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CAROL A. DUDLEY and R. L. MOSS

Department of Physiology, University of Texas Health Science Center at Dallas, Southwestern Medical School, Dallas, Texas 75235, U.S.A.

(Received 11 August 1976)

Luteinizing hormone releasing hormone (LH-RH) has been implicated in the mediation of sexual receptivity in the female rat (Moss & McCann, 1973, 1975; Pfaff, 1973). Furthermore, prostaglandin E2 (PGE2) has been found to release LH-RH (Ojeda, Wheaton & McCann, 1975). Thus, the possibility that PGE2 might potentiate lordosis behaviour in a similar way to LH-RH has been tested.

Forty-seven ovariectomized Wistar rats were injected s.c. with 0·25 mg oestrone in oil (Theelin, 1 mg/ml) and 45 h later, some received a second s.c. injection of 50 μg PGE2/0·5 ml (n = 10), 50 μg PGE1/0–5 ml (n = 7), or 500 ng LH-RH/0–5 ml (n = 10). Of the remaining 20 animals, ten were injected with 2·5 mg progesterone in oil (Lipo-Lutin; 50 mg/ml) 42 h after oestrogen

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I. URBAN, R. L. MOSS, and B. A. CROSS

SUMMARY

Recent investigations in the rabbit have indicated the presence of well-defined afferent nervous pathways for release of neurohypophysial oxytocin. To study these pathways for later use in hypothalamic unit recording, intramammary pressure responses (milk ejection) and arterial pressure were recorded in 45 lactating rabbits with post-pontine brainstem transection. Systematic exploration of the mid-brain tegmentum and ventromedial forebrain areas was carried out with a bipolar stimulating electrode. Release of oxytocin, shown by milk-ejection responses similar to those evoked by a 1–2 mu. synthetic oxytocin occurring in the absence of detectable pressor effects, was recorded in less than half the animals. The location of positive stimulation sites was very variable between animals and reproducibility of responses from the same site was poor. It is concluded that present evidence does not support a discrete afferent path in the mid-brain for the milk-ejection reflex and a possible explanation for the inconstant results of stimulation experiments is suggested.