Fast-synaptic axo-axonal transmission from striatal cholinergic interneurons onto dopamine fibers.

Abstract

Transmission from striatal cholinergic interneurons (CINs) controls dopamine release through nicotinic acetylcholine receptors (nAChRs) on dopaminergic axons. Anatomical studies suggest that cholinergic terminals signal predominantly through non-synaptic volume transmission. However, the influence of cholinergic transmission on electrical signaling in axons remains unclear. We examined axo-axonal transmission from CINs onto dopaminergic axons using perforated-patch recordings which revealed rapid spontaneous EPSPs with properties characteristic of fast synapses. Pharmacology showed that axonal EPSPs (axEPSPs) were mediated primarily by high-affinity 6-containing receptors. Remarkably, axEPSPs triggered spontaneous action potentials locally in dopaminergic axons, suggesting these axons perform integration to convert synaptic input into spiking, a function associated with somatodendritic compartments. We investigated cross-species validity of cholinergic axo-axonal transmission by recording dopaminergic axons in macaque putamen and found similar axEPSPs. Thus, we reveal that fast synaptic-like neurotransmission underlies cholinergic signaling onto dopaminergic axons, providing insight into how nicotinic receptors shape electrical signaling directly in axon terminals.