Further evidence on dual effects of norepinephrine on LH secretion.

To investigate the effects of norepinephrine (NE) on LH secretion, when given into different hypothalamic and extrahypothalamic brain regions, we microinjected 1 microliter of NE in dosages of 10(-4) M concentrations, 10(-6), 10(-8) and 10(-10) M into nine different brain areas in ovariectomized miniature pigs. Microinjections into the dorsal hypothalamus (ADH) and dorsomedial nucleus (DM) had dose-dependent effects. The higher dose of NE (10(-6) M) inhibited and the lower dose of NE (10(-10) M) stimulated LH secretion, when NE was microinjected into the ADH (n = 4 - number of animals). Microinjections into the DM (n = 6) had the opposite effect. Here the lower dose had inhibitory and the higher dose stimulatory effects on plasma LH levels. Microinjections into the ventromedial nucleus of the hypothalamus (n = 4) had no significant effect. Microinjections into the stria terminalis (n = 3) and zona incerta (n = 4) were also ineffective. The effect of NE microinjections into the subthalamic nucleus (n = 4) resembled responses to microinjections into the ADH. NE effects in the amygdala were not dose dependent. Microinjections into the basolateral amygdala (n = 5) decreased and microinjections into the corticomedial part of the amygdala (n = 5) increased plasma LH levels in all four doses of NE. The effects of NE microinjected into the hippocampus (n = 3) were not clear cut. These results indicate a dualism of NE on LH secretion. Thus, the predominant notion that NE is largely or even exclusively stimulatory to LH release should be revised. According to the previous findings and the results of the present study it has been concluded that the effects of NE on LH secretion are not only dependent on the hormonal status of the animal, but also to the dose and site of action of NE.