Colocalization of gamma-aminobutyric acid immunoreactivity and acetylcholinesterase activity in nerve fibers of the mouse adrenal gland

Modulation of spontaneous intracellular Ca²⁺ fluctuations and spontaneous cholinergic transmission in rat chromaffin cells in situ by endogenous GABA acting on GABAA receptors

Using fluorescence [Ca(2+)]i imaging in rat adrenal slices, we characterized the effects of agonists and antagonists of the GABAA receptor (GABAA-R) on resting intracellular Ca(2+) ([Ca(2+)]i) and spontaneous [Ca(2+)]i fluctuations (SCFs) in hundreds of individual chromaffin cells (CCs) recorded simultaneously in situ. Muscimol, a GABAA-R agonist (20 μM; 25 s), induced an increase of resting [Ca(2+)]i in 43 ± 3 % of CCs, a decrease in 26 ± 2 %, and no response in 30 ± 5 %. In Ca(2+)-free external medium, SCFs ceased completely and muscimol failed to elicit [Ca(2+)]i rises. All muscimol-induced [Ca(2+)]i changes were blocked by the GABAA-R antagonist bicuculline, suggesting that they result from changes in membrane potential depending on the cell's Cl(-) equilibrium potential. Unexpectedly, bicuculline increased the amplitude and frequency of SCFs in 54 % of CCs, revealing a tonic inhibition of SCFs by ambient GABA acting through GABAA-R. Mecamylamine (a specific nicotinic cholinergic blocker) decreased basal SCF activity in 18 % of CCs and inhibited bicuculline-induced SCF intensification, suggesting that spontaneous acetylcholine (ACh) release from nerve endings contributes to SCF generation in CCs in situ and that blockade of presynaptic GABAA-Rs intensifies SCFs in part through the disinhibition of spontaneous cholinergic transmission. Electrophysiological experiments confirmed that spontaneous excitatory postsynaptic currents recorded from CCs in situ were enhanced by bicuculline. To our knowledge, this is the first description of a regulatory effect of endogenous GABA on synaptic currents and SCFs of adrenal CCs. These findings denote a novel GABA-mediated presynaptic and postsynaptic regulatory mechanism of CC activity which may participate in the control of catecholamine secretion.

Expression and localization of pChAT as a novel method to study cholinergic innervation of rat adrenal gland

Cholinergic innervation of the rat adrenal gland has been analyzed previously using cholinergic markers including acetylcholinesterase (AChE), choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT). In the present study, we demonstrate putative cholinergic neurons in the rat adrenal gland using an antibody to pChAT, which is the product of a splice variant of ChAT mRNA that is preferentially localized in peripheral cholinergic nerves. Most of the ganglionic neurons as well as small single sporadic neurons in the adrenal gland were stained intensely for pChAT. The density of pChAT-immunoreactive (IR) fibers was distinct in the adrenal cortex and medulla. AChE-, cChAT- and VAChT-immunoreactivities were also observed in some cells and fibers of the adrenal medulla, while the cortex had few positive nerve fibers. These results indicate that ganglionic neurons of the adrenal medulla and nerve fibers heterogeneously express cholinergic markers, especially pChAT. Furthermore, the innervation of the adrenal gland, cortex and medulla, by some cholinergic fibers provides additional morphological evidence for a significant role of cholinergic mechanisms in adrenal gland functions.

Nicotinic receptor Alpha7 expression during mouse adrenal gland development

The nicotinic acetylcholine receptor alpha 7 (α7) is a ligand-activated ion channel that contributes to a diversity of cellular processes involved in development, neurotransmission and inflammation. In this report the expression of α7 was examined in the mouse developing and adult adrenal gland that expresses a green fluorescent protein (GFP) reporter as a bi-cistronic extension of the endogenous α7 transcript (α7(G)). At embryonic day 12.5 (E12.5) α7(G) expression was associated with the suprarenal ganglion and precursor cells of the adrenal gland. The α7(G) cells are catecholaminergic chromaffin cells as reflected by their progressive increase in the co-expression of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) that is complete by E18.5. In the adult, α7(G) expression is limited to a subset of chromaffin cells in the adrenal medulla that cluster near the border with the adrenal cortex. These chromaffin cells co-express α7(G), TH and DBH, but they lack phenylethanolamine N-methyltransferase (PNMT) consistent with only norepinephrine (NE) synthesis. These cell groups appear to be preferentially innervated by pre-ganglionic afferents identified by the neurotrophin receptor p75. No afferents identified by beta-III tubulin, neurofilament proteins or p75 co-expressed α7(G). Occasional α7(G) cells in the pre-E14.5 embryos express neuronal markers consistent with intrinsic ganglion cells and in the adult some α7(G) cells co-express glutamic acid decarboxylase. The transient expression of α7 during adrenal gland development and its prominent co-expression by a subset of NE chromaffin cells in the adult suggests that the α7 receptor contributes to multiple aspects of adrenal gland development and function that persist into adulthood.

Expression and distribution of GABA and GABAB-receptor in the rat adrenal gland

The inhibitory effects of gamma-aminobutyric acid (GABA) in the central and peripheral nervous systems and the endocrine system are mediated by two different GABA receptors: GABAA-receptor (GABAA-R) and GABAB-receptor (GABAB-R). GABAA-R, but not GABAB-R, has been observed in the rat adrenal gland, where GABA is known to be released. This study sought to determine whether both GABA and GABAB-R are present in the endocrine and neuronal elements of the rat adrenal gland, and to investigate whether GABAB-R may play a role in mediating the effects of GABA in secretory activity of these cells. GABA-immunoreactive nerve fibers were observed in the superficial cortex. Some GABA-immunoreactive nerve fibers were found to be associated with blood vessels. Double-immunostaining revealed GABA-immunoreactive nerve fibers in the cortex were choline acetyltransferase (ChAT)-immunonegative. Some GABA-immunoreactive nerve fibers ran through the cortex toward the medulla. In the medulla, GABA-immunoreactivity was seen in some large ganglion cells, but not in the chromaffin cells. Double-immunostaining also showed GABA-immunoreactive ganglion cells were nitric oxide synthase (NOS)-immunopositive. However, neither immunohistochemistry combined with fluorescent microscopy nor double-immunostaining revealed GABA-immunoreactivity in the noradrenaline cells with blue-white fluorescence or in the adrenaline cells with phenylethanolamine N-methyltransferase (PNMT)-immunoreactivity. Furthermore, GABA-immunoreactive nerve fibers were observed in close contact with ganglion cells, but not chromaffin cells. Double-immunostaining also showed that the GABA-immunoreactive nerve fibers were in close contact with NOS- or neuropeptide tyrosine (NPY)-immunoreactive ganglion cells. A few of the GABA-immunoreactive nerve fibers were ChAT-immunopositive, while most of the GABA-immunoreactive nerve fibers were ChAT-immunonegative. Numerous ChAT-immunoreactive nerve fibers were observed in close contact with the ganglion cells and chromaffin cells in the medulla. The GABAB-R-immunoreactivity was found only in ganglion cells in the medulla and not at all in the cortex. Immunohistochemistry combined with fluorescent microscopy and double-immunostaining showed no GABAB-R-immunoreactivity in noradrenaline cells with blue-white fluorescence or in adrenaline cells with PNMT-immunoreactivity. These immunoreactive ganglion cells were NOS- or NPY-immunopositive on double-immunostaining. These findings suggest that GABA from the intra-adrenal nerve fibers may have an inhibitory effect on the secretory activity of ganglion cells and cortical cells, and on the motility of blood vessels in the rat adrenal gland, mediated by GABA-Rs.

GABA(A) Receptors: Post-Synaptic Co-Localization and Cross-Talk with Other Receptors

γ-Aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory neurotransmitter receptors in the central nervous system, and importantly contribute to the functional regulation of the nervous system. Several studies in the last few decades have convincingly shown that GABA can be co-localized with other neurotransmitters in the same synapse, and can be co-released with these neurotransmitters either from the same vesicles or from different vesicle pools. The co-released transmitters may act on post-synaptically co-localized receptors resulting in a simultaneous activation of both receptors. Most of the studies investigating such co-activation observed a reduced efficacy of GABA for activating GABA_ARs and thus, a reduced inhibition of the post-synaptic neuron. Similarly, in several cases activation of GABA_ARs has been reported to suppress the response of the associated receptors. Such a receptor cross-talk is either mediated via a direct coupling between the two receptors or via the activation of intracellular signaling pathways and is used for fine tuning of inhibition in the nervous system. Recently, it was demonstrated that a direct interaction of different receptors might already occur in intracellular compartments and might also be used to specifically target the receptors to the cell membrane. In this article, we provide an overview on such cross-talks between GABA_ARs and several other neurotransmitter receptors and briefly discuss their possible physiological and clinical importance.

Immunohistochemical study on GABAergic system in salivary glands

Gamma-aminobutyric acid (GABA) and its receptors are found in the central nervous system and several peripheral tissues. The purpose of this study was to determine the expression and distribution of GABA and glutamate decarboxylase (GAD), a GABA biosynthetic enzyme, in rat salivary gland. Western blot and real time quantitative RT-PCR revealed that GAD67 was the major isoform of GAD in the salivary glands. Furthermore, both GABA and GAD were detected around the acinar cells in the submandibular glands by immunohistochemical analysis. When both sympathetic and parasympathetic nerves related to the submandibular glands were denervated, the immunoreactivities of GABA and GAD were dramatically depressed, and levels of GAD67 and GABA significantly decreased. However, no morphological changes in the glands were observed after denervation. These results indicate that GAD67 is present around acinar cells in the salivary glands, and suggest that the GABAergic system in the glands is closely related to the autonomic nervous system.

Distribution of gamma-aminobutyric acid receptors in cultured adrenergic and noradrenergic bovine chromaffin cells

Fluorescence imaging techniques for recording cytosolic [Ca(2+)](i) from single chromaffin cells were used to characterize and discriminate between cell subpopulations containing gamma-aminobutyric acid (GABA)(A) and GABA(B) receptor subtypes. By combining this methodology with the immunoidentification of individual chromaffin cells using specific antibodies against tyrosine hydroxylase (TH), phenyl-etanolamine-N-methyl transferase (PNMT), and glutamic acid decarboxylase (GAD) linked to different fluorescent probes, we have been able to ascribe single-cell calcium responses to identified adrenergic and noradrenergic chromaffin cells. GAD enzyme is present in 30% of the chromaffin cell population, located primarily in adrenergic cells; 86% of GAD(+) cells were also PNMT(+). GAD expression was not correlated with the presence of GABA receptors. GABA-responsive cells were found with equal frequency in the GAD(+) and GAD(-) groups. However, the expression of GABA receptors was correlated with the adrenergic phenotype. [Ca(2+)](i) responses to GABA were found more frequently in adrenergic than in noradrenergic cells. GABA(A) receptors are more evenly distributed; about 90% of GABA-responsive cells have them. GABA(B) receptors have a more restricted distribution (present in 45% of responding cells). The coexpression of both GABA(A) and GABA(B) subtypes is the rule; only a minor subpopulation (about 12%) displays exclusively GABA(B) receptors. GABA receptor subtypes are distributed in a similar way when chromaffin cells are separated according to GAD(+)/GAD(-) or PNMT(+)/PNMT(-) classifications, with only minor differences. These data indicate that the intrinsic GABAergic system in the adrenal medulla is not designed as a paracrine model in which a group of cells specializes in transmitter synthesis and a different group serves as a specific target.

Olivocochlear innervation in the mouse: immunocytochemical maps, crossed versus uncrossed contributions, and transmitter colocalization

To further understand the roles and origins of gamma-aminobutyric acid (GABA) and calcitonin gene-related peptide (CGRP) in the efferent innervation of the cochlea, we first produced in the mouse an immunocytochemical map of the efferent terminals that contain acetylcholine (ACh), CGRP, and GABA. Olivocochlear (OC) terminals in inner and outer hair cell (IHC and OHC) regions were analyzed quantitatively along the cochlear spiral via light-microscopic observation of cochlear wholemounts immunostained with antibodies to glutamic acid decarboxylase (GAD), vesicular acetylcholine transporter (VAT), or the peptide CGRP. Further immunochemical characterization was performed in mice with chronic OC transection at the floor of the fourth ventricle to distinguish crossed from uncrossed contributions and, indirectly, the contributions of lateral versus medial components of the OC system. The results in mouse showed that (1) there are prominent GABAergic, cholinergic, and CGRPergic innervations in the OHC and IHC regions, (2) GABA and CGRP are extensively colocalized with ACh in all OC terminals in the IHC and OHC areas, (3) the longitudinal gradient of OC innervation peaks roughly at the 10-kHz region in the OHC area and is more uniform along the cochlear spiral in the IHC area, (4) in contrast to other mammalian species there is no radial gradient of OC innervation of the OHCs, and (5) all OHC efferent terminals arise from the medial OC system and terminals in the IHC area arise from the lateral OC system.