Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, PC65
Descending modulation of cool- and cold-evoked spinal nociception by the periaqueductal grey.
S. Koutsikou1, L. Leith1, B. Lumb1, R. Apps1
1. Physiology & Pharmacology, University of Bristol, Bristol, United Kingdom.
The physiology of cold somatosensation has received much recent interest, and many studies have investigated the peripheral mechanisms of cool and cold detection. However little is known of the central processing of cool and cold-evoked responses, nor whether these responses may be modulated by descending control systems that have profound effects on the processing of other sensory modalities. In particular, descending control that originates from the midbrain periaqueductal grey (PAG) is an important determinant of the pain experience. The current study aimed to further investigate the responses of spinal dorsal horn neurones to cool and cold stimuli in the rat and to determine whether their activity may be modulated by descending control from the PAG. Extracellular recordings were made from lumbar dorsal horn neurones with receptive fields on the hind limb in alphaxalone-anaesthetised (Alfaxan; 15-30mg.kg-1.hr-1, i.v.) male Wistar rats (280-300g; n=19). Cells were characterised according to their responses to low (brush, tap) and high (pinch) threshold mechanical stimulation applied to the receptive field of the cell and classified as class 1 (low threshold, n=3), class 2 (wide dynamic range, n=28) and class 3 (nociceptive-specific, n=6; Menetrey et al, 1977). Cells were then tested for responsiveness to thermal stimuli: acetone (‘cool’), ethyl chloride (‘cold’; both 1ml topically) and noxious heat (55°C water). In class 2 cells that responded to acetone and/or ethyl chloride, the effects of activation of the ventrolateral (vl)-PAG were tested. After 3 control responses to acetone or ethyl chloride (applied at 5min intervals), 60-80nl of D,L-homocysteic acid (DLH; 50mM in physiological saline saturated with pontamine sky blue dye to mark injection sites) was pressure injected under stereotaxic guidance into the vlPAG. Test responses to acetone or ethyl chloride were measured 10s after DLH injection and 3 further recovery responses to acetone or ethyl chloride were measured at 5 min intervals. Of the recorded units, 67% of class 1, 64% of class 2, and 33% of class 3 cells responded to acetone, and 100% of class 1, 68% of class 2, and 33% of class 3 cells responded to ethyl chloride. In class 2 cells, activation of the vlPAG by DLH significantly reduced ethyl chloride-evoked responses to 17±7% of control (mean±S.E.M.; n=8; p<0.05, Kruskal-Wallis). However acetone-evoked responses in class 2 cells were not significantly reduced (92±36%; n=7; p>0.05, Kruskal-Wallis) by activation of the vlPAG. The data show that cold-evoked responses in class 2 dorsal horn neurones, like other sensory modalities, can be modulated by descending control systems that originate in the PAG.
Where applicable, experiments conform with Society ethical requirements