Involvement of the noradrenergic system in modulating the blink reflex in humans

Several researches have shown that the spinal reflex transmission in animals, as well as humans, was inhibited by alpha(2)-agonists, due to a disfacilitation of tonic noradrenergic control on motoneuronal output. To understand better the mechanisms regulating certain aspects of motor activity, here we reinvestigated the possible role of noradrenergic systems in modulating reflex activity of the brainstem in humans. To this aim, blink reflex responses (R1 and R2) evoked by electrical stimulation of the supraorbital nerve were electromyographically recorded in healthy volunteers. Both R1 and R2 areas were measured at 10-min intervals before and after i.v. injection of alpha(2)-agonist clonidine (0.5 microg/kg). The substance induced consistent depression of R1, which reached its maximum 40 min after drug administration (-43% of the control values). Ipsilateral R2 area resulted little affected by clonidine (-15% at 50 min), whereas no effects were observed in contralateral R2. Blood pressure values were never altered by drug injections. These results, taken together with previous observations, support the hypothesis that alpha(2)-agonist substances may cause a transient inactivation of noradrenergic neurons, thus releasing neurons involved in the circuitry of the blink reflex from a facilitatory drive. Since clonidine differentially modulated blink reflex responses, it is likely to assume that such a disfacilitation concerns mostly pontine units mediating the R1. However, the complexity of clonidine's effects at multiple pre- and postsynaptic sites does not allow us to exclude that other systems are involved in the alpha(2)-mediated control of facial motoneurons.