Swelling-activated calcium signalling in cultured mouse primary sensory neurons

The effects of hypo-osmotic membrane stretch on intracellular calcium concentration ([Ca2+](i)), cell volume and cellular excitability were investigated in cultured mouse primary sensory trigeminal neurons. Hypotonic solutions (15-45%) led to rapid cell swelling in all neurons. Swelling was accompanied by dose-dependent elevations in [Ca2+](i) in a large fraction of neurons. Responses could be classified into three categories. (i) In 57% of the neurons [Ca2+](i) responses had a slow rise time and were generally of small amplitude. (ii) In 21% of the neurons, responses had a faster rise and were larger in amplitude. (iii) The remaining cells (22%) did not show [Ca2+](i) responses to hypo-osmotic stretch. Slow and fast [Ca2+](i) changes were observed in trigeminal neurons of different sizes with variable responses to capsaicin (0.5 muM). The swelling-induced [Ca2+](i) responses were not abolished after depletion of intracellular Ca2+ stores with cyclopiazonic acid or preincubation in thapsigargin, but were suppressed in the absence of external Ca2+. They were strongly attenuated by extracellular nickel and gadolinium. Hypotonic stimulation led to a decrease in input resistance and to membrane potential depolarization. Under voltage-clamp, the [Ca2+](i) elevation produced by hypotonic stimulation was accompanied by the development of an inward current and a conductance increase. The time course and amplitude of the [Ca2+](i) response to hypo-osmotic stimulation showed a close correlation with electrophysiological properties of the neurons. Fast [Ca2+](i) responses were characteristic of trigeminal neurons with short duration action potentials and marked inward rectification. These findings suggest that hypo-osmotic stimulation activates several Ca2+-influx pathways, including Gd3+-sensitive stretch-activated ion channels, in a large fraction of trigeminal ganglion neurons. Opening of voltage-gated Ca2+ channels also contributes to the response. The pattern and rate of Ca2+ influx may be correlated with functional subtypes of sensory neurons.