Inositol Trisphosphate Releases Intracellularly Stored Calcium and Modulates Ion Channels in Molluscan Neurons

Stimulation of the bag cell neurons of Aplysia triggers a long-lasting afterdischarge in these cells. In vivo, such a discharge causes the onset of a sequence of reproductive behaviors. We have found that treatments that trigger discharges in vitro stimulate the hydrolysis of phosphoinositides in the bag cell neurons, as measured by increased incorporation of 3H-inositol into fractions containing membrane lipids and water-soluble inositol phosphates. The electrophysiological effects of inositol trisphosphate, one of the products of phosphoinositide turnover that has been shown to mobilize intracellular calcium in non-neuronal cells, were investigated using isolated bag cell neurons in cell culture. Microinjection of inositol trisphosphate into cultured bag cell neurons caused a transient hyperpolarization of the membrane (approximately 35 sec), together with an increase in conductance. This effect of inositol trisphosphate was abolished by 50 mM tetraethylammonium ions. Inositol trisphosphate also reduced the amplitude of action potentials. Injection of calcium ions directly into bag cell neurons mimicked these responses seen after inositol trisphosphate injection. Using the cell-attached patch-clamp technique in conjunction with inositol trisphosphate microinjection, we observed that inositol trisphosphate evoked increases in the activity of a channel carrying outward current at the resting potential and more positive potentials. The estimated slope conductance of the channel modulated by inositol trisphosphate was approximately 40 pS, and its reversal potential was close to that predicted for potassium ions. The increased opening of this channel in response to inositol trisphosphate injection appeared to result from a transient shift of its voltage-dependence to more negative potentials. In a few cases, inositol trisphosphate injection also elicited an increase in the activity of a channel passing inward current at rest. Direct measurements of changes in intracellular calcium in response to inositol trisphosphate were made using digital imaging of isolated neurons loaded with the fluorescent calcium indicator fura-2. These revealed that injection of inositol trisphosphate significantly elevated intracellular calcium levels, and that this inositol trisphosphate-induced rise in cytosolic calcium was not affected by removal of extracellular calcium. In contrast to the effects of trains of action potentials in calcium-containing media, which produced increases in calcium primarily in neurites, the inositol trisphosphate-induced elevation of calcium appeared more localized to the somata of these neurons.(ABSTRACT TRUNCATED AT 400 WORDS)