Mv. Milanes et Ml. Laorden, EFFECTS OF U-50,488H WITHDRAWAL ON CATECHOLAMINERGIC NEURONS OF THE RAT VENTRICLE, British Journal of Pharmacology, 124(6), 1998, pp. 1060-1064
1 In the present study the changes in noradrenaline (NA) and dopamine
(DA) content and turnover during naloxone-induced withdrawal were anal
ysed in the right ventricle of rats chronically treated with the rc-ag
onist U-50,488H. 2 Rats were rendered tolerant by administration of U-
50,488H twice a day for 4 days. On the day of death the animals were i
njected with saline or naloxone (3 mg kg(-1), s.c.) to precipitate a w
ithdrawal syndrome. 3 After naloxone administration to U-50.488H- trea
ted rats we found neither behaviour signs of physical dependence nor c
hanges in the tissue content of noradrenaline (NA). However, naloxone
induced a decrease in both cardiac normetanephrine (NM) levels and NA
turnover. 4 Similarly, naloxone enhanced the dopamine content and decr
eased the 3,4-dihydroxyphenylacetic acid (DOPAC) concentration and dop
amine turnover. 5 Importantly and in contrast to mu-agonists, the pres
ent results demonstrate that U-50,488H withdrawal produced a decrease
in the NA and dopamine turnover, without behavioural signs of physical
dependence.