Highly Localized Ca2+Accumulation Revealed by Multiphoton Microscopy in an Identified Motoneuron and Its Modulation by Dopamine

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Abstract
Calcium is essential for synaptic transmission and the control of the intrinsic firing properties of neurons; this makes Ca2+channels a prime target for neuromodulators. A combination of multiphoton microscopy and voltage-clamp recording was used to determine the localization of voltage-dependent Ca2+accumulation in the two pyloric dilator (PD) neurons of the pyloric network in the spiny lobster,Panulirus interruptus,and its modulation by dopamine. We monitored [Ca2+]iin fine distal branches in the neuropil >350 μm below the surface of the ganglion during controlled voltage steps in voltage clamp. Ca2+accumulation originated mostly from small, fairly rare, spatially restricted varicosities on distal neuritic arborizations. Ca2+diffused from these point sources into adjacent regions. Varicosities with similar morphology in the PD neuron have been shown previously to be sites of synaptic contacts. We have demonstrated in earlier studies that dopamine inhibits activity and greatly reduces synaptic transmission from the PD neuron. In ∼60% of the varicosities, the voltage-activated Ca2+accumulation was reduced by exogenous dopamine (DA) (10−4m). DA decreased the peak amplitude of Ca2+accumulation but had no effect on the rise and decay time. We conclude that DA reduces chemical synaptic transmission from the PD neurons at least in part by decreasing Ca2+entry at neurotransmitter release sites.