Received for publication June 2, 2006.
Revised September 12, 2006.
Accepted for publication September 13, 2006.

ACETYLCHOLINE RELEASE AT NEUROMUSCULAR JUNCTIONS OF ADULT TOTTERING MICE IS CONTROLLED BY N- (Ca_v2.2) AND R- (Ca_v2.3), BUT NOT L-TYPE (Ca_v1.2) Ca²⁺ CHANNELS

Abstract

The mutation in the _1A subunit gene of the P/Q-type (Ca_v 2.1) Ca²⁺ channel present in tottering (tg) mice causes ataxia and motor seizures which resemble absence epilepsy in humans. P/Q-type Ca²⁺channels are primarily involved in acetylcholine (ACh) release at mammalian neuromuscular junctions. Unmasking of L-type (Ca_v 1.1 - 1.2) Ca²⁺ channels occurs in cerebellar Purkinje cells of tg mice. However, whether L-type Ca²⁺ channels are also upregulated at neuromuscular junctions of tg mice is unknown. We characterized thoroughly the pharmacological sensitivity of the Ca²⁺ channels which control ACh release at adult tg neuromuscular junctions. Block of N-and R-type (Ca_v2.2-2.3), but not L-type Ca²⁺ channels, significantly reduced quantal content of EPPs in tg preparations. Neither resting nor KCl-evoked MEPP frequency differed significantly between tg and wild type (wt). Immunolabeling of Ca²⁺ channel subunits _1A, _1B, _1C, and _1E revealed an apparent increase of _1B, and _1E staining, at tg but not wt neuromuscular junctions. This presumably compensates for the deficit of P/Q-type Ca²⁺channels, which localized presynaptically at wt neuromuscular junctions. No _1C subunits juxtaposed with pre- or postsynaptic markers at either wt or tg neuromuscular junctions. Thus, in adult tg mice, immunocytochemical and electrophysiological data indicate that N- and R-type channels both assume control of ACh release at motor nerve terminals. Recruitment of alternate subtypes of Ca²⁺ channels to control transmitter release appears to represent a commonly-occurring method of neuronal plasticity. However, it is unclear which conditions underlie recruitment of Ca_v2 as opposed to Ca_v1-type Ca²⁺ channels.

Key words: acetylcholine release, calcium channel, calcium channel mutation, motor nerve terminal, neuromuscular junction, tottering mutation