B. Fernández

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

Estradiol — induced redistribution of glial fibrillary acidic protein immunoreactivity in the rat brain

The immunohistochemical distribution of the glial fibrillary acidic protein (GFAP), a marker of glial filaments, was studied on coronal sections of the globus pallidus, the area CA4 of the hippocampus and the arcuate nucleus of the hypothalamus, 3 estrogen-sensitive areas of the rat brain. The number and the surface density of the GFAP-immunoreactive cells were evaluated in 6 adult ovariectomized rats injected with a single dose (20 mg/kg) of estradiol valerate (OVX + E2 rats) and in 6 ovariectomized littermates injected with vehicle (OVX rats). Two days after the injection, a similar distribution of the GFAP was observed in the arcuate nucleus of OVX + E2 rats when compared to OVX rats, whereas a significantly (P less than 0.001) increased surface density of GFAP immunoreactive material was observed in the globus pallidus and hippocampus of estradiol-treated rats. Since the number of GFAP-positive cells was unchanged by the estradiol injection, the enhanced surface density of GFAP immunoreactive material in the hippocampus and globus pallidus suggest a possible influence of estradiol on GFAP-immunoreactive glial processes.

Ependyma: phylogenetic evolution of glial fibrillary acidic protein (GFAP) and vimentin expression in vertebrate spinal cord

The phylogenetic evolution was studied of both glial fibrillary acidic protein (GFAP) and vimentin expression in the ependyma of the adult vertebrate spinal cord. Eleven species from different vertebrate groups were examined using different fixatives and fixation procedures to demonstrate any differences in immunoreactivity. GFAP expression in the ependymal cells showed a clear inverse relation with phylogenetic evolution because it was more elevated in lower than in higher vertebrates. GFAP positive cells can be ependymocytes and tanycytes, although depending on their structural characteristics and distribution, the scarce GFAP positive ependymal cells in higher vertebrates may be tanycytes. Ependymal vimentin expression showed a species-dependent pattern instead of a phylogenetic pattern of expression. Vimentin positive ependymal cells were only found in fish and rats; in fish, they were tanycytes and were quite scarce, with only one or two cells per section being immunostained. However, in the rat spinal cord, all the ependymocytes showed positive immunostaining for vimentin. The importance of the immunohistochemical procedure, the cellular nature of GFAP positive ependymal cells and the relationship between tanycytes and ependymocytes are discussed, as well as GFAP and vimentin expression.

Modulation of glutamate transporters (GLAST, GLT-1 and eaac1) in the rat cerebellum following portocaval anastomosis

Glutamate transporters have the important function of removing glutamate released from synapses and keeping extracellular glutamate concentrations below excitotoxic levels. Extracellular glutamate increases in portocaval anastomosis (PCA), so we used a portacaval anastomosis model in rats to analyze the expression of glutamate transporters (GLAST, GLT-1 and EAAC1) in rat cerebellum, 1 and 6 months after PCA, using immunohistochemical methods. In controls, EAAC1 immunoreactivity in Purkinje cells and glial GLAST and GLT-1 immunoreactivities in the molecular layer (ML) increased from young to old rats. One month after PCA, Purkinje cell bodies were not immunostained for neuronal EAAC1 glutamate transporter, whereas glial glutamate transporter expressions (GLAST and GLT-1) were decreased when compared to young controls. In rats with long-term PCA (6 months post-PCA), neuronal and glial glutamate transporter expressions were increased. The expression of the neuronal glutamate transporter EAAC1 was less intense than old controls, whereas glial glutamate transporters (GLAST and GLT-1) increased more than their controls. Since the level of the neuronal glutamate transporter (EAAC1) in long-term PCA did not reach that of the controls, GLAST and GLT-1 glutamate transporters seemed to be required to ensure the glutamate uptake in this type of encephalopathy. EAAC1 immunoreactivity also was expressed by Bergmann glial processes in long-term PCA, but this increase did not suffice to reverse the alterations caused at the early stage. The present findings provide evidence that transitory alteration of glutamate transporter expressions could be a significant factor in the accumulation of excess glutamate in the extracellular space in PCA, which probably makes Purkinje cells more vulnerable to glutamate effect.

Astroglial pattern in the spinal cord of the adult barbel (Barbus comiza)

The distribution and the structural, ultrastructural and immunohistochemical characteristics of the astroglial cells in the spinal cord of the adult barbel (Barbus comiza) have been studied by means of metallic impregnations (Golgi and gold-sublimate), immunohistochemical (GFAP and vimentin) and electron microscopic techniques. GFAP-positive cells were mainly distributed in the ependyma and in the periependymal region, but they have also been observed at subpial level in the anterior column. The ependymocytes were heterogeneous cells because they showed different immunohistochemical characteristics: GFAP-positive, vimentin-positive or non-immunoreactive cells. The radial astrocytes showed only GFAP immunoreactivity, and their processes ended at the subpial zone forming a continuous subpial glia limitans. Desmosomes and gap junctions between soniata and processes of radial astrocytes were numerous, and a relationship between radial astroglial processes and the nodes of Ranvier was also described. The perivascular glia limitans was poorly developed and it was not complete in the blood vessels of the periependymal zone; in this case, the basal lamina was highly developed. An important characteristic in the barbel spinal cord was the existence of a zone with an abundant extracellular space near the ependyma. The presence of radial astroglial somata at subpial level, the existence of vimentin-positive ependymocytes and the abundant extracellular space in the periependymal zone is discussed in relation to the regeneration capacity and the continuous growth showed by fish. Moreover, the abundance of gliofilaments and desmosomes leads us to suggest that mechanical support might be an important function for the astroglial cells in the barbel spinal cord.

Down-regulation of the AMPA glutamate receptor subunits GluR1 and GluR2/3 in the rat cerebellum following pre- and perinatal Δ9-tetrahydrocannabinol exposure

This paper reports the effects of pre- and perinatal exposure to Δ9-tetrahydrocannabinol (THC) on expression levels of specific AMPA glutamate receptor subunits (GluRl and GluR2/3) in the cerebellum of male and female rats. Pregnant rats were administered saline or THC from gestational day 5 (ED5) to postnatal day 20 (PD20). Expression of the GluRl and GluR2/3 subunits of AMPA glutamate receptors was analyzed by immunohistochemistry in THC-exposed rats at three postnatal ages: PD20 (still exposed to THC) to study the direct effect of drug exposure, and PD30 and PD70 (10 and 50 days following THC withdrawal) to analyze the long-term effects of prenatal exposure. Compared to controls, pre- and perinatal THC exposure decreased the immunoreactivity levels of the GluRl subunit in Bergmann glial cells, as well as levels of the GluR2/3 subunit in Purkinje neurons at PD20. These changes in AMPA receptor subunit levels may correlate with the decreased excitatory neurotransmission described in the cerebellum after cannabinoid treatment, which could play a significant role in the biochemical effects of THC. In addition, the reduced glutamate receptor expression observed at PD20 did not return to normal even after THC withdrawal (PD30 and PD70). The results support the idea that THC exposure during critical stages of cerebellar development may alter the glutamatergic system, not only during the drug exposure period itself but also in adults following THC withdrawal. The decreased expressions of glutamate receptors induced by developmental THC exposure could lead to functional alterations through the inhibition of glutamatergic neurotransmission, and clearly demonstrate an interaction between cannabinoids and the glutamatergic system.

Glial fibrillary acidic protein and vimentin immunohistochemistry in the posterior rhombencephalon of the Iberian barb (Barbus comiza)

The gustatory centers (vagal lobes and facial lobe) and the tegment of the posterior rhombencephalon of the Iberian barb (Barbus comiza) have been studied using anti-glial fibrillary acidic protein (anti-GFAP) and anti-vimentin immunohistochemical techniques. GFAP immunoreactivity was found in the tegment and in a part of the vagal lobes while vimentin immunoreactivity was located in the tegment. Two immunopositive cell types were found: ependymocytes and radial astrocytes. Since the distribution of GFAP in the barb rhombencephalon corresponds with zones previously described as cholinergic, the GFAP-immunopositive radial astrocytes might be involved in acetylcholine metabolism.

Prenatal cannabinoid exposure down- regulates glutamate transporter expressions (GLAST and EAAC1) in the rat cerebellum.

Efficient reuptake of synaptically released glutamate is essential for preventing glutamate receptor overstimulation and neuronal death. Glutamate transporters play a vital role in removing extracellular glutamate from the synaptic cleft. This study analyzed the expression of the glial (GLAST) and neuronal (EAAC1) subtypes of glutamate transporter in the cerebellum of male and female offspring exposed pre- and postnatally to Δ9-tetrahydrocannabinol (THC, the main component of marijuana). Pregnant rats were administered saline or THC from gestational day 5 to postnatal day 20 (PD20). The expression of glutamate transporters was examined at PD20, PD30 and PD70 (10 and 50 days after THC withdrawal) to analyze the short- and long-term effects of prenatal THC exposure. The expression of the glutamate transporter GLAST in astroglial cells and EAAC1 in Purkinje neurons decreased in THC-exposed offspring compared to controls. This reduction was observed at all ages but mainly in males. Moreover, the glial glutamate transporter level in THC-exposed rats (quantified by Western blot) was lower than in control rats. These results suggest that THC exposure during cerebellar development may alter the glutamatergic system not only during the period of drug exposure but in the postnatal stage following withdrawal. The down-regulation reported here might reflect an abnormal maturation of the glutamatergic neuron-glia circuitry.

Astroglial Induction of in vivo Angiogenesis

A purified population of astrocytes was prepared from embryonic rat hypothalamus. These cells were transplanted into the cerebral cortex of adult rats with survival time of 5 days and studied by glial fibrillary acidic protein (GFAP) immunohistochemistry and electron microscopy. A progressive gradation from the edges of the implant to the intact host tissue was observed in relation to the vascularization process. The regenerating tissue showed cells with cavities resembling capillary central lumens and their cytoplasms revealed gold. particles when they were immunostained for electron microscopy. Dark processes were seen in capillary-like lumens and, on other occasions, evaginations of endothelial cells were in contact with astroglial processes. These findings lead us to suggest, that capillary-like structures might develop from cavitated astroglial cells, which would permit the migration of endothelial cells into their lumens. Astroglial-endothelial interactions persist until endothelial cells are morphologically differentiated. One possible interpretation of the present data is that astrocytes might participate directly and actively in the regulation of capillary formation.

Neuronal and Astroglial Response to Pre- and Perinatal Exposure to Delta-9-Tetra- Hydrocannabinol in the Rat Substantia nigra

The responses of neurons and astroglial cells to pre- and perinatal exposure to Δ9-tetrahydrocannabinol (Δ9-THC) were evaluated in the substantia nigra (SN) of male and female rats, at three postnatal ages (PD21, PD30 and PD70), by immunohistochemical detection of tyrosine hydroxylase (TH) in dopaminergic neurons and of glial fibrillary acidic protein (GFAP) in astrocytes. Our results showed that the effects of pre- and perinatal exposure to Δ9-THC on neuronal and astroglial immunoreactivities in the SN (compacta and reticulata) varied with sex, with male rats being more susceptible than females. Prenatal exposure to Δ9-THC decreased TH immunoreactivity in the SN of males on PD21 when compared to both their controls and Δ9-THC-exposed females of the same age. Furthermore, the TH expression decreased with age in Δ9-THC-exposed males in the SNc pars compacta, whereas it increased in controls. On the contrary, TH expression was maintained stable in the SN pars compacta of Δ9-THC-exposed females from PD21. These differences in neuronal development caused by prenatal Δ9-THC exposure were associated with significant differences in GFAP expression by astroglial cells in both sexes. On PD21, GFAP immunoreactivity decreased in the SN in Δ9-THC-exposed male rats. Although GFAP expression increased in Δ9-THC-exposed males with age, it did not reach control levels by PD70. On the contrary, significantly increased GFAP expression in the Δ9-THC-exposed females on PD21 was observed, compared to their controls and also to Δ9-THC-exposed male rats; however, the GFAP expression shown by Δ9-THC-exposed females stabilized from PD21. These Δ9-THC-induced changes in the glial development could indicate that Δ9-THC accelerated the maturation of astrocytes in female rats, whereas Δ9-THC delayed astrocytic maturation in Δ9-THC-exposed males. These findings suggest that pre- and perinatal exposure to Δ9-THC can lead to long-term effects in both neurons and glial cells.