CONTRIBUTION OF C-FIBER AFFERENT NERVES AND AUTONOMIC PATHWAYS IN THEURINARY-BLADDER TO SPINAL C-FOS EXPRESSION INDUCED BY BLADDER IRRITATION

Previous studies have revealed that chemical irritation of the urinary bladder and urethral mucosa increases the expression of the immediate -early gene, c-fos, in the lumbosacral spinal cord of the rat. The pre sent experiments were undertaken to determine whether drugs known to s uppress bladder reflex pathways or spinal nociceptive mechanisms would influence c-fos expression induced by chemical irritation of the lowe r urinary tract (LUT). Capsaicin (100 mg/kg subcutaneous (sc), 7 days prior to the experiment) which does not block bladder reflexes but doe s desensitize C-fiber afferents, reduced (89%) the number of Fos-posit ive cells in the lumbosacral spinal cord induced by acetic acid-induce d irritation of the LUT. Morphine (2.5 mg/kg, intravenous (iv)) or a l ow dose of baclofen, a GABA(B) agonist, both of which markedly suppres sed reflex bladder activity, did not alter spinal c-fos expression ind uced by LUT irritation. However, a larger dose of baclofen (10 mg/kg, iv) reduced by 45% the number of Fos-positive cells. Clonidine (200 mu g/kg, iv), an alpha(2) adrenergic agonist, depressed bladder reflexes but produced only a small decrease (25%) in c-fos expression in later al laminae V-VII of the cord. The ganglionic blocking agent, hexametho nium, which blocks autonomic but not afferent pathways to the LUT, dec reased c-fos expression by 50%. The results indicate that certain drug s can differentially affect reflex bladder activity and c-fos expressi on and that analgesic drugs which suppress somatic nociceptive pathway s do not necessarily affect the c-fos expression induced by visceral n ociceptive input.