Marmosets have a greater diversity of c-Fos response after hyperstimulation in distinct cortical regions as compared to rats.

Previous evidence indicated a potential mechanism that might support the fact that primates exhibit greater neural integration capacity as a result of the activation of different structures of the central nervous system, as compared to rodents. The current study aimed to provide further evidence to confirm previous findings by analyzing the patterns of c-Fos expression in more neocortical structures of rats and marmosets using a more robust quantitative technique and evaluating a larger number of brain areas. Nineteen Wistar rats and 21 marmosets (Callithrix jacchus) were distributed among control groups (animals without injections) and animals injected with pentylenetetrazol (PTZ) and euthanized at different time points after stimulus. Immunohistochemical detection of c-Fos was quantified using unbiased and efficient stereological cell counting in eight neocortical regions. Marmosets had a c-Fos expression that was notably more widely expressed (5× more cells) and longer lasting (up to 3 hr) than rats. c-Fos expression in rats presented similar patterns of expression according to the function of the brain cortical structures (associative, sensorial, and motor functions), which was not observed for marmosets (in which no clear pattern could be drawn, and a more diverse profile emerged). Our results provide evidence that the marmoset brain has a greater neuronal activation after intense stimulation by means of PTZ and a more complex pattern of brain activation. We speculate that these functional differences may contribute for the understanding of the different neuronal processing capacities of the neocortex in these mammals' orders.

c-Jun expression after cerebral hyperstimulation differs between rats and marmosets.

Immediate early genes (IEGs) are a fundamental element in the way we respond and adapt to a variety of stimuli. We have recently reported that IEG response, as measured by c-Fos expression, is different between rodents and primates. Here, we further extend this analysis by assessing the expression of c-Jun, one of the main complements of c-Fos, under the same stimulation protocol. For this, we investigated the immunohistochemical expression of c-Jun (and compared with that previously shown for c-Fos) after stimulation with pentylenetetrazol in the cingulate gyrus, motor cortex, piriform cortex, inferior temporal cortex, and visual cortex of rats and marmosets (Callithrix jacchus), both male and female. Overall the immunohistochemical expression of c-Jun was more intense but remained elevated for a shorter duration in marmosets as compared to rats. These results are in contrast to what we had previously shown for c-Fos. Furthermore, in terms of the temporal profile, c-Fos and c-Jun expression occurred in a complementary manner in rats-the peak of c-Fos expression coincided with low levels of c-jun expression-and in a superimposed manner in marmosets-the peak of c-Fos expression coincided with the peak of c-Jun expression. Since Fos proteins may form dimers with Jun proteins and together control late gene expressions in the cell nucleus, this different expression profile between primates and rodents may bear meaningful impact for how the nervous system reacts and adapts to stimulation.