‘Functional’ neuroanatomical tract tracing: Analysis of changes in gene expression of brain circuits of interest

Integrative biological analysis for neuropsychopharmacology

Although advances in psychotherapy have been made in recent years, drug discovery for brain diseases such as schizophrenia and mood disorders has stagnated. The need for new biomarkers and validated therapeutic targets in the field of neuropsychopharmacology is widely unmet. The brain is the most complex part of human anatomy from the standpoint of number and types of cells, their interconnections, and circuitry. To better meet patient needs, improved methods to approach brain studies by understanding functional networks that interact with the genome are being developed. The integrated biological approaches--proteomics, transcriptomics, metabolomics, and glycomics--have a strong record in several areas of biomedicine, including neurochemistry and neuro-oncology. Published applications of an integrated approach to projects of neurological, psychiatric, and pharmacological natures are still few but show promise to provide deep biological knowledge derived from cells, animal models, and clinical materials. Future studies that yield insights based on integrated analyses promise to deliver new therapeutic targets and biomarkers for personalized medicine.

Expression of 5-HT2A receptors in prefrontal cortex pyramidal neurons projecting to nucleus accumbens. Potential relevance for atypical antipsychotic action

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotic drugs modulate information processing in cortico-limbic circuits via dopamine D2 receptor blockade in nucleus accumbens (NAc) whereas atypical antipsychotic drugs preferentially target cortical serotonin (5-HT) receptors. The brain networks involved in the therapeutic action of atypical drugs are not fully understood. Previous work indicated that medial PFC (mPFC) pyramidal neurons projecting to ventral tegmental area express 5-HT2A receptors suggesting that atypical antipsychotic drugs modulate dopaminergic activity distally, via 5-HT2A receptor (5-HT2A-R) blockade in PFC. Since the mPFC also projects heavily to NAc, we examined whether NAc-projecting pyramidal neurons also express 5-HT2A-R. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of mPFC-NAc pyramidal neurons in rat brain express 5-HT2A-R mRNA in a layer- and area-specific manner (up to 68% in layer V of contralateral cingulate). The functional relevance of 5-HT2A-R to modulate mPFC-NAc projections was examined in dual-probe microdialysis experiments. The application of the preferential 5-HT2A-R agonist DOI into mPFC enhanced glutamate release locally (+66 ± 18%) and in NAc (+74 ± 12%) indicating that cortical 5-HT2A-R activation augments glutamatergic transmission in NAc. Since NAc integrates glutamatergic and dopaminergic inputs, blockade of 5-HT2A-R by atypical drugs may reduce cortical excitatory inputs onto GABAergic neurons of NAc, adding to dopamine D2 receptor blockade. Together with previous observations, the present results suggest that atypical antipsychotic drugs may control the activity of the mesolimbic pathway at cell body and terminal level.

Knowledge synthesis with maps of neural connectivity

This paper describes software for neuroanatomical knowledge synthesis based on neural connectivity data. This software supports a mature methodology developed since the early 1990s. Over this time, the Swanson laboratory at USC has generated an account of the neural connectivity of the sub-structures of the hypothalamus, amygdala, septum, hippocampus, and bed nucleus of the stria terminalis. This is based on neuroanatomical data maps drawn into a standard brain atlas by experts. In earlier work, we presented an application for visualizing and comparing anatomical macro connections using the Swanson third edition atlas as a framework for accurate registration. Here we describe major improvements to the NeuARt application based on the incorporation of a knowledge representation of experimental design. We also present improvements in the interface and features of the data mapping components within a unified web-application. As a step toward developing an accurate sub-regional account of neural connectivity, we provide navigational access between the data maps and a semantic representation of area-to-area connections that they support. We do so based on an approach called "Knowledge Engineering from Experimental Design" (KEfED) model that is based on experimental variables. We have extended the underlying KEfED representation of tract-tracing experiments by incorporating the definition of a neuronanatomical data map as a measurement variable in the study design. This paper describes the software design of a web-application that allows anatomical data sets to be described within a standard experimental context and thus indexed by non-spatial experimental design features.

A half century of experimental neuroanatomical tracing

Most of our current understanding of brain function and dysfunction has its firm base in what is so elegantly called the 'anatomical substrate', i.e. the anatomical, histological, and histochemical domains within the large knowledge envelope called 'neuroscience' that further includes physiological, pharmacological, neurochemical, behavioral, genetical and clinical domains. This review focuses mainly on the anatomical domain in neuroscience. To a large degree neuroanatomical tract-tracing methods have paved the way in this domain. Over the past few decades, a great number of neuroanatomical tracers have been added to the technical arsenal to fulfill almost any experimental demand. Despite this sophisticated arsenal, the decision which tracer is best suited for a given tracing experiment still represents a difficult choice. Although this review is obviously not intended to provide the last word in the tract-tracing field, we provide a survey of the available tracing methods including some of their roots. We further summarize our experience with neuroanatomical tracers, in an attempt to provide the novice user with some advice to help this person to select the most appropriate criteria to choose a tracer that best applies to a given experimental design.

A direct projection from the subthalamic nucleus to the ventral thalamus in monkeys

The current basal ganglia model considers the internal division of the globus pallidus and the substantia nigra pars reticulata as the sole sources of basal ganglia output to the thalamus. However, following the delivery of retrograde tracers into the ventral anterior/ventral lateral thalamic nuclei, a moderate number of labeled neurons were found within the subthalamic nucleus (STN) in control cases, MPTP-treated monkeys and animals with levodopa-induced dyskinesias. Furthermore, dual tracing experiments showed that subthalamo-thalamic and subthalamo-pallidal projections arise from different subpopulations of STN efferent neurons. Moreover, upregulated expression of the mRNA coding the vesicular glutamate transporter 2 (vGlut2) was found in retrogradely-labeled STN neurons in MPTP-treated monkeys. By contrast, there is a reduction in vGlut2 mRNA expression in subthalamo-thalamic neurons in dyskinetic monkeys. In conclusion, our findings support the presence of a direct projection from the STN to the ventral thalamus that appears to be functionally modulated by dopaminergic activity.

Neuroanatomical tracing combined with in situ hybridization: analysis of gene expression patterns within brain circuits of interest

Most of our current understanding of brain circuits is based on hodological studies carried out using neuroanatomical tract-tracing. Our aim is to advance one step further by visualizing the functional correlate in a given circuit. In this regard, we believe it is feasible to combine retrograde tracing with fluorescence, non-radioactive in situ hybridization (ISH) protocols. The subsequent detection at the single-cell level of the expression of a given mRNA within retrograde-labeled neurons provides information regarding cellular function. This may be of particular interest when trying to elucidate the performance of brain circuits of interest in animal models of brain diseases. Several combinations of retrograde tracing with either single- and double-ISH are presented here, together with some criteria that influence the selection of the tracer to be used in conjunction with the strong demands of the ISH.

Thalamic transcriptome screening in three psychiatric states

The prefrontal cortex has been implicated in schizophrenia (SZ) and affective disorders by gene expression studies. Owing to reciprocal connectivity, the thalamic nuclei and their cortical fields act as functional units. Altered thalamic gene expression would be expected to occur in association with cortical dysfunction. We screened the expression of the entire human genome of neurons harvested by laser-capture microdissection (LCM) from the thalamic primary relay to dorsolateral prefrontal cortex in three psychiatric disease states as compared with controls. Microarray analysis of gene expression showed the largest number of dysregulated genes was in SZ, followed by major depression (MD) and bipolar mood bipolar (BP) (1152, 385 and 288, respectively). Significantly, IGF1-mTOR-, AKT-, RAS-, VEGF-, Wnt- and immune-related signaling, eIF2- and proteasome-related genes were unique to SZ. Vitamin D receptor and calcium signaling pathway were unique to BP. AKAP95 pathway and pantothenate and CoA biosynthesis were unique to MD. There are significant differences among the three psychiatric disorders in MDNp cells. These findings offer new insights into the transcriptional dysregulation in the thalamus of SZ/BP/MD subjects.

Pyramidal neurons in rat prefrontal cortex projecting to ventral tegmental area and dorsal raphe nucleus express 5-HT2A receptors

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotics modulate cortico-limbic circuits mainly through subcortical D2 receptor blockade, whereas second generation (atypical) antipsychotics preferentially target cortical 5-HT receptors. Anatomical and functional evidence supports a PFC-based control of the brainstem monoaminergic nuclei. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of PFC pyramidal neurons projecting to the dorsal raphe (DR) and/or ventral tegmental area (VTA) express 5-HT(2A) receptors. Cholera-toxin B application into the DR and the VTA retrogradely labeled projection neurons in the medial PFC (mPFC) and in orbitofrontal cortex (OFC). In situ hybridization of 5-HT(2A) receptor mRNA in the same tissue sections labeled a large neuronal population in mPFC and OFC. The percentage of DR-projecting neurons expressing 5-HT(2A) receptor mRNA was approximately 60% in mPFC and approximately 75% in OFC (n = 3). Equivalent values for VTA-projecting neurons were approximately 55% in both mPFC and ventral OFC. Thus, 5-HT(2A) receptor activation/blockade in PFC may have downstream effects on dopaminergic and serotonergic systems via direct descending pathways. Atypical antipsychotics may distally modulate monoaminergic cells through PFC 5-HT(2A) receptor blockade, presumably decreasing the activity of neurons receiving direct cortical inputs.

Origin of calretinin-containing, vesicular glutamate transporter 2-coexpressing fiber terminals in the entorhinal cortex of the rat

The entorhinal cortex of the rat (EC) contains a dense fiber plexus that expresses the calcium-binding protein calretinin (CR). Some CR fibers contain vesicular glutamate transporter 2 (VGluT2, associated with glutamatergic neurotransmission). CR-VGluT2 coexpressing fibers may have an extrinsic origin, for instance, the midline thalamic nucleus reuniens. Alternatively, they may belong to cortical interneurons. We studied the first possibility with anterograde and retrograde neuroanatomical tracing methods combined with CR and VGluT2 immunofluorescence and confocal laser scanning. The alternative possibility was studied with in situ hybridization fluorescence histochemistry for VGluT2 mRNA combined with CR immunofluorescence. In the anterograde tracing experiments, we observed many labeled reuniens fibers in EC expressing CR. Some of these labeled fibers contained immunoreactivity for VGluT2 and CR. In the complementary retrograde tracing experiments, we found retrogradely labeled cell bodies in nucleus reuniens of the thalamus that coexpressed CR. We also examined the colocalization of VGluT2 and CR in the entorhinal cortex by using in situ hybridization and CR immunofluorescence. In these experiments, we observed CR-immunopositive cortical neurons that coexpressed VGluT2. For the same sections, with CR as the principal marker and parvalbumin as a control marker, we found that parvalbumin neurons were negative for VGluT2 mRNA. Thus, CR-VGluT2-expressing axon terminals in EC belong to two sources: projection fibers from the thalamus and axon collaterals of local interneurons. VGluT2 expression is linked to the synaptic transmission of the excitatory neurotransmitter glutamate, so these thalamic CR-VGluT2 projection neurons and entorhinal CR-VGluT2 interneurons should be regarded as excitatory.

Molecular markers distinguishing supragranular and infragranular layers in the human prefrontal cortex

The human neocortex is organized into six layers that are differentiated by the size and packing density of their constituent neurons. The gene products that guide the establishment of this lamination have been studied extensively, but the gene expression gradients present across the layers of the adult human neocortex are mostly unknown. As the supragranular (SG) and infragranular (IG) layers of the human prefrontal cortex (PFC) differ in their connectivity and developmental time course, we hypothesized that the SG and IG layers will show distinct differences in their transcriptomes. To test this prediction, we used laser capture microdissection coupled with DNA microarray transcriptome profiling. Sixty-nine genes exhibited robust and highly consistent expression differences between the SG and IG layers. For six selected markers, in addition to validating the microarray findings, in situ hybridization revealed a complex, subpopulation-specific neuronal distribution. The markers we identified are likely to be related to the functional differences between the SG and IG layers of the human PFC and can be used for assessing alterations in structure and function of this cortical region in human brain disorders.