The neuromuscular junction is a specialized synapse in that every action po
tential in the presynaptic nerve terminal results in an action potential in
the postsynaptic membrane, unlike most interneuronal synapses where a sing
le presynaptic input makes only a small contribution to the population post
synaptic response. The postsynaptic membrane at the neuromuscular junction
contains a high density of neurotransmitter (acetylcholine) receptors and a
high density of voltage-gated Na+ channels. Thus, the large acetylcholine
activated current occurs at the same site where the threshold for action po
tential generation is low. Acetylcholine receptor inducing activity (ARIA),
a 42 kD protein, that stimulates synthesis of acetylcholine receptors and
voltage-gated Na+ channels in cultured myotubes, probably plays the same ro
les at developing and mature motor endplates in vivo. ARIA is synthesized a
s part of a larger, transmembrane, precursor protein called proARIA. Delive
ry of ARIA from motor neuron cell bodies in the spinal cord to the target e
ndplates involves several steps, including proteolytic cleavage of proARIA.
ARIA is also expressed in the central nervous system and it is abundant in
the molecular layer of the cerebellum. In this paper we describe our first
experiments on the processing and release of ARIA from subcellular fractio
ns containing synaptosomes from the chick cerebellum as a model system.