Characterization of presynaptic beta-adrenoceptors facilitating endogenous noradrenaline release in the portal vein of permanently cannulated, freely moving rats

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Neurotransmitter Switching Coupled to β-Adrenergic Signaling in Sympathetic Neurons in Prehypertensive States

Single or combinatorial administration of β-blockers is a mainstay treatment strategy for conditions caused by sympathetic overactivity. Conventional wisdom suggests that the main beneficial effect of β-blockers includes resensitization and restoration of β1-adrenergic signaling pathways in the myocardium, improvements in cardiomyocyte contractility, and reversal of ventricular sensitization. However, emerging evidence indicates that another beneficial effect of β-blockers in disease may reside in sympathetic neurons. We investigated whether β-adrenoceptors are present on postganglionic sympathetic neurons and facilitate neurotransmission in a feed-forward manner. Using a combination of immunocytochemistry, RNA sequencing, Förster resonance energy transfer, and intracellular Ca2+ imaging, we demonstrate the presence of β-adrenoceptors on presynaptic sympathetic neurons in both human and rat stellate ganglia. In diseased neurons from the prehypertensive rat, there was enhanced β-adrenoceptor-mediated signaling predominantly via β2-adrenoceptor activation. Moreover, in human and rat neurons, we identified the presence of the epinephrine-synthesizing enzyme PNMT (phenylethanolamine-N-methyltransferase). Using high-pressure liquid chromatography with electrochemical detection, we measured greater epinephrine content and evoked release from the prehypertensive rat cardiac-stellate ganglia. We conclude that neurotransmitter switching resulting in enhanced epinephrine release, may provide presynaptic positive feedback on β-adrenoceptors to promote further release, that leads to greater postsynaptic excitability in disease, before increases in arterial blood pressure. Targeting neuronal β-adrenoceptor downstream signaling could provide therapeutic opportunity to minimize end-organ damage caused by sympathetic overactivity.

Immunohistochemical changes induced by repeated footshock stress: revelations of gender-based differences

As a growing literature has proven, adverse experiences, particularly when severe and persistent, play a pivotal role in the development of neuronal dysfunctions and psychopathology. In the present study, the neurochemical changes induced by acute and repeated footshock exposure were investigated at the molecular and cellular level, using c-fos and phospho-ERK1/2 immunoreactivity and gene expression arrays. Marked gender-related differences were found following both acute and prolonged footshock exposure. Acute aversive conditioning resulted in significant immunohistochemical changes that might be critically involved in the modulation of fear-related responses, especially in males. Prolonged footshock exposure, on the contrary, was associated with sustained hypothalamic-pituitary-adrenal axis hyperactivity, differential gender-related patterns of cortical-limbic activity, and abnormal neuronal plasticity, especially in medial prefrontocortical regions. These data may provide additional insights into the understanding of the neural circuits underlying the effects of acute and repeated footshock exposure as well as clarify some of the mechanisms involved in the development of stress-related neuronal abnormalities.

Molecular correlates of impaired prefrontal plasticity in response to chronic stress

Disturbed adaptations at the molecular and cellular levels following stress could represent compromised neural plasticity that contributes to the pathophysiology of stress-induced disorders. Evidence illustrates atrophy and cell death of stress-vulnerable neurones in the prefrontal cortex. Reduced plasticity may be realized through the destabilized function of selective proteins involved in organizing the neuronal skeleton and translating neurotrophic signals. To elucidate the mechanisms underlying these effects, rats were exposed to chronic footshock stress. Patterns of c-fos, phospho-extracellular-regulated protein kinases 1/2 (ERK1/2), calcineurin and phospho-cyclic-AMP response-element binding protein (CREB) expression were subsequently investigated. The results indicate chronic stress-induced impairments in prefrontal and cingulate signal transduction cascades underlying neuronal plasticity. The medial prefrontal cortex, demonstrated functional hyperactivity and dendritic phospho-ERK1/2 hyperphosphorylation, while reduced c-fos and calcineurin immunoreactivity occurred in the cingulate cortex. Significantly reduced phospho-CREB expression in both cortical regions, considering its implication in brain-derived neurotrophic factor (BDNF) transcription, suggests reduced synaptic plasticity. This data confirms the damaging effect of stress on cortical activity, on a molecular level. Due to the association of these markers in the regulation of BDNF signalling, these findings suggest a central role for intracellular neurotrophin transduction members in the pathways underlying cellular actions of stress in the brain.

Selective chronic stress-induced in vivo ERK1/2 hyperphosphorylation in medial prefrontocortical dendrites: implications for stress-related cortical pathology?

Stress has been shown to affect brain structural plasticity, promote long-term changes in multiple neurotransmitter systems and cause neuronal atrophy. However, the mechanisms involved in these stress-related neural alterations are still poorly understood. Mitogen-activated protein kinase (MAPK) cascades play a crucial role in the transduction of neurotrophic signal from the cell surface to the nucleus and are implicated in the modulation of synaptic plasticity and neuronal survival. An intriguing possibility is that stress might influence brain plasticity through its effects on selective members of such intracellular signalling cascades responsible for the transduction of neurotrophin signals. Here, we have investigated the effects of stress on the expression of three members of the MAPK/extracellular-regulated kinase (ERK) pathway such as phospho-ERK1, phospho-ERK2 and phospho-cAMP/calcium-responsive element-binding protein (CREB) in the adult rat brain. Male rats were subjected to mild footshocks and the patterns of protein expression were analysed after 21 consecutive days of stress. We found that chronic stress induced a pronounced and persistent ERK1/2 hyperphosphorylation in dendrites of the higher prefrontocortical layers (II and III) and a reduction of phospho-CREB expression in several cortical and subcortical regions. We hypothesized that defects in ERK signalling regulation combined with a reduced phospho-CREB activity may be a crucial mechanism by which sustained stress may induce atrophy of selective subpopulations of vulnerable cortical neurons and/or distal dendrites. Thus, ERK-mediated cortical abnormalities may represent a specific path by which chronic stress affects the functioning of cortical structures and causes selective neural network defects.

Differential role of adrenoceptors in control of plasma glucose and fatty acids in carp, Cyprinus carpio (L.)

Carp were cannulated in the dorsal aorta, and after 2 days of recovery they were infused with 1) norepinephrine, 2) yohimbine (alpha(2)-antagonist) plus norepinephrine, 3) clonidine (alpha(2)-agonist), and 4) isoproterenol (nonselective beta-agonist). Norepinephrine lowered the plasma free fatty acid (FFA) level and raised the plasma glucose level for several hours. Norepinephrine in combination with the alpha(2)-antagonist yohimbine resulted in retardation of the FFA decrease, indicating the involvement of alpha(2)-adrenoceptors. Infusion with the partial alpha(2)-agonist clonidine had a smaller effect. Infusion with isoproterenol caused a marked increase of glucose levels, and unexpectedly a decline of plasma FFA levels, indicating a direct involvement of beta-adrenoceptors. Combination of isoproterenol with either atenolol (beta(1)-antagonist) or ICI-118,551 (beta(2)-antagonist) showed that both beta(1)- and beta(2)-adrenoceptors were involved in the glucose release by isoproterenol. Remarkably, the decline of FFA levels was augmented in the presence of ICI-118,551, whereas with atenolol present plasma FFA levels were increased by isoproterenol. Thus it is concluded that in carp both beta(1)- and beta(2)-adrenoceptors mediate glucose release, whereas lipolysis is controlled by inhibitory beta(1)-adrenoceptors and stimulatory beta(2)-adrenoceptors, as well as by inhibitory alpha(2)-adrenoceptors.

Facilitatory beta2-adrenoceptors on cholinergic and adrenergic nerve endings of the guinea pig trachea

Using electrical field stimulation of epithelium-denuded intact guinea pig tracheal tube preparations, we studied the presence and role of prejunctional beta2-adrenoceptors by measuring evoked endogenous acetylcholine (ACh) and norepinephrine (NE) release directly. Analysis of ACh and NE was through two HPLC systems with electrochemical detection. Electrical field stimulation (150 mA, 0.8 ms, 16 Hz, 5 min, biphasic pulses) released 29.1 +/- 2.5 pmol ACh/g tissue and 70.2 +/- 6.2 pmol NE/g tissue. Preincubation for 15 min with the selective beta2-adrenoceptor agonist fenoterol (1 microM) increased both ACh and NE overflow to 178 +/- 28 (P < 0.01) and 165 +/- 12% (P < 0.01), respectively, of control values, increases that were abolished completely by the selective beta2-adrenoceptor antagonist ICI-118551 (1 microM). Further experiments with increasing fenoterol concentrations (0.1-100 microM) and different preincubation periods (1, 5, and 15 min) showed a strong and concentration-dependent facilitation of NE release, with maximum response levels decreasing (from nearly 5-fold to only 2.5-fold of control value) with increasing agonist contact time. In contrast, sensitivity of facilitatory beta2-adrenoceptors on cholinergic nerves to fenoterol gradually increased when the incubation period was prolonged; in addition, a bell-shaped concentration-response relationship was found at 15 min of preincubation. Fenoterol concentration-response relationships (15-min agonist preincubation) in the presence of atropine and yohimbine (1 microM each) were similar in the case of NE release, but in the case of ACh release, the bell shape was lost. The results indicate a differential capacity and response time profile of facilitatory prejunctional beta2-adrenoceptors on adrenergic and cholinergic nerve terminals in the guinea pig trachea and suggest that the receptors on adrenergic nerves are more susceptible to desensitization.

Effect of clenbuterol on the modulation of noradrenaline release in the rat tail artery

1. Exposure of rat tail arteries to clenbuterol, a beta 2-adrenoceptor agonist, for 20 or 90 min, did not change or increase, respectively, tritium overflow induced by electrical field stimulation in arteries preincubated with [3H]-noradrenaline (NA). This facilitatory effect was antagonized by propranolol. 2. Phentolamine increased the evoked overflow four-fold, which was not modified by 90 min incubation with clenbuterol. In rats pretreated with clenbuterol for 2 weeks, the stimulated overflow was not enhanced by this beta 2-agonist, and the increase produced by phentolamine was markedly diminished. 3. Contractile responses induced by electrical field stimulation were not modified or increased (only at low frequencies) by preincubation with clenbuterol for 20 or 90 min, respectively. This effect was inhibited by propranolol. 4. In arteries precontracted with 5-hydroxytryptamine, clenbuterol (10 nM-10 microM) produced small relaxations, which were reduced by propranolol plus phentolamine and not modified by phentolamine or 90 min exposure to clenbuterol. 5. These results indicate that prolonged exposure of rat tail arteries to clenbuterol produces a facilitation of NA release mediated by activation of presynaptic beta 2-adrenoceptors, which may be involved on the enhancement of contractile responses to electrical stimulation induced by clenbuterol. However, chronic treatment with this beta-agonist desensitizes these receptors.

Co-released adrenaline markedly facilitates noradrenaline overflow through prejunctional beta 2-adrenoceptors during swimming exercise

The effect of intravenously applied (-)-adrenaline, taken up by and released from sympathetic nerves, on swimming exercise-induced noradrenaline overflow in permanently cannulated adrenal demedullated rats was studied. Adrenaline (100 ng/min) was infused for 2 h, during which a plasma concentration of 500 pg/ml (approximately 2.5 nM) was reached. One hour later plasma adrenaline had returned to undetectable levels. During swimming, adrenaline was released into the plasma in concentrations up to 133 pg/ml and the noradrenaline concentration was markedly enhanced as well. The total catecholamine increase amounted to 178% of control (saline infusion) in the first 3 min of swimming and 165% for the whole 20 min. Cocaine (2.5 mg/kg plus 0.05 mg/kg/min), infused together with adrenaline and continued throughout the experiment, prevented the exercise-induced release of adrenaline and no increase in plasma noradrenaline concentration was observed. Yohimbine (0.25 mg/kg) strongly further enhanced the exercise-induced overflow of both noradrenaline and adrenaline. This further increase was completely blocked by the selective beta 2-adrenoceptor antagonist ICI 118,551 ((+/-)-1-[(2,3-dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methyleth yl) amino]-2-butanol) (1.0 mg/kg). These results demonstrate that adrenaline can be taken up by sympathetic nerve endings through cocaine-sensitive uptake carriers and is released from these nerves during swimming exercise. Neuronally released adrenaline markedly enhances exercise-induced catecholamine overflow through activation of prejunctional beta 2-adrenoceptors.

Pre- and postganglionic stimulation-induced noradrenaline overflow is markedly facilitated by a prejunctional beta 2-adrenoceptor-mediated control mechanism in the pithed rat

The aim of the study was to further explore the prejunctional beta-adrenoceptor-mediated control mechanism of noradrenaline release from sympathetic nerves in response to preganglionic nerve stimulation (PNS) and local nerve stimulation of the portal vein, respectively, in the pithed rat. Baseline values as well as the increments of mean arterial blood pressure (delta-BP), heart rate (delta-HR) and plasma noradrenaline levels (delta-NA) in response to four PNS episodes (0.8 Hz, 3 ms, 75 V for 45 s at 20 min intervals), respectively, were evaluated. Fenoterol administration (0.25 mg/kg, i.v.) reduced significantly the basal blood pressure but did not alter delta-BP in response to PNS. Basal heart rate markedly increased after fenoterol without any further change in heart rate induced by PNS. The beta 1-selective antagonist CGP 20712A attenuated delta-BP in response to PNS and prevented the fenoterol-induced increase in basal heart rate. The beta 2-selective antagonist ICI 118,551 per se did not change the blood pressure and heart rate values, but antagonized the fenoterol-induced decrease in basal blood pressure. Fenoterol enhanced plasma delta-NA in response to PNS by 105% in comparison to the corresponding control value. This effect of fenoterol could be blocked by pretreatment with ICI 118,551 but not with CGP 20712A (a selective beta 1-adrenoceptor antagonist) which per se did not significantly change plasma delta-NA. Repeated local stimulation of the portal vein (S1-S3, 2 Hz, 3 ms, 10 mA, for 120 s at 30 min intervals) increased portal plasma noradrenaline without changing mean blood pressure and heart rate in pithed rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization and anatomical localization of prejunctional beta-adrenoceptors in the rat kidney

1. The subtype and anatomical localization of beta-adrenoceptors mediating facilitation of stimulus-induced overflow of noradrenaline ('prejunctional beta-adrenoceptors') are not conclusively known to date. The present study was undertaken to characterize these receptors by use of pharmacological methods as well as to define their localization (prejunctional or postjunctional) with radio-ligand binding and autoradiography techniques combined with surgical denervation of the sympathetic innervation to the rat kidney. 2. Exposure of the kidney to (-)-isoprenaline, the nonselective beta-adrenoceptor agonist, resulted in a dose-dependent facilitation of stimulus-induced neurotransmitter overflow. This response was inhibited by propranolol, the beta 1- and beta 2-adrenoceptor antagonist, with a pA2 of 9.20 suggesting that the prejunctional beta-adrenoceptors are not of the beta 3-subtype. 3. The rank order of potency and potency ratios of beta-adrenoceptor agonists at renal prejunctional beta-adrenoceptors (EC50 for agonist/EC50 for (-)-isoprenaline) were: (-)-isoprenaline (1) > procaterol (2) > salbutamol (3) > adrenaline (10) > (+)-isoprenaline (25). However, dobutamine, the beta 1-adrenoceptor agonist, failed to enhance stimulus-induced overflow of noradrenaline. These results are indicative of the presence of beta 2-adrenoceptors as prejunctional beta-adrenoceptors. 4. Facilitation elicited by (-)-isoprenaline and procaterol, the selective beta 2-adrenoceptor agonist, was inhibited by ICI 118,551, the selective beta 2-adrenoceptor antagonist, with pKb values of 9.20 and 9.35, respectively at renal prejunctional beta-adrenoceptors. Similarly, the pKb values of metoprolol, the selective beta 1-adrenoceptor antagonist, at renal prejunctional beta-adrenoceptors were determined to be 6.25 and 6.18 against (-)-isoprenaline and procaterol, respectively. These results suggest the presence of a homogeneous population of beta 2-adrenoceptors as prejunctional beta-adrenoceptors. 5. Radio-ligand binding analysis of renal beta-adrenoceptors revealed the prevalence of the beta 1-subtype as compared to the beta 2-subtype (63% vs 37%). However, surgical denervation of the rat kidney, resulting in more than 90% reduction in renal noradrenaline content, selectively reduced the beta 2-adrenoceptor population by 80%, implying the presence of beta 2-adrenoceptors on renal sympathetic nerve terminals. 6. Autoradiographic analysis demonstrated the presence of beta 1-adrenoceptors on cortical structures such as glomeruli and tubules. beta-Adrenoceptors were found to be present on tubules (minor population), collecting tubules in outer medulla and the adventitia and adventitial-medial border of intraparenchymal branches of the renal artery. Surgical denervation of the rat kidney resulted in the disappearance of Beta2-adrenoceptors associated with the intraparenchymal branches, without affecting the Beta-adrenoceptor populations at other sites. These results support the notion that the Beta2-subtype is present on renal sympathetic nerve terminals and demonstrate that these prejunctional Beta2-adrenoceptors are associated with the renal vasculature and not with renal tubules.7. The results of the present investigation demonstrate that renal prejunctional Beta-adrenoceptors are of the Beta2-subtype in nature. These receptors are present on sympathetic nerve terminals which are associated with the renal vasculature.

Strong activation of vascular prejunctional beta 2-adrenoceptors in freely moving rats by adrenaline released as a co-transmitter

The effect of adrenaline on the electrically evoked noradrenaline overflow in the portal vein of adrenal demedullated freely moving rats was studied. Adrenaline (100 ng/min) was infused for 2 h into the portal vein. After a 1-h interval when plasma adrenaline had returned to pre-infusion undetectable levels, the portal vein nervus plexus was electrically stimulated. During stimulation (2 Hz, 3 ms, 5 mA) adrenaline and noradrenaline were released. The stimulus-evoked noradrenaline overflow was facilitated to 194% of the control-evoked overflow (infusion of saline). Total catecholamine overflow (noradrenaline plus adrenaline) was enhanced to 258%. The facilitation of the evoked overflow of both noradrenaline and adrenaline was blocked by the selective beta 2-adrenoceptor antagonist, ICI 118,551 (0.3 mg/kg). Cocaine (2.5 mg/kg plus 0.05 mg/kg per min) infused together with adrenaline prevented the evoked release of adrenaline and no facilitation of the stimulus-induced noradrenaline overflow occurred. Inhibition of prejunctional inhibitory alpha 2-adrenoceptors with yohimbine (0.5 mg/kg) further enhanced, to 577%, the electrically evoked catecholamine overflow after adrenaline infusion. The results demonstrate that adrenaline can be taken up by sympathetic nerve endings through cocaine-sensitive uptake carriers, and is released from these nerves during electrical stimulation of the portal vein nervus plexus. Neuronally released adrenaline can strongly facilitate electrically evoked neurotransmitter overflow through activation of prejunctional beta 2-adrenoceptors.

Modulation of noradrenergic transmission in the rat isolated portal vein: Role of prejunctional α2-adrenoceptors and β-adrenoceptors

1. The effect of several adrenoceptor agonists and antagonists on the spontaneous and stimulus-evoked release of [3H]noradrenaline was studied in rat isolated portal vein. 2. Yohimbine (10(-6)M) increased the stimulus-evoked [3H]noradrenaline efflux. Adrenaline alone (3 x 10(-6)M) was without effect, while it increased the resting efflux when added together with yohimbine. 3. Propranolol alone was without effect on the release of [3H]noradrenaline. When added (2 x 10(-6)M) at the same time as yohimbine, it reduced the stimulation-induced 3H efflux. When added before adrenaline and yohimbine, propranolol (10(-5)M) reduced both spontaneous and stimulus-evoked release of [3H]noradrenaline. 4. The effect of several beta-blocking drugs was measured on the enhancing effect of yohimbine on the stimulation-evoked 3H efflux. The beta 1-adrenoceptor blocking drugs: atenolol (5 x 10(-6) and 10(-5) M), metoprolol (5 x 10(-6) and 10(-5) M), like the more selective bisoprolol (2 x 10(-6) and 4 x 10(-6) M) inhibited yohimbine activity. Likewise, propranolol (2 x 10(-6) and 5 x 10(-6)M) and the beta 2-adrenoceptor blocker ICI 118551 exhibited an antagonistic effect. 5. These results indicate the possibility for noradrenaline to activate presynaptic beta-adrenoceptors in rat portal vein. They show an interaction between the presynpatic alpha 2- and beta-adrenoceptor mediated systems in the release of noradrenaline. They suggest the presence and the activity of facilitatory beta 1-adrenoceptors.

Dysfunctional presynaptic alpha 2-adrenoceptors expose facilitatory beta 2-adrenoceptors in the vasculature of spontaneously hypertensive rats

Previous studies on spontaneously hypertensive rats (SHR) have yielded inconsistent information about functional aberrations of the presynaptic alpha 2- and beta 2-adrenoceptor-mediated modulation of sympathetic neurotransmitter release. In the present investigation we studied the capacity of presynaptic beta 2-adrenoceptors that enhance noradrenaline (NA) release in the portal vein of freely moving, unanesthetized SHR and normotensive Wistar rats (WR) using the beta 2-selective agonist fenoterol. The results show that the presynaptic beta 2-adrenoceptor population in SHR responds to significantly lower dosages of fenoterol than that in WR. The reason for this enhanced action, however, could not be attributed to the beta 2-adrenoceptor itself, nor to a diminished neuronal uptake of NA, but to a diminished responsiveness of the presynaptic alpha 2-adrenoceptor. Stimulation of presynaptic alpha 2-adrenoceptors with oxymetazoline (45 micrograms/min) decreased basal NA levels by 46% in WR and by 3% in SHR. Blockade of alpha 2-adrenoceptors, using 0.5 mg/kg yohimbine, induced a 4.86-fold rise in the basal NA level in WR but only a 1.89-fold rise in SHR. A subsequent dose of fenoterol, however, resulted in a further 2.5- and 2.6-fold rise in WR and SHR, respectively, indicating that there is a normal presynaptic beta 2-adrenoceptor population in the vasculature of SHR.