Microiontophoretic studies on noradrenergic inhibition from locus coeruleus of spinal trigeminal nucleus neurons.

Microiontophoretic studies using cats anesthetized with alpha-chloralose and local anaesthetic were undertaken in an attempt to determine whether or not norepinephrine (NE) derived from the locus coeruleus (LC) acts on neurons of the spinal trigeminal nucleus (STN) as an inhibitory transmitter. Both conditioning stimulation of the LC and iontophoretic application of NE inhibited orthodromic spike generation by interior alveolar nerve stimulation in 9 out of 14 STN relay neurons, without affecting antidromic spikes elicited by stimulation of the contralateral medial lemniscus. Glutamate-induced spike firing of the STN relay neurons, however, remained unaltered with iontophoretic application of NE up to 200 nA. The LC conditioning stimulation inhibited orthodromic spikes in 22 out of 25 type-B interneurons in the STN, while iontophoretic NE application seldom had an effect on orthodromic spikes and spontaneous firing of the interneurons. Only 5 out of the 25 interneurons exhibited an inhibition of orthodromic spike generation and spontaneous firing with iontophoretic NE. These neurons fired spikes upon orthodromic stimulation with a relatively short latency, compared with the spike latency of neurons unaffected by iontophoretic NE. The present results strongly suggest that NE released from the terminals of the LC neurons inhibits transmission of STN relay neurons, probably by acting on primary afferent terminals to produce presynaptic inhibition.