Neurite outgrowth in individual neurons of a neuronal population is differentially regulated by calcium and cyclic AMP.

In identified Helisoma neurons, intracellular calcium can regulate neurite elongation and growth cone motility. Neurotransmitters such as 5-HT suppress both neurite elongation and the filopodial and lamellipodial movements of growth cones by causing increases in intracellular calcium (Haydon et al., 1984; Cohan et al., 1987; Mattson and Kater, 1987). Since an additional second messenger, cyclic AMP (cAMP), is known to mediate many physiological effects of neurotransmitters, we tested (1) the possible involvement of cAMP in the regulation of neurite outgrowth from Helisoma buccal neurons and (2) calcium-cAMP interrelationships in the regulation of outgrowth. The cAMP-elevating agents forskolin (5 x 10(-6)-10(-4) M) and dibutyryl cAMP (dbcAMP; 5 x 10(-3)-10(-2) M) suppressed neurite elongation and growth cone movements in identified neurons B19 (5-HT sensitive) and B5 (5-HT insensitive); the suppression was reversible. Exposure of these particular identified neurons to the calcium channel blocker La3+ (10(-5) M) or a culture medium with reduced calcium prevented and reversed the suppressive effects of forskolin and dbcAMP. In order to determine if the results on neurons B5 and B19 were representative of all neurons or only a subset, we examined a larger population of neurons. Calcium ionophore A23187 suppressed outgrowth from all neurons in mass dissociate cultures of buccal neurons, while forskolin or dbcAMP plus IBMX suppressed outgrowth from only one-half of buccal neurons. Finally, we found that 2 subpopulations exist among the neurons whose outgrowth is suppressed by cAMP: One subpopulation requires calcium influx for cAMP to act, while the other does not. Thus, even within the relatively small population of neuronal types comprising the buccal ganglion of Helisoma, second messengers within different neurons can act and interact in different ways to regulate outgrowth.