BDNF and NT-3 applied in the whisker pad reverse cortical changes after peripheral deafferentation in neonatal rats

Neonatal whisker clipping alters behavior, neuronal structure and neural activity in adult rats

Early experience plays critical roles during the development of sensory systems. For example, neonatal surgical manipulations of the whiskers in rodents lead to altered neural activity and behaviors later in life. However, while surgical procedures damage the sensory pathway; it is hard to examine the impact of whisker deprivation on adult animals. To address this issue, we performed a neonatal whisker clipping (WC0-3) paradigm, a non-invasive procedure, from the day of birth (P0) to postnatal day (P) 3, and examined behavioral performances in their adult age. With fully regrown whiskers, the WC0-3 rats exhibited shorter crossable distance than controls in a gap-crossing task, suggesting a defect in their whisker-specific tactile function. In their somatosensory cortex, the layer IV spiny stellate neurons had reduced dendritic complexity and spine density. After exploration in a novel environment, the expression of an activity-dependent immediate early gene, c-fos, increased dramatically in the somatosensory cortex. However, in WC0-3 rats, the number of c-Fos positive cells was less than those in control rats, indicating a fault in transducing sensory-related neural activity between cortical layers in WC0-3 rats. Together, our results demonstrate the roles of early tactile experience on the establishment of layer-specific excitatory connection in the barrel cortex. Early sensory insufficiency would leave long-lasting functional deficits in the sensory system.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Lesion-induced synaptic plasticity in the somatosensory cortex of prenatally stressed rats

Prenatal stress exposure causes long-lasting impairments of the behavioral and neuroendocrine responses to later stressors of the offspring. Although mechanisms underlying these effects remain largely unknown, abnormalities in the neuronal plasticity might be responsible for neurobiological alterations. This study used the whisker-to-barrel pathway as a model system to investigate the effects of prenatal stress on lesion-induced plasticity of neurons. Pregnant rats were subjected to immobilization stress during the trigeminal neurogenesis period, corresponding to gestational days 12 to 17, for three hours a day. After birth, the middle row (C) whisker follicles of pups from the control and stressed groups were electrocauterized. Ten days later, tangentially sectioned cortical hemispheres were stained with cytochrome oxidase histochemistry to calculate the volumes of each barrel row (A-E) in both lesioned and intact sides of the cortex, using stereological methods. The adrenal to body weight ratios were significantly increased in stressed animals, when compared to the controls. The pattern and total volume of the barrel subfield remained unaltered, but the lesion-induced map plasticity index, calculated as the D/C ratio, decreased in stressed animals. In addition, the BDNF (Brain Derived Neurotrophic Factor), NT-3 (neurotrophin-3) and the cyclic AMP response element binding protein (CREB) phosphorylation levels in tissue homogenates of the barrel cortices were measured using the ELISA method. In prenatally stressed animals, the BDNF and NT-3 levels were reduced on the lesioned side, but significant CREB activation was observed on the intact side of the barrel cortex. Taken together, the results show that prenatal stress exposure negatively affects critical period plasticity by reducing the expansion of active barrels following peripheral whisker lesion. These changes arise independent of CREB phosphorylation and appear to be mediated by reduced levels of neurotrophins.

Neonatal whisker trimming causes long-lasting changes in structure and function of the somatosensory system

The significance of very early experience in the maturation of whisker-to-barrel system comes primarily from neonatal whisker or infraorbital nerve lesion studies conducted prior to the formation of cortical barrels. However, the surgical procedures damage the sensory pathway; it is difficult to examine the consequence after the recovery of sensory deprivation. To address this issue, we performed a neonatal whisker-cut (WC) paradigm and examined their behavioral performance during P30 to P35. With fully regrown whiskers, the rats that had whisker cut from the date of birth (P0) to postnatal day (P) 3 (WC 0-3) exhibited shorter crossable distance in the gap-crossing test. However, the rats had whisker cut at P3 only (WC 3) behaved normally in this test, suggesting the critical period for the development of whisker-specific tactile function is P0-P3, agreed with previous findings demonstrated by lesion methods. In the WC 0-3 rats, the cortical areas in the layer IV somatosensory region in relation to the trimmed whiskers were enlarged and the spiny stellate neurons within had larger dendritic span and greater spine density. Furthermore, more long and multiple-head spines were found in these rats. With abnormal structure and function in the somatosensory system, the WC 0-3 rats showed higher explorative activity and more frequent social interactions. Our results have demonstrated that the early tactile deprivation, similar to early visual deprivation, perturbed the developmental program of the brain and affected later behaviors in various aspects.

Does reorganization in the cuneate nucleus following neonatal forelimb amputation influence development of anomalous circuits within the somatosensory cortex?

Neonatal forelimb amputation in rats produces sprouting of sciatic nerve afferent fibers into the cuneate nucleus (CN) and results in 40% of individual CN neurons expressing both forelimb-stump and hindlimb receptive fields. The forelimb-stump region of primary somatosensory cortex (S-I) of these rats contains neurons in layer IV that express both stump and hindlimb receptive fields. However, the source of the aberrant input is the S-I hindlimb region conveyed to the S-I forelimb-stump region via intracortical projections. Although the reorganization in S-I reflects changes in cortical circuitry, it is possible that these in turn are dependent on the CN reorganization. The present study was designed to directly test whether the sprouting of sciatic afferents into the CN is required for expression of the hindlimb inputs in the S-I forelimb-stump field. To inhibit sprouting, neurotrophin-3 (NT-3) was applied to the cut nerves following amputation. At P60 or older, NT-3-treated rats showed minimal sciatic nerve fibers in the CN. Multiunit electrophysiological recordings in the CN of NT-3-treated, amputated rats revealed 6.3% of sites were both stump/hindlimb responsive, compared with 30.5% in saline-treated amputated animals. Evaluation of the S-I following GABA receptor blockade, revealed that the percentage of hindlimb responsive sites in the stump representation of the NT-3-treated rats (34.2%) was not significantly different from that in saline-treated rats (31.5%). These results indicate that brain stem reorganization in the form of sprouting of sciatic afferents into the CN is not necessary for development of anomalous hindlimb receptive fields within the S-I forelimb/stump region.

GAP-43 heterozygous mice show delayed barrel patterning, differentiation of radial glia, and downregulation of GAP-43

GAP-43 heterozygous (HZ) mice exhibit abnormal thalamocortical pathfinding, fasciculation, and terminal arborization at postnatal day 7 (P7). Here we tested whether these defects are correlated with delayed development of HZ cortical patterns. We assessed the rate of barrel segregation and radial glia differentiation in wild-type (WT) and HZ cortices. Since GAP-43 is involved in some forms of neural plasticity, we also compared the duration of the critical period for lesion-induced plasticity in both genotypes. Cytochrome oxidase histochemistry revealed a delay of approximately 1 day in barrel pattern formation in GAP-43 HZ mice. GAP-43 WT barrels showed complete segregation between P2-P3, while HZ barrels did not reach the same level of segregation until P3-P4. We found a similar delay in the transformation of radial glia from monopolar to multipolar phenotypes, from P5 in WT to P7 in HZ cortex. Radial glial cells represent many of the neuronal progenitors in developing cortex and aid in cell migration. Thus, the delay in radial glial differentiation may contribute to the delay in HZ barrel segregation. Interestingly, we found no change in the extent of the critical period for HZ cortical responsiveness to early peripheral damage or in the time course of the cortical response. As expected, GAP-43 expression in HZ cortex is significantly reduced early in development. However, HZ GAP-43 expression remains at maximum levels after P9, when it is normally downregulated. As a result, HZ GAP-43 expression is near-normal by P26, by which time near-normal barrel dimensions have been restored. Our findings indicate that GAP-43 deficiency leads to early delays in barrel development and suggest that these failures are followed by homeostatic responses, including prolonged GAP-43 expression. These compensatory mechanisms may rescue normal cortical reorganization in neonates and near-normal barrel morphology and GAP-43 expression in adulthood.

Whisker maps in marsupials: Nerve lesions and critical periods

In the wallaby, whisker-related patterns develop over a protracted period of postnatal maturation in the pouch. Afferents arrive simultaneously in the thalamus and cortex from postnatal day (P) 15. Whisker-related patterns are first seen in the thalamus at P50 and are well formed by P73, before cortical patterns first appear (P75) or are well developed (P85). This study used the slow developmental sequence and accessibility of the pouch young to investigate the effect of nerve lesions before afferent arrival, or at times when thalamic patterns are obvious but cortical patterns not yet formed. The left infraorbital nerve supplying the whiskers was cut at P0-93 and animals were perfused at P112-123. Sections through the thalamus (horizontal plane) and cortex (tangential) were reacted for cytochrome oxidase to visualize whisker-related patterns. Lesions of the nerve at P2-5, before innervation of the thalamus or cortex, resulted in an absence of patterns at both levels. Lesions from P66-77 also disrupted thalamic and cortical patterns, despite the fact that thalamic patterns are normally well established by P73. Lesions from P82-93 resulted in normal thalamic and cortical patterns. Thus, despite the wallaby having clearly separated times for the development of patterns at different levels of the pathway, these results suggest a single critical period for the thalamus and cortex, coincident with the maturation of the cortical pattern. Possible mechanisms underpinning this critical period could include dependence of the thalamic pattern on corticothalamic activity or peripheral signals to allow consolidation of thalamic barreloids.

Chorda tympani nerve transection at different developmental ages produces differential effects on taste bud volume and papillae morphology in the rat

Chorda tympani nerve transection (CTX) results in morphological changes to fungiform papillae and associated taste buds. When transection occurs during neonatal development in the rat, the effects on fungiform taste bud and papillae structure are markedly more severe than observed following a comparable surgery in the adult rat. The present study examined the potential "sensitive period" for morphological modifications to tongue epithelium following CTX. Rats received unilateral transection at 65, 30, 25, 20, 15, 10, or 5 days of age. With each descending age at the time of transection, the effects on the structural integrity of fungiform papillae were more severe. Significant losses in total number of taste buds and filiform-like papillae were observed when transection occurred 5-30 days of age. Significant reduction in the number of taste pores was indicated at every age of transection. Another group of rats received chorda tympani transection at 10, 25, or 65 days of age to determine if the time course of taste bud degeneration differed depending on the age of the rat at the time of transection. Taste bud volumes differed significantly from intact sides of the tongue at 2, 8, and 50 days post-transection after CTX at 65 days of age. Volume measurements did not differ 2 days post-transection after CTX at 10 or 25 days of age, but were significantly reduced at the other time points. Findings demonstrate a transitional period throughout development wherein fungiform papillae are highly dependent upon the chorda tympani for maintenance of morphological integrity.

Dissociating barrel development and lesion-induced plasticity in the mouse somatosensory cortex

In the mouse somatosensory cortex, thalamocortical axons (TCAs) corresponding to individual whiskers cluster into restricted barrel domains during the first days of life. If whiskers are lesioned before that time, the cortical space devoted to the afferents from the damaged whisker shrinks and becomes occupied by thalamocortical afferents from neighboring unlesioned whiskers. This plasticity ends by postnatal day 3 (P3) to P4 when barrels emerge. To test whether TCA development and lesion-induced plasticity are linked, we used monoamine oxidase A knock-out (MAOA-KO) mice in which normal TCA development is halted by an excess of serotonin. Normal TCA development can be restored when serotonin levels are lowered by parachlorophenylalanine (PCPA). By varying the time of PCPA administration, we found that barrel development can be reinitiated until P11, although the emergence of TCA clusters becomes gradually slower and less complete. In mice in which barrels emerge 3 d later than the normal schedule, at P6 instead of P3, we examined lesion-induced plasticity. We find a progressive decline of the lesion-induced plasticity and a closure at P3, similar to normal mice, showing that this plasticity is not influenced by an excess of serotonin levels. Thus, in MAOA-KO mice, the emergence of barrel patterning can be delayed without a concomitant delay in lesion-induced plasticity, and the cortical space devoted to one whisker representation cannot be modified by the periphery once patterning is imprinted in the subcortical relays. We conclude that the closure of the lesion-induced plasticity period in the barrelfield is probably not determined at the cortical level.

Deafferentation-induced apoptosis of neurons in thalamic somatosensory nuclei of the newborn rat: critical period and rescue from cell death by peripherally applied neurotrophins

This study shows that unilateral transection of the infraorbital nerve (ION) in newborn (P0) rats induces apoptosis in the contralateral ventrobasal thalamic (VB) complex, as evidenced by terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) and electron miscroscopy. Double-labelling experiments using retrograde transport of labelled microspheres injected into the barrel cortex, followed by TUNEL staining, show that TUNEL-positive cells are thalamocortical neurons. The number of TUNEL-positive cells had begun to increase by 24 h postlesion, increased further 48 h after nerve section, and decreased to control levels after 120 h. Lesion-induced apoptosis in the VB complex is less pronounced if ION section is performed at P4, and disappears if the lesion is performed at P7. This time course closely matches the critical period of lesion-induced plasticity in the barrel cortex. Nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), applied on the ION stump alone or in combination, are able to partially rescue thalamic neurons from apoptosis. Total cell counts in the VB complex of P7 animals that underwent ION section at P0 confirm the rescuing effect of BDNF and NGF. Blockade of axonal transport in the ION mimics the effect of ION section. These data suggest that survival-promoting signals from the periphery, maybe neurotrophins, are required for the survival of higher-order neurons in the somatosensory system during the period of fine-tuning of neuronal connections. We also propose that anterograde transneuronal degeneration in the neonatal rat trigeminal system may represent a new animal model for studying the pathways of programmed cell death in vivo.