Monosynaptic inputs to ventral tegmental area glutamate and GABA co-transmitting neurons

A unique population of ventral tegmental area (VTA) neurons co-transmits glutamate and GABA as well as functionally signals rewarding and aversive outcomes. However, the circuit inputs to VTA VGluT2+VGaT+ neurons are unknown, limiting our understanding of the functional capabilities of these neurons. To identify the inputs to VTA VGluT2+VGaT+ neurons, we coupled monosynaptic rabies tracing with intersectional genetic targeting of VTA VGluT2+VGaT+ neurons in mice. We found that VTA VGluT2+VGaT+ neurons received diverse brain-wide inputs. The largest numbers of monosynaptic inputs to VTA VGluT2+VGaT+ neurons were from superior colliculus, lateral hypothalamus, midbrain reticular nucleus, and periaqueductal gray, whereas the densest inputs relative to brain region volume were from dorsal raphe nucleus, lateral habenula, and ventral tegmental area. Based on these and prior data, we hypothesized that lateral hypothalamus and superior colliculus inputs were glutamatergic neurons. Optical activation of glutamatergic lateral hypothalamus neurons robustly activated VTA VGluT2+VGaT+ neurons regardless of stimulation frequency and resulted in flee-like ambulatory behavior. In contrast, optical activation of glutamatergic superior colliculus neurons activated VTA VGluT2+VGaT+ neurons for a brief period of time at high stimulation frequency and resulted in head rotation and arrested ambulatory behavior (freezing). For both pathways, behaviors induced by stimulation were uncorrelated with VTA VGluT2+VGaT+ neuron activity. However, stimulation of glutamatergic lateral hypothalamus neurons, but not glutamatergic superior colliculus neurons, was associated with VTA VGluT2+VGaT+ footshock-induced activity. We interpret these results such that inputs to VTA VGluT2+VGaT+ neurons may integrate diverse signals related to the detection and processing of motivationally-salient outcomes. Further, VTA VGluT2+VGaT+ neurons may signal threat-related outcomes, possibly via input from lateral hypothalamus glutamate neurons, but not threat-induced behavioral kinematics.

Dopamine neurons exhibit emergent glutamatergic identity in Parkinson's disease

Loss of midbrain dopamine neurons causes the cardinal symptoms of Parkinson's disease. However, not all dopamine neurons are equally vulnerable and a better understanding of the cell-type specific properties relating to selective dopamine neuron degeneration is needed. Most midbrain dopamine neurons express the vesicular glutamate transporter VGLUT2 during development and a subset continue to express low levels of VGLUT2 in adulthood, enabling the co-release of glutamate. Moreover, VGLUT2 expression in dopamine neurons can be neuroprotective since its genetic disruption was shown to sensitize dopamine neurons to neurotoxins. Here, we show that in response to toxic insult, and in two distinct models of alpha-synuclein stress, VGLUT2 dopamine neurons were resilient to degeneration. Dopamine neurons expressing VGLUT2 were enriched whether or not insult induced dopamine neuron loss, suggesting that while VGLUT2 dopamine neurons are more resilient, VGLUT2 expression can also be transcriptionally upregulated by injury. Finally, we observed that VGLUT2 expression was enhanced in surviving dopamine neurons from post-mortem Parkinson's disease individuals. These data indicate that emergence of a glutamatergic identity in dopamine neurons may be part of a neuroprotective response in Parkinson's disease.

The Expression Patterns of Cytochrome Oxidase and Immediate-Early Genes Show Absence of Ocular Dominance Columns in the Striate Cortex of Squirrel Monkeys Following Monocular Inactivation

Because at least some squirrel monkeys lack ocular dominance columns (ODCs) in the striate cortex (V1) that are detectable by cytochrome oxidase (CO) histochemistry, the functional importance of ODCs on stereoscopic 3-D vision has been questioned. However, conventional CO histochemistry or trans-synaptic tracer study has limited capacity to reveal cortical functional architecture, whereas the expression of immediate-early genes (IEGs), c-FOS and ZIF268, is more directly responsive to neuronal activity of cortical neurons to demonstrate ocular dominance (OD)-related domains in V1 following monocular inactivation. Thus, we wondered whether IEG expression would reveal ODCs in the squirrel monkey V1. In this study, we first examined CO histochemistry in V1 of five squirrel monkeys that were subjected to monocular enucleation or tetrodotoxin (TTX) treatment to address whether there is substantial cross-individual variation as reported previously. Then, we examined the IEG expression of the same V1 tissue to address whether OD-related domains are revealed. As a result, staining patterns of CO histochemistry were relatively homogeneous throughout layer 4 of V1. IEG expression was also moderate and homogeneous throughout layer 4 of V1 in all cases. On the other hand, the IEG expression was patchy in accordance with CO blobs outside layer 4, particularly in infragranular layers, although they may not directly represent OD clusters. Squirrel monkeys remain an exceptional species among anthropoid primates with regard to OD organization, and thus are potentially good subjects to study the development and function of ODCs.

Molecular Characterization of Superficial Layers of the Presubiculum During Development

The presubiculum, a subarea of the parahippocampal region, plays a critical role in spatial navigation and spatial representation. An outstanding aspect of presubicular spatial codes is head-direction selectivity of the firing of excitatory neurons, called head-direction cells. Head-direction selectivity emerges before eye-opening in rodents and is maintained in adulthood through neurophysiological interactions between excitatory and inhibitory neurons. Although the presubiculum has been physiologically profiled in terms of spatial representation during development, the histological characteristics of the developing presubiculum are poorly understood. We found that the expression of vesicular glutamate transporter 2 (VGluT2) could be used to delimit the superficial layers of the presubiculum, which was identified using an anterograde tracer injected into the anterior thalamic nucleus (ATN). Thus, we immunostained slices from mice ranging in age from neonates to adults using an antibody against VGluT2 to evaluate the VGluT2-positive area, which was identified as the superficial layers of the presubiculum, during development. We also immunostained the slices using antibodies against parvalbumin (PV) and somatostatin (SOM) and found that in the presubicular superficial layers, PV-positive neurons progressively increased in number during development, whereas SOM-positive neurons exhibited no increasing trend. In addition, we observed repeating patch structures in presubicular layer III from postnatal days 12. The abundant expression of VGluT2 suggests that the presubicular superficial layers are regulated primarily by VGluT2-mediated excitatory neurotransmission. Moreover, developmental changes in the densities of PV- and SOM-positive interneurons and the emergence of the VGluT2-positive patch structures during adolescence may be associated with the functional development of spatial codes in the superficial layers of the presubiculum.

Heterogeneous expression of dopaminergic markers and Vglut2 in mouse mesodiencephalic dopaminergic nuclei A8-A13

Co-transmission of glutamate by brain dopaminergic (DA) neurons was recently proposed as a potential factor influencing cell survival in models of Parkinson's disease. Intriguingly, brain DA nuclei are differentially affected in Parkinson's disease. Whether this is associated with different patterns of co-expression of the glutamatergic phenotype along the rostrocaudal brain axis is unknown in mammals. We hypothesized that, as in zebrafish, the glutamatergic phenotype is present preferentially in the caudal mesodiencephalic DA nuclei. Here, we used in mice a cell fate mapping strategy based on reporter protein expression (ZsGreen) consecutive to previous expression of the vesicular glutamate transporter 2 (Vglut2) gene, coupled with immunofluorescence experiments against tyrosine hydroxylase (TH) or dopamine transporter (DAT). We found three expression patterns in DA cells, organized along the rostrocaudal brain axis. The first pattern (TH-positive, DAT-positive, ZsGreen-positive) was found in A8-A10. The second pattern (TH-positive, DAT-negative, ZsGreen-positive) was found in A11. The third pattern (TH-positive, DAT-negative, ZsGreen-negative) was found in A12-A13. These patterns should help to refine the establishment of the homology of DA nuclei between vertebrate species. Our results also uncover that Vglut2 is expressed at some point during cell lifetime in DA nuclei known to degenerate in Parkinson's disease and largely absent from those that are preserved, suggesting that co-expression of the glutamatergic phenotype in DA cells influences their survival in Parkinson's disease.

Roles of dopamine and glutamate co-release in the nucleus accumbens in mediating the actions of drugs of abuse

Projections of ventral tegmental area dopamine (DA) neurons to the medial shell of the nucleus accumbens have been increasingly implicated as integral to the behavioral and physiological changes involved in the development of substance use disorders (SUDs). Recently, many of these nucleus accumbens-projecting DA neurons were found to also release the neurotransmitter glutamate. This glutamate co-release from DA neurons is critical in mediating the effect of drugs of abuse on addiction-related behaviors. Potential mechanisms underlying the role(s) of dopamine/glutamate co-release in contributing to SUDs are unclear. Nevertheless, an important clue may relate to glutamate's ability to potentiate loading of DA into synaptic vesicles within terminals in the nucleus accumbens in response to drug-induced elevations in neuronal activity, enabling a more robust release of DA after stimulation. Here, we summarize how drugs of abuse, particularly cocaine, opioids, and alcohol, alter DA release in the nucleus accumbens medial shell, examine the potential role of DA/glutamate co-release in mediating these effects, and discuss future directions for further investigating these mechanisms.

Impaired Photic Entrainment of Spontaneous Locomotor Activity in Mice Overexpressing Human Mutant α-Synuclein

Parkinson's disease (PD) is characterized by distinct motor and non-motor symptoms. Sleep disorders are the most frequent and challenging non-motor symptoms in PD patients, and there is growing evidence that they are a consequence of disruptions within the circadian system. PD is characterized by a progressive degeneration of the dorsal vagal nucleus and midbrain dopaminergic neurons together with an imbalance of many other neurotransmitters. Mutations in α-synuclein (SNCA), a protein modulating SNARE complex-dependent neurotransmission, trigger dominantly inherited PD variants and sporadic cases of PD. The A53T SNCA missense mutation is associated with an autosomal dominant early-onset familial PD. To test whether this missense mutation affects the circadian system, we analyzed the spontaneous locomotor behavior of non-transgenic wildtype mice and transgenic mice overexpressing mutant human A53T α-synuclein (A53T). The mice were subjected to entrained- and free-running conditions as well as to experimental jet lag. Furthermore, the vesicular glutamate transporter 2 (VGLUT2) in the suprachiasmatic nucleus (SCN) was analyzed by immunohistochemistry. Free-running circadian rhythm and, thus, circadian rhythm generation, were not affected in A53T mice. A53T mice entrained to the light⁻dark cycle, however, with an advanced phase angle of 2.65 ± 0.5 h before lights off. Moreover, re-entrainment after experimental jet lag was impaired in A53T mice. Finally, VGLUT2 immunoreaction was reduced in the SCN of A53T mice. These data suggest an impaired light entrainment of the circadian system in A53T mice.

Circadian Behavioral Responses to Light and Optic Chiasm-Evoked Glutamatergic EPSCs in the Suprachiasmatic Nucleus of ipRGC Conditional vGlut2 Knock-Out Mice

Intrinsically photosensitive retinal ganglion cells (ipRGCs) innervate the hypothalamic suprachiasmatic nucleus (SCN), a circadian oscillator that functions as a biological clock. ipRGCs use vesicular glutamate transporter 2 (vGlut2) to package glutamate into synaptic vesicles and light-evoked resetting of the SCN circadian clock is widely attributed to ipRGC glutamatergic neurotransmission. Pituitary adenylate cyclase-activating polypeptide (PACAP) is also packaged into vesicles in ipRGCs and PACAP may be coreleased with glutamate in the SCN. vGlut2 has been conditionally deleted in ipRGCs in mice [conditional knock-outs (cKOs)] and their aberrant photoentrainment and residual attenuated light responses have been ascribed to ipRGC PACAP release. However, there is no direct evidence that all ipRGC glutamatergic neurotransmission is eliminated in vGlut2 cKOs. Here, we examined two lines of ipRGC vGlut2 cKO mice for SCN-mediated behavioral responses under several lighting conditions and for ipRGC glutamatergic neurotransmission in the SCN. Circadian behavioral responses varied from a very limited response to light to near normal photoentrainment. After collecting behavioral data, hypothalamic slices were prepared and evoked EPSCs (eEPSCs) were recorded from SCN neurons by stimulating the optic chiasm. In cKOs, glutamatergic eEPSCs were recorded and all eEPSC parameters examined (stimulus threshold, amplitude, rise time or time-to-peak and stimulus strength to evoke a maximal response) were similar to controls. We conclude that a variable number but functionally significant percentage of ipRGCs in two vGlut2 cKO mouse lines continue to release glutamate. Thus, the residual SCN-mediated light responses in these cKO mouse lines cannot be attributed solely to ipRGC PACAP release.

Differential projections of the densocellular and intermediate parts of the hyperpallium in the pigeon (Columba livia)

The visual Wulst in birds shows a four-layered structure: apical part of the hyperpallium (HA), interstitial part of HA (IHA), intercalated part of hyperpallium (HI), and densocellular part of hyperpallium (HD). HD also connects with the hippocampus and olfactory system. Because HD is subjacent to HI, the two have been treated as one structure in many studies, and the fiber connections of HD have been examined by afferents and efferents originating outside HD. However, to clarify the difference between these two layers, they need to be treated separately. In the present study, the fiber connections of HD and HI were analyzed with tract-tracing techniques using a combination of injections of cholera toxin subunit B (CTB) for retrograde tracing and biotinylated dextran amine (BDA) for anterograde tracing. When the two tracers were bilaterally injected in HD, a major reciprocal connection was seen with the dorsolateral subdivision (DL) of the hippocampal formation. When CTB and BDA were bilaterally injected in HI, strong reciprocal connections were found between HI and HA. Next, projection neurons in HD and HI were examined by double staining for CTB combined with vesicular glutamate transporter 2 (vGluT2) mRNA in situ hybridization. After CTB was injected in DL or HA, many neurons revealed CTB+/vGluT2+ in HD or HI, respectively. Furthermore, in situ hybridization showed that DL and HA contained neurons expressing various subunits of ionotropic glutamate receptors: AMPA, kainate, and NMDA types. These results suggest that glutamatergic neurons in HD and HI project primarily to DL and HA, respectively.

Causal role for the subthalamic nucleus in interrupting behavior

Stopping or pausing in response to threats, conflicting information, or surprise is fundamental to behavior. Evidence across species has shown that the subthalamic nucleus (STN) is activated by scenarios involving stopping or pausing, yet evidence that the STN causally implements stops or pauses is lacking. Here we used optogenetics to activate or inhibit mouse STN to test its putative causal role. We first demonstrated that optogenetic stimulation of the STN excited its major projection targets. Next we showed that brief activation of STN projection neurons was sufficient to interrupt or pause a self-initiated bout of licking. Finally, we developed an assay in which surprise was used to interrupt licking, and showed that STN inhibition reduced the disruptive effect of surprise. Thus STN activation interrupts behavior, and blocking the STN blunts the interruptive effect of surprise. These results provide strong evidence that the STN is both necessary and sufficient for such forms of behavioral response suppression.

Differential involvement of vesicular and glial glutamate transporters around spinal α-motoneurons in the pathogenesis of SOD1G93A mouse model of amyotrophic lateral sclerosis

From a view point of the glutamate excitotoxicity theory, several studies have suggested that abnormal glutamate homeostasis via dysfunction of glial glutamate transporter-1 (GLT-1) may underlie neurodegeneration in amyotrophic lateral sclerosis (ALS). However, the detailed role of GLT-1 in the pathogenies of ALS remains controversial. To assess this issue, here we elucidated structural alterations associated with dysregulation of glutamate homeostasis using SOD1^G93A mice, a genetic model of familial ALS. We first examined the viability of α-motoneurons in the lumbar spinal cord of SOD1^G93A mice. Measurement of the soma size and density indicated that α-motoneurons might be intact at 9weeks of age (presymptomatic stage), then soma shrinkage began at 15weeks of age (progressive stage), and finally neuronal density declined at 21weeks of age (end stage). Next, we carried out the line profile analysis, and found that the coverage of α-motoneurons by GLT-1-positive (GLT-1⁺) astrocytic processes was decreased only at 21weeks of age, while the reduction of coverage of α-motoneurons by synaptophysin-positive (SYP⁺) presynaptic terminals began at 15weeks of age. Interestingly, the coverage of α-motoneurons by VGluT2⁺ presynaptic terminals was transiently increased at 9weeks of age, and then gradually decreased towards 21weeks of age. On the other hand, there were no time-dependent alterations in the coverage of α-motoneurons by GABAergic presynaptic terminals. These findings suggest that VGluT2 and GLT-1 may be differentially involved in the pathogenesis of ALS via abnormal glutamate homeostasis at the presymptomatic stage and end stage of disease, respectively.

Identification of neural cells activated by mating stimulus in the periaqueductal gray in female rats

Induction of lordosis as typical female sexual behavior in rodents is dependent on a mount stimulus from males and blood levels of estrogen. Periaqueductal gray (PAG) efferent neurons have been suggested to be important for lordosis behavior; however, the neurochemical basis remains to be understood. In this study, we neuroanatomically examined (1) whether PAG neurons activated by mating stimulus project to the medullary reticular formation (MRF), which is also a required area for lordosis; and (2) whether these neurons are glutamatergic. Mating stimulus significantly increased the number of cFos-immunoreactive (ir) neurons in the PAG, particularly in its lateral region. Half of cFos-ir neurons in the lateral PAG were positive for a retrograde tracer (FluoroGold; FG) injected into the MRF. cFos-ir neurons also colocalized with mRNA of vesicular glutamate transporter 2 (vGLUT2), a molecular marker for glutamatergic neurons. Using retrograde tracing and in situ hybridization in conjunction with fluorescent microscopy, we also found FG and vGLUT2 mRNA double-positive neurons in the lateral PAG. These results suggest that glutamatergic neurons in the lateral PAG project to the MRF and are involved in lordosis behavior in female rats.

Acute exercise-induced activation of Phox2b-expressing neurons of the retrotrapezoid nucleus in rats may involve the hypothalamus

The rat retrotrapezoid nucleus (RTN) contains neurons that have a well-defined phenotype characterized by the presence of vesicular glutamate transporter 2 (VGLUT2) mRNA and a paired-like homeobox 2b (Phox2b)-immunoreactive (ir) nucleus and the absence of tyrosine hydroxylase (TH). These neurons are important to chemoreception. In the present study, we tested the hypothesis that the chemically-coded RTN neurons (ccRTN) (Phox2b(+)/TH(-)) are activated during an acute episode of running exercise. Since most RTN neurons are excited by the activation of perifornical and lateral hypothalamus (PeF/LH), a region that regulates breathing during exercise, we also tested the hypothesis that PeF/LH projections to RTN neurons contribute to their activation during acute exercise. In adult male Wistar rats that underwent an acute episode of treadmill exercise, there was a significant increase in c-Fos immunoreactive (c-Fos-ir) in PeF/LH neurons and RTN neurons that were Phox2b(+)TH(-) (p<0.05) compared to rats that did not exercise. Also the retrograde tracer Fluoro-Gold that was injected into RTN was detected in c-Fos-ir PeF/LH (p<0.05). In summary, the ccRTN neurons (Phox2b(+)TH(-)) are excited by running exercise. Thus, ccRTN neurons may contribute to both the chemical drive to breath and the feed-forward control of breathing associated with exercise.

Differential striatal spine pathology in Parkinson's disease and cocaine addiction: a key role of dopamine?

In the striatum, the dendritic tree of the two main populations of projection neurons, called "medium spiny neurons (MSNs)", are covered with spines that receive glutamatergic inputs from the cerebral cortex and thalamus. In Parkinson's disease (PD), striatal MSNs undergo an important loss of dendritic spines, whereas aberrant overgrowth of striatal spines occurs following chronic cocaine exposure. This review examines the possibility that opposite dopamine dysregulation is one of the key factors that underlies these structural changes. In PD, nigrostriatal dopamine degeneration results in a significant loss of dendritic spines in the dorsal striatum, while rodents chronically exposed to cocaine and other psychostimulants, display an increase in the density of "thin and immature" spines in the nucleus accumbens (NAc). In rodent models of PD, there is evidence that D2 dopamine receptor-containing MSNs are preferentially affected, while D1-positive cells are the main targets of increased spine density in models of addiction. However, such specificity remains to be established in primates. Although the link between the extent of striatal spine changes and the behavioral deficits associated with these disorders remains controversial, there is unequivocal evidence that glutamatergic synaptic transmission is significantly altered in both diseased conditions. Recent studies have suggested that opposite calcium-mediated regulation of the transcription factor myocyte enhancer factor 2 (MEF2) function induces these structural defects. In conclusion, there is strong evidence that dopamine is a major, but not the sole, regulator of striatal spine pathology in PD and addiction to psychostimulants. Further studies of the role of glutamate and other genes associated with spine plasticity in mediating these effects are warranted.

Reward and aversion in a heterogeneous midbrain dopamine system

The ventral tegmental area (VTA) is a heterogeneous brain structure that serves a central role in motivation and reward processing. Abnormalities in the function of VTA dopamine (DA) neurons and the targets they influence are implicated in several prominent neuropsychiatric disorders including addiction and depression. Recent studies suggest that the midbrain DA system is composed of anatomically and functionally heterogeneous DA subpopulations with different axonal projections. These findings may explain a number of previously confusing observations that suggested a role for DA in processing both rewarding as well as aversive events. Here we will focus on recent advances in understanding the neural circuits mediating reward and aversion in the VTA and how stress as well as drugs of abuse, in particular cocaine, alter circuit function within a heterogeneous midbrain DA system. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Neurokinin-1 projection cells in the rat dorsal horn receive synaptic contacts from axons that possess alpha2C-adrenergic receptors

Thealpha2C subclass of adrenergic receptor (alpha2C-AR) mediates some of the antinociceptive actions of norepinephrine in the spinal cord. Axon terminals, which possess this receptor, are concentrated in the superficial dorsal horn and originate from spinal interneurons. We performed a series of combined tract-tracing and immunocytochemical studies to determine whether alpha2C-AR-immunoreactive axons target projection neurons that possess the neurokinin-1 (NK-1) receptor because such cells are likely to transmit nociceptive information to the brain. Spinomedullary neurons were labeled by stereotaxic injection of the B-subunit of cholera toxin (CTb) into the caudal ventrolateral medulla of three anesthetized adult rats. After 3 d, the animals were anesthetized again and fixed by perfusion. Sections were cut from midlumbar segments and reacted with antibodies to reveal alpha2C-ARs, CTb, and NK-1 receptors. Retrogradely labeled neurons possessing the NK-1 receptor (n = 45) were examined with confocal microscopy to investigate their relationship with alpha2C-AR-immunoreactive axons. Numerous alpha2C-AR axons were apposed to cell bodies and proximal dendrites of cells in lamina I and also to distal dendrites that originate from labeled cell bodies in lamina III/IV. A combined confocal and electron microscopic method confirmed that these appositions were synaptic. Additional experiments showed that virtually all alpha2C-AR terminals in contact with labeled cells are also immunoreactive for the vesicular glutamate transporter 2 and therefore are glutamatergic. These data suggest that norepinephrine can modulate excitatory synaptic transmission from spinal interneurons to projection cells by acting at alpha2C-ARs. This could be one of the mechanisms that underlie the antinociceptive actions of norepinephrine.