SYNAPTIC-TYPE ACETYLCHOLINE-RECEPTORS RAISE INTRACELLULAR CALCIUM LEVELS IN NEURONS BY 2 MECHANISMS

Nicotinic acetylcholine receptors (AChRs) located in the postsynaptic membrane on neurons are responsible for mediating fast, excitatory syn aptic transmission. If synaptic AChRs are also highly permeable to cal cium as reported recently for several kinds of neuronal AChRs, the syn aptic receptors could regulate calcium-dependent events in the neurons in concert with normal transmission. Chick ciliary ganglion neurons h ave two classes of AChRs, one located predominantly in the synaptic me mbrane and responsible for synaptic signaling through the ganglion and the other located almost exclusively in nonsynaptic membrane and havi ng no known function. The nonsynaptic receptors can readily elevate in tracellular calcium concentrations. The experiments reported here indi cate that synaptic-type receptors can raise intracellular calcium leve ls to the same extent as the nonsynaptic receptors and that they do so not only by being permeable to calcium themselves but also by activat ing voltage-dependent calcium channels (VDCCs). Currents of equivalent amplitude are obtained through the synaptic-type receptors when neuro ns are bathed in solutions containing either sodium or calcium as the sole extracellular cation. Measuring the effect of ion substitutions o n the reversal potential of the receptors and applying the Goldman-Hod gkin-Katz constant field equation indicates the receptors are at least as permeable to calcium as to sodium. When neurons are loaded with th e calcium-sensitive dye fluo-3 and challenged with nicotine, both the synaptic-type and nonsynaptic AChRs substantially elevate intracellula r calcium levels under physiological conditions, and do so largely by activating VDCCs. Confirmation that synaptic-type AChRs can elevate in tracellular calcium levels in the absence of contributions from VDCCs was obtained from voltage-clamp experiments on neurons loaded with flu o-3. The fluorescence signals indicate that the nicotine-induced calci um increases in neurons voltage clamped at rest are nearly as great as those induced in the same neurons when VDCCs are maximally activated by a voltage step. Calcium flux through AChRs may be particularly impo rtant for mediating local changes in calcium concentrations near the p lasma membrane, which, in turn, could regulate specific membrane-assoc iated calcium-dependent events.