Involvement of subtype 1 metabotropic glutamate receptors in apoptosis and caspase-7 over-expression in spinal cord of neuropathic rats
Introduction
Neuropathic pain is caused by an injury or dysfunction in the peripheral or central nervous system [1], [2], [3]. In neuropathic pain conditions, noxious stimuli are perceived as more painful (hyperalgesia), and normal, harmless stimuli may elicit pain (allodynia) [1]. The mechanisms that centrally control such spread of pain stem from neurochemical and functional changes, and therefore, neuropathic pain should be considered a neuropathological condition. Evidence for this classification is offered by several animal models of chronic pain showing that a sustained release of glutamate, cytokines and neurotrophic factors (i.e. neurokinins, BDNF, TNF-alpha, etc.) induces sensitization at both dorsal root ganglion neurons and second-order neurons in the dorsal horn [4], [5], [6]. Indeed, there is evidence that synaptic facilitation requires the release of neurokinins, glutamate, and tropomyosin-related kinase (TRK) family of neurotrophin receptors. Spinal N-methyl-d-aspartate (NMDA) glutamate receptor contributes to triggering intracellular signals that induce long lasting effects at the transcriptional level [3], [7], [8], [9], [10], [11], [12].
Moreover, not only central sensitization, but also a disinhibitory process associated either with excitotoxic-induced neuronal death [2], [10], [13], [14] or with a rearrangement in the pathophysiology of the endogenous antinociceptive pathway [15], have been suggested to be required for the induction of dorsal horn circuitry sensitization. Morphological studies suggested the occurrence of neuronal apoptosis in the spinal cord following peripheral nerve insult [16], [17]. Nevertheless, there is also evidence that astrocytes undergo apoptosis in the spinal cord of neuropathic rat [18]. We have shown that the occurrence of glutamate-dependent apoptosis may be an early and rapid event, as pro-apoptotic bax and bcl-xS mRNA levels increased in the spinal cord of rats within 2–3 days after sciatic nerve chronic constrictive injury (CCI) [19], [20].
Many studies have highlighted the involvement of metabotropic glutamate (mGlu) receptors in nociception control [21], [22], [23], [24], [25], [26]. Group I metabotropic glutamate receptors (mGluRs) (mGlu1 and 5) have been implicated in the processes of central sensitization and persistent nociception [27]. Treatment with selective antagonists for mGlu1 and mGlu5 receptors attenuated the development of mechanical allodynia and decreased extracellular glutamate in the spinal cord [28], [29]. We have previously shown that mGlu5 receptor blockade was transiently anti-allodynic and reduced spinal cord apoptosis in neuropathic rats [20], although an analysis of terminal caspase activation was not carried out in that study.
Caspases, a family of cysteine proteases, play a critical role in the execution phase of apoptosis and are responsible for many of the biochemical and morphological changes associated with apoptosis [30]. Procaspase-9 has been proposed as an initiator caspase that activates the effector caspases-3, -6, and -7 [31]. This apoptotic step is implicated in motorneuron degeneration produced by mutant superoxide dismutase-1 [32], in models of peripheral neuropathy [33], Alzheimer disease [34], [35], [36], Huntington disease [37], and neuropathic pain, suggesting a critical role for these proteases in numerous neurological conditions. To our knowledge, however, there is no study aimed at identifying the possible relationship between mGlu1 receptors, the activation of specific spinal caspases and the development of hyperalgesia/allodynia in neuropathic pain models.
In this study, we have addressed the issue of whether the blockade of mGlu1 receptors might modify early over-expression of pro-apoptotic and gliosis genes in the spinal cord dorsal horn, together with allodynia and hyperalgesia development by CCI of the sciatic nerve.
Section snippets
Animals
Male Wistar rats (Harlan, Udine, Italy) (250–300 g) were housed three per cage under controlled illumination (12:12 h light:dark cycle; light on 06.00 h) and environmental conditions (room temperature 20–22 °C, humidity 55–60%) for at least 1 week before the commencement of experiments. Rat chow and tap water were available ad libitum. Behavioural testing was performed before surgery to establish a baseline for comparison with post-surgical values. CCI and sham rats were assessed for thermal and
AIDA prevents thermal hyperalgesia and mechanical allodynia by 3 and 7 days CCI, JNJ16259685 prevents thermal hyperalgesia, but not mechanical allodynia
Post-surgery mean difference scores for thermal hyperalgesia were significantly lower for the CCI sciatic nerve groups at 3 and 7 days (Fig. 1A and B) than sham rats. The increased sensitivity on the CCI sides was not present on the contralateral sides (data not shown). Similarly, sciatic nerve CCI rats showed mechanical allodynia only on the ipsilateral sides of the nerve ligature at 3 and 7 days post-surgery (Fig. 1C and D). Administration of AIDA, a preferential mGlu1 receptor antagonist,
Discussion
It has been shown that increased expression of group I mGlu receptors parallels the development of thermal hyperalgesia and mechanical allodynia, and this change has been suggested to contribute to the development and maintenance of chronic central pain syndrome [22], [24], [29], [43]. This study provides molecular evidence that, in the model of sciatic nerve ligature in the rat [40], neuropathic pain is associated with marked activation of caspase-7 together with its transient increase in
Acknowledgements
This study was supported by grant from MIUR Italy (PRIN 2005). Dr. Kevin A. Roth was supported by NIH Grant (R01 NS35107).
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