Proceedings of The Physiological Society

Cardiff University (2009) Proc Physiol Soc 17, C14

Oral Communications

Modulation of the monosynaptic H-reflex by the periaqueductal grey

S. Koutsikou1, L. Brock1, O. Ruscombe-King1, B. M. Lumb1, R. Apps1

1. Department of Physiology & Pharmacology, University of Bristol, Bristol, Bristol, United Kingdom.

Outputs from the different functional columns in the periaqueductal grey (PAG) evoke different patterns of co-ordinated motor responses associated with active or passive coping. Active coping is elicited by the dorsolateral/lateral (dl/l) PAG and enables an animal to escape a stressor. It is also characterised by increased mobility (flight or fight behaviours). In contrast, passive coping is elicited by the ventrolateral (vl) PAG and is characterised by cessation of movement and a tense posture. Very little is known about how the PAG influences spinal reflex circuits that contribute to the above repertoire of motor responses. As a first step, the aim of the present study was to examine the effects of neuronal activation in dl/lPAG versus vlPAG on motoneuronal excitability. Specifically, the H-reflex was used as a ‘motor readout’ to measure changes in α-motoneurone excitability during PAG activation. In alphaxalone-anaesthetised (i.v. Alfaxan, Jurox; adult rats (n=14), H-reflex responses were evoked in the hindlimb plantaris muscle by electrical stimulation of the left tibial nerve using subcutaneous needle electrodes inserted at the level of the ankle joint (stimulation rate was once every 6s and the stimulus duration was 0.5ms cf. Gozariu et al. 1998). The H-reflex responses were examined before and following neuronal activation with 50mM DL-homocysteic acid microinjections (max.100nl) in either the ipsilateral dl/l or vlPAG. Activation in the vlPAG significantly increased the amplitude of the H-reflex by 88±11.4% (P<0.001, n=9). In contrast, dl/lPAG activation did not alter the H-reflex significantly (P>0.05, n=5). In the same experiments the M response was used as a control for constancy of the electrical stimulus and for both dl/l and vl PAG activation the M response remained unchanged (P>0.05, n=5 and n=9, respectively). The data reveal differential descending control exerted by the PAG on the excitability of α-motoneurones, which we suggest could contribute to different coping behaviours co-ordinated by the vlPAG.

Where applicable, experiments conform with Society ethical requirements