Projection neurons with shared cotransmitters elicit different motor patterns from the same neural circuit

Specificity in the actions of different modulatory neurons is often attribu ted to their having distinct cotransmitter complements. We are assessing th e validity of this hypothesis with the stomatogastric nervous system of the crab Cancer borealis. In this nervous system, the stomatogastric ganglion (STG) contains a multifunctional network that generates the gastric mill an d pyloric rhythms. Two identified projection neurons [modulatory proctolin neuron (MPN) and modulatory commissural neuron 1 (MCN1)] that innervate the STG and modulate these rhythms contain GABA and the pentapeptide proctolin , but only MCN1 contains Cancer borealis tachykinin-related peptide (CabTRP Ia). Selective activation of each projection neuron elicits different rhyt hms from the STG. MPN elicits only a pyloric rhythm, whereas MCN1 elicits a distinct pyloric rhythm as well as a gastric mill rhythm. We tested the de gree to which CabTRP Ia distinguishes the actions of MCN1 and MPN. To this end, we used the tachykinin receptor antagonist Spantide I to eliminate the actions of CabTRP Ia. With Spantide I present, MCN1 no longer elicited the gastric mill rhythm and the resulting pyloric rhythm was changed. Although this rhythm was more similar to the MPN-elicited pyloric rhythm, these rhy thms remained different. Thus, CabTRP Ia partially confers the differences in rhythm generation resulting from MPN versus MCN1 activation. This result suggests that different projection neurons may use the same cotransmitters differently to elicit distinct pyloric rhythms. It also supports the hypot hesis that different projection neurons use a combination of strategies, in cluding using distinct cotransmitter complements, to elicit different outpu ts from the same neuronal network.