Immunoelectron microscopic technique was used in the present study to observe the μ-opioid re- ceptor-like immunoreactive (MOR-LI) ultrastructures in the medullary subnucleus reticularis dorsalis (SRD) of the rat. The immunoreactive products were located on the surface of membranous cell or- ganelles

or on the surfaces of the vesicles in the cytoplasm. Many dendritic profiles

especially the small dendritic profiles

were immunopositive. The most common synaptic type was asymmetric synapses made by non-immunoreactive a-conal terminals with MOR-LI dendrites. On the other hand

MOR-LI products were occasionally found in neuronal cell bodies and a-conal terminals. For this reason

it was rarely observed that the axo-dendritic synapses were made by MOR-LI a-conal terminals with immunoreactive or non-immunoreactive dendrites. However

we happened to find a axo-axonal synape made by MOR-LI a-conal terminals with non-immunoreactive axonal terminals. The present study provided a morphologic evidence for the involvement of MOR in opioid analgesia by both presynaptic and postsynaptic mechanisms in the rat SRD.