Joint, muscle, and visceral pain: Neck muscle length does not modulate glutamate-evoked reflexes in rat neck and jaw muscles

Abstract

We have documented that the application of the inflammatory irritant mustard oil (MO) to the rat neck paraspinal muscles produces sustained but reversible increases in electromyographic (EMG) activity in neck and jaw muscles that is larger in stretched neck muscles than in non-stretched neck muscles. In this study we explored whether neck muscle length influences the effects of activating peripheral glutamate receptors, which have been previously shown to play a role in MO-evoked reflex activity. EMG activity from the deep neck, trapezius, and digastric muscles was recorded, rectified, and integrated (area under the curve, AUC) in male Sprague-Dawley rats under halothane anaesthesia. Three injections of 20μL 0.5M glutamate (a non-inflammatory excitatory amino acid) or vehicle (isotonic saline) were administered in each of three different sequences of neck position: (1) non-stretched, non-stretched, non-stretched, (2) non-stretched, stretched, non-stretched and (3) stretched, non-stretched, stretched. In the non-stretched neck position, the rat was placed in a straight body position and glutamate injected into the left deep neck muscles. In the stretched neck position the rat was rotated 45° to the right at the neck and glutamate injected into the lengthened left deep neck muscles. Glutamate evoked repeatable and significant increases in neck and jaw EMG activity but there was no significant difference in activity (mean AUC) in the different muscle length conditions (One Way Repeated Measures ANOVA p>0.05). Vehicle did not produce significant neck or jaw EMG activity. Application of glutamate into deep neck muscles evokes EMG activity in neck and jaw muscles that are not modulated by muscle length. This suggests that the modulatory effect of muscle length on neck muscle nociceptive reflexes may depend on the presence of an inflammatory muscle condition and not on the activation of peripheral glutamate receptors. Supported by CIHR MOP-43905 and the CMCC.