Receptors for octopamine in the storage lobe of the locust (Locusta migratoria) corpus cardiacum: Evidence from studies on cyclic nucleotides

The storage lobe of the locust corpus cardiacum consists primarily of neurosecretory cell axons and terminals and serves as a neurohaemal organ for the release of several neuropeptides. Biogenic amines have previously been identified in the storage lobe of the adult corpus cardiacum, and interest in the functional roles of these amines has arisen due to their modulatory actions on numerous physiological processes in insects. It has been suggested that these compounds may be released from the storage lobe as neurohormones, however an alternative possibility is that they serve as neurotransmitters or neuromodulators which act at sites within the corpus cardiacum proper. The present studies were undertaken to determine if specific amine receptor-adenylate cyclases exist in the storage lobe. Octopamine increases the cAMP content of intact cells in the storage lobes of adult and 5th-instar locusts. Forskolin and 5-HT also enhance cAMP levels in the storage lobe however additivity experiments suggest that 5-HT may be interacting with octopamine binding sites. Dopamine is a poor agonist and norepinephrine has no effect on cAMP content. In adult storage lobes octopamine acts on receptors that have pharmacological similarities to octopamine receptors of identified octopaminergic systems. There thus appears to be an octopamine receptor-adenylate cyclase complex in this tissue. These results suggest that octopamine may be involved in regulating some aspect of neurosecretion in the storage lobe of the locust corpus cardiacum. In addition, significantly higher levels of cGMP were found in the storage lobe compared to those reported for the neighbouring glandular lobe, indicating a possible functional role for this cyclic nucleotide.