INSULIN STIMULATES MEMBRANE PHOSPHOLIPID-METABOLISM BY ENHANCING ENDOGENOUS ALPHA-1-ADRENERGIC ACTIVITY IN THE RAT HIPPOCAMPUS

Insulin stimulates membrane phospholipid metabolism and activates protein kinase C (PKC) in its peripheral target tissues. Additionally, insulin can stimulate PKC activity in cultured fetal chick neurons. In the present study, we tested whether insulin can stimulate membrane phospholipid metabolism in the rat hippocampus, a CNS region in which insulin has been reported to stimulate the phosphorylation of a PKC substrate protein and to suppress spontaneous electrical activity of pyramidal cells. Concentrations of 1, 10 and 100 nM insulin significantly stimulated the accumulation of [H-3]inositol phosphate ([H-3]IP1) and [H-3]IP2 in hippocampal slices labelled with [H-3]myoinositol. Significant (P < 0.05) increases of hippocampal diacylglycerol (a product of phosphoinositol hydrolysis) content were observed at 1, 5 or 10 min of incubation with 50 or 100 nM insulin. Addition of tetrodotoxin resulted in a suppression of insulin stimulation of [H-3]IP1 release, suggesting that insulin effects may be indirect and mediated via release of an endogenous neuronal transmitter within the hippocampus. Norepinephrine has been shown to both stimulate PI turnover and suppress the spontaneous electrical activity of pyramidal cells via alpha-1-adrenergic receptors. Therefore, we tested whether the effects of insulin were mediated by norepinephrine. We measured [H-3]IP1 release in the presence or absence of the alpha-1-adrenergic antagonists prazobind and prazosin. These compounds blocked insulin stimulation of IP1 accumulation, suggesting that the action of insulin to stimulate PI turnover is secondary to enhancement of endogenous noradrenergic activity within the hippocampus.