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The hippocampus is a key brain structure for cognitive and emotional functions. Among the hippocampal subregions, the dentate gyrus (DG) is the first station that receives multimodal sensory information from the cortex. Local‐circuit inhibitory GABAergic interneurons (INs) regulate the excitation–inhibition balance in the DG principal neurons (PNs) and therefore are critical for information processing. Similar to PNs, GABAergic INs also receive distinct inhibitory inputs. Among various classes of INs, vasoactive intestinal polypeptide‐expressing (VIP+) INs preferentially target other INs in several brain regions and thereby directly modulate the GABAergic system. However, the morpho‐physiological characteristics and postsynaptic targets of VIP+ INs in the DG are poorly understood. Here, we report that VIP+ INs in the mouse DG are highly heterogeneous based on their morpho‐physiological characteristics. In approximately two‐thirds of morphologically reconstructed cells, their axons ramify in the hilus. The remaining cells project their axons exclusively to the molecular layer (15%), to both the molecular layer and hilus (10%), or throughout the entire DG layers (8%). Generally, VIP+ INs display variable intrinsic properties and discharge patterns without clear correlation with their morphologies. Finally, VIP+ INs are recruited with a long latency in response to theta‐band cortical inputs and preferentially innervate GABAergic INs over glutamatergic PNs. In summary, VIP+ INs in the DG are composed of highly diverse subpopulations and control the DG output via disinhibition. [ABSTRACT FROM AUTHOR]

The chemical synapse is the principal form of contact between neurons of the central nervous system. These synapses are typically configured as presynaptic axon terminations onto postsynaptic dendrites or somata, giving rise to axo‐dendritic and axo‐somatic synapses, respectively. Beyond these common synapse configurations are less‐studied, non‐canonical synapse types that are prevalent throughout the brain and significantly contribute to neural circuit function. Among these are the axo‐axonic synapses, which consist of an axon terminating on another axon or axon terminal. Here, we review evidence for axo‐axonic synapse contributions to neural signaling in the mammalian nervous system and survey functional neural circuit motifs enabled by these synapses. We also detail how recent advances in microscopy, transgenics, and biological sensors may be used to identify and functionally assay axo‐axonic synapses. [ABSTRACT FROM AUTHOR]

Transgenic animals have become a widely used model to identify and study specific cell types in whole organs. Promotor‐driven reporter gene labeling of the cells under investigation has promoted experimental efficacy to a large degree. However, rigorous assessment of transgene expression specificity in these animal models is highly recommended to validate cellular identity and to isolate potentially mislabeled cell populations. Here, we report on one such mislabeled neuron population in a widely used transgenic mouse line in which GABAergic somatostatin‐expressing interneurons (SOMpos INs) are labeled by eGFP (so‐called GIN mouse, FVB‐Tg(GadGFP)45704Swn/J). These neurons represent a subpopulation of all SOMpos INs. However, we report here on GFP labeling of non‐GABAergic neurons in the nucleus endopiriformis of this mouse line. [ABSTRACT FROM AUTHOR]

Transplanting embryonic precursors of GABAergic neurons from the medial ganglionic eminence (MGE) into adult mouse spinal cord ameliorates mechanical and thermal hypersensitivity in peripheral nerve injury models of neuropathic pain. Although Fos and transneuronal tracing studies strongly suggest that integration of MGE-derived neurons into host spinal cord circuits underlies recovery of function, the extent to which there is synaptic integration of the transplanted cells has not been established. Here, we used electron microscopic immunocytochemistry to assess directly integration of GFP-expressing MGE-derived neuronal precursors into dorsal horn circuitry in intact, adult mice with short- (5-6 weeks) or long-term (4-6 months) transplants. We detected GFP with pre-embedding avidin-biotin-peroxidase and GABA with post-embedding immunogold labeling. At short and long times post-transplant, we found host-derived synapses on GFP-immunoreactive MGE cells bodies and dendrites. The proportion of dendrites with synaptic input increased from 50% to 80% by 6 months. In all mice, MGE-derived terminals formed synapses with GFP-negative (host) cell bodies and dendrites and, unexpectedly, with some GFP-positive (i.e., MGE-derived) dendrites, possibly reflecting autoapses or cross talk among transplanted neurons. We also observed axoaxonic appositions between MGE and host terminals. Immunogold labeling for GABA confirmed that the transplanted cells were GABAergic and that some transplanted cells received an inhibitory GABAergic input. We conclude that transplanted MGE neurons retain their GABAergic phenotype and integrate dynamically into host-transplant synaptic circuits. Taken together with our previous electrophysiological analyses, we conclude that MGE cells are not GABA pumps, but alleviate pain and itch through synaptic release of GABA. [ABSTRACT FROM AUTHOR]

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study was to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus (IC) of the mouse revealed that most of the subcortical descending projections originated in the brachium of the IC and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium-binding proteins calbindin, parvalbumin, or calretinin. Using two different strains of GAD-GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the IC is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the IC and amygdala plus corpus striatum as well into the IC and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium-binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the IC. J. Comp. Neurol. 525:885-900, 2017. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The vulnerability and plasticity of hippocampal GABAergic interneurons is a topic of broad interest and debate in the field of epilepsy. In this experiment, we used the electrical kindling model of epilepsy to determine whether seizures that originate in different brain regions have differential effects on hippocampal interneuron subpopulations. Long-Evans rats received 99 electrical stimulations of the hippocampus, amygdala, or caudate nucleus, followed by sacrifice and immunohistochemical or western blot analyses. We analyzed markers of dendritic (somatostatin), perisomatic (parvalbumin), and interneuron-selective (calretinin) inhibition, as well as an overall indicator (GAD67) of interneuron distribution across all major hippocampal subfields. Our results indicate that kindling produces selective effects on the number and morphology of different functional classes of GABAergic interneurons. In particular, limbic kindling appears to enhance dendritic inhibition, indicated by a greater number of somatostatin-immunoreactive (-ir) cells in the CA1 pyramidal layer and robust morphological sprouting in the dentate gyrus. We also found a reduction in the number of interneuron-selective calretinin-ir cells in the dentate gyrus of hippocampal-kindled rats, which suggests a possible reduction of synchronized dendritic inhibition. In contrast, perisomatic inhibition indicated by parvalbumin immunoreactivity appears to be largely resilient to the effects of kindling. Finally, we found a significant induction in the number of GAD67-cells in caudate-kindled rats in the dentate gyrus and CA3 hippocampal subfields. Taken together, our results demonstrate that kindling has subfield-selective effects on the different functional classes of hippocampal GABAergic interneurons. J. Comp. Neurol. 525:389-406, 2017. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

ABSTRACT This study compared the expression pattern, laminar distribution, and cell specificity of several rAAV serotypes (2/1, 2/5, 2/7, 2/8, and 2/9) injected in the primary visual cortex (V1) of adult C57Bl/6J mice. In order to obtain specific expression in certain neuron subtypes, different promoter sequences were evaluated for excitatory cell specificity: a universal cytomegalovirus (CMV) promoter, and two versions of the excitatory neuron-specific Ca2+/calmodulin-dependent kinase subunit α (CaMKIIα) promoter, CaMKIIα 0.4 and CaMKIIα 1.3. The spatial distribution as well as the cell type specificity was immunohistochemically verified. Depending on the rAAV serotype used, the transduced volume expressing reporter protein differed substantially (rAAV2/5 ≫ 2/7 ≈ 2/9 ≈ 2/8 ≫ 2/1). Excitatory neuron-specific targeting was promoter-dependent, with a surprising difference between the 1.3 kb and 0.4 kb CaMKIIα promoters. While CaMKIIα 1.3 and CMV carrying vectors were comparable, with 78% of the transduced neurons being excitatory for CMV and 82% for CaMKIIα 1.3, the shorter CaMKIIα 0.4 version resulted in 95% excitatory specificity. This study therefore puts forward the CaMKIIα 0.4 promoter as the best choice to target excitatory neurons with rAAVs. Together, these results can be used as an aid to select the most optimal vector system to deliver transgenes into specific rodent neocortical circuits, allowing further elucidation of their functions. J. Comp. Neurol. 523:2019-2042, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

ABSTRACT The amino acid L-aspartate (ASP) is one of the most abundant excitatory neurotransmitters in the mammalian brain, but its distribution in other vertebrates has not yet been well characterized. We investigated the distribution of ASP in the brainstem and rostral spinal cord of the adult sea lamprey by using ASP immunohistochemistry. Our results indicate that ASP is accumulated in specific neurons, but not in glia (tanycytes). ASP-immunoreactive neuronal populations were rather similar as the glutamatergic populations reported in the adult sea lamprey (Villar-Cerviño et al. [2013] J Comp Neurol 521:522-557), although some important differences were noted. Characteristically, the largest reticular neurons of the lamprey brainstem (Müller cells) showed ASP immunoreactivity in perikarya and processes, in contrast to the absence or faint glutamate immunoreactivity reported in these perikarya. We also compared the distribution of ASP and γ-aminobutyric acid (GABA) in brainstem neurons by using double immunofluorescence methods. In regions such as the midbrain tectum, dorsal isthmus, and motor nuclei, ASP and GABA immunoreactivity was mostly located in different neurons, whereas in other nuclei (torus semicircularis, octavolateralis area, parvocellular reticular formation), many of the ASP-immunonegative neurons displayed colocalization with GABA. These results, together with those of our previous studies of colocalization of glutamate and GABA, suggest that some lamprey neurons may co-release both excitatory and inhibitory neurotransmitters. Further investigation is needed to elucidate the pathways of uptake and release of ASP by ASP-immunoreactive neurons. Our results indicate that ASP is a neurotransmitter in the central nervous system representative of agnathans, the earliest vertebrate group. J. Comp. Neurol. 522:1209-1231, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

ABSTRACT The retina contains at least 30 different types of amacrine cells but not many are well characterized. In the present study the calcium-binding protein secretagogin was localized in a population of regular and displaced amacrine cells in the retina of the common marmoset Callithrix jacchus. Irrespective of their soma location, the dendrites of secretagogin amacrine cells occupy strata 2, 3, and 4 of the inner plexiform layer, between the two bands formed by cholinergic amacrine cells. Segretagogin amacrine cells are also immunopositive to antibodies against glutamic acid decarboxylase, suggesting that they use γ-aminobutyric acid (GABA) as their neurotransmitter. The spatial density of secretagogin amacrine cells decreases from a peak of about 400 cells/mm2 near 1 mm eccentricity to less than 100 cells/mm2 in peripheral retina; these densities account for about 1% of amacrine cells in the inner nuclear layer and for up to 27% of displaced amacrine cells. The cell bodies form a regular mosaic, suggesting that they constitute a single amacrine cell population. Secretagogin cells have varicose dendrites, which are decorated with small spines. Intracellular injection of DiI into secretagogin cells revealed an average dendritic field diameter of 170 μm and an average coverage factor of 3.2. In summary, secretagogin cells in marmoset retina are medium-field amacrine cells that share their stratification pattern with narrow-field amacrine cells and their neurotransmitter with wide-field amacrine cells. They may mediate spatial inhibition spanning the centralmost (on and off) bands of the inner plexiform layer. J. Comp. Neurol. 522:435-455, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Mice lacking the Dlx1 and Dlx2 homeobox genes (Dlx1/2 mutants) have severe deficits in subpallial differentiation, including overexpression of the Gsx1 and Gsx2 homeobox genes. To investigate whether Gsx overexpression contributes to the Dlx1/2 mutant phenotypes, we made compound loss-of-function mutants. Eliminating Gsx2 function from the Dlx1/2 mutants rescued the increased expression of Ascl1 and Hes5 (Notch signaling mediators) and Olig2 (oligodendrogenesis mediator). In addition, Dlx1/2;Gsx2 mutants, like Dlx1/2;Ascl1 mutants, exacerbated the Gsx2 and Dlx1/2 patterning and differentiation phenotypes, particularly in the lateral ganglionic eminence (LGE) caudal ganglionic eminence (CGE), and septum, including loss of GAD1 expression. On the other hand, eliminating Gsx1 function from the Dlx1/2 mutants (Dlx1/2;Gsx1 mutants) did not severely exacerbate their phenotype; on the contrary, it resulted in a partial rescue of medial ganglionic eminence (MGE) properties, including interneuron migration to the cortex. Thus, despite their redundant properties, Gsx1 and -2 have distinct interactions with Dlx1 and -2. Gsx2 interaction is strongest in the LGE, CGE, and septum, whereas the Gsx1 interaction is strongest in the MGE. From these studies, and earlier studies, we present a model of the transcriptional network that regulates early steps of subcortical development. J. Comp. Neurol. 521:1561-1584, 2013. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Glutamate is the major excitatory neurotransmitter in vertebrates, and glutamatergic cells probably represent a majority of neurons in the brain. Physiological studies have demonstrated a wide presence of excitatory (glutamatergic) neurons in lampreys. The present in situ hybridization study with probes for the lamprey vesicular glutamate transporter (VGLUT) provides an anatomical basis for the general distribution and precise localization of glutamatergic neurons in the sea lamprey brainstem. Most glutamatergic neurons were found within the periventricular gray layer throughout the brainstem, with the following regions being of particular interest: the optic tectum, torus semicircularis, isthmus, dorsal and medial nuclei of the octavolateral area, dorsal column nucleus, solitary tract nucleus, motoneurons, and reticular formation. The reticular population revealed a high degree of cellular heterogeneity including small, medium-sized, large, and giant glutamatergic neurons. We also combined glutamate immunohistochemistry with neuronal tract-tracing methods or γ-aminobutyric acid (GABA) immunohistochemistry to better characterize the glutamatergic populations. Injection of Neurobiotin into the spinal cord revealed that retrogradely labeled small and medium-sized cells of some reticulospinal-projecting groups were often glutamate-immunoreactive, mostly in the hindbrain. In contrast, the large and giant glutamatergic reticulospinal perikarya mostly lacked glutamate immunoreactivity. These results indicate that glutamate immunoreactivity did not reveal the entire set of glutamatergic populations. Some spinal-projecting octaval populations lacked both VGLUT and glutamate. As regards GABA and glutamate, their distribution was largely complementary, but colocalization of glutamate and GABA was observed in some small neurons, suggesting that glutamate immunohistochemistry might also detect non-glutamatergic cells or neurons that co-release both GABA and glutamate. J. Comp. Neurol. 521:522-557, 2013. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]