Cortical spreading depolarization is a potential target for rat brain excitability modulation by Galanin

The inhibitory neuropeptide Galanin (Gal) has been shown to mediate anticonvulsion and neuroprotection. Here we investigated whether Gal affects cortical spreading depolarization (CSD). CSD is considered the pathophysiological neuronal mechanism of migraine aura, and a neuronal mechanism aggravating brain damage upon afflictions of the brain. Immunohistochemistry localized Gal and the Gal receptors 1-3 (GalR1-3) in native rat cortex and evaluated microglial morphology after exposure to Gal. In anesthetized rats, Gal was applied alone and together with the GalR antagonists M40, M871, or SNAP 37889 locally to the exposed cortex. The spontaneous electrocorticogram and CSDs evoked by remote KCl pressure microinjection were measured. In rat cortex, Gal was present in all neurons of all cortical layers, but not in astrocytes, microglia and vessels. GalR2 and GalR3 were expressed throughout all neurons, whereas GalR1 was preponderantly located at neurons in layers IV and V, but only in about half of the neurons. In susceptible rats, topical application of Gal on cortex decreased CSD amplitude, slowed CSD propagation velocity, and increased the threshold for KCl to ignite CSD. In some rats, washout of previously applied Gal induced periods of epileptiform patterns in the electrocorticogram. Blockade of GalR2 by M871 robustly prevented all Gal effects on CSD, whereas blockade of GalR1 or GalR3 was less effective. Although microglia did not express GalRs, topical application of Gal changed microglial morphology indicating microglial activation. This effect of Gal on microglia was prevented by blocking neuronal GalR2. In conclusion, Gal has the potential to ameliorate CSD thus reducing pathophysiological neuronal events caused by or associated with CSD.