Actions of a pair of identified cerebral-buccal interneurons (CBI-8/9) in Aplysia that contain the peptide myomodulin

A combination of biocytin back-fills of the cerebral-buccal connectives and immunocytochemistry of the cerebral ganglion demonstrated that of the 13 b ilateral pairs of cerebral-buccal interneurons in the cerebral ganglion, a subpopulation of 3 are immunopositive for the peptide myomodulin. The prese nt paper describes the properties of two of these cells, which we have term ed CBI-8 and CBI-9. CBI-8 and CBI-9 were found to be dye coupled and electr ically coupled. The cells have virtually identical properties, and conseque ntly we consider them to be "twin" pairs and refer to them as CBI-8/9. CBI- 8/9 were identified by electrophysiological criteria and then labeled with dye. Labeled cells were found to be immunopositive for myomodulin, and, usi ng high pressure liquid chromatography, the cells were shown to contain aut hentic myomodulin. CBI-8/9 were found to receive synaptic input after mecha nical stimulation of the tentacles. They also received excitatory input fro m C-PR, a neuron involved in neck lengthening, and received a slow inhibito ry input from CC5, a cell involved in neck shortening, suggesting that CBI- 8/9 may be active during forward movements of the head or buccal mass. Firi ng of CBI-8 or CBI-9 resulted in the activation of a relatively small numbe r of buccal neurons as evidenced by extracellular recordings from buccal ne rves. Firing also produced local movements of the buccal mass, in particula r a strong contraction of the 17 muscle, which mediates radula opening. CBI -8/9 were found to produce a slow depolarization and rhythmic activity of B 48, the motor neuron for the 17 muscle. The data provide continuing evidenc e that the small population of cerebral buccal interneurons is composed of neurons that are highly diverse in their functional roles. CBI-8/9 may func tion as a type of premotor neuron, or perhaps as a peptidergic modulatory n euron, the functions of which are dependent on the coactivity of other neur ons.