Prejunctional modulation of NANC inhibitory responses in the guinea pig stomach

It is well known that the cholinergic excitatory innervation to the gastrointestinal muscle is pharmacologically modulated by different presynaptic receptors, including 2-adrenoceptors, histamine H3 receptors and adenosine A1 receptors, which control the release of acetylcholine from nerve endings. Conversely, lesser is known about the presynaptic modulation of the inhibitory nerves, which also play a pivotal role in the control gastrointestinal peristalsis. To this aim, we tested the ability of different substances acting at presynaptic level to influence the nerve-mediated relaxation in the gastric smooth muscle. Longitudinal fundic strips were excised from the guinea pig stomach, suspended into isolated organ chambers under isometric conditions and field-stimulated by a pair of coaxial electrodes (5 sec trains at 5 Hz, 0.5 ms, supramax. amplitude, every 120 sec). In preparations pretreated with the muscarinic M3 receptor blocker hexahydro-sila-difenidol (1 M) and with the antiadrenergic compound guanetidine (1 M), field stimulation induced phasic relaxatory responses, which remained stable in amplitude for more than 180 min. Such response was prevented by tetrodotoxin (1 M) and almost completely abolished by L-NAME and methylene blue, suggesting the involvement of nitric oxide (NO) as a final inhibitory neurotransmitter. A series of agonists acting at prejunctional receptors, including UK 14,304 and clonidine (2), oxotremorine (muscarinic), R()-methylhistamine (H3), -aminobutirric acid (GABA), N6-cyclopentyladenosine (A1) were tested on the NO-mediated inhibitory response. However, none of them significantly modified the amplitude of the inhibitory neurogenic response. In contrast, we observed a reduction of the inhibitory response by the L-type calcium channel blocker nifedipine, at a concentrations (1-100 nM), which did not cause mayor changes in the basal tone of the preparations. Conversely, the N-type calcium channel -conotoxin GVIA (1-100 nM), caused only a slight and erratic inhibition of the relaxatory response. These data suggest that the NANC inhibitory innervation in the guinea pig stomach utilises NO as a neurotransmitter, even though the participation of other substances (VIP and/or purines), which may involve NO as a final mediator, cannot be excluded. Such response seems to be calcium-dependent, but cannot be modified by the activation of the prejunctional receptors which usually modulate the cholinergic and non-cholinergic excitatory innervation of the gastrointestinal tract. Finally, the L-type calcium channel, but not the N-type, seems to play a mayor role in the in the triggering processes of the neurosecretory machinery.