"Learned" changes in the responses of the rat barrel field neurons

The Sensory Neocortex and Associative Memory

Most behaviors in mammals are directly or indirectly guided by prior experience and therefore depend on the ability of our brains to form memories. The ability to form an association between an initially possibly neutral sensory stimulus and its behavioral relevance is essential for our ability to navigate in a changing environment. The formation of a memory is a complex process involving many areas of the brain. In this chapter we review classic and recent work that has shed light on the specific contribution of sensory cortical areas to the formation of associative memories. We discuss synaptic and circuit mechanisms that mediate plastic adaptations of functional properties in individual neurons as well as larger neuronal populations forming topographically organized representations. Furthermore, we describe commonly used behavioral paradigms that are used to study the mechanisms of memory formation. We focus on the auditory modality that is receiving increasing attention for the study of associative memory in rodent model systems. We argue that sensory cortical areas may play an important role for the memory-dependent categorical recognition of previously encountered sensory stimuli.

Stimulus-timing-dependent plasticity of cortical frequency representation

Adult cortical circuits possess considerable plasticity, which can be induced by modifying their inputs. One mechanism proposed to underlie changes in neuronal responses is spike-timing-dependent plasticity (STDP), an up- or downregulation of synaptic efficacy contingent upon the order and timing of presynaptic and postsynaptic activity. The repetitive and asynchronous pairing of a sensory stimulus with either another sensory stimulus or current injection can alter the response properties of visual and somatosensory neurons in a manner consistent with STDP. To examine whether such plasticity also exists in the auditory system, we recorded from neurons in the primary auditory cortex of anesthetized and awake adult ferrets. The repetitive pairing of pure tones of different frequencies induced shifts in neuronal frequency selectivity, which exhibited a temporal specificity akin to STDP. Only pairs with stimulus onset asynchronies of 8 or 12 ms were effective and the direction of the shifts depended upon the order in which the tones within a pair were presented. Six hundred stimulus pairs (lasting approximately 70 s) were enough to produce a significant shift in frequency tuning and the changes persisted for several minutes. The magnitude of the observed shifts was largest when the frequency separation of the conditioning stimuli was < approximately 1 octave. Moreover, significant shifts were found only in the upper cortical layers. Our findings highlight the importance of millisecond-scale timing of sensory input in shaping neural function and strongly suggest STDP as a relevant mechanism for plasticity in the mature auditory system.

Conditioned lick behavior and evoked responses using whisker twitches in head restrained rats

To examine whisker barrel evoked response potentials in chronically implanted rats during behavioral learning with very fast response times, rats must be calm while immobilized with their head restrained. We quantified their behaviors during training with an ethogram and measured each individual animals' progress over the training period. Once calm under restraint, rats were conditioned to differentiate between a reward and control whisker twitch, then provide a lick response when presented with the correct stimulus, rewarded by a drop of water. Rats produced the correct licking response (after reward whisker twitch), and learned not to lick after a control whisker was twitched. By implementing a high-density 64-channel electrocorticogram (ECoG) electrode array, we mapped the barrel field of the somatosensory cortex with high spatial and temporal resolution during conditioned lick behaviors. In agreement with previous reports, we observe a larger evoked response after training, probably related to mechanisms of cortical plasticity.

Rapid plasticity follows whisker pairing in barrel cortex of the awake rat

Synaptic plasticity can be induced easily throughout life in the rodent somatic sensory cortex. Trimming all but two whiskers on one side of an adult rat's face, called 'whisker pairing', causes the active (intact) whiskers to develop a stronger drive on cortical cells in their respective barrel columns, while inactive (trimmed) whisker efficacy is down-regulated. To date, this type of activity-dependent plasticity has been induced by trimming all but two whiskers, letting the rats explore their environment from 1 day to 1 month, after which cortical responses were analyzed physiologically under anesthesia. Such studies have enhanced our understanding of cortical plasticity, but the anesthesia complicates the examination of changes that occur in the first few hours after whisker trimming. Here we assayed the short-term changes that occur in alert, active animals over a period of hours after whisker trimming. The magnitude of barrel cortex evoked responses was measured in response to stimulation of the cut and paired whiskers of rats under several conditions: (a) whisking in air (control), (b) active whisking of an object by the rat, and (c) epochs of passive whisker stimulation to identify the onset of whisker pairing plasticity changes in cortex. The main difference between whisking in air without contact and passive whisker stimulation is that the former condition induces an increased response to stimulation of inactive cut whiskers, while the latter condition increases the responses to the stimulated whiskers. The results support the conclusion that whisker pairing plasticity in barrel cortex occurs within 4 h after whisker trimming in an awake, alert animal.

Short exposure to an enriched environment accelerates plasticity in the barrel cortex of adult rats

Cortical sensory neurons adapt their response properties to use and disuse of peripheral receptors in their receptive field. Changes in synaptic strength can be generated in cortex by simply altering the balance of input activity, so that a persistent bias in activity levels modifies cortical receptive field properties. Such activity-dependent plasticity in cortical cell responses occurs in rat cortex when all but two whiskers are trimmed for a period of time at any age. The up-regulation of evoked responses to the intact whiskers is first seen within 24 h in the supragranular layers [Laminar comparison of somatosensory cortical plasticity. Science 265(5180):1885-1888] and continues until a new stable state is achieved [Experience-dependent plasticity in adult rat barrel cortex. Proc Natl Acad Sci U S A 90(5):2082-2086; Armstrong-James M, Diamond ME, Ebner FF (1994) An innocuous bias in whisker use in adult rat modifies receptive fields of barrel cortex neurons. J Neurosci 14:6978-6991]. These and many other results suggest that activity-dependent changes in cortical cell responses have an accumulation threshold that can be achieved more quickly by increasing the spike rate arising from the active region of the receptive field. Here we test the hypothesis that the rate of neuronal response change can be accelerated by placing the animals in a high activity environment after whisker trimming. Test stimuli reveal an highly significant receptive field bias in response to intact and trimmed whiskers in layer IV as well as in layers II-III neurons in only 15 h after whisker trimming. Layer IV barrel cells fail to show plasticity after 15-24 h in a standard cage environment, but produce a response bias when activity is elevated by the enriched environment. We conclude that elevated activity achieves the threshold for response modification more quickly, and this, in turn, accelerates the rate of receptive field plasticity.

Acetylcholine-dependent potentiation of temporal frequency representation in the barrel cortex does not depend on response magnitude during conditioning

The response properties of neurons of the postero-medial barrel sub-field of the somatosensory cortex (the cortical structure receiving information from the mystacial vibrissae can be modified as a consequence of peripheral manipulations of the afferent activity. This plasticity depends on the integrity of the cortical cholinergic innervation, which originates at the nucleus basalis magnocellularis (NBM). The activity of the NBM is related to the behavioral state of the animal and the putative cholinergic neurons are activated by specific events, such as reward-related signals, during behavioral learning. Experimental studies on acetylcholine (ACh)-dependent cortical plasticity have shown that ACh is needed for both the induction and the expression of plastic modifications induced by sensory-cholinergic pairings. Here we review and discuss ACh-dependent plasticity and activity-dependent plasticity and ask whether these two mechanisms are linked. To address this question, we analyzed our data and tested whether changes mediated by ACh were activity-dependent. We show that ACh-dependent potentiation of response in the barrel cortex of rats observed after sensory-cholinergic pairing was not correlated to the changes in activity induced during pairing. Since these results suggest that the effect of ACh during pairing is not exerted through a direct control of the post-synaptic activity, we propose that ACh might induce its effect either pre- or post-synaptically through activation of second messenger cascades.

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.

Dynamics of neuronal processing in rat somatosensory cortex

Recently, the study of sensory cortex has focused on the context-dependent evolution of receptive fields and cortical maps over millisecond to second time-scales. This article reviews advances in our understanding of these processes in the rat primary somatosensory cortex (SI). Subthreshold input to individual rat SI neurons is extensive, spanning several vibrissae from the center of the receptive field, and arrives within 25 ms of vibrissa deflection. These large subthreshold receptive fields provide a broad substrate for rapid excitatory and inhibitory multi-vibrissa interactions. The 'whisking' behavior, an approximately 8 Hz ellipsoid movement of the vibrissae, introduces a context-dependent change in the pattern of vibrissa movement during tactile exploration. Stimulation of vibrissae over this frequency range modulates the pattern of activity in thalamic and cortical neurons, and, at the level of the cortical map, focuses the extent of the vibrissa representation relative to lower frequency stimulation (1 Hz). These findings suggest that one function of whisking is to reset cortical organization to improve tactile discrimination. Recent discoveries in primary visual cortex (VI) demonstrate parallel non-linearities in center-surround interactions in rat SI and VI, and provide a model for the rapid integration of multi-vibrissa input. The studies discussed in this article suggest that, despite its original conception as a uniquely segregated cortex, rat SI has a wide array of dynamic interactions, and that the study of this region will provide insight into the general mechanisms of cortical dynamics engaged by sensory systems.

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.

Learning-induced physiological plasticity in the thalamo-cortical sensory systems: a critical evaluation of receptive field plasticity, map changes and their potential mechanisms

The goal of this review is to give a detailed description of the main results obtained in the field of learning-induced plasticity. The review is focused on receptive field and map changes observed in the auditory, somatosensory and visual thalamo-cortical system as a result of an associative training performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the auditory and somatosensory system are reviewed. In the visual system, as there is no RF and map analysis during learning per se, the evidence presented are from increased neuronal responsiveness, and from the effects of perceptual learning in human and non human primates. Across sensory modalities, the re-tuning of neurons to a significant stimulus or map reorganizations in favour of the significant stimuli were observed at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neuronal changes and (ii) of neuromodulators as "gating" factors of the neuronal changes, is evaluated. The weakness of the Hebbian schema to explain learning-induced changes and the need to better define what the word "learning" means are stressed. It is suggested that future research should focus on the dynamic of information processing in sensory systems, and the concept of "effective connectivity" should be useful in that matter.

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.

Spatio-temporal subthreshold receptive fields in the vibrissa representation of rat primary somatosensory cortex

Spatio-temporal subthreshold receptive fields in the vibrissa representation of rat primary somatosensory cortex. J. Neurophysiol. 80: 2882-2892, 1998. Whole cell recordings of synaptic responses evoked by deflection of individual vibrissa were obtained from neurons within adult rat primary somatosensory cortex. To define the spatial and temporal properties of subthreshold receptive fields, the spread, amplitude, latency to onset, rise time to half peak amplitude, and the balance of excitation and inhibition of subthreshold input were quantified. The convergence of information onto single neurons was found to be extensive: inputs were consistently evoked by vibrissa one- and two-away from the vibrissa that evoked the largest response (the "primary vibrissa"). Latency to onset, rise time, and the incidence and strength of inhibitory postsynaptic potentials (IPSPs) varied as a function of position within the receptive field and the strength of evoked excitatory input. Nonprimary vibrissae evoked smaller amplitude subthreshold responses [primary vibrissa, 9.1 +/- 0.84 (SE) mV, n = 14; 1-away, 5. 1 +/- 0.5 mV, n = 38; 2-away, 3.7 +/- 0.59 mV, n = 22; 3-away, 1.3 +/- 0.70 mV, n = 8] with longer latencies (primary vibrissa, 10.8 +/- 0.80 ms; 1-away, 15.0 +/- 1.2 ms; 2-away, 15.7 +/- 2.0 ms). Rise times were significantly faster for inputs that could evoke action potential responses (suprathreshold, 4.1 +/- 1.3 ms, n = 8; subthreshold, 12.4 +/- 1.5 ms, n = 61). In a subset of cells, sensory evoked IPSPs were examined by deflecting vibrissa during injection of hyperpolarizing and depolarizing current. The strongest IPSPs were evoked by the primary vibrissa (n = 5/5), but smaller IPSPs also were evoked by nonprimary vibrissae (n = 8/13). Inhibition peaked by 10-20 ms after the onset of the fastest excitatory input to the cortex. This pattern of inhibitory activity led to a functional reversal of the center of the receptive field and to suppression of later-arriving and slower-rising nonprimary inputs. Together, these data demonstrate that subthreshold receptive fields are on average large, and the spatio-temporal dynamics of these receptive fields vary as a function of position within the receptive field and strength of excitatory input. These findings constrain models of suprathreshold receptive field generation, multivibrissa interactions, and cortical plasticity.

Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex

Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex. J. Neurophysiol. 80: 3261-3271, 1998. In mature rat primary somatic sensory cortical area (SI) barrel field cortex, the thalamic-recipient granular layer IV neurons project especially densely to layers I, II, III, and IV. A prior study showed that cells in the supragranular layers are the fastest to change their response properties to novel changes in sensory inputs. Here we examine the effect of removing supragranular circuitry on the responsiveness and synaptic plasticity of cells in the remaining layers. To remove the layer II + III (supragranular) neurons from the circuitry of barrel field cortex, N-methyl--aspartate (NMDA) was applied to the exposed dura over the barrel cortex, which destroys those neurons by excitotoxicity without detectable damage to blood vessels or axons of passage. Fifteen days after NMDA treatment, the first responsive cells encountered were 400-430 micrometers below the pial surface. In separate cases triphenyltetrazolium chloride (TTC), a vital dye taken up by living cells, was absent from the lesion area. Cytochrome oxidase (CO) activity was absent in the first few tangential sections through the barrel field in all cases before arriving at the CO-dense barrel domains. These findings indicate that the lesions were quite consistent from animal to animal. Controls consisted of applying vehicle without NMDA under similar conditions. Responses of D2 barrel cells were assessed for spontaneous activity and level of response to stimulation of the principal D2 whisker and four surround whiskers D1, D3, C2, and E2. In two additional groups of animals treated in the same way, sensory plasticity was assessed by trimming all whiskers except D2 and either D1 or D3 (called Dpaired) for 7 days before recording cortical responses. Such whisker pairing normally potentiates D2 barrel cell responses to stimulation of the two intact whiskers (D2 + Dpaired). After NMDA lesions, cortical cells still responded to all whiskers tested. Cells in lesioned cortex showed reduced response amplitude compared with sham-operated controls to all D-row whiskers. In-arc surround whisker (C2 or E2) responses were normal. Spontaneous activity did not change significantly in any remaining layer at the time tested. Modal latencies to stimulation of principal D2 or surround D1 or D3 whiskers showed no significant change after lesioning. These findings indicate that there is a reasonable preservation of the response properties of layer IV, V, VI neurons after removal of layer II-III neurons in this way. Whisker pairing plasticity in layer IV-VI D2 barrel column neurons occurred in both lesioned and sham animals but was reduced significantly in lesioned animals compared with controls. The response bias generated by whisker trimming (Dpaired/Dcut + Dpaired ratio) was less pronounced in NMDA-lesioned than sham-lesioned animals. Proportionately fewer neurons in layer IV (52 vs. 64%) and in the infragranular layers (55 vs. 68%) exhibited a clear response bias to paired whiskers. We conclude that receptive-field plasticity can occur in layers IV-VI of barrel cortex in the absence of the supragranular layer circuitry. However, layer I-III circuitry does play a role in normal receptive-field generation and is required for the full expression of whisker pairing plasticity in granular and infragranular layer cells.

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.

Plasticity of cerebral metabolic whisker maps in adult mice after whisker follicle removal--I. Modifications in barrel cortex coincide with reorganization of follicular innervation

We investigated alterations of the metabolic whisker map of barrel cortex after the removal of the follicles of left whiskers C1, C2 and C3 in adult albino mice. The quantitative autoradiographic [14C]deoxyglucose method was used to measure local cerebral metabolic rates for glucose in barrel cortex of mice two, four, eight, 64, 160 and 250 days after the lesion. Metabolic rates were measured in three groups of animals: (i) mice with lesions that had all whiskers clipped; (ii) mice with lesions that had left whiskers B1-3 and D1-3 stimulated; and (iii) unoperated mice that had left whiskers B1-3 and D1-3 stimulated. Compared with the metabolic rates in barrels C1-3 of stimulated unoperated mice, barrels C1-3 of stimulated mice with lesions showed the first discernible increase in metabolic rate four days after the lesion. The increase became distinct at 64 days, but attained statistical significance only approximately 160 days after the lesion. The lesion per se, i.e. without whisker stimulation, caused only a small increase in metabolic rate in barrels C1-3 accounting for not more than one fourth of the increase in metabolic rate measured after whisker deflection. The removal of whisker follicles C1-3 led, therefore, to an enlargement of the metabolic representations of the adjacent whiskers into the barrels deprived by the lesion. The gradual consolidation of the alterations of the metabolic whisker map coincided with the regeneration of follicular nerves in the whiskerpad. We detected anomalous deep nerves innervating follicles surrounding the lesion at approximately 64 days, and the number of myelinated nerve fibres in the deep nerves of these follicles was increasing with increasing time after the lesion. The coincidence of peripheral and central change suggests that the reorganization of the innervation of the sensory periphery plays an important role in the persistent alterations of the cortical somatotopy in adults following a lesion in the sensory periphery.

Transient changes of electrical activity in the rat barrel cortex during conditioning

To reveal the dynamics of neurophysiological changes in the rat barrel cortex induced by conditioned stimulation we recorded the local micro-electroencephalographic (EEG) activity and evoked potentials (EPs) in barrel cortex to stimulation of a single vibrissa before and after pairing it with a mild electric shock applied to the rat's tail. Following the introduction of the reinforcing stimulus, the amplitude of the first negative component of evoked potentials in the cortex on the conditioned side grew in relation to the same component of control potentials, evoked by stimulation of the opposite symmetrical vibrissa. This change was accompanied by a latent decrease in spectral power of the EEG within the alpha and beta frequency bands in both hemispheres. The observed changes in both of these electrical manifestations of enhanced neuronal activity reverted after two (EP) or three (EEG) days of conditioning. These results are discussed in relation to the putative activity of neuromodulatory systems.

Does Hebbian synaptic plasticity explain learning-induced sensory plasticity in adult mammals?

Over the last decade, a large number of studies have demonstrated that sensory systems undergo functional reorganizations in adult mammals. In the auditory system, highly specific reorganizations were observed during learning situations in which a particular tone frequency predicts the occurrence of an aversive event. After a brief overview of the specific receptive field changes observed after associative learning in cortical and thalamic neurons, I will raise the question concerning whether or not Hebbian synaptic plasticity adequately accounts for these data. The required conditions for Hebbian synaptic plasticity to act do not seem to be met in situations in which learning-induced receptive field plasticity occurs. This analysis points out the weakness of the traditional Hebbian scheme to provide realistic bases for learning-induced neuronal plasticity and stresses the need to look for other potential mechanisms involving neuromodulators.

Lateralized effect of unilateral somatosensory cortex contusion on behavior and cortical reorganization

Previous studies have shown that rats recover function after unilateral somatosensory cortex lesions, possibly by transfer of information processing to other brain areas not normally involved in those functions. In the present study, adult rats underwent unilateral contusions of the somatosensory cortex with ablation of the barrel receptor field. Behavioral testing with modified beam-walking and sensory neglect tasks demonstrated persistent somatosensory deficits in rats with left contusions but no apparent deficits in right injured animals. After 2 months, the [14C]2-deoxyglucose (2-DG) method was used to show the metabolic activity produced by unilateral stimulation of the facial vibrissae. In left injured animals, cortical metabolic activity rostral and caudal to the injury site was depressed both under basal conditions and during right vibrissal stimulation. On the other hand, comparison of the pattern of [14C]2-DG uptake in the intact, right cortex revealed changes in the pattern of glucose utilization associated with left injury combined with right vibrissal stimulation. Pattern changes were quantified by measuring the area in which glucose utilization was within the highest 25% of this range (high activity area; HAA). Right vibrissal stimulation in left injured rats caused an expansion of this HAA in the intact occipital/temporal cortex. Also, in the intact somatosensory cortex of left injured rats, there was an enlarged HAA whether or not vibrissal stimulation was performed. Thus, a combination of depressed peri-injury metabolic activity and aberrant activity in remote brain areas occurs following unilateral somatosensory cortex injury. It remains to be shown whether these factors ameliorate or contribute to persistent behavioral deficits.

Laminar comparison of somatosensory cortical plasticity

During tactile learning there is a transformation in the way the primary somatosensory cortex integrates, represents, and distributes information from the skin. To define this transformation, the site of earliest modification has been identified in rat somatosensory cortex after a change in sensory experience. Afferent activity was manipulated by clipping all except two whiskers on one side of the snout ("whisker pairing"), and the receptive fields of neurons at different cortical depths were mapped 24 hours later. Neurons in layer IV, the target of the primary thalamic pathway, were unaltered, whereas neurons located above and below layer IV showed significant changes. These changes were similar to those that occur in layer IV after longer periods of whisker pairing. The findings support the hypothesis that the layers of cortex contribute differently to plasticity. Neurons in the supragranular and infragranular layers respond rapidly to changes in sensory experience and may contribute to subsequent modification in layer IV.

Neurobiology of associative learning in the neonate: early olfactory learning

Mammalian neonates have been simultaneously described as having particularly poor memory, as evidenced by infantile amnesia, and as being particularly excellent learners with unusually plastic nervous systems that are easily influenced by experience. An understanding of the neurobiological constraints and mechanisms of early learning may contribute to a unified explanation of these two disparate views. Toward that end, we review here our work on the neurobiology of learning and memory in neonates. Specifically, we have examined the neurobiology of early learning using an olfactory classical conditioning paradigm. Olfactory classical conditioning in neonates at the behavioral level conforms well with the requirements and outcomes of classical conditioning described in adults. Furthermore, specific neural correlates of this behavioral conditioning have been described including anatomical and physiological changes, neural pathways, and modulatory systems. In this Review, we outline the behavioral paradigm, the identified neural correlates, and apparent mechanisms of this learning. Finally, we compare the neurobiology of early learning with that reported for mature animals, with specific reference to the role of US-CS convergence, memory modulation, consolidation, and distributed memory.

Developmental plasticity in cerebellar tactile maps: fractured maps retain a fractured organization

Plasticity following deafferentation has been repeatedly demonstrated in topographic sensory maps in the mammalian brain. In this paper we investigated the developmental plasticity of the fractured somatotopic map found in the tactile regions of the rat cerebellum. At various stages of postnatal development between postnatal days 1 and 30, we cauterized the infraorbital branch of the trigeminal nerve, which innervates the upper lip, furry buccal pad, and vibrissae that are represented within cerebellar folium crus IIa. The organization of the crus IIa map was then examined 2 to 3 months after denervation. We found that tactile receptive fields had reorganized throughout the denervated area but maintained a fractured somatotopy. Comparison of the reorganization in different animals showed that the denervated upper lip region was consistently and predominantly replaced by representation of the upper incisors. Analysis of evoked field potentials revealed an alteration, in denervated animals, of the response of the granule cell layer to brief tactile stimulation. This response in normal animals consists of two components at different latencies. Animals lesioned later in development were less likely to have the short latency component. This result suggests a difference in the developmental sensitivity of different cerebellum-related pathways to nerve lesions.

Responses of hippocampal cells can be conditioned during paradoxical sleep

To test the hypothesis that new associations can be acquired during sleep, we developed a conditioning paradigm in which both conditioned (CS) and unconditioned (US) stimuli were non-awakening intra-cerebral stimulations. The CS was a stimulation of the Medical Geniculate body and the US a stimulation of the Central Grey. An increase in hippocampal multiunit activity to CS was taken as the conditioned response. CS-US pairings were presented across 14 sessions, with 15 trials per session and a 24-h inter-session interval. Three groups were studied: in a group the CS-US pairings were given during the awake state (group W), and in two groups pairings were presented during sleep, either slow-wave sleep (group SWS) or paradoxical sleep (group PS). In the last group, to test the possibility of transfer to the awake state of the hippocampal response acquired in PS, the CS alone were presented interspersed with periods of wakefulness. Results showed that, before pairing, CS presentation induced no change in hippocampal multiunit activity in the three groups. After pairing, no hippocampal response to CS presentation occurred in SWS. In contrast, in the W group and in the PS group, a marked increase in hippocampal activity appeared to CS. The hippocampal response in the PS group developed progressively across sessions; it occurred only two sessions later than in the W group. Moreover, when the CS-evoked response reached the asymptotic level in PS, the presentation of CS alone in awake animals elicited the hippocampal response. These results suggest that a cellular conditioning can be established during PS and that the cellular conditioned response developed in PS can be transferred to the awake state.

Classical conditioning induces CS-specific receptive field plasticity in the auditory cortex of the guinea pig

To determine if classical conditioning produces general or specific modification of responses to acoustic conditioned stimuli (CS), frequency receptive fields (RF) of neurons in guinea pig auditory cortex were determined before and up to 24 h after fear conditioning. Highly specific RF plasticity characterized by maximal increased responses to the CS frequency and decreased responses to the pretraining best frequency (BF) and other frequencies was observed in 70% of conditioning cases. These opposing changes were often sufficient to produce a shift in tuning such that the frequency of the CS became the new BF. CS frequency specific plasticity was maintained as long as 24 h. Sensitization training produced general increased responses across the RF without CS specificity. The findings indicate that associative processes produce systematic modification of the auditory system's processing of frequency information and exemplify the advantages of combining receptive field analysis with behavioral training in the study of the neural bases of learning and memory.

Intracortical processes regulating the integration of sensory information

The mechanisms that link sensory inputs in spatially separated regions of cortex can be elucidated by analyzing the mechanisms that generate receptive field properties in cortical neurons under conditions that mimic the waking state; a state when learning, memory and the modification of synaptic strength can be most readily demonstrated. Important advances in understanding receptive field mechanisms in sensory cortex have arisen from studying the precise relationship between the mystacial vibrissae or "whiskers" and their neural representation in separate cortical domains or "barrels". The anatomical precision of whisker projections to barrels permits a unique delineation of thalamocortical and intracortical components of cortical cell responses based on latency and security of response to peripheral receptor stimulation. When recorded in awake animals or even under very light anesthesia, cortical neurons show two components to their response to whisker movement. Neurons in layer IV of a whisker's primary projection zone respond with short latency (7-10 msec) and a high response magnitude (two or more action potentials (spikes) per stimulus). This "Center Receptive Field" (CRF) for layer IV cells is generated in large part by sensory fiber inputs from the thalamus. The CRF is restricted to 1.4 whiskers on average and is the only response detectable when cortical responses are depressed by deep anesthesia. In the "waking state" the same neuron often will respond to deflection of 4-6 surrounding whiskers, but only at longer latency (15-40 msec) and with fewer spikes per stimulus. These more labile responses form an excitatory surround receptive field (SRF). Sensory information that is transduced by individual whiskers and that generates the SRF of a cortical neuron achieves this added response complexity through intracortical mechanisms. The control of the mechanisms that determine the dissemination of sensory information within cortex include: (1) regulating the level of GABAergic inhibition; and (2) potentiation or depression of the response level generated by repeated sensory experience. State-dependent "modulatory" inputs to cortex, such as the noradrenergic and cholinergic fiber system, could regulate the degree of horizontal spread of a sensory input, in part through global changes in the level of inhibition and/or regulating the amplitude of cortical responses, thereby determining the level of associative interactions between sensory inputs.

Modifications of the responses of barrel field neurons to vibrissal stimulation during theta in the awake and undrugged rat

In partially restrained but awake and undrugged rats, excitatory unit responses of the somatic cortex barrel field to vibrissal stimulation, were recorded in two conditions: during spontaneous episodes of theta and in the absence of this rhythm. Two main variables were considered: a signal-to-noise ratio and an index of the "afferent inhibition". Both measures were extracted from peristimulus time histograms. "Theta effects" were characterized by an increase in signal-to-noise ratio and afferent inhibition. They were most important in neurons located in infragranular layers of the cortex; they went in the same direction but only approached significance in supragranular neurons; neurons of the granular layer were not affected. Spontaneous unit activity and latencies were not modified in any group. These data were obtained during a preliminary step of a sensory-sensory conditioning procedure which in some cases modified the receptive field of the neurons. Theta effects were less marked in future "conditioned" than in future non-conditioned neurons but this was probably due to the fact that conditioned neurons had significantly higher signal-to-noise ratio and afferent inhibition. The origin of these "theta effects", hippocampal versus non-hippocampal, and their functional significance, relation to selective attention, are discussed.

Evidence for a cholinergic mechanism of "learned" changes in the responses of barrel field neurons of the awake and undrugged rat

Due to its functional importance and its large and highly differentiated central projections, the vibrissal system of rodents is a prime object for the study of sensory plasticity, especially at the cortical level: the representation of vibrissae in the "barrel field", a part of the somatic cortex, is exceptionally precise and is susceptible to experience-induced changes. In a previous series of experiments, we found that a sensory-sensory conditioning procedure, pairing two vibrissal stimulations, produces significant changes in responses of single neurons of the barrel field in the chronic awake and undrugged rat: (1) the appearance of an excitatory response to a stimulus that was ineffective before pairing ("conditioned response"); (2) the modifications of pre-existing responses consisting of the suppression of afferent inhibition and the appearance of long-latency excitatory components. We report here that the micro-iontophoretic application of atropine abolishes "conditioned responses" and restores afferent inhibition. Acetylcholine facilitates an enlargement of the receptive field and induces a sustained mode of discharge to stimuli. These data provide a new and direct support to the hypothesis that cholinergic mechanisms are involved in the sensory cortex plasticity.