Autoradiographic localization of binding sites for second messengers on neurones and astrocytes of cultured rat cerebellum

Antagonists of the Vasopressin V1 Receptor and of the β(1)-Adrenoceptor Inhibit Cytotoxic Brain Edema in Stroke by Effects on Astrocytes - but the Mechanisms Differ

Brain edema is a serious complication in ischemic stroke because even relatively small changes in brain volume can compromise cerebral blood flow or result in compression of vital brain structures on account of the fixed volume of the rigid skull. Literature data indicate that administration of either antagonists of the V1 vasopressin (AVP) receptor or the β1-adrenergic receptor are able to reduce edema or infarct size when administered after the onset of ischemia, a key advantage for possible clinical use. The present review discusses possible mechanisms, focusing on the role of NKCC1, an astrocytic cotransporter of Na(+), K(+), 2Cl(-) and water and its activation by highly increased extracellular K(+) concentrations in the development of cytotoxic cell swelling. However, it also mentions that due to a 3/2 ratio between Na(+) release and K(+) uptake by the Na(+),K(+)-ATPase driving NKCC1 brain extracellular fluid can become hypertonic, which may facilitate water entry across the blood-brain barrier, essential for development of edema. It shows that brain edema does not develop until during reperfusion, which can be explained by lack of metabolic energy during ischemia. V1 antagonists are likely to protect against cytotoxic edema formation by inhibiting AVP enhancement of NKCC1-mediated uptake of ions and water, whereas β1-adrenergic antagonists prevent edema formation because β1-adrenergic stimulation alone is responsible for stimulation of the Na(+),K(+)-ATPase driving NKCC1, first and foremost due to decrease in extracellular Ca(2+) concentration. Inhibition of NKCC1 also has adverse effects, e.g. on memory and the treatment should probably be of shortest possible duration.

Long-term depression: a learning-related type of synaptic plasticity in the mammalian central nervous system

Studies of various forms of synaptic plasticity in the central nervous system have provided insights into the cellular and molecular mechanisms for certain types of learning and memory. Activity-induced decreases and increases in synaptic efficacy can be elicited in mammalian neurons. Long-term depression (LTD) and long-term potentiation (LTP) are two major forms of activity-dependent synaptic plasticity in the brain. LTD of excitatory synaptic transmission in the cerebellum in the most well studied form of synaptic depression. The induction of cerebellar LTD requires conjunctive activation of alpha-amino-3-hydroxy-5-methyl-4-isoxalepropionate (AMPA) receptors, metabotropic glutamate receptors (mGluRs) and L-type voltage-dependent Ca2+ channels. Several intracellular second messengers and protein kinases are critical for cerebellar LTD, including cGMP, cGMP-dependent protein kinase and protein kinase C (PKC). A novel intercellular messenger, nitric oxide (NO), is found in the cerebellum, is released durinng synaptic stimulation, and may contribute to cerebellar LTD. The expression of cerebellar LTD is mediated by postsynaptic desensitization of AMPA receptors. Recently, a form of homosynaptic LTD has been described in the CA1 region of the hippocampus. The induction of hippocampal LTD is postsynaptic. N-Methyl-D-aspartate receptors and mGluRs are important for induction of hippocampal LTD. Other intracellular and intercellular messengers, such as NO, cGMP and cAMP, might act downstream from glutamate receptors during hippocampal LTD. The expression of hippocampal LTD is likely to be in part presynaptic. While cerebellar LTD may be important for motor learning, the behavioral role of hippocampal LTD remains to be explored.

Adenosine enhances intracellular Ca2+ mobilization in conjunction with metabotropic glutamate receptor activation by t-ACPD in cultured hippocampal astrocytes

2Cl-Adenosine, a non-metabolized adenosine agonist, enhanced the increase in intracellular Ca2+ concentration ([Ca2+]i) in cultured hippocampal astrocytes induced by (+-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), a metabotropic glutamate agonist. In the absence of 2Cl-adenosine, the half effective concentration (EC50) of t-ACPD was about 80 microM. On the other hand, in the presence of 1 microM 2Cl-adenosine, the EC50 of t-ACPD shifted to about 5 microM, although the maximum [Ca2+]i did not change. The synergistic effect of 2Cl-adenosine with t-ACPD on [Ca2+]i elevation was not inhibited by the elimination of extracellular Ca2+, but was inhibited by A1-specific adenosine antagonists. These results indicate that adenosine can act via the A1 receptor as an endogenous co-activator of the metabolic processes induced by metabotropic glutamate receptor activation.

Autoradiographic localization of binding sites for arginine vasopressin and atrial natriuretic peptide on astrocytes and neurons of cultured rat central nervous system

The cellular localization of binding sites for [125I]arginine vasopressin and [125I]atrial natriuretic peptide was studied in explant cultures of rat spinal cord, brain stem and cerebellum by means of autoradiography. In brain stem cultures, especially in the nucleus of the solitary tract, a great number of neurons revealed binding sites for both peptides. In spinal cord cultures, many neurons of various sizes were labelled by [125I]arginine vasopressin, whereas only a small number of cells showed binding sites for [125I]atrial natriuretic peptide. Neurons in cerebellar cultures revealed little or no binding for the peptides. In addition to neurons, binding sites for [125I]arginine vasopressin and [125I]atrial natriuretic peptide were also observed on glial cells. Simultaneous staining of the cultures with glial fibrillary acidic protein has shown that the labelled cells were glial fibrillary acidic protein-positive and could therefore be identified as astrocytes. Labelling of the cells by [125I]arginine vasopressin and [125I]atrial natriuretic peptide was more intense in spinal cord and brain stem cultures than in cultures of cerebellum, providing evidence for a heterogeneity of astrocytes in different regions of the central nervous system. Binding of both [125I]arginine vasopressin and [125I]atrial natriuretic peptide to neurons and astrocytes could be competed by the unlabelled peptides, suggesting specific binding of the radioligands. Our autoradiographic studies provide good evidence that in addition to neurons, astrocytes also express receptors for arginine vasopressin and atrial natriuretic peptide.