Immunocytochemical study of the GABAergic innervation of the mouse pituitary by use of antibodies against gamma-aminobutyric acid (GABA)

Distribution of GABA-immunoreactive neurons in the forebrain of the goldfish, Carassius auratus

The distribution of gamma-aminobutyric acid (GABA) immunoreactivity was studied in the forebrain (tel- and diencephalon) of the goldfish by means of immunocytochemistry on Vibratome sections using antibodies against GABA. Positive perikarya were detected in the olfactory bulbs and in all divisions of the telencephalon, the highest density being found along the midline. In the diencephalon, GABA-containing cell bodies were found in the hypothalamus, in particular in the preoptic and tuberal regions. The inferior lobes, the nucleus recessus lateralis, and more laterodorsal regions, such as the nucleus glomerulosus and surrounding structures, also exhibited numerous GABA-positive perikarya. Cell bodies were also noted in the thalamus, in particular in the dorsomedial, dorsolateral and ventromedial nuclei. The relative density of immunoreactive fibers was evaluated for each brain nucleus and classified into five categories. This ubiquitous distribution indicates that, as in higher vertebrates, GABA most probably represents one of the major neurotransmitters in the brain of teleosts.

Long-lasting intrinsic optical changes observed in the neurointermediate lobe of the mouse pituitary reflect volume changes in cells of the pars intermedia

BACKGROUND/AIMS

Complex intrinsic optical changes (light scattering) are readily observed in the neurointermediate lobe of the mouse pituitary gland following electrical stimulation of the infundibular stalk. Our laboratory has previously identified three distinct phases within the light scattering signal: two rapid responses to action potential stimulation and a long duration recovery. The rapid light scattering signals, restricted to the neurohypophysial portion (posterior pituitary) of the neurointermediate lobe, consist of an E-wave and an S-wave that reflect excitation and secretion, respectively. The E-wave has the approximate shape of the action potential and includes voltage- and current-related components and is independent of Ca(2+) entry. The S-wave is related to Ca(2+) entry and exocytosis. The slow recovery phase of the light scattering signal, which we designated the R-wave, is less well characterized.

METHODS

Using high temporal resolution light scattering measurements, we monitored intrinsic optical changes in the neurointermediate lobe of the mouse pituitary gland. Pharmacological interventions during the measurements were employed.

RESULTS

The data presented here provide optical and pharmacological evidence suggesting that the R-wave, which comprises signals from the posterior pituitary as well as from the pars intermedia, mirrors volume changes in pars intermedia cells following a train of stimuli applied to the infundibular stalk. These volume changes were blocked by the GABA-receptor antagonists bicuculline and picrotoxin, and were mimicked by direct application of GABA in the absence of electrical stimulation.

CONCLUSIONS

These results emphasize the importance of central GABAergic projections into the neurointermediate lobe, and the potential role of GABA in effecting hormone release from the pars intermedia.

Differential sensitivity of excitatory and inhibitory synaptic transmission to modulation by nitric oxide in rat nucleus tractus solitarii

The nucleus tractus solitarii (NTS) is a key central link in control of multiple homeostatic reflexes. A number of studies have demonstrated that exogenous and endogenous nitric oxide (NO) within NTS regulates visceral function, but further understanding of the role of NO in the NTS is hampered by the lack of information about its intracellular actions. We studied effects of NO in acute rat brainstem slices. Aqueous NO solution (NO(aq)) potentiated electrically evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different neuronal subpopulations and, in some neurones, caused a depolarization. Similar effects were observed using the NO donor diethylamine NONOate (DEA/NO). The threshold NO concentration as determined using an NO electrochemical sensor was estimated as approximately 0.4 nm (EC(50) approximately 0.9 nm) for potentiating glutamatergic EPSPs but approximately 3 nm for monosynaptic GABAergic IPSPs. Bath application of the soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) abolished NO(aq)- and DEA/NO-induced potentiation of evoked EPSPs, IPSPs and depolarization. All NO actions were mimicked by the non-NO-dependent guanylate cyclase activator Bay 41-2272. The effects of NO on EPSPs and IPSPs persisted in cells where postsynaptic sGC was blocked by ODQ and therefore were presynaptic, owing to a direct modulation of transmitter release combined with depolarization of presynaptic neurones. Therefore, while lower concentrations of NO may be important for fine tuning of glutamatergic transmission, higher concentrations are required to directly engage GABAergic inhibition. This differential sensitivity of excitatory and inhibitory connections to NO may be important for determining the specificity of the effects of this freely diffusible gaseous messenger.

Intracellular distribution of GABA in the rat anterior pituitary. An electron microscopic autoradiographic study

We studied the internalization and intracellular distribution of [3H] GABA in rat anterior pituitary cells. Electron microscopic autoradiography of anterior pituitary fragments or dispersed pituitary cells incubated with [3H] GABA showed that lactotrophs and, to a lesser extent, somatotrophs were the only cells that contained radioactive grains. Grain density analysis performed on dispersed pituitary cells after a pulse-chase experiment (10 min pulse and then change to a medium without radioactive GABA for various periods up to 2 h) revealed that GABA internalized by lactotrophs was distributed in various intracellular membranous organelles. Of the cell compartments examined, plasma membrane, Golgi apparatus, mitochondria and secretory granules had different time-dependent labeling patterns. The highest grain density values were associated with plasma membrane (at the first chase time) and the Golgi apparatus. Mitochondria and secretory granules also showed significant grain density values. A similar pattern of distribution was observed when fragments of prolactin-secreting pituitary adenomas were incubated with [3H] GABA. These results provide morphological data on the cellular specificity and intracellular distribution of GABA in anterior pituitary cells.

Distribution of GAD-immunoreactive neurons in the diencephalon of the african lungfish Protopterus annectens: colocalization of GAD and NPY in the preoptic area

The distribution of GABAergic neurons was investigated in the diencephalon of the African lungfish, Protopterus annectens, by using specific antibodies directed against glutamic acid decarboxylase (GAD). A dense population of immunoreactive perikarya was observed in the periventricular preoptic nucleus, whereas the caudal hypothalamus and the dorsal thalamus contained only scattered positive cell bodies. Clusters of GAD-positive cells were found in the intermediate lobe of the pituitary. The diencephalon was richly innervated by GAD-immunoreactive fibers that were particularly abundant in the hypothalamus. In the periventricular nucleus, GAD-positive fibers exhibited a radial orientation, and a few neurons extended processes toward the third ventricle. More caudally, a dense bundle of GAD-immunoreactive fibers coursing along the ventral wall of the hypothalamus terminated into the median eminence and the neural lobe of the pituitary. Double-labeling immunocytochemistry revealed that GAD and neuropeptide tyrosine (NPY)-like immunoreactivity was colocalized in a subpopulation of perikarya in the periventricular preoptic nucleus. The proportion of neurons that coexpressed GAD and NPY was higher in the caudal region of the preoptic nucleus. The distribution of GAD-immunoreactive elements in the diencephalon and pituitary of the African lungfish indicates that GABA may act as a hypophysiotropic neurohormone in Dipnoans. The coexistence of GAD and NPY in a subset of neurons of the periventricular preoptic nucleus suggests that GABA and NPY may interact at the synaptic level.

Autoradiographic distribution of radioactivity from (14)C-GABA in the mouse

We investigated the distribution of radioactivity from (14)C-labeled gamma-aminobutyric acid (GABA) in the mouse by in vivo autoradiography to clarify the tissues that show GABA uptake and/or GABA binding. Male mice were injected intravenously with (14)C-GABA in both the absence and presence of an excess of unlabeled GABA, baclofen and isoguvacine. Whole-body autoradiography of (3)H-baclofen, a GABA(B) receptor agonist was also performed. At short intervals after (14)C-GABA injection ( 3 and 6 minutes), very high radioactivity was detected in the kidney cortex, liver, pineal gland, hypophysis, median eminence of the hypothalamus, and cervical ganglion. The hyaline cartilage and glandular part of the stomach showed moderate radioactivity. In the presence of an excess amount of unlabeled GABA, radioactivity in most of tissues decreased significantly, but no significant difference in radioactivity was observed in the presence of baclofen and isoguvacine, agonists of GABA(A) and GABA(B) receptors, respectively. Autoradiography of (3)H-baclofen showed that the kidney had high level of radioactivity, whereas the activity in other tissues and organs was similar or lower than in the blood except for the content of the urinary bladder and the pancreas at 15 minutes after injection. These data indicate that radioactivity from incorporated (14)C-GABA into a variety of cells is much higher than that from bound (14)C-GABA to the receptor sites. Our results suggest that GABA can be quickly localized in many organs of the mouse body after 3 minutes following injection, and GABA may serve multiple functions in those organs.

Differential innervation of individual melanotropes suggests a role for nonsynaptic inhibitory regulation of the developing and adult rat pituitary intermediate lobe

Dopamine and GABA were detected in intermediate lobe axons around birth, and early axons were closely apposed to glial cells and processes, possibly using them for guidance. In the adult, axons containing colocalized dopamine and GABA were distributed in a distinct pattern within the lobe, with plexuses located dorsally and ventrally. Axons preferentially followed glial processes in interlobular septa, yet were also interspersed between melanotropes. Individual melanotropes were contacted by varying numbers of axon terminals, with some devoid of contacts. Boutons contained both small clear vesicles and large dense-cored vesicles; membrane specializations were not well-developed. From these findings we concluded that in addition to direct synaptic inhibition, dopamine and GABA could stimulate their receptors by mechanisms similar to "parasynaptic" [Schmitt (1984) Neuroscience, 13:991-1001] or "volume" [Agnati et al. (1995) Neuroscience, 69:711-726] transmission as proposed for the CNS. Humoral agents passing into the intermediate lobe from portal vessels, thus acting as classical hormones, further regulate the melanotropes. Moreover, approximately 50% of the axonal elements were closely apposed to glia, suggesting that glia could have regulatory roles. Previous studies from our laboratory [Chronwall et al. (1987) Endocrinology, 120:1201-1211; Chronwall et al. (1988) Endocrinology, 123:1992:1202] demonstrated heterogeneity in proopiomelanocortin (POMC) biosynthesis among individual melanotropes, prompting the hypothesis that the degree of innervation could govern the expression of certain molecules. We combined immunohistochemistry and in situ hybridization histochemistry to evaluate whether melanotrope molecular heterogenity is spatially correlated with axons and terminals. Tentatively, melanotropes expressing low levels of POMC and alpha1A subunit P/Q type Ca2+ channel mRNAs often were apposed to axons, whereas those with low levels of D2L receptor mRNA rarely were contacted by axons, suggesting that innervation could be one of the factors inducing and maintaining heterogeneity.

The distribution of GABA-immunoreactive neurons in the brain of the silver eel (Anguilla anguilla L.)

The distribution of GABA-immunoreactivity was studied in the brain of the silver eel (Anguilla anguilla) by means of antibodies directed against GABA. Immunoreactive neuronal somata were distributed throughout the brain. Positive perikarya were detected in the internal cellular layer of the olfactory bulb, and in all divisions of the telencephalon, the highest density being observed along the midline. Numerous GABA-reactive cell bodies were found in the diencephalon, particularly in the preoptic and tuberal regions of the hypothalamus, and the dorsolateral, dorsomedial and ventromedial thalamic nuclei. In the optic tectum, the majority of GABA-positive cell bodies were located in the periventricular layer. A number of immunolabeled cell bodies were observed in different tegmental structures, notably the torus semicircularis. In the cerebellum, the Purkinje cells were either very intensely or very weakly immunoreactive. In the rhombencephalon, reactive cell bodies were observed in the eminentia granularis, the valvula cerebellaris, the octavolateral nucleus, the lobus vagus and in the vagal and glossopharyngeal motor nuclei. Intensely immunoreactive axons and terminals were observed in the external granular layer and internal cellular layer of the olfactory bulb. In the telencephalon, the highest density of reactive fibres and boutons was found in the fields of the medial wall. Many immunolabeled fibres were seen in the medial and lateral forebrain bundles. In the diencephalon, intense labelling of fibres and terminals were observed in the nuclei situated close to the midline. In the optic tectum the highest density of reactive fibres was seen in the sfgs, the layer to which the retina projects massively. Finally, in the rhombencephalon the strongest labelling of neurites was observed in the nuclei of the raphé, the nucleus octavocellularis magnocellularis and the nuclei of the IXth and Xth cranial nerves. The GABAergic system of the eel, which is well developed, appears to be generally comparable to that described in tetrapod vertebrates.

Ultrastructural localization of galanin immunoreactivity in the rat median eminence

The aim of this study was to examine, by use of a pre-embedding immunoperoxidase technique, the ultrastructural localization of galanin immunoreactivity in the external layer of the rat median eminence. Galanin immunoreactivity was only observed in axonal profiles. Immunoreactive fibers were found in contact with the following non-immunoreactive structures: (1) axonal profiles that contain dense granular vesicles and clear vesicles, (2) axonal profiles that contain predominantly clear vesicles, (3) glial cell bodies, and (4) processes of tanycytes. Labeled terminals were also observed in the proximity of the perivascular space of the portal vessels. The results suggest possible interactions between galanin-immunoreactive terminals and other terminals containing peptide and/or other transmitters in the external layer of the median eminence.

An immunoelectron microscopic study of peptide substances in the rat neurohypophysis

The ultrastructural localization of peptide substances, such as vasopressin (VP), substance P (SP), enkephalin (ENK) and somatostatin (SRIF), and the distribution of peptidergic nerves in rat neurohypophysis were studied by immunoelectron microscopy. The results show that VP, SP, ENK and SRIF immunoreactive products are located in large granular vesicles (110-150 nm in diameter), at the periphery of microvesicles and on the outer membrane of mitochondria. The VP-, SP-, ENK- and SRIF- containing nerve fibers are distributed at the periphery of capillaries and in the vicinity of pituicytes. VP- and ENK-positive nerve terminals form axo-axonic synapses with negative terminals. VP- and ENK-terminals are pre- or postsynaptic elements. Axo-axonic synapses formed by two SRIF- positive terminals were also found. In addition, ENK- and SP-positive terminals with pituicytes form synaptoid structures respectively. The authors have discovered VP-positive pituicytes for the first time.

The distribution of GABA-like-immunoreactive neurons in the brain of the newt, Triturus cristatus carnifex, and the green frog, Rana esculenta

The distribution of gamma-aminobutyric acid (GABA) immunoreactivity was studied in the brain of two amphibian species (Triturus cristatus carnifex, Urodela; Rana esculenta, Anura) by employing a specific GABA antiserum. A noteworthy immunoreactive neuronal system was found in the telencephalic dorsal and medial pallium (primordium pallii dorsalis and primordium hippocampi) and in the olfactory bulbs. In the diencephalic habenular nuclei there was a rich GABAergic innervation, and immunoreactive neurons were observed in the dorsal thalamus. In the hypothalamus the GABA immunoreactivity was found in the preoptic area, the paraventricular organ and in the hypothalamo-hypophysial complex. In the preoptic area of the frog some GABA-immunoreactive CSF-contacting cells were shown. In the optic tectum immunolabeled neurons were present in all the cellular layers. A rich GABAergic innervation characterized both the fibrous layers of the tectum and the neuropil of the tegmentum and interpeduncular nucleus. In the cerebellum, in addition to the Purkinje cells showing a variable immunopositivity, some immunoreactive cells bodies appeared in the central grey. Abundant immunolabeled nerve fibers in the acoustico-lateral area and some immunopositive neurons in the region of the raphe nucleus were observed. In conclusion, the GABAergic central systems, well-developed in the amphibian species studied, were generally characterized by close similarities to the pattern described in mammals.

Immunohistochemical localization of gamma-aminobutyric acid in the rat pituitary gland and related hypothalamic regions

The distribution of gamma-aminobutyric acid (GABA) containing neurons in the rat pituitary gland and related hypothalamic areas was immunohistochemically investigated using antibodies raised against GABA conjugated to bovine serum albumin by glutaraldehyde. A dense network of GABA-like immunoreactive fine varicose nerve fibers was observed within the posterior and intermediate lobes of the pituitary gland, surrounding endocrine cells and capillaries, but not in the anterior lobe. In the pituitary stalk, the dense varicose fibers ran along the anterior wall of the posterior lobe into the posterior and intermediate lobes. A small number of GABA-like immunoreactive cell bodies were evident in the intermediate lobe. GABA-like immunoreactive fibers occurred at low to high density in most parts of the hypothalamus. GABA-like immunoreactive neurons were observed in some regions related to the pituitary gland (such as periventricular nucleus, paraventricular nucleus, arcuate nucleus and accessory magnocellular nucleus). These results provide morphological evidence for the presence of GABAergic neurons in the rat hypothalamo-pituitary system.

Abundant GABAergic innervation of rat posterior pituitary revealed by inhibition of GABA-transaminase

An antibody against gamma-aminobutyric acid (GABA) was used to identify GABAergic elements immunocytochemically in the rat posterior pituitary. In order to increase the intracellular concentration of GABA, rats were treated with the GABA-transaminase inhibitor gamma-vinyl-GABA (GVG). Light-microscopic observations of Vibratome and semithin sections revealed the presence of numerous immunoreactive nerve fibers throughout the neural lobe; the mean number and length of these fibers increased by 90% after GVG treatment. Electron microscopy demonstrated the immunostained axons to be of small diameter. The reaction product was confined to small vesicles. No immunostaining occurred in pituicytes. The richness of the GABAergic innervation of the neural lobe contrasts with previous reports using antibodies against glutamate decarboxylase and supports the idea that GABA participates in the presynaptic control of neurosecretion.

Central GABAergic innervation of the pituitary in goldfish: a radioautographic and immunocytochemical study at the electron microscope level

The GABAergic innervation of the goldfish pituitary was studied at the light and electron microscope levels by means of radioautography after in vitro incubation in tritiated gamma-aminobutyric acid (GABA) and immunocytochemistry using antibodies against GABA. Following incubation of pituitary fragments in a medium containing tritiated GABA, a selective uptake of the tracer was observed within the digitations of the neurohypophysis. Silver grain clusters were also observed in the adenohypophyseal tissue. At the electron microscope level, this uptake was found to correspond to nerve endings containing small clear and dense-core vesicles. These labeled profiles were located mainly in neurohypophyseal digitations in close apposition with the basement membrane separating the neurohypophysis from the adenohypophysis. However, they were also encountered in direct contact with most adenohypophyseal cell types in the different lobes. These results were confirmed by immunocytochemical data demonstrating the presence of numerous GABA immunoreactive fibers in both anterior and neurointermediate lobes. They were found either in the digitations of the neurohypophysis or in the adenohypophysis in direct contact with the glandular cells with a distribution and an ultrastructural aspect similar to those observed by radioautography. These data demonstrate that the pituitary of teleosts receives a massive GABAergic innervation. Although physiological data providing a functional significance for such an innervation are lacking, the present study suggests that, as already documented in mammals, GABA may be involved in the neuroendocrine regulation of pituitary functions in teleosts.

Colocalization of GABA and tyrosine hydroxylase immunoreactivities in the axons innervating the neurointermediate lobe of the rat pituitary: an ultrastructural immunogold study

Distribution of the gamma-aminobutyric acid (GABA)ergic and dopaminergic innervations was studied in the rat neurointermediate lobe using antibodies against GABA and tyrosine hydroxylase. In light microscopy, immunoperoxidase staining revealed similar distribution patterns of the axons reacting with both antibodies. Diffusely scattered in both lobes, they were more concentrated along the marginal zone of the neural lobe. Application of a double, recto-verso, immunogold labelling method in electron microscopy revealed systematic colocalization of GABA and tyrosine hydroxylase (TH) immunoreactivities in the axons innervating the intermediate lobe; in the neural lobe, almost all GABA-immunoreactive axons were also labelled for TH. Thus, GABA and dopamine, hitherto reported to occur in distinct axons, in fact colocalize in the axonal systems which innervate the pituitary neurointermediate lobe. These observations suggest possible interactions (pre- or postsynaptic) of both transmitters in the functional regulation of the intermediate and neural lobes.