A pair of reciprocally inhibitory histaminergic sensory neurons are activated within the same phase of ingestive motor programs in Aplysia

Previous studies have shown that each buccal ganglion in Aplysia contains t wo B52 neurons, one in each hemiganglion. We now show that there are two B5 2 neurons in a single buccal hemiganglion and four cells in an animal. We a lso show that the B52 neurons are histamine-immunoreactive and use reverse phase HPLC to show that the histamine-immunoreactive substance is authentic histamine, Previous studies have shown that the B52 neurons make numerous inhibitory synaptic connections with neurons active during the radula closi ng/retraction phase of ingestive motor programs. A computational model of t he Aplysia feeding central pattern generator has, therefore, suggested that the B52 neurons play a role in terminating closing/retraction. Consistent with this idea we show that both B52 neurons fire at the beginning of radul a opening/protraction. We also show that both B52 neurons are sensory neuro ns. They are depolarized when a flap of connective tissue adjacent to the b uccal commissural arch is stretched. During ingestive feeding this is likel y to occur at the peak of closing/retraction as opening/protraction begins. In the course of this study we compare the two ipsilateral B52 neurons and show that these cells are virtually indistinguishable; e.g., they use a co mmon neurotransmitter, make the same synaptic connections, and are both sen sory as well as premotor neurons. Nevertheless we show that the B52 neurons are reciprocally inhibitory. Our results, therefore, strikingly confirm th eoretical predictions made by others that neurons that inhibit each other w ill not necessarily participate in antagonistic phases of behavior.