Effects of electrical stimulation of brainstem nuclei on dorsal horn neuron responses to mechanical stimuli in a rat model of neuropathic pain

ABSTRACT

The aim of the present study is to examine the brainstem sites where the electrical stimulation produces a suppression of dorsal horn neuron responses of neuropathic rats. An experimental neuropathy was induced by a unilateral ligation of L5-L6 spinal nerves of rats. Ten to 15 days after surgery, the spinal cord was exposed and single-unit recording was made on wide dynamic range (WDR) neurons in the dorsal horn. Neuronal responses to mechanical stimuli applied to somatic receptive fields were examined to see if they were modulated by electrical stimulation of various brainstem sites. Electrical stimulation of periaqueductal gray (PAG), n. raphe magnus (RMg) or n. reticularis gigantocellularis (Gi) significantly suppressed responses of WDR neurons to both noxious and non-noxious stimuli. Electrical stimulation of other brainstem areas, such as locus coeruleus. (LC) and n. reticularis paragigantocellularis lateralis (LPGi), produced little or no suppression. Microinjection of morphine into PAG, RMg, or Gi also produced a suppression as similar pattern to the case of electrical stimulation, whereas morphine injection into LC or LPGi exerted no effects. The results suggest that PAG, NRM and Gi are the principle brainstem nuclei involved in the descending inhibitory systems responsible for the control of neuropathic pain. These systems are likely activated by endogenous opioids and exert their inhibitory effect by acting on WDR neurons in the spinal cord.