Catecholamine involvement in episodic luteinizing hormone release in adult ovariectomized rats

This study was intended to examine the role of hypothalamic norepinephrine (HNE) and dopamine (HDA) in episodic luteinizing hormone (LH) release in adult ovariectomized rats. Unrestrained, unanesthetized rats with indwelling right atrial cannulae were bled continuously (30, 50, or 100 mul of whole blood/4-6 min for 3-4 hours), and the blood samples were analyzed for LH by radio-immunoassay. In other individual rats, changes in the hypothalamic levels of norepinephrine and dopamine after drug administration were determined by a radioisotopic-enzymatic catechol-O-methyl transferase assay. alpha-Methyl-p-tyrosine significantly decreased HNE and HDA concentrations but failed to alter episodic LH release. Two dopamine-beta-hydroxylase inhibitors (U-14,624 and FLA-63) caused marked reductions in HNE, small but not statistically significant increases in HDA, and an inhibition of episodic LH secretion. Apomorphine, a dopamine receptor stimulator, caused a transient (50-60 min) but marked inhibition of episodic LH release. Saline injection had no effect. Pimozide, a blocker of dopamine receptors, prevented the inhibitory effects seen following apomorphine. Although not studied in detail, pimozide alone did not appear to alter episodic LH secretion. These data suggest that in adult ovariectomized rats norepinephrine may be an excitatory neurotransmitter in the modulation of episodic LH release. The activation of dopamine receptors may be capable of inhibiting this release process. However, the apparent inability of pimozide alone to alter episodic LH discharge suggests that under physiological conditions dopamine may not play a role in the modulation of episodic LH secretion.