Effects of tricyclic antidepressants on mechanosensitive pelvic nerve afferent fibers innervating the rat colon

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Abstract
The aim of this study was to examine the effects of tricyclic antidepressants on responses of mechanosensitive afferent fibers innervating the rat colon. A total of 53 fibers in the decentralized S1 dorsal root were studied. The effects of the non-specific monoamine reuptake inhibitor imipramine (IMI), the noradrenaline reuptake inhibitor desipramine (DES), and the serotonin reuptake inhibitor clomipramine (CLO) were tested on responses of 22 mechanosensitive afferent fibers to noxious colorectal distension (CRD; 80 mmHg). Cumulative doses of 16 mg/kg of IMI, DES and of CLO reduced responses to noxious CRD to a mean 20%, 22% and 46% of control, respectively. The mean inhibitory doses of the three antidepressants did not differ significantly. Inhibitory effects were independent of potential effects on neurotransmitter reuptake: the effects of IMI and DES were not blocked by the adrenoreceptor antagonist phentolamine, and the effects of IMI and CLO were not affected by the serotonin receptor antagonist metergoline. Attenuation of afferent nerve activity was not mimicked by the anticholinergic glycopyrrolate; the cholinesterase inhibitor neostigmine did not attenuate the effect of IMI on responses to noxious CRD. Interestingly, the opioid receptor antagonist naloxone partially reversed the effects of IMI, and the NMDA receptor channel blocker MK-801 enhanced the inhibitory effects of DES and CLO. These results document that responses of mechanosensitive pelvic nerve afferent fibers to noxious CRD are significantly attenuated by tricyclic antidepressants, a peripheral action that may contribute to the beneficial effects of tricyclic antidepressants in treatment of irritable bowel syndrome.