Neurotransmission at a mature neuromuscular junction requires the precise alignment of pre- and postsynaptic elements across the synaptic cleft. One prominent synaptic organiser and mediator of presynaptic nerve terminal differentiation is β2-laminin. β2-laminin deficient neuromuscular junctions exhibit severe presynaptic aberrations (Noakes et al., 1995) and reduced neural transmission (Knight et al., 2003). As a consequence of these studies we investigated the calcium cooperativity of transmitter release in β2-laminin deficient mice. The power relationship between transmitter release and extracellular calcium concentration is a fourth order of cooperativity in mammalian and amphibian neuromuscular junctions (Dodge & Rahamimoff, 1967; Hubbard et al., 1968). We examined transmitter release using extracellular recording techniques, confirming the previously reported reduction in release at mutant terminals and found no difference in the cooperativity relationship between wild-type (3.20 ± 0.085, n=11) and mutant (3.54 ± 0.20, n=17) release sites. We also found that mutant nerve terminals show no significant differences in the paired-pulse facilitation ratios at low frequency of stimulation, even though evoked transmitter release was reduced by over 50%. However, we did observe a rightward parallel shift of the extracellular calcium vs transmitter release relationship, indicating a change in the calcium sensitivity in β2-laminin deficient terminals. The results suggest that the reduction in calcium sensitivity leads to a drop in transmitter release, resulting from a possible drop in the density of presynaptic voltage-gated calcium channels (P/Q type) at release sites (active zones). Our hypothesis is supported by the observation that β2-laminin binds to presynaptic voltage-gated calcium channels (VGCCs), which results in VGCC clustering and subsequent synaptic vesicle accumulation, i.e. formation of the active zone (Nishimune et al., 2004).
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