The α2c-subclass of adrenergic receptor mediates some of the antinociceptive actions of noradrenaline in the spinal cord. This receptor is present on axon terminals in the superficial dorsal horn. A series of double-labelling experiments for confocal microscopy was performed in the rat (Wistar) to investigate the relationship between the α2c -adrenergic receptor and each of seventeen chemical markers that label various types of axon terminal in the dorsal horn. Quantitative analysis revealed that α2c -adrenergic receptors are neither present on terminals of unmyelinated, myelinated or peptidergic primary afferents nor on descending noradrenergic or serotoninergic terminals, whereas they are present on terminals of spinal origin. α2c -adrenergic receptors are predominantly found on axon terminals of excitatory interneuronal populations and also to a lesser extent they are present on terminals of inhibitory interneurons, hi addition, the receptor is present on terminals that contain certain peptides, which indicates that subpopulations of interneurons possessing the α2c -adrenergic receptor can be differentiated on the basis of their peptidergic content. Electron microscopic analysis revealed that imnunoreactivity is predominantly associated with axon terminals that are presynaptic to dendrites while a small proportion of immunoreactive terminals formed axoaxonic synaptic arrangements. Experimental techniques were combined in order to investigate the relationship of terminals possessing α2c -adrenergic receptors with supraspinally projecting neurons. The techniques included retrograde labelling, multiple and sequential-immunolabelling, correlated confocal-electron microscopy and induction of the immediate early gene c-Fos by peripheral noxious stimulation. The findings indicated that axon terminals containing the α2c -adrenergic receptor densely innervate spinomedullary neurons that express the substance P receptor, neurokinin-1. The latter terminals are glutamatergic (excitatory) and form synapses with this type of neuron. In addition, a substantial number of neurokinin-1 projection neurons in lamina I that are responsive to peripheral thermal noxious stimulation, i.e. express c-Fos, receives innervation from axon terminals containing α2c - adrenergic receptors. The α2c -adrenergic receptor is also present in axon terminals in the lateral spinal nucleus. This nucleus is found in the rat and other rodents and contains projection neurons that are densely innervated by peptidergic varicosities. Double-labelling immunostaining experiments showed that α2c -adrenergic receptors are present on axon terminals of mainly excitatory interneurons but also of inhibitory interneurons, and frequently contain peptides. Electron microscopy revealed that terminals possessing the receptor are presynaptic to dendrites and somata of neurons in the lateral spinal nucleus. The involvement of lateral spinal nucleus neurons in nociceptive transmission and their relationship with axons that possess α2c -adrenergic receptors was investigated. By combining retrograde labelling of projection neurons with induction of c-Fos expression by peripheral noxious stimulation and multiple-immunolabelling, it was possible to identify NK-1 projection neurons in the lateral spinal nucleus that express c-Fos and to determine if such cells receive contacts from terminals possessing the α2c receptor. The results show that neurons in this nucleus are densely innervated by axons that possess the receptor and that a small proportion can be activated by thermal noxious stimulation. In conclusion, noradrenaline is likely to modulate nociceptive transmission by acting on terminals of interneurons that contain the α2c -adrenergic receptor in the superficial dorsal horn and also in the lateral spinal nucleus
