Basal forebrain lesions with or without reserpine injection inhibit cortical reorganization in rat hindpaw primary somatosensory cortex following sciatic nerve section

Cholinergic Deep Brain Stimulation for Memory and Cognitive Disorders

Memory and cognitive impairment as sequelae of neurodegeneration in Alzheimer's disease and age-related dementia are major health issues with increasing social and economic burden. Deep brain stimulation (DBS) has emerged as a potential treatment to slow or halt progression of the disease state. The selection of stimulation target is critical, and structures that have been targeted for memory and cognitive enhancement include the Papez circuit, structures projecting to the frontal lobe such as the ventral internal capsule, and the cholinergic forebrain. Recent human clinical and animal model results imply that DBS of the nucleus basalis of Meynert can induce a therapeutic modulation of neuronal activity. Benefits include enhanced activity across the cortical mantle, and potential for amelioration of neuropathological mechanisms associated with Alzheimer's disease. The choice of stimulation parameters is also critical. High-frequency, continuous stimulation is used for movement disorders as a way of inhibiting their output; however, no overexcitation has been hypothesized in Alzheimer's disease and lower stimulation frequency or intermittent patterns of stimulation (periods of stimulation interleaved with periods of no stimulation) are likely to be more effective for stimulation of the cholinergic forebrain. Efficacy and long-term tolerance in human patients remain open questions, though the cumulative experience gained by DBS for movement disorders provides assurance for the safety of the procedure.

Intermittent stimulation in the nucleus basalis of meynert improves sustained attention in rhesus monkeys

Sustained attention is essential in important behaviors in daily life. Many neuropsychiatric disorders are characterized by a compromised ability to sustain attention, making this cognitive domain an important therapeutic target. In this study, we tested a novel method of improving sustained attention. Monkeys were engaged in a continuous performance task (CPT) while the nucleus basalis of Meynert (NB), the main source of cholinergic innervation of the neocortex, was stimulated. Intermittent NB stimulation improved the animals' performance by increasing the hit rate and decreasing the false alarm rate. Administration of the cholinesterase inhibitor donepezil or the muscarinic antagonist scopolamine alone impaired performance, whereas the nicotinic antagonist mecamylamine alone improved performance. Applying NB stimulation while mecamylamine or donepezil were administered impaired CPT performance. Methylphenidate, a monoaminergic psychostimulant, was applied in conjunction with intermittent stimulation as a negative control, as it does not directly modulate cholinergic output. Methylphenidate also improved performance, and it produced further improvement when combined with NB stimulation. The additive effect of the combination suggested NB stimulation altered behavior independently from methylphenidate effects. We conclude that basal forebrain projections contribute to sustained attention, and that intermittent NB stimulation is an effective way of improving performance.

Cholinergic systems are essential for late-stage maturation and refinement of motor cortical circuits

Previous studies reported that early postnatal cholinergic lesions severely perturb early cortical development, impairing neuronal cortical migration and the formation of cortical dendrites and synapses. These severe effects of early postnatal cholinergic lesions preclude our ability to understand the contribution of cholinergic systems to the later-stage maturation of topographic cortical representations. To study cholinergic mechanisms contributing to the later maturation of motor cortical circuits, we first characterized the temporal course of cortical motor map development and maturation in rats. In this study, we focused our attention on the maturation of cortical motor representations after postnatal day 25 (PND 25), a time after neuronal migration has been accomplished and cortical volume has reached adult size. We found significant maturation of cortical motor representations after this time, including both an expansion of forelimb representations in motor cortex and a shift from proximal to distal forelimb representations to an extent unexplainable by simple volume enlargement of the neocortex. Specific cholinergic lesions placed at PND 24 impaired enlargement of distal forelimb representations in particular and markedly reduced the ability to learn skilled motor tasks as adults. These results identify a novel and essential role for cholinergic systems in the late refinement and maturation of cortical circuits. Dysfunctions in this system may constitute a mechanism of late-onset neurodevelopmental disorders such as Rett syndrome and schizophrenia.

Forelimb amputation-induced reorganization in the ventral posterior lateral nucleus (VPL) provides a substrate for large-scale cortical reorganization in rat forepaw barrel subfield (FBS)

In this study, we examined the role of the ventral posterior lateral nucleus (VPL) as a possible substrate for large-scale cortical reorganization in the forepaw barrel subfield (FBS) of primary somatosensory cortex (SI) that follows forelimb amputation. Previously, we reported that, 6 weeks after forelimb amputation in young adult rats, new input from the shoulder becomes expressed throughout the FBS that quite likely has a subcortical origin. Subsequent examination of the cuneate nucleus (CN) 1 to 30 weeks following forelimb amputation showed that CN played an insignificant role in cortical reorganization and led to the present investigation of VPL. As a first step, we used electrophysiological recordings in forelimb intact adult rats (n=8) to map the body representation in VPL with particular emphasis on the forepaw and shoulder representations and showed that VPL was somatotopically organized. We next used stimulation and recording techniques in forelimb intact rats (n=5) and examined the pattern of projection (a) from the forelimb and shoulder to SI, (b) from the forepaw and shoulder to VPL, and (c) from sites in the forepaw and shoulder representation in VPL to forelimb and shoulder sites in SI. The results showed that the projections were narrowly focused and homotopic. Electrophysiological recordings were then used to map the former forepaw representation in forelimb amputated young adult rats (n=5) at 7 to 24 weeks after amputation. At each time period, new input from the shoulder was observed in the deafferented forepaw region in VPL. To determine whether the new shoulder input in the deafferented forepaw VPL projected to a new shoulder site in the deafferented FBS, we examined the thalamocortical pathway in 2 forelimb-amputated rats. Stimulation of a new shoulder site in deafferented FBS antidromically-activated a cell in the former forepaw territory in VPL; however, similar stimulation from a site in the original shoulder representation, outside the deafferented region, in SI did not activate cells in the former forepaw VPL. These results suggest that the new shoulder input in deafferented FBS is relayed from cells in the former forepaw region in VPL. In the last step, we used anatomical tracing and stimulation and recording techniques in forelimb intact rats (n=9) to examine the cuneothalamic pathway from shoulder and forepaw receptive field zones in CN to determine whether projections from the shoulder zone might provide a possible source of shoulder input to forepaw VPL. Injection of biotinylated dextran amine (BDA) into physiologically identified shoulder responsive sites in CN densely labeled axon terminals in the shoulder representation in VPL, but also gave off small collateral branches into forepaw VPL. In addition, microstimulation delivered to forepaw VPL antidromically-activated cells in shoulder receptive field sites in CN. These results suggest that forepaw VPL also receives input from shoulder receptive sites in CN that are latent or subthreshold in forelimb intact rats. However, we speculate that following amputation these latent shoulder inputs become expressed, possibly as a down-regulation of GABA inhibition from the reticular nucleus (RTN). These results, taken together, suggest that VPL provides a substrate for large-scale cortical reorganization that follows forelimb amputation.

Modularity of mind and the role of incentive motivation in representing novelty

Animal and human brains contain a myriad of mental representations that have to be successfully tracked within fractions of a second in a large number of situations. This retrieval process is hard to explain without postulating the massive modularity of cognition. Assuming that the mind is massively modular, it is then necessary to understand how cognitive modules can efficiently represent dynamic environments-in which some modules may have to deal with change-induced novelty and uncertainty. Novelty of a stimulus is a problem for a module when unknown, significant stimuli do not satisfy the module's processing criteria-or domain specificity-and cannot therefore be included in its database. It is suggested that the brain mechanisms of incentive motivation, recruited when faced with novelty and uncertainty, induce transient variations in the domain specificity of cognitive modules in order to allow them to process information they were not prepared to learn. It is hypothesised that the behavioural transitions leading from exploratory activity to habit formation are correlated with (and possibly caused by) the organism's ability to counter novelty-induced uncertainty.

Cholinergic modulation of working memory activity in primate prefrontal cortex

The prefrontal cortex, a cortical area essential for working memory and higher cognitive functions, is modulated by a number of neurotransmitter systems, including acetylcholine; however, the impact of cholinergic transmission on prefrontal activity is not well understood. We relied on systemic administration of a muscarinic receptor antagonist, scopolamine, to investigate the role of acetylcholine on primate prefrontal neuronal activity during execution of working memory tasks and recorded neuronal activity with chronic electrode arrays and single electrodes. Our results indicated a dose-dependent decrease in behavioral performance after scopolamine administration in all the working memory tasks we tested. The effect could not be accounted for by deficits in visual processing, eye movement responses, or attention, because the animals performed a visually guided saccade task virtually error free, and errors to distracting stimuli were not increased. Performance degradation under scopolamine was accompanied by decreased firing rate of the same cortical sites during the delay period of the task and decreased selectivity for the spatial location of the stimuli. These results demonstrate that muscarinic blockade impairs performance in working memory tasks and prefrontal activity mediating working memory.

The basal forebrain cholinergic system is required specifically for behaviorally mediated cortical map plasticity

The basal forebrain cholinergic system has been implicated in the reorganization of adult cortical sensory and motor representations under many, but not all, experimental conditions. It is still not fully understood which types of plasticity require the cholinergic system and which do not. In this study, we examine the hypothesis that the basal forebrain cholinergic system is required for eliciting plasticity associated with complex cognitive processing (e.g., behavioral experiences that drive cortical reorganization) but is not required for plasticity mediated under behaviorally independent conditions. We used established experimental manipulations to elicit two distinct forms of plasticity within the motor cortex: facial nerve transections evoke reorganization of cortical motor representations independent of behavioral experience, and skilled forelimb training induces behaviorally dependent expansion of forelimb motor representations. In animals that underwent skilled forelimb training in conjunction with a facial nerve lesion, cholinergic mechanisms were required for mediating the behaviorally dependent plasticity associated with the skilled motor training but were not necessary for mediating plasticity associated with the facial nerve transection. These results dissociate the contribution of cholinergic mechanisms to distinct forms of cortical plasticity and support the hypothesis that the forebrain cholinergic system is selectively required for modulating complex forms of cortical plasticity driven by behavioral experience.

Errorless practice as a possible adjuvant to donepezil in Alzheimer's disease

Six individuals with probable Alzheimer's disease (AD) participated in a phase 1 study employing a repeated measures, parallel baseline design testing the hypothesis that error-free experience during word production practice combined with an acetyl cholinesterase inhibitor would improve confrontation naming ability. While acetyl cholinesterase inhibitors are safe and delay cognition decline associated with AD, improvement over baseline cognition is less evident; clinically significant cognitive deficits persist and progress. Both animal and clinical research strongly implicate acetylcholine in learning, a form of neuroplasticity. In clinical practice, however, people with AD are given cholinergic medications without concomitant systematic/targeted retraining. In this study six participants with probable AD and taking donepezil participated in targeted word production practice using an errorless learning strategy. Results showed that combining behavioral enrichment training and an acetyl cholinesterase inhibitor resulted in significant improvements in verbal confrontation naming of trained items for three of six participants. Differences in baseline dementia severity, living conditions, and medications may have influenced the training response. Detection of substantial treatment effects in 50% of subjects suggests further language treatment studies in AD in combination with an acetyl cholinesterase inhibitor are warranted and provide useful information on inclusion/exclusion criteria for use in subsequent studies.

Sensory experience determines enrichment-induced plasticity in rat auditory cortex

Our previous studies demonstrated that only a few days of housing in an enriched environment increases response strength and paired-pulse depression in the auditory cortex of awake and anesthetized rats [Engineer, N.D., Percaccio, C.R., Pandya, P.K., Moucha, R., Rathbun, D.L., Kilgard, M.P., 2004. Environmental enrichment improves response strength, threshold, selectivity, and latency of auditory cortex neurons. J Neurophysiol. 92, 73-82 and Percaccio, C.R., Engineer, N.D., Pruette, A.L., Pandya, P.K., Moucha, R., Rathbun, D.L., Kilgard, M.P., 2005. Environmental enrichment increases paired-pulse depression in rat auditory cortex. J Neurophysiol. 94, 3590-3600]. Multiple environmental and neurochemical factors likely contribute to the expression of this plasticity. In the current study, we examined the contribution of social stimulation, exercise, auditory exposure, and cholinergic modulation to enrichment-induced plasticity. We recorded epidural evoked potentials from awake rats in response to tone pairs and noise bursts. Auditory evoked responses were not altered by social stimulation or exercise. Rats that could hear the enriched environment, but not interact with it, exhibited enhanced responses to tones and increased paired-pulse depression. The degree to which enrichment increased response strength and forward masking was not reduced after a ventricular injection of 192 IgG-saporin. These results indicate that rich auditory experience stimulates physiological plasticity in the auditory cortex, despite persistent deficits in cholinergic activity. This conclusion may be beneficial to clinical populations with sensory gating and cholinergic abnormalities, including individuals with autism, schizophrenia, and Alzheimer's disease.

Auditory cortical plasticity: does it provide evidence for cognitive processing in the auditory cortex?

The past 20 years have seen substantial changes in our view of the nature of the processing carried out in auditory cortex. Some processing of a cognitive nature, previously attributed to higher-order "association" areas, is now considered to take place in auditory cortex itself. One argument adduced in support of this view is the evidence indicating a remarkable degree of plasticity in the auditory cortex of adult animals. Such plasticity has been demonstrated in a wide range of paradigms, in which auditory input or the behavioural significance of particular inputs is manipulated. Changes over the same time period in our conceptualization of the receptive fields of cortical neurons, and well-established mechanisms for use-related changes in synaptic function, can account for many forms of auditory cortical plasticity. On the basis of a review of auditory cortical plasticity and its probable mechanisms, it is argued that only plasticity associated with learning tasks provides a strong case for cognitive processing in auditory cortex. Even in this case the evidence is indirect, in that it has not yet been established that the changes in auditory cortex are necessary for behavioural learning and memory. Although other lines of evidence provide convincing support for cognitive processing in auditory cortex, that provided by auditory cortical plasticity remains equivocal.

Receptor autoradiographic correlates of deafferentation-induced reorganization in adult primate somatosensory cortex

The primate somatosensory system provides an excellent model system with which to investigate adult neural plasticity. We have previously shown that transection of the median and ulnar nerves is followed by an expansion in the representation of radial nerve skin, and that this plasticity proceeds in stages. Immediately following nerve injury, new receptive fields are "unmasked" in a fraction of the affected cortex. The remaining deprived cortex regains responsiveness to tactile stimulation over the following days to weeks. Given these progressive changes, it has been suggested that different mechanisms might account for the earlier and later phases of reorganization. In the present experiments, we quantified receptor autoradiographic binding data for GABAA and GABAB, AMPA, and NMDA receptors in the primary somatosensory cortices of adult squirrel monkeys at four postnerve injury survival durations: immediately (1-3 hours), 3 days, 1 month, and 2 months. We found dramatic reductions in GABAA binding in layer IV within hours following nerve injury, and this reduction was maintained across all survival durations. This finding is consistent with the idea that the earliest reorganizational changes are due to a relaxation in tonic inhibitory mechanisms permitting the expression of formerly subthreshold receptive fields. GABAB receptor binding is decreased in layer IV by 1 month after nerve injury, while binding for AMPA receptors is increased in layer IV by this time. These findings are consistent with our previous suggestion that the second stage of reorganization proceeds via mechanisms comparable to those revealed to account for NMDA-dependent long-term potentiation in the hippocampus.

Short-term reorganization of input-deprived motor vibrissae representation following motor disconnection in adult rats

It has been proposed that abnormal vibrissae input to the motor cortex (M1) mediates short-term cortical reorganization after facial nerve lesion. To test this hypothesis, we cut first the infraorbital nerve (ION cut) and then the facial nerve (VII cut) in order to evaluate M1 reorganization without any aberrant, facial-nerve-lesion-induced sensory feedback. In each animal, M1 output was assessed in both hemispheres by mapping movements induced by intracortical microstimulation. M1 output was compared in different types of peripheral manipulations: (i) contralateral intact vibrissal pad (intact hemispheres), (ii) contralateral VII cut (VII hemispheres), (iii) contralateral ION cut (ION hemispheres), (iv) contralateral VII cut after contralateral ION cut (ION + VII hemispheres), (v) contralateral pad botulinum-toxin-injected after ION cut (ION + BTX hemispheres). Right and left hemispheres in untouched animals were the reference for normal M1 map (control hemispheres). Findings demonstrated that: (1) in ION hemispheres, the mean size of the vibrissae representation was not significantly different from those in intact and control hemispheres; (2) reorganization of the vibrissae movement representation clearly emerged only in hemispheres where the contralateral vibrissae pad had undergone motor output disconnection (VII cut hemispheres); (3) the persistent loss of vibrissae input did not change the M1 reorganization pattern during the first 48 h after motor paralysis (ION + VII cut and ION + BTX hemispheres). Thus, after motor paralysis, vibrissa input does not provide the gating signal necessary to trigger M1 reorganization.

Effect of N-methyl-d-aspartate receptor blockade on plasticity of frontal cortex after cholinergic deafferentation in rat

Cholinergic projections from the nucleus basalis play a critical role in cortical plasticity. For instance, cholinergic deafferentation increases dendritic spine density and expression of the GluR1 subunit of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor in frontal cortex. Acetylcholine modulates glutamatergic activity in cortex, and the N-methyl-d-aspartate subtype of glutamate receptor plays a role in many forms of synaptic plasticity. To assess whether N-methyl-d-aspartate receptors mediate the increase in GluR1 and spine density resulting from cholinergic deafferentation, we examined the effect of N-methyl-d-aspartate receptor blockade on nucleus basalis lesion-induced upregulation of GluR1 and dendritic spines. Rats received unilateral sham or 192 IgG saporin lesions of the nucleus basalis. Half of the rats in each group were treated with the N-methyl-d-aspartate antagonist MK-801 or phosphate-buffered saline. Two weeks later, brains were processed for either immunohistochemical staining of the GluR1 subunit or Golgi histology. In layer II-III of frontal cortex, neuronal GluR1 expression was assessed using an unbiased stereological technique, and spine density was assessed on basilar branches of pyramidal neurons. GluR1 expression was increased after nucleus basalis lesion, but this increase was prevented with MK-801. Similarly, nucleus basalis-lesioned animals had significantly higher spine densities, and this effect was also prevented by treatment with MK-801. Thus, N-methyl-d-aspartate receptor blockade prevented both GluR1 and spine density upregulation following cholinergic deafferentation, suggesting that these effects are N-methyl-d-aspartate receptor-mediated.

Cortical map reorganization without cholinergic modulation

Acetylcholine has been shown to modulate many forms of cortical plasticity. New evidence indicates that reorganization of adult primary auditory cortex is still possible after removal of cholinergic inputs. This finding suggests that acetylcholine may act less as a gate and more as a gain control on cortical plasticity.

Basal Forebrain Cholinergic Input Is Not Essential for Lesion-Induced Plasticity in Mature Auditory Cortex

The putative role of the basal forebrain cholinergic system in mediating lesion-induced plasticity in topographic cortical representations was investigated. Cholinergic immunolesions were combined with unilateral restricted cochlear lesions in adult cats, demonstrating the consequence of cholinergic depletion on lesion-induced plasticity in primary auditory cortex (AI). Immunolesions almost eliminated the cholinergic input to AI, while cochlear lesions produced broad high-frequency hearing losses. The results demonstrate that the near elimination of cholinergic input does not disrupt reorganization of the tonotopic representation of the lesioned (contralateral) cochlea in AI and does not affect the normal representation of the unlesioned (ipsilateral) cochlea. It is concluded that cholinergic basal forebrain input to AI is not essential for the occurrence of lesion-induced plasticity in AI.

Aging and cholinergic deafferentation alter GluR1 expression in rat frontal cortex

Previously, we demonstrated that plasticity of frontal cortex is altered in aging rats: lesions of the nucleus basalis magnocellularis (NBM) produce larger declines in dendritic morphology in frontal cortex of aged rats compared to young adults. Cholinergic afferents from the NBM modulate glutamatergic transmission in neocortex, and glutamate is known to be involved in dendritic plasticity. To begin to identify possible mechanisms underlying age-related differences in plasticity after NBM lesion, we assessed the effect of cholinergic deafferentation on expression of the AMPA receptor subunit GluR1 in frontal cortex of young adult and aging rats. Young adult, middle-aged, and aged rats received sham or 192 IgG-saporin lesions of the NBM, and an unbiased stereological technique was used to estimate the total number of intensely GluR1-immunopositive neurons in layer II-III of frontal cortex. While the number of GluR1-positive neurons was increased in both middle-aged and aged rats, lesions markedly increased the number of intensely GluR1-immunopositive neurons in frontal cortex of young adult rats only. This age-related difference in lesion-induced expression of AMPA receptor subunit protein could underlie the age-related differences in dendritic plasticity after NBM lesions.

The basal forebrain cholinergic system is essential for cortical plasticity and functional recovery following brain injury

A reorganization of cortical representations is postulated as the basis for functional recovery following many types of nervous system injury. Neuronal mechanisms underlying this form of cortical plasticity are poorly understood. The present study investigated the hypothesis that the basal forebrain cholinergic system plays an essential role in enabling the cortical reorganization required for functional recovery following brain injury. The results demonstrate that functional recovery following cortical injury requires basal forebrain cholinergic mechanisms and suggest that the basis for this recovery is the cholinergic-dependent reorganization of motor representations. These findings raise the intriguing possibility that deficits in cholinergic function may limit functional outcomes following nervous system injury.

Effects of sensorimotor restriction and anoxia on gait and motor cortex organization: implications for a rodent model of cerebral palsy

Chronic or acute perinatal asphyxia (PA) has been correlated with the subsequent development of cerebral palsy (CP), a developmental neurological disorder characterized by spasticity and motor abnormalities often associated with cognitive deficits. Despite the prevalence of CP, an animal model that mimics the lifetime hypertonic motor deficits is still not available. In the present study, the consequences of PA on motor behavior, gait and organization of the primary motor cortex were examined in rats, and compared with the behavioral and neurological consequences of early postnatal movement-restriction with or without oxygen deprivation. Rats subjected to PA had mild increases in muscular tone accompanied by subtle differences in walking patterns, paralleled by significantly altered but relatively modest disorganization of their primary motor cortices. Movement-restricted rats, suffering PA or not, had reduced body growth rate, markedly increased muscular tone at rest and with active flexion and extension around movement-restricted joints that resulted in abnormal walking patterns and in a profoundly distorted representation of the hind limbs in the primary motor cortex. Within the sensorimotor-restricted groups, non-anoxic rats presented the most abnormal pattern and the greatest cortical representational degradation. This outcome further supports the argument that PA per se may represent a substrate for subtle altered motor behaviors, and that PA alone is sufficient to alter the organization of the primary motor cortex. At the same time, they also show that early experience-dependent movements play a crucial role in shaping normal behavioral motor abilities, and can make a powerful contribution to the genesis of aberrant movement abilities.

Age-dependent effect of cholinergic lesion on dendritic morphology in rat frontal cortex

Previously, we demonstrated that plasticity of frontal cortex is altered in aging rats: 3 months after surgery, excitotoxic lesions of the nucleus basalis magnocellularis (NBM) produce larger declines in dendritic morphology in frontal cortex of aged rats relative to young adults. To determine whether the differential effect of the lesion was due specifically to loss of cholinergic input from the NBM, we assessed dendritic morphology in frontal cortex after specific cholinergic depletion in young adult, middle-aged, and aged male rats. Rats received unilateral sham or 192-IgG-saporin lesions of the NBM. Two weeks after surgery, brains were stained using a Golgi-Cox procedure. Dendritic morphology was quantified in pyramidal neurons in layers II-III of frontal cortex. Although lesions altered apical dendrites at all ages, these effects were most pronounced in aged rats. In addition, lesions produced marked atrophy of basilar dendrites in middle-aged and aged rats only. Thus, the differential dendritic atrophy resulting from NBM lesions in aged rats occurs within 2 weeks after lesion, and results specifically from loss of cholinergic innervation.

Contribution of acetylcholine to visual cortex plasticity

Acetylcholine is involved in a variety of brain functions. In the visual cortex, the pattern of cholinergic innervation varies considerably across different mammalian species and across different cortical layers within the same species. The physiological effects of acetylcholine in the visual cortex display complex responses, which are likely due to cholinergic receptor subtype composition in cytoplasm membrane as well as interaction with other transmitter systems within the local neural circuitry. The functional role of acetylcholine in visual cortex is believed to improve the signal-to-noise ratio of cortical neurons during visual information processing. Available evidence suggests that acetylcholine is also involved in experience-dependent visual cortex plasticity. At the level of synaptic transmission, activation of muscarinic receptors has been shown to play a permissive role in visual cortex plasticity. Among the muscarinic receptor subtypes, the M(1) receptor seems to make a predominant contribution towards modifications of neural circuitry. The signal transduction cascade of the cholinergic pathway may act synergistically with that of the NMDA receptor pathway, whose activation is a prerequisite for cortical plasticity.

Progressive degradation and subsequent refinement of acoustic representations in the adult auditory cortex

Correlated neuronal activity is believed to play an important role in refining and maintaining cortical circuitry during early development. Here we provide evidence that globally and locally correlated activity mediate different forms of adult plasticity. Pulses of broad-spectrum noise were used to activate time-locked responses across large areas of the rat auditory cortex, globally synchronizing cortical activity. Brief tone pips were used to activate relatively small groups of neurons, generating locally correlated activity. Pairing pulsed noises with nucleus basalis (NB) stimulation in awake rats for 4 weeks broadened spectral tuning, disrupted tonotopic maps, and reduced spontaneous discharge correlation in the primary auditory cortex (AI), as examined under anesthesia. Those effects caused AI neurons to appear qualitatively similar to neurons in nonprimary auditory fields of naive animals. Subsequent pairing of tone pips with NB stimulation for a period of 4 weeks completely reversed these effects induced by previous noise-NB pairing. These findings further demonstrate that the adult auditory cortex retains a substantial capacity for receptive field plasticity and tonotopic map reorganization and that locally correlated activity plays an important role in plasticity in the adult, as in the developing cortex.

Differential effects of cholinergic lesions on dendritic spines in frontal cortex of young adult and aging rats

Previously, we demonstrated that plasticity of frontal cortex is altered in aging rats: cholinergic lesions of the nucleus basalis magnocellularis (NBM) produce larger declines in dendritic morphology in frontal cortex of middle-aged and aged rats relative to young adults. To more closely examine the interactive effects of age and cholinergic deafferentation on synaptic connectivity in frontal cortex, we assessed the effects of specific cholinergic lesions on spine density of frontal cortical neurons in young adult, middle-aged, and aged rats. Rats received unilateral sham or 192 IgG-saporin lesions of the NBM. Two weeks after surgery, brains were stained using a Golgi-Cox procedure, and spine density was quantified in second-, third-, and fourth-order basilar dendrites of pyramidal neurons in layer II-III of frontal cortex. Spine density was reduced at all branch orders in aged, sham-lesioned rats. In addition, whereas lesions produced a marked increase in spine density on second- and third-order branches in young adult rats, lesions failed to significantly alter spine density in middle-aged and aged rats. Thus, the upregulation of dendritic spines may be a compensatory response to deafferentation, which is lost with advancing age.

Electrophysiological evidence for the existence of a posterior cortical-prefrontal-basal forebrain circuitry in modulating sensory responses in visual and somatosensory rat cortical areas

The prefrontal cortex (PFC) receives input from sensory neocortical regions and sends projections to the basal forebrain (BF). The present study tested the possibility that pathways from sensory cortical regions via the PFC-BF and from the BF back to specific sensory cortical areas could modulate sensory responses. Two prefrontal areas that responded to stimulation of the primary somatosensory and visual cortices were delineated: an area encompassing the rostral part of the cingulate cortex that responded to visual cortex stimulation, and a region dorso-lateral to the first in the precentral-motor association area that reacted to somatosensory cortex stimulation. Moreover, BF neurons responded to PFC electrical stimulation. They were located in the ventral pallidum, substantia innominata and the horizontal limb of the diagonal-band areas. Of the responsive BF neurons 42% reacted only to stimulation of 'visually-responsive,' 33% responded only to the 'somatosensory-responsive' prefrontal sites and the remaining neurons reacted to both prefrontal cortical areas. The effect of BF and PFC stimulations on somatosensory and visual-evoked potentials was tested. BF stimulation increased the amplitude of both sensory-evoked potentials. However, stimulation of the 'somatosensory-responsive' prefrontal area increased only somatosensory-evoked potentials while 'visually-responsive' prefrontal-area stimulation increased only visual-evoked potentials. Atropine blocked both facilitatory effects. The proposed cortico-prefronto-basalo-cortical circuitry may have an important role in cortical plasticity and selective attention.

Lesions of the Basal forebrain cholinergic system impair task acquisition and abolish cortical plasticity associated with motor skill learning

The contribution of the basal forebrain cholinergic system in mediating plasticity of cortical sensorimotor representations was examined in the context of normal learning. The effects of specific basal forebrain cholinergic lesions upon cortical reorganization associated with learning a skilled motor task were investigated, addressing, for the first time, the functional consequences of blocking cortical map plasticity. Results demonstrate that disrupting basal forebrain cholinergic function disrupts cortical map reorganization and impairs motor learning. Cholinergic lesions do not impair associative fear learning or overall sensorimotor function. These results support the hypothesis that the basal forebrain cholinergic system may be specifically implicated in forms of learning requiring plasticity of cortical representations.

Mechanisms for acute changes in sensory maps

Many studies have examined changes in the topographic representations of the special senses in cerebral cortex following partial peripheral deafferentations. This approach has demonstrated the short- medium- and long-term aspects of plasticity. However, the extensive capacity for immediate plasticity, while first demonstrated more than 15 years ago, still challenges explanation. What such studies indicate is that each locus in sensory cortex receives viable input from a far wider area of the sensory epithelium than is represented in the normal receptive field, with the implication that much of this input is normally inhibited. Consideration of the geometric and temporal aspects of receptive field plasticity suggests that this inhibition must be tonic and must derive its driving input from a tonically active periphery.

Suppression of cortical representation through backward conditioning

Temporal stimulus reinforcement sequences have been shown to determine the directions of synaptic plasticity and behavioral learning. Here, we examined whether they also control the direction of cortical reorganization. Pairing ventral tegmental area stimulation with a sound in a backward conditioning paradigm specifically reduced representations of the paired sound in the primary auditory cortex (AI). This temporal sequence-dependent bidirectional cortical plasticity modulated by dopamine release hypothetically serves to prevent the over-representation of frequently occurring stimuli resulting from their random pairing with unrelated rewards.

Representation of the hand in the cerebral cortex

In this brief review, the body of work on hand use and cortical plasticity is reviewed. The hand movements and sensory inputs are represented in the mammalian primary motor cortex and the anterior parietal strip. The dominant organizational rules are that representational area is proportional to peripheral innervation, and that cortical architecture is columnar with limited horizontal spread. The representational area and columnar structure can be shaped by behavior and other input manipulations. The central core systems, and especially cholinergic inputs, act as teachers of the cerebral cortex by marking behavioral reinforcers with the release of acetylcholine. This marking is both necessary and sufficient for plasticity to occur in sensory cortex. As a result of this temporal marking of reinforcing events, nearly coincident inputs over restricted sensory, or motor, segments form coherent representations in primary sensory or motor cortex. Focal dystonia is a problem in which overuse of the hand leads to a lack of motor control, and especially inappropriate use of sensory feedback for motor control. Receptive field size, and columnar architecture, are highly abnormal in this disorder. The deficiencies in focal dystonia, and their appropriate treatment, can be understood by applying the principles of cortical plasticity to the behavioural manipulations that cause focal dystonia.

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Central noradrenergic blockade prevents autotomy in rat: implication for pharmacological prevention of postdenervation pain syndrome

Following transection of peripheral nerve, rats exhibit autotomy, which is considered to be the animal model of postdenervation pain syndrome. It has been suggested that phantom limb pain is a result of peripheral denervation leading to reorganization of somatosensory pathways, particularly in the cerebral cortex, which is shown to depend upon central noradrenergic activity. In this study, sciatic and saphenous nerves were sectioned in the left hindpaw of 30 adult rats resulting in complete loss of pain sensation in the hindpaw. A group of rats received normal saline, compared to another group which received N-(2-) Chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) injection 24 h prior to transection. The latter group was also compared to a third group whose central noradrenergic system were also blocked by bilateral injection of 6-OHDA into the ascending noradrenergic bundle 1 week prior to transection. A fourth group received contralateral cortical ablation in addition to peripheral nerve transection and was compared to the first group whose cortex remained intact. The animals were observed daily for 60 days and autotomy was scored in accordance to the system of Wall et al. After 1 week, control animals began to exhibit autotomy. In contrast, autotomy was absent in rats treated with DSP4, similar to rats which received 6-OHDA. Rats which had contralateral cortical ablation showed a considerably delayed onset of autotomy and a reduction in final autotomy scores. We conclude that autotomy, as a model of postdenervation pain syndrome, can be prevented by blockade of noradrenergically mediated cortical reorganization. The clinical implications of this finding are discussed.

Atropine prevents the changes in the hindlimb cortical area induced by hypodynamia-hypokinesia

It has been demonstrated that hypodynamia-hypokinesia (HH), a model of sensory disruption, induced a decrease in the cortical hindpaw representation and an enlargement of the cutaneous receptive fields (RFs). The present study was carried out to determine whether chronic application of atropine could prevent this reorganisation. The extent of the hindlimb representation on the somatosensory cortex was determined in control rats (C), rats submitted to HH (HH), and rats submitted to HH with a chronic cortical infusion of atropine (70 mM, HH-ATR). Our results show that the hindpaw cortical area was similar for the HH-ATR and C rats, and was smaller for the HH rats. The distribution of RFs was comparable for the C and HH-ATR groups with a high percentage of small RFs. In contrast, for the HH rats, the percentage of large RFs was higher. Atropine can thus prevent the reduction in the hindlimb cortical area induced by HH. These results suggest that cholinergic mechanisms contribute to cortical plasticity.

Rapid differential conditioning of the somatosensory evoked potential by changed patterns of brief innocuous tactile stimuli in waking rats is altered by atropine sulfate

Air puffs delivered to the nose of an awake, lightly restrained rat every 15 s produced evoked potentials that changed gradually over time so that the averaged response to the last 40 stimuli was measurably different from the first 40. This habituation-like paradigm increased the size of an early component of the potential in several places. When measured with respect to the time of stimulus onset (there was a 21.6 ms delay in the time of arrival of the stimulus maximum at the nose), one of the largest increases occurred 46 ms later (39 ms latency to onset, and 55 ms latency to offset). As well, a late component of the waveform became more positive, showing a maximum between 156 and 185 ms (133 ms latency to onset, and more than 250 ms latency to offset). Changing the pattern but not the number of stimuli accelerated the rate of this positive shift with a maximum at 37 ms (21 ms latency to onset, and 42 ms latency to offset), but did not affect the rate of change in the late component. This effect of altering the temporal pattern of the stimuli was blocked by systemic injections of atropine sulfate, a blocker of central muscarinic receptors, whereas, neither saline injections nor atropine methyl nitrate injections (an atropine analog that does not cross the blood-brain barrier) could produce these changes. These observations suggest that the adaptive changes of the somatosensory evoked potential induced by novel patterns intercalated in otherwise monotonous repetitive somatic stimuli depend upon central muscarinic mechanisms.

Sensory input directs spatial and temporal plasticity in primary auditory cortex

The cortical representation of the sensory environment is continuously modified by experience. Changes in spatial (receptive field) and temporal response properties of cortical neurons underlie many forms of natural learning. The scale and direction of these changes appear to be determined by specific features of the behavioral tasks that evoke cortical plasticity. The neural mechanisms responsible for this differential plasticity remain unclear partly because important sensory and cognitive parameters differ among these tasks. In this report, we demonstrate that differential sensory experience directs differential plasticity using a single paradigm that eliminates the task-specific variables that have confounded direct comparison of previous studies. Electrical activation of the basal forebrain (BF) was used to gate cortical plasticity mechanisms. The auditory stimulus paired with BF stimulation was systematically varied to determine how several basic features of the sensory input direct plasticity in primary auditory cortex (A1) of adult rats. The distributed cortical response was reconstructed from a dense sampling of A1 neurons after 4 wk of BF-sound pairing. We have previously used this method to show that when a tone is paired with BF activation, the region of the cortical map responding to that tone frequency is specifically expanded. In this report, we demonstrate that receptive-field size is determined by features of the stimulus paired with BF activation. Specifically, receptive fields were narrowed or broadened as a systematic function of both carrier-frequency variability and the temporal modulation rate of paired acoustic stimuli. For example, the mean bandwidth of A1 neurons was increased (+60%) after pairing BF stimulation with a rapid train of tones and decreased (-25%) after pairing unmodulated tones of different frequencies. These effects are consistent with previous reports of receptive-field plasticity evoked by natural learning. The maximum cortical following rate and minimum response latency were also modified as a function of stimulus modulation rate and carrier-frequency variability. The cortical response to a rapid train of tones was nearly doubled if BF stimulation was paired with rapid trains of random carrier frequency, while no following rate plasticity was observed if a single carrier frequency was used. Finally, we observed significant increases in response strength and total area of functionally defined A1 following BF activation paired with certain classes of stimuli and not others. These results indicate that the degree and direction of cortical plasticity of temporal and receptive-field selectivity are specified by the structure and schedule of inputs that co-occur with basal forebrain activation and suggest that the rules of cortical plasticity do not operate on each elemental stimulus feature independently of others.

A computational model of mechanisms controlling experience-dependent reorganization of representational maps in auditory cortex

Cortical representations of sound can be modified by repeatedly pairing presentation of a pure tone with electrical stimulation of neuromodulatory neurons located in the basal forebrain (Bakin & Weinberger, 1996; Kilgard & Merzenich, 1998a). We developed a computational model to investigate the possible effects of basal forebrain modulation on map reorganization in the auditory cortex. The model is a self-organizing map with acoustic response characteristics mimicking those observed in the mammalian auditory cortex. We simulated the effects of basal forebrain modulation, using parameters intrinsic to the self-organizing map, such as the learning rate (controlling the adaptability of map nodes) and the neighborhood function (controlling the excitability of map nodes). Previous research has suggested that both parameters can be useful for characterizing the effects of neuromodulation on plasticity (Kohonen, 1993; Myers et al., 1996; Myers, Ermita, Hasselmo, & Gluck, 1998). The model successfully accounts for experimentally observed effects of pairing basal forebrain stimulation with the presentation of a single tone, but not of two tones, suggesting that auditory cortical plasticity is constrained in ways not accounted for by current theories. Despite this limitation, the model provides a useful framework for describing experience-induced changes in auditory representations and for relating such changes to variations in the excitability and adaptability of cortical neurons produced by neuromodulation.

Changes in sensitivity of cholinoceptors and adrenoceptors during transhemispheric cortical reorganisation in rat SmI

The reorganisation of primary somatosensory cortex that occurs after lesioning the corresponding cortex of the contralateral hemisphere in rat has been termed, 'transhemispheric cortical reorganisation'. Cholinergic and noradrenergic innervations are hypothesized to be involved in cortical plasticity. The present study investigated the change in responses of somatosensory neurones in the hindpaw representation area to muscarinic cholinoceptor and beta-adrenoceptor receptor stimulation, by iontophoretic application of acetylcholine, noradrenaline, propranolol and atropine, during the process of transhemispheric cortical reorganization at 3-4 days and at 20-21 days after lesioning the corresponding area in the contralateral hemisphere. Most neurones in control rats showed excitatory atropine-sensitive responses to acetylcholine, and inhibitory propranolol-sensitive responses to noradrenaline. A marked reduction in neurones exhibiting muscarinic responses (from 69% to 22%) and beta-noradrenoceptor-mediated responses (from 62% to 24%) were seen in rats 3-4 days post lesion. The proportion of neurones responding had recovered by 3 weeks but the direction of the responses had changed with muscarinic response becoming predominantly inhibitory and beta-noradrenoceptor responses predominantly excitatory. It is concluded that transhemispheric cortical reorganization involves both receptor types and that the reciprocal changes at different stages after injury maintain cortical plasticity.

The role of acetylcholine in cortical synaptic plasticity

This review examines the role of acetylcholine in synaptic plasticity in archi-, paleo- and neocortex. Studies using microiontophoretic application of acetylcholine in vivo and in vitro and electrical stimulation of the basal forebrain have demonstrated that ACh can produce long-lasting increases in neural responsiveness. This evidence comes mainly from models of heterosynaptic facilitation in which acetylcholine produces a strengthening of a second, noncholinergic synaptic input onto the same neuron. The argument that the basal forebrain cholinergic system is essential in some models of plasticity is supported by studies that have selectively lesioned the cholinergic basal forebrain. This review will examine the mechanisms whereby acetylcholine might induce synaptic plasticity. It will also consider the neural circuitry implicated in these studies, namely the pathways that are susceptible to cholinergic plasticity and the neural regulation of the cholinergic system.

Treatment effectiveness for patients with a history of repetitive hand use and focal hand dystonia: a planned, prospective follow-up study

Recent studies show that rapid, nearly simultaneous, stereotypical repetitive fine motor movements can degrade the sensory representation of the hand and lead to a loss of normal motor control with a target task, referred to as occupational hand cramps or focal hand dystonia. The purpose of this prospective follow-up study was to determine whether symptomatic patients in jobs demanding high levels of repetition could be relieved of awkward, involuntary hand movements following sensory discriminative retraining complemented by a home program of sensory exercises, plus traditional posture, relaxation, mobilization, and fitness exercises. Twelve patients participated in the study. They all had occupational hand cramps, as diagnosed by a neurologist. Each patient was evaluated by a trained, independent research assistant before treatment and three to six months after treatment, by use of a battery of sensory, motor, physical, and functional performance tests. Care was provided by a physical therapist or a supervised physical therapist student in an outpatient clinic. Patients were asked to stop performing the target task and to come once a week for supervised treatment that included 1) heavy schedules of sensory training with and without biofeedback to restore the sensory representation of the hand, and 2) instructions in stress-free hand use, mirror imagery, mental rehearsal, and mental practice techniques designed to stop the abnormal movements and facilitate normal hand control. Patients were instructed in therapeutic exercises to be performed in the home to improve postural alignment, reduce neural tension, facilitate relaxation, and promote cardiopulmonary fitness. Following the defined treatment period, all patients were independent in activities of daily living, and all but one patient returned to work. Significant gains were documented in motor control, motor accuracy; sensory discrimination, and physical performance (range of motion, strength, posture, and balance). This descriptive study that includes patients with occupation-related focal hand dystonia provides evidence that aggressive sensory discriminative training complemented by traditional fitness exercises to facilitate musculoskeletal health can improve sensory processing and motor control of the hand.

Partial denervation of the whiskerpad in adult mice: pattern and origin of reinnervation

We studied sensory organ reinnervation after nerve transection in the mouse whisker-to-barrel pathway. In one set of adult mice, we determined at light microscopy level the number of fibres reaching the caudal whisker follicles 5, 15, 20, 60, 100 days and 1 year after transection of the sensory nerve of row C. Regenerated fibres were first detected 15 days post lesionem (p.l.) and myelin first observed at 20 days. Between 60 and 100 days, the number of fibres stayed at approximately 80% of the values obtained in control animals. At that time, myelinated fibres reached only 58% of their number in controls. At the electron microscopy level, these fibres differ from control ones by a smaller fibre diameter. The innervation of follicles of adjacent rows was not modified, indicating that follicular reinnervation is row specific. We checked this feature by injecting in another set of mice the denervated follicles and the adjacent ones with distinct retrograde tracers 45 days and 1 year after nerve transection. The percentage of double-labelled neurons in the Gasserian ganglion did not increase in experimental animals. This confirms the absence of colonization of intact follicles by regenerating fibres and indicates that reinnervation of the whisker follicles takes place by regeneration of the degenerated axons without collateral reinnervation. The companion paper describes the pattern of activation of the barrel cortex relative to the present findings.

Differential effects of nucleus basalis lesions in young adult and aging rats

To characterize age-related changes in frontal cortical plasticity, we assessed maze learning and frontal cortical pharmacology in young adult, middle-aged, and aged rats. Rats received either ibotenic acid or sham lesions of the nucleus basalis magnocellularis (NBM) and were then trained on a radial maze task. After training, we assessed [3H]desmethylimipramine (DMI), [3H]muscimol, [3H]AMPA, and [3H]QNB binding using quantitative autoradiography. Both middle-aged and aged rats were impaired on the radial maze task. DMI binding was increased in both middle-aged and aged rats, while QNB binding was decreased in aged rats. While lesions impaired maze performance at all ages, middle-aged and aged rats showed more profound lesion-induced deficits. Lesions increased GABA, and AMPA receptor binding in young adult rats only. These lesion-induced changes may reflect a compensatory response that is lost with advancing age.

Use-dependent exaggeration of brain injury: is glutamate involved?

Extreme overreliance on the impaired forelimb following unilateral lesions of the forelimb representation area of the rat sensorimotor cortex (FL-SMC) leads to exaggeration of the initial cortical injury. Glutamate has repeatedly been implicated in the secondary processes leading to neuronal death following traumatic insult, chiefly because of the neuroprotective properties of excitatory amino acid antagonists in a variety of animal models of brain injury. The present study investigated the possibility that NMDA receptor-mediated processes are involved in use-dependent exaggeration of neuronal injury. Rats were fitted with one-sleeved casts that immobilized the intact forelimb for the first 7 days following FL-SMC lesion, a procedure previously shown to result in use-dependent exaggeration of injury and more severe and persistent limb-use deficits. In the present investigation, administration of MK-801 (1 mg/kg ip once daily on alternate days) during the casting period spared neural tissue surrounding the lesion and enhanced functional recovery of the impaired forelimb. These results suggest a role for NMDA receptor-mediated processes in use-dependent exaggeration of injury.

Experience-dependent corticofugal adjustment of midbrain frequency map in bat auditory system

Recent studies of corticofugal modulation of auditory information processing indicate that cortical neurons mediate both a highly focused positive feedback to subcortical neurons "matched" in tuning to a particular acoustic parameter and a widespread lateral inhibition to "unmatched" subcortical neurons. This cortical function for the adjustment and improvement of subcortical information processing is called egocentric selection. Egocentric selection enhances the neural representation of frequently occurring signals in the central auditory system. For our present studies performed with the big brown bat (Eptesicus fuscus), we hypothesized that egocentric selection adjusts the frequency map of the inferior colliculus (IC) according to auditory experience based on associative learning. To test this hypothesis, we delivered acoustic stimuli paired with electric leg stimulation to the bat, because such paired stimuli allowed the animal to learn that the acoustic stimulus was behaviorally important and to make behavioral and neural adjustments based on the acquired importance of the acoustic stimulus. We found that acoustic stimulation alone evokes a change in the frequency map of the IC; that this change in the IC becomes greater when the acoustic stimulation is made behaviorally relevant by pairing it with electrical stimulation; that the collicular change is mediated by the corticofugal system; and that the IC itself can sustain the change evoked by the corticofugal system for some time. Our data support the hypothesis.

Potential circuit mechanisms underlying concurrent thalamic and cortical plasticity

During the last two decades, plastic reorganization of both sensory and motor representations in the adult central nervous system has been demonstrated following a large variety of manipulations, ranging from partial lesions of the sensory receptor surface to modifications in sensory experience (see /14/ for review). Yet, little is known about the neural circuit mechanisms underlying such reorganization process. Despite the difficulty in addressing this issue, recent studies have provided some insights into this fundamental question. Altogether, these studies suggest that the process of plastic reorganization is a system-wide phenomenon, involving both cortical and subcortical representations. Contrary to classical beliefs, recent work also suggests that the final outcome of the reorganization process is not necessarily beneficial, since it can lead to abnormal perceptual experiences /31/, such as the phantom limb sensation, and even pain /31,32/. In this review, we focus on recent insights into the possible circuit mechanisms underlying sensory plasticity and discuss the potential implications of these findings. We then present physiological evidence supporting the view that the process of plasticity observed at the cortical level may reflect simultaneous changes in many subcortical structures.

Role of the basal forebrain cholinergic projection in somatosensory cortical plasticity

Trimming all but two whiskers in adult rats produces a predictable change in cortical cell-evoked responses characterized by increased responsiveness to the two intact whiskers and decreased responsiveness to the trimmed whiskers. This type of synaptic plasticity in rat somatic sensory cortex, called "whisker pairing plasticity," first appears in cells above and below the layer IV barrels. These are also the cortical layers that receive the densest cholinergic inputs from the nucleus basalis. The present study assesses whether the cholinergic inputs to cortex have a role in regulating whisker pairing plasticity. To do this, cholinergic basal forebrain fibers were eliminated using an immunotoxin specific for these fibers. A monoclonal antibody to the low-affinity nerve growth factor receptor 192 IgG, conjugated to the cytotoxin saporin, was injected into cortex to eliminate cholinergic fibers in the barrel field. The immunotoxin reduces acetylcholine esterase (AChE)-positive fibers in S1 cortex by >90% by 3 wk after injection. Sham-depleted animals in which either saporin alone or saporin unconjugated to 192 IgG is injected into the cortex produces no decrease in AChE-positive fibers in cortex. Sham-depleted animals show the expected plasticity in barrel column neurons. In contrast, no plasticity develops in the ACh-depleted, 7-day whisker-paired animals. These results support the conclusion that the basal forebrain cholinergic projection to cortex is an important facilitator of synaptic plasticity in mature cortex.

Cortical map reorganization enabled by nucleus basalis activity

Little is known about the mechanisms that allow the cortex to selectively improve the neural representations of behaviorally important stimuli while ignoring irrelevant stimuli. Diffuse neuromodulatory systems may facilitate cortical plasticity by acting as teachers to mark important stimuli. This study demonstrates that episodic electrical stimulation of the nucleus basalis, paired with an auditory stimulus, results in a massive progressive reorganization of the primary auditory cortex in the adult rat. Receptive field sizes can be narrowed, broadened, or left unaltered depending on specific parameters of the acoustic stimulus paired with nucleus basalis activation. This differential plasticity parallels the receptive field remodeling that results from different types of behavioral training. This result suggests that input characteristics may be able to drive appropriate alterations of receptive fields independently of explicit knowledge of the task. These findings also suggest that the basal forebrain plays an active instructional role in representational plasticity.