Responses of rat dorsal horn neurons to natural stimulation and to iontophoretically applied norepinephrine.

Extracellular recordings were obtained of 177 neurons throughout the lumbar spinal dorsal horn of urethane- or halothane-anesthetized rats. These neurons all responded to iontophoretically applied L-glutamate and their responses to natural stimulation of the ipsilateral hindlimb were characterized. Iontophoretically applied norepinephrine was tested on 94 of these neurons. Fifty-one neurons were inhibited and 22 were excited. Norepinephrine produced a biphasic inhibitory/excitatory effect on nine neurons. Norepinephrine was exclusively inhibitory on superficial dorsal horn neurons that responded only to innocuous brush and touch and on neurons in the nucleus proprius that responded to brush, touch, and noxious skin pinch. Norepinephrine excited some superficial brush/touch/pinch neurons and produced short inhibitions that were followed by prolonged excitations of some nucleus proprius neurons that responded only to noxious skin pinch. Neurons in the base of the dorsal horn that responded to low-threshold proprioceptive stimulation were excited by norepinephrine. Both the inhibitory and excitatory effects of norepinephrine were stereoselective, but they were not blocked by receptor subtype-selective antagonists. Desensitization to norepinephrine occurred for 30% of the neurons. This study demonstrates that the inhibitory effects of norepinephrine on rat dorsal horn neurons are not restricted to neurons that are responsive to noxious stimuli and that some of these neurons are primarily excited by norepinephrine. The excitatory effects of norepinephrine on low-threshold proprioceptive neurons may contribute to norepinephrine's known enhancement of spinal flexor reflex activity.