An unique model for the analysis of neuronal nitric oxide signaling: the leech CNS

An hypothesis for the presence of nitric oxide synthase (NOS) and the involvement of nitric oxide (NO) in neuronal signaling in the leach (Hirudo medicinalis) CNS is based on (i) the presence of fixative resistant NADPH-diaphorase (NADPH-d) in a specific neuronal population in the segmental ganglion; (ii) direct measurements with NO-sensitive electrodes giving an estimated NO concentration of 400-600 nM; (iii) biochemical assay for NOS using L-arginine/L-citrulline conversion in the CNS which gave a comparable value to that of the rat cerebellum and (iv) specific pharmacological effects of the NO precursor L-arginine and a NOS inhibitor (NG-Nitro-L-arginine) in modulation of chemical connections between NADPH-d-positive and NADPH-d-negative neurones. The very high value of estimated NO concentration and NOS activity and the relatively small number of putative NOS-containing (NADPH-d-positive) cells, including identified sensory (N, P, T cells), motor (AE) and interneurones (151) with known monosynaptic connections, make these neurones an ideal preparation on which to study the possible effects of NO on synaptic interactions.