Effects of pirenzepine, AF-DX 116 and gallamine on the release of catecholamines from the dog adrenal gland in response to splanchnic nerve stimulation: interaction of M1 and M2 receptors with nicotinic receptors.

The present study was undertaken to examine how muscarinic antagonists modify the release of catecholamines evoked by splanchnic nerve stimulation (SNS) from the dog adrenal gland in vivo, in an attempt to elucidate whether muscarinic receptors play a functional role in catecholamine release. Output of epinephrine and norepinephrine was determined from adrenal venous blood by using high-performance liquid chromatography with electrochemical detection. SNS (1 and 3 Hz) produced increases in catecholamine output in a frequency-dependent manner. Intravenous administration of pirenzepine (10-100 micrograms/kg), a selective M1 receptor antagonist, or AF-DX 116 (30-300 micrograms/kg) and gallamine (0.3-3 mg/kg), selective M2 receptor antagonists, did not modify the SNS-induced increases in catecholamine output. C6 (hexamethonium) inhibited the SNS-induced increases in catecholamine output partially in a dose of 1 mg/kg and remarkably in a dose of 10 mg/kg. The combination of C6 (1 mg/kg) with pirenzepine (10 micrograms/kg), AF-DX 116 (30 micrograms/kg) or gallamine (0.3 mg/kg) inhibited the SNS-induced increases in catecholamine output more potently than C6 did by itself. The inhibition by C6 alone was about 50%, but that by each combination reached to about 80%. These results suggest that M2 receptors as well as M1 receptors play a facilitatory role in catecholamine release from the adrenal gland in response to SNS when the nicotinic receptor-mediated mechanism is partially inhibited.     

Inhibition by opioid agonists and enhancement by antagonists of the release of catecholamines from the dog adrenal gland in response to splanchnic nerve stimulation: evidence for the functional role of opioid receptors

The aim of the present study is to examine how opioid agonists and antagonists modify the splanchnic nerve stimulation (SNS)-induced release of catecholamines from the dog adrenal gland in vivo, in an attempt to elucidate whether opioid receptors play a functional role in controlling catecholamine release. Output of epinephrine (EPI) and norepinephrine (NE) was determined from adrenal venous blood by using high-performance liquid chromatography with electrochemical detection. SNS (0.3, 1 and 3 Hz) produced increases in both EPI and NE output in a frequency-dependent manner. Leu-enkephalin (10-100 micrograms/kg i.v.) and morphine (10-100 micrograms/kg i.v.) attenuated the increase in EPI and NE output induced by 1 or 3 Hz of SNS without affecting the basal catecholamine output. A 25 to 40% reduction of the SNS-induced increase in catecholamine output was observed after the treatment with 100 micrograms/kg of leu-enkephalin or morphine. The increase in EPI and NE output induced by 1 and 3 Hz of SNS was enhanced markedly by naloxone (10-1000 micrograms/kg i.v.) and by naltrexone (10-1000 micrograms/kg i.v.). The SNS-induced increase in catecholamine output doubled after treatment with 100 and 1000 micrograms/kg of naloxone or naltrexone. Basal catecholamine output and the increase in output induced by 1 Hz of SNS were unaffected by naloxone or naltrexone. These results suggest that endogenously released opioid peptides inhibit the release of catecholamines by activating opioid receptors in the adrenal gland of the dog.     

Enhancement by yohimbine of nicotine- and dimethylphenylpiperadinium-induced release of norepinephrine from cardiac sympathetic nerves of the dog: interaction of presynaptic alpha and nicotinic receptors

The effect of yohimbine on nicotine- and dimethylphenylpiperadinium (DMPP)-induced release of norepinephrine (NE) from sympathetic cardiac nerves of the dog was examined in order to elucidate the interaction of presynaptic alpha and nicotinic receptors. Intracoronary infusion of nicotine (300 or 500 micrograms/min) or DMPP (100 or 300 micrograms/min) into the left circumflex artery increased coronary sinus output of NE (NE output), left ventricular dp/dt maximum (LV dp/dt max) and coronary sinus blood flow. The nicotine-induced increases in NE output, LV dp/dt max and coronary sinus blood flow were enhanced by simultaneous yohimbine infusion in doses of 10 and 30 micrograms/min into the same artery, and were attenuated by the drug in a dose of 100 micrograms/min. The DMPP-induced increases were enhanced by 10 micrograms/min of yohimbine infusion, but the enhancement was decreased by 30 and 100 micrograms/min. Yohimbine did not modify increases in NE output, LV dp/dt max and coronary sinus blood flow induced by intracoronary infusion of tyramine (100 micrograms/min). We have reported previously that yohimbine in the dose range used enhanced cardiac sympathetic nerve stimulation-induced increases in these parameters in a dose-dependent manner in the same preparation. The enhancement by yohimbine of nicotinic receptor-mediated NE release would be due to the blockade of presynaptic alpha-2 adrenoceptors. Therefore, it is suggested that presynaptic alpha-2 adrenoceptor-mediated feedback control operates on the process of NE release induced by nicotinic receptor activation as well as nerve stimulation. Although the mechanism of loss of the enhancement of the nicotinic effect by the large dose of yohimbine is not known, the possible mechanisms are discussed.     

Metabolic and haemodynamic effects of dopamine plus domperidone in volunteers

There are no studies of the relationship between infusion rate of dopamine and the arterial and venous dopamine plasma concentration and the resulting haemodynamic and metabolic effects. Dopamine was administered to seven volunteers using five infusion rates (1, 3, 6, 9, 13 μg/kg per minute) in an escalating sequence lasting for 30 min for each step. Since dopamine can cause nausea and vomiting, this relationship was investigated after administration of domperidone for infusion rates above 3μg/kg per minute. Haemodynamic effects were assessed using 2-dimensional echocardiography. During the highest infusion rate the arterial plasma dopamine concentration reached 1,379±181 nmol/l. There was a linear correlation between the dopamine infusion rate and both the arterial and the venous plasma concentration. There was no significant change in heart rate or diastolic blood pressure. Systolic blood pressure, ejection fraction and cardiac index increased in a dose-dependent manner. Systemic vascular resistance decreased during the two low doses of dopamine and was not different from baseline values during the three high infusion rates. The plasma concentrations of glucose and non-esterified fatty acids increased from 5.3±0.4 to 6.8±0.9 nmol/l, and from 360±119 to 971±307 μmol/l, respectively, during the 13 μg/kg per minute infusion rate. As the plasma noradrenaline concentration increased up to 7.84±2.46 nmol/l in correlation to the dopamine plasma concentration, an indirect sympathomimetic effect may contribute to the actions of dopamine plasma concentration.     

Voltage and current clamp studies of muscarinic and nicotinic excitation of the rat adrenal chromaffin cells

Characteristics of the muscarinic and nicotinic excitation of chromaffin cells that had been freshly isolated from the rat adrenal medullae were analyzed using voltage and current clamp techniques. A dose-dependent increase in the extracellularly recorded firing of cells was observed when 10(-6) to 10(-4) M acetylcholine (ACh) were locally applied to the cells in the vicinity of the target cell being recorded using a microinflow method. During voltage clamp recording at the resting membrane potential, ACh induced two different sequential inward currents: a transient current with a rapid rising phase (fast response) and an apparent inward current with a slow rising phase (slow response). The membrane conductance increased during the ACh-induced fast response, and it subsequently decreased during the slow response. The amplitude of the fast response decreased when the holding potential was shifted to depolarized levels, whereas the amplitude of the slow response increased with depolarization. Nicotine produced fast depolarization and a transient inward current that was reduced by the membrane depolarization. In contrast, muscarine induced a slow depolarization and an apparent inward current that increased with depolarization. Muscarine also reduced the inward K+ current that had been induced by the application of a high K+ medium to the outside of the cell at the resting membrane potential. It is suggested that muscarinic excitation is triggered by the suppression of K+ channels that are open at potentials near the resting membrane potential. The present results indicate that ACh-induced excitation of adrenal chromaffin cells involves two separate mechanisms mediated by nicotinic and muscarinic receptors.     

Nicotinic receptor-evoked release of acetylcholine and somatostatin in the myenteric plexus is coupled to calcium influx via N-type calcium channels.

Entry of extracellular calcium (Ca++) via voltage-gated Ca++ channels is essential for neurotransmitter release. In this study, we examined whether nicotinic receptor-stimulated release of acetylcholine (ACh) and somatostatin (S14) are coupled to calcium influx via distinct calcium channel subtypes in the myenteric plexus. Isolated ganglia from the guinea pig ileal myenteric plexus were prepared and placed in perfusion chambers under standard conditions. The ganglionic agonist dimethylphenylpiperazinium (DMPP, 10(-6) to 10(-3) M) stimulated the release of [3H]ACh in a concentration-dependent manner. This release was blocked by hexamethonium or Ca(++)-free medium containing 1 mM EGTA and was antagonized by omega-conotoxin, a preferential N calcium channel blocker, but was not affected by nifedipine (L channel antagonist) or nickel (T calcium channel antagonist). DMPP-evoked release of somatostatin was also antagonized by omega-conotoxin, but was not affected by nifedipine or nickel. These observations indicate that neurosecretion of ACh and S14 evoked by DMPP is mediated by calcium entry via voltage-sensitive N-type Ca++ channels. To provide additional evidence that nicotinic receptor stimulation is associated with Ca++ entry via the N-type Ca++ channels, we examined the intracellular calcium [Ca++]i concentration of the myenteric plexus neurons using fura-2 microspectrofluorometry. Basal [Ca++]i of single ileal myenteric neurons was 65 +/- 5 nM. Perfusion with DMPP (10(-6) to 10(-3) M) caused a rapid, transient elevation in [Ca++]i which was abolished by Ca(++)-free medium containing 1 mM EGTA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phytoestrogen cimicifugoside-mediated inhibition of catecholamine secretion by blocking nicotinic acetylcholine receptor in bovine adrenal chromaffin cells

We investigated the effect of the phytoestrogen cimicifugoside, one of the pharmacologically active ingredients of the medicinal plant Cimicifuga racemosa (black cohosh) that has been used to treat many kinds of neuronal and menopausal symptoms, such as arthritis, menopausal depression, and nerve pain. Cimicifugoside inhibited calcium increase induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), a nicotinic acetylcholine receptor (nAChR) agonist in bovine adrenal chromaffin cells with a half-maximal inhibitory concentration (IC(50)) of 18 +/- 2 microM. In contrast, cimicifugoside did not affect the calcium increases evoked by high K(+), veratridine, and bradykinin. The DMPP-induced sodium increase was also inhibited by cimicifugoside with an IC(50) of 2 +/- 0.3 microM, suggesting that the activity of nAChRs is inhibited by cimicifugoside. Cimicifugoside did not affect the KCl-induced secretion but markedly inhibited the DMPP-induced catecholamine secretion that was monitored by carbon-fiber amperometry in real time and high-performance liquid chromatography through electrochemical detection. The results suggest that cimicifugoside selectively inhibits nAChR-mediated response in bovine chromaffin cells.     

Effects of rolipram and cilostamide on renal functions and cyclic AMP release in anesthetized dogs.

The present study was undertaken to examine whether phosphodiesterases III and IV regulate renal cAMP level and whether inhibition of these enzymes influences renal functions in anesthetized dogs. The intrarenal arterial infusion of rolipram (0.1, 0.3, and 1 microgram/kg/min), a selective phosphodiesterase IV inhibitor, increased renal blood flow, glomerular filtration rate, urine flow rate, and urinary Na+ excretion with elevating arterial and renal venous plasma cAMP concentrations and urinary cAMP excretion. However, cilostamide (0.1, 0.3, and 1 microgram/kg/min), a selective phosphodiesterase III inhibitor, did not affect the values of these parameters. Indomethacin (3 mg/kg i.v. bolus and 1 mg/kg/min i.v. infusion), a cyclooxygenase inhibitor, reduced the basal arterial and renal venous plasma cAMP concentrations and blunted the rolipram-induced elevation of cAMP concentrations and urinary cAMP excretion. The effects of rolipram on renal hemodynamics and urine formation were attenuated in the presence of indomethacin. These results suggest that in the dog kidney in vivo, 1) phosphodiesterase IV, but not phosphodiesterase III, participates in degradation of cAMP and 2) the inhibition of phosphodiesterase IV enhances glomerular filtration and urinary Na+ excretion, the responses of which depend in part on indomethacin-susceptible (prostaglandin-mediated, probably) control of basal cAMP level.     

Determinants of agonist binding affinity on neuronal nicotinic receptor beta subunits

The alpha and beta subunits of heteromeric neuronal nicotinic acetylcholine receptors (nAChRs) are thought to contribute "principal" and "complementary" components to the agonist binding site, respectively. At least six loops of amino acid sequence (A, B, and C from alpha; D, E, and F from beta) are involved. We demonstrated previously that receptors containing the beta2 subunit had consistently higher affinities for a variety of agonists than beta4-containing receptors. For example, the affinity of the alpha2beta2 receptor for epibatidine, ACh, nicotine, and dimethylphenylpiperazinium (DMPP) exceeds that of alpha2beta4 by 9-, 61-, 87-, and 120-fold, respectively. Using saturation and competition analysis of receptors formed by chimeric beta subunits coexpressed with alpha2 in Xenopus laevis oocytes, we have now identified sequence segment 54-63 (corresponding to loop D) as a major determinant of affinity for epibatidine, ACh, nicotine, and DMPP. We then analyzed a series of mutant beta2 subunits in which each residue that differs between beta2 and beta4 in this region was changed from what occurs in beta2 to what occurs in beta4. The N55S, V56I, and E63T mutations each resulted in a loss of affinity for ACh and nicotine of 3- to 4-fold, whereas the T59K mutation resulted in a 7-fold loss of ACh and nicotine affinity. These mutations had little or no effect on epibatidine and DMPP affinity. The positive charge introduced by the T59K mutation does not appear to underlie loss of agonist affinity, because a similar loss of affinity was observed when a negative charge (T59D) was introduced at this position.     

Cortisol levels in patients with severe community-acquired pneumonia

Objectives To evaluate cortisol levels and prevalence of adrenal insufficiency in patients with severe community-acquired pneumonia (CAP). Design and setting Retrospective cohort study in a 24-bed medical-surgical intensive care unit (ICU). Patients Forty patients with severe CAP admitted to the ICU from March 2003 and May 2005. Measurements and results Random cortisol levels were measured up to 72 h after ICU admission. A threshold of 20 μg/dl was considered for the diagnosis of adrenal insufficiency. Median cortisol levels were 15.5 μg/dl (IQR 10.8–25.1), and 26 patients (65%) met the criteria for adrenal insufficiency. Other cutoff levels of cortisol were evaluated, and 30 patients (75%) had cortisol levels below 25 μg/dl and 19 (47.5%) had cortisol levels below 15 μg/dl. When only patients with septic shock (n = 19) were evaluated, 12 (63%) had adrenal insufficiency. Conclusions Relative adrenal insufficiency occurs in a high proportion of patients with severe CAP. This finding highlights the importance of measuring cortisol levels and may help explain the potential benefits of hydrocortisone infusion in these patients.     

Comparison of the potentiating effects of nicorandil and its denitrated metabolite (SG-86) on the adenosine-induced vasodepression in rats

Summary— The potentiating activity of SG-86[N-(2-hydroxyethyl)nicotinamide], a denitrated metabolite of nicorandil, on the adenosine-induced vasodepression was compared with that of nicorandil in anesthetized rats. Single bolus iv adenosine (3–100 μg/kg) produced dose-dependent reductions of blood pressure, accompanied by slight decreases (except for 100 μg/kg) in heart rate. The adenosine-induced vasodepression was significantly enhanced during iv infusion of either SG-86 (100 μg/kg per min) as well as nicorandil (10 μg/kg per min). The enhancement of adenosine action by them did not occur in the presence of glibenclamide (20 mg/kg iv). Single bolus iv injections of SG-86 (0.3–30 mg/kg), except for 30 mg/kg, which caused a glibenclamide-sensitive decrease by about 5–10 mm Hg in mean arterial blood pressure, had no effects on blood pressure and heart rate, whereas those of nicorandil (30–300 μg/kg) elicited overt reduction of blood pressure, accompanied by decreases in heart rate. The present results revealed that SG-86, like nicorandil, significantly enhanced the vasodepressor response to adenosine, probably in part through K_ATP channel activation, and that the activity of SG-86 was about 10 times less potent than that of nicorandil.     

Sympathoadrenal responses to acute and chronic hypoxia in the rat.

The sympathoadrenal responses to acute and chronic hypoxic exposure at 10.5 and 7.5% oxygen were determined in the rat. Cardiac norepinephrine (NE) turnover was used to assess sympathetic nervous system (SNS) activity, and urinary excretion of epinephrine (E) was measured as an index of adrenal medullary activity. The responses of the adrenal medulla and SNS were distinct and dependent upon the degree and duration of hypoxic exposure. Chronic hypoxia at 10.5% oxygen increased cardiac NE turnover by 130% after 3, 7, and 14 d of hypoxic exposure. Urinary excretion of NE was similarly increased over this time interval, while urinary E excretion was marginally elevated. In contrast, acute exposure to moderate hypoxia at 10.5% oxygen was not associated with an increase in SNS activity; in fact, decreased SNS activity was suggested by diminished cardiac NE turnover and urinary NE excretion over the first 12 h of hypoxic exposure, and by a rebound increase in NE turnover after reexposure to normal oxygen tension. Adrenal medullary activity, on the other hand, increased substantially during acute exposure to moderate hypoxia (2-fold increase in urinary E excretion) and severe hypoxia (greater than 10-fold). In distinction to the lack of effect of acute hypoxic exposure (10.5% oxygen), the SNS was markedly stimulated during the first day of hypoxia exposure at 7.5% oxygen, an increase that was sustained throughout at least 7 d at 7.5% oxygen. These results demonstrate that chronic exposure to moderate and severe hypoxia increases the activity of the SNS and adrenal medulla, the effect being greater in severe hypoxic exposure. The response to acute hypoxic exposure is more complicated; during the first 12 h of exposure at 10.5% oxygen, the SNS is not stimulated and appears to be restrained, while adrenal medullary activity is enhanced. Acute exposure to a more severe degree of hypoxia (7.5% oxygen), however, is associated with stimulation of both the SNS and adrenal medulla.     

Low-dose theophylline increases urine output in diuretic-dependent critically ill children

Objective: Determine the effect of low-dose theophylline on urine output and the urinary adenosine: cAMP (cyclic adenosine monophosphate) excretion ratio (a measure of phosphodiesterase inhibition) in diuretic-dependent critically ill children. Design: Observational clinical case series and animal laboratory experiment. Setting: A university pediatric intensive care unit and a pharmacology research laboratory. Patients: 10 consecutive oliguric patients treated with theophylline for diuresis. Interventions: Urine output, fluid intake, diuretic dosages, and number of pressors (including dopamine) were monitored over the 24-h period prior to and the 24-h period immediately after theophylline was started. Hourly collections of urine were obtained at baseline and 1 and 3 h after theophylline was started and urinary excretion rates of adenosine and cAMP were measured and calculated. Measurements and results: Mean theophylline level in the children was 5.0 μg/ml. Urine output increased from 1.58 ± 0.46 to 3.75 ± 0.77 ml/kg per h (p = 0.008, paired t-test) after theophylline administration. There was no significant change in fluid intake, vasoactive agents, or dosages of other diuretics during the study periods. Intrarenal infusion of the IC50 concentration of isobutylmethylxanthine for phosphodiesterase activity resulted in a reduction of the adenosine: cAMP urinary excretion ratio in rats (p < 0.05). Low-dose theophylline had no effect on the adenosine: cAMP urinary excretion ratio in children. Concurrent therapy with dopamine was associated with an enhanced diuretic effect of theophylline (with dopamine, 1.30 ± 0.30 to 5.07 ± 0.77 ml/kg per h vs without dopamine, 1.77 ± 0.76 to 2.86 ± 1.08 ml/kg per h; p = 0.03, two-way ANOVA). There was no interaction between dopamine and low-dose theophylline on the urinary adenosine: cAMP excretion ratio (p = 0.56, two-way ANOVA). Conclusions: Theophylline increased urine output in diuretic-dependent critically ill children and the diuretic effect may have been potentiated by concurrent use of dopamine. Adenosine receptor antagonism may be a more likely mechanism for the diuretic effect of theophylline than phosphodiesterase inhibition. Received: 14 October 1997 Accepted: 9 July 1998     

Effects of cyclo-oxygenase inhibition on vasodilatory response to acetylcholine in patients with type 1 diabetes and nondiabetic subjects

OBJECTIVE: Studies examining vasodilatory responses to acetylcholine (ACh) and its derivatives have been conflicting. Enhanced activation of the cyclo-oxygenase pathway and increased availability of vasodilatory prostanoids may occur in type 1 diabetes, and this may compensate for the observed reduction in nitric oxide (NO) activity We examined the role of cyclo-oxygenase inhibition on vasodilatory responses in 12 healthy normotensive type 1 diabetic adults and 12 nondiabetic control subjects of similar age, sex, and BMI. RESEARCH DESIGN AND METHODS: Forearm blood flow was measured using a venous occlusion plethysmography technique at baseline and after brachial artery infusions of ACh (7.5, 15, and 30 microg/min). Forearm blood flow at baseline and after ACh was then reexamined after local intra-arterial infusion of indomethacin (0.3 mg/100 ml forearm volume), a cyclo-oxygenase inhibitor. RESULTS: Baseline blood flow in the diabetic and control groups were similar (2.65 +/- 0.26 vs. 2.59 +/- 0.20 ml/min per 100 ml, respectively; P = 0.4). After indomethacin infusion, the vasodilatory responses to all doses of ACh were reduced in both the diabetic (by 25.30 +/- 4.90%) and control group (by 11.23 +/- 5.45%). However, the reduction in blood flow response to ACh after indomethacin was greater in diabetic patients compared with control subjects (P = 0.03). CONCLUSIONS: Our findings suggest that vasodilatory, prostanoids are important in determining endothelial response to ACh in diabetic and nondiabetic subjects. Increased prostaglandin-mediated vasodilation may compensate for attenuated responses to NO previously reported in diabetic subjects. These findings may partly explain the conflicting reports of endothelial dysfunction in patients with type 1 diabetes.     

Electrophysiological Effects of E 4031, a Drug with Selective Class III Properties, in Man

We studied the electrophysiological effects ofE 4037, given in a dose ascending manner (1.5, 3.0, and 6.0 μg/kg over 5 min followed by 0.1, 0.2, and 0.4 μg/kg per min for 60 min, respectively) to 19 volunteers. There were significant, dose related linear increases in QT and QT_C intervals, in atrial functional and effective refractory periods (ERPs) at a paced cycle length of 400 ms, and in ventricular functional and ERPs at a paced cycle length of 600 ms. There was no significant change in the AH and HV intervals or QRS duration. No significant proarrhythmic or other side effects were encountered during the administration of the drug. E 4031 prolongs atrial and ventricular refractoriness without significantly affecting AV or intraventricular conduction, consistent with selective Class III properties. At the doses used in the present study, intravenous infusion of E 4031 appears to be safe and well tolerated.     

Effects of microdialyzed oxotremorine, carbachol, epibatidine, and scopolamine on intraspinal release of acetylcholine in the rat

Intrathecally administered cholinergic agonists such as oxotremorine (muscarinic), carbachol (mixed nicotinic and muscarinic agonist), and epibatidine (nicotinic) have all been shown to reduce nociception in behavioral studies. Thus, there is substantial evidence for a role of acetylcholine (ACh) in the control of nociception in the spinal cord, but the mechanisms regulating ACh release are not known. The present study was initiated to establish a rat model to study which mechanisms are involved in the control of ACh release. Spinal microdialysis probes were inserted intraspinally at the C1-C5 spinal level in isoflurane-anesthetized rats. The probes were perfused with Ringer's solution containing 10 microM neostigmine to prevent degradation of ACh. Oxotremorine, carbachol, epibatidine, and scopolamine, dissolved in Ringer's solution, were administered intraspinally via dialysis and 30 microliter/10-min samples of dialysate were collected for HPLC analysis of ACh content. The release of ACh was found to be constant in the control (Ringer's only) situation during the experimental period of 150 min. Oxotremorine (100-1000 microM), carbachol (1 mM), and epibatidine (50-5000 microM) enhanced but scopolamine (50-200 nM) decreased the intraspinal release of ACh. Oxotremorine (ED(50) = 118 microM) and epibatidine (ED(50) = 175 microM) were found to produce a dose-dependent increase of ACh release. Cholinergic agonists caused an increase of intraspinal ACh and the antagonist scopolamine caused a decreased release of ACh. The data do not support an autoreceptor function of either nicotinic or muscarinic receptors in the spinal cord, contrary to what has been observed in the brain.     

Cholinergic Pairing with Visual Activation Results in Long-Term Enhancement of Visual Evoked Potentials

Acetylcholine (ACh) contributes to learning processes by modulating cortical plasticity in terms of intensity of neuronal activity and selectivity properties of cortical neurons. However, it is not known if ACh induces long term effects within the primary visual cortex (V1) that could sustain visual learning mechanisms. In the present study we analyzed visual evoked potentials (VEPs) in V1 of rats during a 4–8 h period after coupling visual stimulation to an intracortical injection of ACh analog carbachol or stimulation of basal forebrain. To clarify the action of ACh on VEP activity in V1, we individually pre-injected muscarinic (scopolamine), nicotinic (mecamylamine), α7 (methyllycaconitine), and NMDA (CPP) receptor antagonists before carbachol infusion. Stimulation of the cholinergic system paired with visual stimulation significantly increased VEP amplitude (56%) during a 6 h period. Pre-treatment with scopolamine, mecamylamine and CPP completely abolished this long-term enhancement, while α7 inhibition induced an instant increase of VEP amplitude. This suggests a role of ACh in facilitating visual stimuli responsiveness through mechanisms comparable to LTP which involve nicotinic and muscarinic receptors with an interaction of NMDA transmission in the visual cortex.     

Stimulation of acetylcholine release from myenteric neurons of guinea pig small intestine by forskolin and cyclic AMP

Forskolin, an activator of adenylate cyclase, was used to examine the regulation of [3H]acetylcholine (ACh) release by cyclic AMP (cAMP)-related mechanisms in myenteric plexus-longitudinal muscle preparations of guinea pig small intestine. Forskolin evoked a dose-related increase in [3H]ACh release. Both dibutyryl-cAMP and 8-Br-cAMP significantly elevated [3H]ACh secretion. In the presence of phosphodiesterase inhibitors (theophylline and 3-isobutyl-1-methylxanthine), the basal [3H]ACh output was increased. There was a significantly greater stimulation when forskolin was used to incite endogenous cAMP synthesis and phosphodiesterase inhibitors were simultaneously applied to prevent cAMP breakdown. The enhancement of forskolin-stimulated release by theophylline or 3-isobutyl-1-methylxanthine strongly implicates a synergistic interaction between the two. These findings suggest that forskolin acts to increase ACh release by a modulation of endogenous cAMP and further support a cAMP-mediated mechanism in the secretion of ACh from myenteric cholinergic neurons.     

Inhibition of vagal transmission by cardiac sympathetic nerve stimulation in the dog: possible involvement of opioid receptor

The inhibitory effect of cardiac sympathetic nerve stimulation (SNS) on bradycardic response to vagal nerve stimulation was examined in anesthetized dogs pretreated with atenolol or propranolol to prevent tachycardia caused by SNS or norepinephrine infusion. The amplitudes of vagal bradycardia (0.5, 1, 2, and 4 Hz) during 10 and 30 Hz of SNS were significantly smaller than those during the resting state. The inhibitory effect of 10 Hz of SNS was neither affected by prazosin (10, 30 and 100 micrograms/kg i.v.) nor by yohimbine (10, 30 and 100 micrograms/kg i.v.). The SNS-induced inhibition of vagal bradycardia was inhibited by naloxone (0.3 and 1 mg/kg i.v.) or by naltrexone (0.3 and 1 mg/kg i.v.). Vagal bradycardia was unaffected by intracoronary infusion of norepinephrine (3 micrograms/min) into the right coronary artery, whereas it was effectively inhibited by leu-enkephalin (10, 30 and 100 micrograms/kg i.v.). Bradycardia induced by intracoronary injection of methacholine (0.3, 1 and 3 micrograms) was unaffected by SNS. These results suggest that a presynaptic alpha adrenoceptor mechanism is not involved in the SNS-induced inhibition of vagal bradycardia in the dog, and suggest further that an opioid receptor mechanism may be responsible for the inhibition.     

Plasma free and conjugated catecholamines in diagnosis and localisation of pheochromocytoma

In six patients urinary excretion of vanillylmandelic acid and catecholamines (CA) could establish the diagnosis of pheochromocytoma. Free norepinephrine (NE) in plasma was within the normal range in two patients and plasma free epinephrine (E) was only marginally elevated in one of them. The degree of CA conjugation was not altered and scattered as in controls and was therefore not complementary to the usual determination of plasma free CA. The intermittent nature of CA secretion by the tumour could be demonstrated by multiple blood samplings during a 48-h study period in two patients, e.g. normal plasma values might be associated with pheochromocytoma if measurements are made during a trough. Thus a single peripheral CA determination cannot be of discriminative value in the diagnosis of pheochromocytoma unless it shows marked elevation.Ten patients subjected to intracardial measurements and five patients suspected of having a pheochromocytoma underwent venous catheterisation to determine their free and conjugated plasma CA. In controls CA values near the orifices of adrenal veins differed enormously and partly overlapped with corresponding levels of patients with pheochromocytoma. In one patient with surgically proven left adrenal tumour highest concentrations of CA were measured in the vena cava superior. These high CA concentrations, caused by paroxysmal release of CA by the tumour arouse suspicion of an additional, ectopic tumour. Because venous catherisation cannot be relied on implicitly we propose computed tomographic scanning as a first step in localisation of a pheochromocytoma.