Ultrastructural and neurochemical analysis of synaptic input to trigemino-thalamic projection neurones in lamina I of the rat: a combined immunocytochemical and retrograde labelling study.

The synaptology of lamina I thalamic projection neurones in the spinal trigeminal nucleus of the rat was investigated by combining electron microscopic immunocytochemistry with the retrograde transport of horseradish peroxidase. Fifteen retrogradely labelled neurones were serially sectioned and their dendrites were traced for up to 160 microns in order to characterise the synaptic input to their cell bodies and proximal dendrites. Projection neurones receive synapses from dome-shaped substance P and enkephalin immunoreactive terminals, which make simple axosomatic or axodendritic synapses. In addition, the cells receive synapses from numerous nonimmunoreactive terminals including a wide range of different dome-shaped terminals and various scalloped or glomerular terminals. Dome-shaped terminals synapse with small stubby spines in addition to cell bodies or dendritic shafts and they are probably derived from lamina II interneurones and from descending bulbospinal pathways. Glomerular terminals occur in two main classes: small type A terminals with dark axoplasm and larger type B terminals. Type B terminals participate in synaptic triads in which a peripheral terminal synapses both axoaxonically with the glomerular terminal and axodendritically with the projection neurone. Type A and type B terminals closely resemble the central terminals of spinal cord lamina II glomeruli and are probably derived from C and A delta I degrees afferent fibers. The results indicate that lamina I projection neurones are under pre- and postsynaptic control from diverse sources. Their complex synaptic organisation highlights the key role that such cells play in the rostrad transmission of somatosensory information.