Effect of dopamine receptor blockade or norepinephrine synthesis inhibition on acute, ovariectomy-induced increases in pulsatile luteinizing hormone release in the rat

The initial aim of the present studies was to examine the influence of blockade of dopamine (DA) receptors with pimozide or inhibition of norepinephrine (NE) synthesis with U-14,624 on acute, ovariectomy (OVX)-induced changes in pulsatile LH release. Either treatment instituted at the time of OVX suppressed or inhibited the rapid increase in LH pulse amplitude and frequency normally occurring within 24 hr following ovarian removal on diestrus 1. While administration of pimozide at either 24 hr or 48 hr following OVX suppressed pulsatile LH release by selectively reducing LH pulse frequency, by 8 days following OVX pimozide failed to exert any effect on LH pulse frequency and therefore on pulsatile LH secretion. To determine if there was a transient critical period following OVX of at least 2 days but less than 8 when endogenous DA was excitatory to pulsatile LH release, piribedil (a DA receptor agonist) was given 24 hr following OVX. Rather than increase LH secretion, piribedil markedly suppressed pulsatile LH release indicating that DA does not stimulate LH secretion in acutely ovariectomized rats. These experiments indicate that

1. (1) NE is involved in stimulating the acute, OVX-induced increase that occurs in pulsatile LH release;

2. (2) DA receptorblockade by pimozide has a differential effect on pulsatile LH secretion which depends on the time following OVX when the compound is administered;

3. (3) this differential effect cannot be explained by a transient critical period of a few days duration following OVX during which DA is excitatory to pulsatile LH release.

Keywords: Pulsatile; Luteinizing hormone; Ovariectomy; Dopamine; Norepinephrine; Pimozide