Changes in GABA-immunoreactivity during development of the rostral subdivision of the nucleus of the solitary tract

Identification of a GABAergic neuroblast lineage modulating sweet and bitter taste sensitivity

In Drosophila melanogaster, processing of gustatory information and controlling feeding behavior are executed by neural circuits located in the subesophageal zone (SEZ) of the brain.1 Gustatory receptor neurons (GRNs) project their axons in the primary gustatory center (PGC), which is located in the SEZ.1,2,3,4 To address the function of the PGC, we need detailed information about the different classes of gustatory interneurons that frame the PGC. In this work, we screened large collections of driver lines for SEZ interneuron-specific labeling and subsequently used candidate lines to access the SEZ neuroblast lineages. We converted 130 Gal4 lines to LexA drivers and carried out functional screening using calcium imaging. We found one neuroblast lineage, TRdm, whose neurons responded to both sweet and bitter tastants, and formed green fluorescent protein (GFP) reconstitution across synaptic partners (GRASP)-positive synapses with sweet sensory neurons. TRdm neurons express the inhibitory transmitter GABA, and silencing these neurons increases appetitive feeding behavior. These results demonstrate that TRdm generates a class of inhibitory local neurons that control taste sensitivity in Drosophila.

Nucleus incertus promotes cortical desynchronization and behavioral arousal

Arousal and vigilance are essential for survival and relevant regulatory neural circuits lie within the brainstem, hypothalamus and forebrain. The nucleus incertus (NI) is a distinct site within the pontine periventricular gray, containing a substantial population of GABAergic neurons with long-range, ascending projections. Existing neuroanatomical data and functional studies in anesthetized rats, suggest the NI is a central component of a midline behavioral control network well positioned to modulate arousal, vigilance and exploratory navigation, yet none of these roles have been established experimentally. We used a chemogenetic approach-clozapine-N-oxide (CNO) activation of virally delivered excitatory hM3Dq-DREADDs-to activate the NI in rats and examined the behavioral and physiological effects, relative to effects in naïve rats and appropriate viral-treated controls. hM3Dq activation by CNO resulted in long-lasting depolarization of NI neurons with action potentials, in vitro. Peripheral injection of CNO significantly increased c-Fos immunoreactivity in the NI and promoted cortical electroencephalograph (EEG) desynchronization. These brain changes were associated with heightened arousal, and increased locomotor activity in the homecage and in a novel environment. Furthermore, NI activation altered responses in a fear conditioning paradigm, reflected by increased head-scanning, vigilant behaviors during conditioned fear recall. These findings provide direct evidence that the NI promotes general arousal via a broad behavioral activation circuit and support early hypotheses, based on its connectivity, that the NI is a modulator of cognition and attention, and emotional and motivated behaviors.

Postnatal development of chorda tympani axons in the rat nucleus of the solitary tract

The chorda tympani nerve (CT), one of three nerves that convey gustatory information to the nucleus of the solitary tract (NTS), displays terminal field reorganization after postnatal day 15 in the rat. Aiming to gain insight into mechanisms of this phenomenon, CT axon projection field and terminal morphology in NTS subdivisions were examined using tract tracing, light microscopy, and immunoelectron microscopy at four postnatal ages: P15, P25, P35, and adult. The CT axons that innervated NTS rostrolateral subdivision both in the adult and in P15 rats were morphologically distinct from those that innervated the rostrocentral, gustatory subdivision. In both subdivisions, CT terminals reached morphological maturity before P15. Rostrolateral, but not rostrocentral axons, went through substantial axonal branch elimination after P15. Rostrocentral CT synapses, however, redistribute onto postsynaptic targets in the following weeks. CT terminal preference for GABAergic postsynaptic targets was drastically reduced after P15. Furthermore, CT synapses became a smaller component of the total synaptic input to the rostrocentral NTS after P35. The results underlined that CT axons in rostrocentral and rostrolateral subdivisions represent two distinct populations of CT input, displaying different morphological properties and structural reorganization mechanisms during postnatal development.

Inhibitory neurons in the anterior piriform cortex of the mouse: classification using molecular markers

The primary olfactory cortex (or piriform cortex, PC) is attracting increasing attention as a model system for the study of cortical sensory processing, yet little is known about inhibitory neurons in the PC. Here we provide the first systematic classification of GABA-releasing interneurons in the anterior PC of mice, based on the expression of molecular markers. Our experiments used GAD67-GFP transgenic mice, in which gamma-aminobutyric acid (GABA)-containing cells are labeled with green fluorescent protein (GFP). We first confirmed, using paired whole-cell recordings, that GFP(+) neurons in the anterior PC of GAD67-GFP mice are functionally GABAergic. Next, we performed immunolabeling of GFP(+) cells to quantify their expression of every possible pairwise combination of seven molecular markers: calbindin, calretinin, parvalbumin, cholecystokinin, neuropeptide Y, somatostatin, and vasoactive intestinal peptide. We found that six main categories of interneurons could be clearly distinguished in the anterior PC, based on the size and laminar location of their somata, intensity of GFP fluorescence, patterns of axonal projections, and expression of one or more of the seven markers. A number of rarer categories of interneurons could also be identified. These data provide a road map for further work that examines the functional properties of the six main classes of interneurons. Together, this information elucidates the cellular architecture of the PC and provides clues about the roles of GABAergic interneurons in olfactory processing.

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Early GABA(A) receptor clustering during the development of the rostral nucleus of the solitary tract

While there is an abundance of gamma-aminobutyric acid (GABA) in the gustatory zone of the nucleus of the solitary tract of the perinatal rat, we know that GABAergic synapse formation is not complete until well after birth. Our recent results have shown that GABA(B) receptors are present at birth in the cells of the nucleus; however, they do not redistribute and cluster at synaptic sites until after PND10. The present study examined the time course of appearance and redistribution of GABA(A) receptors in the nucleus. GABA(A) receptors were also present at birth. However, in comparison to GABA(B) receptors, GABA(A) receptors underwent an earlier translocation to synaptic sites. Extrasynaptic label, for example, of GABA(A) receptors was non-existent compared to GABA(B) receptors at PND10 and well-defined clusters of GABA(A) receptors could be seen as early as PND1. We propose that while GABA(A), receptors may play an early neurotransmitter role at the synapse, GABA(B) receptors may play a non-transmitter neurotrophic role.

Differences in ratios of GABA, glycine and glutamate immunoreactivities in nerve terminals on rat hindlimb motoneurons: a possible source of post-synaptic variability

Previous pharmacological and physiological data on GABA and glycine receptor-dependent components of miniature inhibitory post-synaptic currents show that the electrophysiological characteristics of synaptic transmission from inhibitory synapses on spinal motoneurons are highly variable. Although post-synaptic factors are thought to be the major underlying cause of this variability, quantitative immunohistochemical data suggest that the transmitter content of afferents also vary from terminal to terminal. To examine whether ratios of amino acid staining densities vary similar to those of components of post-synaptic currents mediated by the corresponding receptors, we quantified immunogold labeling for GABA, glycine and the major excitatory transmitter, glutamate, in nerve terminals contacting the dendrites of motoneurons retrogradely labeled from the rat hindlimb muscle, biceps femoris. Nearly all terminals (94%) were immunoreactive for at least one amino acid and 64% of these contained two or three amino acids. All possible combinations of GABA, glycine and glutamate labeling were found. Over 70% of the terminals contained glycine, of which 60% also labeled for GABA. Of these GABA/glycine boutons, 40% also had glutamate. Half of all terminals contained GABA, but terminals immunoreactive for GABA alone were extremely rare. Immunoreactivity for glutamate occurred in 48% of all terminals and nearly 60% of these also contained glycine. Labeling densities for GABA, glycine and glutamate varied over a wide range from terminal to terminal. We hypothesize that this diversity in amino acid content may be a major underlying cause of variability in GABA- and glycine receptor-mediated components of miniature inhibitory post-synaptic currents in motoneurons.

Subcellular localization of GABA receptors in the central nervous system using post-embedding immunohistochemistry

The following detailed protocol can be applied to demonstrate the localization of GABA receptors in CNS neurons at the ultrastructural level. While others have investigated receptors at the electron microscopic level using immunocytochemical techniques, the appearance of the tissue is usually poor and analyses of the distribution of receptors is limited. The methodology described in this paper allows for optimal preservation of the tissue while retaining immunogenicity. It does this, in part, by utilizing a balanced salt solution washout in conjunction with fixation. When the ionic composition of a fixative solution differs from extracellular fluids, like in most fixation protocols for electron microscopy, ultrastructural changes may occur in the tissue. Balanced salt solutions, like the Tyrode solution used here, helps maintain the normal extracellular environment allowing the fixing agent to reach sufficient concentration to bring about permanent and more optimal fixation even when reduced amounts of glutaraldehyde are required to preserve antigenicity. Therefore, unlike many protocols for post-embedding immunoelectronmicroscopy, this method allows for superior preservation of tissue ultrastructure compared to results previously published by others.

Redistribution and increased specificity of GABA(B) receptors during development of the rostral nucleus of the solitary tract

Recent results show that there is an abundance of gamma-aminobutyric acid (GABA) before GABAergic synapses have formed in the gustatory zone of the nucleus of the solitary tract. These results suggest that a non-synaptic, developmental function may exist for GABA prior to synaptogenesis. However, GABA exerts its physiological effect via its receptors, the development of which is a largely unknown process. The developmental expression of one of the GABA receptors in the young nucleus of the solitary tract is the focus of this study. The development of GABA(B) receptors was investigated by light and electron microscopy. The results suggest that before the development of GABAergic synapses, GABA(B) receptors are diffusely distributed. When GABAergic synapses form, the receptors become clustered. Quantitative postembedding immunohistochemical studies at the electron microscopic level show that extrasynaptic labeling for GABA(B) receptors decreases during development, but synaptic labeling increases. Increased specificity of neurotransmitter receptors at synapses has been shown in other systems during development, including other central nervous system structures, but this may be the first demonstration of the phenomenon using quantitative electron microscopy.

Patterns of colocalization of GABA, glutamate and glycine immunoreactivities in terminals that synapse on dendrites of noradrenergic neurons in rat locus coeruleus

Amino acid transmitters play a key role in regulating the activity of noradrenergic neurons in the locus coeruleus. We investigated the anatomical substrate for this regulation by quantifying immunoreactivity for GABA, glutamate and glycine in terminals that contacted the dendrites of tyrosine hydroxylase-immunoreactive principal neurons in rat locus coeruleus. Pre-embedding peroxidase immunocytochemistry was used to detect tyrosine hydroxylase-immunoreactivity in Vibratome sections of tissue perfused with 2.5% glutaraldehyde. GABA, glutamate and glycine were localized with postembedding immunogold labelling. Gold particle densities over terminals were measured in three semiserial ultrathin sections, each reacted for a different amino acid. More than 90% (range among rats, 89%-95%) of the terminals analyzed (n = 288) were immunoreactive for at least one amino acid. A high proportion (39%-49%) were positive for two or three amino acids. About two-thirds (60%-69%) of the boutons contained GABA, of which more than half (51%-55%) also contained glycine. More than one-third (36%-38%) of the terminals were positive for glycine. Terminals immunoreactive for glycine alone were rare (0%-2%). About one-third of the terminals showed glutamate-immunoreactivity (32%-37%). GABA and/or glycine occurred in one-fifth to one-third of these. These results show that amino acid-immunoreactivity is present in almost all of the terminals that synapse on tyrosine hydroxylase-positive dendrites in locus coeruleus. Glutamate provides a major excitatory input. The almost complete colocalization of glycine with GABA suggests that the inhibitory input to locus coeruleus is predominantly GABAergic with a contribution from glycine in about half of the GABAergic boutons.