DIFFERENCES IN KETANSERIN BINDING IN THE VENTROMEDIAL HYPOTHALAMUS OFEWES RESPONSIVE OR REFRACTORY TO SHORT DAYS

Participation of central 5HT receptors in the inhibition of LH pulsati lity during refractoriness to short days (SD) in ewes has been suggest ed by previous in vivo studies using various SHT-antagonist such as ke tanserin. In the present study, binding of [H-3]ketanserin in ewe brai n sections was similar to that described in the brain of other species and could correspond with an interaction at 5HT(2) receptors sites. R osenthal analysis from the caudate nucleus was linear (Kd = 3 nM). The displacement studies from the cortex slices showed that the 5HT antag onists such as methysergide, ketanserin, cyproheptadine and spiperone competed with the labelled ligand at nanomolar concentrations whereas serotonin was less active. However, the first 3 drugs recognized diffe rent populations of binding sites. Prazosin, an alpha(1)-adrenergic an tagonist was inactive, but a slight inhibition of [H-3]ketanserin bind ing was induced by pyrilamine, an H-1 histaminic antagonist, within a nanomolar range. Methysergide (10(-6) M), which does not bind to H-1 r eceptors, was therefore used to determine the nonspecific binding. Qua ntitative analysis of the binding of 3 nM [H-3]ketanserin on sections of the ewe brain at the preopticohypothalamic level was then carried o ut by autoradiography. The highest binding densities were observed in the caudate nuclei (64.0 fmol/mg tissue Eq) and the mammillary bodies (52.7 fmol/mg tissue Eq) whereas intermediate or low densities were fo und in the other structures. The anatomical distribution of the labell ing was similar to that described in other species for 5HT(2) receptor s. Ketanserin binding in these areas was compared between two groups o f ovariectomized estradiol-treated Ile-de-France ewes, submitted to ar tificial short days (SD: 8L:16D), one group with a high LH pulsatility (responsive to SD) and the other one with a low LH pulsatility (photo refractory to SD). Binding densities were similar for each one of the studied regions between the two groups, except in the ventrolateral pa rt of the mediobasal hypothalamus, where ewes exhibiting high LH pulsa tility had a more than 2-fold higher binding density than those with a low LH pulsatility (mean +/- SEM, 14.6 +/- 1.4 vs. 5.7 +/- 1.0 fmol/m g tissue Eq, respectively; p < 0.0016). These results suggest that [H- 3]ketanserin binding sites in the ventromedial part of the mediobasal hypothalamus could be associated to the regulation of the photoperiodi c inhibition of LH at the time of establishment of refractoriness to s hort days in the Ile-de-France ewe.