Ontogenesis of nicotinic acetylcholine receptors and presynaptic cholinergic neurons in mammalian brain

We find a significantly lower level of specific [3H]nicotine binding in the fetal rat forebrain of 20 day gestational age as compared to adult rat tissue, and a progressive increase in the [3H]nicotine binding in the early neonatal forebrain, reaching adult level at 14-28 days of age. The maximal binding sites (Bmax) for the brain [3H]-nicotine binding at the ages of 3, 14 and 28 days were 36%, 74% and 98% respectively of the adult value. Neosurugatoxin (NSTX) was a potent inhibitor of [3H]nicotine binding in the neonatal forebrain (IC50 = 205, 81 and 103 nM at the 3, 14 and 28 days of age) as well as in the adult tissue (IC50 = 79 nM). Both the choline acetyltransferase (ChAT) activity and high affinity uptake of [14C]choline were not detectable or extremely low in the 1 week neonatal forebrain, and then increased progressively with age to reach the adult level at about 6 weeks of age. These data indicate that central nicotinic receptors may mature prior to the development of presynaptic cholinergic elements.

Possibilities for adenosine modulation of peristaltic reflex in guinea pig isolated ileum

The possibilities of participation of adenosine in the peristaltic reflex of guinea pig ileum was studied pharmacologically and histologically. Adenosine apparently depressed the peristaltic activity induced by elevation of intraluminal pressure from zero to 1-8 cm H2O, and the extent of the depression somewhat decreased in proportion to the elevation in intraluminal pressure. Also, dipyridamole depressed the peristaltic activity by itself; however, the extent of the depression significantly increased in proportion to the elevation in intraluminal pressure. On the other hand, in the presence of dipyridamole, the elevation of the intraluminal pressure from zero to 1-8 cm H2O elicited an 3H-output increase from 3H-adenosine preloaded guinea pig isolated ileum, with the effect being more pronounced as the pressure was increased. In contrast, the peristaltic activity was more pronounced at lower pressurization. Atropine greatly depressed the peristaltic response but did not affect 3H-output induced by 4 cm H2O pressurization. Tetrodotoxin depressed both markedly. Fluorescence histochemical localization of quinacrine, which binds to adenosin triphosphate (ATP), revealed dense nerve cell bodies and fine interconnecting strands in the ileal myenteric plexus of Auerbach. Also, in microradioautographs of ileal longitudinal muscle incubated with 3H-adenosine, the concentration of developed silver grains was localized in the ganglion cells and in the musculature as varicose fibre. From these results, evidence is provided that the peristalsis of guinea pig ileum may be physiologically modulated by endogenous adenosine, which may be released from neuronal elements of the myenteric plexus in response to the applied intraluminal pressure.

Specific increase of hypothalamic alpha 1-adrenoceptors in spontaneously hypertensive rats: effect of hypotensive drug treatment

To study potential central adrenoceptor alterations in the hypertension, we have determined alpha 1, alpha 2 and beta-adrenoceptors using [3H]WB4101, [3H]yohimbine and [3H]DHA in the brain regions of spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP) and renal hypertensive rats. There was a significant increase in specific [3H]WB4101 binding only in the hypothalamus of SHR and SHRSP at 16-24 weeks of age compared to that of age-matched Wistar-Kyoto rats (WKY). Scatchard analysis revealed a 28-33% increase in the Bmax value for hypothalamic [3H]WB4101 binding without a change in the Kd value, suggesting a change in the receptor density. An increased density of alpha 1-adrenoceptors was consistently observed in the prehypertensive (5 weeks) and developmental (10 weeks) stages of spontaneous hypertension. In contrast, there was no alpha 1-adrenoceptor alteration in the hypothalamus of rats with renal hypertension. The receptor alteration in the SHRSP hypothalamus was not abolished by a chronic hypotensive treatment which prevented the development of hypertension, thereby suggesting that an increased density of the alpha 1-adrenoceptors in spontaneous hypertension does not occur secondarily to the elevation of blood pressure. The SHRSP hypothalamus showed significantly lowered levels of noradrenaline. There was no change in specific binding of [3H]yohimbine and [3H]DHA in the brain regions of SHRSP, except the brainstem which showed a significant decrease in the [3H]yohimbine binding. Thus, the present study suggests an important role for hypothalamic alpha 1-adrenoceptors in the pathogenesis of spontaneous hypertension.

Reduction of 7-ketolithocholic acid to chenodeoxycholic acid by rat liver preparations in vitro

The formation of chenodeoxycholic acid via 7-ketolithocholic acid by rat liver preparations was examined in vitro. Results showed that a rat liver preparation reduced 7-ketolithocholic acid mainly to chenodeoxycholic acid and to ursodeoxycholic acid in a smaller amount, and that the reductase required NADPH but not NADH as coenzyme and was mainly localized in the microsomes.

Brain nicotinic acetylcholine receptors. Biochemical characterization by neosurugatoxin

Specific [3H]nicotine binding to rat forebrain membranes was saturable and of high affinity, and it exhibited pharmacological specificity as well as stereoselectivity for nicotinic agents. There was a regional variation of specific [3H]nicotine binding in rat brain. Low concentrations of neosurugatoxin markedly inhibited specific [3H]nicotine binding in rat forebrain (IC50 = 78 nM) and the competition curve by the toxin was biphasic (pseudo-Hill slope, 0.44). Approximately 50% of [3H]nicotine binding in rat forebrain was inhibited by low concentrations (0.3-100 nM) of neosurugatoxin and the residual binding was inhibited by higher concentrations (0.3-10 microM). In the presence of 3 nM, 100 nM, and 1 microM neosurugatoxin, there was a concentration-dependent (28, 54, and 71%, respectively) loss of [3H]nicotine-binding sites (Bmax) in rat forebrain with little change in the dissociation constant (Kd). The blockade of brain [3H]nicotine-binding sites induced by neosurugatoxin was not reversed by washing. Further, the toxin (10 microM) considerably accelerated the dissociation of [3H]nicotine from its receptor sites initiated by nonlabeled (-)nicotine. These observations suggest that neosurugatoxin may allosterically regulate [3H]nicotine binding rather than competing directly. In contrast to a marked inhibition of [3H]nicotine binding, neosurugatoxin (100 nM-10 microM) had no effect on brain [3H]quinuclidinyl benzilate binding. In conclusion, the present study has shown that [3H]nicotine selectively labels nicotinic cholinergic receptors in rat brain and that neosurugatoxin is a potent noncompetitive antagonist of these receptors.

Effects of adenosine on contractile response of circular muscle in electrically stimulated guinea-pig ileum

The action of adenosine on the electrically induced mechanical response of circular muscle in isolated guinea-pig ileum has been investigated. Electrical stimulation (0.1 Hz) elicited the twitch response, which was completely abolished by tetrodotoxin (0.2 microM), morphine (1 microM) and atropine (0.1 microM). Adenosine (0.1-100 microM) markedly depressed the twitch response in a concentration-dependent manner, and the concentration-depression curve for adenosine was significantly shifted to the right in the presence of theophylline (30 microM). On the other hand, the contractile responses induced by acetylcholine (1-300 microM) were not effected by adenosine at all. The present investigation suggests that the twitch response is mediated through acetylcholine released from the intramural cholinergic nerves supplying the circular muscle of guinea-pig ileum, and adenosine has an inhibitory effect on the cholinergic transmission, probably via P1-purinoceptors.

Effects of adenosine on 45Ca uptake and [3H]acetylcholine release in synaptosomal preparation from guinea-pig ileum myenteric plexus

The effects of adenosine on acetylcholine (ACh) release and calcium uptake were examined in a synaptosomal fraction prepared from guinea-pig ileum myenteric plexus-longitudinal muscle. A high concentration of potassium (40 mM) and electrical pulses (ES:10Hz) caused a marked increase in the output of [3H]ACh from [3H]choline-preloaded crude synaptosomes. This [3H]ACh output was calcium- and temperature-dependent. Adenosine reduced the high potassium-induced release significantly, and the electrically stimulated release completely. When the preparation was depolarized by high potassium or electrical pulses, the 45Ca uptake by synaptosomes was significantly enhanced. The uptake of 45Ca induced by high potassium was significantly reduced and that induced by electrical stimulation was completely abolished by adenosine. From these results, it may be suggested that adenosine inhibits neurotransmitter release by suppressing the presynaptic influx of calcium ion during depolarization of the cholinergic nerve terminals in guinea-pig ileum.

Adenosine and dipyridamole: actions and interactions on the contractile response of guinea-pig ileum to high frequency electrical field stimulation

The action of adenosine on the myenteric plexus-longitudinal muscle strips from guinea-pig ileum to high frequency electrical field stimulation (10 Hz) was investigated. Electrically induced contractions were reduced markedly by tetrodotoxin (0.2 microM) and atropine (1 microM), and partially by noradrenaline (3 microM) and morphine (3 microM). Adenosine, adenosine 5'-monophosphate (AMP) and adenosine triphosphate (ATP) produced a concentration-dependent inhibition of the high frequency contractions over the range of 0.1-100 microM, the most potent being adenosine. The concentration-response curve for adenosine was significantly shifted to the left by dipyridamole (10 nM), while dipyridamole at higher concentrations (30 nM-10 microM), depressed the contraction markedly by itself. Dipyridamole decreased [3H]-adenosine uptake into strips of ileum in a concentration-dependent manner. There was a significant correlationship between the reduction of adenosine uptake and the inhibition of the contraction induced by dipyridamole (r = 0.970). In strips desensitized to adenosine or treated with adenosine deaminase, the inhibitory effect of dipyridamole was significantly reduced. The present investigation revealed that adenosine depressed responses of guinea-pig ileum to high frequency electrical stimulation and suggested that the inhibitory effect of dipyridamole may be closely associated with the behaviour of endogenous adenosine or related compounds.

Effect of 2,4,6-trinitrophenol on catecholamine secretion from perfused bovine adrenal glands

2,4,6-Trinitrophenol (PA) evoked a prolonged catecholamine (CA) secretion from perfused bovine adrenal glands. The PA-evoked CA secretion was concentration-dependent, required the presence of extracellular calcium and resulted from a direct action of PA on the chromaffin cells. Furthermore, PA reduced Mg2+-ATPase activities in the plasma membrane-rich microsome and granule-rich fraction from the adrenal medulla. These results indicate that PA evokes CA secretion through the actions on both the chromaffin cell membranes and granule membranes.

The appearance of exhausted platelets due to a duration of hypertension in stroke-prone spontaneously hypertensive rats

Using strains of spontaneously hypertensive rats with different degrees of hypertension, the influence of the duration of hypertension on platelets was examined through changes in platelet serotonin contents. The blood pressures of these strains were in the descending order of m-SHRSP greater than SHRSP greater than SHR greater than WKY. Serotonin content in normotensive WKY platelets was maintained in the range of 0.715 +/- 0.048(17) n mole/10(8) through ages 5-50 weeks in both sexes. In contrast with WKY of the same age and sex, a significant decrease in platelet serotonin content began to be observed in male m-SHRSP at 18-weeks of age, in female m-SHRSP and male SHRSP at 22-weeks of age, and in female SHRSP at 32-weeks of age, respectively. The content in SHR platelets of both sexes was unaltered up to 40-weeks of age. The time of the appearance of these exhausted platelets coincided with the reported time of scanning electron microscopic observation of vascular injuries in each strain of rats. It has been concluded that a long duration of hypertension causes platelets to become degranulated and exhausted due to in vivo activation of platelets at sites of arterial injury. Thus the changes of platelet contents could be an indicator of vascular injuries.

Enhancement in cholesterol-esterase activity and lipolysis due to 17 beta-estradiol treatment in rat adipose tissue

The effects of 17 beta-estradiol and testosterone administration on cholesterol esterase, lipoprotein lipase activities and adrenalin-induced lipolysis were examined in rat adipose tissues with the change in serum lipid level. The administration of 17 beta-estradiol (500 micrograms/kg, 2 or 4 weeks) to male rats significantly reduced the body weight, and markedly increased serum cholesterol, triacylglycerols and phospholipids. Cholesterol esterase activity was significantly enhanced in the epididymal adipose tissue from estradiol treated rats and the effect was greater with duration of the treatment. In contrast, lipoprotein lipase activity was markedly reduced. Testosterone reduced cholesterol esterase activity in the parametrial adipose tissue through the treatment with 500 micrograms/kg for 6 weeks, but it did neither influence serum lipids nor lipoprotein lipase activity. Basal lipolysis and adrenalin-induced lipolysis were also significantly enhanced in the epididymal adipose tissue from the male rat treated either with 7 mg/kg estradiol 12 h ahead or with 500 micrograms/kg estradiol for 2 weeks. These results indicate that estradiol exerts strong effects on metabolism of the adipose and these effects seems to be mediated through cyclic-AMP. An alteration of adrenergic functions by gonadal steroids might be intervened.

Sex difference in cholesterol esterase activity in rats adipose tissues and changes of the activity following orchiectomy and ovariectomy

Sex differences in cholesterol esterase activity and changes in the activity in response to orchiectomy and ovariectomy were investigated in rat adipose tissue in association with serum lipid levels. Cholesterol esterase activity in the parametrial adipose tissue was 65% higher than that in the epididymal adipose tissue, but there were no sex differences in the activity of subcutaneous and perirenal adipose tissue. Pre- and post-pubertal orchiectomy resulted in a marked enhancement of cholesterol esterase activity, whereas ovariectomy significantly reduced the activity. Serum cholesterol and triacylglycerol levels were elevated through orchiectomy but ovariectomy had no effect. These results suggest that cholesterol esterase activity in the adipose tissue around sexual organs is regulated by gonadal hormones and that the increase in serum cholesterol due to orchiectomy is unlikely to be related to the change in the enzyme activity.

Impaired function of TXA2 and/or calcium in the platelets from SHRSP at hypertensive ages

Aggregation of washed platelets from stroke-prone spontaneously hypertensive rats(SHRSP) was markedly reduced with the development of hypertension in comparison with age-matched normotensive Wistar Kyoto rats(WKY) (Tomita et al. Stroke 15, 70-75, 1984). The mechanism of the hypoaggregability of SHRSP platelets was studied. Ca2+-dependence of thrombin-induced aggregation and MDA formation, and Ionophore A23187-induced aggregation of the platelets from SHRSP at a hypertensive age(16-weeks) was similar to that of the aggregation of platelets from age-matched WKY. Optimum Ca2+ concentration for aggregation and MDA formation was 1-2 mM. There was no difference in aggregation in Ca2+-free medium between the two strains. The enhancement by Ca2+ of both thrombin- and Ionophore A23187- induced aggregation, however, was markedly less in SHRSP than in WKY, whereas their MDA formation was equally enhanced by Ca2+. At a prehypertensive age (4-weeks) the degree of enhancement of aggregation by Ca2+ did not differ in the two strains. The magnitude of phospholipid(PI, PC, PE) degradation, and MDA formation IN SHRSP at early- and late- hypertensive ages(11- and 17-weeks) were either the same as or greater than that of age-matched WKY. A linear correlation line between the amount of MDA formed and the degree of platelet aggregation of SHRSP shifted to the right of WKY. In addition, thrombin-induced thromboxane B2 formation in SHRSP platelets was similar to that in WKY in the concentration range of 0.22 - 0.44 U/ml, and became significantly higher at 0.65 U/ml despite severe hypoaggregability of SHRSP platelets in all the concentrations examined. The overproduction of MDA or thromboxane B2 appears to be a compensatory mechanism. These results suggest that abnormalities of SHRSP platelets at hypertensive ages are due to an impaired function of thromboxane A2 and/or calcium concerned in aggregation and secretion but not due to a defect in cyclo-oxygenase and thromboxane synthetase pathway.

Muscarinic cholinoceptors and choline acetyltransferase activity in the hypothalamus of spontaneously hypertensive rats

To study the role of central cholinergic mechanisms in hypertension, we have determined muscarinic receptors using [3H] (-)quinuclidinyl benzilate (QNB) and choline acetyltransferase (ChAT) activity in the brain regions of spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP) and renal hypertensive rats. The number of muscarinic receptors was significantly (33-38 %) elevated in the hypothalamus of SHR and SHRSP at the ages of 16 and 24 weeks compared to that of Wistar-Kyoto rats (WKY). An increased density of muscarinic receptors was consistently observed in the prehypertensive (5 weeks) and developmental (10 weeks) stages of hypertension. In contrast, in the hypothalamus of rats with renal hypertension there was no muscarinic receptor alteration. The receptor alteration in the SHRSP hypothalamus was not abolished by a chronic hypotensive treatment which prevented the development of hypertension, suggesting that an enhancement of the muscarinic receptors in spontaneous hypertension does not occur secondarily to the elevation of blood pressure. The hypothalamus of SHR and SHRSP at the ages of 5 and 24 weeks showed significantly less activity of ChAT. These data demonstrate that there is a specific increase in muscarinic receptors and a decrease in cholinergic activity in the hypothalamus of SHR and SHRSP. Thus, the present study suggests an important role for hypothalamic cholinergic receptors in the pathogenesis of spontaneous hypertension.

Neosurugatoxin, a specific antagonist of nicotinic acetylcholine receptors

Neosurugatoxin (NSTX) (3 nM-30 nM), recently isolated from the Japanese ivory mollusc (Babylonia japonica) exerted a potent antinicotinic action in the isolated guinea pig ileum. Specific [3H]nicotine binding to rat forebrain membranes was saturable, reversible, and of high affinity. Nicotinic cholinergic agonists exhibited a markedly greater affinity for [3H]nicotine binding sites than a muscarinic agonist, oxotremorine. Although alpha-bungarotoxin had no effect on [3H]nicotine binding, low concentrations (1 nM-1 microM) of NSTX inhibited [3H]nicotine binding in the forebrain membranes and its IC50 value was 69 +/- 6 nM. On the other hand, NSTX did not affect muscarinic receptor binding in the brain. These data indicate that NSTX may be of appreciable interest as a neurotoxin with a selective affinity for ganglionic nicotinic receptors.

Inhibition by chlorpromazine, metals and I-ascorbic acid of calcium-ATPase and magnesium-ATPase in bovine adrenal medullary microsomes

Effects of chlorpromazine, metals and I-ascorbic acid (AA) on Ca2+-ATPase and Mg2+-ATPase in microsomal and granular fractions obtained from the bovine adrenal medulla were studied. Marker enzyme analysis on microsomal subfractions in a discontinuous sucrose density gradient showed a correlation of distribution between ATPase activities and plasma membrane. The two ATPase activities in such plasma membrane-rich microsomes were reduced by chlorpromazine, Hg2+ and Cu2+ (0.3 mM of each), and their effects were greater on the Mg2+-ATPase activity. Zn2+ (0.3 mM) also reduced only the Mg2+-ATPase activity. AA (3 mM) reduced the two ATPase activities to an equal extent. Nevertheless, the inhibitions of ATPases by Hg2+, Cu2+ and Zn2+ were decreased, unaltered and additively enhanced in combination with AA, respectively. We also observed high Mg2+-ATPase activity in the granule-rich fraction, but this ATPase activity was unaffected by all of the above agents. These results indicate that Mg2+-ATPase in the plasma membrane-rich microsome of adrenal medulla is inhibited by chlorpromazine, Hg2+, Cu2+ and Zn2+ more significantly than Ca2+-ATPase, but Mg2+-ATPase in the granular fraction is unaffected, and that AA changes the potency of inhibition by some metals of ATPases diversely.

Alterations in cardiac alpha and beta adrenoceptors during the development of spontaneous hypertension

To study potential cardiac receptor alterations during the development of spontaneous hypertension, specific binding of [3H]-2-N(2,6-dimethoxyphenoxyethyl)amino-methyl-1,4-benzodioxane, (-)-[3H]dihydroalprenolol and (-)-[3H]quinuclidinyl benzilate in ventricles of Wistar Kyoto rats (WKY), spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) at different ages was determined. The Kd and maximal binding for specific binding of [3H]-2-N(2,6-dimethoxyphenoxyethyl)amino-methyl-1,4-benzodioxane and (-)-[3H]dihydroalprenolol in ventricular homogenates of SHR and SHRSP at prehypertensive ages were similar to those of age-matched WKY. With the development of spontaneous hypertension in SHR and SHRSP, there was a significant decrease in the maximal binding for both ligands without a change in Kd. The decrease in maximal binding in SHR and SHRSP at 10 weeks of age was 29 to 38%, compared with age-matched WKY. There was no difference in ventricular (-)-[3H]quinuclidinyl benzilate binding between WKY and SHRSP. Hofstee analysis of the inhibition of ventricular (-)-[3H]dihydroalprenolol binding by practolol demonstrated a specific 51% decrease in ventricular beta-1 receptor density in 10-week-old SHRSP. In addition, the inotropic response to isoproterenol in isolated papillary muscles from SHRSP was significantly smaller than that in WKY. Thus, it is concluded that during the development of spontaneous hypertension in SHR and SHRSP, there is a specific loss in number of cardiac alpha and beta-1 adrenoceptors with a consequently reduced responsiveness of isolated papillary muscles to isoproterenol in SHRSP. These results are compatible with the reported increase in sympathetic outflow to the cardiovascular system in spontaneous hypertension.

Hypoaggregability of washed platelets from stroke-prone spontaneously hypertensive rats (SHRSP)

The aggregation properties of washed SHRSP platelets were investigated in comparison with normotensive WKY platelets at prehypertensive (4 weeks), early hypertensive (11 weeks) and late hypertensive (17 weeks) ages in the absence of plasma factors. The number of platelets in SHRSP was markedly lower with the development of hypertension than that in WKY. The thrombin- and collagen-induced aggregation was markedly reduced in the platelets from 11 and 17 week old SHRSP compared with that of age-matched WKY, whereas the degree of platelet aggregation in 4 week old SHRSP showed a tendency to be even greater than that in WKY. The changes in blood pressure and platelet aggregability were correlated inversely. ADP did not induce aggregation in the same system used for thrombin and collagen stimulation but in another system it aggregated washed rat platelets. Aggregation responses to ADP and ionophore A23187 were also significantly lower in 14 week old SHRSP platelets than age-matched WKY platelets. Together with other evidence, these results suggest that defective Ca2+ function, rather than the presence of exhausted platelets, is responsible for hypoaggregability in SHRSP platelets.

Effect of picrotoxin on adrenal catecholamine secretion

The effect of picrotoxin (PT) on catecholamine (CA) secretion was investigated in perfused bovine adrenal glands. A low dose of PT (3 microM) enhanced the CA secretion evoked by a 15-min exposure to 1,1-dimethyl-4-phenylpiperazinium (DMPP, a nicotinic agonist; 0.1 mM), but a higher dose (0.3 mM) of PT inhibited the DMPP-evoked CA secretion. The rate of decline of secretory response to the prolonged DMPP stimulation was also accelerated by a higher dose (0.1 mM) of PT. In the dose-response curves for DMPP-evoked CA secretion, the inhibitory action of PT (0.3-1 mM) was more prominent at high doses than at low doses of DMPP. The inhibition pattern was similar to the pattern of a barbiturates blockade. In separate experiments, PT (0.1 mM) augmented calcium (10 mM)- and high potassium (56 mM)-evoked secretory responses. Spontaneous CA secretion was unaffected by PT at the concentrations indicated above. These results indicate that a low dose of PT potentiates, but higher doses inhibit, the adrenal CA secretion by a nicotinic agonist and that the inhibitory effect of PT resembles that of barbiturates.