A fluorescence-based double retrograde tracer strategy for charting central neuronal connections

Ventromedial medullary pathway mediating cardiac responses evoked from periaqueductal gray

Periaqueductal gray (PAG) is a midbrain region that projects to areas controlling behavioral and autonomic outputs and is involved in the behavioral and physiological components of defense reactions. Since Raphe Pallidus (RPa) is a medial medullary region comprising sympathetic premotor neurons governing heart function, it is worth considering the PAG-RPa path. We assessed: i) whether PAG projects to RPa; ii) the amplitude of cardiac responses evoked from PAG; iii) whether cardiovascular responses evoked from PAG rely on RPa. Experiments conducted in Wistar rats (±300 g) were approved by Ethics Committee CEUA-UFG (092/18). Firstly, (n = 3), monosynaptic retrograde tracer Retrobeads was injected into RPa; PAG slices were analyzed. Other two groups (n = 6 each) were anesthetized with urethane (1.4 g/kg) and chloralose (120 mg/kg) and underwent craniotomy, tracheostomy, catheterization of femoral artery and vein and of cardiac left ventricle. In one group, we injected the GABA_A receptor antagonist, bicuculline methiodide (BMI - 40 pmol/100 nL) into lateral/dorsolateral PAG. Another group was injected (100 nL) with the GABA_A receptor agonist muscimol (20 mM) into RPa, 20 min before BMI into PAG. The results were: i) retrogradely labelled neurons were found in PAG; ii) PAG activation by BMI caused positive chronotropism and inotropism, which were accompanied by afterload increases; iii) RPa inhibition with Muscimol reduced heart rate, arterial and ventricular pressures; iv) the subsequent PAG activation still increased arterial pressure, cardiac chronotropy and inotropy, but these responses were significantly attenuated. In conclusion, PAG activation increases cardiac chronotropy and inotropy, and these responses seem to rely on a direct pathway reaching ventromedial medullary RPa neurons.

Synaptic Targets of Medial Septal Projections in the Hippocampus and Extrahippocampal Cortices of the Mouse

Temporal coordination of neuronal assemblies among cortical areas is essential for behavioral performance. GABAergic projections from the medial septum and diagonal band complex exclusively innervate GABAergic interneurons in the rat hippocampus, contributing to the coordination of neuronal activity, including the generation of theta oscillations. Much less is known about the synaptic target neurons outside the hippocampus. To reveal the contribution of synaptic circuits involving the medial septum of mice, we have identified postsynaptic cortical neurons in wild-type and parvalbumin-Cre knock-in mice. Anterograde axonal tracing from the septum revealed extensive innervation of the hippocampus as well as the subiculum, presubiculum, parasubiculum, the medial and lateral entorhinal cortices, and the retrosplenial cortex. In all examined cortical regions, many septal GABAergic boutons were in close apposition to somata or dendrites immunopositive for interneuron cell-type molecular markers, such as parvalbumin, calbindin, calretinin, N-terminal EF-hand calcium-binding protein 1, cholecystokinin, reelin, or a combination of these molecules. Electron microscopic observations revealed septal boutons forming axosomatic or axodendritic type II synapses. In the CA1 region of hippocampus, septal GABAergic projections exclusively targeted interneurons. In the retrosplenial cortex, 93% of identified postsynaptic targets belonged to interneurons and the rest to pyramidal cells. These results suggest that the GABAergic innervation from the medial septum and diagonal band complex contributes to temporal coordination of neuronal activity via several types of cortical GABAergic interneurons in both hippocampal and extrahippocampal cortices. Oscillatory septal neuronal firing at delta, theta, and gamma frequencies may phase interneuron activity.

A critical period of vulnerability to adolescent stress: epigenetic mediators in mesocortical dopaminergic neurons

The molecular basis of vulnerability to stress during the adolescent period is largely unknown. To identify potential molecular mediators that may play a role in stress-induced behavioral deficits, we imposed social isolation on a genetically vulnerable mouse model. We report that 3-week (5-8 weeks of age) adolescent stress in combination with disrupted-in-schizophrenia 1 (Disc1) genetic risk elicits alterations in DNA methylation of a specific set of genes, tyrosine hydroxylase, brain-derived neurotrophic factor and FK506 binding protein 5. The epigenetic changes in the mesocortical dopaminergic neurons were prevented when animals were treated with a glucocorticoid receptor (GR) antagonist RU486 during social isolation, which implicates the role for glucocorticoid signaling in this pathological event. We define the critical period of GR intervention as the first 1-week period during the stress regimen, suggesting that this particular week in adolescence may be a specific period of maturation and function of mesocortical dopaminergic neurons and their sensitivity to glucocorticoids. Our study may also imply the clinical significance of early detection and prophylactic intervention against conditions associated with adolescent social stress in individuals with genetic risk.

Spino-olivary projections in the rat are anatomically separate from postsynaptic dorsal column projections

The gracile nucleus (GN) and lateral part of rostral dorsal accessory olive (rDAO) are important relays for indirect, postsynaptic dorsal column, and direct ascending pathways, respectively, that terminate as climbing fibers in the “hindlimb-receiving” parts of the C1 and C3 zones in the cerebellar cortex. While the spinal cells of origin of that project to GN and rDAO are from largely separate territories in the spinal cord, previous studies have indicated that there could be an area of overlap between these two populations in the medial dorsal horn. Given the access of these two ascending tracts to sensory (thalamic) versus sensorimotor (precerebellar) pathways, the present study therefore addresses the important question of whether or not individual neurons have the potential to contribute axons to both ascending pathways. A double-fluorescent tracer strategy was used in rats (red Retrobeads and Fluoro-Ruby or green Retrobeads and Fluoro-Emerald) to map the spatial distribution of cells of origin of the two projections in the lumbar spinal cord. The two pathways were found to receive input from almost entirely separate territories within the lumbar cord (levels L3–L5). GN predominantly receives input from lamina IV, while rDAO receives its input from three cell populations: medial laminae V–VI, lateral lamina V, and medial laminae VII–VIII. Cells that had axons that branched to supply both GN and rDAO represented only about 1% of either single-labeled cell population. Overall, the findings therefore suggest functional independence of the two ascending pathways. J. Comp. Neurol. 522:2179–2190, 2014. © 2013 Wiley Periodicals, Inc.

Excitatory amino acid receptors mediate asymmetry and lateralization in the descending cardiovascular pathways from the dorsomedial hypothalamus

The dorsomedial hypothalamus (DMH) and lateral/dorsolateral periaqueductal gray (PAG) are anatomically and functionally connected. Both the DMH and PAG depend on glutamatergic inputs for activation. We recently reported that removal of GABA-ergic tone in the unilateral DMH produces: asymmetry, that is, a right- (R-) sided predominance in cardiac chronotropism, and lateralization, that is, a greater increase in ipsilateral renal sympathetic activity (RSNA). In the current study, we investigated whether excitatory amino acid (EAA) receptors in the DMH-PAG pathway contribute to the functional interhemispheric difference. In urethane (1.2 to 1.4 g/kg, i.p.) anesthetized rats, we observed that: (i) nanoinjections of N-methyl D-aspartate (NMDA 100 pmol/100 nl) into the unilateral DMH produced the same right-sided predominance in the control of cardiac chronotropy, (ii) nanoinjections of NMDA into the ipsilateral DMH or PAG evoked lateralized RSNA responses, and (iii) blockade of EAA receptors in the unilateral DMH attenuated the cardiovascular responses evoked by injection of NMDA into either the R- or left- (L-) PAG. In awake rats, nanoinjection of kynurenic acid (1 nmol/100 nL) into the L-DMH or R- or L-PAG attenuated the tachycardia evoked by air stress. However, the magnitude of stress-evoked tachycardia was smallest when the EAA receptors of the R-DMH were blocked. We conclude that EAA receptors contribute to the right-sided predominance in cardiac chronotropism. This interhemispheric difference that involves EAA receptors was observed in the DMH but not in the PAG.

Genetic strategies to investigate neuronal circuit properties using stem cell-derived neurons

The mammalian brain is anatomically and functionally complex, and prone to diverse forms of injury and neuropathology. Scientists have long strived to develop cell replacement therapies to repair damaged and diseased nervous tissue. However, this goal has remained unrealized for various reasons, including nascent knowledge of neuronal development, the inability to track and manipulate transplanted cells within complex neuronal networks, and host graft rejection. Recent advances in embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) technology, alongside novel genetic strategies to mark and manipulate stem cell-derived neurons, now provide unprecedented opportunities to investigate complex neuronal circuits in both healthy and diseased brains. Here, we review current technologies aimed at generating and manipulating neurons derived from ESCs and iPSCs toward investigation and manipulation of complex neuronal circuits, ultimately leading to the design and development of novel cell-based therapeutic approaches.

Spatial localization and projection densities of brainstem mossy fibre afferents to the forelimb C1 zone of the rat cerebellum

The present study uses a double retrograde tracer technique in rats to examine the spatial localization and pattern of axonal branching in mossy fibres arising from three major sources in the medulla-the external cuneate nucleus, the sensory trigeminal nucleus and the reticular formation, to two electrophysiologically-identified parts of the cerebellar cortex that are linked by common climbing fibre input - the forelimb-receiving parts of the C1 zone in lobulus simplex and the paramedian lobule. In each experiment a small injection of rhodamine-tagged beads was injected into one cortical region and an injection of fluorescein-tagged beads was injected into the other region. The main findings were: (i) the proportion of double-labelled cells in each of the three precerebeller sources of mossy fibres was positively correlated with those in the inferior olive; and (ii) the C1 zone in lobulus simplex was found to receive a greater density of projections from all three sources of mossy fibres than the C1 zone in the paramedian lobule. These data suggest that two rostrocaudally separated but somatotopically corresponding parts of the C1 zone receive common mossy fibre and climbing fibre inputs. However, the differences in projection densities also suggest that the two parts of the zone differ in the extent to which they receive mossy fibre signals arising from the same precerebellar nuclei. This implies differences in function between somatotopically corresponding parts of the same cortical zone, and could enable a higher degree of parallel processing and integration of information within them.

Subnuclear organization of parabrachial efferents to the thalamus, amygdala and lateral hypothalamus in C57BL/6J mice: a quantitative retrograde double labeling study

The present study investigated the subnuclear organization of collateralized efferent projection patterns from the mouse parabrachial nucleus (PbN), the second taste relay in rodents, to higher gustatory centers, including the ventroposteromedial nucleus of the thalamus (VPMpc), central nucleus of the amygdala (CeA) and lateral hypothalamus (LH). We made injections of the retrograde tracer red and green latex microspheres into the VMPpc and CeA (VPMpc-CeA group), VMPpc and LH (VPMpc-LH group) or CeA and LH (CeA-LH group, n=6 for each group). Injections into these areas preferentially resulted in retrograde labeling in the ipsilateral PbN in all groups. Cells projecting to the VPMpc, CeA, and LH were generally found in all subnuclei, but were differentially distributed. VPMpc-projecting cells predominated in gustatory-related subnuclei, CeA-projecting neurons predominated in the external lateral (el) subnucleus, and concentrated labeling was observed in the dorsal lateral subnucleus (dl) following LH injection. Double-labeled neurons were found for all groups, almost entirely ipsilaterally and primarily in the medial (m), waist area (wa), ventral lateral (vl) and el subnuclei. These results suggest that PbN neurons in different subdivisions have different projection and collateralization patterns to the VPMpc, CeA and LH. Functional implications of these projections are discussed with an emphasis on their roles in taste.

Multiple neuroanatomical tract-tracing using fluorescent Alexa Fluor conjugates of cholera toxin subunit B in rats

Cholera toxin subunit B (CTB) is a highly sensitive retrograde neuroanatomical tracer. With the new availability of fluorescent Alexa Fluor (AF) conjugates of CTB, multiple neuroanatomical connections can be reliably studied and compared in the same animal. Here we provide a protocol that describes the use of AF-CTB for studying connections in the central nervous system of rats. The viscous properties of CTB allow small and discreet injection sites yet still show robust retrograde labeling. Furthermore, the AF conjugates are extremely bright and photostable, compared with other conventional fluorescent tracers. This protocol can also be adapted for use with other neuroanatomical tracers. Including a 7-d survival period, this protocol takes approximately 11 to 12 d to complete in its entirety.

Projection of reconstructed single Purkinje cell axons in relation to the cortical and nuclear aldolase C compartments of the rat cerebellum

Although the overall topography of the cerebellar corticonuclear projection formed by Purkinje cell (PC) axons has been described, only a few studies have dealt with the organization of this projection at the level of individual PC axons. Thus, we reconstructed 65 single PC axons that were labeled with biotinylated dextran amine in the rat. We then analyzed the relationship between the projections of these PCs and the compartmentalization of the cerebellar cortex and nuclei based on the topography of olivocerebellar projection and aldolase C expression in PCs. After giving rise to short local recurrent collaterals near the soma, a PC axon formed a terminal arbor in a specific small area in the cerebellar nuclei (CN). The terminal arbors of vermal PCs were spread more widely than those of hemispheric PCs and sometimes extended to extracerebellar targets. PCs located in any of the aldolase C-positive (Groups I and II) and -negative (Groups III and IV) stripes consistently projected to the caudoventral and rostrodorsal parts of the CN, respectively, precisely in accordance with the compartmentalization of the cortex and nuclei. Mediolateral segregation and rostrocaudal convergence were seen between projections of separate PCs in a single aldolase C compartment. The results revealed a tight link between the projection patterns of individual PC axons, the topography of the olivocerebellar pathway, and the aldolase C expression pattern. Their overall correspondence seems to reflect a basic aspect of cerebellar organization, although some area-dependent variation in the relationship of these three entities was also present.

A light microscope-based double retrograde tracer strategy to chart central neuronal connections

This protocol describes a double retrograde tracing method to chart divergent projections in the CNS using light microscope techniques. It is based on immunohistochemical visualization of retrograde transport of cholera toxin b-subunit (CTb) and silver enhancement of a gold-lectin conjugate. Production of the gold-lectin is explained in detail, and a technique is offered to record through the injection pipettes, to help guide accurate placement of injections. Visualization of the two tracers results in light brown staining of CTb-labeled neurons and labeling by black particles of gold-lectin-containing neurons. Both types of label are easily recognized in the same neuron. The labeling is permanent and is well suited for studies in which large areas of the brain need to be surveyed. The whole procedure (excluding survival time) takes approximately 5-7 d to complete.