Interactions between neuropeptide Y and adrenergic systems in the stimulation of luteinizing hormone release in steroid-primed ovariectomized rats.

Abstract

Intraventricular injection of neuropeptide Y (NPY) stimulates LH release in estradiol benzoate- and progesterone-primed (EBP) ovariectomized rats. Because adrenergic neurotransmitters, norepinephrine (NE) and epinephrine (E), show intraneuronal coexistence with NPY in certain brain regions of the rat and there are similarities in the effects of NPY and NE/E on LH release, we investigated the possible interaction of NPY and adrenergic receptor systems in the stimulation of LH release in EBP-treated ovariectomized rats. The experiments were designed to determine whether NPY exerted its effects via adrenergic receptors and whether combined administrations of NPY and NE can act synergistically or in an additive manner to enhance the LH release response. Permanent stainless steel cannulae were placed in the third ventricle of the brain, and the rats were ovariectomized. Two weeks after surgery, rats were injected with EB (30 micrograms/rat) and P (15 mg/rat). Two days later, the effects of either vehicle alone (control) or various adrenergic and dopaminergic receptor antagonists and an opiate receptor agonist on stimulation of LH release by NPY were assessed. Intraventricular injection of 0.47 nmol NPY increased plasma LH levels at 10, 20, and 30 min in control rats. The NPY-induced LH response was not blocked by pretreatment with any of the following drugs: the alpha-adrenoreceptor antagonist phenoxybenzamine, the alpha 1-adrenoreceptor antagonist prazosin, beta-adrenoreceptor antagonist propranolol, the dopamine receptor antagonist pimozide, or the opiate receptor agonist morphine. All of these drugs affect LH release in other circumstances. On the other hand, the alpha 2-adrenoreceptor antagonist yohimbine significantly attenuated the NPY-induced LH increments. In the second study dealing with the possible synergistic or additive interactions between NPY and NE, we observed that when doses of NPY and NE that separately were only minimally effective in stimulating LH release were administered together, the amounts of LH secreted were greater than the sum of the individual responses. However, when NE and NPY were given together in doses that alone had either no stimulatory or maximal stimulatory effects, there were no additive or synergistic effects on LH release.(ABSTRACT TRUNCATED AT 400 WORDS)