Neuropeptide receptor transcript expression levels and magnitude of ionic current responses show cell type- specific differences in a small motor circuit

We studied the relationship between neuropeptide receptor transcript expression and current responses in the stomatogastric ganglion (STG) of the crab, Cancer borealis. We identified a transcript with high sequence similarity to crustacean cardioactive peptide (CCAP) receptors in insects and mammalian neuropeptide S receptors. This transcript was expressed throughout the nervous system, consistent with the role of CCAP in a range of different behaviors. In the STG, single-cell qPCR showed expression in only a subset of neurons. This subset had previously been shown to respond to CCAP with the activation of a modulator-activated inward current (IMI), with one exception. In the one cell type that showed expression but no I_MI responses, we found CCAP modulation of synaptic currents. Expression levels within STG neuron types were fairly variable, but significantly different between some neuron types. We tested the magnitude and concentration dependence of I_MI responses to CCAP application in two identified neurons, the lateral pyloric (LP) and the inferior cardiac (IC) neurons. LP had several-fold higher expression and showed larger current responses. It also was more sensitive to low CCAP concentrations and showed saturation at lower concentrations, as sigmoid fits showed smaller EC₅₀ values and steeper slopes. In addition, occlusion experiments with proctolin, a different neuropeptide converging onto I_MI, showed that saturating concentrations of CCAP activated all available I_MI in LP, but only approximately two-thirds in IC, the neuron with lower receptor transcript expression. The implications of these findings for comodulation are discussed.