge generator. After i.p. injection of PB, the first characteristic increase (volley) in the hypothalamic multiunit activity (MUA), each of which was associated with the initiation of an LH pulse, appeared at the mean interval of 42 min, which was significantly greater than the value, 30 min, in the rat given i.p. injection of saline. The second and third MUA volleys appeared, however, at similar intervals to those in rats given saline. There were no significant changes in the amplitude of LH pulses which followed. In the OVX rat implanted with a silastic tube packed with estradiol-benzoate (E2) for 1 day, MUA volleys were unclear, mingled with high background activities, but clear volleys appeared after PB injection. The mean interval of LH pulses in the rat given i.p. injection of saline was 51 min, by 24 min longer than the value in the E2-unimplanted OVX rat. But, after PB injection, the 1st LH pulse appeared at the interval of 37 min, which was by 25 min smaller than that after saline and the overall mean of 3 intervals was 34 min, showing a significant shortage. No PB effect was seen in the LH pulse amplitude in the E2-implanted OVX rat. The presults show that the LHRH pulse generator is much more resistant to PB than the surge generator in the OVX rat either with or without estrogen priming, and in the rat with estrogen priming, the pulse generator is rather activated by PB.