Changes in the binding characteristics of the μ, δ and κ subtypes of the opioid receptor in the hypothalamus of the normal cyclic ewe and in the ovariectomised ewe following treatment with ovarian steroids
The μ, δ and κ opioid receptor subtypes were measured across the oestrous cycle of the ewe and in ovariectomised (OVX) ewes treated with oestrogen and/or progesterone. We have used a subtype-specific opioid receptor binding assay, in which [3H]diprenorphine non-preferentially labelled each receptor subtype in the presence of blocking concentrations of site-specific opioid analogues. The density and affinity of each receptor subtype was measured in the preoptic area (POA) of the hypothalamus and the mediobasal hypothalamus (MBH). Normally cycling ewes were killed during the luteal phase of the oestrous cycle and at various times after an injection of a synthetic prostaglandin (cloprostenol) to synchronise the onset of the follicular phase. OVX ewes were either untreated as controls (n=4) or treated with oestrogen (n=4), progesterone (n=4) or oestrogen and progesterone combined (n=4). The total number of opioid receptors did not alter across the oestrous cycle or with steroid hormone treatment. In the POA, the mean (± s.e.m.) number of δ receptors was significantly (P<0·05) greater during the luteal phase than 24 h into the follicular phase (133 ± 45 vs 35 ± 8 fmol/mg protein). A significantly (P<0·05) greater number of δ receptors was also found in the OVX progesterone-treated ewes compared with the control animals (172 ± 9 vs 39 ± 4 fmol/mg protein). In the MBH, the number of δ receptors was significantly (P<0·01) greater in ewes killed 56 h after prostaglandin than luteal-phase ewes (184 ± 40 vs 51 ± 7 fmol/mg protein). The number of μ receptors in both the POA and the MBH was also significantly (P<0·05) higher in the 56-h group than in the 12-h group. A similar trend was also observed in the steroid-treated animals, although differences did not reach statistical significance. The δ:μ ratio in the POA was significantly (P<0·05) higher in the luteal-phase animals than any of the other groups killed after a cloprostenol injection that causes luteolysis. Similarly the ratio of δ receptor density to μ receptor density was greater (P<0·05) in the OVX progesterone-treated ewes than in the OVX control ewes. No differences were found in the κ receptor density across the cycle or with different steroid treatments. These data suggest that the relative proportions of the δ and μ subtypes of the opioid receptor in the hypothalamus change during the oestrous cycle. Regulation appears to be due to the feedback effects of ovarian steroids with progesterone altering the δ:μ ratio. In the MBH, there was a general increase in both δ and μ subtypes during the follicular phase of the oestrous cycle. This may explain, in part, how the responsiveness of the GnRH/LH axis to opioid peptides and antagonists changes across the cycle.