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Median Eminence Dopaminergic Activation Is Critical for the Early Long-Day Inhibition of Luteinizing Hormone Secretion in the Ewe¹

Institut National de la Recherche Agronomique, Unité de Recherche Associeé Centre National de la Recherche Scientifique 1291, Laboratoire de Neuroendocrinologie Sexuelle, 37380 Nouzilly, France

Address all correspondence and requests for reprints to: Dr. Benoît Malpaux, Institut National de la Recherche Agronomique, PRMD, Laboratoire de Neuroendocrinologie Sexuelle, 37380 Nouzilly, France. E-mail: malpaux{at}tours.inra.fr

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 -methyl-paratyrosine [MPT, 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. MPT 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, MPT 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, MPT 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.