Multimodality multidimensional image analysis of cortical and subcortical plasticity in the rat brain

In this work, we developed and implemented a multimodality multidimensional imaging system which is capable of generating and displaying anatomical and functional images of selected structures and processes within a vertebrate's central nervous system (CNS). The functional images are generated from [¹⁴C]-2-deoxy-d-glucose (2DG) autoradiography whereas the anatomic images are derived from cytochrome oxidase (CO) histochemistry. This multi-modality imaging system has been used to study mechanisms underlying information processing in the rat brain. We have applied this technique to visualize and measure the plasticity (deformation) observed in the rat's whisker system due to neonatal lesioning of selected peripheral sensory organs. Application of this imaging system revealed detailed information about the shape, size, and directionality of selected cortical and subcortical structures. Previous 2-D imaging techniques were unable to deliver such holistic information. Another important issue addressed in this work is related to image registration problems. We developed an image registration technique which employs extrinsic fiduciary marks for alignment and is capable of registering images with subpixel accuracy. It uses the information from all available fiduciary marks to promote alignment of the sections and to avoid propagation of errors across a serial data set.     

Transcranial magnetic stimulation and the motor learning-associated cortical plasticity

It has been well established that repetitive motor performance and skill learning alter the functional organization of human corticomotoneuronal system. Over the past decade, transcranial magnetic stimulation (TMS) has helped to demonstrate motor practice and learning-related changes in corticomotoneuronal excitability and representational plasticity. It has also provided some insights into the mechanisms underlying such plasticity. TMS-derived indices show that motor practice, skill acquisition and learning are associated with an increase in cortical excitability and a modulation of intracortical inhibition partly related to the amount of GABA-related inhibition. It has been suggested that these changes in excitability might be related to learning and motor memory formation in the motor cortex. However, it has proved difficult to relate different aspects of TMS-derived representational plasticity with specific behavioral outcomes. A better understanding of the relationship between TMS measurements of practice-related cortical plasticity and underlying mechanisms, in the context of associated changes in behavior, will facilitate the development of techniques and protocols that will allow predictable modulation of cortical plasticity in health and disease.     

Coactivation of thalamic and cortical pathways induces input timing-dependent plasticity in amygdala

Long-term synaptic enhancements in cortical and thalamic auditory inputs to the lateral nucleus of the amygdala (LAn) mediate encoding of conditioned fear memory. It is not known, however, whether the convergent auditory conditioned stimulus (CSa) pathways interact with each other to produce changes in their synaptic function. We found that continuous paired stimulation of thalamic and cortical auditory inputs to the LAn with the interstimulus delay approximately mimicking a temporal pattern of their activation in behaving animals during auditory fear conditioning resulted in persistent potentiation of synaptic transmission in the cortico-amygdala pathway in rat brain slices. This form of input timing-dependent plasticity (ITDP) in cortical input depends on inositol 1,4,5-trisphosphate (InsP(3))-sensitive Ca(2+) release from internal stores and postsynaptic Ca(2+) influx through calcium-permeable kainate receptors during its induction. ITDP in the auditory projections to the LAn, determined by characteristics of presynaptic activity patterns, may contribute to the encoding of the complex CSa.     

Interindividual variability and age-dependency of motor cortical plasticity induced by paired associative stimulation

Paired associative stimulation (PAS) can increase motor cortical excitability, possibly by long-term potentiation (LTP)-like mechanisms. As the capability of the cortex for plasticity decreases with age, we were interested here in testing interindividual variability and age-dependency of the PAS effect. Motor-evoked potentials (MEPs) were recorded from the resting right abductor pollicis brevis muscle before and for 30 min after PAS in 27 healthy subjects (22–71 years of age). PAS consisted of 225 pairs (rate, 0.25 Hz) of right median nerve stimulation followed at an interval equaling the individual N20-latency of the median nerve somatosensory-evoked cortical potential plus 2 ms by transcranial magnetic stimulation of the hand area of left primary motor cortex (PAS_N20+2). The PAS_N20+2-induced changes in MEP amplitude (ratio post PAS/pre PAS) were highly variable (1.00 ± 0.07, range 0.36–1.68). Fourteen subjects showed the expected LTP-like MEP increase (responders) while 13 subjects showed a long-term depression (LTD)-like MEP decrease (non-responders). Responders had a significantly lower resting motor threshold (RMT) and minimum stimulus intensity to elicit MEPs of 1 mV (MEP_{1 mV}) than non-responders. RMT and MEP_{1 mV} correlated significantly negatively with the PAS_N20+2 effect. The absolute PAS_N20+2 effect size irrespective of its direction decreased with age (r = −0.57, P = 0.002), i.e., LTP-like and LTD-like plasticity were large in young subjects but substantially smaller in elderly subjects. In conclusion, measures of motor cortical excitability (RMT, MEP_{1 mV}) and age determine direction and magnitude of PAS effects in individual subjects.     

Long-term depression induced by sensory deprivation during cortical map plasticity in vivo

Cortical map plasticity is thought to involve long-term depression (LTD) of cortical synapses, but direct evidence for LTD during plasticity or learning in vivo is lacking. One putative role for LTD is in the reduction of cortical responsiveness to behaviorally irrelevant or unused sensory stimuli, a common feature of map plasticity. Here we show that whisker deprivation, a manipulation that drives map plasticity in rat somatosensory cortex (S1), induces detectable LTD-like depression at intracortical excitatory synapses between cortical layer 4 (L4) and L2/3 pyramidal neurons. This synaptic depression occluded further LTD, enhanced LTP, was column specific, and was driven in part by competition between active and inactive whiskers. The synaptic locus of LTD and these properties suggest that LTD underlies the reduction of cortical responses to deprived whiskers, a major component of S1 map plasticity.     

Distinct populations of NMDA receptors at subcortical and cortical inputs to principal cells of the lateral amygdala

Fear conditioning involves the transmission of sensory stimuli to the amygdala from the thalamus and cortex. These input synapses are prime candidates for sites of plasticity critical to the learning in fear conditioning. Because N-methyl-D-aspartate (NMDA)-dependent mechanisms have been implicated in fear learning, we investigated the contribution of NMDA receptors to synaptic transmission at putative cortical and thalamic inputs using visualized whole cell recording in amygdala brain slices. Whereas NMDA receptors are present at both of these pathways, differences were observed. First, the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor-mediated component of the synaptic response, relative to the NMDA component, is smaller at thalamic than cortical input synapses. Second, thalamic NMDA responses are more sensitive to Mg2+. These findings suggest that there are distinct populations of NMDA receptors at cortical and thalamic inputs to the lateral amygdala. Differences such as these might underlie unique contributions of the two pathways to fear conditioning.     

Bidirectional long-term motor cortical plasticity and metaplasticity induced by quadripulse transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) has emerged as a promising tool to induce plastic changes that are thought in some cases to reflect N -methyl- d -aspartate-sensitive changes in synaptic efficacy. As in animal experiments, there is some evidence that the sign of rTMS-induced plasticity depends on the prior history of cortical activity, conforming to the Bienenstock–Cooper–Munro (BCM) theory. However, experiments exploring these plastic changes have only examined priming-induced effects on a limited number of rTMS protocols, often using designs in which the priming alone had a larger effect than the principle conditioning protocol. The aim of this study was to introduce a new rTMS protocol that gives a broad range of after-effects from suppression to facilitation and then test how each of these is affected by a priming protocol that on its own has no effect on motor cortical excitability, as indexed by motor-evoked potential (MEP). Repeated trains of four monophasic TMS pulses (quadripulse stimulation: QPS) separated by interstimulus intervals of 1.5–1250 ms produced a range of after-effects that were compatible with changes in synaptic plasticity. Thus, QPS at short intervals facilitated MEPs for more than 75 min, whereas QPS at long intervals suppressed MEPs for more than 75 min. Paired-pulse TMS experiments exploring intracortical inhibition and facilitation after QPS revealed effects on excitatory but not inhibitory circuits of the primary motor cortex. Finally, the effect of priming protocols on QPS-induced plasticity was consistent with a BCM-like model of priming that shifts the crossover point at which synaptic plasticity reverses from depression to potentiation. The broad range of after-effects produced by the new rTMS protocol opens up new possibilities for detailed examination of theories of metaplasticity in humans.     

Steroid-dependent plasticity in the medial amygdala

Behavioral sex differences have traditionally been thought to arise from gonadal steroids during a neonatal sensitive period. However, it is possible to sex-reverse certain behaviors by reversing the levels of circulating androgen in adult males and females. These results suggest that the sexually dimorphic substrates of sex behavior are subject to a high degree of plasticity, even in adulthood. I have found that circulating androgen exerts a trophic effect on the Nissl-stained morphology of an important nucleus in the control of sex behavior, namely, the posterodorsal subnucleus of the medial amygdala. First, sex-reversing the level of circulating androgen reversed the sex difference in soma size and regional volume of the posterodorsal subnucleus of the medial amygdala in adult rats. Interestingly, activation of both androgen and estrogen receptors was necessary for the post-castration maintenance of a masculine phenotype in terms of posterodorsal subnucleus of the medial amygdala cell size, whereas only estrogen receptor activity was necessary to maintain a masculine posterodorsal subnucleus of the medial amygdala volume. Then, we showed that seasonal variation in androgen was correlated with morphologic plasticity in the posterodorsal subnucleus of the medial amygdala of the Siberian hamster. However, if the experimental males were housed with females, their posterodorsal subnucleus of the medial amygdalas failed to regress in response to winter-like short daylengths. Furthermore, when male hamsters were castrated and treated with testosterone, the posterodorsal subnucleus of the medial amygdala responded to the hormone only if the animals were in summer-like photoperiods. Overall, these findings indicate that circulating androgens are critical for the maintenance of greater posterodorsal subnucleus of the medial amygdala regional volumes and soma sizes, and that environmental variables can regulate testosterone secretion and responsiveness.     

Comparison of motor effects following subcortical electrical stimulation through electrodes in the globus pallidus internus and cortical transcranial magnetic stimulation

Current concepts of transcranial magnetic stimulation (TMS) over the primary motor cortex are still under debate as to whether inhibitory motor effects are exclusively of cortical origin. To further elucidate a potential subcortical influence on motor effects, we combined TMS and unilateral subcortical electrical stimulation (SES) of the corticospinal tract. SES was performed through implanted depth electrodes in eight patients treated with deep brain stimulation (DBS) for severe dystonia. Chronaxie, conduction velocity (CV) of the stimulated fibres and poststimulus time histograms of single motor unit recordings were calculated to provide evidence of an activation of large diameter myelinated fibres by SES. Excitatory and inhibitory motor effects recorded bilaterally from the first dorsal interosseus muscle were measured after SES and focal TMS of the motor cortex. This allowed us to compare motor effects of subcortical (direct) and cortical (mainly indirect) activation of corticospinal neurons. SES activated a fast conducting monosynaptic pathway to the alpha motoneuron. Motor responses elicited by SES had significantly shorter onset latency and shorter duration of the contralateral silent period compared to TMS induced motor effects. Spinal excitability as assessed by H-reflex was significantly reduced during the silent period after SES. No ipsilateral motor effects could be elicited by SES while TMS was followed by an ipsilateral inhibition. The results suggest that SES activated the corticospinal neurons at the level of the internal capsule. Comparison of SES and TMS induced motor effects reveals that the first part of the TMS induced contralateral silent period should be of spinal origin while its later part is due to cortical inhibitory mechanisms. Furthermore, the present results suggest that the ipsilateral inhibition is predominantly mediated via transcallosal pathways.This paper is dedicated to Bernd-Ulrich Meyer, who died in a plane accident     

Electrophysiological classes of cat primary visual cortical neurons in vivo as revealed by quantitative analyses

To facilitate the characterization of cortical neuronal function, the responses of cells in cat area 17 to intracellular injection of current pulses were quantitatively analyzed. A variety of response variables were used to separate the cells into subtypes using cluster analysis. Four main classes of neurons could be clearly distinguished: regular spiking (RS), fast spiking (FS), intrinsic bursting (IB), and chattering (CH). Each of these contained significant subclasses. RS neurons were characterized by trains of action potentials that exhibited spike frequency adaptation. Morphologically, these cells were spiny stellate cells in layer 4 and pyramidal cells in layers 2, 3, 5, and 6. FS neurons had short-duration action potentials (<0.5 ms at half height), little or no spike frequency adaptation, and a steep relationship between injected current intensity and spike discharge frequency. Morphologically, these cells were sparsely spiny or aspiny nonpyramidal cells. IB neurons typically generated a low frequency (<425 Hz) burst of spikes at the beginning of a depolarizing current pulse followed by a tonic train of action potentials for the remainder of the pulse. These cells were observed in all cortical layers, but were most abundant in layer 5. Finally, CH neurons generated repetitive, high-frequency (350-700 Hz) bursts of short-duration (<0.55 ms) action potentials. Morphologically, these cells were layer 2-4 (mainly layer 3) pyramidal or spiny stellate neurons. These results indicate that firing properties do not form a continuum and that cortical neurons are members of distinct electrophysiological classes and subclasses.     

Intrinsic connections in the medial amygdala as revealed by complete deafferentation

Using a newly designed knife, the medial amygdala of rats was completely deafferented to exclude the synapses of the afferents coming from the outside of the medial amygdaloid nucleus (MAN). Semiquantitative analysis by electron microscopy showed that the number of the intact dendritic synapses per unit neuropil area of the MAN of the islands was markedly reduced to about one-third of that of the controls. These intact synapses appeared to be intrinsic or of local origin. In contrast, degenerating synapses, two-thirds of the dendritic synapses in the MAN, were considered as synapses with the fibers from outside the islands.     

Multiple forms of activity-dependent intrinsic plasticity in layer V cortical neurones in vivo

Synaptic plasticity is classically considered as the neuronal substrate for learning and memory. However, activity-dependent changes in neuronal intrinsic excitability have been reported in several learning-related brain regions, suggesting that intrinsic plasticity could also participate to information storage. Compared to synaptic plasticity, there has been little exploration of the properties of induction and expression of intrinsic plasticity in an intact brain. Here, by the means of in vivo intracellular recordings in the rat we have examined how the intrinsic excitability of layer V motor cortex pyramidal neurones is altered following brief periods of repeated firing. Changes in membrane excitability were assessed by modifications in the discharge frequency versus injected current ( F–I ) curves. Most (∼64%) conditioned neurones exhibited a long-lasting intrinsic plasticity, which was expressed either by selective changes in the current threshold or in the slope of the F–I curve, or by concomitant changes in both parameters. These modifications in the neuronal input–output relationship led to a global increase or decrease in intrinsic excitability. Passive electrical membrane properties were unaffected by the intracellular conditioning, indicating that intrinsic plasticity resulted from modifications of voltage-gated ion channels. These results demonstrate that neocortical pyramidal neurones can express in vivo a bidirectional use-dependent intrinsic plasticity, modifying their sensitivity to weak inputs and/or the gain of their input–output function. These multiple forms of experience-dependent intrinsic changes, which expand the computational abilities of individual neurones, could shape new network dynamics and thus might participate in the formation of mnemonic motor engrams.     

Orienting behavior by rats with visual cortical and subcortical lesions

Summary The effect of visual cortical and subcortical lesions on orienting behavior was assessed by examining the rats' ability to interrupt an ongoing response and perform appropriate head and postural adjustments to repeatedly presented auditory or apparently moving visual stimuli. Large lesions of the entire superior colliculus (SC) or the deep layers of the SC did not result in visual agnosia or the inability to perform the motor responses involved in orienting. Rather, the orienting response simply was not emitted to visual stimuli that the intact rat treated as less salient, but was to those it treated as more salient. Lesions of either the superficial layers of the SC or visual cortex also did not completely prevent orienting to very salient, apparently moving visual stimuli, but did produce changes in the number of responses made to such stimuli and in the occurrence of other components of orienting behavior. It was suggested that the SC and visual cortex play a modulatory role in orienting behavior and that stimulus characteristics must be considered in the development of neuronal models of orienting behavior.This investigation was supported by the Natural Sciences and Engineering Research Council of Canada (Grant AO-179) to R.C. Tees, Canada Council Doctoral Fellowship and Killam Postdoctoral Fellowship to G. C. Midgley     

A cortical area that responds specifically to optic flow, revealed by fMRI

The continuously changing optic flow on the retina provides information about direction of heading and about the three-dimensional structure of the environment. Here we use functional magnetic resonance imaging (fMRI) to demonstrate that an area in human cortex responds selectively to components of optic flow, such as circular and radial motion. This area is within the region commonly referrred to as V5/MT complex, but is distinct from the part of this region that responds to translation. The functional properties of these two areas of the V5/MT complex are also different; the response to optic flow was obtained only with changing flow stimuli, whereas response to translation occurred during exposure to continuous motion.     

Increased cortical plasticity in the elderly: changes in the somatosensory cortex after paired associative stimulation

A fundamental feature of the human cortex is the capability to express plastic changes that seem to be present even during physiological aging. The paired associative stimulation (PAS) protocol is a paradigm capable of inducing neuroplastic changes, possibly by mechanisms related to spike timing-dependent associative neuronal activity, and represents a suitable tool for investigating age-dependent neuroplastic modulations of the primary somatosensory cortex (S1). To examine age dependency of S1 plasticity, the amplitude changes of median nerve somatosensory evoked potential (SEP) before and after PAS intervention were investigated in young and elderly subjects. The main finding of our study is that low-frequency medial nerve stimulation paired with transcranial magnetic stimulation over the contralateral cortex enhances S1 excitability. Moreover, the S1 long term potentiation–like plasticity changes as a function of aging, with a significant increase of N20–P25 complex in the elderly compared to young subjects. These results are congruent with the hypothesis that some elderly subjects retain a high level of plasticity in specific neuronal circuits. Such plasticity could represent a compensatory mechanism, in terms of functional reserve of somatosensory cortex, used by the aging brain to counterbalance the cortical degeneration associated with aging.     

Rostro-caudal gradient in the amygdala structural and functional organisation

New data supporting the assumptions of the hypothesis stating the rostro-caudal gradient in the amygdala structural and functional organization are presented. First formulated on the basis of particular location of amygdala zones possessing sexual dimorphism, the above hypothesis is corroborated by the works which characterize its structural organization, participation of amygdala rostral and caudal parts in ensuring various functions including behavior. The authors believe that the amygdala rostro-caudal gradient is predetermined by the phase character of its formation in phylogenesis.     

Spatio-temporal plasticity of cortical receptive fields in response to repetitive visual stimulation in the adult cat

Many psychophysical experiments on perceptual learning in humans show increases of performance that are most probably based on functions of early visual cortical areas. Long-term plasticity of the primary visual cortex has so far been shown in vivo with the use of visual stimuli paired with electrical or pharmacological stimulation at the cellular level. Here, we report that plasticity in the adult visual cortex can be achieved by repetitive visual stimulation. First, spatial receptive field profiles of single units (n=38) in area 17 or 18 of the anesthetized cat were determined with optimally oriented flashing light bars. Then a conditioning protocol was applied to induce associative synaptic plasticity. The receptive field center and an unresponsive region just outside the excitatory receptive field were synchronously stimulated ('costimulation', repetition rate 1 Hz; for 10-75 min). After costimulation the receptive field and its adjacent regions were mapped again. We observed specific increases of the receptive field size, changes of the receptive field subfield structure as well as shifts in response latency.In 37% of the cells the receptive field size increased specifically towards the stimulated side but not towards the non-stimulated opposite side of the receptive field. In addition, changes in the relative strength and size of the on and off subfield regions were observed. These specific alterations were dependent on the level of neuronal activity during costimulation. During recovery, the new responses dropped down to 120% of the preconditioning value on average within 103 min; however, the decay times significantly depended on the response magnitude after costimulation. In the temporal domain, the latency of new responses appeared to be strongly influenced by the latency of the response during costimulation.Twenty-nine percent of the units displayed no receptive field enlargement, most likely because the activity during costimulation was significantly lower than in the cases with enlarged receptive fields. An unspecific receptive field enlargement towards both the stimulated and non-stimulated side was observed in 34% of the tested cells. In contrast to the cells with specifically enlarged receptive fields, the unspecific increase of receptive field size was always accompanied by a strong increase of the general activity level.We conclude that the receptive field changes presumably took place by strengthening of synaptic inputs at the recorded cells in a Hebbian way as previously shown in the visual cortex in vitro and in vivo. The observed receptive field changes may be related to preattentive perceptual learning and could represent a basis of the 'filling in' of cortical scotomas obtained with specific training procedures in human patients suffering from visual cortex lesions.