PLASTICITY IN THE MYENTERIC PLEXUS OF THE RAT ILEUM AFTER LONG-TERM SYMPATHECTOMY

To investigate the effect of chronic sympathectomy on the innervation of a tissue with an extensive intrinsic component, 1-week-old rat pups were treated with 50 mg/kg guanethidine for 3 weeks, a treatment show n to produce complete and long-lasting sympathectomy, and the ileum ex amined. Changes in the levels of noradrenaline, neuropeptide Y, calcit onin gene-related peptide, substance P and vasoactive intestinal polyp eptide in the external muscle layers containing the myenteric plexus o f the ileum were determined between 6 and 20 weeks of age. After sympa thectomy, noradrenalin levels were initially depleted (3% of age-match ed controls at 6 weeks, P<0.001, and 18% of age-matched controls at 12 weeks, P<0.001), but were not significantly reduced at 20 weeks (67% of age-matched controls). Such increases in noradrenaline content with time after sympathectomy did not occur in the mesenteric vein (levels in 20-week-old sympathectomized rats were 2% of the control values (P <0.001). In the myenteric plexus, catecholamine fluorescent nerve fibr es were seen in the 12-week-old sympathectomized rats, although tyrosi ne hydroxylase-immunoreactivity was absent. Guanethidine sympathectomy had no effect on the neuropeptide levels in 6-week-old rat ileum but there was a selective increase at 20 weeks; the levels of calcitonin g ene-related peptide and substance P were increased (x3, P<0.001 and x1 .6, P<0.05, respectively) while vasoactive intestinal polypeptide and neuropeptide Y levels were unchanged. Short-term sympathectomy (destru ction of sympathetic nerve terminals by acute 6-hydroxydopamine treatm ent) had no affect on noradrenaline or peptide levels in this tissue. The changes in innervation noted after long-term sympathectomy may rep resent a part of the adaptive response of the enteric nervous system w hich permits normal intestinal function in the absence of extrinsic ne uronal inputs.