Localization of L-glutamate decarboxylase immunoreactivity in the major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex of the rat

GABA and neuroactive steroid interactions in glia: new roles for old players?

In recent years it has becoming clear that glial cells of the central and peripheral nervous system play a crucial role from the earliest stages of development throughout adult life. Glial cells are important for neuronal plasticity, axonal conduction and synaptic transmission. In this respect, glial cells are able to produce, uptake and metabolize many factors that are essential for neuronal physiology, including classic neurotransmitters and neuroactive steroids. In particular, neuroactive steroids, which are mainly synthesized by glial cells, are able to modulate some neurotransmitter receptors affecting both glia and neurons. Among the signaling systems that are specialized for neuron-glial communication, we can include neurotransmitter GABA.The main focus of this review is to illustrate the cross-talk between neurons and glial cells in terms of GABA neurotransmission and actions of neuroactive steroids. To this purpose, we will review the presence of the different GABA receptors in the glial cells of the central and peripheral nervous system. Then, we will discuss their modulation by some neuroactive steroids.

Expression of GABAA receptor β2/3 subunits in the rat major pelvic ganglion

Several pharmacological and physiological studies have suggested that GABA(A) receptors (GABA(A) Rs) may exist in the rat major pelvic ganglion (MPG), a large coalescent pelvic ganglion that contains both sympathetic and parasympathetic components which innervates pelvic organs. However, the presence of GABA(A) R in the MPG has never been demonstrated directly by morphological studies. In the present study, we used immunohistochemistry to demonstrate the existence of GABA(A) R beta2/3 subunits for the first time in the rat MPG. We also analyzed the neurochemical properties of MPG neurons expressing GABA(A) R beta2/3 subunits. GABA(A) R beta2/3-immunoreactive (-IR) neurons occupied 27.4+/-7.0% of the whole neuronal population, and many of these (77.6%) were co-localized with tyrosine hydroxylase (TH). Likewise, most (86.5%) of TH-IR neurons were GABA(A) R beta2/3-positive. GABA(A) R beta2/3 subunits were also expressed in a few VIP- or NOS-IR neurons, the cholinergic or non-adrenergic, non-cholinergic (NANC) neurons. These results suggest that GABA(A) Rs are involved in the modulation of most sympathetic, noradrenergic neurons and also a subset of VIP and NOS neurons of the rat MPG.

Role of the central and peripheral nervous system in the ovarian function

This review attempts to give a comprehensive overview of ovarian innervation, considering the whole nervous system and its different levels that may modify the ovarian function. The connection between the ovary and the central nervous system through the autonomic pathways, including the peripheral ganglia, is highlighted. The evidence obtained over the last years highlights the role of the superior ovarian nerve (SON) in the ovarian phenomena. Besides, the effect on the ovary of conventional neurotransmitters and others such as indolamines and peptides, which have been found in this organ, are discussed. Various reproductive diseases have been studied almost exclusively from the endocrine point of view. It is evident that a better knowledge about the role of the neural factors involved in the ovarian physiology may facilitate the understanding of some of these. A review of the concepts and an update of some experimental designs is made that permits clarifying several aspects of the relationship between the neural system and the ovary. At present, there is no doubt that the innervation of the ovary is involved in several physiological aspects of this gland function. However, the relationship of some levels of the nervous system and the ovary offer a wide avenue for future research.

Unusual autonomic ganglia: connections, chemistry, and plasticity of pelvic ganglia

The pelvic ganglia provide the majority of the autonomic nerve supply to reproductive organs, urinary bladder, and lower bowel. Of all autonomic ganglia, they are probably the least understood because in many species their anatomy is particularly complex. Furthermore, they are unusual autonomic ganglia in many ways, including their connections, structure, chemistry, and hormone sensitivity. This review will compare and contrast the normal structure and function of pelvic ganglia with other types of autonomic ganglia (sympathetic, parasympathetic, and enteric). Two aspects of plasticity in the pelvic pathways will also be discussed. First, the influence of gonadal steroids on the maturation and maintenance of pelvic reflex circuits will be considered. Second, the consequences of nerve injury will be discussed, particularly in the context of the pelvic ganglia receiving distributed spinal inputs. The review demonstrates that in many ways the pelvic ganglia differ substantially from other autonomic ganglia. Pelvic ganglia may also provide a useful system in which to study many fundamental neurobiological questions of broader relevance.

gamma-Aminobutyric acid (GABA) metabolism in mammalian neural and nonneural tissues

4-Aminobutyric acid (GABA), a major inhibitory neurotransmitter of mammalian central nervous system, is found in a wide range of organisms, from prokaryotes to vertebrates. GABA is widely distributed in nonneural tissue including peripheral nervous and endocrine systems. GABA acts on GABAA and GABAB receptors. GABAA receptors are ligand-gated chloride channels modulated by a variety of drugs. GABAB receptors are essentially presynaptic, usually coupled to potassium or calcium channels, and they function via a GTP binding protein. In neural and nonneural tissues, GABA is metabolized by three enzymes--glutamic acid decarboxylase (GAD), which produces GABA from glutamic acid, and the catabolic enzymes GABA-transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). Production of succinic acid by SSADH allows entry of the GABA carbon skeleton into the tricarboxylic acid cycle. Alternate sources of GABA include putrescine, spermine, spermidine and ornithine, which produce GABA via deamination and decarboxylation reactions, while L-glutamine is an additional source of glutamic acid via deamination. GAD from mammalian brain occurs in two molecular forms, GAD65 and GAD67 (referring to subunit relative molecular weight (Mr) in kilodaltons). These different forms of GAD are the product of different genes, differing in nucleotide sequence, immunoreactivity and subcellular localization. The presence and characteristics of GAD have been investigated in a wide variety of nonneural tissues including liver, kidney, pancreas, testis, ova, oviduct, adrenal, sympathetic ganglia, gastrointestinal tract and circulating erythrocytes. In some tissues, one form (GAD65 or GAD67) predominates. GABA-T has been located in most of the same tissues, primarily through histochemical and/or immunochemical methods; GABA-T is also present in a variety of circulating cells, including platelets and lymphocytes. SSADH, the final enzyme GABA catabolism, has been detected in some of the tissues in which GAD and GABA-T have been identified, although the presence of this enzyme has not been in mammalian pancreas, ova, oviduct, testis or sympathetic ganglia.

Distribution of GABA-immunoreactive nerve fibers and cells in the cervical and thoracic paravertebral sympathetic trunk of adult rat: evidence for an ascending feed-forward inhibition system

Neurochemical and immunohistochemical evidence suggests that the superior cervical ganglion (SCG) contains all components of a gamma-aminobutyric acid (GABA)ergic transmission system, which includes GABAergic axons of unknown origin. The number of nerve fibers with and without GABA-like immunoreactivity was determined in interganglionic connectives at all cervical and thoracic levels of the paravertebral sympathetic trunk. In addition, the distribution of GABA-immunoreactive (IR) neurons was established within the ganglion chain and compared with the relative frequency of principal neurons richly innervated by GABA-IR axon terminals. The following results were obtained: 1) the total number of nerve fibers in cross sections did not significantly vary between the cervical levels, but it increased steadily from upper to lower thoracic segments; 2) in contrast, the number of GABA-IR fibers decreased from the cervical sympathetic trunk below the SCG (approximately 300 fibers) down to the seventh to tenth thoracic ganglion, below which no such fiber was seen; 3) GABA-IR nerve fibers originate from a subclass of GABA-IR cells; these are small, bipolar neurons with predominantly ascending, unmyelinated axon-like processes; 4) the number of principal neurons richly innervated by GABA-IR nerve fibers decreased from the SCG to the upper thoracic ganglia, and was very small below; and 5) apart from basket-like innervation, GABA-IR axons also formed diffuse networks around GABA-negative principal neurons predominantly in cervical and upper thoracic ganglia. These data suggest that the GABAergic innervation of paravertebral sympathetic ganglia is more complex than previously suspected. What appears as preganglionic afferents from several spinal segments (C8-Th7) innervate GABAergic neurons in the sympathetic trunk which have ascending axons and focus their inhibitory effects on the cervical sympathetic ganglia, predominantly the SCG. These data suggest that GABAergic small interganglionic neurons form a feed-forward inhibition system, which may be driven by multisegmental spinal input in the paravertebral sympathetic ganglion chain.

Modulation by GABA of neuroplasticity in the central and peripheral nervous system

Apart from being a prominent (inhibitory) neurotransmitter that is widely distributed in the central and peripheral nervous system, gamma-aminobutyric acid (GABA) has turned out to exert trophic actions. In this manner GABA may modulate the neuroplastic capacity of neurons and neuron-like cells under various conditions in situ and in vitro. In the superior cervical ganglion (SCG) of adult rat, GABA induces the formation of free postsynaptic-like densities on the dendrites of principal neurons and enables implanted foreign (cholinergic) nerves to establish functional synaptic contacts, even while preexisting connections of the preganglionic axons persist. Apart from postsynaptic effects, GABA inhibits acetylcholine release from preganglionic nerve terminals and changes, at least transiently, the neurochemical markers of cholinergic innervation (acetylcholinesterase and nicotinic receptors). In murine neuroblastoma cells in vitro, GABA induces electron microscopic changes, which are similar in principle to those seen in the SCG. Both neuroplastic effects of GABA, in situ and in vitro, could be mimicked by sodium bromide, a hyperpolarizing agent. In addition, evidence is available that GABA via A- and/or B-receptors may exert direct trophic actions. The regulation of both types of trophic actions (direct, receptor-mediated vs. indirect, bioelectric activity dependent) is discussed.

L-glutamate decarboxylase immunoreactivity in developing sympathetic tissues of the rat

An indirect immunofluorescence method was used to study the appearance and distribution of L-glutamate decarboxylase (GAD), the enzyme synthesizing gamma-aminobutyric acid, in developing rat retroperitoneal sympathetic tissues. GAD immunoreactivity was analyzed in correlation with immunoreactivity to the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), in the main retroperitoneal paraganglion, adrenal medullae and abdominal sympathetic ganglia. In different abdominal sympathetic tissues TH-immunoreactive cells first appeared on embryonic days 12.5-14.5, while GAD immunoreactivity was first observed in all these tissues in 14.5-day-old embryos (E 14.5). This suggests that the first expression of GAD is not coupled to the onset of catecholamine synthesis. In developing chain ganglia, GAD immunoreactivity was localized prenatally only in cell clusters with bright TH immunoreactivity, suggesting that GAD is expressed only in the cell lineage leading to ganglionic small intensely fluorescent (SIF) cells. The coeliac-superior mesenteric ganglion complex developed from the preaortic sympathetic tissue, starting from E 16.5 embryos, when the cranial, moderately TH-immunoreactive cells of this tissue were seen to form compact cell islets around the branches of the abdominal aorta. The caudal, intensely TH-immunoreactive cells of preaortic sympathetic tissue were seen to form the main retroperitoneal paraganglion from day E 15.5. During the prenatal period GAD immunoreactivity in preaortic sympathetic tissue was present caudally only in these paraganglionic cells and cranially in some brightly TH-immunoreactive cells, representing SIF and/or paraganglionic cells. In the adrenal medulla, only some of the TH-immunoreactive cells showed GAD immunoreactivity during early developmental stages. The moderately TH-immunoreactive, noradrenaline-synthesizing, cell clusters were seen for the first time in E 16.5 embryos, and they exhibited no GAD immunoreactivity. Thereafter, GAD was expressed only in the intensely TH-immunoreactive, adrenaline-synthesizing, cell clusters. The results of this study indicate that in the developing rat sympathetic tissues GAD is present only in the cell lineages which differentiate into SIF cells of abdominal sympathetic ganglia, preaortic paraganglionic cells and adrenaline cells of the adrenal medulla.

Immunocytochemical localization of L-glutamate decarboxylase and catecholamine-synthesizing enzymes in the retroperitoneal sympathetic tissue of the newborn rat

The localization of L-glutamate decarboxylase (GAD), the enzyme synthesizing gamma-aminobutyric acid, was studied in newborn rat retroperitoneal sympathetic tissue, i.e. the main retroperitoneal paraganglion, adrenal medullae and abdominal sympathetic ganglia using the indirect immunofluorescence method. The coexistence of GAD with the catecholamine-synthesizing enzymes tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) was analyzed in consecutive sections or by staining one section consecutively with different antisera. GAD immunoreactivity was observed only in some cell types of each organ studied. In the main retroperitoneal paraganglion, the small, intensely TH-immunoreactive, paraganglion-type cells were GAD-immunoreactive, while the larger moderately TH-immunoreactive, neuron-like cells were non-reactive for GAD. In the adrenal medulla, GAD immunoreactivity was localized only in the adrenaline-synthesizing, PNMT-immunoreactive chromaffin cells. The noradrenaline-synthesizing, i.e. the TH-immunoreactive cells with no PNMT immunoreactivity, were non-reactive for GAD. In the abdominal sympathetic ganglia, some small intensely TH-immunoreactive cells were GAD-immunoreactive, while the principal neurons were non-reactive for GAD. These results provide immunohistochemical evidence that GAD is present and is colocalized with catecholamine-synthesizing enzymes in various sympathetic tissues of the newborn rat. The present results indicate that GAD is localized in adrenaline-synthesizing cells of all the sympathetic tissues studied. A fraction of noradrenaline-synthesizing cells of retroperitoneal sympathetic tissues, excluding the adrenal medulla, also contains GAD.