Functional Characteristics of the Luteinizing Hormone-Releasing Hormone Pulse Generator in Conscious Unrestrained Female Rabbits: Activation by Norepinephrine*

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Abstract
We probed the activity of the neural LHRH apparatus by means of push-pull cannulae implanted in the hypothalamus of conscious intact anestrous unrestrained New Zealand White female rabbits raised under natural light. The data revealed that the neuropeptide LHRH is released in a pulsatile manner (one pulse per 46.4 .+-. 3.9 min; amplitude, 1.63 .+-. 0.39 pg; n = 7) from a rather extensive area within the basal hypothalamus of the rabbit covering the rostral, medial, and posterior regions in a rostro-caudal direction and about 1 mm lateral. A remarkable change in mean LHRH output was observed during the year, and two distinct types of LHRH output were detected: low (0.72 .+-. 0.07 pg/10 min; n = 7) when these reflex ovulators were perfused during late spring to early summer and high (2.92 .+-. 0.29 pg/10 min; n = 5) when perfused during late summer, early fall to spring months. This 4-fold annual change in LHRH output was mainly due to changes in the amplitude of the LHRH signal and was not correlated with the position of the push-pull cannula within a restricted area in the hypothalamus. These changes may be related to the state of the ovaries, since rabbits with high LHRH output showed larger ovaries with well developed follicles than rabbits with low LHRH output. Norepinephrine, a hypothalamic neurotransmitter known to be involved in the mounting-evoked ovulation reflex in this induced ovulator, elicited a 2-fold greater mean LHRH release (P < 0.05) when infused in doses of 10-7-10-6 M directly into the hypothalamic perfusion area. The data indicate that the activity of the LHRH pulse generator in this species presents remarkable annual variations in its mean LHRH output mainly due to a greater amplitude of the LHRH signal and that norepinephrine is a potent stimulator of LHRH release in conscious unrestrained female rabbits.