Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC298

Poster Communications

Descending pathways influence sensitization of reflex responses in the rat

K. L. Dobson1, J. Harris1

1. Division of Animal Sciences, University of Nottingham, Loughborough, United Kingdom.

In decerebrate spinalized rabbits, application of the noxious chemical mustard oil (MO) anywhere on the ipsilateral (IL) hindlimb causes prolonged enhancement of reflexes evoked in the knee flexor semitendinosus and the ankle flexor tibialis anterior (TA). In non-spinal animals however, only MO applied to the plantar surface of the foot causes sensitization of flexor reflexes, indicating a descending inhibitory control of reflex sensitization fields that restricts sensitization to sites which make ground contact (Clarke & Harris, 2003). The present study investigated whether a similar differential pattern of reflex sensitization by MO could be found in rats, a species more widely used in pain research.<p>Experiments were performed on 74 rats decerebrated under nitrous oxide/isoflurane (2.25-2.8%) anaesthesia. In addition, 32 rats were spinalized at spinal level T9. Reflexes were evoked in the left knee flexor biceps femoris (BF) and TA by electrical stimulation of skin near the toes, whilst responses in the ankle extensor medial gastrocnemius (MG) were evoked by stimulation of skin at the heel. Responses were recorded as compound EMGs which were averaged and integrated by computer. Conditioning stimuli (5 µl 20% mustard oil), separated by intervals of at least 63 mins, were applied to the IL toe tips, metatarsophalangeal joints (MTJ), mid-sole, heel, dorsal MTJ, instep, ankle joint, knee and lateral gastrocnemius (LG) muscle; the contralateral heel and toes; and the snout and tail. No more than 4 stimuli were applied per animal. Statistical significance was assessed using repeated measures ANOVA and durations of change are expressed as a mean value (minimum n=6) ± SEM.<p>In decerebrate spinalized rats, facilitation of all three reflexes was only elicited by MO application to sites in the IL peritalar region i.e. heel, instep, ankle joint and LG; MO-induced inhibition of responses was absent. For example, MO applied to the heel augmented the heel-MG, toes-BF and toes-TA reflexes (all P<0.001) for periods of ≥22 mins. In decerebrate non-spinal rats potentiation of reflex responses was more restricted. Heel-MG responses were sensitized by MO application to the heel (P<0.03) whilst flexor responses were facilitated from more distal plantar sites. For example, MO applied to the MTJ increased toes-BF and toes-TA reflexes (P<0.03) for 31 ± 9 and 25 ± 7 mins respectively. In contrast to spinalized rats, inhibition of reflexes was observed following MO application to the IL hindlimb as well as off-limb sites.<p>The present data confirm that organization of sensitization of hindlimb reflexes is controlled by descending pathways. Their removal leads to a proximal shift in the sensitization fields of flexor reflexes which implies that both descending inhibitory and facilitatory pathways influence the pattern of MO-induced sensitization in the rat.

Where applicable, experiments conform with Society ethical requirements