| Abstract: | Since the initial observations in the 1950s a large number of neurotransmitters and hormones have been shown to influence phosphatidylinositol (PI) metabolism in brain and peripheral ganglia (see ref. 3 for review). This has led to the suggestion that PI is part of an intracellular second messenger system for some types of diffusible chemical factors. Consistent with this are recent reports that one of the products of PI turnover (diacylglycerol) stimulates the Ca-dependent phospholipid-dependent protein kinase (kinase C) while a second (inositol trisphosphate) causes the release of calcium from intracellular stores. Thus it is possible that at least some brain neurotransmitters utilize the PI system to produce functional effects that are in addition to and which outlast the very brief physiological responses they elicit. Although it had been anticipated that another class of receptors might inhibit receptor-mediated stimulation of PI breakdown, no clear examples of such effects have been described. We now report that acidic amino acids, which are that acidic amino acids, which are thought to be excitatory neurotransmitters at the majority of brain synapses, strongly inhibit the stimulation of PI metabolism elicited by carbachol, histamine, or by potassium-induced depolarization, without changing the response to noradrenaline. As well as indicating a novel function for the excitatory amino acids, these results suggest that the central nervous system possesses cell-cell interactions of a previously unsuspected type. |
