Distribution of mu receptors in the ventral respiratory group neurons; immunohistochemical and pharmacological studies in decerebrate cats.

Immunoreactivity for mu receptors was investigated in 21 bulbar respiratory neurons, individually identified by intracellular recording and labeling with neurobiotin. In 14 of these neurons, effects of iontophoresed morphine were examined. Morphine hyperpolarized the membrane and decreased spike discharges in 4/6 augmenting inspiratory (aug-I), 4/5 postinspiratory (post-I) and 3/3 augmenting expiratory (aug-E) neurons. It had no effect on two aug-I and one post-I neurons. Strong immunoreactivity for mu receptor was detected in the soma and dendrites of 5/8 aug-I, 5/7 post-I and 6/6 aug-E neurons. In the remaining three aug-I and two post-I neurons that included cells unresponsive to morphine, weak immunoreactivity was detected only in the dendrites. These results demonstrated wide, but uneven, distribution of mu receptors in bulbar respiratory neurons and suggest their contribution to respiratory depression by opioids.