Muscarinic and quisqualate receptor-induced phosphoinositide hydrolysis in primary cultures of striatal and hippocampal neurons. Evidence for differential mechanisms of activation.

Several neurotransmitters activate polyphosphoinositide (PPI) hydrolysis in CNS neurons as the first step of a transmembrane signalling cascade that may lead to neuronal circuit modulation. Muscarinic and quisqualate receptor-triggered PPI hydrolysis was investigated in neuronal primary cultures. A clear increase in inositol phosphates (Ins-Ps) was detected as early as 15 s after the agonist addition; at this time, the increases of inositol 1,4,5-trisphosphate (measured by HPLC) were relatively larger with respect to the other Ins-Ps. Ins-P accumulation was maintained in part in a Ca2+-free medium, excluding that Ca2+ entry is the fundamental step of the receptor-induced PPI hydrolysis. Acute cell pretreatment with phorbol dibutyrate, an activator of protein kinase C, was able to inhibit 50% of the response to carbachol, and almost completely the quisqualate effect, suggesting a negative feedback modulation by the enzyme. Finally, pertussis toxin failed to inhibit muscarinic responses, whereas it blocked greater than 70% of the quisqualate stimulation. The two receptors therefore appear coupled to phosphodiesterase by two different G proteins. The comparison of the results obtained by stimulating the two receptor systems suggests that the generation of the same intracellular signal at two distinct receptor types may occur by different coupling mechanisms, and be differently regulated even in the same neuronal preparations.