Atrial natriuretic peptide expression is increased in rat cerebral cortex following spreading depression: possible contribution to sd-induced neuroprotection

Abstract

Cortical spreading depression (CSD) is characterised by slowly propagating waves of cellular depolarization and depression and involves transient changes in blood flow, ion balance and metabolism. In cerebral ischaemia, peri-infarct CSD-like depolarization potentiates infarct growth, whereas preconditioning with a CSD episode protects against subsequent ischaemic insult. Thus, many of the long-lasting molecular changes that occur in CSD-affected tissue are presumed to be part of a 'neuroprotective cascade.' 3',5'-Cyclic guanosine monophosphate (cGMP) has been shown to be a neuroprotective mediator and the nitric oxide system, which increases cGMP production by soluble guanylate cyclase, is up-regulated by CSD. Atrial and C-type natriuretic peptide (ANP/CNP) are present in cerebral cortex and their actions are mediated via particulate guanylate cyclase receptors and cGMP production. Therefore, in further efforts to characterise the role of cGMP-related systems in CSD and neuroprotection, this study investigated possible changes in cortical natriuretic peptide expression following acute, unilateral CSD in rats. Using in situ hybridisation, significant 20-80% increases in ANP mRNA were detected in layers II and VI of ipsilateral cortex at 6 h and 1-14 days after CSD. Ipsilateral cortical levels were again equivalent to control contralateral values after 28 days. Assessment of cortical concentrations of ANP immunoreactivity by radioimmunoassay revealed a significant 57% increase at 7 days after CSD. Despite using a sensitive signal-amplification protocol, authentic ANP-like immunostaining was readily detected in subcortical nerve fibres, but was not reliably detected in normal or CSD-affected neocortex, suggesting the presence of very low levels, and/or active or differential processing of the peptide. Cortical CNP mRNA levels are not altered by CSD, indicating the specificity of the observed effects.Overall, these novel findings demonstrate a prolonged increase in cortical ANP expression after an acute episode of CSD. The overlap between the described time course of CSD-induced protection against ischaemic insult and demonstrated increases in ANP levels, suggest that ANP (like nitric oxide) may contribute to CSD-induced neuroprotection, via effects on cGMP production and other signal-transduction pathways.