The relative efficacy of monopolar vs. bipolar electrodes in stimulation-produced analgesia

Focal brain stimulation (FBS) of the periaqueductal gray (PAG) produces reliable antinociception. The use of different electrode configurations alters the distribution of excitation as well as the locus of cells being stimulated, making it difficult to compare results across laboratories. This study compared the analgesic properties of bipolar electrodes delivering biphasic current and monopolar electrodes delivering either a biphasic or a monophasic current to the ventral PAG. Naloxone reversibility of the analgesia was also tested. Results indicate that biphasic current with either monopolar or bipolar electrodes is more likely to elicit analgesia than monophasic current using monopolar electrodes. Naloxone reversed the analgesia produced by the monopolar/monophasic model, but only attenuated the monopolar/biphasic FBS and did not affect the analgesia produced by the bipolar/biphasic configuration. Biphasic current delivered through bipolar electrodes results in the sequential activation of different cell populations. Use of bipolar electrodes may widen the distribution of excitation beyond that of either monopolar configuration. Thus, a wider neural field of excitation may cause a bleedover of the field of stimulation into two systems (one opiate and one nonopiate).