Insulin Synthesis by Isolated Rabbit Neurons*

Abstract

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.

Insulin in the Brain: Sources, Localization and Functions

“…Moreover, some studies tried to further localize the cellular origin of insulin in the CNS and showed that the synthesis of insulin is confined to a subset of neurons and not glia [55]. In another study conducted in primary neuronal cell culture, Clarke et al have shown that depolarization of neuronal cells from rat brain by potassium ions in the presence of calcium or by veratridine caused more than threefold stimulation of release of immunoreactive insulin [56].…”

Knock-in of diphteria toxin A chain gene at Ins2 locus: effects on islet development and localization of Ins2 expression in the brain

“…In invertebrates which lack an endocrine pancreas, insulin-related molecules have been described mainly in the central nervous system (Smit et al, 1988;Kromer-Metzger et al, 1994;Pimentel et al, 1996;Wolkow et al, 2000). In vertebrates, insulin or insulin-like transcripts have been found for instance in rat periventricular hypothalamic cells, in rabbit or rat neuron cell cultures and in an human neuroectodermal tumor (Young, 1986;Schechter et al, 1988;Nakamura et al, 2001).…”

In situ autoradiography and ligand-dependent tyrosine kinase activity reveal insulin receptors and insulin-like growth factor I receptors in prepancreatic chicken embryos.

“…(17). Neurons have been reported to synthesize insulin in neonatal rabbit brain (24). IGF-I mRNA was detected in whole rat brain at embryonic day 14 (25) and found in both neuronal and glial cultures of rat fetuses (25,26), although it was not detected in human fetal cerebral cortex (27).…”