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 leech (H irudo medicinalis) CNS is based on (i) the presence of fixative resist ant NADPH-diaphorase (NADPH-d) in a specific neuronal population in th e segmental ganglion; (ii) direct measurements with NO-sensitive elect rodes giving an estimated NO concentration of 400-600 nM; (iii) bioche mical assay for NOS using L-arginine/L-citrulline conversion in the CN S 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 (N-G-Nitro-L-arginine) in modulation of chemical connec tions between NADPH-d-positive and NADPH-d-negative neurones. The very high value of estimated NO concentration and NOS activity and the rel atively small number of putative NOS-containing (NADPH-d-positive) cel ls, including identified sensory (N, P, T cells), rotor (AE) and inter neurones (151) with known monosynaptic connections, make these neurone s an ideal preparation an which to study the possible effects of NO on synaptic interactions.