Effect of sympathomimetic drugs and corresponding receptor antagonists on the rate of cerebrospinal fluid production

The Cerebrospinal Fluid Secretion Rate Increases in Awake and Freely Moving Rats but Differs With Experimental Methodology

Cerebrospinal fluid (CSF) dynamics hold implications for neurological health. Despite its importance, accurate quantification of the CSF secretion rate remains a challenge due to methodological controversies and the influence of anesthesia. A novel technique is established to determine CSF dynamics in awake and freely moving rats, and the CSF secretion is quantified with three different methodologies. The CSF secretion rate is higher in awake rats than in anesthetized rats, the latter demonstrating no requirement for mechanical ventilation for optimal CSF quantification. The CSF secretion rate is ≈10-fold lower with the "direct method" than with the ventriculo-cisternal perfusion assay, although the relative acetazolamide-mediated reduction in CSF secretion is similar across three tested methods. The findings demonstrate the importance of awake models for optimal quantification of the absolute rate of CSF secretion but highlight the versatility of methodologies for the determination of relative changes in CSF secretion associated with inhibitors, age, sex, and various pathologies.

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Fluid transport in the brain

The brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully toward processing information and behaving, while homeostatic functions predominate during sleep. The blood-brain barrier establishes a stable environment that is optimal for neuronal function, yet the barrier imposes a physiological problem; transcapillary filtration that forms extracellular fluid in other organs is reduced to a minimum in brain. Consequently, the brain depends on a special fluid [the cerebrospinal fluid (CSF)] that is flushed into brain along the unique perivascular spaces created by astrocytic vascular endfeet. We describe this pathway, coined the term glymphatic system, based on its dependency on astrocytic vascular endfeet and their adluminal expression of aquaporin-4 water channels facing toward CSF-filled perivascular spaces. Glymphatic clearance of potentially harmful metabolic or protein waste products, such as amyloid-β, is primarily active during sleep, when its physiological drivers, the cardiac cycle, respiration, and slow vasomotion, together efficiently propel CSF inflow along periarterial spaces. The brain's extracellular space contains an abundance of proteoglycans and hyaluronan, which provide a low-resistance hydraulic conduit that rapidly can expand and shrink during the sleep-wake cycle. We describe this unique fluid system of the brain, which meets the brain's requisites to maintain homeostasis similar to peripheral organs, considering the blood-brain-barrier and the paths for formation and egress of the CSF.

Noradrenaline in the aging brain: Promoting cognitive reserve or accelerating Alzheimer's disease?

Many believe that engaging in novel and mentally challenging activities promotes brain health and prevents Alzheimer's disease in later life. However, mental stimulation may also have risks as well as benefits. As neurons release neurotransmitters, they often also release amyloid peptides and tau proteins into the extracellular space. These by-products of neural activity can aggregate into the tau tangle and amyloid plaque signatures of Alzheimer's disease. Over time, more active brain regions accumulate more pathology. Thus, increasing brain activity can have a cost. But the neuromodulator noradrenaline, released during novel and mentally stimulating events, may have some protective effects-as well as some negative effects. Via its inhibitory and excitatory effects on neurons and microglia, noradrenaline sometimes prevents and sometimes accelerates the production and accumulation of amyloid-β and tau in various brain regions. Both α2A- and β-adrenergic receptors influence amyloid-β production and tau hyperphosphorylation. Adrenergic activity also influences clearance of amyloid-β and tau. Furthermore, some findings suggest that Alzheimer's disease increases noradrenergic activity, at least in its early phases. Because older brains clear the by-products of synaptic activity less effectively, increased synaptic activity in the older brain risks accelerating the accumulation of Alzheimer's pathology more than it does in the younger brain.

Direct Measurement of Cerebrospinal Fluid Production in Mice

The emerging interest in brain fluid transport has prompted a need for techniques that provide an understanding of what factors regulate cerebrospinal fluid (CSF) production. Here, we describe a methodology for direct quantification of CSF production in awake mice. We measure CSF production by placing a catheter in a lateral ventricle, while physically blocking outflow from the 4^th ventricle. Using this methodology, we show that CSF production increases during isoflurane anesthesia, and to a lesser extent with ketamine/xylazine anesthesia, relative to the awake state. Aged mice have reduced CSF production, which is even lower in aged mice overexpressing amyloid-β. Unexpectedly, CSF production in young female mice is 30% higher than in age-matched males. Altogether, the present observations imply that a reduction in CSF production might contribute to the age-related risk of proteinopathies but that the rate of CSF production and glymphatic fluid transport are not directly linked.

Liu et al. develop a method for direct quantification of cerebrospinal fluid (CSF) production in awake mice. Using this method, the authors evaluate the effect of brain states, ages, sex, anesthetic types, and amyloid-β burden on CSF production.

Physiological roles of aquaporins in the choroid plexus

The choroid plexus is a specialized tissue that lines subdomains within the four ventricles of the brain where most of the cerebrospinal fluid is produced. Maintenance of an equilibrium in volume and composition of the cerebrospinal fluid (CSF) is vital for a normal brain function, ensuring an optimal environment for the neurons. The necessarily high water permeability of the choroid plexus barrier is made possible by the abundant expression of a water channel, Aquaporin-1 (AQP1), on the apical side of the membrane from early stages of development through adulthood. Data from studies of AQP1 suggest that it also can contribute as a gated ion channel, and suggest that the AQP1-mediated ionic conductance has physiological significance for the regulation of cerebrospinal fluid secretion. The regulation of AQP1 ion channels could be one of several transport mechanisms that contribute to the decreased CSF secretion in response to endogenous signaling molecules such as atrial natriuretic peptide. Numerous classes of ion channels and transporters are targeted specifically to each side of the cellular membrane, and they all work in concert to secrete CSF. Several signaling cascades have a direct effect on transporters and ion channels present in the choroid plexus epithelium, altering their transport activity and therefore modulating the net transcellular movement of solutes and water. Several neurotransmitters, neuropeptides, and growth factors can influence CSF secretion by direct effect on transport mechanisms of the epithelium. The mammalian choroid plexus receives innervation from noradrenergic sympathetic fibers, cholinergic and peptidergic fibers that modulate CSF secretion. Water imbalance in the brain can have life-threatening consequences resulting from altered excitability and neurodegeneration, disruption of the supply of nutrients, loss of signaling molecules, and the accumulation of unwanted toxins and metabolites. Understanding the mechanisms involved in the modulation of CSF secretion is of fundamental importance. An appreciation of AQP1 as an ion channel in addition to its role as a water channel should offer new targets for therapeutic strategies in diseases involving water imbalance in the brain.

Pseudotumor cerebri

Pseudotumor cerebri is a perplexing syndrome of increased intra-cranial pressure without a space-occupying lesion. The terminology for the disorder has changed over the years and the diagnostic criteria revised to reflect advances in diagnostic technology and insights into the disease process. The classification and nomenclature depend on the presence or absence of an underlying cause. When the diagnostic criteria are followed, a secondary etiology is unlikely. When no secondary cause is identified, the syndrome is termed "idiopathic intracranial hypertension."

Dopamine receptor immunohistochemistry in the rat choroid plexus

1. Earlier studies have demonstrated a high density of dopamine D1-like receptor binding in the choroid plexus by light microscope autoradiography, but the dopaminergic specificity of this binding was questioned. 2. In this study the localization of dopamine receptor subtypes was investigated in the rat choroid plexus by Western blot analysis and immunohistochemistry using antibodies raised against dopamine D1-D5 receptor protein. 3. Western blot analysis revealed reactivity with immune bands of approximately 50 and 51 KDa corresponding to dopamine D1 and D5 receptors, respectively. Dopamine D1-like (D1 and D5) receptor protein immunoreactivity insensitive to superior cervical ganglionectomy was located in smooth muscle of choroid arteries and to a larger extent within choroid plexus epithelium. 4. Western blot analysis revealed reactivity with immune bands of approximately 53 KDa and 40-42 KDa corresponding to dopamine D2 and D4 receptors, respectively, and no dopamine D3 receptor reactivity. Dopamine D2-like receptor protein immunoreactivity displayed a distribution similar to that of tyrosine-hydroxylase (TH)-immunoreactive sympathetic fibres and disappeared after superior cervical ganglionectomy. It consisted in the expression of dopamine D2 and to a lesser extent of D4 receptor protein immunoreactivity perivascularly and associated with choroid epithelium. No D3 receptor protein immunoreactivity was found in rat choroid plexus. 5. The above results indicate that rat choroid plexus expresses dopamine receptor protein, being dopamine D1-like receptors predominant in epithelium and arterial smooth muscle and D2-like receptors in sympathetic nerve fibres supplying choroid plexus epithelium and vasculature. 6. These findings suggests that dopamine receptors with a different anatomical localization may modulate production of cerebrospinal fluid.

Vasopressin mediates the inhibitory effect of central angiotensin II on cerebrospinal fluid formation

Angiotensin II infused at low doses into the cerebral ventricles decreases cerebrospinal fluid (CSF) production. Since central angiotensin II also activates the sympathetic nervous system and promotes vasopressin release, the roles of these two factors in mediating the inhibitory effect of angiotensin II on CSF formation were studied. CSF production was measured in rats by the ventriculocisternal perfusion method. During central angiotensin II infusion (5 pg min(-1)), the following adrenoceptor antagonists were administered intravenously (i.v.): phentolamine (alpha1/alpha2, 2 mg/kg per h), prazosin (alpha1, 1 mg/kg per h), and propranolol (beta, 1 mg/kg per h). None of these agents affected the inhibitory effect of angiotensin II on CSF formation. In comparison, in animals administered i.v., the vasopressin V1 receptor antagonist, d(CH2)5Tyr(Me)Arg-vasopressin (10 microg/kg per h), the angiotensin II-induced decrease in CSF production was abolished. Our observations indicate, therefore, that vasopressin mediates the inhibitory effect of central angiotensin II on CSF formation.

Periventricular morphology in the diencephalon of antarctic notothenioid teleosts

We have examined the subependymal region of the diencephalic third ventricle in notothenioid perciforms and report a pattern of neuropil expansions that appears to be phyletically derived for notothenioids and their outgroups but that is otherwise unique among vertebrates. We recognize five types of expansions based on their composition (from less dense neuropil to sacs) and width or protrusion into the third ventricle. In the species with the most elaborate morphology, Trematomus bernacchii, bilateral subependymal expansions fuse along the midline to form a single sac within the ventricular cavity. The extent of these expansions loosely corresponds with phyletic position but also (and perhaps more importantly) is correlated with the habitation of cold water (r2 = 0.48; P = 0.012). Furthermore, subependymal expansion type is positively correlated with the maximum size of the soma of neurons in two hypothalamic nuclei, the preopticus magnocellularis (r2 = 0.54; P = 0.006) and the lateralis tuberis (r2 = 0.40; P = 0.038). These nuclei project to the pituitary and contain cerebrospinal fluid-contacting neurons. In considering the functional consequences of this morphology, we cannot dismiss the possibility that these structures form a specialized enteroceptive system tied to the monitoring of cerebrospinal and extracellular fluid components, including antifreeze glycopeptides and inorganic ions.

Na+,K(+)-ATPase in the choroid plexus. Regulation by serotonin/protein kinase C pathway

In the choroid plexus, the ion pump Na+,K(+)-ATPase regulates the production of cerebrospinal fluid. We now report that incubation of choroid plexus with an activator of protein kinase C, phorbol 12,13-dibutyrate, strongly stimulates the phosphorylation of Na+,K(+)-ATPase and inhibits its activity. Similar effects were obtained with serotonin, which in the choroid plexus stimulates phosphoinositide turnover, thereby activating protein kinase C. Serotonin (10 microM) increased by about 10-fold the amount of phosphorylated Na+,K(+)-ATPase and significantly reduced its activity. Two-dimensional peptide mapping showed comigration of Na+,K(+)-ATPase phosphorylated by either phorbol 12,13-dibutyrate or serotonin in intact cells and by protein kinase C in vitro. These results demonstrate that first messengers can regulate the activity of Na+,K(+)-ATPase through a mechanism involving protein phosphorylation. Moreover, they provide a plausible mechanism for the demonstrated ability of serotonin to decrease cerebrospinal fluid production.

Cl- current activation in choroid plexus epithelial cells involves a G protein and protein kinase A

The involvement of GTP-binding proteins (G proteins) in the regulation of the Cl- conductance in rat choroid plexus epithelial cells was investigated, using the whole cell patch-clamp technique. Intracellular application of a nonhydrolyzable GTP analogue, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 0.1-0.2 mM), evoked a transient increase in the Cl- conductance. The activated Cl- current exhibited inward rectification and was independent of time at hyperpolarizing or depolarizing voltage pulses. The effect of GTP gamma S was inhibited by a nonhydrolyzable GDP analogue, guanosine 5'-O-(2-thiodiphosphate) (2 mM), and by an inhibitor of protein kinase A, H-89, but was not affected by chelation of cytosolic Ca2+ with 5 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. GTP gamma S failed to activate the current when ATP was omitted from the pipette solution. Intracellular application of adenosine 3',5'-cyclic monophosphate (cAMP; 0.25 mM) or the catalytic subunit of protein kinase A activated a similar Cl- current. These results suggest that G proteins activate Cl- channels via a cAMP-dependent pathway in rat choroid plexus.

Angiotensin II decreases the rate of production of cerebrospinal fluid

The choroid plexus, which produces cerebrospinal fluid (CSF), contains receptors for angiotensin II and a very high concentration of angiotensin-converting enzyme. Circulating angiotensin II decreases blood flow to the choroid plexus. The first goal of this study was to examine the hypothesis that angiotensin II decreases the production of CSF. The second goal was to determine whether effects of angiotensin II on the production of CSF were receptor-mediated. Production of CSF was measured in chloralose-anesthetized rabbits using ventriculocisternal perfusion of artificial CSF containing blue dextran. Rabbits received either vehicle, angiotensin II, angiotensin II in the presence of an angiotensin II antagonist (saralasin), or saralasin intravenously. Increases in blood pressure, during administration of angiotensin II, were prevented by withdrawal of blood. Under control conditions, CSF production averaged 7.2 +/- 0.2 microliters/min (mean +/- S.E.). Angiotensin II (100 ng/kg/min i.v.) decreased CSF production by 24 +/- 3% (P < 0.05, n = 8). In the presence of saralasin (1 microgram/kg/min i.v.), angiotensin II had no significant effect on CSF production (-4 +/- 6%, P > 0.05, n = 7). Vehicle did not affect CSF production significantly (-2 +/- 6%, P > 0.05, n = 7). Saralasin alone decreased production of CSF (-21 +/- 5%, P < 0.05, n = 7). To test the specificity of saralasin in blocking effects of angiotensin II receptor stimulation on CSF production, the carbonic anhydrase inhibitor acetazolamide was administered in the presence and absence of saralasin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of atriopeptin on production of cerebrospinal fluid

We reported previously that intravenous infusion of atriopeptin increases blood flow to the choroid plexus. The first goal of this study was to determine whether blood-borne atriopeptin increases the production of CSF. Ventriculocisternal perfusion was used to measure the production of CSF in anesthetized rabbits. Atriopeptin increased blood flow to the choroid plexus (measured with microspheres) but did not alter the production of CSF. The second goal of the study was to determine whether intracerebroventricular injection of atriopeptin affects the production of CSF. Injection of atriopeptin into the cerebral ventricles increased blood flow to the choroid plexus but produced a small decrease in production of CSF. In summary, blood-borne and intraventricular atriopeptin increase blood flow to the choroid plexus, but do not increase the production of CSF.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Serotonergic effects on carbonic anhydrase activity in the choroid plexus

The carbonic anhydrase (CA) activities in the choroid plexus of dogs were investigated by electron microscopy, and the effects of 5-hydroxytryptophan (5-HTP) on them were examined to elucidate the participation of serotonin in the production of cerebrospinal fluid. The reaction products yielded by the method employed proved to be CA activity by elimination tests using acetazolamide (Diamox). Following administration of 5-HTP, the CA activities fell to 43.3% of the control value, that is, approximately 56% of the CA activities in the choroid plexus were affected by serotonin. When tetrabenazine (TBZ) was administered, the CA activities in the choroid plexus decreased to 22.4% of the control value. These results suggest that the CA activity in the choroid plexus is remarkably suppressed when nervous control of the choroid plexus is disturbed by the administration of a monoamine denervator such as TBZ. The present data indicate that the serotonergic inhibitory effect on the CA activity in the choroid plexus may be less predominant than that of TBZ and acetazolamide.

Simultaneous and continuous measurement of choroid plexus blood flow and cerebrospinal fluid production: effects of vasoactive intestinal polypeptide

Using laser-Doppler flowmetry during ventriculocisternal perfusion with inulin-[14C]carboxylic acid, choroid plexus blood flow (CPBF) and CSF production were measured simultaneously in rats during periods of 3 h. Blood flow and CSF production decreased only slightly during control experiments. The effect of vasoactive intestinal polypeptide (VIP) was studied at different concentrations of the peptide given either intraventricularly or intravenously. Intraventricular administration of VIP (10(-9) or 10(-7) M) resulted in a decrease in CSF production of up to 30%, while CPBF increased by 20%, also demonstrating that CSF production and blood flow are not directly coupled in the choroid plexus. When infused intravenously, VIP (10 or 100 pmol/kg/min) increased CPBF, an effect partly antagonized at higher concentrations owing to a VIP-induced systemic hypotension. No effect of VIP on CSF production could be seen with intravenous administration.

Net production of cerebrospinal fluid is decreased by SCH-23390

A high density of binding sites for the ligands 3H-SCH-23390 and 3H-SKF-83566 has been found in the choroid plexus. Although these sites have similar pharmacology to D1 dopamine receptors, the high-affinity component of 3H-SCH-23390 binding in the choroid plexus has been identified as the 5-HT1c subtype of serotonin receptor. We investigated the possible role of these receptors in modulating the production of cerebrospinal fluid (CSF) in rats. (R) SCH-23390 produced up to a 50% decrease in net CSF production, compared to saline. This compound is a partial agonist at 5-HT1c serotonin receptors, and an antagonist at D1 dopamine receptors. The (S) enantiomer of SCH-23390 (SCH-23388) was ineffective. Drugs interacting with receptors for neurotransmitters in the choroid plexus may hold promise for the treatment of patients with increased intracranial pressure, including those with mass lesions, head trauma, acute or chronic hydrocephalus, or pseudotumor cerebri.

Corticosteroid action on choroid plexus: reduction in Na+-K+-ATPase activity, choline transport capacity, and rate of CSF formation

Glucocorticoids have a well-known clinical effect on brain edema and intracranial hypertension, but the mechanism of action is still poorly understood. In the present report the effect of beta-methasone on choroid plexus transport and CSF formation was studied. Following 5 days of daily treatment with betamethasone the CSF production rate in rabbits was reduced by 43% as measured by ventriculo-cisternal perfusion with radioactive inulin. Accordingly, the transport capacity in the choroid plexus, measured in terms of choline uptake and accumulation in vitro, and the activity of Na+--K+-ATPase decreased in both rabbit (in the lateral ventricles by 31 and 31%, respectively) and rat (by 16 and 24%, respectively). Thus, the demonstrated influence of glucocorticoids on these functions of the choroid plexus seem to be important components in their therapeutic effect on intracranial hypertension.

Changes in transport functions of isolated rabbit choroid plexus under the influence of oestrogen and progesterone

Isolated choroid plexuses from rabbits were used to determine uptake and accumulation of 10(-5) M radiolabelled choline (expressed as tissue/medium ratio) and the activities of various types of ATPases (based on ouabain inhibition and bicarbonate stimulation) following pre-treatment of the animals with 0.5 mg kg-1 17-beta-oestradiol, alone or in combination with 2 mg kg-1 progesterone. The combined treatment reduced the choline uptake by 35% and also lowered the activity of Na+,K+-ATPase by 31%, without influencing tissue wet weight. The reduction in HCO3-ATPase was smaller and not statistically significant. There was a tendency also for oestrogen alone to lower these activities, but only by less than 20%. The Ca2+,Mg2+-ATPase activity was not significantly affected by any of the hormones.

Effect of 5-hydroxytryptamine on the rate of cerebrospinal fluid production in rabbit

Infusion of 5-hydroxytryptamine in mock cerebrospinal fluid of rabbits lowered cerebrospinal fluid production dose-dependently by nearly 30%. The effect was amplified in the presence of the monoamine oxidase inhibitor, nialamide. The 5-hydroxytryptamine-induced inhibition was partly counteracted by ketanserin, and further addition of practolol completely blocked the reduction in cerebrospinal fluid formation, suggesting that both 5-hydroxytryptamine receptors and beta 1-adrenergic receptors were involved in the response mediated by both the secretory epithelium and the vascular bed of the plexus.

Effect of adrenergic agonists on eicosanoid output from isolated rabbit choroid plexus and iris-ciliary body

Prostanoid production by rabbit choroid plexus (CP) and iris-ciliary body (ICB), and the effects of adrenergic agonists thereon, were studied in vitro. Immunoreactive prostaglandin (PG) E2 was the major prostanoid released by both tissues; the output from ICB was some two orders of magnitude greater than from CP. Immunoreactive 6-keto PGF1 alpha and thromboxane (TX) B2, the dehydration products of prostacyclin and TXA2, respectively, were detected in smaller quantities. Epinephrine stimulated the outputs of PGE2 and 6-keto PGF1 alpha, but not of TXB2, from both tissues. ICB responded to epinephrine concentrations of 10(-4) and 10(-5), while only 10(-4) was effective in stimulating prostanoid synthesis in the CP. Phenylephrine, an adrenergic agonist, stimulated prostanoid output from the ICB, but not from the CP. It is concluded that adrenergic mechanisms stimulate the biosynthesis of prostanoids in the rabbit CP and ICB. The implications of such interactions to aqueous humor and cerebrospinal fluid dynamics, or to other processes in brain and ocular physiology, are discussed.

Agonist-induced phosphoinositide hydrolysis in choroid plexus

5-Hydroxytryptamine (5-HT, serotonin) stimulates phosphoinositide hydrolysis in choroid plexus by interacting with the 5-HTlc site. In the present study, the effects of 5-HT were compared with those of other agonists. 5-HT stimulates a rapid release of all three inositol sugars in a mianserin-sensitive manner. Inositol bisphosphate and inositol trisphosphate levels increase about twofold within 2.5 min, whereas inositol monophosphate levels are not appreciably elevated until 5 min. In contrast, glutamate, carbachol, histamine, substance P, and vasopressin, agents that increase phosphoinositide hydrolysis in other tissues, do not stimulate this response in choroid plexus. High concentrations of norepinephrine increase inositol phosphate release in choroid plexus, but this effect is apparently mediated by activation of the 5-HTlc site. The depolarizing agents KCl and veratrine also fail to stimulate phosphoinositide hydrolysis in choroid plexus. These results, combined with the finding that the phosphoinositide response to 5-HT is insensitive to tetrodotoxin, suggest that the effects of 5-HT are not secondary to neurotransmitter release. Furthermore, an indirect effect mediated via arachidonic acid metabolism is unlikely, since inhibitors of cyclooxygenase and lipoxygenase do not reduce the 5-HT response. We conclude, therefore, that phosphoinositide hydrolysis is the transducing mechanism of the 5-HT 5-HTlc receptor and that the choroid plexus will serve as a useful model system for studies of this receptor.

A unique serotonin receptor in choroid plexus is linked to phosphatidylinositol turnover

A novel serotonergic binding site, the 5-HT1C site, has been characterized recently in choroid plexus and several brain regions. The biochemical and physiological roles of this site have not been previously described. In this report we show that serotonin (5-hydroxytryptamine, 5-HT) stimulates phosphatidylinositol turnover in rat choroid plexus. The pharmacology of serotonin-stimulated phosphatidylinositol hydrolysis in choroid plexus was compared to the pharmacology in cerebral cortex, where this response is mediated by the serotonin 5-HT2 receptor. Serotonin increased phosphatidylinositol turnover in choroid plexus by 6-fold and in cerebral cortex by 2.5-fold. Serotonin was greater than 10-fold more potent in choroid plexus (EC50 = 46 nM) than in cerebral cortex (EC50 = 540 nM). The serotonin antagonists ketanserin, mianserin, and spiperone inhibited the response in the two tissues with different potencies. In cerebral cortex all three exhibited nanomolar affinities consistent with their potencies at the 5-HT2 site. In choroid plexus, however, the rank order (mianserin greater than ketanserin much greater than spiperone) and absolute potencies were consistent with binding to the 5-HT1C site. These data suggest that the 5-HT1C site in choroid plexus is a functional receptor that utilizes phosphatidylinositol turnover as its biochemical effector system.

A comparative study of the innervation of the choroid plexus in amphibia

The aminergic and cholinergic innervation of choroid plexuses in three species of amphibia was investigated. Plexuses of the Japanese toad and the bullfrog had poor innervation by adrenergic nerves of sympathetic origin, but in the clawed toad, these plexuses were heavily innervated by adrenergic axons from ganglion cells located in the plexus stroma. Nerve fibers positive for acetylcholinesterase were not found in the plexuses, except for a few fibers with very weak enzyme activity in the clawed toad.

Influence of thyroid hormones on transport function and Na+-K+-ATPase activity in the rat choroid plexus

The sympathomimetic stimulation of choroid plexus transport and secretion in rat seems to be mediated by beta-adrenergic receptors. In the present report the effect of induced changes in thyroid function on transport mechanisms in the rat choroid plexus was studied. Following induction of hyperthyroidism (treatment with T3 for 10 days) the tissue/medium ratio (T/M) for choline uptake in choroid plexus in vitro decreased significantly by 68%. The Na+-K+-ATPase activity showed a statistically significant increase of about 16%. Also following cervical sympathectomy, T3 caused a reduction of the T/M for choline, to the same level as in the non-sympathectomized animals, while the effect of T3 on the Na+-K+-ATPase activity was changed into a 22% decrease. Hypothyroidism (administration of PTU in the drinking water) had little or no effect on the uptake and accumulation of choline in the choroid plexus. The Na+-K+-ATPase activity was reduced by 40%, in contrast to the stimulating effect of T3. There is, hence, reason to believe that the transport of choline in the choroid plexus is only partly regulated by adrenergic mechanisms acting via Na+-K+-ATPase. The major effect of T3 on the choline uptake may be exerted by a mechanism different from the ATPase activity and not involving adrenergic receptors.

Stimulation of cyclic adenosine 3',5'-monophosphate formation in rabbit choroid plexus by beta-receptor agonists and vasoactive intestinal polypeptide

The formation of cyclic adenosine 3',5'-monophosphate (cAMP) was determined in blood-free choroid plexus homogenates from all ventricles of rabbit using a competitive protein binding technique. Previous stimulation of the intact plexus tissue in vitro with sympathomimetic drugs or vasoactive intestinal polypeptide (VIP) leads to increased local synthesis of cAMP. Tests with selective beta-receptor agonists and antagonists suggested that beta 1-receptors predominate, which is consistent with studies of sympathomimetic effects on cerebrospinal fluid production in vivo.

Sympathetic nervous control of cerebrospinal fluid production in experimental obstructive hydrocephalus

Hydrocephalus was induced in rabbits by cisternal injection of kaolin (0.5 ml of a 30 g/100 ml solution of hydrated aluminium silicate). The rate of bulk production of cerebrospinal fluid (CSF) was measured with a modified Pappenheimer [14C]inulin dilution technique in a ventriculoventricular perfusion system 3 days and 3 weeks after treatment. In the acute state of high-pressure hydrocephalus, CSF formation was reduced to almost one-half normal. Bilateral electrical stimulation of the cervical sympathetic nerves lowered the production rate by another 18%, with only an insignificant tendency to normalization after cessation of stimulation. In the chronic state of low-pressure hydrocephalus at 3 weeks after the kaolin injection, the rate of CSF formation was of the same magnitude as in animals studied after 3 days. However, sympathetic nerve stimulation now reduced the production rate by a further 39%, and prestimulation values were almost restored within 1 h after termination of stimulation. Thus, cranial sympathetic stimulation was very effective in reducing CSF formation in the chronic stage of low-pressure hydrocephalus. during acute high-pressure hydrocephalus, the sympathetic defence mechanisms were probably already recruited to such an extent that electrical activation of the sympathetic nerves was not able to further affect significantly the rate of CSF formation.

Sympathetic influence on sodium-potassium activated adenosine triphosphatase activity of rabbit and rat choroid plexus

Ouabain-sensitive Na+-K+-ATPase was measured spectrophotometrically in the lateral choroid plexuses of rabbit and rat. In the rabbit, a significant increase in the enzyme activity was seen at one week after unilateral sympathectomy (removal of the superior cervical ganglion), but not at three days or two weeks postoperatively, as compared with the intact, contralateral plexus. Unilateral sympathetic denervation of the rat's choroid plexus induced a nearly 40% decrease in Na+-K+-ATPase activity at 6 days after the operation, while no effect was seen after 12 days. The results agree with a local sympathetic inhibition of CSF production in rabbit (corresponding studies on rat have not been performed), and favor the assumption that the adrenergic nerves in the choroid plexus mediate direct effects on transport functions in the plexus epithelium.

Increased cerebrospinal fluid norepinephrine in essential hypertension

Studies in man and animals suggest increased central noradrenergic activity in hypertension. We can now measure norepinephrine (NE) in cerebrospinal fluid (CSF) accurately and this should provide an index of central noradrenergic activity. NE in CSF is very labile and altered by anxiety, stress, drugs, sodium balance and other factors. When these factors are controlled, NE in CSF appears elevated in essential hypertensives, particularly among young patients. Neurologic diseases that alter NE in CSF cause parallel alterations in blood pressure. Increased central noradrenergic activity is reflected in CSF NE levels and may cause some forms of hypertension.

Sympathetic influence on transport functions in the choroid plexus of rabbit and rat

In vitro uptake of an organic base (choline) and an organic acid (p-aminohippuric acid; PAH), both radiolabelled, was measured in isolated choroid plexus of rat and rabbit, and expressed as tissue/medium ratios. A significant increase in the tissue accumulation of choline was seen in denervated rabbit choroid plexus 1 week following unilateral cervical sympathectomy. The accumulation of PAH was not affected. The uptake of both test substances was significantly reduced after sympathetic denervation of the rat choroid plexus. The results agree with the local sympathetic inhibition of cerebrospinal fluid production in rabbit (corresponding studies on rat have not been performed), and favour the assumption that the adrenergic nerves in the choroid plexus mediate direct effects on the plexus epithelium.

Autonomic nerves in the mammalian choroid plexus and their influence on the formation of cerebrospinal fluid

The choroid plexuses of all ventricles receive a well-developed adrenergic and cholinergic innervation reaching both the secretory epithelium and the vascular smooth muscle cells. Also peptidergic nerves, containing vasoactive intestinal polypeptide, are present but primarily associated only with the vascular bed. A sympathetic inhibitory effect on the plexus epithelium has been indicated in determinations of carbonic anhydrase activity and by studies of various aspects of active transport in isolated plexus tissue. Pharmacological analysis in vitro has shown the choroidal arteries to possess both vasoconstrictory alpha-adrenergic and vasodilatory beta-adrenergic receptors. Electrical stimulation of the sympathetic nerves, which originate in the superior cervical ganglia, induces as much as 30% reduction in the net rate of cerebrospinal fluid (CSF) production, while sympathectomy results in a pronounced increase, about 30% above control, in the CSF formation. There is strong reason to believe that the choroid plexus is under the influence of a considerable sympathetic inhibitory tone under steady-state conditions. From pharmacological and biochemical experiments it is suggested that the sympathomimetic reduction in the rate of CSF formation is the result of a combined beta-receptor-mediated inhibition of the secretion from the plexus epithelium and a reduced blood flow in the choroid plexus tissue resulting from stimulation of the vascular alpha-receptors. The choroid plexus probably also represents an important inactivation site and gate mechanism for sympathomimetic amines, as evidenced by considerable local activity of catechol-O-methyl transferase and monoamine oxidase, primarily type B. The CSF production rate is also reduced by cholinomimetic agents, suggesting the presence of muscarinic-type cholinergic receptors in the choroid plexus.

The cerebrospinal fluid and plasma concentrations of gamma-trace and beta2-microglobulin at various ages and in neurological disorders

The concentrations of gamma-trace and beta2-microglobulin in cerebrospinal fluid (CSF) and plasma were determined in 64 individuals of various ages without signs of organic disorder in the central nervous system (CNS). A strong connection was found between the CSF level of gamma-trace and the age of the individual, with the CSF level of newborns being 3--4 times that of adults. A similar, but less marked, connection was found for the CSF level of beta2-microglobulin and the age of the individual. The plasma levels of the two proteins also varied with the age of the individual, but the variations were not as great as those of the CSF levels. The results strongly emphasize the necessity of using age-matched reference values when CSF and plasma levels of the proteins are to be evaluated in different groups of patients. Thirteen children and 98 adults with various neurological disorders were also examined. Significantly increased CSF levels of gamma-trace and beta2-microglobulin as well as increased plasma concentration of gamma-trace and CSF/plasma gradient of beta2-microglobulin were found in infectious disorders. Increased gamma-trace concentration in plasma and beta2-microglobulin concentration in CSF were seen in cerebrovascular disorders. The mechanisms which regulate the turnover of proteins in CSF are discussed.