SYNAPTIC RESPONSES OF SUBSTANTIA-GELATINOSA NEURONS TO DORSAL COLUMN STIMULATION IN RAT SPINAL-CORD IN-VITRO

1. To study the mechanism of dorsal column stimulation-induced depress ion of nociceptive transmission in the spinal cord, synaptic responses evoked in dorsal horn neurones by dorsal column and dorsal root stimu lations were examined in a horizontal spinal cord slice of the adult r at. Intracellular recordings were made from substantia gelatinosa (SG) neurones. 2. All SG neurones examined received monosynaptic inputs an d/or polysynaptic inputs from both dorsal column and dorsal root. AS f ibres were probably responsible for the synaptic responses. The respon ses evoked by dorsal column stimulation were similar to those evoked b y primary afferent AS fibre stimulation. 3. Monosynaptic excitatory po stsynaptic potentials (EPSPs) evoked by dorsal column AS fibres were d epressed by 8-cyano-7-nitroquinoxaline-2,3-dione, suggesting that thes e fibres released L-glutamate or a related amino acid as a transmitter . 4. In 38 of 101 SG neurones, dorsal column stimulation evoked an ini tial EPSP followed by fast and/or slow inhibitory postsynaptic potenti als (IPSPs). These IPSPs reversed polarity at a membrane potential of -73 +/- 2 mV. The fast IPSPs observed in 16 of the SG neurones (42 %) that received inhibitory inputs were depressed by strychnine, while th e slow IPSPs observed in 22 SG neurones were depressed by bicuculline. In a few cells, a long-lasting slow IPSP with a much slower time cour se was detected; this IPSP was insensitive to strychnine and bicuculli ne, and reversed polarity at a membrane potential near -90 mV. 5. Repe titive stimulation of the dorsal column depressed the amplitude of mon osynaptic EPSPs evoked by dorsal root stimulation. 6. The responses of SG neurones to dorsal column stimulation had configurations and durat ions similar to responses to dorsal root stimulation, and may be media ted largely by the same AS fibres. However, a C fibre-mediated respons e could not be detected in SG neurones from dorsal column stimulation, although dorsal root stimulation could evoke C fibre-mediated monosyn aptic EPSPs in 18 of 88 SG neurones (20 %). 7. These observations sugg est that XG neurones receive abundant A delta but not C fibre inputs f rom the dorsal column and that dorsal column stimulation inhibits prim ary afferent transmission in the spinal cord both by reducing transmit ter release from primary AS fibres and by hyperpolarizing SQ neurones.