GABA activity mediating cytosolic Ca2+ rises in developing neurons is modulated by cAMP-dependent signal transduction.

In the majority of developing neurons, GABA can exert depolarizing actions, thereby raising neuronal Ca2+. Ca2+ elevations can have broad consequences during development, inducing gene expression, altering neurite outgrowth and growth cone turning, activating enzyme pathways, and influencing neuronal survival. We used fura-2 and fluo-3 Ca2+ digital imaging to assess the effects of inhibiting or activating the cAMP signal transduction pathway on GABA activity mediating Ca2+ rises during the early stages of in vitro hypothalamic neural development. Our experiments stemmed from the finding that stimulation of transmitter receptors shown to either activate or inhibit adenylyl cyclase activity caused a rapid decrease in Ca2+ rises mediated by synaptically released GABA. Both the adenylyl cyclase activator forskolin and the inhibitor SQ-22,536 reduced the Ca2+ rise elicited by the synaptic release of GABA. Bath application of the membrane-permeable cAMP analogs 8-bromo-cAMP (8-Br-cAMP) or 8-(4-chlorophenylthio)-cAMP (0.2-5 mM) produced a rapid, reversible, dose-dependent inhibition of Ca2+ rises triggered by synaptic GABA release. Potentiation of GABAergic activity mediating Ca2+ rises was observed in some neurons at relatively low concentrations of the membrane-permeable cAMP analogs (20-50 microM). In the presence of tetrodotoxin (TTX), postsynaptic Ca2+ rises triggered by the bath application of GABA were only moderately depressed (13%) by 8-Br-cAMP (1 mM), suggesting that the inhibitory effects of 8-Br-cAMP were largely the result of a presynaptic mechanism. The protein kinase A (PKA) inhibitors H89 and Rp-3', 5'-cyclic monophosphothioate triethylamine also caused a large reduction (>70%) in Ca2+ rises triggered by synaptic GABA release. Unlike the short-term depression elicited by activation of the cAMP signal transduction pathway, Ca2+ depression elicited by PKA inhibition persisted for an extended period (>30 min) after PKA inhibitor washout. Postsynaptic depression of GABA-evoked Ca2+ rises triggered by H89 (in the presence of TTX) recovered rapidly, suggesting that the extended depression observed during synaptic GABA release was largely through a presynaptic mechanism. Long-term Ca2+ modulation by cAMP-regulating hypothalamic peptides may be mediated through a parallel mechanism. Together, these results suggest that GABAergic activity mediating Ca2+ rises is dependent on ongoing PKA activity that is maintained within a narrow zone for GABA to elicit a maximal Ca2+ elevation. Thus, neuromodulator-mediated changes in the cAMP-dependent signal transduction pathway (activation or inhibition) could lead to a substantial decrease in GABA-mediated Ca2+ rises during early development.