Differential action of amitriptyline on neurons in the trigeminal nucleus.

We studied the effect of amitriptyline (AMI) on neurons in the spinal trigeminal nucleus caudalis in cats anesthetized with alpha-chloralose. The IV injection of 1.0 to 4.0 mg/kg AMI had a differential effect on the inhibitory mechanisms controlling the responses of these neurons. AMI significantly enhanced the segmental inhibition (SI) of wide dynamic range (WDR) neurons but had little or no effect on low-threshold mechanoceptive neurons. AMI also facilitated the SI of some nociceptive specific (NS) neurons and the periventricular inhibition of some WDR and NS neurons, but these effects were not statistically significant. Our observations suggest that AMI exerts its antineuralgic effect by enhancing the ability of SI to prevent excessive firing of WDR neurons. This supports the notion that neuropathic pain is caused by dysfunction of inhibitory mechanisms in the CNS.