Presynaptic Ca2+ influx at a mouse central synapse with Ca2+ channel subunit mutations

Genetic alterations in Ca2+ channel subunits can be used to study the inter action among channel subunits and their roles in channel function. P/Q- and N-type Ca2+ channels reside at the presynaptic terminal and control the re lease of neurotransmitter at mammalian central synapses. We used fluorescen ce imaging techniques to investigate presynaptic Ca2+ currents and neurotra nsmitter release at hippocampal Schaffer collateral synapses in both totter ing (tg, alpha(1A) subunit) and lethargic (Ih, beta(4) subunit) mutant mice . Application of selective toxins revealed a large reduction in presynaptic P/Q-type Ca2+ transients, from 39% of total in +/+ mice to 6% in tg/tg mic e, whereas the proportion of N-type increased from 35 to 68%, respectively. Neurotransmitter release in the tg/tg mutant relied almost exclusively on N-type channels, as shown by the complete blockade of synaptic transmission with omega-conotoxin GVIA. Remarkably, loss of b4, a subunit predicted to regulate the subcellular targeting and modulation of both P/Q- and N-type c hannels, resulted in no significant difference in the ratio of Ca2+ channel subtypes or Ca2+ dependence of neurotransmitter release in lethargic mice. G-protein-mediated inhibition of Ca2+ channels was also unaltered. These r esults indicate that a profound decrease in presynaptic P/Q- type currents leads to dependence of neurotransmitter release on N-type channels. In cont rast, absence of beta(4) appears not to compromise either P/Q- or N-type ch annel function at this hippocampal synapse, implicating rescue of presynapt ic Ca2+ currents by other available beta subunits. The present study reveal s compensatory molecular mechanisms in the regulation of presynaptic Ca2+ e ntry and neurotransmitter release.