Application of opioids to the brainstem activates a form of descending inhibition that is mediated by serotonin acting at 5-HT1A, 5-HT1B/1D and 5-HT2 receptors (Lo et al. 2003). However, in rat doses of morphine sufficient to activate this inhibition do not alter the firing patterns of physiologically identified serotonergic cells in the rostral ventromedial medulla (Gao et al. 1998), the source of most serotonergic inputs to the spinal cord. A possible explanation for this apparent paradox was offered by the observation that brain application of opioids causes the release of acetylcholine from the spinal cord (Gage et al. 2001), and that activation of spinal nicotinic receptors stimulates the release of 5-HT directly from spinal serotonergic terminals (Cordero-Erausquin & Changeux, 2001). We have investigated this idea indirectly by examining the effects of spinal application of the nicotinic receptor antagonist mecamylamine on fentanyl-evoked descending inhibition.
Nine rabbits were decerebrated under isoflurane (2-4 %)-N2O anaesthesia. Reflexes were evoked in the medial gastrocnemius (MG) muscle nerve by electrical stimulation of the ipsilateral sural nerve at an intensity sufficient to excite C fibres (125 times threshold). Neurograms were averaged and integrated by computer, and analysed in three time bands relative to the stimulus: 5-12 ms (phase 1), 12-100 ms (phase 2) and 100-250 ms (phase 3). Fentanyl was administered to the fourth ventricle in incrementing doses of 3, 7 and 20 µg kg-1 at intervals of 10 min to give a cumulative dose of 30 µg kg-1. At least 1 h after fentanyl administration, when reflexes had recovered to pre-drug values, mecamylamine was given intrathecally in a single dose of 300-600 µg. Forty to 60 min after mecamylamine administration, fentanyl was administered as before. Experiments were terminated by I.V. administration of 2 ml saturated KCL solution.
In the control state, the highest dose of fentanyl inhibited (Friedman’s ANOVA, P ▓le│ 0.01) phase 1, 2 and 3 reflexes to median values of 31 % (inter-quartile range, IQR, 25-41 %), 41 % (IQR 33-67 %) and 32 % (IQR 27-61 %) of pre-fentanyl levels, respectively. Mecamylamine had no significant effects on MG reflex responses per se, but significantly reduced the effects of the highest dose of fentanyl on the reflex (Wilcoxon tests, P < 0.04), so that phase 1, 2 and 3 reflexes were depressed to median values of 69 % (IQR 57-83 %), 80 % (IQR 59-90 %) and 74 % (IQR 45-101 %) of immediate pre-fentanyl values.
These data are consistent with the idea that acetylcholine is a mediator in the descending inhibition activated by opioids applied to the brain.
This work was supported by BBSRC.