Impariment by 6-hydroxydopamine of locus coeruleus-induced monosynaptic potential in the spinal trigeminal nucleus

All experiments were performed on cats immobilized with gallamine. In the rostral part of spinal trigeminal nucleus (STN), single fiber action potential with a consistent and short latency was elicited by electrical stimulation of the locus coeruleus (LC). When 2 mg of 6-hydroxydopamine (6-OHDA) dissolved in 5% ascorbic acid solution was applied into the lateral ventricle, the STN field potential produced by LC stimulation was reduced in amplitude within 20 hr. The STN potential elicited by trigeminal nerve stimulation, however, was unaffected until 24 hr after 6-OHDA. The drug also blocked the inhibitory effect of LC conditioning stimulation on the STN potential elicited by trigeminal nerve stimulation, whereas it did not modify the inhibitory effect of conditioning stimulation of the sensory cortex on the STN potential. Ascorbic acid solution, a solvent, affected neither the STN potential by LC stimulation, nor the inhibitory effect of LC neurons on the STN potential elicited by trigeminal nerve stimulation. These results strongly suggest the existence of noradrenergic fiber from the LC to STN, through which inhibition of the STN neurons is produced.