Parvalbumin expression affects synaptic development and physiology at the Drosophila larval NMJ

Presynaptic Ca²⁺ appears to play multiple roles in synaptic development and physiology. We examined the effect of buffering presynaptic Ca²⁺ by expressing parvalbumin (PV) in Drosophila neurons, which do not normally express PV. The studies were performed on the identified Ib terminal that innervates muscle fiber 5. The volume-averaged, residual Ca²⁺ resulting from single action potentials (APs) and AP trains was measured using the fluorescent Ca²⁺ indicator, OGB-1. PV reduced the amplitude and decay time constant (τ) for single-AP Ca²⁺ transients. For AP trains, there was a reduction in the rate of rise and decay of [Ca²⁺]_i but the plateau [Ca²⁺]_i was not affected. Electrophysiological recordings from muscle fiber 5 showed a reduction in paired-pulse facilitation, particularly the F₁ component; this was likely due to the reduction in residual Ca²⁺. These synapses also showed reduced synaptic enhancement during AP trains, presumably due to less buildup of synaptic facilitation. The transmitter release for single APs was increased for the PV-expressing terminals and this may have been a homeostatic response to the decrease in facilitation. Confocal microscopy was used to examine the structure of the motor terminals and PV expression resulted in smaller motor terminals with fewer synaptic boutons and active zones. This result supports earlier proposals that increased AP activity promotes motor terminal growth through increases in presynaptic [Ca²⁺]_i.