In the gastrointestinal tract, tachykinins are peptide neurotransmitters in nerve circuits that regulate intestinal motility, secretion, and vascular functions. Tachykinins also contribute to transmission from spinal afferents that innervate the gastrointestinal tract and have roles in the responses of the intestine to inflammation. Tachykinins coexist with acetylcholine, the primary transmitter of excitatory neurons innervating the muscle, and act as a co-neurotransmitter of excitatory neurons. Excitatory transmission is mediated through NK1 receptors (primarily on interstitial cells of Cajal) and NK2 receptors on the muscle. Tachykinins participate in slow excitatory transmission at neuro-neuronal synapses, through NK1 and NK3 receptors, in both ascending and descending pathways affecting motility. Activation of receptors (NK1 and NK2) on the epithelium causes fluid secretion. Tachykinin receptors on immune cells are activated during inflammation of the gut. Finally, tachykinins are released from the central terminals of gastrointestinal afferent neurons in the spinal cord, particularly in nociceptive pathways.

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It is concluded that acylation of the ghrelin peptide is essential to promote propulsive contractions of the colorectum and that des-acyl ghrelin opposes this effect. At most other sites of ghrelin action, des-acyl ghrelin either has no effect or it mimics ghrelin. This is the first evidence that non-acylated ghrelin opposes the action of the acylated peptide in the spinal cord.

Key pointsr The pathophysiological roles of the CNS in bowel dysfunction in patients with irritable bowel syndrome and Parkinson's disease remain obscure.r In the present study, we demonstrate that dopamine in the lumbosacral defaecation centre causes strong propulsive motility of the colorectum.r The effect of dopamine is a result of activation of sacral parasympathetic preganglionic neurons via D2-like dopamine receptors.r Considering that dopamine is a neurotransmitter of descending pain inhibitory pathways, our results highlight the novel concept that descending pain inhibitory pathways control not only pain, but also the defaecation reflex.r In addition, severe constipation in patients with Parkinson's disease can be explained by reduced parasympathetic outflow as a result of a loss of the effect of dopaminergic neurons. AbstractWe have recently demonstrated that intrathecally injected noradrenaline caused propulsive contractions of the colorectum. We hypothesized that descending pain inhibitory pathways control not only pain, but also the defaecation reflex. Because dopamine is one of the major neurotransmitters of descending pain inhibitory pathways in the spinal cord, we examined the effects of the intrathecal application of dopamine to the spinal defaecation centre on colorectal motility. Colorectal intraluminal pressure and expelled volume were recorded in vivo in anaesthetized rats. Slice patch clamp and immunohistochemistry were used to confirm the existence of dopamine-sensitive neurons in the sacral parasympathetic nuclei. Intrathecal application of dopamine into the L6-S1 spinal cord, where the lumbosacral defaecation centre is located, caused propulsive contractions of the colorectum. Inactivation of spinal neurons using TTX blocked the effect of dopamine. Although thoracic spinal transection had no effect on the enhancement of colorectal motility by intrathecal dopamine, the severing of the pelvic nerves abolished the enhanced motility. Pharmacological experiments revealed that the effect of dopamine is mediated primarily by D2-like dopamine receptors. Neurons labelled with retrograde dye injected at the colorectum showed an inward current in response to dopamine in slice patch clamp recordings. Furthermore, immunohistochemical analysis revealed that neurons immunoreactive to choline acetyltransferase express D2-like dopamine receptors. Taken together, our findings demonstrate that dopamine activates sacral parasympathetic preganglionic neurons via D2-like dopamine receptors and causes propulsive motility of the colorectum in rats. The present study supports the hypothesis that descending pain inhibitory pathways regulate defaecation reflexes.