MODULATION OF EXCITATORY AMINO-ACID RESPONSES IN RAT DORSAL HORN NEURONS BY TACHYKININS

1. In freshly isolated spinal dorsal horn (DH) neurons (laminae I-III) of the young rat, the effects of tachykinins (substance P, neurokinin A) on inward current induced by excitatory amino acids were studied under whole-cell voltage-clamp conditions. 2. When the cells were clamped to a holding potential of -60 mV, a simultaneous application of N-methyl-D-aspartate (NMDA) (10(-4) M) and substance P (SP) (2 x 10(-9)-10(-7) M) for 10 s reversibly enhanced (by 129.6 +/- 8.2%, mean +/- SE) the peak amplitude of the initial transient component of the NMDA-induced current in approximately 60% of the examined cells and reduced it (to 83.3 +/- 2.7%) in 27% of the cells. In addition, SP produced an increase (by 133.6 +/- 11.7%) or a small decrease (to 85.9 +/- 1.4%) in the steady-state component of the NMDA response. In difference to SP, a simultaneous application of NMDA (10(-4) M) and neurokinin A (NKA) (10(-10)-10(-7) M) reversibly suppressed (to 86.6 +/- 2.1%) the peak amplitude of the NMDA-induced current in 75% of the examined cells. 3. The NMDA-induced currents were modulated by tachykinins not only during the coadministration but up to 20 min after the removal of the peptide. SP potentiated the initial peak NMDA current by 147.9 +/- 8.1% in 78% of examined cells and decreased it (76.3 +/- 5.7%) in 11% of cells. The potentiating effect was concentration-dependent (range: 10(-11)-10(-8) M) and reversible, but it was reduced with repeated applications. In addition, SP increased (by 125.4 +/- 3.6%) or reduced (to 86.0 +/- 1.8%) the steady-state component of the NMDA response. 4. When the single DH neurons were exposed to SP or NKA for 30 s-7 min before the testing of the NMDA responses, tachykinins had two distinct effects on the peak amplitude of the transient component of the NMDA-induced current, consisting of an initial depression (SP: to 64.8 +/- 2.1%; NKA: to 76.3 +/- 4.4%) followed by a potentiation (SP: by 146.6 +/- 6.8%; NKA: by 178.4 +/- 35.2%). The enhancing effect in some cells lasted less-than-or-equal-to 1 h. 5. A claimed novel nonselective tachykinin antagonist, spantide II (10(-8) M) coadministered with NMDA (10(-4) M), slightly depressed the peak component of NMDA-induced current. In addition, it effectively blocked the SP-induced potentiation of the responses of DH neurons to NMDA. 6. The development of the SP enhancement of the NMDA-induced current is prevented in the presence of glycine. 7-Chlorokynurenic acid, a competitive antagonist at the glycine allosteric site of the NMDA receptor, led to a reestablishment of the SP effect. 7. When bis-(omicron-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) concentration in the pipette solution was increased from 1 to 10 mM, both the initial depressant effect and the late potentiation of the NMDA-induced current caused by SP was noticeably reduced. This result is consistent with a possibility that the SP enhancement of NMDA responses of DH neurons could result from a rise of [Ca2+]i and subsequent activation of Ca2+-dependent processes. 8. 4-beta-Phorbol-12,13-dibutyrate, an activator of protein kinase C, and forskolin, an activator of adenylate cyclase, applied for 1-4 min, mimic the effects of tachykinins by producing an initial depression followed by a marked and prolonged potentiation of the NMDA-induced current responses of isolated DH neurons. Staurosporine, the agent known to inhibit both protein kinase C and cyclic AMP-dependent protein kinase, reduced the tachykinin-caused potentiation of the NMDA response. These results suggest that in the rat spinal DH protein kinase C and the cyclic AMP system may both be involved in the regulation of postsynaptic NMDA receptor function. In addition, phosphorylation of the NMDA receptor-ion channel complex or a related protein by a staurosporine-sensitive protein kinase(s) may be involved in the mediation of the tachykinin effects. 9. SP (2 x 10(-10)-2 x 10(-6) M) applied for 30 s-2.5 min produced a small increase in the transient and sustained components of quisqualic acid-induced currents in a proportion of DH neurons. Pretreatment with SP or NKA (5 x 10(-10)-10(-8) M) for 1-5 min enhanced (SP: by 118.3 +/- 4.0%; NKA: by 134.0 +/- 12.2%) the amplitude of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced current in 42% of the tested cells. However, KA-induced currents appear to be little affected by tachykinins. 10. Our results suggest that tachykinins modulate the NMDA and the AMPA receptors, signaling function in a proportion of the rat spinal DH neurons. Possible mechanisms of action are discussed in relation to tachykinin-dependent regulation of excitatory amino acid-mediated primary afferent neurotransmission, including nociception.