A Potential Role of Postsynaptic Limitation in Activity-Dependent Homeostasis

The distribution of neurotransmitter release at amphibian neuromuscular junctions ( NMJ ) is characterised by a shift from Poisson to a tight binomial distribution as calcium concentration is increased. Despite advances in our understanding of neurotransmission, a paradox remains about how the large number of active zones ( AZs ) that contribute to release have such a low variance, resulting in a tight binomial distribution. Muscle-specific sodium channel-blocking neurotoxins have allowed us to examine the release characteristics when the unit size of transmitter release (quantal size) is not altered. Our present study has compared the neurotransmitter release characteristics when D-tubocurarine (curare) or the Na-channel blocking conotoxin (μ-conotoxin GIIIB) was used as the extracellular calcium concentration was increased. Quantal neurotransmitter release from toad iliofibularis motor nerve terminals was examined using intracellular electrodes and focal extracellular electrodes. Motor nerve terminal branches for recordings were located using DiOC ₂ (5)-fluorescent imaging. Muscle action potentials were suppressed by using μ-conotoxin GIIIB (μ-CgTx GIIIB, 2.0 x 10 ^-6 M) or curare, 6.5 x 10 ^-6 M. Under μ-CgTx GIIIB, the maximum transmitter release reached was 25 quanta at high extracellular calcium concentration (2.1 mM); conversely, under curare, quantal content reached 148 quanta ( P =0.0067). Using binomial analysis, it was found that this quantal increase was due to a significant increase in the number of release sites ‘n’ rather than the average probability of release ‘p’. Based on these observations, along with analyses of ECP shape and duration, we propose a mechanism in which a fast autoinhibitory response (< 5 ms from the nerve terminal impulse) limits the maximum level of depolarisation by voltage-dependent regulation of the acetylcholine receptor ( AChR ) channel conductance.