Avoidance of neonatal cortical lesions by developing somatosensory barrels

Neocortical Expansion: An Attempt toward Relating Phylogeny and Ontogeny

The neocortex is the most characteristic feature of the human brain. On gross inspection, its convoluted surfaces can be seen to have overgrown and covered most other brain structures. In the functional sphere, it is to the neocortex that we attribute those behaviors assumed to be most uniquely human such as cognition and linguistic behavior. This essay is an attempt to understand how this structure expanded during the course of mammalian evolution. At present, any attempt must be more speculative than definitive, but it is offered in the hope that it will generate more discussion on a topic that is central to all neurobiology, as well as a number of allied disciplines. I will proceed by outlining current views on the evolution of the brain, briefly review the organization of the somatosensory cortex in several mammalian forms, and then discuss in some detail ontogenetic mechanisms that may have some bearing on neocortical phylogeny. The primary proposition put forth is that the mammalian neocortex is relatively unspecified by strict genetic means, and that this allowed the neocortex to expand and adapt to a variety of circumstances during the course of phylogeny.

A sealed cranial window system for simultaneous recording of blood flow, and electrical and optical signals in the rat barrel cortex

We have developed a new sealed cranial window technique which allows the manipulation of simultaneously and independently multiple sensor probes, such as a glass microelectrode and a laser-Doppler probe. possible. Furthermore, normal intracranial pressure (4 mmHg) can be maintained throughout the craniectomy and the experiment. Using this technique, we have measured the neuronal activity and local cerebral blood flow together with the intrinsic optical properties in the rat barrel cortex during mechanical stimulation of the whiskers. The onset of the field response recorded by an extracellular electrode in the principal barrel columns occurred about 8 ms from the beginning of stimulation. These responses were well correlated with the whisker displacements (3 Hz, 2 s). The local cerebral blood flow, measured by laser-Doppler flowmetry, started to increase about 0.5 s after the first field response, peaked at about 1.7 s, and then gradually waned. A similar time-course of changes in the local blood volume was observed by simultaneous intrinsic optical imaging at the hemoglobin-isosbestic wavelength (570 nm). These results suggest that our technique would be useful for assessing the mechanism underlying neurovascular coupling under physiological conditions in vivo.

Development of geniculocortical projections to visual cortex in rat: evidence early ingrowth and synaptogenesis

Anterograde movement of DiI and transneuronal transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were used to study the temporal and laminar patterns of ingrowth of the geniculocortical projection to visual cortex in fetal and postnatal rats. The development of this projection was compared to patterns of migration and settling of [3H]-thymidine-labeled neurons destined for cortical layer IV, and to geniculocortical synapse formation. DiI-labeled geniculocortical axons were found in the intermediate zone beneath the lateral cerebral mantle at embryonic day (E)17 and in the subplate layer underlying visual cortex by E18. On E19 they appeared to accumulate and grow radially into an expanding subplate layer and into the deep part of developing cortical layer VI. By postnatal day (P)0, DiI or WGA-HRP-labeled geniculocortical axons were found in developing cortical layers VI and V. By P1, they invaded the deep portion of the cell-dense cortical plate, where they were in position to make initial contact with neurons that would later form layer IV. A few axons traversed the cortical plate to reach the marginal zone. Layer IV became an identifiable layer on P2, and a clear projection to layer IV was evident by P3. These results suggest that geniculocortical afferents grow continuously from the intermediate zone, initially into an expanding subplate layer and then sequentially into each of the developing cortical layers without evidence of "waiting." Electron microscopic data suggest that geniculocortical axons begin to form immature synapses with dendrites and neuronal perikarya as they first encounter cortical neurons, first in the subplate layer and then in developing layers VI, V and marginal zone, in addition to the primary target layer IV. The precise targeting and overall temporal and laminar patterns of ingrowth and synaptogenesis suggest that geniculocortical axons are directed to the visual cortex by guidance cues within the internal capsule and subplate. Further, they reach the occipital pole early enough to influence the specification and histogenesis of cortical area 17, perhaps by exerting an influence on the deep-to-superficial "wave" of neuronal differentiation in sequentially developing subplate and cortical layers VI, V and IV.

Peripheral nerve damage facilitates functional innervation of brain grafts in adult sensory cortex

The neural pathways that relay information from cutaneous receptors to the cortex provide the somatic sensory information needed for cortical function. The last sensory relay neurons in this pathway have cell bodies in the thalamus and axons that synapse on neurons in the somatosensory cortex. After cortical lesions that damage mature thalamocortical fibers in the somatosensory cortex, we have attempted to reestablish somatosensory cortical function by grafting embryonic neocortical cells into the lesioned area. Such grafts survive in adult host animals but are not innervated by thalamic neurons, and consequently the grafted neurons show little if any spontaneous activity and no responses to cutaneous stimuli. We have reported that transection of peripheral sensory nerves prior to grafting "conditions" or "primes" the thalamic neurons in the ventrobasal complex so that they extend axons into grafts subsequently placed in the cortical domain of the cut nerve. In this report we present evidence that the ingrowth of ventrobasal fibers leads to graft neurons that become functionally integrated into the sensory circuitry of the host brain. Specifically, the conditioning lesions made prior to grafting produce graft neurons that are spontaneously active and can be driven by natural activation of cutaneous receptors or electrical stimulation of the transected nerve after it regenerates. Furthermore, oxidative metabolism in these grafts reaches levels that are comparable to normal cortex, whereas without prior nerve cut, oxidative metabolism is abnormally low in neocortical grafts. We conclude that damage to the sensory periphery transsynaptically stimulates reorganization of sensory pathways through mechanisms that include axonal elongation and functional synaptogenesis.

Maintenance of discrete somatosensory maps in subcortical relay nuclei is dependent on an intact sensory cortex

Lesions of the rat barrelfield cortex drastically alter the discrete representations of the somatosensory periphery in the central nervous system. We have found that lesions placed in the parietal cortex, after the formation of barrels (postnatal day 5), can irreversibly abolish vibrissae- and extremity-related patterns of cytochrome oxidase activity in the principal sensory nucleus of the trigeminal nerve and in the dorsal column nuclei. Furthermore, abnormal patterns of enzymatic activity occur in the remaining primary somatosensory cortex and the ventrobasal nucleus of the thalamus. We conclude that cortical barrels are essential in maintenance of periphery-related discrete morphological organization in the rodent somatosensory system.

Removal and reimplantation of the parietal cortex of the neonatal mouse: consequences for the barrelfield

The barrelfield, i.e., the cortical representation of the contralateral whiskerpad, develops in the mouse between postnatal days 4 and 6 (P4 and P6). The pattern of barrels in the barrelfield is dependent on that of the whiskers on the whiskerpad. We removed (unilaterally) and reimplanted, with normal orientation (n = 23) or after 180 degrees rotation (n = 23), that part of the pallium where the barrelfield is to develop. These operations were made at birth (PO), at P1 or at P3. Our aim was to know whether a homeomorphic representation of the periphery would be established, by regeneration or by retarded growth of surviving thalamic fibers into the cortex, and if so, whether it could be modified in its orientation. Of group (i) 16 animals survived of which 14 killed between P26 and P29 could be analyzed. We operated 11 at PO and 4 of these had a normal barrelfield, two a rotated one (by about 30 degrees) with some barrels in odd positions, one a wildly disorganized one and 4 no barrels at all. Three mice operated at P3 presented a normal barrelfield. In group (ii) operated at PO, the 17 survivors, killed between P13 and P68, had no barrels. The results indicate that a homeomorphic representation of the periphery may be established in the somatosensory cortex after an early, complete and massive interruption not only of all its afferent and efferent fibers, but also of its vascularization. With respect to animals of group (ii) we cannot rule out the ingrowth of thalamic afferents into the implant.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cytotoxic deletions of somatic sensory cortex in fetal rats

Pregnant rats were injected on the 14th day of gestation with the cytotoxic drug methylazoxymethanol acetate. This compound causes the death of neural precursor cells that were synthesizing DNA at the time of injection. After birth, the progeny of treated mothers grew to maturity with a neocortex that was greatly reduced in area by the death of all cells, particularly at the frontal and occipital poles but at medial and lateral margins of neocortex as well. In the remaining cortex layers II through IV failed to develop. The experiment deprived growing thalamocortical axons, which innervate the somatic sensory cortex late in development, of part of their normal target area and of a substantial number of their definitive target cells. It also deprived them of any cues they might have received from these target cells migrating through them as the axons accumulate beneath the cortical plate. Anatomical experiments indicated that, despite these defects, thalamocortical axons could still colonize the sensorimotor areas and form synapses in their typically bilaminar pattern, though the outer, denser lamina of terminations occurred abnormally at the level of the apices of layer V pyramidal cell bodies. Receptive field mapping of single and multiunit responses in the somatic sensory region showed brisk responses and receptive fields of normal size. It also indicated the formation of a body map that was topographically intact except for deletions at its periphery; that is, a total map was not compressed into a smaller area. This suggests that somatic sensory thalamocortical fibers recognize only remaining cortical target cells in appropriate fields. Moreover, successful ones among them seem to recognize neighborhood relations and conserve synaptic space at the expense of those that would have innervated the deleted peripheral parts of the area. Pyramidal neurons in the remaining cortical layers and in ectopic islands of cells that had incompletely migrated from the neuroepithelium, probably on account of destruction of radial glial cell precursors, were shown by retrograde labeling to send their axons only to appropriate subcortical targets. Pyramidal neurons, though recognized as such, also adopted a variety of abnormal orientations, such as inversion, apparently in an attempt to grow apical dendrites toward major zones of synaptic terminations.