Glenn D. R. Watson

The role of the medial prefrontal cortex in the acquisition, retention, and reversal of a tactile visuospatial conditional discrimination task.

The medial prefrontal cortex (mPFC) is responsible for executive functions such as abstract rule coding, strategy switching, and behavioral flexibility; however, there is some debate regarding the extent to which mPFC is involved in reversal learning, especially in complex multisensory tasks such as conditional discrimination. Therefore, we investigated the effects of mPFC inactivation on the acquisition, retention, and reversal of a visuospatial conditional discrimination (CD) task. In experiment 1, muscimol was infused through bilateral cannulae on days 1, 2, and 3 to test the effects of mPFC inactivation on task acquisition and days 19, 20, and 21 to test the effects on retention of the task. For experiment 2, rats were trained on the CD task for 21 days with no infusions given, after which the reward contingency was reversed, with infusions given during the first six days of reversal. The results of experiment 1 showed that the muscimol and saline groups did not differ on acquisition or retention. However, experiment 2 showed that the muscimol group displayed significantly more performance errors than the control group during reversal. Compared to the control group, the muscimol group also showed a decreased tendency to use a side-bias strategy during the intermediate stages of reversal. The failure of the muscimol group to exhibit a side bias suggests that the mPFC is necessary for sampling strategies necessary for the reversal of a visuospatial CD task.

Thalamostriatal projections from the medial posterior and parafascicular nuclei have distinct topographic and physiologic properties

The dorsolateral striatum (DLS) is critical for executing sensorimotor behaviors that depend on stimulus-response (S-R) associations. In rats, the DLS receives it densest inputs from primary somatosensory (SI) cortex, but it also receives substantial input from the thalamus. Much of rat DLS is devoted to processing whisker-related information, and thalamic projections to these whisker-responsive DLS regions originate from the parafascicular (Pf) and medial posterior (POm) nuclei. To determine which thalamic nucleus is better suited for mediating S-R associations in the DLS, we compared their input-output connections and neuronal responses to repetitive whisker stimulation. Tracing experiments demonstrate that POm projects specifically to the DLS, but the Pf innervates both dorsolateral and dorsomedial parts of the striatum. The Pf nucleus is innervated by whisker-sensitive sites in the superior colliculus, and these sites also send dense projections to the zona incerta, a thalamic region that sends inhibitory projections to the POm. These data suggest that projections from POm to the DLS are suppressed by incertal inputs when the superior colliculus is activated by unexpected sensory stimuli. Simultaneous recordings with two electrodes indicate that POm neurons are more responsive and habituate significantly less than Pf neurons during repetitive whisker stimulation. Response latencies are also shorter in POm than in Pf, which is consistent with the fact that Pf receives its whisker information via synaptic relays in the superior colliculus. These findings indicate that, compared with the Pf nucleus, POm transmits somatosensory information to the DLS with a higher degree of sensory fidelity.

The Zona Incerta Regulates Communication between the Superior Colliculus and the Posteromedial Thalamus: Implications for Thalamic Interactions with the Dorsolateral Striatum

There is uncertainty concerning the circuit connections by which the superior colliculus interacts with the basal ganglia. To address this issue, anterograde and retrograde tracers were placed, respectively, into the superior colliculus and globus pallidus of Sprague-Dawley rats. In this two-tracer experiment, the projections from the superior colliculus terminated densely in the ventral zona incerta (ZIv), but did not overlap the labeled neurons observed in the subthalamic nucleus. In cases in which anterograde and retrograde tracers were placed, respectively, in sensory-responsive sites in the superior colliculus and posteromedial (POm) thalamus, the labeled projections from superior colliculus innervated the ZIv regions that contained the labeled neurons that project to POm. We also confirmed this colliculo–incertal–POm pathway by depositing a mixture of retrograde and anterograde tracers at focal sites in ZIv to reveal retrogradely labeled neurons in superior colliculus and anterogradely labeled terminals in POm. When combined with retrograde tracer injections in POm, immunohistochemical processing proved that most ZIv projections to POm are GABAergic. Consistent with these findings, direct stimulation of superior colliculus evoked neuronal excitation in ZIv and caused inhibition of spontaneous activity in POm. Collectively, these results indicate that superior colliculus can activate the inhibitory projections from ZIv to the POm. This is significant because it suggests that the superior colliculus could suppress the interactions between POm and the dorsolateral striatum, presumably to halt ongoing behaviors so that more adaptive motor actions are selected in response to unexpected sensory events. SIGNIFICANCE STATEMENT By demonstrating that the zona incerta regulates communication between the superior colliculus and the posteromedial thalamus, we have uncovered a circuit that partly explains the behavioral changes that occur in response to unexpected sensory stimuli. Furthermore, this circuit could explain why deep brain stimulation of the zona incerta is beneficial to patients who suffer from Parkinsons disease.

Corticofugal projection patterns of whisker sensorimotor cortex to the sensory trigeminal nuclei

The primary (S1) and secondary (S2) somatosensory cortices project to several trigeminal sensory nuclei. One putative function of these corticofugal projections is the gating of sensory transmission through the trigeminal principal nucleus (Pr5), and some have proposed that S1 and S2 project differentially to the spinal trigeminal subnuclei, which have inhibitory circuits that could inhibit or disinhibit the output projections of Pr5. Very little, however, is known about the origin of sensorimotor corticofugal projections and their patterns of termination in the various trigeminal nuclei. We addressed this issue by injecting anterograde tracers in S1, S2 and primary motor (M1) cortices, and quantitatively characterizing the distribution of labeled terminals within the entire rostro-caudal chain of trigeminal sub-nuclei. We confirmed our anterograde tracing results by injecting retrograde tracers at various rostro-caudal levels within the trigeminal sensory nuclei to determine the position of retrogradely labeled cortical cells with respect to S1 barrel cortex. Our results demonstrate that S1 and S2 projections terminate in largely overlapping regions but show some significant differences. Whereas S1 projection terminals tend to cluster within the principal trigeminal (Pr5), caudal spinal trigeminal interpolaris (Sp5ic), and the dorsal spinal trigeminal caudalis (Sp5c), S2 projection terminals are distributed in a continuum across all trigeminal nuclei. Contrary to the view that sensory gating could be mediated by differential activation of inhibitory interconnections between the spinal trigeminal subnuclei, we observed that projections from S1 and S2 are largely overlapping in these subnuclei despite the differences noted earlier.

Interhemispheric resting-state functional connectivity of the claustrum in the awake and anesthetized states

The claustrum is a brain region whose function remains unknown, though many investigators suggest it plays a role in conscious attention. Resting-state functional magnetic resonance imaging (RS-fMRI) has revealed how anesthesia alters many functional connections in the brain, but the functional role of the claustrum with respect to the awake versus anesthetized states remains unknown. Therefore, we employed a combination of seed-based RS-fMRI and neuroanatomical tracing to reveal how the anatomical connections of the claustrum are related to its functional connectivity during quiet wakefulness and the isoflurane-induced anesthetic state. In awake rats, RS-fMRI indicates that the claustrum has interhemispheric functional connections with the mediodorsal thalamus (MD) and medial prefrontal cortex (mPFC), as well as other known connections with cortical areas that correspond to the connections revealed by neuroanatomical tracing. During deep isoflurane anesthesia, the functional connections of the claustrum with mPFC and MD were significantly attenuated, while those with the rest of cortex were not significantly altered. These changes in claustral functional connectivity were also observed when seeds were placed in mPFC or MD during RS-fMRI comparisons of the awake and deeply anesthetized states. Collectively, these data indicate that the claustrum has functional connections with mPFC and MD-thalamus that are significantly lessened by anesthesia.

Interhemispheric connections between the infralimbic and entorhinal cortices: The endopiriform nucleus has limbic connections that parallel the sensory and motor connections of the claustrum

We have previously shown that the claustrum is part of an interhemispheric circuit that interconnects somesthetic–motor and visual–motor cortical regions. The role of the claustrum in processing limbic information, however, is poorly understood. Some evidence suggests that the dorsal endopiriform nucleus (DEn), which lies immediately ventral to the claustrum, has connections with limbic cortical areas and should be considered part of a claustrum–DEn complex. To determine whether DEn has similar patterns of cortical connections as the claustrum, we used anterograde and retrograde tracing techniques to elucidate the connectivity of DEn. Following injections of retrograde tracers into DEn, labeled neurons appeared bilaterally in the infralimbic (IL) cortex and ipsilaterally in the entorhinal and piriform cortices. Anterograde tracer injections in DEn revealed labeled terminals in the same cortical regions, but only in the ipsilateral hemisphere. These tracer injections also revealed extensive longitudinal projections throughout the rostrocaudal extent of the nucleus. Dual retrograde tracer injections into IL and lateral entorhinal cortex (LEnt) revealed intermingling of labeled neurons in ipsilateral DEn, including many double‐labeled neurons. In other experiments, anterograde and retrograde tracers were separately injected into IL of each hemisphere of the same animal. This revealed an interhemispheric circuit in which IL projects bilaterally to DEn, with the densest terminal labeling appearing in the contralateral hemisphere around retrogradely labeled neurons that project to IL in that hemisphere. By showing that DEn and claustrum have parallel sets of connections, these results suggest that DEn and claustrum perform similar functions in processing limbic and sensorimotor information, respectively. J. Comp. Neurol. 525:1363–1380, 2017.

Simultaneous GCaMP6-based fiber photometry and fMRI in rats

BACKGROUND Understanding the relationship between neural and vascular signals is essential for interpretation of functional MRI (fMRI) results with respect to underlying neuronal activity. Simultaneously measuring neural activity using electrophysiology with fMRI has been highly valuable in elucidating the neural basis of the blood oxygenation-level dependent (BOLD) signal. However, this approach is also technically challenging due to the electromagnetic interference that is observed in electrophysiological recordings during MRI scanning. NEW METHOD Recording optical correlates of neural activity, such as calcium signals, avoids this issue, and has opened a new avenue to simultaneously acquire neural and BOLD signals. RESULTS The present study is the first to demonstrate the feasibility of simultaneously and repeatedly acquiring calcium and BOLD signals in animals using a genetically encoded calcium indicator, GCaMP6. This approach was validated with a visual stimulation experiment, during which robust increases of both calcium and BOLD signals in the superior colliculus were observed. In addition, repeated measurement in the same animal demonstrated reproducible calcium and BOLD responses to the same stimuli. COMPARISON WITH EXISTING METHOD(S) Taken together, simultaneous GCaMP6-based fiber photometry and fMRI recording presents a novel, artifact-free approach to simultaneously measuring neural and fMRI signals. Furthermore, given the cell-type specificity of GCaMP6, this approach has the potential to mechanistically dissect the contributions of individual neuron populations to BOLD signal, and ultimately reveal its underlying neural mechanisms. CONCLUSIONS The current study established the method for simultaneous GCaMP6-based fiber photometry and fMRI in rats.

Sensory Processing in the Dorsolateral Striatum: The Contribution of Thalamostriatal Pathways

The dorsal striatum has two functionally-defined subdivisions: a dorsomedial striatum (DMS) region involved in mediating goal-directed behaviors that require conscious effort, and a dorsolateral striatum (DLS) region involved in the execution of habitual behaviors in a familiar sensory context. Consistent with its presumed role in forming stimulus-response (S-R) associations, neurons in DLS receive massive inputs from sensorimotor cortex and are responsive to both active and passive sensory stimulation. While several studies have established that corticostriatal inputs contribute to the stimulus-induced responses observed in the DLS, there is growing awareness that the thalamus has a significant role in conveying sensory-related information to DLS and other parts of the striatum. The thalamostriatal projections to DLS originate mainly from the caudal intralaminar region, which contains the parafascicular (Pf) nucleus, and from higher-order thalamic nuclei such as the medial part of the posterior (POm) nucleus. Based on recent findings, we hypothesize that the thalamostriatal projections from these two regions exert opposing influences on the expression of behavioral habits. This article reviews the subcortical circuits that regulate the transmission of sensory information through these thalamostriatal projection systems, and describes the evidence that indicates these circuits could be manipulated to ameliorate the symptoms of Parkinson’s disease (PD) and related neurological disorders.

A diencephalic pathway for movement initiation and rescue of Parkinsonian symptoms

The parafascicular nucleus (Pf) of the thalamus projects to the subthalamic nucleus (STN), a major target for deep brain stimulation (DBS) in Parkinson’s disease (PD), but the function of this projection remains unknown. Here, we used optogenetics, 3D motion capture, in vivo electrophysiology and 1-photon calcium imaging, unsupervised behavioral classification, and viral-based neuroanatomical tracing to examine the contribution of Pf efferents to movement generation in mice. We discovered that Pf neurons are highly correlated with movement velocity and excitation of Pf neurons generates turning and orienting movements. Movement initiation was not due to Pf projections to the striatum, but rather its projections to the STN. Optogenetic excitation of the Pf-STN pathway restores movement in a common mouse model of PD with complete akinesia. Collectively, our results reveal a thalamo-subthalamic pathway regulating movement initiation, and demonstrate a circuit mechanism that could potentially explain the clinical efficacy of DBS for relief of PD motor symptoms.

The relationship between the claustrum and endopiriform nucleus: a perspective towards consensus on cross-species homology

With the emergence of interest in studying the claustrum, a recent special issue of the Journal of Comparative Neurology dedicated to the claustrum (Volume 525, Issue 6, pp. 1313–1513) brought to light questions concerning the relationship between the claustrum (CLA) and a region immediately ventral known as the endopiriform nucleus (En). These structures have been identified as separate entities in rodents but appear as a single continuous structure in primates. During the recent Society for Claustrum Research meeting, a panel of experts presented data pertaining to the relationship of these regions and held a discussion on whether the CLA and En should be considered (a) separate unrelated structures, (b) separate nuclei within the same formation, or (c) subregions of a continuous structure. This review article summarizes that discussion, presenting comparisons of the cytoarchitecture, neurochemical profiles, genetic markers, and anatomical connectivity of the CLA and En across several mammalian species. In rodents, we conclude that the CLA and the dorsal endopiriform nucleus (DEn) are subregions of a larger complex, which likely performs analogous computations and exert similar effects on their respective cortical targets (e.g., sensorimotor versus limbic). Moving forward, we recommend that the field retain the nomenclature currently employed for this region but should continue to examine the delineation of these structures across different species. Using thorough descriptions of a variety of anatomical features, this review offers a clear definition of the CLA and En in rodents, which provides a framework for identifying homologous structures in primates.