Effects of niravoline (RU 51599), a selective kappa-opioid receptor agonist on intracranial pressure in gradually expanding extradural mass lesion

Kappa Opioid Agonist-Induced Diuresis: Characteristics, Mechanisms, and Beyond

Activation of the kappa opioid receptor (KOR) induces antinociception, anti-pruritic activity, diuresis, sedation, and dysphoria. KOR agonist-induced diuresis is characterized as water diuresis, in which water excretion with urine is increased without altering electrolyte excretion. Both centrally and peripherally acting KOR agonists promote diuresis. KOR antagonists block KOR agonist-evoked diuresis suggesting that the diuretic effect is through activation of the KOR. Studies in different experimental animal species and in humans indicate that KOR agonists decrease antidiuretic hormone (ADH) secretion and release from the hypothalamus and posterior pituitary; decrease response to ADH in kidneys; increase renal sympathetic nerve activity; and increase adrenaline, noradrenaline, and dopamine release from the adrenal medulla. The therapeutic potentials of KOR agonists as water diuretics have been studied in animal models of cerebral edema due to ischemia and intracranial mass, hypertension, and cirrhosis. This chapter reviews characteristics, possible mechanisms, as well as therapeutic potentials of KOR agonist-induced diuresis.

Long-term alterations in mu, delta and kappa opioidergic receptors following middle cerebral artery occlusion in mice

Alterations in the opioidergic system may play a role in the molecular mechanisms underlying neurochemical responses to cerebral ischaemia. The present study aimed to determine the delayed expression of mu, delta and kappa opioid receptors, following 1, 2, 7, and 30 days of middle cerebral artery occlusion (MCAO) in mice. Using quantitative autoradiography, we highlighted significant decreases in mu, delta and kappa opioid receptor expression in ipsilateral cortices from day 1 post-MCAO. Moreover, in contralateral nucleus lateralis thalami pars posterior, ipsi- and contralateral nucleus medialis dorsalis thalami, and ipsilateral substantia nigra, pars reticulata (SNr), kappa receptors were increased; mu receptor densities were decreased in nucleus ventralis thalami, pars posterior (VThP), and SNr. delta-Binding sites were increased in the striatum on day 30 post-MCAO. The alterations in opioid receptors in cortical infarcts were correlated with strong histological damage. Further reductions in opioid receptor densities in cortical infarcts were observed at later time points. In subcortical brain regions, opioid receptor densities were also altered but no histological damage was seen, except in the VThP, in which cell density was increased on day 30. Delayed reductions in opioid receptor densities in the infarct appeared as the continuation of the early processes previously demonstrated. However, changes in subcortical opioid receptor expression may correlate with neuronal alterations in remote brain regions. Changes in opioidergic receptor expression in these regions may be involved in the long-term consequences of stroke and could be used as biomarker of neuronal alteration through the use of imaging techniques in the clinic.

Exofocal alterations in opioidergic receptor densities following focal cerebral ischemia in the mouse

In previous studies of our group, we have reported differential alterations in opioidergic receptor subtypes densities in infarcted and periinfarcted brain tissue following middle cerebral artery occlusion (MCAO) in mice. Other studies have also described subcortical alterations consecutive to focal cortical ischemia. For a better understanding of ischemic processes in exofocal areas, we have investigated the evolution of opioidergic receptors following focal cortical ischemia through the quantification of relative binding densities, B(max) and K(d) values for the mu, delta, and kappa subtypes. Our results demonstrate that opioid receptor subtypes exhibit adaptations at distance from the ischemic core, mainly in the striatum, the thalamus, and the substantia nigra. Indeed, mu and delta B(max) values were increased in ventral thalamic nuclei, while kappa relative binding densities were transiently increased in nucleus medialis dorsalis and nucleus lateralis, pars posterior. Moreover, the B(max) of mu and delta receptors were transiently decreased at 6 h post-MCAO in ipsi- and contralateral patches and matrices of the striatum. Conversely, the mu B(max) values were increased in ipsi- and contralateral substantia nigra, pars compacta, and pars reticulata, 24 h following MCAO. In contralateral substantia nigra, pars compacta, kappa B(max) was found to be decreased at 24 h post-MCAO. These alterations could reflect neuronal dysfunction in exofocal brain structures, consecutively to the degeneration of defined neuroanatomical pathways. Our study indicates that opioidergic receptors could be used as markers of the neuronal reorganization that take place in subcortical areas following an ischemic insult of the brain cortex.

Modulation of ocular hydrodynamics and iris function by bremazocine, a kappa opioid receptor agonist

This study was designed to determine the activity of bremazocine (BRE), a relatively selective kappa opioid receptor agonist, on intraocular pressure (IOP), aqueous humor formation and pupil diameter (PD) in conscious, normal, dark-adapted New Zealand white (NZW) rabbits. IOP was measured in normal and unilaterally sympathectomized rabbits using a calibrated pneumatonometer and the aqueous flow rate was determined by the use of a Fluorotron Master. A masked-design study was conducted in which the rabbits' eyes were treated with BRE topically and unilaterally; the fellow eyes received vehicle. IOP and PD measurements were taken at 0.5 hr and 0 time before BRE and 0.5, 1, 2, 3, 4 and 5 hr post-treatment. Fluorophotometry recordings were taken at 1 hr before and 0.5, 1.5, 2.5 and 3.5 hr after topical application of the drug or vehicle. The effect of the relatively selective kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI), on bremazocine-induced changes in IOP, PD and aqueous flow was also determined. BRE (10 and 100 micrograms 25 microliters-1 vehicle) produced dose-related, bilateral reductions in IOP, PD and aqueous humor flow. A large increase in IOP (14 mmHg) was observed when BRE (100 micrograms) was applied to sympathectomized eyes. This ocular hypertensive effect was antagonized when the sympathectomized eyes were pretreated with naloxone (200 micrograms), a non-selective opioid receptor antagonist. BRE (10 and 100 micrograms) decreased the aqueous humor flow rate bilaterally by approximately 48 and 60%, respectively, at 0.5 hr after administration to the ipsilateral eye. Nor-BNI (100 micrograms) antagonized the effect of BRE (10 micrograms) on IOP and aqueous flow rates more effectively than on PD. These data indicate that bremazocine causes reductions in IOP by suppressing aqueous flow, but the ocular hypotensive effects are dependent on the presence of intact sympathetic nerves. Antagonism of BRE's effects on aqueous humor dynamics by nor-BNI suggests that the mechanism of IOP and aqueous flow reduction may involve, in part, an action on kappa receptors. Further experiments are necessary to fully define the opioid receptor populations in the ciliary body.