Rat striatum contains pure population of ETB receptors

Alcohol reinstatement after prolonged abstinence from alcohol drinking by female adolescent rats: Roles of cyclooxygenase-2 and the prostaglandin E2 receptor 1

BACKGROUND

Adolescent alcohol misuse is a global problem that can significantly increase the reinstatement of alcohol drinking during re-exposure after abstinence, but the mechanism that causes this increase is unknown. Female adolescents are an understudied population but they are particularly vulnerable to adolescent-onset alcohol abuse. We aimed to determine how adolescent-onset alcohol drinking affects pro-inflammatory mediators endothelin-1 (ET-1), cyclooxygenase-2 (COX-2), and prostaglandin E₂ (PGE₂) in the brain and the role of COX-2 and PGE₂ in EtOH reinstatement in adolescent females.

METHODS

Adolescent female rats were exposed to a 2-bottle choice paradigm of water vs 5% ethanol (EtOH) every other day over a 21 day period. ET-1 and COX-2 proteins were measured in the dorsal striatum (DS) after a 4 week abstinence from EtOH drinking. The COX-2 inhibitor nimesulide was then administered during abstinence prior to an EtOH reinstatement or sucrose preference or to measure PGE₂ content. The PGE₂ receptor 1 (EP₁) antagonist SC-51089 was then administered prior to EtOH reinstatement during which EtOH intake was measured.

RESULTS

EtOH drinking significantly increased ET-1 by 33.8 ± 8.9% and COX-2 by 71.4 ± 24.3% in the DS. Treatment with nimesulide during abstinence attenuated EtOH intake during reinstatement after prolonged abstinence by 40.3 ± 12.4% compared to saline controls. Adolescent EtOH drinking and abstinence increased PGE₂ 150.5 ± 30.9% in the DS and nimesulide attenuated this increase. SC-51089 treatment during abstinence attenuated EtOH reinstatement by 48.1 ± 8.4% compared to DMSO controls.

CONCLUSIONS

These experiments identified a prostaglandin-mediated mechanism that offers a putative pharmacological target to attenuate EtOH reinstatement after adolescent-onset EtOH drinking.

Focal Lesions in the Rat Central Nervous System Induced by Endothelin-1

Axon injury following cerebral ischemia has received little scientific attention compared to the abundance of information dealing with the pathophysiology of grey matter ischemia. There are differences in the initial response of grey and white matter to ischemia in vitro. In this study we investigate whether the vasoactive peptide, endothelin-1, can generate a focal ischemic lesion in the white matter and compare the findings with endothelin-1-induced lesions in the grey matter. Using a minimally invasive technique to microinject endothelin-1 into selected brain regions, we observed an acute reduction in local MRI perfusion in the injected hemisphere after 1 hour. Twenty-four hours after microinjection of 10 pmoles of endothelin-1, we observed a loss of neurons in the grey matter. At 72 hours, neutrophils were absent and a macrophage/microglia response and astrocyte gliosis were detected. No breakdown in the blood-brain barrier was detected. After injection of 10 pmoles endothelin-1 into the cortical white matter, we observed prolific amyloid precursor protein-positive immunostaining (indicative of axonal disruption) and an increase in tau-1 immunostaining in oligodendrocytes at 6 hours. Similar to the grey matter lesions, no neutrophils were present, a macrophage/microglia response did not occur until 72 hours and there was no disruption in the blood-brain barrier. Focal injections of endothelin-1 into specific areas of the rat CNS represent a model to investigate therapeutic approaches to white matter ischemia.

Upregulation of endothelin-1 binding in tissues of salt-loaded stroke-prone spontaneously hypertensive rats

Upon maintained on a 1% NaCl drinking solution beginning at 7 weeks of age, the stroke-prone spontaneously hypertensive rat (SHRsp) developed severe hypertension and stroke; most died by 16 weeks. The mechanism by which these diseases evolve remains unclear. Endothelin-1 (ET-1) is a potent, peptidic vasoconstrictor and is implicated in the pathogenesis of various cardiovascular, renal, and central nervous system diseases. The purpose of the present study was to compare the binding of [125I]ET-1 to the brain, heart, kidney, liver, and spleen membrane preparations of 16-week-old SHRsp and age-matched normotensive Wistar-Kyoto rats (WKY). The KD values for [125I]ET-1 binding to the corresponding tissues of the two strains were not significantly different, except in the brain (SHRsp: 17 +/- 1 pM; WKY: 24 +/- 1 pM). In contrast, the Bmax values measured in the brain, heart, kidney, and liver of SHRsp were 1.5- to 2.1-fold greater than those of their WKY counterparts. Competition of [125I]ET-1 binding to the membrane preparations by the specific ETA receptor antagonist BQ-123 or the specific ETB receptor agonist sarafotoxin S6c revealed a similar proportion of ETA and ETB receptor subtypes in the corresponding tissues of the two rat strains. These results indicate that ET-1 binding is upregulated in SHRsp and suggest that ET-1 may play a pathophysiological role in this animal model of genetic hypertension.

Endothelin and dopamine release

Endothelins and endothelin receptors are widespread in the brain. There is increasing evidence that endothelins play a role in brain mechanisms associated with behaviour and neuroendocrine regulation as well as cardiovascular control. We review the evidence for an interaction of endothelin with brain dopaminergic mechanisms. Our work has shown that particularly endothelin-1 and ET(B) receptors are present at significant levels in typical brain dopaminergic regions such as the striatum. Moreover, lesion studies showed that ET(B) receptors are present on dopaminergic neuronal terminals in striatum and studies with local administration of endothelins into the ventral striatum showed that activation of these receptors causes dopamine release, as measured both with in vivo voltammetry and behavioural methods. While several previous studies have focussed on the possible role of very high levels of endothelins in ischemic and pathological mechanisms in the brain, possibly mediated by ET(A) receptors, we propose that physiological levels of these peptides play an important role in normal brain function, at least partly by interacting with dopamine release through ET(B) receptors.

Distinct pharmacological properties of ET-1 and ET-3 on astroglial gap junctions and Ca(2+) signaling

Astrocytes represent a major target for endothelins (ETs), a family of peptides that have potent and multiple effects on signal transduction pathways and can be released by several cell types in the brain. In the present study we have investigated the involvement of different ET receptor subtypes on intercellular dye diffusion, intracellular Ca(2+) homeostasis, and intercellular Ca(2+) signaling in cultured rat astrocytes from hippocampus and striatum. Depending on the ET concentration and the receptor involved, ET-1- and ET-3-induced intracellular Ca(2+) increases with different response patterns. Both ET-1 and ET-3 are powerful inhibitors of gap junctional permeability and intercellular Ca(2+) signaling. The nonselective ET receptor agonist sarafotoxin S6b and the ET(B) receptor-selective agonist IRL 1620 mimicked these inhibitions. The ET-3 effects were only marginally affected by an ET(A) receptor antagonist but completely blocked by an ET(B) receptor antagonist. However, the ET-1-induced inhibition of gap junctional dye transfer and intercellular Ca(2+) signaling was only marginally blocked by ET(A) or ET(B) receptor-selective antagonists but fully prevented when these antagonists were applied together. The ET-induced inhibition of gap junction permeability and intercellular Ca(2+) signaling indicates that important changes in the function of astroglial communication might occur when the level of ETs in the brain is increased.

Endothelin induces dopamine release from rat striatum via endothelin-B receptors

The aim of the present study was to determine whether local administration of endothelin induces the release of dopamine in the rat striatum and to characterize and localize endothelin receptors in this brain region. Local injection of endothelin-1 (10 pmol) into the ventral striatum of urethane-anaesthetized rats caused an increase of 8 microM in the extracellular concentration of dopamine as measured by in vivo chronoamperometry. The peak increase in dopamine concentration occurred within 5 min of endothelin injection. Injection of the selective endothelin-B receptor agonist [Ala1.3,11.15]endothelin-1 (10 pmol) also caused an increase in extracellular dopamine concentration, suggesting that endothelin is acting at the endothelin-B receptor to elicit its effect. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway, the response to local injection of endothelin-1 (10 pmol) was significantly inhibited on the lesioned side as compared to the non-lesioned side. In contrast, pretreatment of the rats with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (5 mg/kg, i.p.) or the nitric oxide synthase inhibitor NG-nitro-L-arginine (3 mg/kg, i.p.) did not alter the endothelin-induced release of dopamine. In binding studies, addition of endothelin-1 displaced [125I]endothelin-1 with a Ki of 220 pM. The endothelin-B receptor antagonist BQ788 displaced [125I]endothelin-1 with a Ki of 120 nM, whereas the endothelin-A receptor antagonist BQ123 produced only a 25% displacement at 10 microM, suggesting that endothelin receptors in the striatum are of the endothelin-B subtype. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopamine system, [125I]endothelin-1 binding was reduced by 53% in lesioned striatum compared to non-lesioned striatum, with no difference in the Kd. These data provide evidence that endothelin acts on a homogeneous population of endothelin-B receptors within the striatum to cause the release of dopamine and that a significant proportion of these receptors is located on dopaminergic neurons.

Endothelin interactions with brain dopamine systems

We examined the interactions of endothelin (ET) with dopaminergic systems in rat brain. Using HPLC and radioimmunoassay, we found that striatum contained the highest levels of predominantly ET-1, whereas highest levels of predominantly ET-3 were found in the pituitary. Dopamine depletion in the striatum did not change the levels of immunoreactive ET, even though we have previously found a decrease in the density of ET receptors. In a comparison of spontaneously hypertensive rats (SHR) with Wistar-Kyoto (WKY) rats, ET levels were lower in cerebellum and medulla, with no difference in striatum or in other brain areas. In conclusion, ET is present in high levels in striatum, but these levels are not affected by dopamine depletion or in SHR.