Article date: August 1995
By: C.Q. Cao, R.H. Evans, P.M. Headley, P.M. Udvarhelyi, in Volume 115, Issue 8, pages 1469-1474
Whole cell synaptic currents were recorded under voltage clamp from a total of 54 ventral horn neurones held near to their resting potential by the patch clamp technique in immature rat spinal cord preparations in vitro. Twenty eight neurones were identified, by antidromic invasion from ventral roots, as motoneurones. Excitatory postsynaptic currents (e.p.s.cs) of peak amplitude −480 pA±66 s.e.mean and −829±124pA were evoked respectively from the unidentified ventral horn neurones and the motoneurones in response to maximal activation of the segmental dorsal root.
The e.p.s.cs were depressed reversibly by the metabotropic glutamate agonists lS3S‐l‐aminocyclo‐ pentane‐l, 3‐dicarboxylate (1S3S‐ACPD) (EC50 17.1μm±0.3 s.e.mean, n =14) and L‐2‐amino‐4‐ phosphonobutanoate (L‐AP4) (EC50 = 2.19±0.19 μm, n=15). Since both agonists independently produced more than 90% depression it is likely that the receptors that mediate their effects are present on the same presynaptic terminals.
When the Mg2+ concentration was raised from 0.75 mM to 2.75 mM together with the addition of 50 μm D‐2‐amino‐5‐phosphonopentanoate (AP5), a treatment which would increase the proportion of monosynaptic component in the e.p.s.c. the concentration‐effect plots for both 1S3S‐ACPD (EC50 1.95 ± 0.4 μm, n = 8) and L‐AP4 (EC50 0.55 ± 0.20 μm, n = 7) were shifted to the left, suggesting that monosynaptic e.p.cs of primary afferents to ventral horn neurones are more susceptible to L‐AP4 and 1S3S‐ACPD than are other synapses in polysynaptic pathways.
1S3S‐ACPD (20 and 50 μm) also caused mean sustained inward currents of 95 ±31 pA (n = 6) and 248 ±49 pA (n =10) respectively. In the combined presence of AP5 (50 μm) and Mg2+ (2.75 mM) the mean response to 50 μm 1S3S‐ACPD was reduced to 106± 18 pA (n = 4). In the presence of tetrodotoxin (1 μm) the corresponding value was 48 ±6 pA (n = 4). Similar sustained inward currents produced by N‐ methyl‐D‐aspartate (NMDA) were almost abolished to <10 pA in the presence of AP5 and 2.75 mM Mg2+. In the presence of tetrodotoxin the maximum inward current produced by NMDA was undiminished. Thus a large component of the excitatory action of 1S3S‐ACPD was mediated at non‐ NMDA receptors both directly at the patch‐clamped neurones and indirectly by synaptic relay.
Whole cell synaptic currents were recorded under voltage clamp from a total of 54 ventral horn neurones held near to their resting potential by the patch clamp technique in immature rat spinal cord preparations in vitro. Twenty eight neurones were identified, by antidromic invasion from ventral roots, as motoneurones. Excitatory postsynaptic currents (e.p.s.cs) of peak amplitude −480 pA±66 s.e.mean and −829±124pA were evoked respectively from the unidentified ventral horn neurones and the motoneurones in response to maximal activation of the segmental dorsal root.
The e.p.s.cs were depressed reversibly by the metabotropic glutamate agonists lS3S‐l‐aminocyclo‐ pentane‐l, 3‐dicarboxylate (1S3S‐ACPD) (EC50 17.1μm±0.3 s.e.mean, n =14) and L‐2‐amino‐4‐ phosphonobutanoate (L‐AP4) (EC50 = 2.19±0.19 μm, n=15). Since both agonists independently produced more than 90% depression it is likely that the receptors that mediate their effects are present on the same presynaptic terminals.
When the Mg2+ concentration was raised from 0.75 mM to 2.75 mM together with the addition of 50 μm D‐2‐amino‐5‐phosphonopentanoate (AP5), a treatment which would increase the proportion of monosynaptic component in the e.p.s.c. the concentration‐effect plots for both 1S3S‐ACPD (EC50 1.95 ± 0.4 μm, n = 8) and L‐AP4 (EC50 0.55 ± 0.20 μm, n = 7) were shifted to the left, suggesting that monosynaptic e.p.cs of primary afferents to ventral horn neurones are more susceptible to L‐AP4 and 1S3S‐ACPD than are other synapses in polysynaptic pathways.
1S3S‐ACPD (20 and 50 μm) also caused mean sustained inward currents of 95 ±31 pA (n = 6) and 248 ±49 pA (n =10) respectively. In the combined presence of AP5 (50 μm) and Mg2+ (2.75 mM) the mean response to 50 μm 1S3S‐ACPD was reduced to 106± 18 pA (n = 4). In the presence of tetrodotoxin (1 μm) the corresponding value was 48 ±6 pA (n = 4). Similar sustained inward currents produced by N‐ methyl‐D‐aspartate (NMDA) were almost abolished to <10 pA in the presence of AP5 and 2.75 mM Mg2+. In the presence of tetrodotoxin the maximum inward current produced by NMDA was undiminished. Thus a large component of the excitatory action of 1S3S‐ACPD was mediated at non‐ NMDA receptors both directly at the patch‐clamped neurones and indirectly by synaptic relay.
DOI: 10.1111/j.1476-5381.1995.tb16639.x
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