Electrophysiological evidences for possible participation of periventricular neurons in anterior pituitary regulation.

Unit activity of neurons in the periventricular area of the third ventricle (PVA), in which the organum vasculosum laminae terminalis is included, was recorded in female rats in proestrus. The units were antidromically driven by electrical stimulation of the arcuate-median eminence region (ARC-ME). In the antidromic responses, a notch was generally observed in the rising phase of the driven wave, and fractionation of A- and B-components at the notch was readily elicited by applying repetitive stimulatory pulses at frequencies higher than 10 Hz, or successive double pulses with intervals less than 3.5 msec. At the same time, ARC-ME efferents to the PVA were suggested by orthodromic responses in the PVA to ARC-ME stimulation. Occasionally, anti- and orthodromic responses appeared in one electrode, indicating a proximate distribution of these two types of neurons in the PVA. Repetitive stimulation of the ARC-ME at 50 Hz facilitated the orthodromically driven units, whereas antidromically driven units were inhibited. This seems to imply that the orthodromic responses might be recorded from an inhibitory interneuron in the ARC-ME efferent pathway to the PVA, judging from the identical time course of the responses of the two types of the units. Both the anti- and orthodromically driven units were tested with microiontophoresis of LH-RH, TRH, LH, FSH and prolactin. Orthodromically driven units showed no response to microiontophoresis of any hormone. In the antidromically driven units, microiontophoresis of LH-RH and FSH elicited inhibition in 15 and 18% of the tests, and facilitation was seen in 15-25% of the tests in response to LH, TRH and prolactin. The demonstration that local application of hypothalamic and pituitary hormones exerts direct effects on the activity of the PVA neurons which send their axons directly to the ARC-ME, provides additional evidence that the PVA may participate in anterior pituitary regulation.