Dual label in situ hybridization studies provide evidence that luteinizing hormone-releasing hormone neurons do not synthesize messenger ribonucleic acid for mu, kappa, or delta opiate receptors

Abundant evidence suggests that opiatergic neurons play an important intermediary role in the regulation of LHRH release by ovarian steroids; however, it is unclear whether opiates communicate directly or indirectly with LHRH neurons. To investigate this issue, we used dual label in situ hybridization histochemistry to determine whether LHRH neurons synthesize messenger RNA (mRNA) for mu, kappa, and/or delta opiate receptors. For these studies, we examined both intact (n = 3) and ovariectomized, steroid-treated rats. Ten of the ovariectomized rats were implanted 1 week later (day 0) with SILASTIC brand (Dow Coming) capsules of estradiol. On the morning of day 2, half of the estradiol-treated rats were injected with 5 mg progesterone. All animals were killed at approximately 1530 h on day 2. We found that cells expressing mu, kappa, and delta opiate receptor mRNAs were in all sections that also contained LHRH neurons. In every case, LHRH neurons were seen to be surrounded by or in close proximity to cells containing mu, kappa, or delta mRNAs. However, regardless of steroid treatment, we found no neurons containing both LHRH mRNA and mRNAs encoding any of the three receptor subtypes. These results support the hypothesis that LHRH neurons are regulated indirectly by opiatergic neurons.