TARGETED DISRUPTION OF THE CA2-TYPE AND L-TYPE CA2+ CHANNEL ACTIVITY AND ALTERS THE VOLTAGE-DEPENDENT ACTIVATION OF P( CHANNEL BETA(3) SUBUNIT REDUCES N)Q-TYPE CA2+ CHANNELS IN NEURONS/

In comparison to the well characterized role of the principal subunit of voltage-gated Ca2+ channels, the pore-forming, antagonist-binding a lpha(1) subunit, considerably less is understood about how beta subuni ts contribute to neuronal Ca2+ channel function. We studied the role o f the Ca2+ channel beta(3) subunit, the major Ca2+ channel beta subuni t in neurons, by using a gene-targeting strategy. The beta(3) deficien t (beta(3)-/-) animals were indistinguishable from the wild type (wt) with no gross morphological or histological differences. However, in s ympathetic beta 3-/- neurons, the L- and N-type current was significan tly reduced relative to wt. Voltage-dependent activation of P/Q-type C a2+ channels was described by two Boltzmann components with different voltage dependence, analogous to the ''reluctant'' and ''willing'' sta tes reported for N-type channels. The absence of the beta(3) subunit w as associated with a hyperpolarizing shift of the ''reluctant'' compon ent of activation. Norepinephrine inhibited wt and beta(3)-/- neurons similarly but the voltage sensitive component was greater for N-type t han P/Q-type Ca2+ channels, The reduction in the expression of N-type Ca2+ channels in the beta(3)-/- mice may be expected to impair Ca2+ en try and therefore synaptic transmission in these animals. This effect may be reversed, at least in part, by the increase in the proportion o f P/Q channels activated at less depolarized voltage levels.