Npas4 is up-regulated in the corticolimbic system of the rodent brain following focal cortical ischaemia

The aim of this study was to investigate the expression of the neural activity-dependent transcription factor Neuronal PAS domain protein 4 (Npas4) in various rodent models of stroke. Two different models of stroke were used to induce focal cerebral ischaemia; middle cerebral artery occlusion in rats and Rose Bengal-mediated photochemical ischaemia in mice. Npas4 protein expression in these animals was then investigated using immunohistochemistry and was compared to sham operated animals. Despite the differences in the size and location of the infarcts produced by these two models, the pattern of Npas4 expression that was observed was remarkably similar across both paradigms. Npas4 was found to be rapidly, robustly and transiently up-regulated in the ipsilateral hemisphere following cerebral ischaemia. No expression was observed in the infarct core or the contralateral hemisphere. Npas4 protein expression peaked at 3.5 hours after the onset of ischaemia before gradually returning to baseline levels. In both of the models the same characteristic pattern of Npas4 expression was observed in discrete corticolimbic regions which at times included sites that were undamaged and were some distance away from the lesion. Highest expression was seen in the frontal cortex (layers II/III and V), piriform cortex (layer II), nucleus accumbens, ventral pallidum, amygdala, thalamus and hypothalamus. Interestingly, no up-regulation was observed in the dentate gyrus. Expression of Npas4 was specific to neurons and was not seen in astrocytes, oligodendrocytes or microglial cells. The type of neuron that expressed Npas4 varied in a brain region-dependent manner and it was generally the principal neuron type of each region that expressed Npas4 most frequently. In the frontal and piriform cortices, Npas4 expression was largely associated with excitatory projection neurons, while in the limbic striatum Npas4 was expressed predominantly by inhibitory medium spiny projection neurons. In summary, this study demonstrates that: (1) following cerebral ischaemia there is a unique differential induction of Npas4 protein expression in corticolimbic regions of the rodent brain that are critically linked to cognition and emotion, and (2) this distinctive pattern of expression is reproduced in two distinct models of stroke that differ vastly in the size and location of the resulting infarct. We hypothesise that Npas4 may be part of a transcriptional regulatory programme that is activated within the corticolimbic circuitry following an ischemic insult and that this transcriptional programme is linked to the cognitive and emotional dysfunction seen in the post-stroke mammalian brain.