Insulin synthesis by isolated rabbit neurons.

Insulin has been identified in the central nervous system of a number of vertebrate species, but the site of synthesis as yet remains unresolved. Two previous studies reported the presence of insulin mRNA in neural tissue, but related efforts to confirm and better localize the cellular origin of the hormone have yielded equivocal results. In the present study we have attempted to clarify this issue by employing both immunocytochemistry and in situ hybridization on isolated enriched cultures of rabbit brain neurons and glia. Our data show that a subset (3-5%) of neurons is positively immunoreactive for insulin, but all of the glial cells are negative. The level of staining intensity can be increased by preincubating the neurons with monensin (a Na+ ionophore that prevents cell secretory activity), but not the fraction of positive cells. Similarly, in situ hybridization reveals the presence of mRNA in 3-5% of neurons, but no such signal is detected in glia. Thus, our data not only confirm previous reports of insulin in the central nervous system, but, more importantly, indicate that the synthesis of the hormone is local and apparently confined to a subset of neurons.