Studies in the in vivo spinal cord have demonstrated the existence of oscillatory activity in the theta frequency (4-15 Hz) band (Lidierth & Wall, 1996). In hippocampal neurones in vitro, a novel protocol involving pressure ejection of a high molarity potassium solution has been utilised to induce transient oscillatory activity (Towers et al. 2000). The aim of the current study was to determine if theta frequency band oscillations could be induced by pressure ejection of potassium into the substantia gelatinosa in vitro, an area important for nociceptive processing. The ability of the putative gap junction blocker, octanol, to affect this oscillatory activity was assessed.

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 µm thickness were cut from lumbar segments and placed in an interface recording chamber perfused with artificial cerebrospinal fluid (ACSF) and equilibrated with 95 % O₂ and 5 % CO₂ at 32 °C. Extracellular AC-coupled field recordings were made from the substantia gelatinosa lamina of the spinal cord using glass microelectrodes (1-2 M&OHgr;) filled with ACSF. Application of high molarity potassium was achieved by pressure ejecting (10-20 ms) from a microelectrode (1-2 M&OHgr;) KCH₂SO₄ (1.5 M) which was placed close to the field recording electrode. Data are expressed as means ± S.E.M.

Pressure ejection of potassium evoked a field theta oscillation of 5-15 s duration and a mean peak frequency of 7.7 ± 0.1 Hz (n = 5). Application of the putative gap junction blocker, octanol (1 mM), significantly reduced the amplitude of the peak power of the spectral analysis (% reduction of 55 ± 16, P < 0.05, paired t test, n = 5) and the area (4-15 Hz, % reduction of 62 ± 15, P < 0.05, paired t test, n = 5). There was no significant effect of octanol on the peak frequency of the theta band oscillation (control 7.7 ± 0.1 Hz versus octanol 7.7 ± 0.1 Hz, P > 0.05, paired t test, n = 5).

These data demonstrate that theta band oscillations can be transiently induced in the substantia gelatinosa of the spinal cord in vitro using pressure ejection of high molarity potassium. This oscillatory activity may play a role in the somatosensory processing of nociceptive signals. A role for gap junctions is implicated in these theta oscillations although further studies will be required to assess the synaptic and non-synaptic contributory mechanisms.