Extracellular recordings in rat spinal nociceptive neurones in vivo by Sandkuhler & Eblen-Zajjur (1994) have shown oscillatory activity in the theta frequency range (6-12 Hz). We have previously demonstrated that theta frequency rhythms can be evoked in the substantia gelatinosa (a dorsal horn lamina important for nociceptive processing) by transient elevation of extracellular potassium (Asghar et al. 2002). GABA and glycine play an important inhibitory role in the spinal dorsal horn and several studies in hippocampal neurones have shown that inhibitory interneurons are important in generating network oscillations. Therefore, the present investigation aimed to determine whether antagonism of GABA_A or glycine inhibitory receptors could influence potassium-induced theta oscillations in the substantia gelatinosa.

Spinal cords were removed from Wistar rats (age 12-14 days) terminally anaesthetised with urethane (2 g kg^-1 I.P.). Transverse slices of 300 mm thickness were cut from lumbar segments and placed in an interface recording chamber perfused with artificial cerebrospinal fluid (ACSF) at 32 °C. Extracellular field recordings were made from the substantia gelatinosa lamina of the spinal cord using ACSF-filled glass microelectrodes (1-2 MΩ). Potassium (KCH₃SO₄, 1.5 M) was pressure ejected (10-20 ms duration) from a microelectrode (1-2 MΩ) placed close to the field recording electrode. Power spectra were analysed to reveal the mean peak frequency and amplitude in the 6-12 Hz band. The area under the peak was calculated between 6 and 12 Hz. Data are expressed as means ± S.E.M.

Brief pressure ejection of potassium evoked a field oscillation of 5-15 s duration which had a mean peak frequency of 7.3 ± 0.3 Hz (n = 12). Bath application of the GABA_A receptor antagonist, bicuculline (30 mM), significantly reduced the peak amplitude (% reduction of 81 ± 4, P < 0.005, paired t test, n = 6) and the area (% reduction of 76 ± 5, P < 0.01, paired t test, n = 6). The glycine receptor antagonist, strychnine (10 mM), also significantly reduced the peak amplitude (% reduction of 69 ± 9, P < 0.05, paired t test, n = 6) and the area (% reduction of 67 ± 8, P < 0.05, paired t test, n = 6). There was no apparent effect of either bicuculline or strychnine on the peak frequency.

These data demonstrate that theta frequency oscillations evoked in the substantia gelatinosa are partly dependent upon GABA_A and glycine inhibitory neurotransmission. This inhibitory neurotransmitter-dependent rhythmic activity could play a role in the somatosensory processing of nociceptive signals.

All procedures accord with current UK legislation.