Intrathecal amitriptyline acts as an N-methyl-D-aspartate receptor antagonist in the presence of inflammatory hyperalgesia in rats.

BACKGROUND

Amitriptyline and other tricyclic antidepressants exhibit high affinity binding to N-methyl-D-aspartate (NMDA) receptors in vitro and inhibit NMDA receptor activation-induced neuroplasticity in hippocampal slices. Because spinal NMDA receptor activation is believed to be central to generation and maintenance of hyperalgesic pain, the purpose of this study was to test whether intrathecal amitriptyline reduced inflammation-induced hyperalgesia in the rat.

METHODS

Rats were prepared with chronic lumbar intrathecal and femoral intravenous catheters and nociceptive threshold was assessed by hind paw withdrawal to a radiant heat stimulus. Rats received an injection of carrageenin in one hind paw followed by thermal paw withdrawal testing 3 hr later and intrathecal amitriptyline and/or intravenous morphine injection. In other rats, intrathecal NMDA injection was preceded by either intrathecal saline or 60 micrograms amitriptyline.

RESULTS

Intrathecal amitriptyline reversed thermal hyperalgesia in a dose-dependent manner, but had no effect on withdrawal latency of the contralateral, noninjected paw. Intrathecal phentolamine plus methysergide did not alter amitriptyline's effect, except at the lowest dose. Intravenous morphine increased paw withdrawal latency in both inflamed and control paws in a dose-dependent fashion, and morphine interacted additively with intrathecal amitriptyline to reverse hyperalgesia. Thermal hyperalgesia induced by NMDA was completely antagonized by intrathecal amitriptyline.

CONCLUSIONS

Amitriptyline and other tricyclic antidepressants have been demonstrated to exhibit modest activity against clinical neuropathic pain after systemic administration. These data suggest that more profound pain relief might be obtained by intrathecal administration. Amitriptyline reverses hyperalgesia in rats by a mechanism unrelated to monoamine reuptake inhibition, and likely due to NMDA receptor antagonism.