Median Eminence Dopaminergic Activation Is Critical for the Early Long-Day Inhibition of Luteinizing Hormone Secretion in the Ewe

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Abstract
In ewes, photoperiod modulates LH release. The median eminence (ME) dopaminergic activity seems to be implicated in the inhibition of LH secretion by photoperiod. This study investigated the functional importance of ME dopaminergic activity for LH secretion inhibition in three inhibitory photoperiodic treatments: after 33 long days (LD) (LD1 treatment), after 72 LD (LD2 treatment), and after 34 short days. Using reverse microdialysis on three groups of seven ewes, a solution of α-methyl-paratyrosine [αMPT, an inhibitor of tyrosine hydroxylase (TH); 10 mm in Ringer’s lactate] was infused into the ME for 5 h, preceded by a 5-h control period during which only vehicle was infused, in each of the three photoperiodic treatments. αMPT dramatically decreased the 3,4-dihydroxyphenylacetic acid concentration, similarly in all three photoperiodic treatments, suggesting a similar inhibition of TH activity. In the LD1 treatment, αMPT significantly increased LH pulse frequency (+1.22 ± 0.46 pulse/5 h from control period, mean± sem, n = 9; P < 0.05) and mean concentration (+51 ± 28%; P < 0.001). In the other two photoperiodic treatments, αMPT had no significant effect on LH release. Thus, blockade of dopamine synthesis in the ME seems to stimulate LH secretion in early, but not long-term, inhibition by LD nor after the transition to short days. Therefore, dopaminergic activity of the ME seems to be critical for LH secretion inhibition in some photoperiodic inhibitory treatments but not in others.

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