Immunocytochemical localization of glutamic acid decarboxylase (GAD) and glutamine synthetase (GS) in the area postrema of the cat. Light and electron microscopy

Anti-emetic effects of thalidomide: Evidence, mechanism of action, and future directions

The rationale for using thalidomide (THD) as a treatment for nausea and vomiting during pregnancy in the late 1950s appears to have been based on its sedative or hypnotic properties. In contrast to contemporaneous studies on the anti-emetic activity of phenothiazines, we were unable to identify publications reporting preclinical or clinical evaluation of THD as an anti-emetic. Our survey of the literature revealed a clinical study in 1965 showing THD reduced vomiting in cancer chemotherapy which was substantiated by similar studies from 2000, particularly showing efficacy in the delayed phase of chemotherapy-induced nausea and vomiting. To identify the mechanism(s) potentially involved in thalidomide's anti-emetic activity we reviewed its pharmacology in the light of nausea and vomiting mechanisms and their pharmacology with a particular emphasis on chemotherapy and pregnancy. The process identified the following potential mechanisms: reduced secretion of Growth Differentiation Factor 15, suppression of inflammation/prostaglandin production, downregulation of cytotoxic drug induced upregulation of iNOS, and modulation of BK (KCa1.1) channels and GABAA/glutamate transmission at critical points in the emetic pathways (nucleus tractus solitarius, area postrema). We propose ways to investigate these hypothesized mechanisms and discuss the associated challenges (e.g., objective quantification of nausea) in addition to some of the more general aspects of developing novel drugs to treat nausea and vomiting.

Three-plane description of astroglial populations of OVLT subdivisions in rat: Tanycyte connections to distant parts of third ventricle

This study demonstrates glial and gliovascular markers of organon vasculosum laminae terminalis (OVLT) in three planes. The distribution of glial markers displayed similarities to the subfornical organ. There was an inner part with vimentin- and nestin-immunopositive glia whereas GFAP and the water-channel aquaporin 4 were found at the periphery. This separation indicates different functions of the two regions. The presence of nestin may indicate stem cell-capabilities whereas aquaporin 4 has been reported to promote the osmoreceptor function. Glutamine synthetase immunoreactivity was sparse like in the area postrema and subfornical organ. The laminin and β-dystroglycan immunolabelings altered along the vessels such as in the subfornical organ indicating altering gliovascular relations. The different subdivisions of OVLT received glial processes of different origins. The posterior periventricular zone contained short vimentin-immunopositive processes from the ependyma of the adjacent surface of the third ventricle. The lateral periventricular zone received forceps-like process systems from the anterolateral part of the third ventricle. Most interestingly, the "dorsal cap" received a mixed group of long GFAP- and vimentin-immunopositive processes from a distant part of the third ventricle. The processes may have two functions: a guidance for newly produced cells like radial glia in immature brain and/or a connection between distant parts of the third ventricle and OVLT.

Glial and perivascular structures in the subfornical organ: distinguishing the shell and core

The subfornical organ (SFO) is a circumventricular organ with a chemosensitive function, and its vessels have no blood-brain barrier. Our study investigated the glial and vascular components in the SFO to determine whether their distributions indicate subdivisions, how to characterize the vessels and how to demarcate the SFO. To this end, we investigated glial markers (GFAP, glutamine synthetase, S100) and other markers, including vimentin and nestin (immature glia), laminin (basal lamina), β-dystroglycan (glio-vascular connections), and aquaporin 4 (glial water channels). We determined that the 'shell' of the SFO was marked by immunoreactivity for S100, GFAP and aquaporin 4. Nestin immunoreactivity was characteristic of the 'core'. Vimentin was almost evenly distributed. Glutamine synthetase immunoreactivity occurred in the shell but its expression was sparse. Vessels in the core were decorated with laminin but showed a discontinuous expression of aquaporin 4. Vimentin and GFAP staining was usually in separate glial elements, which may be related to their functional differences. Similar to other vessels in the brain, β-dystroglycan was detected along the shell vessels but laminin was not. The gradual disappearance of the laminin immunopositivity was attributed to the gradual disappearance of the perivascular space. Thus, our findings suggest that the shell and core glio-vascular structures are adapted to different sensory functions: osmoperception and the perception of circulating peptides, respectively.

Development of the area postrema: an immunohistochemical study in the macaque

The organization and chemical development of the area postrema (AP) in the macaque monkey was studied by immunohistochemistry imaged with conventional and confocal microscopy from day 40 of gestation to adulthood. The thin ependyma of the adult was found to develop from a thick continuous structure beginning in the second trimester. It was later invaded by tyrosine hydroxylase immunoreactive (TH+) and dopamine beta-hydroxylase immunoreactive (DBH+) cells and fibers, suggesting a possible route for release of neurotransmitter directly into ventricular cerebrospinal fluid. Other TH+ and/or DBH+ fibers were found in close approximation to blood vessels. Prominent vascularity of the parenchyma of AP was present late in the first trimester (fetal day (Fd)57 in the macaque) and increased further until birth. By contrast, the underlying solitary nucleus was hypervascular at Fd57, but its vascularity rapidly declined by late in the second trimester. TH+ neurons first appeared late in the first trimester, and DBH+ neurons appeared in the second trimester; these findings are consistent with the view that catecholaminergic cells in AP are the earliest members of the A2 noradrenergic group. Catecholaminergic cells or fibers in AP contained little labeling for synaptic vesicular proteins, suggesting that the release of neurotransmitter there may not involve a synaptic mechanism. Synapses were first observed in mid-second trimester, and most were associated with GABA+ fibers.

An astrocyte toxin influences the pattern of breathing and the ventilatory response to hypercapnia in neonatal rats

Recent in vitro data suggest that astrocytes may modulate respiration. To examine this question in vivo, we treated 5-day-old rat pups with methionine sulfoximine (MS), a compound that alters carbohydrate and glutamate metabolism in astrocytes, but not neurons. MS-treated pups displayed a reduced breathing frequency (f) in baseline conditions relative to saline-treated pups. Hypercapnia (5% CO(2)) increased f in both groups, but f still remained significantly lower in the MS-treated group. No differences between treatment groups in the responses to hypoxia (8% O(2)) were observed. Also, MS-treated rats showed an enhanced accumulation of glycogen in neurons of the facial nucleus, the nucleus ambiguus, and the hypoglossal nucleus, structures that regulate respiratory activity and airway patency. An altered transfer of nutrient molecules from astrocytes to neurons may underlie these effects of MS, although direct effects of MS upon neurons or upon peripheral structures that regulate respiration cannot be completely ruled out as an explanation.

Opioid peptides inhibit excitatory but not inhibitory synaptic transmission in the rat dorsal motor nucleus of the vagus

Opioid peptides produce gastrointestinal inhibition and increase feeding when applied to the brainstem. The present studies were designed to determine the actions of opioid peptides on synaptic transmission within the dorsal motor nucleus of the vagus (DMV) and the localization of mu-opioid receptors. Whole-cell recordings were made from identified gastrointestinal-projecting DMV neurons in thin brainstem slices of the rat. Electrical stimulation of the nucleus of the tractus solitarius evoked EPSCs and IPSCs. In all neurons tested, methionine (Met)-enkephalin (0.003-30 microm) inhibited the peak amplitude of the EPSCs. The effect was prevented by naloxone (1 microm) as well as by naloxonazine (0.2 microm). An increase in the ratio of the evoked paired pulses indicated that the inhibition was attributable to actions at presynaptic receptors. This presynaptic inhibitory action was mimicked by [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (0.1 microm) and the analgesic dipeptide kyotorphin (10 microm) but not by cyclic[d-Pen(2), d-Pen(5)]-enkephalin (1 microm) and trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methanesulfonate (1 microm). In contrast, the amplitude of evoked IPSCs was not altered either by Met-enkephalin or by any of the opioid receptor-selective agonists. Immunohistochemical studies revealed that nerve terminals apposing DMV neurons showed immunoreactivity to mu-opioid receptors colocalized with glutamate immunoreactivity but not glutamic acid decarboxylase immunoreactivity. These results suggest that within the DMV, mu-opioid receptors are present on the nerve terminals of excitatory but not inhibitory inputs to GI motoneurons. Such specificity may imply that the central inhibitory action of opioid peptides on gastrointestinal function targets selected pathways.

The brain response to 2-deoxy glucose is blocked by a glial drug

Two brain regions - the basomedial hypothalamus and area postrema (AP) - react to changes in circulating glucose levels by altering feeding behavior and the secretion of pituitary and non-pituitary hormones. The precise identity of cells responding to glucose in these regions is uncertain. The recent detection of high-capacity glucose transporter proteins in astrocytes in these areas has suggested that astrocytes may play a role in glucose sensing by the brain. To test this hypothesis, rats were injected with either saline or methionine sulfoximine (MS), a compound that produces alterations in carbohydrate and glutamate metabolism in astrocytes. Eighteen hours later, rats were injected with either saline or 2-deoxy glucose (2-DG) and brain sections were stained to demonstrate 2-DG-activated neurons immunoreactive for Fos protein. MS-treated rats showed a 70% reduction in numbers of Fos+ neurons in the AP region (p<0.05). Also, specialized, Gomori+ astrocytes were particularly abundant in both glucose sensitive regions and showed a distribution identical to that reported for high-capacity glucose transporter proteins. These data suggest that specialized astrocytes influence the glucose-sensing function of the brain.

Coordinated transcriptional regulation of the unc-25 glutamic acid decarboxylase and the unc-47 GABA vesicular transporter by the Caenorhabditis elegans UNC-30 homeodomain protein

An important aspect of the specification of neuronal fate is the choice of neurotransmitter. In Caenorhabditis elegans the neurotransmitter GABA is synthesized by the UNC-25 glutamic acid decarboxylase (GAD) and packaged into synaptic vesicles by the UNC-47 transporter. Both unc-25 and unc-47 are expressed in 26 GABAergic neurons of five different types. Previously, we have identified that the unc-30 homeobox gene controls the fate of 19 type D GABAergic neurons. We report here that the UNC-30 homeodomain protein transcriptionally regulates the expression of unc-25 and unc-47 in the 19 type D neurons. UNC-30 bound to the unc-25 and unc-47 promoters sequence-specifically. Mutations in the UNC-30 binding sites of the unc-25 and unc-47 promoters abolished the expression of reporter genes in the D neurons. The ectopic expression of UNC-30 induced the ectopic expression of reporter genes driven by the wild-type unc-25 and unc-47 promoters. Our data establish a mechanism for cell type-specific transcriptional coregulation of genes required for the synthesis and packaging of the neurotransmitter GABA.

Patch-clamp study on membrane properties and transmitter activated currents of rabbit area postrema neurons

Using the patch-clamp technique in combination wit sliced tissue preparation the membrane properties of newborn rabbit area postrema neurons were investigated. The neurons responded upon depolarization with a fast Na+-current followed by an inactivating and non-inactivating K+-current. GABA-activated currents were investigated resulting in a large C1-(-)conductance, indicating the expression of GABAA-receptors. The expression of glutamate receptor mRNA was studied by in situ hybridization and electrophysiological measurements of these receptors by means of the patch-clamp technique. As a main result it was found that ionotropic glutamate receptors in the area postrema are composed of "flop" variants of the GluA-, GluB- and GluC-subunits.

MK-801 affects cardiovascular responses produced by GABAA agents in area postrema

The cardiovascular effects of nanoliter microinfusions of muscimol, bicuculline, and MK-801 (dizocilpine) into area postrema (AP) were tested in urethane-anesthetized male Sprague-Dawley rats. Microinfusion of muscimol (10 nl/min/5min, 1 ng/nl) into the AP produced significant hypertension (+29 +/- 8 mmHg), but no significant change in heart rate (HR). Microinfusion of bicuculline (10 nl/min/5 min, 1 ng/nl) into the AP produced significant hypotension (-27 +/- 4 mmHg) and bradycardia (-53 +/- 10 bpm). Treatment of the AP with MK-801 (20 nl/min/5 min, 6 ng/nl) produced no changes in mean arterial pressure or HR by itself, but it completely blocked the hypotension and bradycardia produced by bicuculline infusion, without affecting the hypertension produced by muscimol microinfusion into AP. Following pretreatment of AP with MK-801, microinfusion of muscimol into the AP produced significant tachycardia (+51 +/- 16 bpm). The data suggest that a) GABAergic neurotransmission in the AP affects its cardiovascular functions; b) functional interactions between NMDA receptor-mediated neurotransmission and GABAA receptor-mediated neurotransmission within the AP modulate blood pressure and HR regulation.

Inhibition of experimental seizures in canines by repetitive vagal stimulation

Repetitive electrical stimulation of the canine cervical vagus nerve interrupts or abolishes motor seizures induced by strychnine and tremors induced by pentylenetetrazol (PTZ). Tremors were defined as rhythmic alternating contractions of opposing muscle groups, exerting much less force than seizure contractions. Seizures were induced by injection boluses of strychnine or PTZ at 1- to 4-min intervals until sustained muscle activity was observed electromyographically (EMG). Vagal stimulation terminated seizures in 0.5-5 s. There were prolonged periods with no spontaneous EMG activity after stimulation. The period of protection was approximately four times the stimulation period. The antiseizure actions of vagal stimulation were not altered by transection of the vagus distal to the stimulating electrode. Optimal stimulus parameters were estimated: strength, approximately 20 V (electrode resistance 1-5 omega); frequency 20-30 Hz; duration, approximately 0.2 ms. These data suggest that the antiseizure effects derive from stimulation of small-diameter afferent unmyelinated fibers in the vagus nerve. These results may form the basis of a new therapeutic approach to epilepsy.

Neuroactive amino acids in the area postrema. An immunocytochemical investigation in rat with some observations in cat and monkey (Macaca fascicularis)

The localization of five neuroactive amino acids in the rat area postrema was studied by postembedding immunocytochemistry in semithin and ultrathin sections. Antisera to GABA, glycine, glutamate and aspartate produced labelling of cells that were identified as neurons in the electron microscope. GABA-like and glycine-like immunoreactivities occurred in about 20% and 60% of the neurons, respectively, and a minor proportion of the cells displayed both immunoreactivities, suggesting a cellular colocalization of GABA and glycine. Immunoreactivities for glutamate and aspartate were found in a large majority of the neurons, including most of the cells that were positive for GABA and/or glycine. Taurine immunoreactivity was highly concentrated in a few small cells with ultrastructural features typical of microglial cells, and in processes that were probably derived from these. Taurine also appeared to be abundant in cells confined to the perivascular space. The electron microscopic, immunogold analysis of the neuropil revealed numerous nerve terminals that were enriched in GABA or glutamate immunoreactivity, compatible with a transmitter role of these amino acids. Glycine immunolabelling was found preferentially in postsynaptic elements, suggesting that the glycine-containing cells lack locally ramifying axon collaterals, and that they mainly project outside the area postrema. Aspartate immunolabelling was also generally low in axon terminals. This is similar to the situation in several other brain areas and could indicate that the latter amino acid primarily serves metabolic functions in the area postrema.

Glutamine synthetase immunoreactivity is present in oligodendroglia of various regions of the central nervous system

Glutamine synthetase immunoreactive oligodendrocytes were identified in the cerebral cortex, cerebellum, brain stem, and spinal cord. They were mostly confined to the gray matter, particularly close to neurons and processes. The white matter showed few immunoreactive oligodendroglia. It was suggested that some type of oligodendrocytes, specially those in perineuronal location, might fulfill a functional role more akin to astrocytes than to the normally myelinating oligodendroglia.

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.

Metabolism of glutamate and ammonia in astrocyte: an immunocytochemical study

Alpha-ketoglutarate (alpha-KG) reductive amination activity in rat brain was found to be mostly absorbed with an antibody against liver glutamate dehydrogenase. With this and anti-glutamine synthetase antibodies, alpha-KG reductive amination activity was immunocytochemically shown to coexist with glutamine synthetase activity in astrocytes. The results suggest that astrocytes de novo synthesize glutamate from alpha-KG and ammonia, and metabolize it to glutamine.