Identification and characterization of adrenergic receptors and catecholamine-stimulated adenylate cyclase in hog pial membranes

Cerebral blood flow decreased by adrenergic stimulation of cerebral vessels in anesthetized newborn pigs with traumatic brain injury

Changes in cerebral blood flow (CBF), pial arteriolar diameter, and arterial blood pressure, gases, and pH were examined before and for 3 hours after fluid-percussion brain injury in alpha-chloralose-anesthetized piglets. The brain injury was induced by a percussion of 2.28 +/- 0.06 atm applied for 23.7 +/- 0.5 msec to the right parietal cortex. Regional CBF was measured with radiolabeled microspheres, and changes in pial arteriolar diameter were monitored in the left parietal cortex using closed cranial windows. Immediately following brain injury, mean blood pressure transiently (for approximately 10 minutes) either increased or decreased and then exhibited a prolonged decrease in all of the animals. The brains showed changes consistent with traumatic brain injury such as subarachnoid hemorrhage, contusions, or reactive axonal swelling; none showed histological evidence of a global alternative pathogenetic mechanism such as hypoxic ischemic damage. While CBF of uninjured control animals did not change over a 3-hour observation period, after brain injury blood flow decreased 30% +/- 1% below the baseline level within 10 minutes and remained there for 2 to 3 hours posttrauma. After adrenergic blockade, CBF did not decrease at any time during the 3-hour period in either the uninjured control or the injured animals. Concomitant with the decreased blood flow after brain injury, pial arteriolar diameter decreased 14% below the preinjury level. However, in piglets treated with adrenoceptor antagonists, uninjured control and brain-injured animals did not show a decrease in pial arteriolar diameter. The present results support the hypothesis that increased sympathetic outflow to the cephalic vasculature following the fluid-percussion brain injury causes cerebral vasoconstriction decreasing pial arteriolar diameter and regional CBF.

The role of the endothelial dependent relaxing factor in the regulation of cerebral circulation

It has recently been demonstrated that vessel dilation induced by several physiological agents is dependent on an intact vascular endothelium. In order to explain this endothelium dependence, it has been hypothesized that a still unknown chemical substance, generically named Endothelium Dependent Relaxing Factor (EDRF) is necessary for physiological vasodilation. The role of this EDRF in the cerebrovascular physiology is not yet well understood. In this article the cerebrovascular physiological control is reviewed in relationship with possible EDRF actions. The importance of endothelial lesions in the cerebrovascular responses is discussed.

Ultrastructure of the meninges at the site of penetration of veins through the dura mater, with particular reference to Pacchionian granulations. Investigations in the rat and two species of New-World monkeys (Cebus apella, Callitrix jacchus)

At the sites where a vein penetrates through the dura mater, two aspects deserve particular attention: (i) The delineation of the perivascular cleft, a space belonging to the interstitial cerebrospinal fluid (CSF) compartment, toward the interior hemal milieu of the dura mater. (ii) The relationship between the perivascular arachnoid layer and the subdural neurothelium at the point of vascular penetration. These problems were investigated in the rat and in two species of New-World monkeys (Cebus apella, Callitrix jacchus). Concerning the first aspect, tight appositions of meningeal cells to the vessel wall, the basal lamina of which is widened and enriched with microfibrils, prevent communication between the interstitial CSF in the perivascular cleft and the hemal milieu in the dura mater. With reference to the second aspect, the perivascular arachnoid cells are transformed into neurothelial cells at the point where they become exposed to the hemal milieu of the dura mater and subsequently continuous with the subdural neurothelium. Leptomeningeal protrusions encompassing outer CSF space can penetrate into the dura mater. These protrusions may expand and branch repeatedly, forming along the wall of the dural sinus Pacchionian granulations. At these sites, however, the structural integrity of the sinus wall and the Pacchionian granulation is not lost. Numerous vesiculations not only in the sinus and vascular walls, but also in the cellular arrays of the Pacchionian granulations or paravascular leptomeningeal protrusions indicate mechanisms of transcellular fluid transport. Moreover, the texture of the leptomeningeal protrusions favors an additional function of these structures as a "volume" buffer.

Exogenous norepinephrine constricts cerebral arterioles via alpha 2-adrenoceptors in newborn pigs

The purpose of this study was to determine whether exogenous norepinephrine mediates cerebrovascular constriction via alpha 1- or alpha 2-adrenoceptors in anesthetized neonatal pigs. Diameters of pial arterioles in anesthetized piglets, 1--6 days old, were investigated using a "closed" cranial window. We examined constrictor effects of norepinephrine on pial arterioles in the absence and presence of relatively selective alpha 1-(prazosin) and alpha 2-(yohimbine) adrenoceptor antagonists (1 mg/kg i.v.). Yohimbine and prazosin inhibited pial arteriolar constriction induced by topical application of clonidine and phenylephrine (10(-6) and 10(-4) M, respectively), and yohimbine did not affect the response to topical phenylephrine. In one group diameter was 188 +/- 13 (mean +/- SEM) micron during control and 146 +/- 12 micron during 10(-5) M norepinephrine (22 +/- 5% constriction). Following yohimbine the same vessels did not constrict significantly. In another group 10(-5) M norepinephrine constricted arterioles by 22 +/- 5%, and this response was unaffected by prazosin (24 +/- 5% constriction). We conclude that pial arterioles are responsive to both alpha 1- and alpha 2-adrenoceptor agonists, that intravenous administration of prazosin and yohimbine results in these drugs crossing the blood-brain barrier and inhibiting constrictor effects of agonists, and that norepinephrine constricts pial arterioles predominantly via alpha 2-adrenoceptors.

Neurochemical coupled actions of transmitters in the microvasculature of the brain

The discovery that monoamine nerves end on the central microvessels of the choroid plexus, pia-arachnoid and parenchyma has prompted an intense investigation as to their physiological and neuropathological roles. The source of the monoamine fibers to the pial vessels and choroid plexus was shown to be the superior cervical ganglion. Ganglionic stimulation causes vasoconstriction or vasodilation of pial vessels, an event depending upon the functional ratio of alpha to beta adrenergic receptors. Moreover, stimulation of the superior cervical ganglion evokes an inhibition of cerebrospinal fluid formation in choroid plexus. The locus coeruleus is the site of adrenergic nerve supply to the parenchymal capillaries and stimulation of this nucleus increases capillary permeability to small molecules and water. Neurotransmitter receptors (adrenergic, histamine, adenosine, dopamine, prostacyclin, prostaglandins and specific amino acids or neuropeptides) have been identified on microvessels and in many instances these transmitter actions are coupled to cyclic AMP synthesis. Moreover, cyclic AMP has been shown to increase the rate of capillary endothelial pinocytosis and produce brain edema. In small vessels containing smooth muscle cells cyclic AMP production improves cerebral blood flow via an initiation of vasodilatory processes. The presence of receptors for serotonin and acetylcholine have likewise been demonstrated to occur on cerebral microvessels. Limited information is available as to the receptor coupled actions of these two transmitters, but cholinergic mechanisms may act to restrict catecholamine-induced formation of cyclic AMP. Altered sensitivity of microvessels to neurotransmitters has been demonstrated following conditions of stroke, hypertension, aging, diabetes and X-irradiation.

Alpha-adrenoreceptors and muscarine receptors in human pial arteries and microvessels: a receptor binding study

Human pial arteries and intraparenchymal microvessels were isolated for enzyme assays and radioligand binding studies of receptors. Special attention was paid to contamination with brain tissue, which was assessed by luxol staining and cerebroside assays for myelin and by scanning electron microscopy. The amount of contamination was approximately 1% for pial vessels and 14% for microvessel preparations. Significant levels of alpha 1-adrenoreceptors (binding sites for [3H]prazosin) and alpha 2-adrenoreceptors (sites labeled by [3H]azidoclonidine) were found in both types of vessels, suggesting that each receptor can modify contractility in these human vessels. Levels of muscarine receptors (sites labeled with [3H]quinuclidinyl benzilate) and choline acetyltransferase activity were considered significant only in pial vessels.

Evidence for alpha 2 adrenergic receptors in bovine cerebral arteries

Specific [3H]yohimbine binding to the homogenates of bovine cerebral arteries was saturable and of high affinity (KD = 21 nM) with a Bmax of 720 fmol/mg protein. On the other hand, there was no detectable specific [3H]prazosin binding to these tissues. Scatchard and Hill plot analyses of specific [3H]yohimbine binding indicated one class of binding sites. Specific binding of [3H]yohimbine was displaced effectively by alpha 2 adrenergic agents and less effectively by alpha 1 adrenergic agents. IC50 values for adrenergic drugs of [3H]yohimbine binding were as follows: yohimbine, 55 nM; tramazoline, 270 nM; clonidine, 580 nM; methoxamine, 15 microM; prazosin, 47 microM, phenylephrine, 49 microM; norepinephrine, 60 microM; epinephrine, 150 microM. These results suggest that alpha adrenergic receptors in bovine cerebral artery are mostly of the alpha 2 subtype.

Characterization of alpha adrenoceptors in pial arteries of the bovine brain

When attempting to characterize the nature of adrenoceptors in bovine pial arteries, we found specific 3H-yohimbine binding was saturable, reversible and of high affinity (KD = 18.3 +/- 1.2 nM) with a Bmax of 687 +/- 27 fmol/mg protein (N = 4). On the other hand, there was no specific 3H-prazosin binding in these tissues. Scatchard and Hill plot analyses of specific 3H-yohimbine binding indicated one class of binding sites. From kinetic analyses of the data, association and dissociation rate constants of 1.6 +/- 0.3 X 10(7) M-1min-1 and 0.51 +/- 0.04 min-1, respectively, were calculated (N = 3). The dissociation constant from the equation KD = K-1/K+1 was 35.7 +/- 7.6 nM, such being in good agreement with the KD value estimated from Scatchard plots. Specific binding of 3H-yohimbine was displaced effectively by alpha 2 adrenergic agents and less effectively by alpha 1 adrenergic agents or beta adrenergic agents. Ki values for adrenergic drugs of 3H-yohimbine binding were as follows: yohimbine, 25 nM; clonidine, 260 nM; methoxamine, 6.8 microM; propranolol, 8.7 microM; prazosin, 21 microM; phenylephrine, 22 microM; noradrenaline, 27 microM; adrenaline, 66 microM; isoproterenol, 3,300 microM. These results indicate that alpha adrenoceptors in the bovine cerebral arteries can be classified as the alpha 2 subtype.

Adenylate cyclase sensitivity to catecholamines and forskolin in rat pia-arachnoid and cerebral microvessels

Adenylate cyclase in homogenates of perfused pia-arachnoid from the rat brain displayed a sensitivity for activation by dl'isoproterenol, l'epinephrine, l'norepinephrine (NE) and fenoterol that was greater than shown by partial beta2 adrenergic agonists (metaproterenol and salbutamol), or partial beta1 adrenergic agonists (tazolol and dobutamine) and an alpha adrenergic agonist (phenylephrine). The addition of the quanosine triphosphate (GTP) analog, Gpp(NH)p(5'-guanylyl imidodiphosphate) to the enzyme preparations resulted in an increased ability of all agents (except tazolol) to activate adenylate cyclase. The agent, forskolin, exerted a very potent activation of the catalytic site of adenylate cyclase in both pia-arachnoid and cerebral microvessels (capillary fraction). The order of potency for adrenergic antagonists to inhibit NE-induced stimulation of the pial enzyme was: propranolol (mixed beta) greater than butoxamine (beta2) greater than phentolamine (alpha1) greater than practolol (beta1). Subchronic injections of reserpine to rats resulted in a pial enzyme that was hyper-responsive to NE and isoproterenol. Similarly, the capillary enzyme displayed an enhanced activity to NE and dopamine (DA). Both high and low Km forms of cyclic AMP (cAMP) phosphodiesterase were detected in developing and adult pia-arachnoid and capillaries. The addition of a calmodulin fraction plus calcium ions did not elicit activation of the high Km enzyme. Moreover, adenylate cyclase during development was sensitive to activation by NE and forskolin. Thus, the pia-arachnoid possesses an adenylate cyclase receptor-coupled system with a mixed response to catecholamines, but which is primarily beta2 in nature. In addition, this system, as well as the capillaries, possesses a capacity to respond to manipulation of monoamine levels.

Neuronal, glial and meningeal localizations of neurotransmitter-sensitive adenylate cyclases in cerebral cortex of mice

The neurotransmitter-sensitive adenylate cyclases, respectively present in the dissociated cells of new-born mouse cerebral cortex (containing both neuronal and glial cells) and in a homogeneous population of glial cells, were compared. The dissociated cells from the cerebral cortex of new-born mice were found to contain Ca2+-, dopamine-, serotonin- and purinergic-sensitive adenylate cyclases. The dopaminergic receptor involved was extensively characterized and was similar to that described in adult animals. Beta-adrenergic-sensitive adenylate cyclase was present but was poorly active. After 3 weeks in culture, the neurons disappeared and a homogeneous population of glial cells was obtained (96% of the cells synthetized glial fibrillary acidic protein). These glial cells contained a highly potent beta-adrenergic-sensitive adenylate cyclase, and adenosine- adn Ca2+-sensitive enzymes. Ca2+ stimulation of the adenylate cyclase was due to the presence of calmodulin. We suggested that the dopaminergic- and serotoninergic-sensitive adenylate cyclases which disappeared during culture are probably localized in neuronal cells. The presence of Ca2+-, adenosine- and beta-adrenergic-sensitive adenylate cyclases in glial cells does not exclude their presence in neuronal cells. For comparison, the same experiments were conducted on meningeal layers of new-born mice and on meningeal cells in culture. They both contained beta-adrenergic- and purinergic-sensitive adenylate cyclases.

Identification of beta-adrenergic receptor binding sites in rat brain microvessels, using [125I]iodohydroxybenzylpindolol

Brain microvessels were prepared from rat cerebral cortex. The purity was confirmed by phase-contrast microscopy and by the measurement of an enzymatic marker, gamma-glutamyltranspeptidase. The microvessel preparation was subjected to radioreceptor assay using a 125I-labelled beta-adrenergic antagonist, hydroxybenzylpindolol (IHYP). The binding was linear with protein concentration up to at least 80% microgram per tube. It was saturated at 200 pM IHYP concentration. The KD value calculated by Scatchard analysis was 69.4 +/- 9.9 pM. The maximum binding (Bmax) was 107 +/- 4 fmol/mg protein. The binding reached equilibrium within 30 min and was dissociated by addition of (-)-propranolol. The inhibitory effects of isomers of propranolol and isoproterenol on this binding showed that (-)-isomers were two orders of magnitude more potent than the (+)-isomers. Other neurotransmitters did not affect IHYP binding. The characteristics of the binding, saturability, high affinity, reversibility and stereospecificity, suggest tha IHYP is bound to beta-adrenergic receptor sites located on brain microvessels.

The sympathetic superior cervical ganglia as peripheral neuroendocrine centers

The superior cervical ganglia (SCG) provide sympathetic innervation to the pineal gland, cephalic blood vessels, the choroid plexus, the eye, carotid body and the salivary and thyroid glands. Removal of the ganglia brings about several neuroendocrine changes in mammals, including the disruption of water balance in pituitary stalk-sectioned rats, and the alteration of normal photoperiodic control of reproduction in hamsters, ferrets, voles, rams and goats. These effects are commonly attributed to pineal denervation. However pinealectomy does not always mimic ganglionectomy in its neuroendocrine sequelae. This paper discusses several examples illustrating the lack of homology of ganglia and pineal removal, including the prolactin release brought about by gonadal steroids in spayed rats, the changes in drinking behaviour caused by ganglionectomy and the control of goitrogenic response to methylmercaptoimidazole in rats. All these examples indicate that SCG removal, at least as far as for neuroendocrinologists and pineal experimenters are concerned, should not be considered simply as "pineal denervation". A functionally relevant link between SCG and the hypothalamus may occur in rats inasmuch as ganglionectomy depresses norepinephrine uptake and increases the number and responses of alpha-adrenoceptors in medial basal hypothalamus. Lastly the SCG are active points of concurrency for hormone signals, as revealed by the metabolic changes induced by steroid and anterior pituitary hormones in these structures even in the absence of intact preganglionic connections, as well as by the existence of putative receptors for some of the hormones, namely, estradiol, testosterone and corticosteroids. The SCG appear to constitute a peripheral neuroendocrine center.

Relation between brain catecholamine receptors and dopaminergic stereotypy in rat strains

In order to examine the potential relation between behaviour elicited by apomorphine and the density of catecholamine receptors, we studied stereotypy and the binding of [3H]spiperone to D2-(or neuroleptic/dopamine) receptors, of [3H]WB-4101 to alpha1-adrenoceptors, and [3H]clonidine to alpha2-adrenoceptors in two strains of rats. The F344 rats exhibited significantly more stereotypy to apomorphine than the Buffalo strain of rats. Other F344 rats which did not receive apomorphine and 34% more D2-receptors in the striatum and 50% more D2-receptors in the olfactory tubercle. Alpha2-Receptors in the frontal cortex were 20% higher in F344 rats when compared to the Buffalo strain, while alpha 1-receptors were the same across both strains. These results suggest a relation between D2-dopamine/neuroleptic receptor density and behavioural responsiveness to the direct-acting and specific dopamine agonist, apomorphine.