A study of the antinociceptive and toxicological effects of intrathecal dexmedetomidine and methoxamine in the rat

Abstract

Alpha (α)-adrenergic agonists represent a novel drug class of spinal analgesics and are most commonly used in combination with local anesthetics and opioids. Spinal noradrenergic antinociception is mediated primarily by α₂- adrenoceptors, although α₁-adrenoceptor-mediated antinociception has never been disproven. As demonstrated vasoconstrictors, however, there are concerns regarding the safety of the spinal administration of α₁- and α₂- agonists, alone and in combination. -- The purpose of this study was to determine if intrathecal (i.t.) methoxamine (MX) (α₁-agonist) potentiates i.t. dexmedetomidine (DX) (α₂- agonist)-induced antinociception, if i.t. DX, alone and in combination with MX, injected twice daily for four days, causes spinal neurotoxicity, and the effect of a sub-toxic combination of i.t. DX and MX on i.t. dynorphin-induced neurotoxicity. Male, Sprague-Dawley rats (300-400g) implanted with i.t. catheters (L1 termination) were used throughout. The i.t injection of dexmedetomidine (0.01-µg) produced dose-dependent antinociception in the tail flick (TF) and paw pressure (PP) tests (ED₅₀ = 45 and 252 ng, respectively). The addition of a fixed dose of MX (10 µg i.t.), which produced <5% maximum percent effect (MPE) in the TF test and was inactive in the PP test, significantly shifted the DX dose response curve to the left (ED₅₀= 8.1 ng; TF test and 10 ng; PP test) but did not prolong DX's duration of action. A fixed dose combination of DX (0.025 µg) + MX (10 µg) producing near maximal antinociception in the TF test and intermediate activity in the PP test, was near completely blocked by prazosin (10 µg i.t.) or Wyeth 27127 (0.5 µg i.t.). -- Repeated i.t. injections of high dose DX (10 µg) and DX+MX (10 µg each) produced sedation but no motor dysfunction, no inflammation, haemorrhage or necrosis of the spinal cord. Immunohistochemical studies revealed no damage to or loss of calcitonin-gene-related peptide immunoreactivity(CGRP-IR) in either the dorsal or ventral horns or substance P immunoreactivity (SP-IR) in the ventral horn as compared to vehicle-treated rats. In contrast, dynorphin A₁₋₁₃ (192 µg;120 nmol) produced: 1) immediate and irreversible hindlimb paralysis; 2) loss of the stepping and tail flick reflexes; and 3) delayed bladder and bowel dysfunction. Forty-eight hours after dynorphin, immunohistochemical examination revealed a marked depletion of CGRP-IR motor neurons in the lumbar ventral horn. CGRP-or SP-IR was unchanged in the dorsal horn. -- To assess the effect of i.t. DX and MX, alone and in combination, on dynorphin neurotoxicity, rats were pretreated with i.t. DX, MX, a combination of DX+MX (10 µg each) or saline 15 min before i.t. dynorphin A₁₋₁₃(192 µg;120 nmol). In saline pretreated rats, dynorphin produced the same effects as described above. Pretreatment with i.t. DX, MX or DX+MX attenuated the hindlimb paralytic effect of dynorphin. Twenty-four hours after injection, motor reflexes were preserved and accelerating rotarod (RR) scores were similar to controls (no dynorphin) in 4 out of 5 DX-, 3 out of 5 MX- pretreated and 4 out of 4 DX+MX-pretreated rats. Histological evaluation of the spinal cords obtained from these rats were consistent with these functional results, indicative of a neuroprotective effect. For DX and DX+MX pretreated rats, there was a corresponding decrease in rectal temperature (up to 3.4°C for DX alone). MX did not alter rectal temperature. These results of this study indicate that 1) a threshold dose of i.t. MX potentiates α₂-mediated antinociception in the rat; 2) repeated spinal administration of this drug combination has no detectable neurotoxic effect and 3) pretreatment with these drugs effects neuroprotection against i.t. dynorphin in the rat.