Ek. Polston et al., DIURNAL FLUCTUATIONS IN MATING-INDUCED OXYTOCINERGIC ACTIVITY WITHIN THE PARAVENTRICULAR AND SUPRAOPTIC NUCLEI DO NOT INFLUENCE PROLACTIN SECRETION, Endocrinology, 139(12), 1998, pp. 4849-4859
Previous studies have implicated oxytocin (OT) in the control of surge
-type PRL secretion in the pregnant and pseudopregnant rat. The presen
t studies examined the relationship between mating-induced activation
of OT neurons in the paraventricular (PVN), supraoptic (SON), and ante
rior commissural (ACN) nuclei and PRL secretion. Activity within OTerg
ic neurons, as measured by increased c-fos expression, was examined im
mediately and 5 days following mating in ovariectomized, estrogen-plus
-progesterone-treated rats at the time when nocturnal PRL surges are e
xpressed (0600 h) and at an intersurge time (2400 h). Females received
fifteen intromissions (15I), 15 mounts-without-intromission (MO), or
no stimulation (homecage, HC) from a sexually experienced male. Receip
t of 15I at 0600 h induced significantly higher numbers of OT immunore
active (OT-IR) cells and FOS/OT-IR double-labeled cells in the parvoce
llular division of the PVN (PVN) and in the SON than did 15I. at 2400
h. Numbers of OT-IR and FOS/OT-IR cells in the ACN and in the magnocel
lular compartment of the PVN (PVN) were not influenced by mating at ei
ther time. In contrast, acute PRL secretion induced within 5-30 min by
15I was not influenced by whether mating occurred at 1800 h (diurnal
surge), 2400 h, or 0600 h, nor were plasma OT levels elevated during t
he 1 h following 15I or MO at these times. Examination of FOS-IR cells
throughout the hypothalamus across the two times of day revealed prev
iously unreported differences between 15I and control MO treatments in
the PVN, SON, and the ventrolateral part of the arcuate nucleus (ARC(
v1)). On day 5 post mating, numbers of OT-IR and FOS/OT-IR cells in th
e PVN, SON, and ACN were very low and were similar between 0600 h and
2400 h and between females that showed (15I) or did not show (MO) mati
ng-induced PRL surges characteristic of pregnancy. The results of thes
e studies demonstrate that intromissive but not mounts-only stimulatio
n from males induces a rapid increase in OT-IR staining and OT neuron
activation in the PVN,, and the SON. These mating-induced responses in
OT neurons occurred within Ih after mating only at 0600 h, suggesting
a diurnal fluctuation in sensitivity to intromissive stimulation. Cha
nges in OTergic function were not seen in response to mating at other
times of day, nor at the time of the nocturnal PRL surge 5 days after
mating. We conclude that OT activity induced by mating does not act to
stimulate PRL secretion directly, but may be involved in the process(
es) by which genitosensory stimulation initiates surge-type PRL secret
ion.