Differential changes in alpha- and beta-adrenoceptors in the cerebral cortex and hippocampus of the Mongolian gerbil after unilateral brain ischemia

The physiologic implications of isolated alpha(1) adrenergic stimulation

Phenylephrine and methoxamine are direct-acting, predominantly α(1) adrenergic receptor (AR) agonists. To better understand their physiologic effects, we screened 463 articles on the basis of PubMed searches of "methoxamine" and "phenylephrine" (limited to human, randomized studies published in English), as well as citations found therein. Relevant articles, as well as those discovered in the peer-review process, were incorporated into this review. Both methoxamine and phenylephrine increase cardiac afterload via several mechanisms, including increased vascular resistance, decreased vascular compliance, and disadvantageous alterations in the pressure waveforms produced by the pulsatile heart. Although pure α(1) agonists increase arterial blood pressure, neither animal nor human studies have ever shown pure α(1)-agonism to produce a favorable change in myocardial energetics because of the resultant increase in myocardial workload. Furthermore, the cost of increased blood pressure after pure α(1)-agonism is almost invariably decreased cardiac output, likely due to increases in venous resistance. The venous system contains α(1) ARs, and though stimulation of α(1) ARs decreases capacitance and may transiently increase venous return, this gain may be offset by changes in afterload, venous compliance, and venous resistance. Data on the effects of α(1) stimulation in the central nervous system show conflicting changes, while experimental animal data suggest that renal blood flow is reduced by α(1)-agonists, and both animal and human data suggest that gastrointestinal perfusion may be reduced by α(1) tone.

alpha2-Adrenoceptor agonists: shedding light on neuroprotection?

Although alpha(2)-adrenoceptor agonists are widely used for analgesia, anxiolysis, sedation, sympatholysis and as anaesthetic-adjuncts for many years, their potential use as neuroprotectants has so far been confined to laboratory experiments. Despite the large body of evidence from both in vivo and in vitro studies, their exact neuroprotective mechanisms remain elusive. Herein, we review the available literature pertaining to the neuroprotective effect of alpha(2)-adrenoceptor agonists and the possible biochemical and physiological cascades involved in their mechanisms of action. The remarkable safety profile of alpha(2)-adrenoceptor agonists and their high potency of neuroprotection should prompt clinical trials to evaluate their neuroprotective efficacy in humans.

beta-adrenergic receptors primarily are located on the dendrites of granule cells and interneurons but also are found on astrocytes and a few presynaptic profiles in the rat dentate gyrus

In the rat dentate gyrus, beta-adrenergic receptor (beta-AR) activation is thought to be important in mediating the effects of norepinephrine (NE). beta-AR-immunoreactivity (beta-AR-I) was localized in this study by light and electron microscopy in the rat dentate gyrus by using two previously characterized antibodies to the beta-AR. By light microscopy, dense beta-AR-I was observed in the somata of granule cells and a few hilar interneurons. Diffuse and slightly granular beta-AR-I was found in all laminae, although it was most noticeable in the molecular layer. Ultrastructurally, the cytoplasm of granule cell and interneuronal perikarya (some of which contained parvalbumin immunoreactivity) contained beta-AR-I. beta-AR-I was associated primarily with the endoplasmic reticula; however, a few patches were observed near the plasmalemma. Quantitative analysis revealed that the greatest proportion of beta-AR-labeled profiles was found in the molecular layer. The majority of beta-AR-labeled profiles were either dendritic or astrocytic. In dendritic profiles, beta-AR-I was prominent near postsynaptic densities in large dendrites, many of which originated from granule cell somata. Moreover, some beta-AR-I was found in dendritic spines, sometimes affiliated with the spine apparati. Astrocytic profiles with beta-AR-I were commonly found next to unlabeled terminals which formed asymmetric (excitatory-type) synapses with dendritic spines. Additionally, beta-AR-I was observed in a few unmyelinated axons and axon terminals, many of which formed synapses with dendritic spines. Dual-labeling studies revealed that axons and axon terminals containing tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme, often were near both neuronal and glial profiles containing beta-AR-I. These studies demonstrate that hippocampal beta-AR-I is localized: 1) principally in postsynaptic sites on granule cells and a few interneurons (some of which were basket cells); and 2) in glial processes. These observations add further support to the contention that beta-AR-activation modulates synaptic function through disparate pathways: directly, at either postsynaptic densities or presynaptic processes, or indirectly, through adjacent glial processes.

Effect of intracerebral norepinephrine depletion on outcome from severe forebrain ischemia in the rat

Manipulations of plasma catecholamine concentrations influence outcome from ischemic brain insults. It has been suggested that these effects are mediated by influences on brain catecholamine concentrations. This study examined whether major changes in brain norepinephrine concentrations can alter outcome from severe forebrain ischemia. Sprague-Dawley rats were administered 50 mg/kg i. p. N-(chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) or were left untreated (control). One week later, these rats were subjected to either 7 or 8 min of normothermic forebrain ischemia (bilateral carotid occlusion and MABP=30 mmHg) and allowed to recover for 4 days. Histologic damage was then evaluated. In other control and DSP-4-treated animals, hippocampal microdialysate norepinephrine concentrations were measured before, during and after 8 min of forebrain ischemia. Norepinephrine concentrations were also determined in brain homogenates from non-ischemic DSP-treated and control rats. A 95% depletion of norepinephrine was observed in brain homogenates from non-ischemic DSP-4-treated rats compared with control. During ischemia, microdialysate norepinephrine concentrations increased in control but not in DSP-4-treated rats (P=0.002). For plasma, intra-ischemic epinephrine concentrations increased 8-10-fold and returned to baseline values post-ischemia with no differences between groups. Plasma norepinephrine values remained unchanged in both groups. Histologic damage resulting from either 7 or 8 min of ischemia in hippocampal structures, caudoputamen, and neocortex was similar between DSP-4-treated and control groups. This study could not identify any effect of major changes in brain norepinephrine concentrations on ischemic brain damage. These data indicate that peripheral catecholamine effects on near-complete forebrain ischemic outcome are unlikely to be mediated by effects on central catecholamine concentrations.

Pre-ischemic depletion of brain norepinephrine decreases infarct size in normothermic rats exposed to transient focal cerebral ischemia

This study examined the importance of brain norepinephrine concentration on outcome from a focal ischemic insult. Fasted temperature-controlled male Wistar rats pretreated with DSP-4, (N-(chloroethyl)-N-ethyl-2-bromobenzylamine), to selectively deplete brain norepinephrine, were subjected to reversible filament occlusion of the middle cerebral artery for 75 min in the awake state. After 3 days recovery, total infarct volume in DSP-4 treated rats (185 +/- 107 mm3) was reduced vs. untreated control animals (242 +/- 71 mm3, P = 0.04). Subcortical infarct volume was also smaller in the DSP-4 group (93 +/- 44 vs. 121 +/- 28 mm3, P = 0.02). Cortical infarct volume was not statistically different between groups. Neurologic function correlated with infarct-size. These findings suggest that brain norepinephrine affects stroke development either by direct neuronal toxicity and/or through influences on the penumbral circulation. Dampening of the central stress response induced by the onset of stroke may thus be advantageous.

Effects of isoflurane, ketamine, and fentanyl/N2O on concentrations of brain and plasma catecholamines during near-complete cerebral ischemia in the rat

We postulated that adrenergic responses to global cerebral ischemia are anesthetic-dependent and similar in both brain and arterial blood. Rats were anesthetized with isoflurane (1.4%), ketamine (1 mg x kg(-1) x min(-1)), or fentanyl (25 microg x kg(-1) x h(-1))/70% N2O. The carotid arteries were occluded for either 20 min with mean arterial pressure (MAP) 50 mm Hg (incomplete ischemia) or 10 min with MAP 30 mm Hg (near-complete ischemia). Norepinephrine was measured in hippocampal microdialysate. Norepinephrine and epinephrine were measured in arterial plasma. In both hippocampus and plasma, basal norepinephrine was similar among anesthetics. During incomplete ischemia, hippocampal norepinephrine was twofold greater with fentanyl/N2O than with isoflurane (P = 0.037), but plasma norepinephrine and epinephrine were similar and unchanged among all three anesthetics. During near-complete ischemia, hippocampal norepinephrine was threefold greater with ketamine than fentanyl/N2O (P = 0.005), whereas plasma norepinephrine and epinephrine were markedly greater with fentanyl/N2O than with ketamine (P < 0.0005) or isoflurane (P = 0.05). There was no correlation between norepinephrine concentrations in hippocampus and plasma for either incomplete or near-complete ischemia. This study demonstrates that adrenergic responses to global ischemia are anesthetic-dependent, particularly during more severe insults. The absence of a correlation between plasma and brain catecholamine concentrations indicates that adrenergic responses to ischemia are independent in brain and blood.

IMPLICATIONS

It has been proposed that anesthetics modulate cerebral ischemic outcome by influencing peripheral adrenergic responses to ischemia. This experiment demonstrates that anesthetics differentially modulate adrenergic responses to ischemia but that effects in plasma and brain are independent. This suggests that events detected in the peripheral circulation do not implicate direct mechanisms of action of catecholamines at the neuronal/glial level.

Temporal changes in glial fibrillary acidic protein messenger RNA and [3H]PK11195 binding in relation to imidazoline-I2-receptor and alpha 2-adrenoceptor binding in the hippocampus following transient global forebrain ischaemia in the rat

Immunohistochemical studies have demonstrated that following global forebrain ischaemia the selective neuronal loss that occurs in the CA1 pyramidal cell layer of the hippocampus is accompanied by a reactive astrocytosis, characterized by increases in glial fibrillary acidic protein, and activation of microglia. In this study the spatial changes in glial fibrillary acidic protein messenger RNA levels in the hippocampus have been mapped four, eight, 12, 16 and 20 days following 10 min of global forebrain ischaemia in the rat and related to changes in [3H]PK11195 binding to peripheral benzodiazepine receptors, a putative marker of activated microglia. Recent studies have suggested that the imidazoline-I2-receptor, one of a class of non-adrenergic receptors related to, but structurally and functionally distinct from alpha 2-adrenoceptors, may have a functional role in controlling the expression of glial fibrillary acidic protein. To explore this possibility further we have also mapped changes in imidazoline-I2-receptor and alpha 2-adrenoceptor binding sites. Following transient ischaemia there was a marked, biphasic increase in glial fibrillary acidic protein messenger RNA levels throughout the vulnerable CA1 region of the hippocampus, peaking four days after ischaemia and then increasing gradually during the remainder of the study period. There was also a sustained increase in [3H]PK11195 binding, however, in contrast to the initial increase in glial fibrillary acidic protein messenger RNA levels that peaked four days after ischaemia the density of [3H]PK11195 binding increased rapidly in all strata of the CA1 region over the first eight days and then increased more slowly throughout days 12 to 20. Despite the marked increase in glial fibrillary acidic protein messenger RNA levels there was no concomitant alteration in imidazoline-I2-receptor binding sites detected using the specific radioligand, [3H]2-(2-benzofuranyl)-2-imidazoline, although alpha 2-adrenoceptor binding was decreased at eight days after ischaemia and did not recover. The time-course and biphasic nature of the changes in the astrocytic marker, glial fibrillary acidic protein messenger RNA, in the hippocampus following ischaemia may reflect different functions of glial fibrillary acidic protein-reactive astrocytes in the post-ischaemic period. Differences in temporal expression of glial fibrillary acidic protein messenger RNA and [3H]PK11195 binding support the proposed localization of peripheral benzodiazepine receptors on activated microglia, as distinct from reactive astrocytes. There was no evidence in the present study that imidazoline-I2-receptors are functionally linked to glial fibrillary acidic protein expression as the reactive astrocytosis in the hippocampus following ischaemia was not associated with changes in imidazoline-I2-receptor binding site density.