Intratelencephalic Neurons Research Articles

Synaptic connectivity to L2/3 of primary visual cortex measured by two-photon optogenetic stimulation.

Understanding cortical microcircuits requires thorough measurement of physiological properties of synaptic connections formed within and between diverse subclasses of neurons. Towards this goal, we combined spatially precise optogenetic stimulation with multicellular recording to deeply characterize intralaminar and translaminar monosynaptic connections to supragranular (L2/3) neurons in the mouse visual cortex. The reliability and specificity of multiphoton optogenetic stimulation were measured across multiple Cre lines, and measurements of connectivity were verified by comparison to paired recordings and targeted patching of optically identified presynaptic cells. With a focus on translaminar pathways, excitatory and inhibitory synaptic connections from genetically defined presynaptic populations were characterized by their relative abundance, spatial profiles, strength, and short-term dynamics. Consistent with the canonical cortical microcircuit, layer 4 excitatory neurons and interneurons within L2/3 represented the most common sources of input to L2/3 pyramidal cells. More surprisingly, we also observed strong excitatory connections from layer 5 intratelencephalic neurons and potent translaminar inhibition from multiple interneuron subclasses. The hybrid approach revealed convergence to and divergence from excitatory and inhibitory neurons within and across cortical layers. Divergent excitatory connections often spanned hundreds of microns of horizontal space. In contrast, divergent inhibitory connections were more frequently measured from postsynaptic targets near each other.

How does TMS “engage” the human cortex? Insights from structural and functional mapping of the precentral motor hand knob

Abstract Background: TMS of the primary motor hand area (M1-HAND) can probe the function of fast corticomotor connections to the contralateral hand. TMS gives rise to indirect (transsynaptic) excitation of large pyramidal tract (PT) neurons in M1-HAND that make monosynaptic connections with spinal motoneurons in the cervical cord. Other cell types are effectively excited by TMS as well. Yet, it is still unclear which neuronal elements (e.g. axon terminals or axon hillock), which cell types (e.g. PT neurons, intra-telencephalic (IT) neurons, or inhibitory interneurons), and which sites in the precentral hand knob (e.g. crown, lip, or sulcus region) are preferentially depolarized by the TMS-induced electrical field in the precentral gyrus. Objective: To address whether non-invasive neuroimaging in human volunteers can help to pinpoint the primary targets for TMS of M1-HAND. Methods: Structural and functional neuroimaging (MRI and PET) were employed in a series of experiments to target the M1-HAND. Structural MRI was used for shaped-based TMS mapping, electrical field modelling, and mapping of cortical myelin content. Functional MRI and PET were used to localize clusters in the precentral hand knob that are functionally activated by focal TMS of M1-HAND. Results: Inter-individual differences in precentral cortical myelin content account for interindividual differences in rostro-caudal “motor hotspot” location, linking cortical myelination to TMS target engagement. Functional brain mapping reveals different peak target sites in the precentral hand knob and indicates that PT and IT neurons as well as inhibitory interneurons are effectively excited by TMS. Discussion: Structural and functional neuroimaging can help to elucidate how TMS engages its local cortical targets in the precentral gyrus. Causal interpretation of the results needs to consider the potential and pitfalls inherent to the neuroimaging method and the physiological implications of the TMS protocol (e.g. intensity, number of stimuli, online-offline TMS). Keywords: Transcranial magnetic stimulation, Motor cortex, Magnetic resonance imaging, Positron emission tomography

Epigenomic diversity of cortical projection neurons in the mouse brain

Neuronal cell types are classically defined by their molecular properties, anatomy and functions. Although recent advances in single-cell genomics have led to high-resolution molecular characterization of cell type diversity in the brain1, neuronal cell types are often studied out of the context of their anatomical properties. To improve our understanding of the relationship between molecular and anatomical features that define cortical neurons, here we combined retrograde labelling with single-nucleus DNA methylation sequencing to link neural epigenomic properties to projections. We examined 11,827 single neocortical neurons from 63 cortico-cortical and cortico-subcortical long-distance projections. Our results showed unique epigenetic signatures of projection neurons that correspond to their laminar and regional location and projection patterns. On the basis of their epigenomes, intra-telencephalic cells that project to different cortical targets could be further distinguished, and some layer 5 neurons that project to extra-telencephalic targets (L5 ET) formed separate clusters that aligned with their axonal projections. Such separation varied between cortical areas, which suggests that there are area-specific differences in L5 ET subtypes, which were further validated by anatomical studies. Notably, a population of cortico-cortical projection neurons clustered with L5 ET rather than intra-telencephalic neurons, which suggests that a population of L5 ET cortical neurons projects to both targets. We verified the existence of these neurons by dual retrograde labelling and anterograde tracing of cortico-cortical projection neurons, which revealed axon terminals in extra-telencephalic targets including the thalamus, superior colliculus and pons. These findings highlight the power of single-cell epigenomic approaches to connect the molecular properties of neurons with their anatomical and projection properties.

Spatially resolved cell atlas of the mouse primary motor cortex by MERFISH

A mammalian brain is composed of numerous cell types organized in an intricate manner to form functional neural circuits. Single-cell RNA sequencing allows systematic identification of cell types based on their gene expression profiles and has revealed many distinct cell populations in the brain1,2. Single-cell epigenomic profiling3,4 further provides information on gene-regulatory signatures of different cell types. Understanding how different cell types contribute to brain function, however, requires knowledge of their spatial organization and connectivity, which is not preserved in sequencing-based methods that involve cell dissociation. Here we used a single-cell transcriptome-imaging method, multiplexed error-robust fluorescence in situ hybridization (MERFISH)5, to generate a molecularly defined and spatially resolved cell atlas of the mouse primary motor cortex. We profiled approximately 300,000 cells in the mouse primary motor cortex and its adjacent areas, identified 95 neuronal and non-neuronal cell clusters, and revealed a complex spatial map in which not only excitatory but also most inhibitory neuronal clusters adopted laminar organizations. Intratelencephalic neurons formed a largely continuous gradient along the cortical depth axis, in which the gene expression of individual cells correlated with their cortical depths. Furthermore, we integrated MERFISH with retrograde labelling to probe projection targets of neurons of the mouse primary motor cortex and found that their cortical projections formed a complex network in which individual neuronal clusters project to multiple target regions and individual target regions receive inputs from multiple neuronal clusters.

Variations in Commissural Input Processing Across Different Types of Cortical Projection Neurons.

To understand how incoming cortical inputs are processed by different types of cortical projection neurons in the medial prefrontal cortex, we compared intrinsic physiological properties of and commissural excitatory/inhibitory influences on layer 5 intratelencephalic (IT), layer 5 pyramidal tract (PT), and layers 2/3 IT projection neurons. We found that intrinsic physiological properties and commissural synaptic transmission varied across the three types of projection neurons. The rank order of intrinsic excitability was layer 5 PT > layer 5 IT > layers 2/3 IT neurons. Commissural connectivity was higher in layers 2/3 than layer 5 projection neurons, but commissural excitatory influence was stronger on layer 5 than layers 2/3 pyramidal neurons. Paired-pulse ratio was also greater in PT than IT neurons. These results indicate that commissural inputs activate deep layer PT neurons most preferentially and superficial layer IT neurons least preferentially. Deep layer PT neurons might faithfully transmit cortical input signals to downstream subcortical structures for reliable control of behavior, whereas superficial layer IT neurons might integrate cortical input signals from diverse sources in support of higher-order cognitive functions.

Parallel processing of working memory and temporal information by distinct types of cortical projection neurons

It is unclear how different types of cortical projection neurons work together to support diverse cortical functions. We examined the discharge characteristics and inactivation effects of intratelencephalic (IT) and pyramidal tract (PT) neurons—two major types of cortical excitatory neurons that project to cortical and subcortical structures, respectively—in the deep layer of the medial prefrontal cortex in mice performing a delayed response task. We found stronger target-dependent firing of IT than PT neurons during the delay period. We also found the inactivation of IT neurons, but not PT neurons, impairs behavioral performance. In contrast, PT neurons carry more temporal information than IT neurons during the delay period. Our results indicate a division of labor between IT and PT projection neurons in the prefrontal cortex for the maintenance of working memory and for tracking the passage of time, respectively.

Cell Type-Specific Decrease of the Intrinsic Excitability of Motor Cortical Pyramidal Neurons in Parkinsonism.

The hypokinetic motor symptoms of Parkinson's disease (PD) are closely linked with a decreased motor cortical output as a consequence of elevated basal ganglia inhibition. However, whether and how the loss of dopamine (DA) alters the cellular properties of motor cortical neurons in PD remains undefined. We induced parkinsonism in adult C57BL/6 mice of both sexes by injecting neurotoxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle. By using ex vivo patch-clamp recording and retrograde tracing approach, we found that the intrinsic excitability of pyramidal tract neurons (PTNs) in the primary motor cortical (M1) layer (L)5b was greatly decreased in parkinsonism; but the intratelencephalic neurons (ITNs) were not affected. The cell type-specific intrinsic adaptations were associated with a depolarized threshold and broadened width of action potentials (APs) in PTNs. Moreover, the loss of midbrain dopaminergic neurons impaired the capability of M1 PTNs to sustain high-frequency firing, which could underlie their abnormal pattern of activity in the parkinsonian state. We also showed that the decreased excitability in parkinsonism was caused by an impaired function of both persistent sodium channels and the large conductance, Ca2+-activated K+ channels. Acute activation of dopaminergic receptors failed to rescue the impaired intrinsic excitability of M1 PTNs in parkinsonian mice. Altogether, our data demonstrated a cell type-specific decrease of the excitability of M1 pyramidal neurons in parkinsonism. Thus, intrinsic adaptations in the motor cortex provide novel insight in our understanding of the pathophysiology of motor deficits in PD.SIGNIFICANCE STATEMENT The degeneration of midbrain dopaminergic neurons in Parkinson's disease (PD) remodels the connectivity and function of cortico-basal ganglia-thalamocortical network. However, whether and how dopaminergic degeneration and the associated basal ganglia dysfunction alter motor cortical circuitry remain undefined. We found that pyramidal neurons in the layer (L)5b of the primary motor cortex (M1) exhibit distinct adaptations in response to the loss of midbrain dopaminergic neurons, depending on their long-range projections. Besides the decreased thalamocortical synaptic excitation as proposed by the classical model of Parkinson's pathophysiology, these results, for the first time, show novel cellular and molecular mechanisms underlying the abnormal motor cortical output in parkinsonism.

Distinct populations of cortical pyramidal neurons mediate drug reward and aversion

Processing within the anterior cingulate cortex (ACC) is crucial for the patterning of appropriate behavior, and ACC dysfunction following chronic drug use is thought to play a major role in drug addiction. However, cortical pyramidal projection neurons can be subdivided into two major types (intratelencephalic (IT) and pyramidal tract (PT)), with distinct inputs and projection targets, molecular and receptor profiles, morphologies and electrophysiological properties. Yet, how each of these cell populations modulate behavior related to addiction is unknown. We demonstrate that PT neurons regulate the positive features of a drug experience whereas IT neurons regulate the negative features. These findings support a revised theory of cortical function in addiction, with distinct cells and circuits mediating reward and aversion.

Anatomically revealed morphological patterns of pyramidal neurons in layer 5 of the motor cortex

Neuronal cell types are essential to the comprehensive understanding of the neuronal function and neuron can be categorized by their anatomical property. However, complete morphology data for neurons with a whole brain projection, for example the pyramidal neurons in the cortex, are sparse because it is difficult to trace the neuronal fibers across the whole brain and acquire the neuron morphology at the single axon resolution. Thus the cell types of pyramidal neurons have yet to be studied at the single axon resolution thoroughly. In this work, we acquire images for a Thy1 H-line mouse brain using a fluorescence micro-optical sectioning tomography system. Then we sample 42 pyramidal neurons whose somata are in the layer 5 of the motor cortex and reconstruct their morphology across the whole brain. Based on the reconstructed neuronal anatomy, we analyze the axonal and dendritic fibers of the neurons in addition to the soma spatial distributions, and identify two axonal projection pattern of pyramidal tract neurons and two dendritic spreading patterns of intratelencephalic neurons. The raw image data are available upon request as an additional asset to the community. The morphological patterns identified in this work can be a typical representation of neuron subtypes and reveal the possible input-output function of a single pyramidal neuron.

Differential Striatal Axonal Arborizations of the Intratelencephalic and Pyramidal-Tract Neurons: Analysis of the Data in the MouseLight Database.

There exist two major types of striatum-targeting neocortical neurons, specifically, intratelencephalic (IT) neurons and pyramidal-tract (PT) neurons. Regarding their striatal projections, it was once suggested that IT axons are extended whereas PT axons are primarily focal. However, subsequent study with an increased number of well-stained extended axons concluded that such an apparent distinction was spurious due to limited sample size. Recent work using genetically labeled neurons reintroduced the differential spatial extent of the striatal projections of IT and PT neurons through population-level analyses, complemented by observations of single axons. However, quantitative IT vs. PT comparison of a large number of axons remained to be conducted. We analyzed the data of axonal end-points of 161 IT neurons and 33 PT neurons in the MouseLight database (http://ml-neuronbrowser.janelia.org/). The number of axonal end-points in the ipsilateral striatum exhibits roughly monotonically decreasing distributions in both neuron types. Excluding neurons with no ipsilateral end-point, the distributions of the logarithm of the number of ipsilateral end-points are considerably overlapped between IT and PT neurons, although the proportion of neurons having more than 50 ipsilateral end-points is somewhat larger in IT neurons than in PT neurons. Looking at more details, among IT subpopulations in the secondary motor area (MOs), layer 5 neurons and bilateral striatum-targeting layer 2/3 neurons, but not contralateral striatum-non-targeting layer 2/3 neurons, have a larger number of ipsilateral end-points than MOs PT neurons. We also found that IT ipsilateral striatal axonal end-points are on average more widely distributed than PT end-points, especially in the medial-lateral direction. These results indicate that IT and PT striatal axons differ in the frequencies and spatial extent of end-points while there are wide varieties within each neuron type.

Reconstruction of Intratelencephalic Neurons in the Mouse Secondary Motor Cortex Reveals the Diverse Projection Patterns of Single Neurons.

The secondary motor cortex (MOs) plays crucial roles in cognitive and executive processes and has reciprocal connections with numerous cortices in rodents. However, descriptions of the neuronal morphologies and projection patterns of the MOs at the level of a single neuron are lacking, severely hindering the comprehensive understanding of the wiring diagram of the MOs. Herein, we used a Cre-dependent adeno-associated virus (AAV) to fluorescently label ~80 pyramidal neurons nearby or in the MOs and acquired an uninterrupted whole-brain 3D dataset at a voxel resolution of 0.2 × 0.2 × 1 μm with a whole-brain fluorescence imaging system (fMOST). Based on our 3D dataset, we reconstructed the complete morphologies of 36 individual intratelencephalic (IT) neurons nearby or in the MOs and analyzed the projection patterns and projection strengths of these neurons at a single-neuron level based on several parameters, including the projection areas, the total number of branches, the fiber length, and the total number of terminal tips. We obtained a neuron with an axonal length of 318.43 mm, which is by far the longest reported axonal length. Our results show that all individual neurons in the MOs, regardless of whether they are located in layer 2/3 or layer 5, display diverse projection patterns and projection strengths, implying that these neurons might be involved in different brain circuits at different intensities. The results lay a solid foundation for exploring the relationship between neuronal morphologies and behavioral functions of the MOs at the level of a single neuron.

Serotonergic Regulation of Corticoamygdalar Neurons in the Mouse Prelimbic Cortex.

Neuromodulatory transmitters, such as serotonin (5-HT), selectively regulate the excitability of subpopulations of cortical projection neurons to gate cortical output to specific target regions. For instance, in the mouse prelimbic cortex, 5-HT selectively excites commissurally projecting (COM) intratelencephalic neurons via activation of 5-HT2A (2A) receptors, while simultaneously inhibiting, via 5-HT1A (1A) receptors, corticofugally projecting pyramidal neurons targeting the pons. Here we characterize the physiology, morphology, and serotonergic regulation of corticoamygdalar (CAm) projection neurons in the mouse prelimbic cortex. Layer 5 CAm neurons shared a number of physiological and morphological characteristics with COM neurons, including higher input resistances, smaller HCN-channel mediated responses, and sparser dendritic arbors than corticopontine neurons. Across cortical lamina, CAm neurons also resembled COM neurons in their serotonergic modulation; focally applied 5-HT (100 μM; 1 s) generated 2A-receptor-mediated excitation, or 1A- and 2A-dependent biphasic responses, in ipsilaterally and contralaterally projecting CAm neurons. Serotonergic excitation depended on extrinsic excitatory drive, as 5-HT failed to depolarize CAm neurons from rest, but could enhance the number of action potentials generated by simulated barrages of synaptic input. Finally, using dual tracer injections, we identified double-labeled CAm/COM neurons that displayed primarily excitatory or biphasic responses to 5-HT. Overall, our findings reveal that prelimbic CAm neurons in layer 5 overlap, at least partially, with COM neurons, and that neurons projecting to either, or both targets, exhibit 2A-dependent serotonergic excitation. These results suggest that 5-HT, acting at 2A receptors, may promote cortical output to the amygdala.

Specialized Subpopulations of Deep-Layer Pyramidal Neurons in the Neocortex: Bridging Cellular Properties to Functional Consequences.

Neocortical pyramidal neurons with somata in layers 5 and 6 are among the most visually striking and enigmatic neurons in the brain. These deep-layer pyramidal neurons (DLPNs) integrate a plethora of cortical and extracortical synaptic inputs along their impressive dendritic arbors. The pattern of cortical output to both local and long-distance targets is sculpted by the unique physiological properties of specific DLPN subpopulations. Here we revisit two broad DLPN subpopulations: those that send their axons within the telencephalon (intratelencephalic neurons) and those that project to additional target areas outside the telencephalon (extratelencephalic neurons). While neuroscientists across many subdisciplines have characterized the intrinsic and synaptic physiological properties of DLPN subpopulations, our increasing ability to selectively target and manipulate these output neuron subtypes advances our understanding of their distinct functional contributions. This Viewpoints article summarizes our current knowledge about DLPNs and highlights recent work elucidating the functional differences between DLPN subpopulations.

Preferential cholinergic excitation of corticopontine neurons.

Phasic activation of M1 muscarinic receptors generates transient inhibition followed by longer lasting excitation in neocortical pyramidal neurons. Corticopontine neurons in the mouse prefrontal cortex exhibit weaker cholinergic inhibition, but more robust and longer lasting excitation, than neighbouring callosal projection neurons. Optogenetic release of endogenous ACh in response to single flashes of light (5ms) preferentially enhances the excitability of corticopontine neurons for many tens of seconds. Cholinergic excitation of corticopontine neurons involves at least three ionic mechanisms: suppression of KV 7 currents, activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Preferential cholinergic excitation of prefrontal corticopontine neurons may facilitate top-down attentional processes and behaviours. Pyramidal neurons in layer 5 of the neocortex comprise two broad classes of projection neurons: corticofugal neurons, including corticopontine (CPn) neurons, and intratelencephalic neurons, including commissural/callosal (COM) neurons. These non-overlapping neuron subpopulations represent discrete cortical output channels contributing to perception, decision making and behaviour. CPn and COM neurons have distinct morphological and physiological characteristics, and divergent responses to modulatory transmitters such as serotonin and acetylcholine (ACh). To better understand how ACh regulates cortical output, in slices of mouse prefrontal cortex (PFC) we compared the responsivity of CPn and COM neurons to transient exposure to exogenous or endogenous ACh. In both neuron subtypes, exogenous ACh generated qualitatively similar biphasic responses in which brief hyperpolarization was followed by longer lasting enhancement of excitability. However, cholinergic inhibition was more pronounced in COM neurons, while excitatory responses were larger and longer lasting in CPn neurons. Similarly, optically triggered release of endogenous ACh from cholinergic terminals preferentially and persistently (for ∼40s) enhanced the excitability of CPn neurons, but had little impact on COM neurons. Cholinergic excitation of CPn neurons involved at least three distinct ionic mechanisms: suppression of KV 7 channels (the 'M-current'), activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Our findings demonstrate projection-specific selectivity in cholinergic signalling in the PFC, and suggest that transient release of ACh during behaviour will preferentially promote corticofugal output.

Serotonin Differentially Regulates L5 Pyramidal Cell Classes of the Medial Prefrontal Cortex in Rats and Mice.

The prefrontal cortex receives a dense serotonergic innervation that plays an important role in its regulation. However, how serotonin regulates different pyramidal and interneuron cell classes in this area is incompletely understood. Previous work in rats has shown that serotonin differentially regulates two classes of pyramidal cells in layer 5. It excites one class by activating 5-HT2A receptors, whereas it more subtly modulates the integrative properties of the other by co-activating 5-HT1A and 5-HT2A receptors. Here we have used electrophysiological recordings, combined with retrograde labeling and morphological reconstruction, to show that the first cell class corresponds to long range corticofugal neurons and the second corresponds to intratelencephalic neurons. These results suggest that, in rats, serotonin facilitates subcortical output while more subtly modulating cortico-cortical and cortico-striatal output. Interestingly, these results obtained in rats differ from those previously reported for mouse prefrontal cortex. Therefore we reinvestigated the effects of serotonin in mice and confirmed that serotonin predominantly activates inhibitory 5-HT1A receptors on long-range corticofugal cells. Thus serotonin exerts opposite effects on these cells in rats and mice. Finally, we determined whether cortical serotonin responsiveness in mice is regulated during development. Serotonin elicited predominantly depolarizing inward current responses during the early postnatal period, whereas inhibitory 5-HT1A receptor-mediated responses did not become evident until the end of the second postnatal week. These results reveal commonalities as well as unexpected differences in the serotonergic regulation of long-range corticofugal and intratelencephalic neurons of layer 5 in rat and mouse.

Distinct Laterality in Forelimb-Movement Representations of Rat Primary and Secondary Motor Cortical Neurons with Intratelencephalic and Pyramidal Tract Projections.

Two distinct motor areas, the primary and secondary motor cortices (M1 and M2), play crucial roles in voluntary movement in rodents. The aim of this study was to characterize the laterality in motor cortical representations of right and left forelimb movements. To achieve this goal, we developed a novel behavioral task, the Right-Left Pedal task, in which a head-restrained male rat manipulates a right or left pedal with the corresponding forelimb. This task enabled us to monitor independent movements of both forelimbs with high spatiotemporal resolution. We observed phasic movement-related neuronal activity (Go-type) and tonic hold-related activity (Hold-type) in isolated unilateral movements. In both M1 and M2, Go-type neurons exhibited bias toward contralateral preference, whereas Hold-type neurons exhibited no bias. The contralateral bias was weaker in M2 than M1. Moreover, we differentiated between intratelencephalic (IT) and pyramidal tract (PT) neurons using optogenetically evoked spike collision in rats expressing channelrhodopsin-2. Even in identified PT and IT neurons, Hold-type neurons exhibited no lateral bias. Go-type PT neurons exhibited bias toward contralateral preference, whereas IT neurons exhibited no bias. Our findings suggest a different laterality of movement representations of M1 and M2, in each of which IT neurons are involved in cooperation of bilateral movements, whereas PT neurons control contralateral movements.SIGNIFICANCE STATEMENT In rodents, the primary and secondary motor cortices (M1 and M2) are involved in voluntary movements via distinct projection neurons: intratelencephalic (IT) neurons and pyramidal tract (PT) neurons. However, it remains unclear whether the two motor cortices (M1 vs M2) and the two classes of projection neurons (IT vs PT) have different laterality of movement representations. We optogenetically identified these neurons and analyzed their functional activity using a novel behavioral task to monitor movements of the right and left forelimbs separately. We found that contralateral bias was reduced in M2 relative to M1, and in IT relative to PT neurons. Our findings suggest that the motor information processing that controls forelimb movement is coordinated by a distinct cell population.

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.

Corticostriatal and mesocortical dopamine systems: do species differences matter?

Gordon M. G. Shepherd recently wrote a stimulating Review1 that provides novel perspectives on the potential role of the corticostriatal systems in neuropsychiatric disorders ranging from autism and schizophrenia to Parkinson’s disease. However, we believe that important differences between the rodent and primate brain are not sufficiently discussed in this Review. Dr Shepherd points out that the striatum is innervated by two different populations of cortical neurons — the intratelencephalic (IT) neurons and the pyramidal tract (PT) neurons — and discusses evidence for the hypothesis that disturbances in firing patterns of IT and PT neurons might contribute to human neuropsychiatric disorders. This discussion is based almost exclusively on data from the rodent literature and on the explicit assumption of an “evolutionary conservation of IT and PT organization across species and cortical areas”. However, the literature suggests that there are, in fact, substantial species differences in the organization of the PT system. In rodents, the axons of cortical PT neurons branch heavily to innervate subcortical targets, such as the striatum, thalamus, subthalamic nucleus, ventral midbrain and brain stem2. As shown in FIG. 1a, PT neurons are differently organized in primates: although the axons of a small subset of PT neurons sparsely branch to the striatum and claustrum, these neurons do not innervate other forebrain or midbrain regions3. Furthermore, careful studies in non-human primates have indicated that the electrophysiological and functional properties of cortical PT neurons in the primary motor cortex (M1) differ strikingly from those of corticostriatal neurons4–6. Importantly, recent data have shown that the electrophysiological activity of primate PT neurons is strongly altered in the (resting) parkinsonian state, whereas the electrophysiological activity of corticostriatal neurons is not affected6. Figure 1 Highly simplified diagram of key anatomical differences between rodents and primates with regard to pyramidal tract neurons (a) and the sources of dopamine innervation of prefrontal/cingulate and M1 cortices (b) Species differences should also be considered when discussing dopaminergic projections to the cerebral cortex (FIG. 1b). In rodents, mesocortical dopamine projections arise almost exclusively from the ventral tegmental area (VTA) and terminate mostly in prefrontal regions. The primary motor cortex, where a large population of PT neurons is located, is only sparsely innervated by these fibres. By contrast, primates (including humans) have gained a substantial dopaminergic innervation of M1 and related motor cortices7,8; this innervation has arisen in large part from the substantia nigra pars compacta (SNc) and the retrorubral area (RRA)9,10, thereby suggesting a prominent role of cortical dopamine on PT neurons in the M1 of primates, but not in rodents. These species differences in cortical dopamine projections might be important for our understanding of the pathophysiology of Parkinson’s disease, in which neurons in the SNc and RRA are far more affected by the neurodegenerative process than those in VTA. Thus, it seems that the organization of the corticostriatal system(s) and its modulatory dopaminergic inputs has undergone substantial evolutionary change. We believe that the resulting species differences deserve attention when interpreting data from animal studies because they affect our understanding of the neural substrates of normal behaviour and of the physiological disturbances that underlie neuropsychiatric disorders in humans.

Intratelencephalic corticostriatal neurons equally excite striatonigral and striatopallidal neurons and their discharge activity is selectively reduced in experimental parkinsonism

Striatonigral and striatopallidal neurons form distinct populations of striatal projection neurons. Their discharge activity is imbalanced after dopaminergic degeneration in Parkinson's disease. Striatal projection neurons receive massive cortical excitatory inputs from bilateral intratelencephalic (IT) neurons projecting to both the ipsilateral and contralateral striatum and from collateral axons of ipsilateral neurons that send their main axon through the pyramidal tract (PT). Previous anatomical studies in rats suggested that IT and PT inputs preferentially excite striatonigral and striatopallidal neurons, respectively. Here we used electrophysiological criteria to identify them with antidromic stimulations. We show that the spontaneous discharge activity of IT neurons is depressed, whereas that of PT neurons is not affected in the rat cortex ipsilateral to 6-hydroxydopamine injection. However, our functional experiments do not support the hypothesis of a differential cortical input to striatal pathways. Firstly, although the conduction velocity of PT neurons is 4.6 times faster than that of IT neurons, identified striatopallidal and striatonigral neurons exhibit identical latencies of their spike responses to electrical stimulation of the ipsilateral cortex. Secondly, although PT neurons are ipsilateral, both striatal populations exhibit similar sensitivity to the stimulation of the ipsilateral and contralateral cortex. We suggest that IT neurons provide the main excitatory input to both striatal populations and that the corticostriatal PT input is weaker. Therefore, our functional data do not support our previous hypothesis that the deficit of IT neurons associated with the dopaminergic depletion might contribute to the striatal imbalance. This imbalance might rather result from intrinsic striatal mechanisms.