Compartmentalized calcium dynamics in a C. elegans interneuron encode head movement

Abstract

The confinement of neuronal activity to specific subcellular regions is a mechanism for expanding the computational properties of neurons. Although the circuit organization underlying compart-mentalized activity has been studied in several systems 1–4 , its cellular basis is still unknown. Here we characterize compartmentalized activity in Caenorhabditis elegans RIA interneurons, which have multiple reciprocal connections to head motor neurons and receive input from sensory pathways. We show that RIA spatially encodes head movement on a subcellular scale through axonal compart-mentalization. This subcellular axonal activity is dependent on acetylcholine release from head motor neurons and is simulta-neously present and additive with glutamate-dependent globally synchronized activity evoked by sensory inputs. Postsynaptically, the muscarinic acetylcholine receptor GAR-3 acts in RIA to compartmentalize axonal activity through the mobilization of intracellular calcium stores. The compartmentalized activity func-tions independently of the synchronized activity to modulate locomotory behaviour. The 'wiring diagram' of the C. elegans nervous system allows the dissection of circuit function from single neurons to behavioural outputs 5,6 . RIA interneurons regulate navigation behaviours and occupy a key circuit position, receiving input from multiple sensory networks and showing reciprocal synaptic connections with head motor neurons (Fig. 1a) 5,7,8 . To characterize the properties of RIA, we expressed the genetically encoded calcium indicator GCaMP3 (ref. 9) under the RIA-specific glr-3 promoter 10 and performed calcium imaging with a microfluidic device in which semi-restrained trans-genic animals could bend their heads in the dorsal–ventral plane, allowing simultaneous monitoring of neural activity and head move-ments 11 . We stimulated the animals with alternating fluid streams of 3-methylbutan-1-ol (also known as isoamyl alcohol; IAA) and buffer, because IAA is detected by AWC olfactory neurons, one of the sensory neurons upstream of RIA and some of its major synaptic inputs 12 . We did not detect any prominent calcium response in the RIA cell body (Supplementary Fig. 1a). The single axon of RIA projects ventrally into the nerve cord, where it forms a hairpin loop, then enters and extends around the nerve ring, forming a near-complete circle (Fig. 1b). C. elegans normally lies on its side such that several distinct portions of the RIA axon lie in the same focal plane: the 'loop' (the ventrally directed hairpin loop), nrD and nrV (the dorsal and ventral segments of the RIA axon in the nerve ring, respectively) (Fig. 1b). Strikingly, we observed robust calcium dynamics in each of these axonal segments that seemed to be independent, and this axonal calcium activity was correlated with head movement (Fig. 1c, d and Supplementary Movie 1).