Responses of hypothalamic neurons to the microiontophoresis of LH-RH, LH and FSH under various levels of circulating ovarian hormones.

Abstract

Iontophoretic application of LH-releasing hormone (LH-RH) elicited changes in the activity of 44 out of 74 MBH units. Each unit was recorded from the neurons which were confirmed to send their axons directly to the median eminence by antidromic activation. The responsiveness of the units to LH-RH fluctuated during the estrous cycle, with a larger percentage of responsive units on proestrus than on diestrus-1. Because estrogen treatment in ovariectomized rats increased the response of the units, this effect of LH-RH might be facilitated by estrogen. On the other hand, only 2 out of 45 medial preoptic (MPO) neurons, which send their axons to the MBH, showed a response to LH-RH. LH and FSH also caused changes in the unit activity in the MBH and MPO. These findings suggest that LH-RH itself may play an auto-regulatory role in the neural control mechanism of LH-RH secretion into the pituitary portal vessel. The responses of MBH and MPO neurons to LH and FSH seem to imply that these neurons may participate as sensitive components of the central nervous system in the feedback control mechanism of gonadotropins.