Possible regeneration of rat medial frontal cortex following neonatal frontal lesions

Abstract

The experiments described here show that the cavity left by midline frontal cortex removals at 10 days of age (P10) fills in with neural tissue. Similar changes are not found at earlier and later ages. This neuronal filling is blocked by prior pretreatment by administration of Bromodeoxyuridine (BrdU) on embryonic day 13. Administration of BrdU following the P10 lesion does not interfere with regrowth. Subsequent immunohistochemical staining for BrdU demonstrates the regrown area to be composed of newly generated cells, which include pyramidal and nonpyramidal neurons. Injections of a retrograde tracer into the striatum or posterior parietal cortex shows that the new neurons have connections similar to those of undamaged brains. The regrowth of this tissue is correlated with recovery of function in a test of forelimb use. Thus, the mammalian brain, during some privileged postnatal stages of growth, is capable of extensive reorganization that includes regeneration of lost neurons. These results are discussed in relation to the proximity of the lesion to the stem cells in the lateral ventricle and their postnatal migrational activities.

Introduction

A large literature now shows that when rats are given frontal cortical lesions over the first 7–12 days of life they show substantial recovery of function 11, 13, 14, 20, 22, 23. In fact, under optimal circumstances rats can nearly completely compensate for the loss of midline frontal cortical tissue [20]. This functional recovery is most extensive if the cortex is injured between the postnatal ages of 7 and 12 days and if they do not include extensive amounts of adjoining motor cortex. Under these circumstances the lesion cavity is filled over a 7–14 day period after the injury ([20]; see Fig. 1). Although the disappearance of the cavity might be thought to be due to collapse of adjacent tissue, it is also possible that there is a generation of new neurons to `regrow' the lost region. The production of cortical neurons is largely confined to the prenatal period in the mammalian brain. Nonetheless, the mammalian brain is capable generating neurons postnatally as granule cells in both the olfactory bulb and hippocampus of rodents, and stem cells in the subventricular zone of rodents and primates, are capable of growing well into adulthood 4, 10, 25, 27. In addition, there are a number of reports of limited neural generation after cortical injury in the rat 1, 8. There is, however, no evidence of substantial neuronal regeneration, reconstitution of complex circuitry, or restoration of pyramidal cells, which are the major output neurons of the cerebral cortex, following postnatal cortical injury in mammals. Nor is there any evidence that lesion-induced neurogenesis plays any role in functional recovery after postnatal injury.

The current series of experiments asked (1) about the characteristics of the replacement tissue; (2) can the replacement be blocked; (3) are the cells filling the lesion cavity newly generated neurons and, if so; (4) do the new neurons form connections to other cerebral regions as in normal control rats. We examined the first two questions by measuring the volume of tissue in normal versus frontal-lesioned brains and by blocking regrowth with E13 bromodeoxyuridine (BrdU) injections. To answer the third question we combined immunohistochemical labeling with the mitotic marker BrdU and the neuron antibody NeuN to identify newly generated neurons. To answer the fourth question we combined BrdU labeling and retrograde tracing techniques to identify the connections of newly generated neurons. Finally, since the behavioral viability of the tissue is the ultimate question in our studies, we compared the forelimb reaching performance of rats with and without regenerated tissue.