Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, PC66
Stimulation of the periaqueductal grey modulates cortical somatosensory evoked potentials elicited by an acute noxious stimulus.
E. K. Davies1, S. Koutsikou1, B. M. Lumb1, J. C. Murrell2
1. Department of Physiology and Pharmacology, University of Bristol, Bristol, United Kingdom. 2. Department of Clinical Veterinary Science, University of Bristol, Bristol, United Kingdom.
The periaqueductal grey (PAG) plays a key role in descending control of spinal nociception, and therefore the pain experience. It is functionally divided into longitudinal columns (including the dorsolateral (DL) and ventrolateral (VL) PAG) which, when activated, produce anti-nociception and distinct behavioural responses, characterised as active and passive coping strategies respectively (Lovick and Bandler, 2005). The effects of PAG stimulation at the level of the spinal dorsal horn have been widely investigated, but concurrent changes in cortical activity are unknown. The aim of this study was to investigate whether PAG stimulation modulates the cortical response to an acute noxious stimulus, using somatosensory evoked potentials (SEPs) as the outcome measure. DL- and VL-PAG were stimulated independently to identify whether these regions, known to cause distinct behavioural responses, also differentially modulate the cortical response to an acute noxious stimulus. Eight male Wistar rats (280-320g), anaesthetised with an I.V. infusion of alfaxalone (~25mg.kg-1.hr-1) were studied. SEPs were recorded from four active dural electrodes, placed bilaterally over the primary somatosensory cortex (S1) and the vertex (Vx) (2.5mm caudal, 2.5mm lateral to bregma and 4.5mm caudal, 1mm lateral to bregma, respectively). Ground and reference electrodes were placed over left and right frontal sinuses respectively (10mm rostral, 1mm lateral to bregma). SEPs were evoked by noxious electrical tail stimulation, and amplitudes of a positive to negative waveform in the 10-30ms range were assessed before and after chemical stimulation of VL- or DL-PAG using DL-homocysteic acid (DLH; 150nl; 50mM in physiological saline saturated with pontamine sky blue dye to mark injection sites). DL-PAG stimulation significantly increased waveform amplitude recorded from both S1 and Vx (to peaks of 118.6±7.5% and 115.1±6.1% of baseline respectively, mean±S.E.M., both p<0.05, ANOVA + Tukey’s post-test, n=14, 8) with no significant difference between ipsilateral and contralateral recording electrodes (p>0.05, t-test, n=7, 4 for S1 and Vx respectively). Conversely VL-PAG stimulation did not significantly alter the waveform amplitude recorded from S1 (p>0.05, ANOVA + Tukey’s post-test, n=11), but significantly decreased the waveform amplitude recorded from Vx (75.0±6.3% of baseline, mean±S.E.M., p<0.01, ANOVA + Tukey’s post-test, n=6). These data show that stimulation of VL- and DL-PAG, to mimic activation occurring in response to environmental stressors, differentially modulate cortical responses to acute noxious stimuli, which may in turn affect the behavioural response.
Where applicable, experiments conform with Society ethical requirements