Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, PC62
Midbrain control of spino-olivary neurones: a role in passive coping behaviour?
J. L. Leith1, S. Koutsikou1, R. Apps1, B. M. Lumb1
1. Department of Physiology & Pharmacology, University of Bristol, Bristol, United Kingdom.
Studies of descending control of spinal nociception have focused primarily on sensory and autonomic functions and have neglected investigation of the control of pain pathways that convey inputs to motor control centres, such as the olivo-cerebellar system. This is a critical gap in our understanding of pain processing given that noxious stimuli drive powerful motor responses, which have important consequences for survival. The aim of the current study was to examine the effects of neuronal activation in the ventrolateral periaqueductal grey (vlPAG), which is a source of descending control and is implicated in coordinating passive coping behaviours, on responses of spino-olivary projection neurones in the dorsal horn to both innocuous and noxious peripheral stimulation. Experiments were carried out in alphaxalone-anaesthetised (Alfaxan, 15-30mg.kg-1.hr-1, i.v) male Wistar rats (280-300g; n=12). Extracellularly recorded antidromically activated (Lipski, 1981) spino-olivary neurones were characterised by their responses to peripheral stimulation and the effects of descending control following neuronal activation in the vlPAG were investigated. Application of low threshold (brush, tap, joint movement) and high threshold (noxious pinch) mechanical stimuli to receptive fields on the hindlimb revealed four classes of neurones (Class 1, n=1; Class 2, n=4; Class 3, n=3; Class 4, n=4; Menetrey et al, 1977). Responses of i) Class 2 and 3 neurones to noxious pinch (3.6N), ii) Class 1 and 2 neurones to innocuous pinch (0.6N) and iii) Class 4 neurones to joint movement were monitored before and after microinjection of an excitatory amino acid (50mM DL-homocysteic acid; 60-80nl) into the vlPAG at sites that evoked depressor responses. Activation of the vlPAG produced a significant decrease in noxious pinch-evoked responses of Class 2 and 3 neurones (to 35.6%, P<0.05, and 1.2%, p<0.01, of control respectively; Kruskal-Wallis). In contrast, activation in the vlPAG increased the responses of Class 2 spino-olivary neurones to innocuous pinch (to 123.5%; n=2). Interestingly, responses of Class 4 spino-olivary neurones to joint movement were significantly increased (to 216%, p<0.001; Kruskal-Wallis) following activation of the vlPAG. The data reveal differential descending control exerted by the vlPAG on responses of spino-olivary neurones to sensory stimuli of different behavioural significance, which could contribute to passive coping behaviours coordinated by the vlPAG.
Where applicable, experiments conform with Society ethical requirements