Whole cell patch-clamp recordings were made from substantia gelatinosa neurons in the thick slice of the adult rat spinal cord, which retained an attached dorsal root to study the pharmacological properties of spontaneous and primary afferent fibre-evoked synaptic currents. The majority of substantia gelatinosa neurons tested exhibited miniature excitatory postsynaptic currents in the presence of tetrodotoxin (0.5 μM). Stimulation of primary afferent Aδ fibres evoked monosynaptic and/or polysynaptic excitatory postsynaptic currents. In Mg²⁺-containing solution, 6-cyano-7-nitroquinoxaline-2,3-dione (10μM) abolished the evoked excitatory postsynaptic currents and the miniature excitatory postsynaptic currents. 2-Amino-5-phosphonovaleric acid (50–100μM) had little effect on the miniature excitatory postsynaptic current. In Mg²⁺-free solution, however, 6-cyano-7-nitroquinoxaline-2,3-dione reduced but did not abolish the miniature excitatory postsynaptic currents, leaving the miniature excitatory postsynaptic currents with a small amplitude and a slow time course, which were abolished by 2-amino-5-phosphonovaleric acid. At holding potentials more positive than −60 mV, stimulation of Aδ fibres evoked outward postsynaptic currents in 11 out of 28 substantia gelatinosa neurons. The evoked inhibitory postsynaptic currents were abolished in seven out of 11 neurons by either strychnine (0.5μM) or bicuculline (10 μM), and in the remaining four neurons by the combination of both antagonists.

These observations suggest that interneurons as well as Aδ fibres releasel-glutamate, which evokes the excitatory postsynaptic current predominantly interacting with the non-N-methyl-d-aspartate receptor of the substantia gelatinosa neurons, and that the glycinergic and GABAergic inhibitory neurons are also likely to be involved in nociceptive transmission in the dorsal horn.