The analgesic effect of clinically used exogenous opioids, such as mor
phine, is mediated primarily through mu-opioid receptors(1-3), but the
function of the kappa-receptor in opioid analgesia is unclear. Althou
gh kappa-receptor agonists can produce analgesia(4,5), behavioural stu
dies indicate that kappa agonists applied intravenously or locally int
o the spinal cord antagonize morphine analgesia (see refs 4, 6 for rev
iews). As morphine, a primary mu agonist(1), also binds to kappa-recep
tors(7) and the analgesic effectiveness of morphine decreases with rep
eated use (tolerance), it is important to understand the mechanism for
the functional interaction between kappa- and mu-opioid receptors in
the central nervous system. Here we present in vitro electrophysiologi
cal and in vivo behavioural evidence that activation of the kappa-rece
ptor specifically antagonizes mu-receptor-mediated analgesia. We show
that in slice preparations of a rat brainstem nucleus, which is critic
al for the action of opioids in controlling pain, functional kappa- an
d mu-receptors are each localized on physiologically different types o
f neuron. Activation of the kappa-receptor hyperpolarizes neurons that
are activated indirectly by the mu-receptor. In rats, kappa-receptor
activation in this brainstem nucleus significantly attenuates local mu
-receptor-mediated analgesia. Our findings suggest a new cellular mech
anism for the potentially ubiquitous opposing interaction between mu-
and kappa-opioid receptors and may help in the design of treatments fo
r pain.