A study of effects of putative neurotransmitters injected into the lateral cerebral ventricle of man

Beta-2 adrenoreceptor gene polymorphisms and sympathetic outflow in humans

OBJECTIVES

Previous association studies suggested that common polymorphisms of the beta-2 adrenoreceptor gene leading to glycine for arginine substitution at position 16 or glutamic acid for glutamine substitution at position 27 affect blood pressure. We reasoned that measurements of resting sympathetic nerve traffic could increase the sensitivity of defining a gene phenotype relationship.

METHODS

We studied 111 Caucasian subjects (70 men, 41 women) with blood pressure<140/90 mmHg. We measured electrocardiogram, beat-by-beat finger blood pressure, brachial blood pressure, and muscle sympathetic nerve activity (MSNA) using microneurography. We genotyped the functionally relevant polymorphisms of the beta-2 adrenoreceptor gene by means of allele-specific polymerase chain reaction.

RESULTS

Sympathetic nerve traffic was similar regardless of genotypes. We obtained similar results when we quantified sympathetic nerve traffic as bursts/100 heart beats or as normalized burst area or when we adjusted resting sympathetic nerve traffic for gender, age, and blood pressure. The polymorphism at position 27 affects sympathetic regulation in men. Men with a Glu/Glu genotype had a significant positive correlation between blood pressure and MSNA.

INTERPRETATIONS

While our study was not sufficiently powered to detect subtle influences of genetic variability in the beta-2 adrenoreceptor gene on resting sympathetic nerve traffic, a large effect is unlikely. However the observation that beta-2 adrenoreceptor genotype may affect coupling between resting sympathetic nerve traffic and systolic blood pressure deserves to be tested in larger populations.

The Importance of Brainstem Cholinergic Neurons in the Pressor Response to Cocaine

After intracisternal injection, 140 nmol (48 microg) of cocaine (but not lidocaine or procaine) evoked an increase in mean arterial pressure (MAP) of 41 mm Hg. The increase in MAP began within 1 min after injection and lasted 10 to 15 min. The pressor response to intracisternal injection of cocaine was not mediated through central alpha-adrenergic receptors, but intracisternal pretreatment with D1 or D2 dopamine receptor antagonists shortened the duration of the response. Pretreatment with intracisternal injection of hemicholinium-3 to deplete medullary acetylcholine produced a dose-dependent inhibition of the pressor and tachycardic responses to intracisternal injection of cocaine. Central pretreatment with hemicholinium-3 also inhibited the pressor response to intravenous injection of 0.5 mg/kg cocaine. Atropine pretreatment was only partly effective in blocking the pressor and tachycardic responses to intracisternal injection of cocaine. However, a single intracisternal injection of the nicotinic ganglionic receptor blocker hexamethonium inhibited the pressor response to cocaine administered intracisternally 24 h later, and on each of the following 4 days. The blocking effect of hexamethonium was not mimicked by the alpha7 selective antagonist methyllycaconitine or by the alpha4beta2 subtype-preferring antagonist dihydro-beta-erythroidine. The data suggest that the pressor response to cocaine is mediated by medullary acetylcholine release on to nicotinic receptors of the ganglionic type, enhancing the output of bulbospinal sympathetic premotor neurons. Our results provide new evidence for the prolonged inactivation of relevant central nicotinic receptors by nicotinic receptor antagonists, and suggest that such compounds might be used safely for cocaine overdose, as well as for antiabuse issues without the concern for autonomic side effects.

Limited effect of systemic beta-blockade on sympathetic outflow

Central beta-adrenoreceptors may augment sympathetic outflow. We tested the hypothesis that beta-blockade attenuates central sympathetic outflow by inhibiting central adrenoreceptors. We studied 18 healthy controls (4 female, 14 male; age, 26+/-6 years, body mass index, 23+/-3 kg/m(2)). ECG, brachial, and finger arterial blood pressure, muscle sympathetic nerve activity, and respiration were measured continuously before and during complete beta-blockade. Subjects received a total intravenous dose of 0.21 mg/kg of propranolol in 15 minutes. Spontaneous baroreflex slopes were calculated using the sequence technique (BRSup, BRSdown). The sympathetic baroreflex slope was determined at baseline using phenylephrine and sodium nitroprusside infusions. The subjects underwent cold pressor testing before and during beta-blockade. The R-R interval increased from 861+/-119 ms at baseline to 952+/-141 ms during beta-blockade (P<0.01). Blood pressure was 117+/-9/65+/-8 mm Hg at baseline and 117+/-10/67+/-8 mm Hg during beta-Blockade (P=NS). beta-Blockade did not affect baroreflex sensitivity (BRSup: 21+/-10 versus 28+/-11 ms/mmHg, P<0.1; BRSdown: 17+/-8 versus 20+/-8 ms/mmHg, P=NS). Muscle sympathetic nerve activity increased significantly during beta-blockade (number of bursts/100 beats: 32+/-9 versus 40+/-14, P<0.05), compared with baseline. However, the operating points of the parasympathetic and sympathetic baroreflex during beta-blockade were on the baroreflex curves obtained at baseline. beta-Blockade blunted the heart rate response to cold pressor testing; blood pressure and muscle sympathetic nerve activity responses were similar. Our study demonstrates that propranolol does not cause an acute decrease in sympathetic activity in normotensive young subjects. This, observation is not consistent with an important tonic stimulatory effect of beta-adrenoreceptors in the brain.

Regulation of blood pressure with calcium-dependent dopamine synthesizing system in the brain and its related phenomena

The effects of calcium on blood pressure regulation remain controversial. Although the mechanism by which calcium increases blood pressure when it is given intravenously and acutely has been elucidated, that by which calcium reduces blood pressure when it is supplemented chronically and slightly through daily diet is unclear. From a number of animal experiments concerning the effects of calcium on blood pressure, we believe that calcium ions have two separate roles in the regulation of blood pressure through both central and peripheral systems: (1) calcium ions reduce blood pressure through a central, calcium/calmodulin-dependent dopamine-synthesizing system and (2) calcium ions increase blood pressure through an intracellular, calcium-dependent mechanism in the peripheral vasculature. These concepts were applied to elucidate the mechanisms underlying hypertension in spontaneously hypertensive rats (SHR) and changes in blood pressure in other experimental animals, and the following conclusions were reached. The decrease of the serum calcium level in spontaneously hypertensive rats (SHR) causes a decrease in calcium/calmodulin-dependent dopamine synthesis in the brain. The subsequent low level of brain dopamine induces hypertension. The increase in susceptibility to epileptic convulsions and the occurrence of hypertension in epileptic mice (El mice) may be linked through a lowering of calcium-dependent dopamine synthesis in the brain, and epilepsy and hypertension may be associated. Exercise leads to increases in calcium-dependent dopamine synthesis in the brain, and the increased dopamine levels induce physiological changes, including a decrease in blood pressure. Cadmium which is not distinguished from calcium by calmodulin, activates calmodulin-dependent functions in the brain, and increased dopamine levels may decrease blood pressure. In this report, our studies are considered in light of reports from many other laboratories.

Effects of body core temperature and brain dopamine activity on timing processes in humans

In a placebo-controlled study, the effects of experimentally induced increase in body core temperature and of the dopamine antagonist haloperidol on judgments of an apparent second, a speeded-tapping task, and temporal discrimination of intervals in the range of milliseconds and seconds were investigated in 40 healthy male subjects. A 0.7 degree C-increase in body core temperature due to 3-h exposure to an ambient temperature of 52 degrees C did not cause any statistically significant changes in timing tasks. Unlike heat exposure, 3 mg of haloperidol caused a pronounced impairment of performance on the temporal discrimination of intervals in the range of milliseconds and seconds (P < 0.01 and P < 0.001, respectively) as well as speeded tapping (P < 0.05). For temporal discrimination of intervals in the range of seconds, a significant interaction between ambient temperature and haloperidol could be established (P < 0.05) indicating that haloperidol caused a significant performance decrement only in subjects exposed to an ambient temperature of 28 degrees C but not in those exposed to 52 degrees C. The overall pattern of results suggests that temporal processing of intervals in the range of milliseconds can be considered a function of dopaminergic activity in the basal ganglia while temporal processing of longer intervals appears to be cognitively mediated. Furthermore, the hypothesis that timing processes in humans are modulated by changes in body core temperature could not be established.