Pharmacological characterisation of place escape/avoidance behaviour in the rat chronic constriction injury model of neuropathic pain

Usefulness of antidepressants for improving the neuropathic pain-like state and pain-induced anxiety through actions at different brain sites

Clinically, it is well known that chronic pain induces depression, anxiety, and a reduced quality of life. There have been many reports on the relationship between pain and emotion. We previously reported that chronic pain induced anxiety with changes in opioidergic function in the central nervous system. In this study, we evaluated the anxiolytic-like effects of several types of antidepressants under a chronic neuropathic pain-like state and searched for the brain site of action where antidepressants show anxiolytic or antinociceptive effects. Sciatic nerve-ligated mice exhibited thermal hyperalgesia and tactile allodynia from days 7 to 28 after nerve ligation. At 4 weeks after ligation, these mice showed a significant anxiety-related behavior in the light-dark test and the elevated plus-maze test. Under these conditions, repeated administration of antidepressants, including the tricyclic antidepressant (TCA) imipramine, the serotonin noradrenaline reuptake inhibitor (SNRI) milnacipran, and the selective serotonin reuptake inhibitor (SSRI) paroxetine, significantly prevented the anxiety-related behaviors induced by chronic neuropathic pain. These antidepressants also produced a significant reduction in thermal hyperalgesia and tactile allodynia. Moreover, the microinjection of paroxetine into the basolateral amygdala or cingulate cortex reduced anxiety-related behavior, and microinjection into the primary somatosensory cortex significantly attenuated thermal hyperalgesia. These findings suggest that serotonergic antidepressants are effective for treating anxiety associated with chronic neuropathic pain and may be useful for treating neuropathic pain with emotional dysfunction such as anxiety. Furthermore, SSRIs show anxiolytic and antinociceptive effects by acting on different brain regions.

Dopamine D1 and D2 receptor agonism enhances antinociception mediated by the serotonin and noradrenaline reuptake inhibitor duloxetine in the rat formalin test

Altered functioning of monoamine-containing pathways descending from supraspinal structures to the spinal dorsal horn contributes to injury-induced sensitization of nociceptive transmission. Antidepressant drugs as typified by the dual serotonin (5-HT) and noradrenaline reuptake inhibitor duloxetine attenuate various signs and symptoms of persistent pain in animals and humans. The current study assessed whether dopamine receptor agonists could further enhance the antinociceptive activity of duloxetine in an animal model of injury-induced central sensitization, the rat formalin test. Duloxetine (3-100 mg/kg, s.c.), the dopamine D(1) receptor agonist SKF-82958 (0.1-1 mg/kg, s.c.) and the dopamine D(2) receptor agonist quinpirole (0.003-0.1 mg/kg, s.c.) all significantly attenuated spontaneous nociceptive behaviours during the second phase of the test; duloxetine and quinpirole also attenuated nociceptive behaviours during first phase and interphase. These antinociceptive actions of SKF-82958 and quinpirole were selectively antagonized by SCH 23390 and eticlopride respectively. Remarkably, when completely inactive doses of duloxetine (3 mg/kg) and SKF-82958 (0.3 mg/kg) were combined, a marked attenuation of second phase nociceptive behaviours occurred (P<0.05 vs vehicle), indicative of analgesic synergy. Similarly, when an active antinociceptive dose of quinpirole (0.03 mg/kg, P<0.05 vs vehicle) was combined with an inactive dose of duloxetine (3 mg/kg), a potentiation of duloxetine-mediated antinociception was observed (P<0.001 vs vehicle). Taken together, these results suggest that antidepressant drugs that can enhance the activity of 5-HT, noradrenaline and dopamine neurotransmission within nociceptive pathways should provide a broader spectrum of antinociception than dual mechanism of action reuptake inhibitors in animal models of injury-induced persistent nociception.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Amygdala GABA-A receptor involvement in mediating sensory-discriminative and affective-motivational pain responses in a rat model of peripheral nerve injury

The contribution of the amygdala to neuropathic pain processing in animals has not been clearly acknowledged. To assess the relative contribution of amygdala GABA-A receptors in mediating sensory-discriminative and affective-motivational pain components, the GABA-A receptor agonist muscimol and the antagonist bicuculline (both 10-25 ng/microl) were administered by acute bilateral injection directly into the central amygdala in rats with a chronic constriction injury (CCI). Escape/avoidance behaviour reflecting the affective-motivational dimension of pain was measured using a light/dark chamber in combination with suprathreshold nociceptive stimulation, and was defined as a shift from the 'non-aversive' dark area of the chamber to the 'aversive' light area. Hindpaw mechanical allodynia and mechanical hyperalgesia thresholds reflecting the sensory-discriminative dimension of pain were determined prior to and following escape/avoidance testing. Muscimol administration into the amygdala attenuated escape/avoidance behaviour and reversed hindpaw mechanical hypersensitivity in CCI rats; the magnitude of reduction in escape/avoidance behaviour was 2- to 3-fold greater than mechanical allodynia. Surprisingly, administration of bicuculline also attenuated escape/avoidance behaviour but had no effect on nociceptive behaviours. The muscimol-induced reversal of hindpaw mechanical hypersensitivity was completely blocked by co-administration of bicuculline, in contrast to escape/avoidance behaviour. Motility behaviour was unaffected by injection of either drug as determined in the open field test. Thus, amygdala GABA-A receptors appear to play an important role in sensory and especially affective pain processing in neuropathic rats. Furthermore, after nerve injury reflex nociceptive behaviours appear to be under tonic control by descending inputs, which originate from or are modulated within the amygdala.