Multiple peptides infrequently coexist in progesterone receptor-containing neurons in the ventrolateral hypothalamic nucleus of the guinea-pig: an immunocytochemical triple-label analysis of somatostatin, neurotensin and substance P.

Progesterone plays an important role in regulating reproductive behaviour in guinea-pigs through actions exerted at the ventrolateral nucleus (VL), an area of the brain that contains progesterone receptors (PR) and neuroactive peptides, somatostatin (SOM), neurotensin (NT) and substance P (SP). Previous double-label analyses provided evidence that a substantial proportion of these neuropeptidergic cells contain PR. By means of triple-label immunofluorescence histochemistry, we examined whether PR are colocalized with two neuropeptides (SOM + NT or SP + SOM or SP + NT) within the same neurons in the VL. Ovariectomized guinea-pigs were primed with estradiol to induce PR immunoreactivity, and treated with colchicine to visualize immunoreactive (IR) neuropeptidergic cells. Both monoclonal mouse PR and polyclonal rabbit neuropeptide antibodies were used in double staining and in elution-restaining experiments. In the whole VL, the proportion of each coexisting peptide with PR obtained after double immunofluorescence appeared in decreasing order as: SOM (34%)>NT (25%)>SP (20%). Occasional colocalization was seen between PR and two neuropeptides throughout the rostrocaudal extent of the VL. Combining our various quantitative observations, we found that, of the total population of PR-IR neurons containing any combination of SOM, NT and SP, only about 1.5% contained SOM and NT, 2% contained SP and SOM and 1.6% contained SP and NT. These results indicate that while many PR-IR neurons also contain SOM or NT or SP in the guinea-pig VL, there may be very few PR-IR neurons that express more than one of these three peptides.