Abstract

Background and aims: Activation of corticotropin releasing factor 1 (CRF₁) receptors is involved in stress related responses and visceral pain, while activation of CRF₂ receptors dampens the endocrine and some behavioural stress responses. We hypothesised that CRF₂ receptor activation may influence visceral pain induced by colorectal distension (CRD) in conscious rats, and assessed the possible sites and mechanisms of action.

Methods: Male Sprague-Dawley rats were exposed to CRDs (60 mm Hg, 10 minutes twice, with a 10 minute rest interval). Visceromotor responses (VMR) were measured by electromyography or visual observation. Spinal (L6–S1) extracellular signal regulated kinase 1/2 (ERK 1/2) activation following in vivo CRD and CRF₂ receptor gene expression in the T13–S1 dorsal root ganglia (DRG) and spinal cord were determined. Inferior splanchnic afferent (ISA) activity to CRD (0.4 ml, 20 seconds) was assessed by electrophysiological recording in an in vitro ISA nerve-inferior mesenteric artery (intra-arterial)-colorectal preparation.

Results: In controls, VMR to the second CRD was mean 31 (SEM 4)% higher than that of the first (p<0.05). The selective CRF₂ agonist, human urocortin 2 (hUcn 2, at 10 and 20 μg/kg), injected intravenous after the first distension, prevented sensitisation and reduced the second response by 8 (1)% and 30 (5)% (p<0.05) compared with the first response, respectively. RT-PCR detected CRF₂ receptor gene expression in the DRG and spinal cord. CRD (60 mm Hg for 10 minutes) induced phosphorylation of ERK 1/2 in neurones of lumbosacral laminae I and IIo and the response was dampened by intravenous hUcn 2. CRD, in vitro, induced robust ISA spike activity that was dose dependently blunted by hUcn 2 (1–3 μg, intra-arterially). The CRF₂ receptor antagonist, astressin₂-B (200 μg/kg subcutaneously or 20 μg intra-arterially) blocked the hUcn 2 inhibitory effects in vivo and in vitro.

Conclusions: Peripheral injection of hUcn 2 blunts CRD induced visceral pain, colonic afferent, and spinal L6-S1 ERK 1/2 activity through CRF₂ receptor activation in rats.