Noradrenaline-mediated inhibition by locus coeruleus of spinal trigeminal neurons

Inhibitory effects of conditioning stimulation of the locus coeruleus (LC) on the neuron activity in spinal trigeminal nucleus (STN) were investigated in gallamine-immobilized cats. Field potentials of STN and spike potentials of single relay neurons in STN were orthodromically elicited by ipsilateral alveolar nerve stimulation and antidromically by stimulation of contralateral medial lemniscus. Conditioning stimuli were applied to LC and sensory cortex (SC) at various C-T intervals.

In tracking experiments near the LC region, conditioning stimulation of LC itself produced the most pronounced decrease in amplitude of the STN field potentials. Orthodromic spikes of STN single neurons were significantly reduced by conditioning stimulation of LC as well as SC. In reserpine-treated animals, however, conditioning stimulation of LC failed to produce a decrease in the number of orthodromic spikes, while the inhibitory effect of SC conditioning stimulation remained unaffected. Under these circumstances, intravenous L-dopa and intraventricular noradrenaline reproduced an inhibitory effect of LC conditioning stimulation on orthodromic spike generation, while such an effect was not seen with either dopamine or serotonin. Antidromic spike was unaltered by any of these treatments. Histochemically, catecholamine fluorescence in LC was entirely eliminated after reserpine-treatment, but was restored after L-dopa injection. These results strongly suggest that noradrenaline released from the terminals of neurons originating in LC produces an inhibition of transmission in the STN relay neurons.