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Contrasting Ca²⁺ channel subtypes at cell bodies and synaptic terminals of rat anterioventral cochlear bushy neurones

Joanne M. Doughty, Margaret Barnes-Davies, Zoltan Rusznák *, Csaba Harasztosi * and Ian D. Forsythe

1. Whole-cell patch clamp recordings were made from bushy cells of the anterioventral cochlear nucleus (aVCN) and their synaptic terminals (calyx of Held) in the medial nucleus of the trapezoid body (MNTB).

2. Both high voltage-activated (HVA) and low voltage-activated (LVA) calcium currents were present in acutely dissociated aVCN neurones and in identified bushy neurones from a cochlear nucleus slice.

3. The transient LVA calcium current activated rapidly on depolarization (half-activation, -59 mV) and inactivated during maintained depolarization (half-inactivation, -89 mV). This T-type current was observed in somatic recordings but was absent from presynaptic terminals.

4. On the basis of their pharmacological sensitivity, P/Q-type Ca²⁺ channels accounted for only 6 % of the somatic HVA, while L-, N- and R-type Ca²⁺ channels each accounted for around one-third of the somatic calcium current.

5. The divalent permeabilities of these native calcium channels were compared. The Ba²⁺/Ca²⁺ conductance ratios of the somatic HVA and LVA channels were 1·4 and 0·7, respectively. The conductance ratio of the presynaptic HVA current was 0·9, significantly lower that that of the somatic HVA current.

6. We conclude that LVA currents are expressed in the bushy cell body, but are not localized to the excitatory synaptic terminal. All of the HVA current subtypes are expressed in bushy cells, but there is a strong polarity to their localization; P-type contribute little to somatic currents but predominate at the synaptic terminal; L-, N- and R-types dominate at the soma, but contribute negligibly to calcium currents in the terminal.

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