Amino acid-mediated EPSPs at primary afferent synapses with substantia gelatinosa neurones in the rat spinal cord.
1. Fast excitatory postsynaptic potentials (EPSPs) evoked by stimulation of A delta and C fibres were examined by intracellular recording from substantia gelatinosa (SG) neurones in a transverse slice preparation of adult rat spinal cord. 2. Single low-intensity stimuli applied to the dorsal root activated A delta fibres and evoked monosynaptic EPSPs in 70% of SG neurones. In 5% of SG neurones, increasing the intensity and duration of stimulation evoked solely C fibre-mediated EPSPs. About 20% of neurones received both A delta and C fibre input from primary afferents. 3. Low concentrations of tetrodotoxin (TTX, approximately 50 nM) blocked EPSPs evoked by stimulation of A delta fibres without affecting those evoked by C fibre stimulation. Higher concentrations of TTX (500 nM) also blocked C fibre-evoked responses. 4. EPSPs evoked by A delta and C fibre stimulation reversed in polarity at membrane potentials near 0 mV, similar to the reversal potential of spontaneous EPSPs and of the potential change evoked by exogenous glutamate. 5. A delta and C fibre-evoked EPSPs were depressed by kynurenate and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); C fibre-evoked EPSPs appeared to be less sensitive. 6. In the presence of TTX, only 50% of SG neurones were depolarized by L-glutamate. However, neurones which exhibited no direct response to L-glutamate received afferent-evoked EPSPs which were sensitive to CNQX. In sensitive neurones, the depolarization evoked by L-glutamate was depressed by only approximately 15% in the presence of CNQX, whereas afferent-evoked EPSPs recorded from the same neurone were almost completely suppressed. Combined application of DL-2-amino-5-phosphonovaleric acid (APV) and CNQX depressed the response to L-glutamate by only approximately 25%. 7. These findings suggest that A delta and C fibres use L-glutamate or a related amino acid as a transmitter at synapses with substantia gelatinosa neurones. The postsynaptic actions of this transmitter are mediated predominantly by non N-methyl-D-aspartic acid (NMDA) receptors. The failure of CNQX and APV to completely block the L-glutamate-evoked depolarization of substantia gelatinosa neurones raises the possibility that exogenously applied L-glutamate activates a non-NMDA receptor distinct from that which mediates the actions of the synaptically released afferent transmitter.
Yoshimura
M
M
Howard Hughes Medical Institute, Columbia University, New York, NY 10032.
Jessell
T
T
D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D000200:Action Potentials / Q000187:drug effects; D000596:Amino Acids / Q000494:pharmacology*; D000818:Animals; D066298:In Vitro Techniques; D008297:Male; D009412:Nerve Fibers / Q000502:physiology; D009413:Nerve Fibers, Myelinated / Q000502:physiology; D009474:Neurons / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013116:Spinal Cord / Q000502:physiology*; D013376:Substantia Gelatinosa / Q000502:physiology*; D013569:Synapses / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
D000596:Amino Acids; D011810:Quinoxalines; D013779:Tetrodotoxin; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015763:2-Amino-5-phosphonovalerate