Different relationship of N- and P/Q-type Ca²⁺ channels to channel-interacting slots in controlling neurotransmission at cultured hippocampal synapses

Synaptic transmission at CNS synapses is often mediated by joint actions of multiple Ca²⁺ channel subtypes, most prominently, P/Q- and N-type. We have proposed that P/Q-type Ca²⁺ channels saturate type-preferring slots at presynaptic terminals, which impose a ceiling on the synaptic efficacy of the channels. To test for analogous interactions for presynaptic N-type Ca²⁺ channels, we overexpressed their pore-forming Ca_v2.2 subunit in cultured mouse hippocampal neurons, recorded excitatory synaptic transmission from transfected cells, and dissected the contributions of N-, P/Q-, and R-type channels with subtype-specific blockers. Overexpression of Ca_v2.2 did not increase the absolute size of the EPSC even though somatic N-type current was augmented by severalfold. Thus, the strength of neurotransmission is saturated with regard to levels of Ca²⁺ channel expression for both N-type and P/Q-type channels. Overexpression of Ca ²⁺-impermeable Ca_v2.2 subunits decreased EPSC size, corroborating competition for channel slots. Striking asymmetries between Nand P/Q-type channels emerged when their relative contributions were compared with channel overexpression. Overexpressed N-type channels could competitively displace P/Q-type channels from P/Q-preferring slots and take over the role of supporting transmission. The converse was not found with overexpression of P/Q-type channels, regardless of their C-terminal domain. We interpret these findings in terms of two different kinds of presynaptic slots at excitatory synapses, one accepting N-type channels but rejecting P/Q-type (N _specific) and the other preferring P/Q-type but also accepting N-type (PQ_preferring). The interaction between channels and slots governs the respective contributions of multiple channel types to neurotransmission and, in turn, the ability of transmission to respond to various stimulus patterns and neuromodulators.