Immunocytological and biochemical localization and biological activity of the newly sequenced cerebral peptide 2 in Aplysia.

Cerebral peptide 2 (CP2), a 41 amino acid neuropeptide, was identified because it was transported from the cerebral ganglia of Aplysia to other central ganglia. Immunocytology indicates that CP2 is distributed widely in the CNS and peripheral tissues of Aplysia. Most CP2-immunoreactive neurons were found in the cerebral ganglia and extensively overlap with the distribution of cerebral peptide 1 (CP1). HPLC analyses confirm that individual cerebral neurons synthesize both CP1 and CP2. In other ganglia, CP1 and CP2 are localized predominantly to different neurons. CP2-immunoreactive fibers and varicosities are present in the neuropil of all ganglia but were found surrounding cell bodies and axon hillocks most often in the buccal and abdominal ganglia. Thus, the effects of CP2 on neurons in these ganglia were determined using intracellular recording. In the buccal ganglia, CP2 evokes rhythmic activity in many motor neurons that seems similar to that observed during ingestion; however, only one identified neuron was found to be depolarized directly. By contrast, in the abdominal ganglion, many neurons are depolarized directly by CP2. A number of these have been shown to be part of the circuit that regulates respiratory pumping. Injection of CP2 into freely behaving Aplysia increases the rate of respiratory pumping and causes other changes in behavior. CP2 is stable in hemolymph, which raises the possibility that it may act as a hormone. Thus, CP2 is a bioactive neuropeptide that is present in many neurons and likely functions as a transmitter or a hormone.