Pregabalin suppresses spinal neuronal hyperexcitability and visceral hypersensitivity in the absence of peripheral pathophysiology

Abstract

Background: Opioid-induced hyperalgesia is recognized in the laboratory and the clinic, generating central hyperexcitability in the absence of peripheral pathology. We investigated pregabalin, indicated for neuropathic pain, and ondansetron, a drug that disrupts descending serotonergic processing in the central nervous system, on spinal neuronal hyperexcitability and visceral hypersensitivity in a rat model of opioid-induced hyperalgesia.

Methods: Male Sprague-Dawley rats (180-200 g) were implanted with osmotic mini-pumps filled with morphine (90 μg · μl⁻¹ · h⁻¹) or saline (0.9% w/v). On days 7-10 in isoflurane anesthetized animals, we evaluated the effects of (1) systemic pregabalin on spinal neuronal and visceromotor responses, and (2) spinal ondansetron on dorsal horn neuronal response. Messenger ribonucleic acid concentrations of α2δ-1, 5HT3A, and μ-opioid receptor in the dorsal root ganglia of all animals were analyzed.

Results: In morphine-treated animals, evoked spinal neuronal responses were enhanced to a subset of thermal and mechanical stimuli. This activity was attenuated by pregabalin (by at least 71%) and ondansetron (37%); the visceromotor response to a subset of colorectal distension pressures was attenuated by pregabalin (52.8%; n = 8 for all measures, P < 0.05). Messenger ribonucleic acid concentrations were unchanged.

Conclusions: The inhibitory action of pregabalin in opioid-induced hyperalgesia animals is neither neuropathy-dependent nor reliant on up-regulation of the α₂δ-1 subunit of voltage-gated calcium channels-mechanisms proposed as being essential for pregabalin's efficacy in neuropathy. In opioid-induced hyperalgesia, which extends to colonic distension, a serotonergic facilitatory system may be up-regulated, creating an environment that is permissive for pregabalin-mediated analgesia without peripheral pathology.

Fig. 1

Altered behavioural activity and response profile of dorsal horn wide dynamic range spinal neurons following sustained morphine exposure in rats. Results are presented as mean ± standard error of the mean. Following 5 – 8 days of chronic morphine- (n = 20) or saline- (n = 16) treatment, hind paw withdrawal frequencies to von Frey (vF, g) filaments of increasing force were recorded (A). Following 7 – 10 days, the evoked neuronal responses to electrical (B), mechanical (C) and thermal (D) stimuli were recorded in naïve (n = 27), saline- (n = 12) and morphine-treated (n = 17) animals. Sustained morphine exposure increased the excitability of spinal neurons to peripherally applied natural stimuli. Based on latency measurements neuronal responses were subdivided into Aβ-, Aδ- and C-fibres, or post-discharge. See methods for further details. Significant differences from saline group baseline responses: * P < 0.05, ** P < 0.01, ***P < 0.001.

Fig. 2

The effect of systemic pregabalin (10 mg/kg and 30 mg/kg) on the mean baseline response values of dorsal horn wide dynamic range spinal neurons in saline- and morphine-treated animals (both groups n = 8). Results are presented as mean ± standard error of the mean. On post-operative days 7 - 10 the evoked neuronal response to electrical (A), mechanical (B and C) and thermal (D and E) stimuli were recorded before and after drug treatment in both animal groups. In morphine- but not saline-treated animals, high dose pregabalin (30 mg/kg) reduced a subset of electrically evoked responses while low dose pregabalin (10 mg/kg) reduced the evoked response of spinal neurons to peripherally applied noxious natural stimuli. Based on latency measurements neuronal responses were subdivided into Aβ-, Aδ- and C-fibres, or post-discharge. See methods for further details. Significant differences from morphine baseline responses: * P < 0.05, ** P < 0.01.

Fig. 3

Quantification of α₂δ-1, 5HT3A and mu opioid receptor 1 messenger RNA levels in morphine- and saline-treated animal dorsal root ganglia (both groups n = 8) on postoperative days 7-10 normalised to the mean of the saline group for the corresponding 3 measures (messenger RNA level = 100%). Results are presented as the mean percentage of control value ± standard error of the mean. There was no significant difference between groups (P > 0.05).

Fig. 4

The effect of spinal ondansetron (100 μg) and subsequent systemic administration of pregabalin (10 mg/kg) on the mean baseline response profiles of dorsal horn wide dynamic range spinal neurons in morphine-treated animals (n = 8). Results are presented as mean ± standard error of the mean. On post-operative days 7 - 10 the evoked neuronal response to electrical (A), mechanical (B) and thermal (C) stimuli were recorded before and after drug treatment. Ondansetron reduced a subset of electrically evoked responses and the evoked response of spinal neurons to peripherally applied noxious natural stimuli. Pregabalin (10 mg/kg) did not further reduce the evoked neuronal response to electrical or natural stimuli. Based on latency measurements neuronal responses were subdivided into Aβ-, Aδ- and C-fibres, or post-discharge. See methods for further details. Significant differences from morphine baseline responses: * P < 0.05, *** P < 0.001.

Fig. 5

Altered visceromotor responses (VMR’s) at noxious colorectal distension (CRD) pressures in morphine-treated animals. Results are presented as mean ± standard error of the mean. On postoperative days 7 - 10, the evoked VMRs to a range of innocuous and noxious CRD pressures were recorded in saline- (n = 8) and morphine-treated (n = 9) animals. Sustained morphine exposure increased the evoked VMRs to noxious CRD pressures. Significant differences from saline group baseline responses: * P < 0.05, ** P < 0.01, ***P < 0.001.

Fig. 6

The effect of systemic administration of pregabalin (30 mg/kg) on the visceromotor responses (VMR’s) of saline- (n = 8) and morphine-treated (n = 9) animals. Results are presented as mean ± standard error of the mean. On postoperative days 7 - 10, the evoked response to innocuous and noxious colorectal distension (CRD) pressures were recorded before and after drug treatment. Pregabalin (30 mg/kg) reduced the VMR to noxious CRD pressures in saline- (A) and morphine-treated (B) animals. After 20 min, pregabalin normalised the elevated response in morphine-treated animals to that of the VMR in saline-treated animals. The VMR’s were not significantly increased or decreased 60 min after pregabalin treatment (C). Significant differences from baseline responses: *^/^ P < 0.05, **^/^^ P < 0.01, ***^{/^^^} P < 0.001. * denotes significant difference from control following 20 min pregabalin treatment. ^ denotes significant difference from control following 60 min pregabalin treatment.