Adrenoceptors: A Focus on Psychiatric Disorders and Their Treatments

Research into the involvement of adrenoceptor subtypes in the cause(s) of psychiatric disorders is particularly challenging. This is partly because of difficulties in developing animal models that recapitulate the human condition but also because no evidence for any causal links has emerged from studies of patients. These, and other obstacles, are outlined in this chapter. Nevertheless, many drugs that are used to treat psychiatric disorders bind to adrenoceptors to some extent. Direct or indirect modulation of the function of specific adrenoceptor subtypes mediates all or part of the therapeutic actions of drugs in various psychiatric disorders. On the other hand, interactions with central or peripheral adrenoceptors can also explain their side effects. This chapter discusses both aspects of the field, focusing on disorders that are prevalent: depression, schizophrenia, anxiety, attention-deficit hyperactivity disorder, binge-eating disorder, and substance use disorder. In so doing, we highlight some unanswered questions that need to be resolved before it will be feasible to explain how changes in the function of any adrenoceptor subtype affect mood and behavior in humans and other animals.

Adrenoceptors in the Eye - Physiological and Pathophysiological Relevance

The autonomic nervous system plays a crucial role in the innervation of the eye. Consequently, it comes as no surprise that catecholamines and their corresponding receptors have been extensively studied and characterized in numerous ocular structures, including the cornea, conjunctiva, lacrimal gland, trabecular meshwork, uvea, and retina. These investigations have unveiled substantial clinical implications, particularly in the context of treating glaucoma, a progressive neurodegenerative disorder responsible for irreversible vision loss on a global scale. The primary therapeutic approaches for glaucoma frequently involve the modulation of α1-, α2-, and β-adrenoceptors, making them pivotal targets. In this chapter, we offer a comprehensive overview of the expression, distribution, and functional roles of adrenoceptors within various components of the eye and its associated structures. Additionally, we delve into the pivotal role of adrenoceptors in the pathophysiology of glaucoma. Furthermore, we provide a concise historical perspective on adrenoceptor research, examine the distinct contributions of individual adrenoceptor subtypes to the treatment of various ocular conditions, and propose potential future avenues of exploration in this field.

The Role of Adrenoceptors in the Retina

The retina is a part of the central nervous system, a thin multilayer with neuronal lamination, responsible for detecting, preprocessing, and sending visual information to the brain. Many retinal diseases are characterized by hemodynamic perturbations and neurodegeneration leading to vision loss and reduced quality of life. Since catecholamines and respective bindings sites have been characterized in the retina, we systematically reviewed the literature with regard to retinal expression, distribution and function of alpha₁ (α₁)-, alpha₂ (α₂)-, and beta (β)-adrenoceptors (ARs). Moreover, we discuss the role of the individual adrenoceptors as targets for the treatment of retinal diseases.

Patulin suppresses α1-adrenergic receptor expression in HEK293 cells

Patulin (PAT) is a common mycotoxin contaminant of apple products linked to impaired metabolic and kidney function. Adenosine monophosphate activated protein kinase (AMPK), abundantly expressed in the kidney, intercedes metabolic changes and renal injury. The alpha-1-adrenergic receptors (α₁-AR) facilitate Epinephrine (Epi)-mediated AMPK activation, linking metabolism and kidney function. Preliminary molecular docking experiments examined potential interactions and AMPK-gamma subunit 3 (PRKAG3). The effect of PAT exposure (0.2-2.5 µM; 24 h) on the AMPK pathway and α₁-AR was then investigated in HEK293 human kidney cells. AMPK agonist Epi determined direct effects on the α₁-AR, metformin was used as an activator for AMPK, while buthionine sulphoximine (BSO) and N-acetyl cysteine (NAC) assessed GSH inhibition and supplementation respectively. ADRA1A and ADRA1D expression was determined by qPCR. α₁-AR, ERK1/2/MAPK and PI3K/Akt protein expression was assessed using western blotting. PAT (1 µM) decreased α₁-AR protein and mRNA and altered downstream signalling. This was consistent in cells stimulated with Epi and metformin. BSO potentiated the observed effect on α₁-AR while NAC ameliorated these effects. Molecular docking studies performed on Human ADRA1A and PRKAG3 indicated direct interactions with PAT. This study is the first to show PAT modulates the AMPK pathway and α₁-AR, supporting a mechanism of kidney injury.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

alpha(1)-Adrenergic receptor subtype function in fetal and adult cerebral arteries

In the developing fetus, cerebral artery (CA) contractility demonstrates significant functional differences from that of the adult. This may be a consequence of differential activities of alpha(1)-adrenergic receptor (alpha(1)-AR) subtypes. Thus we tested the hypothesis that maturational differences in adrenergic-mediated CA contractility are, in part, a consequence of differential expression and/or activities of alpha(1)-AR subtypes. In CA from fetal ( approximately 140 days) and nonpregnant adult sheep, we used wire myography and imaging, with simultaneous measurement of tension and intracellular Ca(2+) concentration ([Ca(2+)](i)), radioimmunoassay, and Western immunoblots to examine phenylephrine (Phe)-induced contractile responses. The alpha(1A)-AR antagonists (5-MU and WB-4101) completely inhibited Phe-induced contraction in adult but not fetal CA; however, [Ca(2+)](i) increase was reduced significantly in both age groups. The alpha(1D)-AR antagonist (BMY-7378) blocked both Phe-induced contractions and Ca(2+) responses to a significantly greater extent in adult compared with fetal CA. In both age groups, inhibition of alpha(1A)-AR and alpha(1B)-AR, but not alpha(1D)-AR, significantly reduced inositol 1,4,5-trisphosphate responses to Phe. Western immunoblots demonstrated that the alpha(1)-AR subtype expression was only approximately 20% in fetal CA compared with the adult. Moreover, in fetal CA, the alpha(1D)-AR was expressed significantly greater than the other two subtypes. Also, in fetal but not adult CA, Phe induced a significant increase in activated ERK1/2; this increase in phosphorylated ERK was blocked by alpha(1B)-AR (CEC) and alpha(1D)-AR (BMY-7378) inhibitors, but not by alpha(1A)-AR inhibitors (5-MU or WB-4101). In conclusion, in the fetal CA, alpha(1B)-AR and alpha(1D)-AR subtypes play a key role in contractile response as well as in ERK activation. We speculate that in fetal CA alpha(1B)-AR and alpha(1D)-AR subtypes may be a critical factor associated with cerebrovascular growth and function.

Expression and function of G-protein-coupled receptorsin the male reproductive tract

This review focuses on the expression and function of muscarinic acetylcholine receptors (mAChRs), α1-adrenoceptors and relaxin receptors in the male reproductive tract. The localization and differential expression of mAChR and α1-adrenoceptor subtypes in specific compartments of the efferent ductules, epididymis, vas deferens, seminal vesicle and prostate of various species indicate a role for these receptors in the modulation of luminal fluid composition and smooth muscle contraction, including effects on male fertility. Furthermore, the activation of mAChRs induces transactivation of the epidermal growth factor receptor (EGFR) and the Sertoli cell proliferation. The relaxin receptors are present in the testis, RXFP1 in elongated spermatids and Sertoli cells from rat, and RXFP2 in Leydig and germ cells from rat and human, suggesting a role for these receptors in the spermatogenic process. The localization of both receptors in the apical portion of epithelial cells and smooth muscle layers of the vas deferens suggests an involvement of these receptors in the contraction and regulation of secretion.

Immunolocalization of alpha(1A)-adrenoceptors in rat and human epididymis

Immunohistochemistry was conducted to analyze the cellular localization of alpha(1A)-adrenoceptors along rat and human epididymis. ADR-A, a polyclonal antibody that recognizes the specific C-terminal region of alpha(1A)-adrenoceptors, immunostained this adrenoceptor subtype in smooth muscle cells surrounding the epididymal tubules and interstitial blood vessels and in subpopulations of epithelial cells from adult rat and human caput and cauda epididymidis. The same cell types from rat epididymidis were immunostained by ADR-1, a polyclonal antibody that recognizes a common region of the three alpha(1)-adrenoceptor subtypes, alpha(1A), alpha(1B), and alpha(1D). Immunostaining with both antibodies was also conducted in adult rat and human vas deferens and seminal vesicle used as positive controls because of the abundance of alpha(1A)-adrenoceptors in these tissues. ADR-A- and ADR-1-positive immunostaining was differentially distributed depending on the antibody, method of tissue fixation (Bouin-fixed and fresh frozen tissues), species (rat and human), tissue (caput and cauda epididymidis), and age (immature and adult rats) analyzed. This is the first report immunolocalizing alpha(1A)-adrenoceptor along rat and human epididymis. The presence of this adrenoceptor subtype in epididymal smooth muscle and epithelial cells indicates their contribution to smooth muscle contractile responses and a possible role in the absorptive and/or secretory activities of the epithelium lining the epididymal duct. Taken together, our results should contribute to a better understanding of the physiological role of alpha(1)-adrenoceptors in the epididymidis and the importance of the sympathetic nervous system for male (in)fertility.

Testosterone protects rat hearts against ischaemic insults by enhancing the effects of alpha(1)-adrenoceptor stimulation

BACKGROUND AND PURPOSE

Testosterone alleviates symptoms in patients with ischaemic heart disease. Androgen receptors are present in the heart, and testosterone upregulates gene expression of cardiac beta(1)-adrenoceptors. We hypothesize that testosterone may confer cardioprotection by interacting with adrenoceptors.

EXPERIMENTAL APPROACH

In isolated perfused hearts and ventricular myocytes from orchidectomized rats without or with testosterone (200 microg/100 g) replacement, we first determined the effect of ischaemia/reperfusion in the presence of noradrenaline (10(-7) M). Then we determined the contribution of interactions between testosterone and alpha(1)- or beta(1)-adrenoceptors in cardiac injury/protection (infarct size, release of lactate dehydrogenase, viability of myocytes, recovery of contractile function and incidence of arrhythmias) upon ischaemia/reperfusion by pharmacological manipulation using selective adrenoceptor agonists (alpha(1)-adrenoceptor agonist: phenylephrine 10(-6) M; non-selective beta-adrenoceptor agonist: isoprenaline 10(-7) M) and antagonists (alpha(1): prazosin or benoxathian 10(-6) M; beta(1): CGP 20712A 5 x 10(-7) M). We also determined the expression of alpha(1) and beta(1)-adrenoceptor in the hearts from rats with and without testosterone.

KEY RESULTS

Testosterone reduced injury induced by ischaemia/reperfusion and noradrenaline. This was achieved by enhancing the beneficial effect of alpha(1)-adrenoceptor stimulation, which was greater than the deleterious effect of beta(1)-adrenoceptor stimulation (also enhanced by testosterone). The effects of testosterone were abolished or attenuated by blockade of androgen receptors. Testosterone also enhanced the expression of alpha(1A) and beta(1)-adrenoceptor.

CONCLUSIONS AND IMPLICATIONS

Testosterone conferred cardioprotection by upregulating the cardiac alpha(1)-adrenoceptor and enhancing the effects of stimulation of this adrenoceptor. The effect of testosterone was at least partly mediated by androgen receptors.

Cellular and subcellular localization of alpha-1 adrenoceptors in the rat visual cortex

Noradrenaline is thought to play modulatory roles in a number of physiological, behavioral, and cellular processes. Although many of these modulatory effects are mediated through alpha-1 adrenoceptors, basic knowledge of the cellular and subcellular distributions of these receptors is limited. We investigated the laminar distribution pattern of alpha-1 adrenoceptors in rat visual cortex, using immunohistochemistry at both light and electron microscopic levels. Affinity-purified anti-alpha-1 antibody was confirmed to react only with a single band of about 70-80 kDa in total proteins prepared from rat visual cortex. Alpha-1 adrenoceptors were widely distributed though all cortical layers, but relatively high in density in layers I, II/III, and V. Immunoreactivity was observed in both neuronal perikarya and processes including apical dendrites. In double-labeling experiments with anti-microtubule-associated protein 2, anti-neurofilament, anti-glial fibrillary acidic protein, anti-glutamic acid decarboxylase 65/67, anti-2-3-cyclic nucleotide 3-phosphodiesterase, and anti-tyrosine hydroxylase antibodies, alpha-1 adrenoceptors were found mainly in dendrites and somata of microtubule-associated protein 2-immunopositive neurons. About 20% of alpha-1 adrenoceptors were in GABAergic neurons. A small number of alpha-1 adrenoceptors were also distributed in axons of excitatory neurons, astrocytes, oligodendrocytes and noradrenergic fibers. Using an immunoelectron microscopic technique, numerous regions of alpha-1 adrenoceptor immunoreactivity were found in cell somata, on membranes of dendrites, and in postsynaptic regions. Moreover, a small number of immunoreaction products were also detected in axons and presynaptic sites. These findings provide the first quantitative evidence regarding the cellular and subcellular localization of alpha-1 adrenoceptor immunoreactivity in visual cortex. Moreover, the ultrastructural distribution of alpha-1 adrenoceptor immunoreactivity suggests that alpha-1 adrenoceptors are transported mainly into dendrites and that they exert effects at postsynaptic sites of neurons.

Ovarian expression of alpha (1)- and beta (2)-adrenoceptors and p75 neurotrophin receptors in rats with steroid-induced polycystic ovaries

Polycystic ovary syndrome (PCOS) is the main cause of infertility in women. Despite extensive research aimed at identifying the pathogenetic mechanism underlying this condition, the aetiology of the disease is still unknown. Evidence from studies on women with PCOS and on an experimental rat polycystic ovary (PCO) model suggests that the sympathetic regulatory drive to the ovary may be unbalanced. The present study was designed to investigate this hypothesis. Accordingly, we used the well-defined rat PCO model, where PCO is induced by a single intramuscular (i.m.) injection of estradiol valerate (EV), and compared the model with oil-injected controls. We studied the ovarian expression of the alpha1- and beta2-adrenoceptors (ARs), the neurotrophin receptor p75 (p75NTR), and the sympathetic marker tyrosine hydroxylase (TH) at two time points: 30 and 60 days after EV injection. Our data demonstrate for the first time that all of the alpha1-AR subtypes are expressed in normal rat ovaries at both the mRNA and the protein levels. Furthermore, the expression of the alpha1-AR subtypes was differentially modulated in a time- and subtype-dependent manner in rats with EV-induced PCO. The ovaries in rats with steroid-induced PCO are characterised by an early overexpression of these molecules and p75NTR, while the beta2-AR was downregulated. An increase in the expression of ovarian TH after EV injection was also detected, suggesting a structural and functional remodelling of ovarian sympathetic innervation in PCO rats. Our evidence strongly indicates that the role of the sympathetic nervous system is crucial in the pathogenesis of EV-induced PCO. Overall, our findings suggest that therapeutical approaches aimed at down-regulating the sympathetic tone to the ovary could be useful in the prevention and clinical treatment of PCOS.

Neonatal blood pressure regulation

Hypertension is often viewed solely as a disease of the adult. However, early indicators of hypertension are frequently observed in young children and neonates. Having an adequate appreciation of the normal range of infant blood pressure is critical for the appropriate management of the conditions associated with elevated or abnormally low blood pressure. In healthy neonates, systolic blood pressure increases rapidly during the first 6 weeks of life with the most rapid rise observed during the first 5 days. A similar pattern is observed for diastolic pressures. The observed increases in blood pressure are positively correlated with birth weight and both gestational and postnatal age. The incidence of hypertension in the neonate has been reported to range from 0.2% to 2.6% and is frequently an indicator of other renal or cardiovascular abnormalities. Systemic hypotension is reported in 24% to 45% of very low birth weight infants and is frequently caused by hypovolemia. The regulation of blood pressure is complex and the mechanisms involved remain to be fully elucidated. The results of several investigations into the molecular mechanism(s) of hypertension are considered.

Allosteric alpha 1-adrenoreceptor antagonism by the conopeptide rho-TIA

A peptide contained in the venom of the predatory marine snail Conus tulipa, rho-TIA, has previously been shown to possess alpha1-adrenoreceptor antagonist activity. Here, we further characterize its pharmacological activity as well as its structure-activity relationships. In the isolated rat vas deferens, rho-TIA inhibited alpha1-adrenoreceptor-mediated increases in cytosolic Ca2+ concentration that were triggered by norepinephrine, but did not affect presynaptic alpha2-adrenoreceptor-mediated responses. In radioligand binding assays using [125I]HEAT, rho-TIA displayed slightly greater potency at the alpha 1B than at the alpha 1A or alpha 1D subtypes. Moreover, although it did not affect the rate of association for [3H]prazosin binding to the alpha 1B-adrenoreceptor, the dissociation rate was increased, indicating non-competitive antagonism by rho-TIA. N-terminally truncated analogs of rho-TIA were less active than the full-length peptide, with a large decline in activity observed upon removal of the fourth residue of rho-TIA (Arg4). An alanine walk of rho-TIA confirmed the importance of Arg4 for activity and revealed a number of other residues clustered around Arg4 that contribute to the potency of rho-TIA. The unique allosteric antagonism of rho-TIA resulting from its interaction with receptor residues that constitute a binding site that is distinct from that of the classical competitive alpha1-adrenoreceptor antagonists may allow the development of inhibitors that are highly subtype selective.

Alpha 1-adrenergic receptor responses in alpha 1AB-AR knockout mouse hearts suggest the presence of alpha 1D-AR

Two functional alpha(1)-adrenergic receptor (AR) subtypes (alpha(1A) and alpha(1B)) have been identified in the mouse heart. However, it is unclear whether the third known subtype, alpha(1D)-AR, is also present. To investigate this, we determined whether there were alpha(1)-AR responses in hearts from a novel mouse model lacking alpha(1A)- and alpha(1B)-ARs (double knockout) (ABKO). In Langendorff-perfused hearts, alpha(1)-ARs were stimulated with phenylephrine. For ABKO hearts, phenylephrine reduced left ventricular pressure and coronary flow (to 87 +/- 2% and 86 +/- 4% of initial, respectively, n = 11, P < 0.01). These effects were blocked by prazosin and 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-8-azaspirol[4,5]decane-7,9-dione] dihydrochloride, suggesting that alpha(1D)-AR-mediated responses were present. In contrast, right ventricular trabeculae from ABKO hearts did not respond to phenylephrine, suggesting that in ABKO perfused hearts, the effects of phenylephrine were not mediated by direct actions on cardiomyocytes. A novel finding was that alpha(1)-AR stimulation caused positive inotropy in the wild-type mouse heart, in contrast to negative inotropy observed in mouse cardiac muscle strips. We conclude that mouse hearts lacking alpha(1A)- and alpha(1B)-ARs retain functional alpha(1)-AR responses involving decreases of coronary flow and ventricular pressure that reflect alpha(1D)-AR-mediated vasoconstriction. Furthermore, alpha(1)-AR inotropic responses depend critically on the experimental conditions.

[11C]RN5: a new agent for the in vivo imaging of myocardial alpha1-adrenoceptors

The radiolabelling with the positron-emitter Carbon-11 and the biological evaluation in rats of 3-[2-[4-(2-[11C]methoxyphenyl)piperazin-1-yl]ethyl]pyrimido[5,4-b]indole-2,4-dione ([11C]RN5), alpha1-adrenoceptor antagonist (K(i)=0.21 nM), as a putative radioligand for the non-invasive assessment of alpha1-adrenoceptors with positron emission tomography (PET) is reported. The radiosynthesis procedure consisted of O-methylation of des-methyl precursor with [11C]methyl iodide in the presence of potassium hydroxide in dimethylformamide (DMF) at 80 degrees C. [11C]RN5 was obtained in >99% radiochemical purity in 25 min with a radiochemical yield in the 20-30% range, end of synthesis (EOS) (non-decay corrected) and a specific radioactivity of 92.5+/-18.5 GBq/micromol. Pre-clinical studies in rats showed a high uptake of [11C]RN5 in heart, spleen, adrenal gland, lung and kidney but not in the brain. Inhibition studies with high doses of different adrenergic antagonists indicate that more than 70% of myocardial uptake of [11C]RN5 is due to specific binding to alpha1-adrenoceptors. Our results indicate that [11C]RN5 is suitable to be further developed as a potential radioligand for the in vivo PET imaging of myocardial alpha1-adrenoceptors in humans.

Developmental regulation of alpha 1A-adrenoceptor function in rat brain oligodendrocyte cultures

In this study, we examined the effect of norepinephrine (NE) on phosphatidylinositol-4,5-bisphosphate (PI) hydrolysis in progenitors and differentiated oligodendrocytes. NE caused a time- and concentration-dependent increase in total inositol phosphate (IP(t)) formation. The magnitude of this response increased as oligodendrocytes matured and was accompanied with an increase in alpha(1)-adrenoceptor (alpha(1)-AR) levels. To pharmacologically characterize the alpha(1)-AR subtype mediating PI hydrolysis in 12-day differentiated oligodendrocytes, various selective antagonists were used. Prazosin, the non-selective 1-AR antagonist, blocked NE-mediated IP(t) formation. Similarly, the alpha(1A)-AR selective competitive antagonists, 5-methyl urapidil (5-MU) and WB4104, were potent blockers of NE-mediated IP(t) formation. In contrast, the alpha(1B)- and alpha(1D)-AR antagonist, chloroethylclonidine and the alpha(1D)-AR antagonist, BMY 7378, had no effect. These results suggest that NE-induced PI hydrolysis in differentiated oligodendrocytes was mediated through the alpha(1A)-AR. Furthermore, this response was prevented by EGTA and CdCl(2), suggesting a requirement for extracellular calcium. The presence of alpha(1)-AR subtypes in oligodendrocytes was confirmed by reverse transcriptase coupled polymerase chain reaction and by immunoprecipitation, with subtype specific antibodies. The results indicated that mRNA and protein for the alpha(1A)-, alpha(1B)- and alpha(1D)-AR subtypes were expressed. In conclusion, our findings show that oligodendrocytes express all three alpha(1)-AR subtypes but that only the alpha(1A)-AR was involved in NE-mediated IP(t) formation.

Alpha1-adrenoceptor subtypes in rat epididymis and the effects of sexual maturation

We have characterized the expression of alpha1-adrenoceptor in epididymis from rats in different stages of sexual maturation: 40 (immature), 60 (young adult), and 120 (adult) days of age. Plasma testosterone levels were low in the immature animals but increased significantly in the 60- and 120-day-old animals. These changes were followed by a progressive increase in rat body weight and in caput and cauda epididymis relative weight. Reverse transcription polymerase chain reaction assay indicated that alpha1a-, alpha1b-, and alpha1d-adrenoceptor transcripts were present in both caput and cauda epididymis from adult rats. Ribonuclease protection assays further indicated that the expression of these alpha1-adrenoceptor transcripts differed with age and epididymal region analyzed. Prazosin (nonselective alpha1 antagonist), 5-methyl urapidil (alpha1A-selective), and BMY 7378 (alpha1D-selective) displaced [3H]prazosin binding curves in caput and cauda epididymis from 40- and 120-day-old rats. The potency order for these antagonists, as calculated from the negative logarithm of the inhibition constant (pK(i)) values for the high-affinity sites, indicated a predominant population of alpha1A-adrenoceptor subtype in caput and cauda epididymis from adult animals. Differences in pK(i) values in caput and cauda epididymis from immature and adult animals also suggested that the relative amount of alpha1-adrenoceptors, at the protein level, is modulated by sexual maturation. Taken together, the changes in alpha1-adrenoceptor expression during sexual maturation may suggest specific roles for these receptors in epididymal function.