We studied the organization of the forepaw barrel subfield (FBS) in layer IV of adult rat somatosensory cortex using the mitochondrial marker cytochrome oxidase and related this organization to the representation of the forepaw. The FBS is an ovoid structure consisting of barrels and barrel-like structures, the most conspicuous of which form four centrally located medio lateral running bands. Each band contains three to four barrels. These centrally located bands are bordered along their entire lateral side by a nebulous zone of undifferentiated labeling. At the anterior border, two small barrels are located laterally and one or two larger barrels are located medially. Medial to the central zone are three well-defined barrels. The posterior border consists of a nebulous field of labeling and occasional barrel-like structures. The results from our electrophysiological recording and mapping revealed that the forepaw representation was topographically organized into a single map and that the forepaw map matches almost precisely with individual barrels and barrel-like structures in the FBS. Each of the four central bands is associated with the representation of a single glabrous digit. Digit two (D2) is represented anteriorly and followed posteriorly by D3 through D5. Within each digit band the digit is somatotopically organized, with the skin over the distal phalanx represented in the two lateral barrels and the middle and proximal phalanges represented in the medial barrel(s). The dorsal hairy digit skin and dorsal hand are represented in the lateral zone. D1 is represented by two small anteriorly located barrels. Medial to the representation of the glabrous digits is the representation of the palmar pads. The representation of these pads, in turn, lies between the representations of the thenar (located anteriorly) and hypothenar (located posteriorly) pads. Posterior to the hypothenar pad representation lie the representations of the wrist and forearm. While the present results support the conclusion that individual barrels are associated with discrete locations on the forepaw, examples were found where the recording site was not precisely located within the predicted barrel. Some of these errors may be accounted for by limitations in the mapping techniques; nevertheless, the FBS offers an excellent model system to study relationships between cortical structure and function.

Neurons in layer IV of rat somatosensory (SI) barrel cortex receive punctate somatic input from well-defined regions of the periphery. Following peripheral deafferentation, SI neurons in deafferented cortex respond to new input from neighboring regions of the skin surface. The precise mechanism(s) through which this occurs is unknown, although corticocortical and barreloid to barrel connections have been suggested as possible substrates. Because layer-IV barrels receive a strong afferent input from ventroposterior (VP) thalamic projection neurons, any divergence in the thalamocortical (TC) projection to multiple cortical barrels could also provide an anatomical substrate for rapid cortical reorganization. We used in-vivo intracellular recording methods to record and physiologically identify neurons in rat VP and to label those neurons with an intracellular tracer. Thalamic neurons (n=117) were impaled with sharp intracellular electrodes, and the receptive field(s) and firing pattern were measured. Cells were then injected with biocytin or biotinylated dextran amine (BDA). A total of 38 labeled TC neurons were quantitatively analyzed for soma size and dendritic arborization size; quantitative analysis of TC-axon arborizations in layer IV of barrel cortex was carried out in a total of 13 TC neurons. Two different axon-arborization patterns were identified in SI cortex: direct-projecting axons (n=6) were observed to project to and arborize within a single cortical barrel as well as extend their fibers into adjacent barrels; bifurcating-type axons (n=7) were seen to bifurcate in the subcortical white matter or in layer VI and then project to multiple barrel columns, where they arborized in layer IV. Axon fibers were always observed in three or more cortical barrels (mean=5, range=3-7). The mean mediolateral extent of arborizations in layer IV for the direct-projecting and bifurcating type axons were 458 microm and 1,302 microm, respectively, and these were significantly different (t=3.78, P<0.01). Axon-fiber length within cortical laminae was measured for each arborization pattern in relationship to the total fiber length within a cortical column. Direct-projecting axons always had greater than 50% of their fiber length within layer IV. Bifurcating-type axons were differentially distributed within multiple columns and always had less than 50% of their total column fiber length in layer IV. Morphological analysis of TC somata and dendrites revealed no correlation between local neuron morphology and axonal-arborization patterns. All intracellularly recorded TC neurons had similar adapting firing patterns when injected with a long-duration pulse. Our results showed that TC neurons project to multiple cortical barrels with one barrel receiving the principal input. This divergent TC projection pattern in SI cortex may provide an anatomical substrate for cortical plasticity and must be considered in any mechanism of rapid cortical reorganization.

The physiological representation of the shoulder and surrounding body was examined in layer IV of somatosensory cortex (SI) in rats that had underground removal of the forelimb, either as newborns on postnatal day three (PND-3) or as adults (at least 8 weeks of age). Electrophysiological recordings were used to map the shoulder and body representations (physiological map), and the mitochondria marker, cytochrome oxidase (CO), was used to visualize recording sites in barrel and barrel-like structures (morphological map) in layer IV of deafferents and intact controls. The SI shoulder representation lies in a nebulously stained region that lies posterior to the forearm, wrist, and forepaw representations; the latter region is associated with the well-defined forepaw barrel subfield (FBS). The major findings are: (1) the shoulder is represented as a single zone located at the posterior extent of the SI body map in intact rats; (2) limb deafferentation in adult or neonatal rats that were physiologically mapped 6-16 weeks post-amputation resulted in two or more islets of "new" representation of the shoulder in the FBS in addition to the representation of the "original" shoulder in the posterior part of the body map; (3) deafferentations made in neonatal rats, physiologically mapped as adults, had a significantly greater (Mann-Whitney U) amount of "new" cortical representation within the FBS than did rats deafferented as adults; (4) fewer unresponsive sites in the FBS were found for neonate deafferents than for adult deafferents; (5) evoked response latencies following electrical stimulation of the shoulder were shortest for cortical sites within the "original" shoulder representation in intact controls, and latencies recorded at the "original" shoulder representation in deafferents were also shorter than latencies recorded in "new" shoulder representations in both groups of deafferents; and (6) morphological maps of the FBS were altered in neonate deafferents to the extent that the barrel structure was poorly formed, as exemplified by the absence of the four mediolateral running bands; however, the overall ovoid shape of the FBS was still apparent, but not as sharply defined as for intact controls or adult deafferents. Possible mechanisms for reorganization following large-scale deafferentation are discussed.

No abstract

Maternal alcohol exposure results in a variety of neurodevelopmental abnormalities that include cognitive and sensorimotor dysfunctions that often persist into adulthood. Many reports of central nervous system disturbances associated within a clinical diagnosis of fetal alcohol syndrome point toward disturbances in central information processing. In this study, we used the rat barrel field cortex as a model system to examine the effects of prenatal alcohol exposure (PAE) on the organization and size of the large whisker representation in layer IV of the posteromedial barrel subfield (PMBSF) in somatosensory cortex. Pregnant rats (Sprague-Dawley) were intragastrically gavaged daily with alcohol doses (6 gm/kg body weight) from gestational day 1 to day 20 in a chronic binge pattern which produced blood alcohol levels ranging between 260 mg/dl and 324 mg/dl. Chow-fed (CF), pair-fed (PF), and cross-foster (XF) groups served as normal, nutritionally matched, and maternal controls, respectively, for the ethanol-exposed (EtOH) treatment group. All pups were examined on gestational day 32 corresponding approximately to postnatal day 9. EtOH and control group pups were weighed, anesthetized, and perfused. Brains were removed and weighed, with and without cerebellum and olfactory bulbs, and the neocortex was removed and weighed. Cortices were then flattened, sectioned tangentially, and stained with a metabolic marker-cytochrome oxidase-to reveal the barrel field. A subset of 27 cortical barrels, associated with the representation of the large whisker pad, was selected to examine in detail. The major results were: (i) the total barrel field area comprising the PMBSF was significantly reduced in EtOH (by 17%) and XF (by 16%) pups compared with CF pups, (ii) the sizes of individual barrels within the PMBSF were also significantly reduced in EtOH (16%) and XF (18%) pups, (iii) the septal region between barrels was also significantly reduced in EtOH (18%) and XF (12%) pups, (iv) anteriorly located barrels underwent greater reduction in size relative to the posteriorly located barrels, (v) body weights were also significantly reduced in EtOH (21%) and XF (27%) pups, (vi) total brain weight [with and without (forebrain) cerebellum/olfactory bulbs] and cortical weights were also significantly reduced in EtOH (total brain weight 15%, forebrain weight 16%, cortical weight 15%) and XF (18%, 19%, 20%) pups, and in contrast (vi) neither the overall barrel field pattern nor the pattern of individual barrels in the PMBSF was altered. These findings suggest that PAE reduces body and brain weight as well as the central cortical representation of the whisker pad, while leaving the overall barrel field pattern unperturbed. While these results might appear to support a miniaturization hypothesis (smaller PMBSF, smaller brain, smaller body weight), PAE also shows regional vulnerability within the PMBSF whereby anteriorly located barrels are most affected.