Effect of enkephalin on the micturition cycle of the cat

The effect of the synthetic opiate {D-Ala2, Me-Phe4}-leu-enkephalin (DAMLE) on the micturition cycle of the cat was studied. In vivo assays were performed with young male cats under two different conditions: 1) decerebrated cats (D-cats), with an intercollicular transection of the brainstem, and 2) spinal cats (S-cats), with a spinal transection between C5-C6. In vitro studies were carried out on bladder strips taken from adult male cats. The D-cats showed two types of voiding patterns: the first type (I) was characterized by a smooth wave of pressure and an incomplete emptying of the bladder; the second type (II) began like the type I, but ended with a series of small contractions accompanied by small jets of liquid, resulting in the complete emptying of the bladder. DAMLE inhibited vesical contractions and completely inhibited voiding in D-cats at doses equal or superior to 250 μg/kg i.v.; no effect was noted with lower doses. Vesical contractions were hardly affected in S-cats, even at high doses (>350 μg/kg i.v.). DAMLE did not affect electrically induced contractions of isolated bladder strips. Naloxone not only antagonized the inhibitory effects of DAMLE, but also induced perse a contraction of the bladder. These results indicate that enkephalinergic neurons are involved in the central neural circuitry of the micturition cycle in the cat, with an inhibitory effect at the level of either the ascending spinal pathways or the pontine Barrington's center.