Activation of Muscarinic and Serotonergic Receptors Results in Phosphoinositide Hydrolysis but Not in Mobilization of Calcium in Sympathetic Neurons

Abstract

The effects of various neurotransmitters on phosphoinositide hydrolysis, mobilization of Ca²⁺ and release of [³H]-norepinephrine ([³H]-NE) were studied in cultures of sympathetic neurons of chick embryos. [³H]-inositol-1,4,5-triphosphate ([³H]-IP₃) was increased in sympathetic neurons by acetylcholine (ACh), muscarine and serotonin (5-HT). Dopamine and norepinephrine did not stimulate phosphoinositide hydrolysis. Intracellular concentration of free Ca²⁺ ([Ca²⁺]_i) was measured in Indo-1-loaded sympathetic neurons at rest and after addition of test agents. Measurements were made in the cell body and growth cone regions since Ca²⁺ mobilization is known to be different in different regions of the sympathetic neurons. ACh (nicotinic component was blocked by hexamethonium) and 5-HT failed to increase the [Ca²⁺]_i, in the cell body as well as in the growth cone. The spontaneous release of [³H]-NE was not affected by ACh and 5-HT. Caffeine increased the [Ca²⁺]_i only in the cell body but not in the growth cone and had no effect on the release of [³H]-NE. These results suggest that an IP3-insensitive but caffeine-sensitive pool of Ca²⁺ is present only in the somatic region of sympathetic neurons and is not coupled to the transmitter release.