Serotonin innervation of physiologically identified lamina I projection neurons.

Physiologically characterized lamina I projection neurons, including antidromically activated spinomesencephalic and spinothalamic tract cells, were intracellularly stained with HRP and then processed and examined for serotonin-immunoreactive contacts. We observed cells with both high and low densities of contacts from serotonergic axons. Serotonin contacts were found on both nociceptive-specific and wide-dynamic-range projection neurons. The density of contacts did not appear to correlate with any physiological characteristic.