Thalamic connectivity of rat somatic motor cortex

Attentional modulation of secondary somatosensory and visual thalamus of mice

Each sensory modality has its own primary and secondary thalamic nuclei. While the primary thalamic nuclei are well understood to relay sensory information from the periphery to the cortex, the role of secondary sensory nuclei is elusive. We trained head-fixed mice to attend to one sensory modality while ignoring a second modality, namely to attend to touch and ignore vision, or vice versa. Arrays were used to record simultaneously from secondary somatosensory thalamus (POm) and secondary visual thalamus (LP). In mice trained to respond to tactile stimuli and ignore visual stimuli, POm was robustly activated by touch and largely unresponsive to visual stimuli. A different pattern was observed when mice were trained to respond to visual stimuli and ignore touch, with POm now more robustly activated during visual trials. This POm activity was not explained by differences in movements (i.e., whisking, licking, pupil dilation) resulting from the two tasks. Post hoc histological reconstruction of array tracks through POm revealed that subregions varied in their degree of plasticity. LP exhibited similar phenomena. We conclude that behavioral training reshapes activity in secondary thalamic nuclei. Secondary nuclei respond to the same behaviorally relevant, reward-predicting stimuli regardless of stimulus modality.

Oscillatory waveform sharpness asymmetry changes in motor thalamus and motor cortex in a rat model of Parkinson's disease

Parkinson's disease (PD) causes bursty and oscillatory activity in basal ganglia output that is thought to contribute to movement deficits through impact on motor thalamus and motor cortex (MCx). We examined the effect of dopamine loss on motor thalamus and motor cortex activity by recording neuronal and LFP activities in ventroanterior-ventrolateral (VAVL) thalamus and MCx in urethane-anesthetised control and parkinsonian rats. Dopamine lesion decreased the firing rate and increased the bursting of putative pyramidal neurons in layer V, but not layer VI, of the MCx without changing other aspects of firing pattern. In contrast, dopamine lesion did not affect VAVL firing rate, pattern or low threshold calcium spike bursts. Slow-wave (~1 Hz) oscillations in LFP recordings were analyzed with conventional power and waveform shape analyses. While dopamine lesion did not influence total power, it was consistently associated with an increase in oscillatory waveform sharpness asymmetry (i.e., sharper troughs vs. peaks) in both motor thalamus and MCx. Furthermore, we found that measures of sharpness asymmetry were positively correlated in paired motor thalamus-MCx recordings, and that correlation coefficients were larger in dopamine lesioned rats. These data support the idea that dysfunctional MCx activity in parkinsonism emerges from subsets of cell groups (e.g. layer V pyramidal neurons) and is evident in the shape but not absolute power of slow-wave oscillations. Hypoactive layer V pyramidal neuron firing in dopamine lesioned rats is unlikely to be driven by VAVL thalamus and may, therefore, reflect the loss of mesocortical dopaminergic afferents and/or changes in intrinsic excitability.

Specific populations of basal ganglia output neurons target distinct brain stem areas while collateralizing throughout the diencephalon

Basal ganglia play a central role in regulating behavior, but the organization of their outputs to other brain areas is incompletely understood. We investigate the largest output nucleus, the substantia nigra pars reticulata (SNr), and delineate the organization and physiology of its projection populations in mice. Using genetically targeted viral tracing and whole-brain anatomical analysis, we identify over 40 SNr targets that encompass a roughly 50-fold range of axonal densities. Retrograde tracing from the volumetrically largest targets indicates that the SNr contains segregated subpopulations that differentially project to functionally distinct brain stem regions. These subpopulations are electrophysiologically specialized and topographically organized and collateralize to common diencephalon targets, including the motor and intralaminar thalamus as well as the pedunculopontine nucleus and the midbrain reticular formation. These findings establish that SNr signaling is organized as dense, parallel outputs to specific brain stem targets concurrent with extensive collateral branches that encompass the majority of SNr axonal boutons.

Parallel and Serial Sensory Processing in Developing Primary Somatosensory and Motor Cortex

It is generally supposed that primary motor cortex (M1) receives somatosensory input predominantly via primary somatosensory cortex (S1). However, a growing body of evidence indicates that M1 also receives direct sensory input from the thalamus, independent of S1; such direct input is particularly evident at early ages before M1 contributes to motor control. Here, recording extracellularly from the forelimb regions of S1 and M1 in unanesthetized rats at postnatal day (P)8 and P12, we compared S1 and M1 responses to self-generated (i.e., reafferent) forelimb movements during active sleep and wake, and to other-generated (i.e., exafferent) forelimb movements. At both ages, reafferent responses were processed in parallel by S1 and M1; in contrast, exafferent responses were processed in parallel at P8 but serially, from S1 to M1, at P12. To further assess this developmental difference in processing, we compared exafferent responses to proprioceptive and tactile stimulation. At both P8 and P12, proprioceptive stimulation evoked parallel responses in S1 and M1, whereas tactile stimulation evoked parallel responses at P8 and serial responses at P12. Independent of the submodality of exafferent stimulation, pairs of S1-M1 units exhibited greater coactivation during active sleep than wake. These results indicate that S1 and M1 independently develop somatotopy before establishing the interactive relationship that typifies their functionality in adults.SIGNIFICANCE STATEMENT Learning any new motor task depends on the ability to use sensory information to update motor outflow. Thus, to understand motor learning, we must also understand how animals process sensory input. Primary somatosensory cortex (S1) and primary motor cortex (M1) are two interdependent structures that process sensory input throughout life. In adults, the functional relationship between S1 and M1 is well established; however, little is known about how S1 and M1 begin to transmit or process sensory information in early life. In this study, we investigate the early development of S1 and M1 as a sensory processing unit. Our findings provide new insights into the fundamental principles of sensory processing and the development of functional connectivity between these important sensorimotor structures.

Secondary Degeneration of White Matter After Focal Sensorimotor Cortical Ischemic Stroke in Rats

Ischemic lesions could lead to secondary degeneration in remote regions of the brain. However, the spatial distribution of secondary degeneration along with its role in functional deficits is not well understood. In this study, we explored the spatial and connectivity properties of white matter (WM) secondary degeneration in a focal unilateral sensorimotor cortical ischemia rat model, using advanced microstructure imaging on a 14 T MRI system. Significant axonal degeneration was observed in the ipsilateral external capsule and even remote regions including the contralesional external capsule and corpus callosum. Further fiber tractography analysis revealed that only fibers having direct axonal connections with the primary lesion exhibited a significant degeneration. These results suggest that focal ischemic lesions may induce remote WM degeneration, but limited to fibers tied to the primary lesion. These “direct” fibers mainly represent perilesional, interhemispheric, and subcortical axonal connections. At last, we found that primary lesion volume might be the determining factor of motor function deficits.

The N-Methyl d-Aspartate Glutamate Receptor Antagonist Ketamine Disrupts the Functional State of the Corticothalamic Pathway

The non-competitive N-methyl d-aspartate glutamate receptor (NMDAR) antagonist ketamine elicits a brain state resembling high-risk states for developing psychosis and early stages of schizophrenia characterized by sensory and cognitive deficits and aberrant ongoing gamma (30-80 Hz) oscillations in cortical and subcortical structures, including the thalamus. The underlying mechanisms are unknown. The goal of the present study was to determine whether a ketamine-induced psychotic-relevant state disturbs the functional state of the corticothalamic (CT) pathway. Multisite field recordings were performed in the somatosensory CT system of the sedated rat. Baseline activity was challenged by activation of vibrissa-related prethalamic inputs. The sensory-evoked thalamic response was characterized by a short-latency (∼4 ms) prethalamic-mediated negative sharp potential and a longer latency (∼10 ms) CT-mediated negative potential. Following a single subcutaneous injection of ketamine (2.5 mg/kg), spontaneously occurring and sensory-evoked thalamic gamma oscillations increased and decreased in power, respectively. The power of the sensory-related gamma oscillations was positively correlated with both the amplitude and the area under the curve of the corresponding CT potential but not with the prethalamic potential. The present results show that the layer VI CT pathway significantly contributes in thalamic gamma oscillations, and they support the hypothesis that reduced NMDAR activation disturbs the functional state of CT and corticocortical networks.

The pattern of thalamocortical and brain stem projections to the vibrissae-related sensory and motor cortices in de-whiskered congenital hypothyroid rats

The present study is designed to investigate the plastic organization of the thalamo-cortical (TC) and brain stem afferents of whisker primary sensory (wS1) and motor (wM1) cortical areas in congenital hypothyroid (CH) pups following whisker deprivation (WD) from neonatal to adolescence period. Maternal hypothyroidism was induced by adding propylthiouracil (PTU) to the drinking water from early embryonic day 16 to postnatal day (PND) 60. Pregnant rats were divided into intact and CH groups (n = 8). In each group, the total whiskers of pups (4 of 8) were trimmed continuously from PND 1 to PND 60. Retrograde tracing technique with WGA-HRP was performed in the present study. Retrogradely labeled neurons were observed in the specific thalamic nuclei (VPM and VL) following separately WGA-HRP injections into wS1/M1 cortical areas. The number of labeled cells in the VPM, VL, VM and PO nuclei of the thalamus significantly decreased in CH offsprings rats (P < 0.05). Neonatal WD did not show any significant effects on the number of VPM, VL, VM and PO labeled projection neurons to wS1 and wM1 cortical areas. In addition, retrogradely labeled neurons in dorsal raphe (DR) and locus coeruleus (LC) nuclei were observed in all experimental groups. The number of DR and LC labeled neurons were higher in the CH and whisker deprived groups compared to their matching controls (P < 0.05). Upon our results, CH and WD had no synergic or additive effects on the TC and brain stem afferent patterns of barrel sensory and motor cortices.

Intracortical signal processing of periodontal ligament sensations in rat

The somatosensory information from the orofacial region, including the periodontal ligament (PDL), is processed in a manner that differs from that used for other body somatosensory information in the related cortices. It was reported that electrical stimulation to rat PDL elicited activation of the insular oral region (IOR) and the primary (S1) and secondary (S2) somatosensory cortices. However, the physiological relationship between S1 and S2/IOR is not well understood. To address this issue, we performed in vivo optical imaging using a voltage-sensitive dye. Our results demonstrated that the electrical stimulation to the PDL of the mandibular incisor evoked the simultaneous activation of S1 and the S2/IOR. The stimulation to the initial response area of the S1 evoked responses in the S2/IOR, and vice versa. An injection of tetrodotoxin (TTX) to the cortical region between S1 and S2/IOR attenuated such elicited responses only in the non-stimulated cortical partner site. The cortico-cortical interaction between S1 and S2/IOR was suppressed by the application of TTX, indicating that these two cortical regions bi-directionally communicate the signal processing of PDL sensations. An injection of FluoroGold™ (FG) to the initial response area in S1 or the S2/IOR showed that FG-positive cells were scattered in the non-injected cortical counterpart. This morphological result demonstrated the presence of a bi-directional intracortical connection between the initial response areas in S1 and the S2/IOR. These findings suggest the presence of a mutual connection between S1 and the S2/IOR as an intracortical signal processing network for orofacial nociception.

Whisking-Related Changes in Neuronal Firing and Membrane Potential Dynamics in the Somatosensory Thalamus of Awake Mice

The thalamus transmits sensory information to the neocortex and receives neocortical, subcortical, and neuromodulatory inputs. Despite its obvious importance, surprisingly little is known about thalamic function in awake animals. Here, using intracellular and extracellular recordings in awake head-restrained mice, we investigate membrane potential dynamics and action potential firing in the two major thalamic nuclei related to whisker sensation, the ventral posterior medial nucleus (VPM) and the posterior medial group (Pom), which receive distinct inputs from brainstem and neocortex. We find heterogeneous state-dependent dynamics in both nuclei, with an overall increase in action potential firing during active states. Whisking increased putative lemniscal and corticothalamic excitatory inputs onto VPM and Pom neurons, respectively. A subpopulation of VPM cells fired spikes phase-locked to the whisking cycle during free whisking, and these cells may therefore signal whisker position. Our results suggest differential processing of whisking comparing thalamic nuclei at both sub- and supra-threshold levels.

Synaptic circuit organization of motor corticothalamic neurons

Corticothalamic (CT) neurons in layer 6 constitute a large but enigmatic class of cortical projection neurons. How they are integrated into intracortical and thalamo-cortico-thalamic circuits is incompletely understood, especially outside of sensory cortex. Here, we investigated CT circuits in mouse forelimb motor cortex (M1) using multiple circuit-analysis methods. Stimulating and recording from CT, intratelencephalic (IT), and pyramidal tract (PT) projection neurons, we found strong CT↔ CT and CT↔ IT connections; however, CT→IT connections were limited to IT neurons in layer 6, not 5B. There was strikingly little CT↔ PT excitatory connectivity. Disynaptic inhibition systematically accompanied excitation in these pathways, scaling with the amplitude of excitation according to both presynaptic (class-specific) and postsynaptic (cell-by-cell) factors. In particular, CT neurons evoked proportionally more inhibition relative to excitation (I/E ratio) than IT neurons. Furthermore, the amplitude of inhibition was tuned to match the amount of excitation at the level of individual neurons; in the extreme, neurons receiving no excitation received no inhibition either. Extending these studies to dissect the connectivity between cortex and thalamus, we found that M1-CT neurons and thalamocortical neurons in the ventrolateral (VL) nucleus were remarkably unconnected in either direction. Instead, VL axons in the cortex excited both IT and PT neurons, and CT axons in the thalamus excited other thalamic neurons, including those in the posterior nucleus, which additionally received PT excitation. These findings, which contrast in several ways with previous observations in sensory areas, illuminate the basic circuit organization of CT neurons within M1 and between M1 and thalamus.

Systematic comparison of adeno-associated virus and biotinylated dextran amine reveals equivalent sensitivity between tracers and novel projection targets in the mouse brain

As an anterograde neuronal tracer, recombinant adeno-associated virus (AAV) has distinct advantages over the widely used biotinylated dextran amine (BDA). However, the sensitivity and selectivity of AAV remain uncharacterized for many brain regions and species. To validate this tracing method further, AAV (serotype 1) was systematically compared with BDA as an anterograde tracer by injecting both tracers into three cortical and 15 subcortical regions in C57BL/6J mice. Identical parameters were used for our sequential iontophoretic injections, producing injections of AAV that were more robust in size and in density of neurons infected compared with those of BDA. However, these differences did not preclude further comparison between the tracers, because the pairs of injections were suitably colocalized and contained some percentage of double-labeled neurons. A qualitative analysis of projection patterns showed that the two tracers behave very similarly when injection sites are well matched. Additionally, a quantitative analysis of relative projection intensity for cases targeting primary motor cortex (MOp), primary somatosensory cortex (SSp), and caudoputamen (CP) showed strong agreement in the ranked order of projection intensities between the two tracers. A detailed analysis of the projections of two brain regions (SSp and MOp) revealed many targets that have not previously been described in the mouse or rat. Minor retrograde labeling of neurons was observed in all cases examined, for both AAV and BDA. Our results show that AAV has actions equivalent to those of BDA as an anterograde tracer and is suitable for analysis of neural circuitry throughout the mouse brain.

Evolution of mammalian sensorimotor cortex: thalamic projections to parietal cortical areas in Monodelphis domestica

The current experiments build upon previous studies designed to reveal the network of parietal cortical areas present in the common mammalian ancestor. Understanding this ancestral network is essential for highlighting the basic somatosensory circuitry present in all mammals, and how this basic plan was modified to generate species specific behaviors. Our animal model, the short-tailed opossum (Monodelphis domestica), is a South American marsupial that has been proposed to have a similar ecological niche and morphology to the earliest common mammalian ancestor. In this investigation, we injected retrograde neuroanatomical tracers into the face and body representations of primary somatosensory cortex (S1), the rostral and caudal somatosensory fields (SR and SC), as well as a multimodal region (MM). Projections from different architectonically defined thalamic nuclei were then quantified. Our results provide further evidence to support the hypothesized basic mammalian plan of thalamic projections to S1, with the lateral and medial ventral posterior thalamic nuclei (VPl and VPm) projecting to S1 body and S1 face, respectively. Additional strong projections are from the medial division of posterior nucleus (Pom). SR receives projections from several midline nuclei, including the medial dorsal, ventral medial nucleus, and Pom. SC and MM show similar patterns of connectivity, with projections from the ventral anterior and ventral lateral nuclei, VPm and VPl, and the entire posterior nucleus (medial and lateral). Notably, MM is distinguished from SC by relatively dense projections from the dorsal division of the lateral geniculate nucleus and pulvinar. We discuss the finding that S1 of the short-tailed opossum has a similar pattern of projections as other marsupials and mammals, but also some distinct projections not present in other mammals. Further we provide additional support for a primitive posterior parietal cortex which receives input from multiple modalities.

Laminar-specific distribution of zinc: evidence for presence of layer IV in forelimb motor cortex in the rat

The rat is the most widely studied pre-clinical model system of various neurological and neurodegenerative disorders affecting hand function. Although brain injury to the forelimb region of the motor cortex in rats mostly induces behavioral abnormalities in motor control of hand movements, behavioral deficits in the sensory-motor domain are also observed. This questions the prevailing view that cortical layer IV, a recipient of sensory information from the thalamus, is absent in rat motor cortex. Because zinc-containing neurons are generally not found in pathways that run from the thalamus, an absence of zinc (Zn) in a cortical layer would be suggestive of sensory input from the thalamus. To test this hypothesis, we used synchrotron micro X-ray fluorescence imaging to measure Zn distribution across cortical layers. Zn maps revealed a heterogeneous layered Zn distribution in primary and secondary motor cortices of the forelimb region in the adult rat. Two wider bands with elevated Zn content were separated by a narrow band having reduced Zn content, and this was evident in two rat strains. The Zn distribution pattern was comparable to that in sensorimotor cortex, which is known to contain a well demarcated layer IV. Juxtaposition of Zn maps and the images of brain stained for Nissl bodies revealed a "Zn valley" in primary motor cortex, apparently starting at the ventral border of pyramidal layer III and ending at the close vicinity of layer V. This finding indicates the presence of a conspicuous cortical layer between layers III and V, i.e. layer IV, the presence of which previously has been disputed. The results have implications for the use of rat models to investigate human brain function and neuropathology, such as after stroke. The presence of layer IV in the forelimb region of the motor cortex suggests that therapeutic interventions used in rat models of motor cortex injury should target functional abnormalities in both motor and sensory domains. The finding is also critical for future investigation of the biochemical mechanisms through which therapeutic interventions can enhance neural plasticity, particularly through Zn dependent pathways.

Two whisker motor areas in the rat cortex: evidence from thalamocortical connections

In primates, the motor cortex consists of at least seven different areas, which are involved in movement planning, coordination, initiation, and execution. However, for rats, only the primary motor cortex has been well described. A rostrally located second motor area has been proposed, but its extent, organization, and even definitive existence remain uncertain. Only a rostral forelimb area (RFA) has been definitively described, besides few reports of a rostral hindlimb area. We have previously proposed existence of a second whisker area, which we termed the rostral whisker area (RWA), based on its differential response to intracortical microstimulation compared with the caudal whisker area (CWA) in animals under deep anesthesia (Tandon et al. [2008] Eur J Neurosci 27:228). To establish that RWA is distinct from the caudally contiguous CWA, we determined sources of thalamic inputs to the two proposed whisker areas. Sources of inputs to RFA, caudal forelimb area (CFA), and caudal hindlimb region were determined for comparison. The results show that RWA and CWA can be distinguished based on differences in their thalamic inputs. RWA receives major projections from mediodorsal and ventromedial nuclei, whereas the major projections to CWA are from the ventral anterior, ventrolateral, and posterior nuclei. Moreover, the thalamic nuclei that provide major inputs to RWA are the same as for RFA, and the nuclei projecting to CWA are same as for CFA. The results suggest that rats have a second rostrally located motor area with RWA and RFA as its constituents.

The neuroinvasive profiles of H129 (herpes simplex virus type 1) recombinants with putative anterograde-only transneuronal spread properties

The use of viruses as transneuronal tracers has become an increasingly powerful technique for defining the synaptic organization of neural networks. Although a number of recombinant alpha herpesviruses are known to spread selectively in the retrograde direction through neural circuits only one strain, the H129 strain of herpes simplex virus type 1, is reported to selectively spread in the anterograde direction. However, it is unclear from the literature whether there is an absolute block or an attenuation of retrograde spread of H129. Here, we demonstrate efficient anterograde spread, and temporally delayed retrograde spread, of H129 and three novel recombinants. In vitro studies revealed no differences in anterograde and retrograde spread of parental H129 and its recombinants through superior cervical ganglion neurons. In vivo injections of rat striatum revealed a clear bias of anterograde spread, although evidence of deficient retrograde transport was also present. Evidence of temporally delayed retrograde transneuronal spread of H129 in the retina was observed following injection of the lateral geniculate nucleus. The data also demonstrated that three novel recombinants efficiently express unique fluorescent reporters and have the capacity to infect the same neurons in dual infection paradigms. From these experiments we conclude that H129 and its recombinants not only efficiently infect neurons through anterograde transneuronal passage, but also are capable of temporally delayed retrograde transneuronal spread. In addition, the capacity to produce dual infection of projection targets following anterograde transneuronal passage provides an important addition to viral transneuronal tracing technology.

Towards a circuit mechanism for movement tuning in motor cortex

The firing rates of neurons in primate motor cortex have been related to multiple parameters of voluntary movement. This finding has been corroborated by stimulation-based studies that have mapped complex movements in rodent and primate motor cortex. However, it has been difficult to link the movement tuning of a neuron with its role within the cortical microcircuit. In sensory cortex, neuronal tuning is largely established by afferents delivering information from tuned receptors in the periphery. Motor cortex, which lacks the granular input layer, may be better understood by analyzing its efferent projections. As a primary source of cortical output, layer 5 neurons represent an ideal starting point for this line of experimentation. It is in these deep output layers that movements can most effectively be evoked by intracortical microstimulation and recordings can obtain the most useful signals for the control of motor prostheses. Studies focused on layer 5 output neurons have revealed that projection identity is a fundamental property related to the laminar position, receptive field and ion channel complement of these cells. Given the variety of brain areas targeted by layer 5 output neurons, knowledge of a neuron's downstream connectivity may provide insight into its movement tuning. Future experiments that relate motor behavior to the activity of neurons with a known projection identity will yield a more detailed understanding of the function of cortical microcircuits.

The thalamocortical vestibular system in animals and humans

The vestibular system provides the brain with sensory signals about three-dimensional head rotations and translations. These signals are important for postural and oculomotor control, as well as for spatial and bodily perception and cognition, and they are subtended by pathways running from the vestibular nuclei to the thalamus, cerebellum and the "vestibular cortex." The present review summarizes current knowledge on the anatomy of the thalamocortical vestibular system and discusses data from electrophysiology and neuroanatomy in animals by comparing them with data from neuroimagery and neurology in humans. Multiple thalamic nuclei are involved in vestibular processing, including the ventroposterior complex, the ventroanterior-ventrolateral complex, the intralaminar nuclei and the posterior nuclear group (medial and lateral geniculate nuclei, pulvinar). These nuclei contain multisensory neurons that process and relay vestibular, proprioceptive and visual signals to the vestibular cortex. In non-human primates, the parieto-insular vestibular cortex (PIVC) has been proposed as the core vestibular region. Yet, vestibular responses have also been recorded in the somatosensory cortex (area 2v, 3av), intraparietal sulcus, posterior parietal cortex (area 7), area MST, frontal cortex, cingulum and hippocampus. We analyze the location of the corresponding regions in humans, and especially the human PIVC, by reviewing neuroimaging and clinical work. The widespread vestibular projections to the multimodal human PIVC, somatosensory cortex, area MST, intraparietal sulcus and hippocampus explain the large influence of vestibular signals on self-motion perception, spatial navigation, internal models of gravity, one's body perception and bodily self-consciousness.

The effect of spinal cord stimulation on seizure susceptibility in rats

OBJECTIVES

Spinal cord stimulation (SCS) activates the thalamus, which may be involved in generation of seizures. SCS may therefore influence seizure susceptibility. We investigated the effect of SCS on seizure susceptibility when performed at low frequency (4 Hz) and a frequency in the typical range of SCS treatment (54 Hz).

MATERIALS AND METHODS

Rats were divided in three groups: control (N = 8), 4 Hz SCS (N = 6), and 54 Hz SCS (N = 8). Tonic-clonic seizures were induced by 10-min intravenous infusion of pentylenetetrazole (PTZ). SCS was started 5 min prior to PTZ infusion and continued for 5 min after infusion offset. Seizure susceptibility was accessed via the latency, number, and total duration of seizures.

RESULTS

Four Hz SCS significantly increased seizure susceptibility. Fifty-four Hz SCS produced a nonsignificant trend toward decreased seizure susceptibility.

CONCLUSIONS

Low-frequency SCS is proconvulsive in rats. Further research needs to investigate if this also applies to humans.

Thalamocortical connections of functional zones in posterior parietal cortex and frontal cortex motor regions in New World monkeys

Posterior parietal cortex (PPC) links primate visual and motor systems and is central to visually guided action. Relating the anatomical connections of PPC to its neurophysiological functions may elucidate the organization of the parietal-frontal network. In owl and squirrel monkeys, long-duration electrical stimulation distinguished several functional zones within the PPC and motor/premotor cortex (M1/PM). Multijoint forelimb movements reminiscent of reach, defense, and grasp behaviors characterized each functional zone. In PPC, functional zones were organized parallel to the lateral sulcus. Thalamocortical connections of PPC and M1/PM zones were investigated with retrograde tracers. After several days of tracer transport, brains were processed, and labeled cells in thalamic nuclei were plotted. All PPC zones received dense inputs from the lateral posterior nucleus and the anterior pulvinar. PPC zones received additional projections from ventral lateral (VL) divisions of motor thalamus, which were also the primary source of input to M1/PM. Projections to PPC from rostral motor thalamus were sparse. Dense projections from ventral posterior (VP) nucleus of somatosensory thalamus distinguished the rostrolateral grasp zone from the other PPC zones. PPC connections with VL and VP provide links to cerebellar nuclei and the somatosensory system, respectively, that may integrate PPC functions with M1/PM.

Cannabinoids reveal separate controls for whisking amplitude and timing in rats

Whisking is controlled by multiple, possibly functionally segregated, motor sensory-motor loops. While testing for effects of endocannabinoids on whisking, we uncovered the first known functional segregation of channels controlling whisking amplitude and timing. Channels controlling amplitude, but not timing, were modulated by cannabinoid receptor type 1 (CB1R). Systemic administration of CB1R agonist Δ(9)-tetrahydrocannabinol (Δ(9)-THC) reduced whisking spectral power across all tested doses (1.25-5 mg/kg), whereas whisking frequency was affected at only very high doses (5 mg/kg). Concomitantly, whisking amplitude and velocity were significantly reduced in a dose-dependent manner (25-43 and 26-50%, respectively), whereas cycle duration and bilateral synchrony were hardly affected (3-16 and 3-9%, respectively). Preadministration of CB1R antagonist SR141716A blocked Δ(9)-THC-induced kinematic alterations of whisking, and when administered alone, increased whisking amplitude and velocity but affected neither cycle duration nor synchrony. These findings indicate that whisking amplitude and timing are controlled by separate channels and that endocannabinoids modulate amplitude control channels.

Thalamic afferent and efferent connectivity to cerebral cortical areas with direct projections to identified subgroups of trigeminal premotoneurons in the rat

The roles of supramedullary brain mechanisms involved in the control of jaw movements are not fully understood. To address this issue, a series of retrograde (Fluorogold, FG) and anterograde (biotinylated dextran amine, BDA) tract-tracing studies were done in rats. At first, we identified projection patterns from defined sensorimotor cortical areas to subgroups of trigeminal premotoneurons that are located in defined brainstem areas. Focal injections of FG into these brainstem areas revealed that the rostralmost part of lateral agranular cortex (rmost-Agl), the rostralmost part of medial agranular cortex (rmost-Agm), and the rostralmost part of primary somatosensory cortex (rmost-S1) preferentially project to brainstem areas containing jaw-closing premotoneurons, jaw-opening premotoneurons and a mixture of both types of premotoneurons, respectively. The thalamic reciprocal connectivities to rmost-Agl, rmost-Agm, and rmost-S1 were then investigated following cortical injections of FG or BDA. We found many retrogradely FG-labeled neurons and large numbers of axons and terminals labeled anterogradely with BDA in the dorsal thalamus mainly on the side ipsilateral to the injection sites. The rmost-Agl had strong connections with the ventral lateral nucleus (VL), ventromedial nucleus (VM), parafascicular nucleus, and posterior nucleus (Po); the rmost-Agm with the ventral anterior nucleus, VL, VM, central lateral nucleus, paracentral nucleus, central medial nucleus, mediodorsal nucleus and Po; and the rmost-S1 with the ventral posteromedial nucleus and Po. The present results suggest that the descending multiple pathways from the cerebral cortex to jaw-closing and jaw-opening premotoneurons have unique functional roles in jaw movement motor control.

Differential topography of the bilateral cortical projections to the whisker and forepaw regions in rat motor cortex

Whisker and forelimb movements in rats have distinct behavioral functions that suggest differences in the neural connections of the brain regions that control their movements. To test this hypothesis, retrograde tracing methods were used to characterize the bilateral distribution of the cortical neurons that project to the whisker and forelimb regions in primary motor (MI) cortex. Tracer injections in each MI region revealed labeled neurons in more than a dozen cortical areas, but most labeling was concentrated in the sensorimotor areas. Cortical projections to the MI forepaw region originated primarily from the primary somatosensory (SI) cortex in the ipsilateral hemisphere. In contrast, most projections to the MI whisker region originated from the MI whisker region in the contralateral hemisphere. Tracer injections in the MI whisker region also revealed a higher proportion of labeled neurons in the claustrum and in the posterior parietal cortex. Injections of different tracers into the MI whisker and forepaw regions of some rats revealed a topographic organization of neuronal labeling in several sensorimotor regions. Collectively, these findings indicate that the MI whisker and forepaw regions receive different sets of cortical inputs. Whereas the MI whisker region is most strongly influenced by callosal projections, presumably to mediate bilateral coordination of the whiskers, the MI forepaw region is influenced mainly by ipsilateral SI inputs that convey somatosensory feedback.

Tracing thalamo-cortical connections in tenrecA further attempt to characterize poorly differentiated neocortical regions, particularly the motor cortex

The hedgehog tenrec (Afrosoricidae) has a very poorly differentiated neocortex. Previously its primary sensory regions have been characterized with hodological and electrophysiological techniques. Unlike the marsupial opossum the tenrec may also have a separate motor area as far as there are cortico-spinal cells located rostral to the primary somatosensory cortex. However, not knowing its thalamic input it may be premature to correlate this area with the true (mirror-image-like) primary motor cortex in higher mammals. For this reason the tenrec's thalamo-cortical connections were studied following tracer injections into various neocortical regions. The main sensory areas were confirmed by their afferents from the principal thalamic nuclei. The dorsal lateral geniculate nucleus, in addition, was connected with the retrosplenial area and a rostromedial visual region. Unlike the somatosensory cortex the presumed motor area did not receive afferents from the ventrobasal thalamus but fibers from the cerebello-thalamic target regions. These projections, however, were not restricted to the motor area, but involved the entire somatosensorimotor field as well as adjacent regions. The projections appeared similar to those arising in the rat thalamic ventromedial nucleus known to have a supporting function rather than a specific motor task. The question was raised whether the input from the basal ganglia might play a crucial role in the evolution of the mammalian motor cortex? Certainly, in the tenrec, the poor differentiation of the motor cortex coincides with the virtual absence of an entopeduncular projection to the ventrolateral thalamus.

Region and sex differences in constituent dopamine neurons and immunoreactivity for intracellular estrogen and androgen receptors in mesocortical projections in rats

Many cortical and prefrontal functions show sex differences in their development, adult capacity, and dysfunction in disorders like schizophrenia. Correlations between circulating gonadal hormones and certain prefrontal functions have also been identified in humans and experimental animal models. Although multiple mechanisms may be involved, such hormone sensitivities/sex differences could be related to gonadal steroid actions on another regulator of cortical/prefrontal cortical function, the mesocortical dopamine system. Thus, although it is well known that perturbations in prefrontal dopamine signaling induce behavioral deficits, it is also known that several endpoints of these afferents are sensitive to gonadal steroids and/or are sexually dimorphic. This study explored possible substrates for this in two ways: by comparing the distributions of immunoreactivity for intracellular estrogen (alpha and beta) and androgen receptors among retrogradely labeled dopaminergic and nondopaminergic mesocortical neurons projecting to prefrontal, premotor, and primary motor cortices, areas in which male rat dopamine axons are differentially hormone-sensitive; and by comparing anatomical data in males and females. These analyses revealed region-, cell-, and sex-specific specializations in receptor localization that paralleled established patterns of mesocortical hormone sensitivity, including the androgen sensitivity of dopamine axons and dopamine-dependent functions in prefrontal cortex. It was also found that the proportions of dopamine neurons making up mesocortical projections were approximately 30% in males, whereas in females, significantly more constituent cells were dopaminergic. Together, these features may be part of the neurobiology giving mesocortical afferents their hormone sensitivities and/or sex differences in physiology, function, and dysfunction in disease.

Contralateral corticothalamic projections from MI whisker cortex: potential route for modulating hemispheric interactions

Rat whisking behavior is characterized by high amounts of bilateral coordination in which whisker movements on both sides of the face are linked. To elucidate the neural substrate that might mediate this bilateral coordination, neuronal tracers were used to characterize the bilateral distribution of corticothalamic projections from primary motor (MI) cortex. Some rats received tracers in the MI whisker region, whereas others received tracers in the MI forepaw region. The MI whisker region projects bilaterally to the anteromedial (AM), ventromedial (VM), and ventrolateral (VL) nuclei, and to parts of the intralaminar nuclei. By contrast, the MI forepaw region sends virtually no projections to the contralateral thalamus. Consistent with these findings, bilateral injections of different tracers into the MI whisker region of each hemisphere produced tracer overlap on both sides of the thalamus. Furthermore, MI whisker projections to the contralateral thalamus terminate in close proximity to the thalamocortical neurons that project to the MI whisker region of that contralateral hemisphere. The terminal endings of the contralateral corticothalamic projections contain small synaptic varicosities and other features that resemble the modulator pathways described for other corticothalamic projection systems. In addition, tracer injections into AM, VM, and VL revealed dense clusters of labeled neurons in layer VI of the medial agranular (Agm) zone, which corresponds to the MI whisker region. These results suggest that projections from the MI whisker region to the contralateral thalamus may modulate the callosal interactions that are presumed to play a role in coordinating bilateral whisking behavior.

MI neuronal responses to peripheral whisker stimulation: relationship to neuronal activity in si barrels and septa

The whisker region in the rodent primary motor (MI) cortex receives dense projections from neurons aligned with the layer IV septa in the whisker region of the primary somatosensory (SI) cortex. To compare whisker-induced responses in MI with respect to the SI responses in the septa and adjoining barrel regions, we used several experimental approaches in anesthetized rats. Reversible inactivation of SI and the surrounding cortex suppressed the magnitude of whisker-induced responses in the MI whisker region by 80%. Subsequent laminar analysis of MI responses to electrical or mechanical stimulation of the whisker pad revealed that the most responsive MI neurons were located >or=1.0 mm below the pia. When layer IV neurons in SI were recorded simultaneously with deep MI neurons during low-frequency (2-Hz) deflections of the whiskers, the neurons in the SI barrels responded 2-6 ms earlier than those in MI. Barrel neurons displayed similar response latencies at all stimulus frequencies, but the response latencies in MI and the SI septa increased significantly when the whiskers were deflected at frequencies of 8 Hz. Finally, cross-correlation analysis of neuronal activity in SI and MI revealed greater amounts of time-locked coordination among septa-MI neuron pairs than among barrel-MI neuron pairs. These results suggest that the somatosensory corticocortical inputs to MI cortex convey information processed by the SI septal circuits.

Spatiotemporal expression pattern of non-clustered protocadherin family members in the developing rat brain

Protocadherins (PCDHs) consist of the largest subgroup of the cadherin superfamily, and most PCDHs are expressed dominantly in the CNS. Because PCDHs are involved in the homophilic cell-cell adhesion, PCDHs in the nervous system have been suggested to play roles in the formation and maintenance of the synaptic connections. Although many PCDHs (>50) are in tandem arranged as a cluster in a specific chromosome locus, there are also considerable numbers of non-clustered PCDH members (approximately 20). In this study, we examined the spatiotemporal distribution of mRNAs for 12 non-clustered PCDHs in rat brain using in situ hybridization. Some of them (PCDH1, PCDH7, PCDH9, PCDH10, PCDH11, PCDH17, and PCDH20) exhibited region-dependent expression pattern in the cerebral cortex during the early postnatal stage (P3), which is a critical period for the establishment of specific synaptic connections: PCDH7 and PCDH20 mRNAs were predominantly expressed in the somatosensory (parietal) and visual (occipital) cortices, whereas PCDH11 and PCDH17 mRNAs were preferentially expressed in the motor (forelimb and hindlimb areas) and auditory (temporal) cortices, and PCDH9 mRNA was highly expressed in the motor and main somatosensory cortices. These PCDHs were also expressed in the specific regions of the connecting thalamic nuclei. These cortical regionalization and thalamic nuclei-specificity appeared to be most distinct in P3 compared with those of embryonic and adult stages. Taken together, these results suggest that PCDHs may play specific roles in the establishment of selective synaptic connections of specific modality of cerebral cortex with other communicating brain regions such as the thalamus.

Topography of cortical projections to the dorsolateral neostriatum in rats: multiple overlapping sensorimotor pathways

In rodents, the whisker representation in primary somatosensory (SI) cortex projects to the dorsolateral neostriatum, but the location of these projections has never been characterized with respect to layer IV barrels and their intervening septa. To address this issue, we injected a retrograde tracer into the dorsolateral neostriatum and then reconstructed the location of the labeled corticostriatal neurons with respect to the cytochrome oxidase (CO)-labeled barrels in SI. When the tracer was restricted to a small focal site in the neostriatum, the retrogradely labeled neurons formed elongated strips that were parallel to the curvilinear orientation of layer IV barrel rows. After larger tracer injections, labeled neurons were distributed uniformly across layer V and were aligned with both the barrel and septal compartments. Labeled projections from the contralateral SI barrel cortex, however, were much fewer in number and were disproportionately associated with the septal compartments. A comparison of the labeling patterns in the ipsilateral and contralateral hemispheres revealed symmetric, mirror-image distributions that extended across primary motor cortex (MI) and multiple somatosensory cortical regions, including the secondary somatosensory (SII) cortex, the parietal ventral (PV) and parietal rhinal (PR) areas, and the posteromedial (PM) region. Examination of the thalamus revealed labeled neurons in the intralaminar nuclei, in the medial part of the posterior nucleus (POm), and in the ventrobasal complex. These results indicate that the dorsolateral neostriatum integrates sensorimotor information from multiple sensorimotor representations in the thalamus and cortex.

Differential origin of projections from SI barrel cortex to the whisker representations in SII and MI

We have previously shown that projections from SI barrel cortex to the MI whisker representation originate primarily from columns of neurons that are aligned with the layer IV septa. SI barrel cortex also projects to SII cortex, but the origin of these projections has not been characterized with respect to the barrel and septal compartments. To address this issue, we injected retrograde tracers into the SII whisker representation and then reconstructed the location of the labeled neurons in SI with respect to the layer IV barrels. In some animals, two different tracers were injected into the whisker representations of SII and MI to detect double-labeled neurons that would indicate that some SI neurons project to both of these cortical areas. We found that the projections to SII cortex originate from sites that are uniformly distributed throughout the extragranular layers of barrel cortex. In cases in which different tracers were injected in SII and MI, double-labeled neurons appeared above and below the layer IV septal compartment and at sites aligned with the boundaries of the layer IV barrels. To the extent that the columns of neurons aligned with the barrel and septal compartments represent functionally distinct circuits, these results indicate that SII receives information from both circuits, whereas MI receives inputs primarily from the septal circuits.

Cortical and thalamic coherence during spike–wave seizures in WAG/Rij rats

The study examines cortico-cortical and cortico-thalamic network synchronization at the onset of spike-wave discharges (SWD) in a genetic model of absence epilepsy, WAG/Rij rats. Coherence was measured between multiple cortical areas (intracortical), reticular and rely thalamic nuclei (intrathalamic) and between the cortex and the thalamus. SWD-related increase of coherence (5-60 Hz) was found in all investigated pairs. The highest increase of coherence was around the mean frequency of SWD (8-11.5 Hz) and in the harmonic band 16-21.5 Hz with two central maxima around 10 and 20 Hz. The frequency profile of coherence was different in different intracortical networks, therefore latter were divided into local, global and transhemispheric networks. The presumable source of SWD in the somatosensory cortex and its closest surroundings formed a minimal (local) circuit, in which occurrence of SWD was facilitated by a consistent shift of network synchrony from delta to alpha/beta frequencies. Transhemispheric coherence revealed the largest increase with an additional 16 Hz peak, suggesting a crucial involvement of the corpus callosum in the pathophysiology of absence seizures. The increase in interhemispheric coherence was largest between relatively remote somatosensory or frontal areas, supporting the assumption that SWD originate from the lateral fronto-parietal cortical area.

Cortical, callosal, and thalamic connections from primary somatosensory cortex in the naked mole-rat (Heterocephalus glaber), with special emphasis on the connectivity of the incisor representation

We investigated the distribution of cortical, callosal, and thalamic connections from the primary somatosensory area (S1) in naked mole-rats, concentrating on lower incisor and forelimb representations. A neuronal tracer (WGA-HRP) was injected into the center of each respective representation under guidance from microelectrode recordings of neuronal activity. The locations of cells and terminals were determined by aligning plots of labeled cells with flattened cortical sections reacted for cytochrome oxidase. The S1 lower incisor area was found to have locally confined intrahemispheric connections and longer connections to a small cluster of cells in the presumptive secondary somatosensory (S2) and parietal ventral (PV) incisor fields. The S1 incisor area also had sparse connections with anterior cortex, in presumptive primary motor cortex. Homotopic callosal projections were identified between the S1 lower incisor areas in each hemisphere. Thalamocortical connections related to the incisor were confined to ventromedial portions of the ventral posterior medial subnucleus (VPM) and posterior medial nucleus (Po). Injections into the S1 forelimb area revealed reciprocal intrahemispheric connections to S2 and PV, to two areas in frontal cortex, and to two areas posterior to S1 that appear homologous to posterior lateral area and posterior medial area in rats. The S1 forelimb representation also had callosal projections to the contralateral S1 limb area and to contralateral S2 and PV. Thalamic distribution of label from forelimb injections included ventral portions of the ventral posterior lateral subnucleus (VPL), dorsolateral Po, the ventral lateral nucleus, and the ventral medial nucleus and neighboring intralaminar nuclei.

Intracellularly labeled pyramidal neurons in the cortical areas projecting to the spinal cord. I. Electrophysiological properties of pyramidal neurons

To study cortical motor control, we examined electrical characteristics of pyramidal neurons in the present report, and intra- or juxta-columnar connections of the pyramidal neurons to corticospinal neurons in the accompanying report. Pyramidal neurons were intracellularly recorded and stained in slices of rat motorsensory cortices (areas FL, HL and M1) where many corticospinal neurons were labeled retrogradely. They were morphologically classified into classical, star and other modified pyramidal neurons, and electrophysiologically into regular-spiking (RS), intrinsic bursting (IB) and irregular-spiking (IS) neurons on the basis of spiking pattern in response to 500 ms depolarizing current pulses. RS responses were further divided into RS with slow adaptation (RS-SA) and RS with fast adaptation (RS-FA). The electrical properties were associated with the laminar location of the neurons; RS-SA responses were observed frequently in layer II/III and less frequently in layers IV-VI, and IB and IS responses were exclusively found in layers V and VI, respectively. Interestingly, all layer IV neurons in area FL/HL were RS-FA star-pyramidal neurons, whereas layer IV neurons in area M1 were RS-SA classical pyramidal neurons. Although weak stimulation of areas FL/HL and M1 is known to elicit movement, these results suggest different information processings between the two areas.

Area specificity and topography of thalamocortical projections are controlled by ephrin/Eph genes

The mechanisms generating precise connections between specific thalamic nuclei and cortical areas remain poorly understood. Using axon tracing analysis of ephrin/Eph mutant mice, we provide in vivo evidence that Eph receptors in the thalamus and ephrins in the cortex control intra-areal topographic mapping of thalamocortical (TC) axons. In addition, we show that the same ephrin/Eph genes unexpectedly control the inter-areal specificity of TC projections through the early topographic sorting of TC axons in an intermediate target, the ventral telencephalon. Our results constitute the first identification of guidance cues involved in inter-areal specificity of TC projections and demonstrate that the same set of mapping labels is used differentially for the generation of topographic specificity of TC projections between and within individual cortical areas.

Nigrothalamostriatal and nigrothalamocortical pathways via the ventrolateral parafascicular nucleus

The present tract-tracing study in the rat indicated that neurons in the ventrolateral part of the parafascicular thalamic nucleus (PF), where nigral fibers from the dorsolateral part of the substantia nigra pars reticulata (SNr) terminated, sent their axons to the ventrolateral part of the striatum as well as to the rostrolateral part of the lateral agranular cortex (AGl). We further demonstrated that symmetrical synaptic contacts were made between these nigral axons and striatum- or AGl-projecting PF neurons. Since the dorsolateral part of the SNr, ventrolateral part of the striatum and rostrolateral part of the AGl are responsible regions for orofacial behaviors, the nigrothalamostriatal and nigrothalamo-cortical pathways via the ventrolateral part of the PF may play a role in the control of orofacial motor function.

Synaptic organization of GABAergic projections from the substantia nigra pars reticulata and the reticular thalamic nucleus to the parafascicular thalamic nucleus in the rat

The ventrolateral part of the parafascicular thalamic nucleus (PF), which is considered to take part in the control mechanism of orofacial motor functions, receives projection fibers not only from the dorsolateral part of the substantia nigra pars reticulata (SNr) but also from the ventral part of the reticular thalamic nucleus (RT) [Tsumori et al., Brain Res. 858 (2000) 429]. In order to better understand the influence of these fibers upon the PF projection neurons, the morphology, synaptology and chemical nature of them were examined in the present study. After ipsilateral injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into the dorsolateral part of the SNr and biotinylated dextran amine (BDA) into the ventral part of the RT, overlapping distributions of PHA-L-labeled SNr fibers and BDA-labeled RT fibers were seen in the ventrolateral part of the PF. At the electron microscopic level, the SNr terminals made synapses predominantly with the medium to small dendrites and far less frequently with the somata and large dendrites, whereas approximately half of the RT terminals made synapses with the somata and large dendrites and the rest did with the medium to small dendrites of PF neurons. Some of single dendritic as well as single somatic profiles received convergent synaptic inputs from both sets of terminals. These terminals were packed with pleomorphic synaptic vesicles and formed symmetrical synapses. After combined injections of PHA-L into the dorsolateral part of the SNr, BDA into the ventral part of the RT and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the ventrolateral part of the striatum or into the rostroventral part of the lateral agranular cortex, WGA-HRP-labeled neurons were embedded in the plexus of PHA-L- and BDA-labeled axon terminals within the ventrolateral part of the PF, where the PHA-L- and/or BDA-labeled terminals were in synaptic contact with single somatic and dendritic profiles of the WGA-HRP-labeled neurons. Furthermore, the SNr and RT axon terminals were revealed to be immunoreactive for gamma-aminobutyric acid (GABA), by using the anterograde BDA tracing technique combined with immunohistochemistry for GABA. The present data suggest that GABAergic SNr and RT fibers may exert different inhibitory influences on the PF neurons for regulating the thalamic outflow from the PF to the cerebral cortex and/or striatum in the control of orofacial movements.

Visceral region in the rat primary somatosensory cortex identified by vagal evoked potential

Recent noninvasive human studies have reconfirmed the presence of a viscerally responsive region in the most lateral part of the primary somatosensory cortex (S1). The present electrophysiological study identified the corresponding area in rats as a vagal afferent projection region and examined the cytoarchitecture. Electrical stimulation of the cervical vagus nerve elicited a field potential comparable in waveform, latency, and amplitude to the simultaneously evoked potential in the insular visceral sensory cortex. The potential field adjoined the S1 trigeminal region without overlap, and was rostroventral to the lip representation barrel field, which was identified histochemically, and rostrodorsal to the tongue representation region, which was identified electrophysiologically. The vagal potential underwent a phase reversal in the middle layers; thus, the current sink site was cytoarchitectonically identified as the most rostral part of the parietal granular cortex or the S1, where no somatosensory input has previously been demonstrated. The rat S1 contains a region representing general visceral information, topographically located as if the visceral organs protruded from the mouth.

Organization of corticostriatal projections from the vibrissal representations in the primary motor and somatosensory cortical areas of rodents

To characterize corticostriatal projections from rodent sensorimotor cortex, the anterograde tracers biotinylated dextran amine (BDA) and fluororuby (FR) were injected into the whisker representations of the primary motor (MI) and somatosensory (SI) cortices. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to determine the degree of labeled overlap in both of these subcortical structures. Corticostriatal projections from the vibrissal representation in MI were more extensive than corresponding projections from SI. Both cortical areas sent dense projections to the dorsolateral neostriatum, but the MI vibrissal representation also projected to regions located more rostrally and medially. Despite these differences, both MI and SI projected to overlapping parts of the dorsolateral neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the medial sector of the posterior complex of the thalamus (POm). Because POm is somatotopically organized and has reciprocal connections with both SI and MI cortices, the amount of labeled overlap in POm was used to indicate whether the tracers were injected into corresponding whisker representations of MI and SI. We found that the proportion of labeled overlap in the neostriatum was highly correlated with the amount of labeled overlap in POm. These results indicate that the rodent neostriatum receives convergent projections from corresponding regions in MI and SI cortex. Furthermore, the thalamocortical projections of the POm indicate that it may modulate corticostriatal outputs from corresponding representations in MI and SI.

Organization of projections from the medial agranular cortex to the superior colliculus in the rat: a study using anterograde and retrograde tracing methods

The organization of corticotectal projections from the medial agranular cortex (AGm), which has been considered to contain rat's frontal eye field, was examined using anterograde and retrograde tracing techniques. When biotinylated dextranamine (BDA) injections were made into the rostral part of the AGm, small numbers of BDA-labeled axons were found in the rostral two-thirds of the superior colliculus (SC) while some labeled axons were seen in the caudal one-third of the SC. These labeled axons were distributed mainly in the lateral part of the stratum griseum intermediale. On the other hand, after BDA injections into the caudal part of the AGm, moderate to dense plexuses of labeled axons were found in the rostral two-thirds of the SC while some labeled axons were seen in the caudal one-third of the SC. These labeled axons were distributed in the ventromedial and dorsolateral marginal zones of the stratum griseum intermediale as well as in the stratum griseum profundum. The corticotectal projections were largely uncrossed. After combined injections of BDA into the caudal part of the AGm on one side and cholera toxin B subunit (CTb) into the paramedian pontine reticular formation on the opposite side or into the interstitial nucleus of Cajal on the same side, the overlapping distributions of BDA-labeled axons and CTb-labeled neurons were found in the ventromedial marginal zone of the stratum griseum intermediale ipsilateral to the site of BDA injection. These results suggest that the caudal part of the AGm plays a more significant role in the oculomotor function than does the rostral part of the AGm.

The organization of lateral ventromedial thalamic connections in the rat: a link for the distribution of nociceptive signals to widespread cortical regions

We have used several anatomical tracing techniques to study the organization of the lateral ventromedial thalamic nucleus in the rat, a region that is selectively activated by cutaneous nociceptive inputs from any part of the body. The lateral ventromedial thalamic projections are organized as a widespread dense band covering mainly layer I of the dorsolateral anterior-most aspect of the cortex. This band diminishes progressively as one moves caudally, disappearing completely at 1mm caudal to bregma level. These widespread projections contrast with the circumscribed projections to the deep layers of the primary somatosensory and insular cortices from the adjacent ventral posteromedial and ventroposterior parvicellular thalamic regions, respectively. Injections into the lateral part of the ventromedial thalamic nucleus of an anterograde/retrograde tracer showed that the cortical layer I areas showing the densest projections from this thalamic region also contain the greatest number of retrogradely labeled cells in cortical layers V and VI. The same injections retrogradely labeled numerous cells which were confined to the dorsal subnucleus reticularis dorsalis in an area that contains a concentration of neurons with widespread nociceptive convergence. Finally, the lateral part of the ventromedial thalamic nucleus was also differentially labeled following a topical application of tetramethylrhodamine-labeled dextran on the dorsolateral anterior cortex. These findings suggest that lateral ventromedial thalamic neurons could be part of a spino-reticulo-thalamo-cortical network that allows signals of pain from any part of the body surface to spread across widespread cortical areas.

Morphological evidence for a vestibulo-thalamo-striatal pathway via the parafascicular nucleus in the rat

We observed by anterograde and retrograde tracing techniques that projection fibers originating from the medial vestibular nucleus (MVe) of the rat terminated in the dorsal two-thirds of the lateral part of the parafascicular thalamic nucleus (PF), where neurons sending their axons to the dorsolateral part of the striatum existed. It was further revealed that the vestibular fibers made asymmetrical synaptic contacts mainly with dendrites and additionally with soma of the striatum-projecting PF neurons. These data suggest that output signals from the MVe may be transmitted disynaptically to the striatal neurons via the PF neurons.

The role of nigral and thalamic output pathways in the expression of oral stereotypies induced by amphetamine injections into the striatum

Microinjections of amphetamine into the ventrolateral striatum (VLS) elicit a striking behavioral syndrome characterized by compulsive oral and forelimb motor stereotypies. The neural pathways that mediate these behavioral responses downstream from the striatum have not yet been identified. In a series of experiments, we investigated the involvement of the substantia nigra pars reticulata (SNr) and the ventromedial nucleus of the thalamus (VMT) in the mediation of this behavioral syndrome. We demonstrated that lidocaine-induced reversible inactivation of the SNr reduced amphetamine-induced stereotyped biting and gnawing behaviors, suggesting that the nigral output pathway plays a significant role in the expression of these behavioral responses. In turn, injections of lidocaine into the VMT only transiently reduced amphetamine-stimulated biting and increased stereotyped gnawing and paw nibbling, suggesting that the expression of oral stereotypies induced by amphetamine injections into the VLS is not dependent on thalamocortical feedback.

Immunohistochemical distribution of the two isoforms of synaphin/complexin involved in neurotransmitter release: localization at the distinct central nervous system regions and synaptic types

The cellular and subcellular localization of the two synaphin isoforms, proteins associated with the docking/fusion complex crucial to neurotransmitter release, was studied in the rat central nervous system by using light microscopic and electron microscopic immunohistochemistry with monoclonal antibodies specific to each isoform. Synaphin 1 (complexin II) was predominantly expressed in neurons of the central nervous system regions such as cerebral cortex (the II, III and VI cortical layers), claustrum, hippocampus, entorhinal cortex, amygdaloid nuclei, substantia nigra pars compacta, superior colliculus, pontine reticulotegmental nucleus and inferior olive, whereas synaphin 2 (complexin I) was in the cerebral cortex (the IV cortical layer), thalamus, locus coeruleus, gigantocellular reticular field, cuneate nucleus and cerebellar basket and stellate cells. In some regions, including the caudate-putamen, globus pallidus, pontine reticular nucleus, cerebellar nuclei and spinal gray matter, synaphin 1 was mainly present in small or medium-sized neurons, while synaphin 2 was in large cells. Medial habenular nucleus and cerebellar granule cells showed both immunoreactivities. In the neuropil of the cerebral cortex and hippocampus, synaphin 1 expression was accentuated in the axon terminals of axospinal and axodendritic synapses, while synaphin 2 was predominant in the axon terminals of axosomatic synapses. In the axon terminals, both immunolabelings were associated with synaptic vesicles and the plasma membrane, being accentuated in the vicinity of synaptic contacts. In the cerebral cortex, both immunoreactivities were also present occasionally in dendrites and dendritic spines, associated with microtubules and the plasma membrane including the postsynaptic densities. These results suggest that the two isoforms of synaphin are involved in synaptic function at the distinct presynaptic regions in the central nervous system, and that some dendrites are another functional site for the proteins.

Quantitative analyses of neurons projecting to primary motor cortex zones controlling limb movements in the rat

The objective was to determine if projections of single neurons to primary motor cortex preferentially terminate in several efferent zones that could form synergies for the execution of limb movements. Intracortical microstimulation was used to identify zones evoking hip flexion (HF), elbow flexion (EF), and both plantarflexion (PF) and dorsiflexion (DF) about the ankle. Histological examination showed that the zones from which some movements were evoked extended beyond the agranular cortex into granular cortex. Fluorogold, Fast blue, and propridium iodide or rhodamine-labeled dextran were injected into three of these four efferent zones in each rat. There was a virtual absence of multiple-labeled cells despite having an intermingling of different-colored cells of which 15% in frontal cortex were less than 1.2 mm away from a neighboring neuron that projected to a different efferent zone. This suggests that single neurons projecting to the motor cortex do not hard-wire specific synergies but rather project to single efferent zones in order to offer the greatest degree of freedom for the generation of movements. The distribution of ventral posterolateral and ventrolateral thalamic nucleus labeling depended on whether the injections were in granular or agranular cortex. Conversely, frontal cortex projections to motor efferent zones were made irrespective of their location in either granular or agranular cortex and thereby supporting their presumed role in the control of movements. Hindlimb motor cortex injections yielded retrograde labeling that extended into the more localised distribution of frontal cortex neurons retrogradely labeled from forelimb injections. This may allow hindlimb movements to be synchronized by forelimb movements during walking on challenging terrain.

Transplants of fetal frontal cortex grafted into the occipital cortex of newborn rats receive a substantial thalamic input from nuclei normally projecting to the frontal cortex

A number of molecular and hodological experiments have provided evidence that there is an early commitment of neocortical neurons to express features unique to a certain cortical area. However, the findings of several transplantation experiments have indicated that late embryonic cortical tissue heterotopically grafted into the neocortex of newborn rats receives a set of thalamic projections appropriate for the host cortical locus within which it develops. To provide further information on the extent to which neocortical neurons are predetermined to develop area-specific systems of connections, in this study we have compared the pattern of thalamic afferents to grafts of embryonic day 16 occipital or frontal neocortex transplanted into the occipital cortex of newborn rats. Two months after grafting, a retrograde neurotracer (cholera toxin, subunit b) was injected into the grafts to precisely assess the number of cells in the visual- and/or motor-related nuclei of the host thalamus projecting to each category of transplants (occipital-to-occipital or frontal-to-occipital). Transplants of embryonic occipital cortex received significant input from several visual-related thalamic nuclei, i.e. the lateral posterior and lateral dorsal nuclei, and no input from motor-related thalamic nuclei. However, only few labeled cells were found in the dorsal lateral geniculate nucleus, which was systematically affected by a severe atrophy, probably in response to the lesion of the occipital cortex performed prior to the transplantation. By comparison, transplants of frontal origin received a substantial input from the ventrolateral and ventromedial thalamic nuclei, which normally project to the frontal cortex, but received a weak input from the lateral posterior and lateral dorsal nuclei. Neocortical neurons grafted heterotopically into the neocortex of newborn hosts are not only able to contact cortical and subcortical targets appropriate for their embryonic site of origin, but are also susceptible to derive thalamic inputs closely related to their embryonic origin.