The effects of dorsal root stimulation and of substance P (SP), neurokinin A (NKA), and calcitonin gene-related peptide (CGRP) on the basal release of 9 endogenous amino acids, including glutamate (Glu) and aspartate (Asp), have been investigated using the rat spinal cord slice-dorsal root ganglion preparation and high-performance liquid chromatography with fluorimetric detection. High-intensity repetitive electrical stimulation of a lumbar dorsal root produced a Ca2(+)- dependent increase in the basal release of Asp, Glu, glycine (Gly), serine (Ser), and threonine (Thr). Low concentrations of SP (2 x 10(-7) M) caused a selective increase in the rate of basal release of Glu, whereas higher concentrations (1–5 x 10(-6) M) produced, in addition, an increase in the basal release of Asp. The SP-induced increase of Glu persisted in the absence of external Ca2+, but the effect was blocked by (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP, an SP analog claimed to be an antagonist of the synthetic SP. NKA (5 x 10(-7) - 10(-6) M), a related tachykinin coexpressed with SP in primary sensory neurons, enhanced the basal release of Gly. CGRP (10(-7) M) caused a significant, largely Ca2(+)-independent increase in the basal release of Glu and Asp and a decrease in asparagine. SP and CGRP potentiated the electrically evoked release of Glu and Asp. Neonatal capsaicin treatment did not significantly alter the basal efflux of 9 endogenous amino acids from the spinal slices, but it prevented the dorsal root stimulation-evoked release of Asp, Glu, Gly, and Thr and the SP-induced increase in the basal release of Glu. However, the effect of CGRP was not significantly modified by the capsaicin treatment. These results indicate that tachykinins (SP and NKA) and CGRP are capable of modulating the basal and electrically evoked release of endogenous Glu and Asp, and these actions may provide an important mechanism by which the peptides contribute to the regulation of the primary afferent synaptic transmission. The enhancement of the basal and the dorsal root stimulation-evoked release of Glu and Asp by tachykinins and CGRP may have important physiological implications for strengthening the synaptic connections in the spinal dorsal horn.