Actions of neuropeptide K and calcitonin gene-related peptide on inferior mesenteric ganglion cells--tachykinin interactions with non-cholinergic potentials evoked by ureteric nerve stimulation

A ganglionic intestinointestinal reflex activated by acute noxious challenge

To investigate noxious stimulation-responsive neural circuits that could influence the gut, we recorded from intestinally directed (efferent) nerve filaments dissected from mesenteric nerves close to the small intestine in anesthetized rats. These exhibited baseline multiunit activity that was almost unaffected by vagotomy (VagX) and reduced only slightly by cutting the splanchnic nerves. The activity was halved by hexamethonium (Hex) treatment. When an adjacent gut segment received an intraluminal stimulus 2,4,6-trinitrobenzenesulfonate (TNBS) in 30% ethanol, mesenteric efferent nerve activity increased for more than 1 h. The increased activity was almost unaffected by bilateral vagotomy or splanchnic nerve section, indicating a lack of central nervous involvement, but it was 60% reduced by hexamethonium. Spike sorting discriminated efferent single and predominantly single-unit spike trains that responded to TNBS, were unaffected by splachnectomy but were silenced by hexamethonium. After noxious stimulation of one segment, the adjacent segment showed no evidence of suppression of gut motility or vasoconstriction. We conclude that luminal application of a noxious stimulus to the small intestine activates an entirely peripheral, intestinointestinal reflex pathway. This pathway involves enteric intestinofugal neurons that excite postganglionic sympathetic neurons via a nicotinic synapse. We suggest that the final sympathetic efferent neurons that respond to a tissue damaging stimulus are distinct from vasoconstrictor, secretomotor, and motility inhibiting neurons.NEW & NOTEWORTHY An intraluminal noxious chemical stimulus applied to one segment of small intestine increased mesenteric efferent nerve activity to an adjacent segment. This was identified as a peripheral ganglionic reflex that did not require vagal or spinal connections. Hexamethonium blocked most, but not all, ongoing and reflex mesenteric efferent activity. The prevertebral sympathetic efferent neurons that are activated likely affect inflammatory and immune functions of other gut segments.

Patterns of innervation of sympathetic vascular neurons by peptide-containing primary sensory fibers

The purpose of this study was to determine whether there is a specific organization of the primary sensory innervation on to identified vascular neurons in the inferior mesenteric ganglion (IMG) in guinea-pig. Retrograde tracers were placed intraluminally in inferior mesenteric artery (IMA) or inferior mesenteric vein (IMV) in vitro to identify ganglionic neurons as arterial, venous or unlabeled neurons. The distribution of primary sensory nerve fibers containing calcitonin gene-related peptide (CGRP), neuronal nitric oxide synthase (NOS) and substance P immunoreactivity (SP-IR) was compared before and after treatment with capsaicin. In control animals the density of immunoreactivity varied both with the transmitter and the type of neuron innervated. The density of immunoreactivity for all the three substances was reduced by capsaicin treatment. The degree of reduction of immunoreactivity in the fibers varied with the transmitter and the type of neuron. The density of CGRP and SP immunoreactive fibers was greatest around unlabeled neurons; 78% of the CGRP fibers were of primary sensory origin and all of the SP fibers were primary sensory. Around arterial neurons 44% of the CGRP fibers were of primary sensory origin and around venous 68% were primary sensory. NOS positive innervation around venous neurons was denser than around arterial neurons and all of it was completely (97%) eliminated by capsaicin, indicating that it was solely of primary sensory origin. We conclude that the primary sensory fibers innervating the IMG are differentially distributed to arterial and venous neurons and that the pattern of distribution is characteristic for each sensory neurotransmitter.

Neurons in the chicken ureter are innervated by substance P- and calcitonin gene-related peptide-containing nerve fibres: immunohistochemical and electrophysiological evidence

Numerous ganglia or single neurones immunoreactive to protein gene-product 9.5 (PGP) were demonstrated in the chicken ureter. Ganglia were observed in the main nerve trunks accompanying the ureter (400-2,000 cells), in the adventitia (1-45 cells; density; 79 +/- 12 ganglia/cm2; mean +/- S.E.M.), in the circular muscle (1-9 cells; 76 +/- 10 ganglia/cm2) and in the longitudinal muscle (1-8 cells; 232 +/- 41 ganglia/cm2). Most of the PGP-positive neurones in the nerve trunk ganglia (approximately 66%) and in the smooth muscle layers (85%) were encircled by a dense plexus of varicose nerve fibres containing both substance P (SP) and calcitonin gene-related peptide (CGRP). SP-positive somata were rarely observed. Immunogold electron microscopy revealed that SP- and CGRP-immunoreactivity were colocalised in the same dense core vesicles. A strong reduction of SP-positive nerve fibres was observed in organ cultures of the ureter, indicating their extrinsic origin. The fibres might originate from the dorsal root ganglia, where SP and CGRP were colocalised in 20-30% of the neurones. The sensitivity of ureteric neurones to SP and CGRP was investigated in recordings obtained from mechanosensitive nerve fibres with cell bodies located in or adjacent to the ureter (U-G units). The majority (71%) of the U-G units was excited by local application of SP in a dose-dependent manner. The SP-sensitive U-G neurones had higher mechanical thresholds (29 +/- 5 mmHg) as opposed to the SP-insensitive ones (10 +/- 3 mmHg). Repeated applications of high doses of SP to the U-G units resulted in desensitisation and reduced the response to mechanical stimuli. None of the U-G units responded to local application of CGRP, but all U-G units were excited by acetylcholine. The data support the hypothesis that SP-containing primary afferents are involved in the modulation of the activity of ureteric neurons in the chicken.

Characterization of the peripheral action of neuropeptide K on the rat cardiovascular system

The effects of neuropeptide K (NPK) were measured on mean arterial pressure (MAP) and heart rate (HR) after i.v. injection in urethane-anesthetized rats. NPK (6.5 and 32.5 nmol/kg) produced sustained decreases in MAP and elicited increases in HR. Whereas the NPK-induced tachycardia lasted more than 30 min at 32.5 nmol/kg, a latent and long-lasting bradycardia appeared from 20 min after injection of 6.5 nmol/kg. The initial tachycardia was converted to bradycardia by metoprolol but remained unaffected by hexamethonium, atropine and naloxone. These four treatments, however, prevented the bradycardiac response to NPK at 30 min. Whereas phentolamine, idazoxan, bilateral adrenalectomy and chemical sympathectomy with 6-hydroxydopamine (6-OHDA) preserved the initial tachycardia induced by NPK, they converted the decrease in HR to a tachycardiac response at 30 min. The vasodepressor response to NPK was significantly enhanced by bilateral adrenalectomy, chemical sympathectomy and metoprolol but remained unaffected by all other treatments. Neither the MAP nor the HR responses to NPK were affected by indomethacin. These results suggest that NPK can accelerate HR through non-reflex activation of the sympathoadrenal system. The secondary bradycardia induced by NPK may be due to a vagal reflex while the vasodepressor response to NPK is probably attributable to a direct action mediated by specific receptors on arterial blood vessels. Thus, NPK is considered as the most potent biologically active tachykinin so far described on the rat cardiovascular system.