Nitric oxide: a co-modulator of efferent peptidergic neurosecretory cells including a unique octopaminergic neurone innervating locust heart.

Our findings suggest that nitric oxide (NO) acts as peripheral neuromodulator in locusts, in which it is commonly co-localized with RF-like peptide in neurosecretory cells. We also present the first evidence for NO as a cardio-regulator in insects. Putative NO-producing neurones were detected in locust pre-genital free abdominal ganglia by NADPH-diaphorase histochemistry and with an antibody against NO synthase (NOS). With both methods, we identified the same 14 somata in each examined ganglion: two dorsal posterior midline somata; six ventral posterior midline somata; and three pairs of lateral somata. A combination of NOS-detection methods with nerve tracing and transmitter immunocytochemistry revealed that at least 12 of these cells were efferent, of which four were identified as peptidergic neurosecretory cells with an antiserum detecting RFamide-like peptides. One of the latter was unequivocally identified as an octopaminergic dorsal unpaired median (DUM) neurone, which specifically projected to the heart ("DUM-heart"). Its peripheral projections revealed by axon tracing appeared as a meshwork of varicose endings encapsulating the heart. NOS-like immunoreactive profiles were found in the heart nerve. NO donors caused a dose-dependent increase in heart rate. This cardio-excitatory effect was negatively correlated to resting heart rate and seemed to be dependent on the physiological state of the animal. Hence, NO released from neurones such as the rhythmically active DUM-heart might exert continuous control over the heart. Possible mechanisms for the actions of NO on the heart and interactions with other neuromodulators co-localized in the DUM-heart neurone (octopamine, taurine, RF-amide-like peptide) are discussed.