The muscarinic stimulation of phospholipid labeling in hippocampus is independent of its cholinergic input.

The enhanced incorporation of 32Pi into two minor acidic phospholipids, phosphatidate and phosphatidylinositol, is a response characteristic of a number of effectors, including neurotransmitters and hormones, with their receptors on the cell membrane. In the nervous system, this is perhaps best documented in the case of acetylcholine action on brain synaptosomes, slices or on sympathetic ganglia (for review, see ref. 14). This 'phospholipid labeling effect' (PLE) appears to require tissue structural integrity since it is not observed in cell-free preparations. The PLE elicited by muscarinic agonists in nerve ending fractions 19 might be considered an exception. However, isolated synaptosomes retain considerable internal structure and metabolic viability, and as such may be considered as resealed anucleate neurons. Neurotransmitter receptors have traditionally been considered to be localized on the postsynaptic membrane, and a number of indirect approaches suggest that the neurotransmitter-directed PLE in pineal gland 21 and superior cervical ganglion 11 is associated with this site. In the case of synaptosomes, this conclusion is not readily made. For example, the assumption that added azPi must first be converted to y-32P-ATP before phospholipids can be labeled suggests that intrasynaptosomal oxidative phosphorylation is an obligate step in the synaptosomal PLE (see ref. 19), an argument that would appear to favor a presynaptic localization. Furthermore, postsynaptic membranes are not prominent in the 'light' synaptosomal fraction shown to mediate the PLE in guinea pig brain fractions 2°. The case for a presynaptic site is however by no means conclusive. In a recent study, we concluded that calcium mobilization accompanies the muscarinic PLE (see ref. 5) in synaptosomes. The increase in calcium availability that parallels the PLE is at variance with the probable physiological role of presynaptic muscarinic receptors in cerebral cortex in reducing neurotransmitter release 1~,z2. by appearing to limit calcium availability 13. That is, we would have anticipated that ac-