The relationship between substance P release and the activation of its receptor in the spinal cord remains unclear. Substance P release is usually measured by radioimmunoassay, whereas the internalization of the neurokinin 1 (NK1) receptor has been used to assess its activation by noxious stimuli. Our objective was to compare substance P release and NK1 receptor internalization produced by capsaicin in rat spinal cord slices. Superfusion of the slices with capsaicin for 3 min produced a gradual increase in substance P release that peaked 3-7 min afterward, and then decreased to baseline levels. The concentration-response curve for capsaicin was biphasic, with concentrations above 10 microM producing significantly less release. The effective concentration for 50% of response (EC(50)) for capsaicin, calculated from its stimulatory phase, was 2.3 microM. However, the potency of capsaicin to elicit NK1 receptor internalization in the same slices was one order of magnitude higher (EC(50)=0.37 microM) in lamina I, probably because NK1 receptors become saturated at relatively low concentrations of substance P. The potency of capsaicin to produce internalization was progressively lower in lamina III (EC(50)=1.9 microM) and lamina IV (EC(50)=14.5 microM), suggesting that neurokinins released in laminae I-II become diluted as they diffuse to the inner dorsal horn. To study the correlation between these two measures, we plotted substance P release against NK1 receptor internalization and fitted a saturation binding function to the points. The correlation was good for laminae I (R(2)=0.82) and III (R(2)=0.78), but it was poor (R(2)=0.35) for lamina IV because NK1 receptor internalization kept on increasing at high concentrations of capsaicin, whereas substance P release decreased. In conclusion, amounts of substance P able to activate NK1 receptors may fall under the threshold of detection of radioimmunoassay. Conversely, radioimmunoassay often detects levels of substance P release well over those required to saturate NK1 receptors in the superficial dorsal horn, but that may be able to activate these receptors in nearby regions of the spinal cord.