Lack of modulation by presynaptic alpha 2-adrenoceptors of adrenergic transmitters release evoked by activation of 5-hydroxytryptamine and nicotine receptors

Quantification of the voltage-response relationship between punctate and field electrical stimulation and the function of isolated rat left atria and papillary muscles

The effects of varying the electrical stimulation voltage on the noradrenergic tone and systolic and diastolic cardiac function of isolated rat cardiac preparations is unknown. If a wide range of voltages substantially alters the basal noradrerengic tone of isolated cardiac preparations, highly variable responses may complicate the interpretation of the cardiac effects of interventions, such as drugs or toxins. This study was designed to determine whether field or punctate electrode stimulation altered systolic and diastolic cardiac function of isolated rat atrial and ventricular muscle preparations through a beta-adrenergic receptor-mediated mechanism. Isolated left atria and right ventricular papillary muscles from adult Sprague-Dawley rats were stimulated to contract isometrically in Krebs bicarbonate buffer (30 degrees C, pH 7.4, 0.5 Hz). Cardiac muscle function was assessed by determining cardiac contractility (dF/dt), relaxation (80% relaxation time), and muscle stiffness (changes in resting force). Muscles were electrically stimulated to contract using either punctate or field electrodes. Basal values of cardiac function were determined at threshold voltages, and voltage was increased in stepwise fashion, over a wide range, to obtain a voltage response-relation to cardiac function. In left atrial preparations, both punctate and field electrical stimulation caused a 200% increase in cardiac contractility. The greatest changes in contractility occurred at near threshold voltages (less than 5 volts for punctate and 19 volts for field stimulation). The voltage-dependent increases in cardiac contractility were attenuated or abolished by pretreatment with atenolol (10 micromol/L; selective beta 1-antagonist) or reserpine (5 mg/kg i.p., 24 hr before euthanasia). In contrast to left atrial preparations, neither field or punctate electrical stimulation had any effect on the cardiac contractility of papillary muscle preparations. Neither field or punctate electrical stimulation had any effect on cardiac relaxation (80% relaxation time) or muscle stiffness (changes in resting force) of left atrial or papillary muscle preparations. This study demonstrates that punctate and field electrical stimulation can significantly increase cardiac contractility of rat left atrial preparations at near threshold voltages by increasing noradrenergic tone. Thus, control of the voltage range may be required to reduce variation in cardiac function of isolated rat left atrial preparations.

Auditory-evoked response of the cortex after yohimbine administration: phase advance effect of central noradrenergic activation

The effects of central noradrenergic activation on an auditory-evoked cortical response were studied using systemic administration of yohimbine (2 mg/kg intravenously, IV), a noradrenergic stimulant, in 13 anesthetized rats. To analyze changes of the response, surface and intracortical evoked potentials (EP) as well as extracellular single-unit recordings with tungsten microelectrodes were employed. It was noted that the initial-positive wave of the surface EP corresponded to unit firing responses in a restricted area of the auditory cortex, where the surface EP was largest and a polarity inversion of the intracortical EP was observed. The following effects were produced by yohimbine: 1) The initial-positive surface potential (n = 10) and corresponding intracortical potential with inverted polarity (n = 6) both showed an increase in amplitude and a decrease in peak latency; 2) the unit firing response (n = 10) tended to show an increase in peak frequency and a decrease in peak firing latency; and 3) yohimbine produced an earlier ending of the firing period, and in paired stimulation experiments (n = 7) it prolonged the period during which the second response was suppressed, indicating an augmentation of postexcitation inhibition. Later histological examination suggested that most of the units recorded were pyramidal cells. These findings indicate that chemical stimulation of the central noradrenergic system by yohimbine enhances both the initial excitatory and following inhibitory processes in the auditory-evoked response of the cortical units (probably pyramidal cells), resulting not only in amplification of the response but also in advancement of the response phase.(ABSTRACT TRUNCATED AT 250 WORDS)

The problem with autoreceptors

There are numerous problems with the concept that antagonists enhance transmitter release by blockade of feedback. It was shown that antagonist enhancement of transmitter release does not correlate satisfactorily with the intensity of stimulation or with other indices of biophase transmitter concentration. Wide variations were shown to exist between antagonists in the amount of enhancement of release they induce. Also, antagonists enhance transmitter release or the effector response with a single stimulation pulse, a condition under which no feedback is possible. A study of agonist/antagonist relationships indicates different sites of action, and it was determined that the antagonist effect has negligible or minimal latency and that enhancement by antagonists is maximal under minimal condition of stimulation. Antagonists were shown to enhance release by a direct action, not by passive occupancy of agonist sites. Experiments were described in which acetylcholine and cold selectively antagonized antagonist but not agonist effects. Further, experiments with pulse duration shifts and with veratridine pointed to a direct action of antagonists on Na+ (also Ca++?) channel gating mechanisms, which results in a shift in the voltage dependence of activation. If antagonists, in some particular instances, enhance release by blockade of sites involved in negative feedback this is likely lost or mired in their more prominent direct actions on neurosecretion--and these must be sorted out. The acceptance of the fact that antagonists act directly to alter transmitter release (and not only as passive occupiers of presynaptic receptors), as the present study shows, both in the central nervous system and in the periphery, opens a new area for future investigation, and may be exploitable for therapeutic purposes and to gain an enriched understanding of the mechanism of neurosecretion.

Activation of alpha 2-adrenoceptors attenuates the inotropic effect of field stimulation in atria

Guinea-pig left atria were driven to contract at a rate of 0.5 Hz by stimulation with punctate electrodes. For additional field stimulation, a train of one to eight field pulses (30 V; 0.05-0.4 ms duration) was applied during each refractory period. Cholinergic effects were blocked by atropine and thus the resulting increase of the contractile force was caused by noradrenaline released from sympathetic nerve endings. Trains of several short field pulses delivered in the refractory period after each contraction produced a significantly greater inotropic effect than one single pulse of the same total duration delivered in each refractory period. Phentolamine, rauwolscine and idazoxan, by blocking prejunctional alpha 2-adrenoceptors selectively increased the inotropic effect of single field pulses. Selective blockers of alpha 1-adrenoceptors (prazosin, corynanthine) were ineffective in this respect. The effect of alpha 2-adrenoceptor blockers persisted in the presence of noradrenaline uptake blockers (cocaine or desipramine plus corticosterone) or phenylephrine, but was overcome by the alpha 2-adrenoceptor agonists, clonidine and guanabenz. It is concluded that activation of presynaptic alpha 2-adrenoceptors limits the release of noradrenaline by long-lasting single field pulses. Autoinhibition of transmitter release was not seen with trains of short field pulses.

The determination of presynaptic pA2 values of yohimbine and phentolamine on the perfused rat heart under conditions of negligible autoinhibition

1 Rat isolated perfused hearts with the right sympathetic nerves attached were loaded with [3H]-(-)-noradrenaline. The nerves were stimulated with up to 40 trains of 10 pulses every min at 1 Hz, and the evoked increases of [3H-]noradrenaline overflow into the perfusate, of right atrial tension development and ventricular beating frequency were measured. 2 Oxymetazoline inhibited the evoked transmitter overflow (IC50: 10 nM) and decreased the postsynaptic responses in a concentration-dependent manner. It behaved as a full against in abolishing the evoked transmitter overflow. 3 Yohimbine up to 1 microM neither enhanced the evoked [3H]-noradrenaline overflow nor the postsynaptic parameters. Phentolamine (1 microM) caused a transient, minor (less than 30%) increase in [3H]-noradrenaline overflow. 4 Yohimbine (0.03-1.0 microM) and phentolamine (0.1-5.0 microM) shifted to the right the concentration-response curve of oxymetazoline for the inhibition of [3H]-noradrenaline overflow in response to nerve stimulation without depressing the maxima. The pA2 values were 7.82 and 7.52, respectively. 5 Yohimbine (0.1 microM) also antagonized the decrease induced by oxymetazoline in the postsynaptic responses to nerve stimulation. 6 The results confirm the existence of presynaptic inhibitory alpha 2-adrenoceptors at the adrenergic nerve fibres of the rat heart in vitro. Under the stimulation and perfusion conditions selected, the released endogenous transmitter apparently does not activate a negative feedback mechanism, thus permitting the determination of pA2 values.

Differences between receptors for 5-hydroxytryptamine on autonomic neurones revealed by nor-(-)-cocaine

1 The interaction between nor-(-)-cocaine and 5-hydroxytryptamine (5-HT) at autonomic neuronal 5-hydroxytryptamine (5-HT) receptors has been investigated on the rabbit heart and guinea-pig ileum. 2 The chronotropic response to 5-HT mediated through activation of receptors on the terminal sympathetic fibres of the rabbit heart was antagonized selectively and surmountably by nor-(-)-cocaine. Schild analysis yielded a pA2 value for the interaction of 7.79 +/- 0.11, n = 13. 3 On the guinea-pig ileum induced to contract by activation of 5-HT receptors on the intramural cholinergic nerves, nor-(-)-cocaine was also a selective and surmountable antagonist of 5-HT, although the concentrations needed were higher than those shown to be effective on the heart. The pA2 value for the interaction was 6.49 +/- 0.07, n = 16. 4 The differential blocking effect of nor-(-)-cocaine in the two tissues suggests that the receptors for 5-HT on the cholinergic nerves of the ileum and the sympathetic fibres of the heart are different. The data thus lend support to the earlier similar conclusion based on the observation that 5-methoxytryptamine is a potent agonist on the ileum yet inactive on the heart.

Inhibition of 5-hydroxytryptamine-evoked autonomic transmitter release by apomorphine

Apomorphine inhibited chronotropic responses of the isolated rabbit heart to 5-HT by 40% at 1.17 micrometers and by 90% at 4.68 micrometers and strongly inhibited the outflow of 3H following preloading of hearts with [3H]- (-)-noradrenaline. Apomorphine, 4.68 micrometers, had no significant effects on transmitter release evoked by DMPP or tyramine but inhibited the responses to SNS at frequencies up to 3.2 Hz. The inhibitory effects of apomorphine on 5-HT were resistant to blockade by chlorpromazine, 1.4 micrometers, haloperidol, 1.6 micrometers, spiroperidol, 2.5 micrometers, or yohimbine, 2.8 micrometers. In contrast, the inhibitory effects of apomorphine on low frequency SNS were abolished by yohimbine. On the guinea-pig ileum treated with methysergide, apomorphine, 1.17-4.68 micrometers, blocked the indirect cholinergic responses to 5-HT less markedly than it blocked the indirect sympathomimetic responses to 5-HT on the rabbit heart. Moreover, the effects were non-selective since responses to DMPP and transmural stimulation of the intramural cholinergic nerves were similarly reduced. Modification of 5-HT receptor function is the most likely explanation for the action of apomorphine with the differential effect on 5-HT in the heart and ileum reflecting differences in the receptors and/or post receptorial events at the two sites.