Stress exacerbates neuron loss and microglia proliferation in a rat model of excitotoxic lower motor neuron injury.

All individuals experience stress and hormones (e.g., glucocorticoids/GCs) released during stressful events can affect the structure and function of neurons. These effects of stress are best characterized for brain neurons; however, the mechanisms controlling the expression and binding affinity of glucocorticoid receptors in the spinal cord are different than those in the brain. Accordingly, whether stress exerts unique effects on spinal cord neurons, especially in the context of pathology, is unknown. Using a controlled model of focal excitotoxic lower motor neuron injury in rats, we examined the effects of acute or chronic variable stress on spinal cord motor neuron survival and glial activation. New data indicate that stress exacerbates excitotoxic spinal cord motor neuron loss and associated activation of microglia. In contrast, hypertrophy and hyperplasia of astrocytes and NG2+ glia were unaffected or were modestly suppressed by stress. Although excitotoxic lesions cause significant motor neuron loss and stress exacerbates this pathology, overt functional impairment did not develop in the relevant forelimb up to one week post-lesion. These data indicate that stress is a disease-modifying factor capable of altering neuron and glial responses to pathological challenges in the spinal cord.

Glucocorticoids and macrophage migration inhibitory factor (MIF) are neuroendocrine modulators of inflammation and neuropathic pain after spinal cord injury.

Traumatic spinal cord injury (SCI) activates the hypothalamic-pituitary-adrenal (HPA) axis, a potent neuroendocrine regulator of stress and inflammation. SCI also elicits a profound and sustained intraspinal and systemic inflammatory response. Together, stress hormones and inflammatory mediators will affect the growth and survival of neural and non-neural cells and ultimately neurologic recovery after SCI. Glucocorticoids (GCs) are endogenous anti-inflammatory steroids that are synthesized in response to stress or injury, in part to regulate inflammation. Exogenous synthetic GCs are often used for similar purposes in various diseases; however, their safety and efficacy in pre-clinical and clinical SCI is controversial. The relatively recent discovery that macrophage migration inhibitory factor (MIF) is produced throughout the body and can override the anti-inflammatory effects of GCs may provide unique insight to the importance of endogenous and exogenous GCs after SCI. Here, we review both GCs and MIF and discuss the potential relevance of their interactions after SCI, especially their role in regulating maladaptive mechanisms of plasticity and repair that may contribute to the onset and maintenance of neuropathic pain.

Opioid regulation of spinal cord plasticity: evidence the kappa-2 opioid receptor agonist GR89696 inhibits learning within the rat spinal cord.

Spinal cord neurons can support a simple form of instrumental learning. In this paradigm, rats completely transected at the second thoracic vertebra learn to minimize shock exposure by maintaining a hindlimb in a flexed position. Prior exposure to uncontrollable shock (shock independent of leg position) disrupts this learning. This learning deficit lasts for at least 24h and depends on the NMDA receptor. Intrathecal application of an opioid antagonist blocks the expression, but not the induction, of the learning deficit. A comparison of selective opioid antagonists implicated the kappa-opioid receptor. The present experiments further explore how opioids affect spinal instrumental learning using selective opioid agonists. Male Sprague-Dawley rats were given an intrathecal injection (30 nmol) of a kappa-1 (U69593), a kappa-2 (GR89696), a mu (DAMGO), or a delta opioid receptor agonist (DPDPE) 10 min prior to instrumental testing. Only the kappa-2 opioid receptor agonist GR89696 inhibited acquisition (Experiment 1). GR89696 inhibited learning in a dose-dependent fashion (Experiment 2), but had no effect on instrumental performance in previously trained subjects (Experiment 3). Pretreatment with an opioid antagonist (naltrexone) blocked the GR89696-induced learning deficit (Experiment 4). Administration of GR89696 did not produce a lasting impairment (Experiment 5) and a moderate dose of GR89696 (6 nmol) reduced the adverse consequences of uncontrollable nociceptive stimulation (Experiment 6). The results suggest that a kappa-2 opioid agonist inhibits neural modifications within the spinal cord.