Neuronal organization of rat thalamus for processing information of vibrissal movements

Comprehensive mapping of whisker-evoked responses reveals broad, sharply tuned thalamocortical input to layer 4 of barrel cortex

Cortical neurons are organized in columns, distinguishable by their physiological properties and input-output organization. Columns are thought to be the fundamental information-processing modules of the cortex. The barrel cortex of rats and mice is an attractive model system for the study of cortical columns, because each column is defined by a layer 4 (L4) structure called a barrel, which can be clearly visualized. A great deal of information has been collected regarding the connectivity of neurons in barrel cortex, but the nature of the input to a given L4 barrel remains unclear. We measured this input by making comprehensive maps of whisker-evoked activity in L4 of rat barrel cortex using recordings of multiunit activity and current source density analysis of local field potential recordings of animals under light isoflurane anesthesia. We found that a large number of whiskers evoked a detectable response in each barrel (mean of 13 suprathreshold, 18 subthreshold) even after cortical activity was abolished by application of muscimol, a GABA(A) agonist. We confirmed these findings with intracellular recordings and single-unit extracellular recordings in vivo. This constitutes the first direct confirmation of the hypothesis that subcortical mechanisms mediate a substantial multiwhisker input to a given cortical barrel.

Subbarrel patterns of thalamocortical innervation in rat somatosensory cortical barrels: Organization and postnatal development

Barrel hollows in the posteromedial barrel subfield of adult rat somatosensory cortex typically encompass two or three metabolically and structurally distinct regions, termed subbarrels. We used immunohistochemical staining for vesicular glutamate transporter 2 and the neuronal serotonin transporter, in conjunction with cytochrome oxidase (CO) histochemistry, to investigate the distribution of thalamocortical (TC) axon terminals in relation to subbarrel domains. We found, first, that CO-dark subbarrels are more intensely immunoreactive for thalamocortical terminals than the CO-light clefts that separate them. Second, during the first postnatal week, immunoreactivity for markers of TC terminals is relatively homogeneous throughout the barrel hollow; subbarrel patterns of distribution only become recognizable between P-8 and P-10. These observations extend previous findings that subbarrels denote barrel regions enriched in synaptic contacts. The data also indicate that allocation of TC terminals into subbarrel domains does not occur immediately upon thalamic axon ingrowth. Instead, refinement of TC arbors into subbarrels is a gradual process, the outcome of which is not manifest until the second week of postnatal life.

Response transformation and receptive-field synthesis in the lemniscal trigeminothalamic circuit

To understand how the lemniscal trigeminothalamic circuit (PrV --> VPM) of the rodent whisker-to-barrel pathway transforms afferent signals, we applied ramp-and-hold deflections to individual whiskers of lightly narcotized rats while recording the extracellular responses of neurons in either the ventroposterior medial (VPM) thalamic nucleus or in brain stem nucleus principalis (PrV). In PrV, only those neurons antidromically determined to project to VPM were selected for recording. We found that VPM neurons exhibited smaller response magnitudes and greater spontaneous firing rates than those of their PrV inputs, but that both populations were similarly well tuned for stimulus direction. In addition, fewer VPM (74%) than PrV neurons (93%) responded with sustained, or tonic, discharges during the plateau phase of the stimulus. Neurons in both populations responded most robustly to deflections of a single, "principal whisker" (PW), and the majority of cells in both PrV (90%) and VPM (73%) also responded to deflections of at least one adjacent whisker (AW). AW responses in both nuclei occurred on average at longer latencies and were more temporally dispersed than PW responses. Lateral inhibition, as evidenced by AW-evoked activity suppression, was rare in PrV but prevalent in VPM. In both nuclei, however, suppression was weak, with AW responses being on average excitatory. Our results suggest that the receptive-field structures and response properties of individual VPM neurons can be explained in large part by input from one or a small number of PrV neurons, but that intrathalamic mechanisms act to further transform the afferent signal.

State-dependent processing of sensory stimuli by thalamic reticular neurons

Inhibitory neurons of the thalamic reticular (RT) nucleus fire in two activity modes, burst and tonic, depending on an animal's behavioral state. In tonic mode, depolarized RT cells fire single action potentials continuously, whereas burst firing consists of grouped discharges separated by periods of quiescence. To determine how these firing modes affect sensory-evoked RT responses, single-unit responses to controlled whisker deflections were analyzed according to the burst versus tonic mode of spontaneous activity (SA) preceding the response. After burst mode activity (i.e., either quiescence or spontaneous bursts), responses exhibited a slow approximately 15 msec rise to peak firing rates followed by a approximately 35 msec decay. Interspike intervals within the response exhibited accelerando-decelerando patterns similar to those of spontaneous bursts. After tonic mode activity (i.e., single spikes), responses had a nearly instantaneous approximately 1.5 msec rise-to-peak followed by a approximately 40 msec decay, with large spike counts (5.2 spikes per stimulus) similar to those evoked in burst mode (6.2 spikes per stimulus). Interspike intervals were longer in tonic mode and exhibited a decelerando pattern. Initial evoked spikes, however, had shorter latencies and greater synchrony, contributing to the rapid onset of tonic population response. Shifts from quiescent (presumed burst mode) to tonic SA could be induced by either previous whisker deflections or iontophoretic application of NMDA; both manipulations effected appropriate shifts from burst to tonic response spike patterns. In awake animals, burst and tonic firing in RT, as in thalamocortical relay nuclei, may reflect sensory processing strategies appropriate for different behavioral and attentional states.

The Somatotopic Organization Within the Cat's Thalamic Reticular Nucleus

The organization of the somatosensory representation within the cat's thalamic reticular nucleus (TRN) was studied. Focal injections of horseradish peroxidase (HRP), wheatgerm agglutinin conjugated to HRP, and/or [3H]proline were made into somatosensory cortical areas 1 (S1) and 2 (S2). The resultant labelling in the thalamus was analysed. Single injections into S1 result in single zones of terminal labelling in TRN that are restricted to the centroventral part of the sheet-like nucleus. In reconstructions from horizontal sections these zones of labelling resemble thin 'slabs', which lie in the plane of the nucleus parallel to its borders, occupy only a fraction of the thickness of the reticular sheet, and are broadly elongated in the dorsoventral and oblique rostrocaudal dimensions. Thus, the slabs of S1 terminals, which represent large loci of the body surface, and the main distribution of the reticular dendrites have a similar orientation. In comparisons of the zones of labelling following single or double injections at different cortical sites in S1, an inner (medial) to outer (lateral) shift in labelling in the ventrobasal complex (VB) is accompanied by an inner (medial) to outer (lateral) shift in labelling along the thickness of the reticular sheet. Thus, like VB the reticular nucleus receives a topographically accurate projection from S1. Further, the somatotopic map conveyed from S1 to TRN is orientated perpendicular to the plane of the nucleus and repeats the spatial organization of the map in VB. S2 injections result in zones of terminal labelling in that part of TRN that receives S1 inputs. On the basis of these findings, together with those in other mammalian species, two conclusions can be reached about corticoreticular relations. First, although there can be continuity in individual maps of cortical inputs to TRN, there are discontinuities in cortical representations at the inner and outer borders of the reticular sheet. Second, TRN can receive a significant convergence of inputs from different cortical areas.

Whisker maps of neuronal subclasses of the rat ventral posterior medial thalamus, identified by whole-cell voltage recording and morphological reconstruction

Whole-cell voltage recordings were made in vivo in the ventral posterior medial nucleus (VPM) of the thalamus in urethane-anaesthetised young (postnatal day 16-24) rats. Receptive fields (RFs) on the whisker pad were mapped for 31 neurones, and 10 cells were recovered for morphological reconstruction of their dendritic arbors. Most VPM neurones had antagonistic subthreshold RFs that could be divided into excitatory and inhibitory whiskers. VPM cells comprised different classes, the most frequently occurring being single-whisker excitation (SWE) and multi-whisker excitation (MWE) cells. In SWE cells (36 % of VPM neurones), only principal whisker (PW) deflection evoked an EPSP and was followed by a single action potential (AP) or remained subthreshold. The depolarisation was terminated by a large, delayed IPSP. A stimulus evoked on average 0.74 +/- 0.46 APs (mean +/- S.D.) with short latency (8.1 +/- 1.0 ms) and small temporal scatter (0.31 +/- 0.23 ms dispersion of 50 % of the first APs). In MWE cells (29 % of VPM neurones), deflection of several whiskers evoked EPSPs. PW responses were either subthreshold EPSPs or consisted of an EPSP followed by one or several APs (0.96 +/- 0.99 APs per stimulus). AP responses were often associated with putative low-threshold calcium-dependent regenerative potentials and were followed by a small delayed IPSP. AP responses had a longer latency (12.3 +/- 2.6 ms) and larger temporal scatter (2.5 +/- 1.6 ms) than responses of SWE cells. MWE cells had a lower input resistance than SWE cells. The elongation of dendritic arbors along the representation fields of rows and arcs in VPM barreloids was weakly correlated with the subthreshold RF elongation along whisker rows and arcs, respectively. Evoked EPSP-AP responses exhibited a sharper directional tuning than subthreshold EPSPs, which in turn exhibited a sharper directional tuning than IPSPs. In conclusion, we document two main classes of VPM neurones. SWE cells responded with a precisely timed single AP to the deflection of the PW. In contrast, MWE cell RFs were more broadly tuned and the temporally dispersed multiple AP responses of these cells represented the degree of collective deflection of the PW and several adjacent whiskers.

High responsiveness and direction sensitivity of neurons in the rat thalamic reticular nucleus to vibrissa deflections

The thalamic reticular nucleus (Rt) is strategically positioned to integrate descending and ascending signals in the control of sensorimotor and other thalamocortical activity. Its prominent role in the generation of sleep spindles notwithstanding, relatively little is known of Rt function in regulating interactions with the sensory environment. We recorded and compared the responses of individual Rt and thalamocortical neurons in the ventroposterior medial (VPm) nucleus of the rat to controlled deflections of mystacial vibrissae. Transient Rt responses to the onset (ON) and offset (OFF) of vibrissa deflection are larger and longer in duration than those of VPm and of all other populations studied in the whisker/barrel pathway. Magnitudes of ON and OFF responses in Rt were negatively correlated with immediately preceding activities, suggesting a contribution of low-threshold T-type Ca(2+) channels. Rt neurons also respond with high tonic firing rates during sustained vibrissa deflections. By comparison, VPm neurons are less likely to respond tonically and are more likely to exhibit tonic suppression. Rt and VPm populations are similar to each other, however, in that they retain properties of directional sensitivity established in primary afferent neurons. In both populations neurons are selective for deflection angle and exhibit directional consistency, responding best to a particular direction of movement regardless of the starting position of the vibrissal hair. These findings suggest a role for Rt in the processing of detailed sensory information. Temporally, Rt may function to limit the duration of stimulus-evoked VPm responses and to focus them on rapid vibrissa perturbations. Moreover, by regulating the baseline activity of VPm neurons, Rt may indirectly enhance the response selectivity of layer IV barrel neurons to synchronous VPm firing.

Spontaneous activity and responses to sensory stimulation in ventrobasal thalamic neurons in the rat: an in vivo intracellular recording and staining study

Spontaneous activity and responses to sensory stimulation in ventrobasal (VB) thalamic neurons were studied in barbiturate-anesthetized rats through intracellular recordings. The recordings were carried out with micropipettes filled with K acetate KCl plus horseradish peroxidase (HRP), our KCl plus biocytin. Two types of spontaneous depolarizing events were observed: fast potentials (FPs), characterized by a low amplitude (5.3 +/- 1.8 mV [mean and standard deviation]), a fast rising slope (1.15 +/- 0.19 msec), and a short duration (8.47 +/- 0.89 msec); and slow potentials (SPs), characterized by a larger and more variable amplitude (9.1 +/- 5.6 mV) and a longer duration (62.5 +/- 27.2 msec), with a slower rising slope (26.2 +/- 6.4 msec). The potential changes elicited by sensory stimuli delivered manually were similar to those elicited by electronically gated short air jets to the receptive fields. FPs were evoked by sensory stimulation in 62.7% of the recorded neurons, and SPs in the remaining 37.3%. Both types of events could occur spontaneously in the same neuron, but only one of them was triggered by stimulation of the receptive field. Five neurons that were successfully stained with either HRP or biocytin were studied in detail. All were medium-sized stellate cells, with spine-like appendages sparsely distributed along slender radiating dendrites. The axons took a rostrolateral course across the VB, and all but one left one or two thin collaterals in the reticular thalamic nucleus. No overt morphological differences were observed between VB neurons that responded with FPS or SPs to sensory stimulation.

Effects of neonatal transection of the infraorbital nerve upon the structural and functional organization of the ventral posteromedial nucleus in the rat

The present study examined the way in which an indirect partial deafferentation of the medial portion of the ventrobasal complex (VPM/VPL) induced by neonatal transection of the infraorbital nerve (ION) altered the structural and functional properties of its constituent neurons. This manipulation significantly reduced the volume of the contralateral VPM/VPL. In addition, cell counts in Nissl-stained material revealed a significant reduction of the number of VPM/VPL neurons contralateral to neonatal ION transection. We also analyzed the effect of neonatal ION transection on the soma-dendritic morphology of individual neurons in the ventral posteromedial nucleus of the thalamus (VPM) by intracellular injection of horseradish peroxidase (HRP) in vivo and Lucifer yellow in fixed slices. Neonatal transection of the ION resulted in increased dendritic length, area, and volume of VPM neurons in both preparations; however only the changes observed in fixed slices reached statistical significance. Alterations in the functional characteristics of VPM neurons were also observed following neonatal nerve damage. There was a significant decrease in the percentage of vibrissae-sensitive neurons and a corresponding increase in the percentages of neurons responsive to guard hair deflection or that were unresponsive to peripheral stimulation. Neonatal nerve damage also resulted in significantly longer latencies of VPM cells after stimulation of either trigeminal nucleus principalis or subnucleus interpolaris. The present results indicate that the development of normal response properties and soma-dendritic morphology of VPM neurons is dependent upon intact afferent input during development. Indirect partial deafferentation of VPM/VPL by neonatal transection of the ION results in reduced neuron number, which may result in decreased competition among the dendrites of these neurons. This proposal is consistent with observations of increased dendritic dimensions of VPM neurons contralateral to neonatal ION damage.

Thalamic processing of vibrissal information in the rat: II. Morphological and functional properties of medial ventral posterior nucleus and posterior nucleus neurons

Extracellular recording, intracellular recording, intracellular horseradish peroxidase injection, and receptive field mapping techniques were employed to evaluate the physiological and morphological properties of medial ventral posterior nucleus (VPM) and posterior nucleus (POm) neurons in normal adult rats. Overall, we physiologically characterized 148 VPM and 121 POm neurons. Over 82% of the VPM cells were excited only by deflection of one or more mystacial vibrissae, 10% were activated by displacement of guard hairs, and the remainder were either excited by indentation of the skin or were unresponsive. Less than 40% of the POm cells were activated by vibrissa deflection, 18% were excited by displacement of guard hairs, and another 17% were unresponsive. Most of the rest of the POm cells were excited by stimulation of skin, mucosa, or activation of muscle-related afferents. Small percentages of POm cells responded only to noxious stimulation, were classified as having a wide dynamic range, or were inhibited by peripheral stimulation. Electrical stimulation of either PrV or SpI activated most neurons in both VPM and POm. This excitation was almost invariably followed by a long-lasting hyperpolarization which was generally strong enough to prevent responses to either electrical stimuli delivered in the brainstem or mechanical stimulation of the periphery. The receptive fields of vibrissa-sensitive cells in POm were generally much larger than those of cells in VPM. Data obtained with extracellular recording indicated that VPM and POm cells responded to an average of 1.4 and 4.0 vibrissae, respectively. Intracellular recording from smaller samples of VPM and POm cells demonstrated the existence of inputs that were insufficient to produce spikes from the cell, but did yield epsp's. When both sub- and suprathreshold excitation were considered, the average number of vibrissa in the receptive field of a VPM cell was 2.7 and the value for POm cells became 7.8. HRP-filled neurons recovered in POm (N = 20) generally had much larger dendritic arbors than neurons in VPM (N = 31). For the former cells, the size of the dendritic tree was significantly correlated with the number of vibrissa to which the cell responded; for the latter neurons, it was not.

Thalamo-cortical processing of vibrissal information in the rat. II. spatiotemporal convergence in the thalamic ventroposterior medial nucleus (VPm) and its relevance to generation of receptive fields of S1 cortical "barrel" neurones

One hundred and twenty-six cells, sampled in the vicinity of the D1 barreloid in the ventroposterior medial nucleus of the thalamus, were tested for magnitude and latency of response to brief deflections (3 ms; 1.14 degrees) of vibrissae in adult rats under controlled conditions of light urethane anaesthesia. Similar results were achieved for D1 and non-D1-dominant cells. D1-dominant cells (N = 76) responded to the centre-receptive field (D1) vibrissa with a mean of 1.08 spikes per stimulus at modal latencies of 3-12 ms (inter-quartile range 4-5 ms) and to surrounding vibrissae with a mean of 0.26 spikes per stimulus at latencies of 3-41 ms (interquartile range 5-8 ms). Surround-receptive fields showed extensive overlap but were reduced and finally eliminated by deepening anaesthesia. A cell-by-cell analysis showed no correlation between latency and response magnitude for responses to surround vibrissae. Response magnitudes to the surround- and centre-receptive field inputs for D1-dominant barrel cells were some 2.5- and 1.7-fold greater, respectively, than for thalamic cells under identical experimental conditions. The latencies to centre- and surround-receptive field inputs for D1-dominant barrel cells were 2.5 and 10-20 ms later than for thalamus, respectively. These data on a mismatch of latencies for surround- and centre-receptive fields in thalamus and cortex support the notion that surround-receptive fields of cortical barrel cells are almost entirely constructed intracortically during light anaesthesia (Armstrong-James et al., '91), although it is argued that surround-receptive fields of thalamic cells conceivably could be relayed in other cortical states or serve a role in plasticity.

Modulation of vibrissa-evoked cortical potentials after infraorbital nerve crush in rats

Cortical potentials evoked by unilateral stimulation of the major vibrissae were recorded in 12 rats subjected to unilateral crush of the infraorbital nerve. Immediately after nerve crushing, the latency of the initial positive potential evoked at contralateral scalp sites by stimulating the vibrissae of the nerve-crushed side was increased. In contrast, the latency of the ipsilaterally evoked potential was shortened. The relative amplitude of the negative component to the positive one of the evoked potentials tended, immediately after the nerve crush, to be smaller on the contralateral cortex (N/P-contra) and greater on the ipsilateral cortex (N/P-ipsi). These changes disappeared largely by the 2nd post-operative week. It is suggested that reduction of the tactile signals transmitted through the crossed pathway is responsible for the prolonged latency and the smaller N/P-contra. Shortening of the ipsilateral latency and the enhanced N/P-ipsi may be due to liberation of the ipsilateral sensory system from inhibition by the contralateral one.

Structure-function relationships in rat brainstem subnucleus interpolaris: IV. Projection neurons

In a companion paper (Jacquin et al., '89), the structure and function of local circuit (LC) neurons in spinal trigeminal (V) subnucleus interpolaris (Sp Vi) were described. The present report provides similar data for 44 projection neurons in Sp Vi. Of these, 25 thalamic, 16 cerebellar, 2 superior collicular, and 1 inferior olivary projecting neurons were studied. The majority responded to vibrissa(e) deflection, and all except 4 of these had multivibrissae receptive fields. The remainder were responsive to either guard hair deflection or indentation of glabrous skin. Latencies to V ganglion shocks were suggestive of monosynaptic activation from the periphery. Sp Vi projection neurons were topographically organized in a manner consistent with that of their primary afferent inputs. Nonvibrissa sensitive cells had diverse morphologies. Morphometric analyses of the more heavily sampled thalamic and cerebellar projecting, vibrissa(e)-sensitive cells indicated the following. (1) As compared to LC neurons, projection neurons had bigger receptive fields, cell bodies, dendritic trees, and axons; less circular dendritic trees; a greater preponderance of spiny dendrites and fewer axon collaterals in Sp Vi. (2) Dendritic tree extent correlated significantly with receptive field size, thus suggesting that dendritic tree size is one mechanism contributing to receptive field size in vibrissae-sensitive projection neurons. (3) V thalamic cells had significantly bigger receptive fields and dendritic trees, and also give off more local axon collaterals, than V cerebellar neurons. Collicular and inferior olivary projecting neurons shared structural and functional attributes with other Sp Vi long-range projecting cells. Structure-function relationships exist for vibrissa-sensitive projection neurons in Sp Vi. The relevant parameters correlating with projection neuron morphology are receptive field size and projection status, whereas for Sp Vi LC neurons the relevant correlative parameter is peripheral receptor association.

Burst discharges of thalamic reticular neurons: an intracellular analysis in anesthetized rats

In order to analyze the mechanism of burst discharges intracellular recordings were made from 27 somatosensory thalamic reticular (S-TR) neurons in urethane-anesthetized rats. Burst discharges, composed of 2-7 spikes, were always superposed on a slow depolarization (SD) lasting for 40-60 ms, which appeared only when the membrane was hyperpolarized. The number of spikes superposed on an SD varied depending upon the amplitude of the SD. A single shock stimulation of the lemniscus medialis elicited a series of SDs, each without being preceded by a phasic hyperpolarizing potential. The SDs were repeated with spindle rhythms. Evidence has been provided that EPSPs contribute to the mechanism for triggering SDs. In spontaneous rhythmic SDs occurring with the rhythm of EEG spindles, steps representing EPSPs were recordable on the rising phase of each SD. It is suggested that excitatory synaptic inputs to S-TR neurons with the spindle rhythm are responsible for the rhythmic generation of SDs. Ventrobasal relay neurons are presumed as the source of the inputs.

Burst discharges associated with phasic hyperpolarizing oscillations of rat ventrobasal relay neurons

Intracellular recordings were made from ventrobasal relay neurons in urethane-anesthetized rats. A series of phasic hyperpolarizations repeated with the spindle rhythm appeared in response to single shocks to the medial lemniscus or spontaneously. On the recovery slope of some phasic hyperpolarizations slow depolarizations (SDs) lasting for 30-50 ms with burst discharges were generated as rebound excitation. The voltage dependency of SDs was proved by changing the membrane potential by current injection. The number of spikes triggered by the SD increased as the SD became larger in amplitude and faster in rising speed.