Comparison of the electrophysiology and morphology of layers III and II neurons of the rat medial entorhinal cortex in vitro

The basic membrane characteristics of neurons in layers II and III of the medial entorhinal cortex (MEA) were recorded using the intracellular current clamp technique in in vitro slices of the rat brain. Two types of cells were distinguished according to the presence of a time-dependent inward rectification (SAG current) with hyperpolarizing current pulses. The cells in which this inward rectification was not observed (No-SAG cells) had a larger input resistance, a more negative resting membrane potential and a more depolarized firing threshold. They more often displayed a strongly adapting firing pattern, and their action potentials had a slower decay rate and lacked a depolarizing afterpotential, compared with the SAG cells. SAG cells typically had a prominent rebound depolarization at the end of a hyperpolarizing current and membrane potential oscillations (7 Hz) upon subthreshold depolarizations. Cs⁺ blocked the time-dependent inward rectification. The rebound depolarization persisted, even in the presence of tetrodotoxin. Biocytin labelling showed that layer III consisted mainly of pyramidal-shaped cells. Most layer III cells were of the No-SAG type. All cells in layer II, stellate and pyramidal cells, were classified as SAG cells. We conclude that the cells in MEA layers II and III display different electroresponsiveness, but that this appears to be more related to the layer where they are located than to a specific morphology. As layer III consisted mainly of cells of the No-SAG type, we suggest that layer III cells are less excitable than the SAG type layer II cells.     

Electrophysiological characterization of interlaminar entorhinal connections: an essential link for re-entrance in the hippocampal-entorhinal system

The hippocampal formation communicates with the neocortex mainly through the adjacent entorhinal cortex. Neurons projecting to the hippocampal formation are found in the superficial layers of the entorhinal cortex and are largely segregated from the neurons receiving hippocampal output, which are located in deep entorhinal layers. We studied the communication between deep and superficial entorhinal layers in the anaesthetized rat using field potential recordings, current source density analysis and single unit measurements. We found that subiculum stimulation was able to excite entorhinal neurons in deep layers. This response was followed by current sinks in superficial layers. Both responses were subject to frequency dependent facilitation, but not depression. Selective blockade of deep layer responses also abolished subsequent superficial layer responses. This clearly demonstrates a functional deep-to-superficial layer communication in the entorhinal cortex, which can be triggered by hippocampal output. This pathway may provide a means by which processed hippocampal output is integrated or compared with new incoming information in superficial entorhinal layers, and it constitutes an important link in the process of re-entrance of activity in the hippocampal-entorhinal network, which may be important for consolidation of memories or retaining information for short periods.     

Morphology of globus pallidus neurons: Its correlation with electrophysiology in guinea pig brain slices

Intracellular recordings obtained from globus pallidus neurons in guinea pig revealed, on the basis of their membrane properties, the existence of at least two major (types I and II) and one minor (type III) groups of neurons. Type I neurons were silent at the resting membrane level and generated a burst of spikes with strong accommodation to depolarizing current injection. Type II neurons fired at the resting membrane level or with small membrane depolarization, and their repetitive firing (≤200 Hz) was very sensitive to the amplitude of injected current and showed weak accomodation. Type III neurons did not fire spontaneously at the resting membrane level. These neurons were morphologically characterized by intracellular injection of biocytin following the electrophysiological recordings. Among the major groups, the soma size of type I neurons (40 × 23 μm) was larger than that of type II neurons (29 × 17 μm). Both types of neurons had three to six primary dendrites. Dendritic spines were very sparse. Occasionally, dendrites exhibited varicosities, especially in their terminal branches. Dendritic fields were disc-like in shape and were perpendicular to striopallidal fibers. Most of the axons had intranuclear collaterals. Main axonal branches projected rostrally or caudally, and in some neurons one axonal branch could be followed caudally, and another rostrally, into the striatum. These two types were major neurons in the globus pallidus and were considered to be projection neurons. Type III neurons were small (18 × 12 μm), and their dendrites were covered with numerous spines. They were considered to be interneurons. J. Comp. Neurol. 377:85-94, 1997. © 1997 Wiley-Liss, Inc.     

Microcircuitry of posterior cingulate cortex in vitro: electrophysiology and laminar analysis using the current source density method

We used current source density (CSD) analysis of a laminar profile of subicular stimulus-evoked field potentials recorded in cortical slices in vitro to characterize the interlaminar microcircuitry of posterior cingulate cortex. Neuroanatomic and electrophysiologic data indicate that subiculocingulate tract (SCT) afferents monosynaptically excite apical dendrites of deep laminae (V–VI) neurons, evoking pure EPSPs, while superficial laminae (II/III–IV) neurons are driven polysynaptically, evoking a mixture of longer latency EPSPs and IPSPs. Consistent with this model, CSD analysis of field potential laminar profiles supports the conclusion that activation of excitatory subicular afferent terminal fields in superfical laminae of cingulate cortex elicits primary monosynaptic activation of apical dendrites of deep lamina (V–VI) pyramids. Subsequent EPSP propagation to the somata of these pyramids generated synchronous action potential discharges which appeared to elicit delayed polysynaptic activation of superficial laminae pyramids and interneurons. Latency differences between SCT-stimulus-evoked EPSPs and action potentials in superficial and deep laminae were minimized by stimulus train frequencies of 5–8 Hz, indicating that the proposed microcircuitry can show functional tuning at frequencies characteristic of hippocampal neuronal activity (theta). Such tuning suggests that hippocampal output activity frequency and phase locked to theta rhythm will be preferentially gated through cingulate cortex.     

Generation of theta rhythm in medial entorhinal cortex of freely moving rats

A regular slow wave theta rhythm can be recorded in the medial entorhinal cortex (MEC) of freely moving rats during voluntary behaviors and paradoxical sleep. Electrode penetrations normal to the cortical layers proceeding from the deeper to the more superficial layers reveal a continuous theta rhythm in layers IV-III (deep MEC theta rhythm) with an amplitude maximum in layer III, a null between the outer one-third of layer III and the inner one-half of layer I, and a continuous phase-reversed theta rhythm in layers II-I (superficial MEC theta rhythm) with an amplitude maximum there. Deep MEC theta rhythm is similar in phase and wave shape to CA1 theta rhythm; superficial MEC theta rhythm is similar in phase to DG theta rhythm. Laminar profiles throughout MEC show that the theta rhythm is generated there; it is not volume conducted from hippocampus.     

Propagation dynamics of epileptiform activity acutely induced by bicuculline in the hippocampal-parahippocampal region of the isolated Guinea pig brain

PURPOSE: Aim of the study is to investigate the involvement of parahippocampal subregions in the generation and in the propagation of focal epileptiform discharges in an acute model of seizure generation in the temporal lobe induced by arterial application of bicuculline in the in vitro isolated guinea pig brain preparation. METHODS: Electrophysiological recordings were simultaneously performed with single electrodes and multichannel silicon probes in the entorhinal, perirhinal, and piriform cortices and in the area CA1 of the hippocampus of the in vitro isolated guinea pig brain. Interictal and ictal epileptiform discharges restricted to the temporal region were induced by a brief (3-5 min) arterial perfusion of the GABA(A) receptor antagonist, bicuculline methiodide (50 microM). Current source density analysis of laminar field profiles performed with the silicon probes was carried out at different sites to establish network interactions responsible for the generation of epileptiform potentials. Nonlinear regression analysis was conducted on extracellular recordings during ictal onset in order to quantify the degree of interaction between fast activities generated at different sites, as well as time delays. RESULTS: Experiments were performed in 31 isolated guinea pig brains. Bicuculline-induced interictal and ictal epileptiform activities that showed variability of spatial propagation and time course in the olfactory-temporal region. The most commonly observed pattern (n = 23) was characterized by the initial appearance of interictal spikes (ISs) in the piriform cortex (PC), which propagated to the lateral entorhinal region. Independent and asynchronous preictal spikes originated in the entorhinal cortex (EC)/hippocampus and progressed into ictal fast discharges (around 25 Hz) restricted to the entorhinal/hippocampal region. The local generation of fast activity was verified and confirmed both by CSD and phase shift analysis performed on laminar profiles. Fast activity was followed by synchronous afterdischarges that propagated to the perirhinal cortex (PRC) (but not to the PC). Within 1-9 min, the ictal discharge ceased and a postictal period of depression occurred, after which periodic ISs in the PC resumed. Unlike preictal ISs, postictal ISs propagated to the PRC. CONCLUSIONS: Several studies proposed that reciprocal connections between the entorhinal and the PRC are under a very efficient inhibitory control (1). We report that ISs determined by acute bicuculline treatment in the isolated guinea pig brain progress from the PC to the hippocampus/EC just before ictal onset. Ictal discharges are characterized by a peculiar pattern of fast activity that originates from the entorhinal/hippocampal region and only secondarily propagates to the PRC. Postictal propagation of ISs to the PRC occurred exclusively when an ictal discharge was generated in the hippocampal/entorhinal region. The results suggest that reiteration of ictal events may promote changes in propagation pattern of epileptiform discharges that could act as trigger elements in the development of temporal lobe epilepsy.     

Morphology and projection pattern of medial and dorsal pallial neurons in the frog Discoglossus pictus and the salamander Plethodon jordani

In the frog Discoglossus pictus and the salamander Plethodon jordani, the morphology and axonal projection pattern of neurons in the medial and dorsal pallium were determined by intracellular biocytin labeling. A total of 77 pallial neurons were labeled in the frog and 58 pallial neurons in the salamander. Within the medial pallium (MP) of the frog, four types of neurons were identified on the basis of differences in their axonal projection pattern. Type I neurons have bilateral projections into telencephalic and diencephalic areas; type II neurons have bilateral projections to telencephalic areas and ipsilaterally descending projections to diencephalic regions; type III neurons have only intratelencephalic connections, and a single type IV neuron has ipsilaterally descending projections. The somata of the four types occupy four nonoverlapping zones. Neurons of the dorsal pallium (DP) project exclusively to the ipsilateral MP and to the dorsal edge of the lateral pallium. In the ventral MP of the salamander, neurons have mostly intratelencephalic projections. Neurons in the dorsal MP project bilaterally to diencephalic and telencephalic regions. Neurons in the medial DP project ipsilaterally to the MP, lateral septum, nucleus accumbens, medial amygdala, and the internal granule layer of the olfactory bulb. In five cases, fibers were found in the commissura hippocampi, but in only two cases could these fibers be followed toward the contralateral MP and septum. Neurons in the lateral DP had no contralateral projections; they projected to the ipsilateral MP and in eight cases to the ipsilateral septum as well. Based on similarities of cytoarchitecture and projection pattern in neurons of the MP and DP, it is proposed that both frogs and salamanders have an MP subdivided into a ventral and dorsal portion, and a DP subdivided into a medial and a lateral portion.     

Effects of pilocarpine on the cortical and hippocampal theta rhythm in different vigilance states in rats

It has been suggested that theta rhythm gates the flow of information between the hippocampus and cortex during memory processes. The cholinergic system plays an important role in regulating vigilance states and in generating theta rhythm. This study aims to analyse the effects of the muscarinic agonist pilocarpine (120 and 360 microg, i.c.v.) on hippocampal and frontal cortical theta rhythm during several vigilance states in rats. Pilocarpine injection increased the duration and number of episodes with theta activity, particularly when theta rhythm appeared during waking states in the cortex and hippocampus simultaneously. It seems that the effects of pilocarpine are related to the appearance of cortical theta activity in waking states, and suggest that pilocarpine could modify the transference rate of information from the hippocampus to cortex in rats during wakefulness states, in relation to the postulated effect of cholinergic system modulating memory consolidation.     

Intracellular and current source density analysis of pretectal input to the optic tectum of the frog

Objective

Electrophysiological examination of the ipsilateral pretectotectal projection has proved that pretectal cells elicit strong suppressive responses to the ipsilateral tectum. However, the neural mechanisms underlying the contralateral pretectotectal prejection are still obscure. The present study aimed to examine the synaptic nature of pretectal nuclei and contralateral tectal cells, and to demonstrate the spatiotemporal pattern of neuronal activity in the 2 main brain structures.

Methods

Intracellular recording and current source density (CSD) analysis were used to test the complexity of neuronal mechanism of pretectotectal information transfer.

Results

The pretectal stimulation elicited only one type of response on the contralateral tectum, the inhibitory postsynaptic potential (IPSP). The majority of contra-induced IPSPs were assumed to be polysynaptically driven. In the CSD analysis, only one sink with short latency was observed in each profile. The ipsilateral projection produced a prominent monosynaptic sink in layer 8 of tectum. Recipient neurons were located in layers 6 and 7 of tectum. The result confirmed former findings from ipsilateral intracellular recordings.

Conclusion

These results suggest the following neuronal circuit: afferents from the pretectal nuclei broadly inhibit both tectal neuron, and since no second sink occurs in tectal layers, the pretectotectal excitatory afferents probably do not extend over the whole tectum, but are within limited state. The results of intracellular recording and CSD analysis further provide evidence of how pretectal afferent activity flows within the tectal laminae.     

Interactions of Dopamine with Glutamate- and GABA-mediated Synaptic Transmission in the Rat Entorhinal Cortex In Vitro

We have studied the interactions between dopamine and glutamate-mediated transmission in the entorhinal cortex using intracellular recording in a slice preparation from the rat brain. High concentrations (0.1 – 1 mM) of dopamine had weak, direct effects on the membrane potential with predominantly hyperpolarizing responses in layer II neurons and depolarizing responses in layer V. Studies with the dopamine antagonists sulpiride (D2 antagonist, 10 – 50 μM) and SCH-23390 (D1 antagonist, 50 μM) indicated that the hyperpolarization by dopamine could be mediated by D2 receptors, although the pharmacology was not clear-cut. The depolarizing response was not affected by either D1 or D2 antagonists. Synaptic responses of layer II and layer V cells were complex, consisting of both inhibitory and excitatory potentials. In untreated slices, dopamine reduced all components of the synaptic responses. However, when components of the responses were pharmacologically isolated, only the excitatory, glutamate-mediated potentials were consistently affected and the GABAergic inhibitory potentials were more resistant to reduction by dopamine. Excitatory potentials mediated by both N -methyl- d -aspartate and α-amino-3-hydroxy-5-methyl-isoxazolepropionic acid receptors were reduced by dopamine, but the former were more strongly affected. Studies with antagonists suggested that the D1 receptor is more likely to be involved in the decrement of glutamate transmission. Thus, dopamine appears to modulate glutamate-mediated synaptic transmission in the entorhinal cortex, and it is conceivable that a disturbance in this interaction could be involved in the aetiology of schizophrenia.     

Olfactory input to the parahippocampal region of the isolated guinea pig brain reveals weak entorhinal-to-perirhinal interactions

The processing of olfactory inputs by the parahippocampal region has a central role in the organization of memory in mammals. The olfactory input is relayed to the hippocampus via interposed synapses located in the piriform and entorhinal cortices. Whether olfactory afferents directly or indirectly project to other areas of the parahippocampal region beside the entorhinal cortex (EC) is uncertain. We performed an electrophysiological and imaging study of the propagation pattern of the olfactory input carried by the fibres that form the lateral olfactory tract (LOT) into the parahippocampal region of the in vitro isolated guinea pig preparation. Laminar analysis was performed on field potential depth profiles recorded with 16-channel silicon probes at different sites of the insular-parahippocampal cortex. The LOT input induced a large amplitude polysynaptic response in the lateral EC. Following appropriate LOT stimulation, a late response generated by the interposed activation of the hippocampus was observed in the medial EC. LOT stimulation did not induce any local response in area 36 of the perirhinal cortex (PRC), while a small amplitude potential with a delay similar to the lateral EC response was inconsistently observed in PRC area 35. No PRC potentials were observed following the responses evoked by LOT stimulation in either the lateral or the medial EC. These findings were substantiated by current source density analysis of PRC laminar profiles. To further verify the absence of EC-to-PRC field interactions after LOT stimulation, high-resolution optical imaging of neuronal activity was performed after perfusion of the isolated brain with the voltage-sensitive dye RH-795. The optical recordings confirmed that olfactory-induced activity in the EC does not induce massive PRC activation. The present findings suggest that the olfactory input into the parahippocampal region is confined to the entorhinal cortex. The results also imply that, as demonstrated for the PRC-to-EC pathway, the propagation of neuronal activity from the EC to the PRC is hindered, possibly by a powerful inhibitory control generated within the EC.     

Concentration and storage of biotin in the amphibian brain

Prominent displays of endogenous biotin reactivity can be observed at specific locations in histochemical preparations of the forebrain and midbrain in the northern leopard frog (Rana pipiens) and common American toad (Bufo americanus). At the light microscopic level, the biotin reactivity appears in clusters of darkly stained puncta of either spherical or rodlike shape in the olfactory cortex, nucleus isthmi, and hypothalamus. With the electron microscope, the biotin reactive spheres are identified as neuronal varicosities and synaptic boutons and the rods as short segments of axons. Appropriate controls demonstrate that the punctate biotin-reactive structures are sites of concentration of biotin or a biotin analog in the processes of certain neurons. These data represent the first observation on the selective concentration of a vitamin in vertebrate neurons and suggest that biotin may have specialized functions in anatomically delimited areas of the central nervous system. Localization of the densest concentration of the biotin-reactive puncta in the dorsolateral prominence of the olfactory cortex may have relevance to the functional organization of the olfactory system. The distributions of biotin-reactive puncta were observed in laboratory-housed frogs and in wild toads captured in the summer months but were sparse or absent in batches of commercially obtained frogs examined immediately upon arrival in the laboratory. Systemic administration of biotin or biocytin hydrochloride did not alter the appearance or numbers of the biotin-reactive structures either in newly received or laboratory-housed frogs. These findings suggest that the capacity of the biotin storage mechanism in the amphibian brain may be set by environmental factors and may be readily saturable from natural dietary or enteric sources. © 1996 Wiley-Liss, Inc.     

Synapsin I Gene expression in the adult rat brain with comparative analysis of mRNA and protein in the hippocampus

Synapsin I is the best characterized member of a family of neuron-specific phosphoproteins thought to be involved in the regulation of neurotransmitter release. In this report, we present the first extensive in situ hybridization study detailing the regional and cellular distribution of synapsin I mRNA in the adult rat brain. Both the regional distribution and relative levels of synapsin I mRNA established by in situ hybridization were confirmed by RNA blot analysis. Our data demonstrate the widespread yet regionally variable expression of synapsin I mRNA throughout the adult rat brain. The greatest abundance of synapsin I mRNA was found in the pyramidal neurons of the CA3 and CA4 fields of the hippocampus, and in the mitral and internal granular cell layers of the olfactory bulb. Other areas abundant in synapsin I mRNA were the layer II neurons of the piriform cortex and layer II and V neurons of the entorhinal cortex, the granule cell neurons of the dentate gyrus, the pyramidal neurons of hippocampal fields CA1 and CA2, and the cells of the parasubiculum. In general, the pattern of expression of synapsin I mRNA paralleled those encoding other synaptic terminal-specific proteins, such as synaptophysin, VAMP-2, and SNAP-25, with noteworthy exceptions. To determine specificall how synapsin I mRNA levels are related to levels of synapsin I protein, we examined in detail the local distribution patterns of both synapsin I mRNA and protein in the rat hippocampus. These data revealed differential levels of expression of synapsin I mRNA and protein within defined synaptic circuits of the rat hippocampus. © 1993 Wiley-Liss, Inc.     

Overview of computational models of hippocampus and related structures: Introduction to the special issue

Extensive computational modeling has focused on the hippocampal formation and associated cortical structures. This overview describes some of the factors that have motivated the strong focus on these structures, including major experimental findings and their impact on computational models. This overview provides a framework for describing the topics addressed by individual articles in this special issue of the journal Hippocampus.     

Barrelfield of the prenatally X-irradiated rat somatosensory cortex: A histochemical and electrophysiological study

The effect of prenatal X-irradiation on the vibrissal cortical barrelfield of the brain of rats exposed to 200 R on the embryonic day 17 was studied morphologically and electrophysiologically. Cytoarchitectural barrels fail to appear in adult rats that have been subjected to this in utero treatment. However, sections cut in a plane tangential to the vibrissal cortex and examined for cytochrome oxidase (CO), a mitochondrial enzyme, contained a matrix of patterned CO activity which, albeit smaller and weaker in intensity, is similar to CO barrels in normal controls. Current source density analysis of cortical field potentials indicated that, as in the normal cortex, the earliest sink following peripheral stimulation appears in association with this high CO activity. These results suggest that the specific vibrissal thalamocortical pathway sets up an excitatory synaptic activity in the cortex of the irradiated animal. Efficacy of this route in eliciting postsynaptic spikes in the cortical output neurons was confirmed by recording extracellular spike responses to vibrissa displacements from layer Vb pyramidal neurons that were then injected intracellularly with horseradish peroxidase for later anatomical identification. © 1995 Wiley-Liss, Inc.     

Interactions between Associative Synaptic Potentials in the Piriform Cortex of the In Vitro Isolated Guinea Pig Brain

The interaction between synaptic potentials generated by the activation of separate sets of associative fibres was investigated in the piriform cortex of an in vitro isolated guinea pig brain preparation. Restricted regions of the piriform cortex served by separate contingents of afferent fibres of the lateral olfactory tract were isolated surgically. The activity generated by these patches of cortex in response to afferent stimulation propagates to remote cortical regions along cortico-cortical associative fibres. Current source density (CSD) analysis of field potential laminar profiles evoked by lateral olfactory tract stimulation confirmed that the synaptic sinks induced by distinct associative fibre contingents converge on the apical dendrites of piriform cortex neurons in the superficial lb layer. Pairing between potentials evoked by activation of two separate sets of associative fibres resulted in an almost linear summation when the two responses coincided. For interstimulus intervals of <100 ms, heterosynaptic pairing of independent associative inputs induced a facilitation of the conditioned associative potential, which correlated with an increase in the associative sink located in layer lb, as demonstrated by CSD analysis. The evaluation of the pairing intervals suggests that the heterosynaptic facilitation of the conditioned associative potentials may be due to the summation of local and remote associative synaptic events. It is concluded that separate associative inputs converge on the apical dendrites of piriform cortex pyramidal neurons to generate synaptic potentials through the activation of spatially close but independent synapses. The role of associative synaptic integration in the functional organization of the olfactory cortex is discussed.     

Memory in the aging brain: doubly dissociating the contribution of the hippocampus and entorhinal cortex

Since the time of Aristotle it has been thought that memories can be divided into two basic types; conscious recollections and familiarity-based judgments. Neuropsychological studies have provided indirect support for this distinction by suggesting that different regions within the human medial temporal lobe (MTL) are involved in these two forms of memory, but none of these studies have demonstrated that these brain regions can be fully dissociated. In a group of nondemented elderly subjects, we found that performance on recall and recognition tests was predicted preferentially by hippocampal and entorhinal volumes, respectively. Structural equation modeling revealed a double dissociation, whereby age-related reductions in hippocampal volume resulted in decreases in recollection, but not familiarity, whereas entorhinal volume was preferentially related to familiarity. The results demonstrate that the forms of episodic memory supported by the human hippocampus and entorhinal cortex can be fully dissociated, and indicate that recollection and familiarity reflect neuroanatomically distinct memory processes.     

Ultrastructure and aspects of functional organization of pyramidal and nonpyramidal entorhinal projection neurons contributing to the perforant path

Identified entorhino-hippocampal projection neurons were investigated for their ultrastructure. Spinous projection neurons (pyramidal and spiny stellate cells) display common features such as symmetric axosomatic terminals on their somata, asymmetric synapses on the spines, and both types of synapses on the dendritic shafts. Their axons descend towards the white matter, branching occasionally via collaterals which establish contact with local spines and rarely on dendritic shafts and somata. The sparsely spinous projection neurons (multipolar and horizontal-bipolar) typically show deep nuclear infolds and symmetric and asymmetric synapses on their somata and dendritic shafts. Axons also collateralize in the soma vicinity and form local synapses. It is concluded that the entorhino-hippocampal projection neurons (both spiny and sparsely spinous) act locally and distally thus performing simultaneously as local-circuit and as projection neurons. In accordance with other morphological and electrophysiological reports it appears likely that the generation, modulation, and suppression of entorhinal excitation waves is mediated by these neurons through direct excitation, feed-forward and feed-back inhibition, and disinhibition.     

Mechanical vibratory stimulation of feline forepaw skin induces long-lasting potentiation in the secondary somatosensory cortex

We investigated the long-lasting effects of mechanical vibratory stimulation of the skin on the excitability of feline cortical neurons in the forelimb areas of the primary (SI) and secondary (SII) somatosensory cortices. Conditioning mechanical stimuli were 300 bursts of 10 pulses at 200 Hz delivered with a 10-s interburst interval from a mechanical stimulator. Test field potentials and unit discharges were evoked by electrical stimulation to the ventral posterolateral thalamic nucleus (VPL) or by single mechanical stimuli applied to the skin. In SII, the mechanical burst stimulation to the skin increased the amplitudes of field potentials and the frequency of unit discharges elicited by single mechanical stimuli applied to the skin. The vibratory conditioning stimulus also produced a similar potentiation of the VPL-evoked field potentials (126-139% increase in amplitude, P < 0.05) with an associated increase in firing rates of extracellularly recorded neuronal activity (117%, P < 0.001). These potentiations persisted through the entire experimental period of 120 min. The translaminar current source density analysis calculated from the VPL-evoked field potentials increased to 127% of the control value (P < 0.01). In contrast, in SI we observed no significant changes in the field potential amplitudes or in the currents generated in superficial layers (91-117%). Taken together with the previous finding that tetanic electrical stimulation of VPL induces long-lasting potentiation of the VPL-evoked cortical responses in SII but not of those in SI, the present results suggest that SII has a large capacity for the rapid functional plasticity involved in the learning that occurs during repeated tactile experiences.