Comparison of the effect of electrochemical stimulation of the medial preoptic area and the hypothalamic arcuate nucleus upon LH release in ovariectomized and proestrous rats.

The effect of electrochemical stimulation of the medial preoptic area (MPO) and the hypothalamic arcuate nucleus (ARC) on LH release was compared among long-term ovariectomized (OVX), ovariectomized rats treated with estradiol benzoate (OVX-E) and proestrous (PE) rats under pentobarital anesthesia. The MPO stimulation failed to facilitate LH release on OVX rats whereas the stimulation increased serum LH levels in both OVX-E and PE rats. The ARC stimulation resulted in an elevation of serum LH in OVX and PE rats. The time course of serum LH elevation after the electrochemical stimulation of the MPO or the ARC was different between OVX-E and PE rats. In PE rats serum LH concentrations began to rise 20-30 min after the stimulation of the MPO or the ARC with peak values obtained around 70-90 min. Serum LH started to elevate immediately with peak values in 10-20 min following the ARC stimulation in OVX rats and the MPO stimulation in OVX-E rats. The time courses for changes in serum LH concentrations following LH-RH administration were similar in OVX and PE rats. From these results, the lack of circadian rhythm in serum LH levels in long-term OVX rats may be due to the inability of the MPO to re- spond to the stimulus generated from circadian pacemaker(s). The cause of the delay observed between the stimulation of the MPO and LH rise in PE rats may not exist in the levels of neural transmission between the MPO and the ARC or the pituitary, but in the level of the mediobasal hypothalamus.