Partial blocking of NMDA receptors restricts plastic changes in adult mouse barrel cortex

Nursing-induced somatosensory cortex plasticity: temporally decoupled changes in neuronal receptive field properties are accompanied by modifications in activity-dependent protein expression

This study is an attempt to gain insight into the malleability of representational maps in the primary somatosensory cortex in relation to the expression of proteins involved in inhibitory and excitatory neurotransmitter systems that contribute to maintain these maps in a dynamic state. Malleability of somatosensory maps is characterized by changes in the sizes of neuron receptive fields (RFs) affecting the representational grain and in the locations and submodalities of these RFs modifying the map extent. The concomitance of these alterations remains so far hypothetical. We used nursing as an evolving source of ethologically significant cutaneous stimulation. This cyclic behavior is particularly suited to investigating the time course of experience-dependent cortical changes. Electrophysiological maps of the ventrum skin were recorded twice in the same lactating rats between nursing initiation and several weeks after nursing. We found that reduction in RF size occurred earlier than map expansion. As nursing time declined, the map expansion was maintained longer than the RF sharpening. Based on this difference in time course, we compared the expression patterns of several activity-dependent proteins in relation to the RF plasticity. Western blot analysis showed an increase in glutamic acid decarboxylase expression that was concomitant with RF contraction. In contrast, NR2A subunit of NMDA and alpha calcium/calmodulin kinase type II were upregulated at times when map expansion was observed. We propose that inhibitory and excitatory plasticity mechanisms operating with different time courses may contribute to the temporal dissociation of nursing-induced RF reshaping and map expansion.

N-methyl-D-aspartate receptor subunit changes after traumatic injury to the developing brain

Traumatic brain injury (TBI) is a major cause of disability in the pediatric population and can result in abnormal development. Experimental studies conducted in animals have revealed impaired plasticity following developmental TBI, even in the absence of significant anatomical damage. The N-methyl-D-aspartate receptor (NMDAR) is clearly involved in both normal development and in the pathophysiology of TBI. Following lateral fluid percussion injury in postnatal day (PND) 19 rats, we tested the hypothesis that TBI sustained at an early age would result in impaired NMDAR expression. Using immunoblotting and reverse transcriptase-polymerase chain reaction (RT-PCR), protein and RNA levels of NMDAR subunits were measured in the cerebral cortex and hippocampus on post-injury days (PID) 1, 2, 4, and 7 (though the PID7 analysis was only for protein) and compared with age-matched shams. Significant effects of hemisphere (analysis of variance [ANOVA], p<0.01), and interactions between hemisphere and injury (ANOVA, p<0.05) and hemisphere and PID (ANOVA, p<0.05) were found for synaptic protein levels of the NR2A subunit in hippocampus. Specifically, within the ipsilateral hippocampus, NR2A was reduced by 9.9%, 47.9%, 40.8%, and 6.3% on PID1, PID2, PID4, and PID7, respectively. Within the cortex, there was a significant effect of injury (ANOVA, p<0.05) without any hemispheric differences. These bilateral cortical reductions measured 30.5%, 3.2%, 5.7%, and 13.4% at the same timepoints after injury. Injury had no significant main effect on NR1 or NR2B protein levels. RT-PCR analysis showed no significant changes in NR1, NR2A, or NR2B gene expression; however, as a positive control, hsp70 was induced more than twofold in ipsilateral cortex and hippocampus on PID1. It is known that NR2A expression levels increase during normal development, and in response to environmental stimuli. Our data suggest that injury-induced reduction in the expression of NR2A is one likely mechanism for the impaired experience-dependent neuroplasticity seen following traumatic injury to the immature brain.

Deficits in experience-dependent cortical plasticity and sensory-discrimination learning in presymptomatic Huntington's disease mice

Huntington's disease (HD) is one of a group of neurodegenerative diseases caused by an expanded trinucleotide (CAG) repeat coding for an extended polyglutamine tract. The disease is inherited in an autosomal dominant manner, with onset of motor, cognitive, and psychiatric symptoms typically occurring in midlife, followed by unremitting progression and eventual death. We report here that motor presymptomatic R6/1 HD mice show a severe impairment of somatosensory-discrimination learning ability in a behavioral task that depends heavily on the barrel cortex. In parallel, there are deficits in barrel-cortex plasticity after a somatosensory whisker-deprivation paradigm. The present study demonstrates deficits in neocortical plasticity correlated with a specific learning impairment involving the same neocortical area, a finding that provides new insight into the cellular basis of early cognitive deficits in HD.

Differential regulation of cortical NMDA receptor subunits by sensory learning

NMDA receptor is an important player in neuronal plasticity, including cortical reorganization. In the adult cerebral cortex, the receptor properties are regulated by relative expression of NR2A and NR2B subunits. We have previously found that 3 days of sensory conditioning, in which stimulation of whiskers was paired with a tail shock, induce NMDA-receptor-dependent expansion of metabolically labeled cortical representations of the stimulated vibrissae. Here, we examined the effect of learning-induced cortical reorganization upon expression of NR2A and NR2B NMDA receptor subunits. An increase in NR2A mRNA expression in the barrel of the "trained" row of vibrissae was observed with in situ hybridization 24 h after sensory conditioning. NR2B mRNA expression level did not change. Protein level of both regulatory subunits and obligatory NR1 subunit were examined in P2 fraction. NR2A protein level was found elevated 1 h and 24 h after the sensory conditioning, but not in controls which received only whisker stimulation, signifying that the change was associated with cortical map reorganization. NR2B protein level was transiently elevated in both trained and stimulated control groups. NR1 protein level did not change. The results show that simple sensory learning induces a change in expression of regulatory NMDA receptor subunits, indicating a potential for receptor channel properties modification.

Reduced plasticity of cortical whisker representation in adult tenascin-C-deficient mice after vibrissectomy

The effect of the extracellular matrix recognition molecule tenascin-C on cerebral plasticity induced by vibrissectomy was investigated with 2-deoxyglucose (2DG) brain mapping in tenascin-C-deficient mice. Unilateral vibrissectomy sparing row C of vibrissae was performed in young adult mice. Two months later, cortical representations of spared row C vibrissae and control row C on the other side of the snout were visualized by [(14)C]2DG autoradiography. In both wild-type and tenascin-C-deficient mice, cortical representation of the spared row was expanded in all layers of the barrel cortex. However, the effect was significantly more extensive in wild-type animals than in the mutant. Elimination of tenascin-C by genetic manipulation thus reduces the effect of vibrissectomy observed in the somatosensory cortex. No increase in number of fibres in the vibrissal nerve of spared vibrissae was seen, and occurrence of additional nerve to the spared follicle was very rare. Thus, in tenascin-C-deficient mice functional plasticity seems to be impaired within the CNS.

Large-scale plasticity in barrel cortex following repeated whisker trimming in young adult hamsters

Using the 2DG/immunostaining method [McCasland, J.S., Graczyk, G.M., 2000. Metabolic mapping-Unit 1.6. In: Gerfen, C.R. (Ed.), Current Protocols in Neuroscience. Wiley, New York, pp 1.6.1-1.6.15], we have previously demonstrated large-scale plasticity in whisker/barrel fields of young adult hamsters subject to follicle ablation on postnatal day 7 (P7) [Somatosens. Motor Res. 13 (1996) 245]. This plasticity occurs after the barrel field has formed, but before neuronal differentiation and synaptogenesis are complete. The present study tested for similar large-scale plasticity following whisker deprivation in young adult hamsters, when neuronal and synaptic development are more mature. Beginning around P40, animals had all whiskers except row C trimmed on alternating days for periods ranging from 1 h to 2 weeks, after which they were administered (3)H 2DG (i.p.) and allowed to explore a fresh empty cage. Autoradiograms from these animals showed a clear expansion in the zone of heavy 2DG labeling with continued whisker trimming. Hamsters with row C spared overnight showed markedly higher labeling in the row C barrels, as expected. After 2 weeks of repeated trimming, the pattern of 2DG labeling in the barrel field ranged from complete activation of all large-whisker columns, as in a previous study of P7 follicle ablation, down to a more localized activation of rows B, C, and D. Intermediate periods of trimming produced more localized label in the region of row C. There was a clear trend toward larger areas of activation with longer periods of trimming. Because inhibitory neurons are strongly activated in all cases, this large-scale neuronal plasticity must take place in the presence of strong inhibition. The data show that simple trimming of all but a few whiskers in normally reared adults leads to abnormally widespread metabolic labeling encompassing virtually the entire barrel field. Taken together, our findings suggest that a large-scale synaptic reorganization occurs in barrel fields deprived of normal sensory input in the adult as well as during postnatal development.

A morphogenetic role for acetylcholine in mouse cerebral neocortex

Recently, cholinergic afferents to cerebral cortex have met renewed attention regarding the regulation of plasticity as well as cognitive processing. My laboratory has developed a mouse neonatal basal forebrain lesion paradigm that has contributed considerably to the understanding of cholinergic mechanisms in cortical development. We have shown that transient cholinergic deafferentation, beginning at birth, precipitates alterations in neuronal differentiation and synaptic connectivity that persist into maturity, and contribute to altered cognitive behavior. These data are in general agreement with studies in rats in which the cholinergic basal forebrain is lesioned very early in development but contrast with effects of later developmental lesions. Moreover, in mouse, both morphological and behavioral consequences of the lesion are sex dependent. Studies of receptors and secondary messengers that are instrumental in morphogenesis and plasticity suggest that sex dependent molecular alterations occur within days if not hours following cortical cholinergic deafferentation.

Effects of insulin-like growth factor I on climbing fibre synapse elimination during cerebellar development

Functional neural circuit formation includes the process by which redundant synaptic connections formed earlier during development are subsequently eliminated. We report that insulin-like growth factor I (IGF-I) is a candidate factor that influences the developmental transition from multiple to mono innervation of cerebellar Purkinje cells (PCs) by climbing fibres (CFs). Continuous local application of exogenous IGF-I to the mouse cerebellum by means of ethylene-vinyl acetate copolymer (Elvax) significantly increased the degree of multiple CF innervation, when the IGF-I containing Elvax was implanted at postnatal day 8 (P8). In contrast, the IGF-I application starting at P12 had no effect on CF innervation. Conversely, continuous local application of antisera against IGF-I and its receptor significantly decreased the degree of multiple CF innervation when the application started at P8. We found that chronic treatment of exogenous IGF-I from P8 significantly enhanced the CF-mediated excitatory postsynaptic currents (CF-EPSCs). This effect was manifest for the smaller CF-EPSCs but not for the largest CF-EPSC of the multiple-innervated PCs. Conversely, chronic application of antisera from P8 caused attenuation of the largest CF-EPSCs. Other parameters for basic synaptic functions and cerebellar morphology were largely normal after the IGF-I or antisera treatment. These results suggest that IGF-I enhances the strength of developing CF synapses and may promote their survival, whereas the shortage of IGF-I impairs the development of CF synapses and, as a result, may facilitate their elimination. Thus, IGF-I is a potentially important factor among various signalling molecules that can influence CF synapse elimination during cerebellar development.

Rapid, experience-dependent changes in levels of synaptic zinc in primary somatosensory cortex of the adult mouse

Electrophysiological studies have established that the adult cerebral cortex undergoes immediate functional reorganizations after perturbations of the sensory periphery. These activity-dependent modifications are thought to be mediated via the rapid regulation of the synaptic strength of existing connections. Recent studies have implicated synaptic zinc as contributing to activity-dependent mechanisms of cortical plasticity, such as long-term potentiation and long-term depression, by virtue of its potent ability to modulate glutamatergic neurotransmission. To investigate the role of synaptic zinc in cortical plasticity, we examined changes in the barrel-specific distribution of zinc in axon terminals innervating the primary somatosensory cortex of adult mice at different time points after whisker plucking. In layer IV of normal adult mice, zinc staining in the barrel field was characterized by intense staining in inter-barrel septae and low levels of staining in barrel hollows. Within 3 hr, and up to 1 week after the removal of a row of whiskers, zinc staining increased significantly in barrel hollows corresponding to the plucked whiskers. With longer survival times, levels of zinc staining gradually declined in deprived barrel hollows, returning to normal levels by 2-3 weeks after whisker removal. Increased levels of zinc staining in deprived barrel hollows were highly, negatively correlated with the length of whiskers as they regrew. These results indicate that levels of synaptic zinc in the neocortex are rapidly regulated by changes in sensory experience and suggest that zinc may participate in the plastic changes that normally occur in the cortex on a moment-to-moment basis.

Neither peripheral nerve input nor cortical NMDA receptor activity are necessary for recovery of a disrupted barrel pattern in rat somatosensory cortex

Elevating cortical serotonin (5-HT) in rats from postnatal day (P-) 0 to P-6 by administering the monoamine oxidase (MAO(A)) inhibitor, clorgyline, produces a dose-dependent spectrum of effects on rat somatosensory organization, ranging from enlarged with indistinct septa to a complete lack of vibrissae-related patterns. However, if clorgyline treatment is stopped on P-6, a qualitatively and quantitatively normal vibrissae-related pattern of thalamocortical afferents appears in somatosensory cortex (S-I) on P-10. We employed high performance liquid chromatography (HPLC), infraorbital nerve (ION) transection, N-methyl-D-aspartate (NMDA) receptor blockade, 1,1'-dioctadecyl-3,3,3"3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamic afferents, and CO histochemistry to determine whether peripheral nerve input and/or cortical NMDA receptor activity were required for the recovery of vibrissae-related patterns in clorgyline-treated animals. Clorgyline administration from P-0 to P-6 produced a 1589.4+/-53.3% increase in cortical 5-HT over control animals on P-6 and a 268.8+/-6.3% elevation over controls at P-10. Postnatal day 6 pups had significantly altered vibrissae-related patterns in S-I following 6 days of clorgyline treatment but by P-10, the characteristic vibrissae-related patterns were restored. Neither transection of the ION nor application of the NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (APV), to the cortices of P-6 pups that were treated with clorgyline from birth had any significant effect on the recovery of the vibrissae-related patterns by P-10. These results indicate that neither peripheral nerve input nor cortical NMDA receptor activity are necessary for the restoration of cortical vibrissae-related patterns in rats that have sustained transient elevations of 5-HT.

Eyeblink conditioning in the rabbit (Oryctolagus cuniculus) with stimulation of the mystacial vibrissae as a conditioned stimulus

Eyeblink conditioning is a well-understood paradigm for the study of learning and memory and has been successfully employed with the use of auditory and visual conditioned stimuli (CSs). In this study, vibrotactile stimulation of the mystacial vibrissae was examined as an alternative CS in the rabbit (Oryctolagus cuniculus). The technique is described and acquisition of eyeblink conditioning (EBC) with stimulation of a single row of vibrissae in a delay paradigm is reported. Extinction of EBC with presentation of the CS alone is demonstrated, as well as reacquisition with stimulation of a single whisker. Finally, control experiments ensure that the CS has no auditory components. Ipsilateral presentation of the CS and airpuff is a more effective combination for training than contralateral presentations. Vibrotactile stimulation of the vibrissae as a CS will enable further examination of the neural correlates of learning in a well-characterized sensory system.

Critical period for activity-dependent synapse elimination in developing cerebellum

Synapse elimination is considered to be the final step in neural circuit formation, by causing refinement of redundant connections formed at earlier developmental stages. The developmental loss of climbing fiber innervation from cerebellar Purkinje cells is an example of such synapse elimination. It has been suggested that NMDA receptors are involved in the elimination of climbing fiber synapses. In the present study, we probed the NMDA receptor-dependent period of climbing fiber synapse elimination by using daily intraperitoneal injections of the NMDA receptor antagonist MK-801. We found that blockade of NMDA receptors during postnatal day 15 (P15) and P16, but not before or after this period, resulted in a higher incidence of multiple climbing fiber innervation and caused a mild but persistent loss of motor coordination. Neither basic synaptic functions nor cerebellar morphology were affected by this manipulation. Chronic local application of MK-801 to the cerebellum during P15 and P16 also yielded a higher incidence of multiple climbing fiber innervation. During P15-P16, large NMDA receptor-mediated EPSCs were detected at the mossy fiber-granule cell synapse, but not at the parallel fiber-Purkinje cell or climbing fiber-Purkinje cell synapse. It is therefore likely that the NMDA receptors located at the mossy fiber-granule cell synapse mediate signals leading to the elimination of surplus climbing fibers. These results suggest that an NMDA receptor-dependent phase of climbing fiber synapse elimination lasts 2 d at most. During this phase, the final refinement of climbing fiber synapses occurs, and disruption of this process leads to permanent impairment of cerebellar function.

GABA immunoreactivity in mouse barrel field after aversive and appetitive classical conditioning training involving facial vibrissae

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced an expansion of the cortical representation of the "trained row", labeled with 2-deoxyglucose (2DG), in layer IV of the barrel field. The present study has examined the pattern of GABA immunoreactivity (GABA-IR) in the cortical representation of row B of the facial vibrissae after (i) 3 days of aversive training, and (ii) 2 months of appetitive training, where stimulation of row B of vibrissae on one side of the snout was used as a conditioned stimulus. The most notable observation was a greater density of GABA-IR cells concentrated in the hollows of the "trained row" B barrels compared to the hollows in the barrel field of the opposite hemisphere in the same mouse. After aversive training, we noted a 2-fold increase in the density of GABA-IR neurons in the hollows of row B; after reward training, the increase amounted to 49%. In contrast, GABA-IR was unchanged in the control groups, which received only stimulation of vibrissae without the unconditioned stimulus. The classification of labeled neurons according to size revealed that the increase in density of GABA-IR neurons was confined to the small (12-15 microm) diameter group. We concluded that the GABAergic system undergoes up-regulation, after both associative learning paradigms, and that the population of small, GABAergic neurons plays an active role in use-dependent plasticity.

Partial denervation of the whiskerpad in adult mice: altered patterns of metabolic activity in barrel cortex

One hundred days after unilateral C-row nerve transection in the adult mouse whiskerpad, the caudal follicles of row C are reinnervated with approximately 80 % of the original number of axons [Corthésy, M.-E., Bronchti, G. & Welker, E. (1999) Eur. J. Neurosci. , 11, 2835-2846]. To what extent is this reinnervation functional, and how does it interact with the enlargement of the functional representation of neighbouring rows subsequent to the denervation? Using the autoradiographic deoxyglucose method, we studied the whisker representation at the level of the barrel cortex 100 days post lesionem. We stimulated whiskers belonging to the denervated row C, the neighbouring rows B and D, or to all five rows A-E. The deoxyglucose uptake was measured in tangential sections through layer IV. The results indicate that, 100 days post lesionem, whiskers of row C reactivate their cortical barrels. However, (i) the magnitude of this cortical response was reduced; (ii) row C barrels were equivalently activated by the stimulation of the neighbouring rows; and (iii) when all whiskers were stimulated, we observed a significantly reduced deoxyglucose uptake over the representation of nonlesioned whiskers of rows D and E. Therefore, 100 days after the peripheral nerve lesion the reinnervation of the whiskerpad had not restored a normal pattern of activation at the level of the barrel cortex. We propose that this is due to a modified interaction between the representations of the various rows of follicles at the cortical level that does not return to normal.

Anatomical correlates of representational map reorganization induced by partial vibrissectomy in the barrel cortex of adult mice

We examined the potential for changes in cortical connectivity to accompany long-term plastic changes in functional cortical representations of mystacial vibrissae. Plasticity in the barrel cortex of young adult mice was evoked by vibrissectomy that spared row C of whiskers. We found that 2-deoxyglucose brain mapping causes a progressive expansion of cortical representation of the spared vibrissae. Two months after vibrissectomy, when the width of the cortical map of the spared row of vibrissae doubled, living cortical slices of the barrel cortex were injected with fluorescent dextrans. The injections were centered on spared, deprived and control vibrissal columns. The injections labeled three intracortical projection systems: (i) local connections from one vibrissal column to neighboring columns; (ii) long-range projections running in the septa and walls of the barrels and spanning several barrels; and (iii) very-long-range fibers running horizontally in the lower part of layer V. The local, short-range projection system was analysed following small injections into the centers of columns in layers III and IV. We found that injections into spared barrels labeled axons extending for significantly greater distances in all layers (except layer V), and labeled cell bodies situated significantly further, than after injections into deprived or control barrels. Also, the total axonal density labeled by injections into the spared barrel was higher by 70% than for the deprived or control barrels. Alterations of topographical maps in adult somatosensory cortex may occur immediately after functional denervation, but may also increase with time, as in the case of our experimental situation. Our results indicate that persistent, long-term plastic change can remodel connectivity in the barrel cortex.

Vibrissae-evoked behavior and conditioning before functional ontogeny of the somatosensory vibrissae cortex

The following experiments determined that the somatosensory whisker system is functional and capable of experience-dependent behavioral plasticity in the neonate before functional maturation of the somatosensory whisker cortex. First, unilateral whisker stimulation caused increased behavioral activity in both postnatal day (P) 3-4 and P8 pups, whereas stimulation-evoked cortical activity (14C 2-deoxyglucose autoradiography) was detectable only in P8 pups. Second, neonatal rat pups are capable of forming associations between whisker stimulation and a reinforcer. A classical conditioning paradigm (P3-P4) showed that the learning groups (paired whisker stimulation-shock or paired whisker stimulation-warm air stream) exhibited significantly higher behavioral responsiveness to whisker stimulation than controls. Finally, stimulus-evoked somatosensory cortical activity during testing [P8; using 14C 2-deoxyglucose (2-DG) autoradiography] was assessed after somatosensory conditioning from P1-P8. No learning-associated differences in stimulus-evoked cortical activity were detected between learning and nonlearning control groups. Together, these experiments demonstrate that the whisker system is functional in neonates and capable of experience-dependent behavioral plasticity. Furthermore, in contrast to adult somatosensory classical conditioning, these data suggest that the cortex is not required for associative somatosensory learning in neonates.

Experience-dependent plasticity of adult rat S1 cortex requires local NMDA receptor activation

The effect of blocking NMDA glutamate receptors in adult rat cortex on experience-dependent synaptic plasticity of barrel cortex neurons was studied by infusing D-AP5 with an osmotic minipump over barrel cortex for 5 d of novel sensory experience. In acute pilot studies, 500 microM D-AP5 was shown to specifically suppress NMDA receptor (NMDAR)-dependent responses of single cells in cortical layers I-IV. To induce plasticity, all whiskers except D2 and D1 were cut close to the face 1 d after pump insertion. The animals were housed with 2 cage mates before recording 4 d later. This pairing of two whiskers for several days in awake animals generates highly significant biases in responses from D2 layer IV (barrel) cells to the intact D1 whisker as opposed to the cut D3 whisker. D-AP5 completely prevented the D1/D3 surround whisker bias from occurring in the D2 barrel cells (p > 0.6 for D1 > D3, Wilcoxon). Fast-spike and slow-spike barrel cells were affected equally, suggesting parity for inhibitory and excitatory cell plasticity. D-AP5 only partially suppressed the D1/D3 bias in supragranular layers (layers II-III) in the same penetrations (p < 0.042 for D1 > D3). In control animals, the inactive L-AP5 isomer allowed the bias to develop normally toward the intact surround whisker (p < 0.001 for D1 > D3) for cells in all layers. We conclude that experience-dependent synaptic plasticity of mature barrel cortex is cortically dependent and that modification of local cortical NMDARs is necessary for its expression.

Effects of neonatal cholinergic basal forebrain lesions on excitatory amino acid receptors in neocortex

The role of cholinergic basal forebrain projections in the modulation of cortical plasticity and associated functional changes is currently the subject of renewed attention. Excitatory amino acid receptors have been identified as mediators of cortical topographic efferent and afferent information. In addition some of these receptors, notably the NMDA and metabotropic [mGluR] type, participate in cortical plasticity. Growing evidence suggests that interactions between cholinergic and glutamatergic systems contribute to cognitive cortical functions and their anatomical and physiological substrates. Though cholinergic and glutamatergic mechanisms have both been shown to be involved in cortical morphogenesis, few studies have attempted to study their interactions in development. The present study investigates the effect of neonatal lesions to the cholinergic basal forebrain on NMDA, AMPA and mGluR receptors in BALB/CByJ mice, at two different developmental ages. We demonstrated previously that nBM lesions at birth result in transient cholinergic depletion for the first two postnatal weeks, substantial morphogenetic alterations in neocortex and cognitive deficits by adulthood. We show here that unilateral neonatal lesions result in decreases in NMDA and AMPA receptors but increases in mGluRs during the second postnatal week (PND 14). At 30 days postnatal, lesion mediated changes were attenuated, compared with PND 14, but significant sex differences in control and nBM lesioned mice were apparent. These data support the notion that cholinergic/glutamatergic interactions are important during early cortical morphogenesis. Moreover, our results highlight the fact that cholinergic as well glutamatergic developmental mechanisms are sexually dimorphic.

Effects of D-AP5 and NMDA microiontophoresis on associative learning in the barrel cortex of awake rats

Experiments involving single-unit recordings and microiontophoresis were carried out in the barrel cortex of awake, adult rats subjected to whisker pairing, an associative learning paradigm where deflections of the recorded neuron's principle vibrissa (S2) are repeatedly paired with those of a non-adjacent one (S1). Whisker pairing with a 300 ms interstimulus interval was applied to 61 cells. In 23 cases, there was no other manipulation whereas in the remaining 38, pairing occurred in the presence of one of three pharmacological agents previously shown to modulate learning, receptive field plasticity and long-term potentiation: N-methyl-D-aspartic acid (NMDA) (n=8), the NMDA receptor antagonist AP5 (n=17) or the nitric oxide synthase inhibitor L-nitro-arginine-N-methyl-ester (L-NAME) (n=13). Non-associative (unpaired) experiments (n=14) and delivery of pharmacological agents without pairing (n=14) served as controls. Changes in neuronal responsiveness to S1 following one of these procedures were calculated and adjusted relative to changes in the responses to S2. On average, whisker pairing alone yielded a 7% increase in the responses to S1. This enhancement differed significantly from the 17% decrease obtained in the non-associative control condition and could not be attributed to variations in the state of the animals because analysis of the cervical and facial muscle electromyograms revealed that periods of increased muscular activity, reflecting heightened arousal, were infrequent (less than 4% of a complete experiment on average) and occurred randomly. The enhancement of the responses to S1 was further increased when whisker pairing was performed in the presence of L-NAME (27%) or NMDA (35%) whereas AP5 reduced it to 1%. During the delivery period, NMDA enhanced both neuronal excitability and responsiveness to S1 whereas AP5 depressed them. However, the effects of both substances disappeared immediately after administration had ended. L-NAME did not affect the level of ongoing activity and responses to S1 significantly. From these data, we concluded that, since the changes in the responses to S1 lasted longer than the periods of both whisker pairing and drug delivery, they were not residual excitatory or inhibitory drug effects on neuronal excitability. Thus, our results indicate that, relative to the unpaired controls, whisker pairing led to a 24% increase in the responsiveness of barrel cortex neurons to peripheral stimulation and that these changes were modulated by the local application of pharmacological agents that act upon NMDA receptors and pathways involving nitric oxide. We can infer that somatosensory cerebral cortex is one site where plasticity emerges following whisker pairing.

Development of laminar distributions of kainate receptors in the somatosensory cortex of mice

Kainate receptors were present at birth in the murine somatosensory cortex as revealed by quantitative in vitro autoradiography. During the first five postnatal days [3H]kainate binding rapidly increased and the maximum density in layer IV was reached at P12. The adult laminar pattern of receptor binding distribution was established by the third postnatal week with the heaviest labeling of infragranular layers. The sharp increase of kainate receptor during the first postnatal week coincides with the critical period for cytoarchitectonic plasticity of the barrels and establishment of functional thalamo-cortical connections in the barrel field.

Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: II. Alpha-AMPA and kainate receptors

The ontogeny of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate (KA) glutamate receptors in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry. A somatotopic pattern of AMPA receptors with fewer [3H]AMPA sites in barrel centers than in surrounding cortex did not emerge until postnatal day 10 (P10). After reaching a peak density at P14, the density of [3H]AMPA receptors declined in both barrel centers and surrounding cortex. Compared with AMPA receptors, the density of [3H]KA sites at all ages was low, a somatotopic expression of [3H]KA sites was missing, and the developmental curve for [3H]KA sites was more shallow than that for [3H]AMPA binding sites. A differential ontogeny of AMPA and KA receptors in barrel field cortex was also demonstrated in immunocytochemical studies with antibodies to the AMPA receptor subunits GluR1 and GluR2,3 and the KA receptor subunits GluR6,7. GluR1 and GluR2,3 staining was more dense in barrel septa than in barrel centers; this pattern persisted into adulthood. GluR1 and GluR2,3 receptors were localized to cell bodies and dendrites as well as the neuropil, but different populations of cortical neurons expressed these receptors. At P10, KA receptor subunits GluR6,7 exhibited a contrasting pattern to that of AMPA receptor subunits, with slightly more neuropil staining in barrel centers than in surrounding cortex. After that point, the somatotopic pattern of GluR6,7 subunit expression was lost. The contrasting developmental patterns of expression of the AMPA and KA receptors in the barrel field suggest that they may play different roles in the whisker-to-barrel pathway.

Implants for sustained drug release over the somatosensory cortex of the newborn rat: a comparison of materials and surgical procedures

Drugs that interfere with neural transmission are an important tool in assessing the role of specific neurotransmitters in the development of the nervous system. Systemic drug treatments often produce neurodevelopmental effects with questionable specificity. Furthermore, many compounds of interest do not cross the blood-brain barrier. To overcome these limitations, either elvax or gelfoam implants have been previously employed to produce sustained drug release over specific brain regions. In this paper, stereotaxic coordinates are provided for reproducible insertion of drug-delivery systems over the rat somatosensory cortex at birth (P0), prior to the appearance of the cortical barrel pattern; a novel and simpler method for preparation of elvax 40p sheets is described; a new implantation technique is provided. Furthermore, we compare the efficiency and tolerability of elvax vs gelfoam implants, showing that gelfoam, but not elvax, significantly disrupts cortical cytoarchitecture. Finally, successful destruction of serotonin-containing terminals in layer IV of the primary somatosensory cortex of the newborn rat is demonstrated by application of parachloroamphetamine-containing elvax implants.

Electrophysiological effects of temporary deafferentation on two characterized cell types in the nucleus gracilis of the rat

Single- and multiunit recordings were made in the nucleus gracilis of anaesthetized rats in order to study the characteristics of the responses to natural cutaneous stimulation before and during local anaesthetic-induced deafferentation. Two types of cells were found which exhibited different electrophysiological features at rest and in response to stimulation of their receptive fields (RFs). Low-frequency (LF) neurons (77%) had very low spontaneous activity, and most could be antidromically activated from the medial lemniscus. High-frequency (HF) cells (23%) had a much higher spontaneous discharge rate, with shorter spike duration, and did not project through the lemniscus. Both cell types generated phasic responses with similar latencies following cutaneous stimulation. Recordings of lemniscal axons had similar characteristics to those of LF neurons. Within minutes after anaesthetizing the functional centre of the RF, the LF and HF cells displayed new RFs, and enhanced responses to stimuli delivered at the periphery of the original fields. Firing rates increased during anaesthesia, but only in LF cells. Using a paired-stimulation paradigm, many LF neurons displayed during anaesthesia a decrease of the normal inhibition that the conditioning stimulus evoked on the responses to the test stimulus; the opposite effect was observed in all HF cells. These results suggest that (i) LF neurons correspond to thalamic projection cells, and HF neurons may be inhibitory interneurons; (ii) by disinhibiting LF (but not HF) cells, temporary deafferentation may increase neuronal responsiveness to peripheral stimulation, and thus contribute to reveal new RFs, and (iii) these changes in the nucleus gracilis may partly account for the reorganization of representational maps at higher levels of the somatosensory pathways.

Elevated AP-1 transcription factor DNA binding activity at the onset of functional plasticity during development of rat sensory cortical areas

The patterns of three transcription factor DNA binding activities, namely AP-1, Octamer and CREB, were examined in barrel and visual cortices of rat brain during early postnatal development, when activity-dependent plasticity of neuronal responses and connectivity was described. Main peak levels of AP-1 DNA binding activity have been observed at 21 days postnatally in both cortical areas. In addition, slightly elevated AP-1 levels were detected at 3-7 postnatal days in the barrel and in the visual cortex. In contrast, Octamer DNA binding activities were at the highest levels in both areas at 7 days postnatally, and CREB DNA binding activities were not appreciably modulated throughout the development. The AP-1 protein complex at 21 days postnatally was composed of JunD, JunB, Fra-2, FosB and to much lesser extent of c-Fos in both cortical areas. Treatment of 21 day old rats with MK-801, an NMDA receptor antagonist, provoked a dramatic decrease in AP-1 DNA binding activities in the barrel cortex, but not in the visual cortex. Elevated AP-1 DNA binding activity can be taken as a good correlate of an onset of functional plasticity in the rat sensory cortex, although in the two primary sensory cortices examined it seems to be regulated in different ways.

NMDA receptors in mouse barrel cortex during normal development and following vibrissectomy

The development of N-methyl-D-aspartate (NMDA) receptors and the effects of vibrissectomy upon [3H]MK-801 binding were examined in the barrel cortex of mice. Autoradiographic studies showed that initially very low binding of [3H]MK-801 sharply increased during the second postnatal week reaching the adult level by the end of the third week. Scatchard analysis performed on cortical membrane preparations indicated that this rise of [3H]MK-801 labelling was due to an increase in the number of binding sites and a decrease of Kd at postnatal day 15 and 28. The interlaminar differences of labelling were registered from postnatal day 8. Changes of interlaminar distribution were found during the second and third postnatal weeks. In adult barrel cortex the highest binding was found in supragranular layers. In layer IV of the cortex, the pattern of binding resembled the pattern of barrels. Unilateral denervation of vibrissae performed in neonatal and adult mice did not alter the intensity of [3H]MK-801 labelling or the laminar distribution of binding sites. These results suggest that NMDA receptor binding does not reflect the plastic changes occurring in the barrel cortex.