Imaging of spreading depression and anoxic depolarization in rat neocortical slice: Prevention by sigma(1) receptor agonists

Spreading depression (SD) is a profound but transient depolarization of neurons and glia that arises focally and migrates across the cortical and subcortical gray. In vivo it occurs under normoxic conditions during migraine aura where it precedes migraine pain but does not damage tissue. During focal ischemia, the anoxic depolarization (AD), which our laboratory has shown arises and propagates similar to SD, induces cell damage in an ischemic core. From here repeated SD-like events arise and may expand the core by promoting neuronal damage. Pharmacological blockade of AD and the subsequent damage has proven elusive. Sigma receptor ligands can be neuroprotective, reducing infarct size following focal ischemia, and reducing hypoxia or glutamate toxicity. This suggested a potential role for sigma receptors in the prevention of SD and AD.; Sigma receptors were first proposed to regulate psychomimetic behaviours in animal models of schizophrenia. While the physiological role of sigma receptors is unknown, several studies have implicated them in modulating immunosuppresant, antipsychotic and neuroprotective effects. These receptors differ from phencyclidine binding sites, NMDA and opioid receptors. Radioligand binding studies indicate that at least two subtypes of the σR exist with selective binding affinity of carbetapentane and dextromethorphan (DM) by σ₁ receptors. The neuroprotective properties of the therapeutically well-tolerated antitussive DM were of particular interest in this study.; In the rat neocortical slice preparation, our normoxic slice model of SD repeatedly generated robust SD during superfusion with elevated K+ aCSF. Intrinsic optical signals were monitored to determine the effect of various compounds on the generation and propagation of SD. Exposure of slices to the either O₂/glucose deprivation or metabolic inhibitors simulated global ischemia and induced AD to propagate across gray matter. In our focal ischemia model, slices were pre-incubated in 12 mM K+ to slightly depolarize the tissue, before 100 μM of the Na+/K+-ATPase inhibitor ouabain was focally applied to layers II/III with a micropipette. A ‘gradient of damage’ was revealed in the wake of the propagating signal, showing neuronal damage (reduced light transmittance) in the most metabolically compromised tissue near the drop site and dissipating in the unstressed tissue distant to the drop site. The partially compromised tissue between damaged and undamaged regions, we propose, represents a penumbra-like region.; Our IOS imaging demonstrates that SD and AD are focally initiated and propagating phenomena whose outcome is dependent upon the metabolic state of the tissue through which they migrate. Our results indicate that activation of sigma₁ receptors prevents SD or AD in rat neocortex independent of activity at NMDA receptors. The underlying implications that sigma, receptor activation may prevent the development of migraine aura and subsequent pain, while at the same time being neuroprotective against ischemic damage during stroke has important implications. It is significant that both SD and AD are essentially shut down by DM, a well tolerated therapeutic drug.