L-Citrulline Immunoreactivity Reveals Nitric Oxide Production in the Electromotor and Electrosensory Systems of the Weakly Electric Fish, <i>Apteronotus leptorhynchus</i>

Abstract

Weakly electric fish produce electric organ discharges (EODs) used for electrolocation and communication. In the brown ghost knifefish, Apteronotus leptorhynchus, several neuron types in brain regions that control the EOD or process electrosensory information express nitric oxide synthase (NOS). The present study used immunoreactivity for L-citrulline, a byproduct of the production of nitric oxide (NO) by NOS, to assess NO production in NOS-expressing neurons. A polyclonal antibody against L-citrulline produced specific labeling in most neuronal populations previously identified to express NOS. Specifically, several cell types that precisely encode temporal information and/or fire at high frequencies, including spherical cells in the electrosensory lateral line lobe, giant cells in layer VI of the dorsal torus semicircularis, and pacemaker and relay cells in the pacemaker nucleus, were strongly immunoreactive for L-citrulline. This suggests that these neurons produced high levels of NO. Notably, electromotor neurons, which also strongly express NOS, were not immunoreactive for L-citrulline, suggesting that NOS did not produce high levels of NO in these neurons. No apparent differences in L-citrulline distribution or intensity were observed between socially isolated fish and fish exposed to playback stimuli simulating the presence of a conspecific. This suggests that social stimulation by electrocommunication signals is not necessary for high levels of NO production in many NOS-positive neurons. Future studies focusing on regulation of NO production in these systems, and the effects of NO on electrosensory processing and electromotor pattern generation will help elucidate the function of NO signaling pathways in this system.

Cell type-specific plasticity at parallel fiber synapses onto Purkinje cells in the posterior caudal lobe of the mormyrid fish cerebellum

“…In mammals, the diffusible messenger nitric oxide (NO) has been found necessary for the PF-LTP induced by 1 Hz PF stimulation alone (Lev-Ram et al 2002), while the presynaptic LTP generated by 4-Hz simulation requires an adenylate cyclase cascade involving activated PKA in PF terminals (Salin et al 1996). Studies in gymnotid fish have shown that the activity of the NO synthase-related enzyme NADPH-d (Smith et al 2005;Turner and Moroz 1995) and [ 3 H]forskolin-adenylate cyclase binding (Maler 1999) are both widely distributed in the cerebellar cortex, indicating that NO and PKA are potential factors in the cerebellar plasticity of teleosts. But again, it is unknown whether these molecules are expressed in the CLp or elsewhere in the mormyrid cerebellum.…”