The imidazoline receptor in control of blood pressure by clonidine and allied drugs

Agmatine modulates spontaneous activity in neurons of the rat medial habenular complex-a relevant mechanism in the pathophysiology and treatment of depression?

The dorsal diencephalic conduction system connects limbic forebrain structures to monaminergic mesencephalic nuclei via a distinct relay station, the habenular complexes. Both habenular nuclei, the lateral as well as the medial nucleus, are considered to play a prominent role in mental disorders like major depression. Herein, we investigate the effect of the polyamine agmatine on the electrical activity of neurons within the medial habenula in rat. We present evidence that agmatine strongly decreases spontaneous action potential firing of medial habenular neurons by activating I1-type imidazoline receptors. Additionally, we compare the expression patterns of agmatinase, an enzyme capable of inactivating agmatine, in rat and human habenula. In the medial habenula of both species, agmatinase is similarly distributed and observed in neurons and, in particular, in distinct neuropil areas. The putative relevance of these findings in the context of depression is discussed. It is concluded that increased activity of the agmatinergic system in the medial habenula may strengthen midbrain dopaminergic activity. Consequently, the habenular-interpeduncular axis may be dysregulated in patients with major depression.

Noradrenergic modulation of risk/reward decision making

RATIONALE

Catecholamine transmission modulates numerous cognitive and reward-related processes that can subserve more complex functions such as cost/benefit decision making. Dopamine has been shown to play an integral role in decisions involving reward uncertainty, yet there is a paucity of research investigating the contributions of noradrenaline (NA) transmission to these functions.

OBJECTIVES

The present study was designed to elucidate the contribution of NA to risk/reward decision making in rats, assessed with a probabilistic discounting task.

METHODS

We examined the effects of reducing noradrenergic transmission with the α2 agonist clonidine (10-100 μg/kg), and increasing activity at α2A receptor sites with the agonist guanfacine (0.1-1 mg/kg), the α2 antagonist yohimbine (1-3 mg/kg), and the noradrenaline transporter (NET) inhibitor atomoxetine (0.3-3 mg/kg) on probabilistic discounting. Rats chose between a small/certain reward and a larger/risky reward, wherein the probability of obtaining the larger reward either decreased (100-12.5 %) or increased (12.5-100 %) over a session.

RESULTS

In well-trained rats, clonidine reduced risky choice by decreasing reward sensitivity, whereas guanfacine did not affect choice behavior. Yohimbine impaired adjustments in decision biases as reward probability changed within a session by altering negative feedback sensitivity. In a subset of rats that displayed prominent discounting of probabilistic rewards, the lowest dose of atomoxetine increased preference for the large/risky reward when this option had greater long-term utility.

CONCLUSIONS

These data highlight an important and previously uncharacterized role for noradrenergic transmission in mediating different aspects of risk/reward decision making and mediating reward and negative feedback sensitivity.

Brain stem adenosine receptors modulate centrally mediated hypotensive responses in conscious rats: A review

Adenosine is implicated in the modulation of cardiovascular responses either at the peripheral or at central level in experimental animals. However, there are no dedicated reviews on the involvement of adenosine in mediating the hypotensive response of centrally administered clonidine in general and specifically in aortically barodenervated rats (ABD). The conscious ABD rat model exhibits surgically induced baroreflex dysfunction and exaggerated hypotensive response, compared with conscious sham-operated (SO) rats. The current review focuses on, the role of adenosine receptors in blood pressure (BP) regulation and their possible crosstalk with other receptors e.g. imidazoline (I₁) and alpha (α_2A) adrenergic receptor (AR). The former receptor is a molecular target for clonidine, whose hypotensive effect is enhanced approx. 3-fold in conscious ABD rats. We also discussed how the balance between the brain stem adenosine A₁ and A_2A receptors is regulated by baroreceptors and how such balance influences the centrally mediated hypotensive responses. The use of the ABD rat model yielded insight into the downstream signaling cascades following clonidine-evoked hypotension in a surgical model of baroreflex dysfunction.

Agmatine : metabolic pathway and spectrum of activity in brain

Agmatine is an endogenous neuromodulator that, based on animal studies, has the potential for new drug development. As an endogenous aminoguanidine compound (1-amino-4-guanidinobutane), it is structurally unique compared with other monoamines. Agmatine was long thought to be synthesised only in lower life forms, until its biosynthetic pathway (decarboxylation of arginine) was described in the mammalian brain in 1994. Human arginine decarboxylase has been cloned and shown to have 48% identity to ornithine decarboxylase. In neurons of the brain and spinal cord, agmatine is packaged into synaptic vesicles and released upon neuronal depolarisation. Other evidence of a neuromodulation role for agmatine is the presence of a specific cellular uptake mechanism and a specific metabolic enzyme (agmatinase; which forms putrescine).Initially, agmatine was conceptualised as an endogenous clonidine-displacing substance of imidazoline receptors; however, it has now been established to have affinity for several transmembrane receptors, such as alpha(2)-adrenergic, imidazoline I(1) and glutamatergic NMDA receptors. In addition to activity at these receptors, agmatine irreversibly inhibits neuronal nitric oxide synthase and downregulates inducible nitric oxide synthase. Endogenous agmatine is induced in response to stress and/or inflammation. Stressful conditions that induce agmatine include hypoxic-ischaemia and cold-restraint stress of ulcerogenic proportion. Induction of agmatine in the brain seems to occur in astrocytes, although neurons also synthesise agmatine. The effects of injected agmatine in animals include anticonvulsant-, antineurotoxic- and antidepressant-like actions. Intraperitoneal or intracerebroventricular injections of agmatine rapidly elicit antidepressant-like behavioural changes in the rodent forced swim test and tail suspension test. Intraperitoneal injections of agmatine into rats and mice also elicit acute anxiolytic-like behavioural changes in the elevated plus-maze stress test. In an animal model of acute stress disorder, intraperitoneal agmatine injections diminish contextual fear learning. Furthermore, intraperitoneal injections of agmatine reduce alcohol and opioid dependence by diminishing behaviour in a rat conditioned place preference paradigm. Based on these findings, agmatine appears to be an endogenous neuromodulator of mental stress. The possible roles and/or beneficial effects of agmatine in stress-related disorders, such as depression, anxiety and post-traumatic stress disorder, merit further investigation.

TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo [4,5-g]isoquinoline): discovery, pharmacological effects, and therapeutic potential

Chemically, TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline) can be viewed as a conformationally restricted phenylalkylamine that is related in structure to amphetamine but does not stimulate (or depress) locomotor activity in rodents. In radioligand binding studies TDIQ displays selective affinity for alpha(2)-adrenergic receptor subsites (i.e., alpha(2A)-, alpha(2B)-, and alpha(2C)-adrenergic receptors), and behavioral data suggest that it might exert an agonist (or partial agonist) effect at alpha(2)-adrenergic receptors or interact at alpha(2)-adrenergic heteroreceptors. Drug discrimination studies in rats indicate that TDIQ: (1) serves as a discriminative stimulus, (2) may be useful in the treatment of symptoms associated with the abuse of cocaine, and (3) exhibits a low potential for abuse. In addition, TDIQ exhibits a dose-dependent and wide dissociation between doses that produce an anxiolytic-like effect or an inhibition of "snack" consumption in mice and doses that produce minimal, if any, effects in tests that measure a potential for disruption of coordinated movement or motor activity. Also, TDIQ displays negligible effects on the heart rate (HR) and blood pressure (BP) of mice. Taken together, the preclinical data suggest that TDIQ exhibits a favorable ratio of therapeutic-like effects (anxiolytic, therapeutic adjunct in the treatment of cocaine abuse, and appetite suppression) to side effect-like activities (behavioral impairment, drug abuse, or adverse cardiovascular effect). As such, TDIQ could: (1) be a forerunner for a new type of chemical entity in the treatment of certain forms of anxiety and/or obesity and (2) serve as a structural template in the discovery and development of additional agents that might be selective for alpha(2)-adrenergic receptors.

TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline) exhibits anxiolytic-like activity in a marble-burying assay in mice

Numerous studies have suggested that the central alpha2-adrenergic receptor system may exert an important role in some types of human anxiety. The anxiolytic-like activity and potential side effect-like activities of the novel and purported alpha2-adrenergic compound TDIQ (5,6,7,8-1,3-dioxolo[4,5-g]isoquinoline) were compared to those of the anxiolytic drugs diazepam and buspirone, and the nonselective alpha2-adrenergic agent clonidine. Anxiolytic-like behavior was assessed in an object (marble)-burying assay, a selective test for the evaluation of known anxiolytics and identification of putative antianxiety compounds, that used mice housed either alone or in groups (5/cage). The rodents' antianxiety-like effect was defined as dose-related increases in the number of marbles that remained uncovered in their bedding material without concomitant disruption of their motor activities. Rotarod and inclined screen procedures were employed as potential indicators of side effects. An additional test monitored the heart rate (HR) and blood pressure (BP) of mice after the intravenous (IV) administration of doses of TDIQ. The reference compounds inhibited marble-burying behavior in a dose-related manner and produced various degrees of impairment in the side effect tests. TDIQ also inhibited object burying and displayed a wide separation between doses that produced anxiolytic-like activity and doses that produced some, if any, disruption of coordinated movement and/or motor activity. Moreover, the IV administration of TDIQ, up to 10 mg/kg, produced negligible effects on the HR and BP of mice. TDIQ could be a lead candidate for a new type of structural compound in the treatment of certain forms of anxiety.

Physiological significance of alpha(2)-adrenergic receptor subtype diversity: one receptor is not enough

Alpha(2)-adrenergic receptors mediate part of the diverse biological effects of the endogenous catecholamines epinephrine and norepinephrine. Three distinct subtypes of alpha(2)-adrenergic receptors, alpha(2A), alpha(2B), alpha(2C), have been identified from multiple species. Because of the lack of sufficiently subtype-selective ligands, the specific biological functions of these receptor subtypes were largely unknown until recently. Gene-targeted mice carrying deletions in the genes encoding for individual alpha(2)-receptor subtypes have added important new insight into the physiological significance of adrenergic receptor diversity. Two different strategies have emerged to regulate adrenergic signal transduction. Some biological functions are controlled by two counteracting alpha(2)-receptor subtypes, e.g., alpha(2A)-receptors decrease sympathetic outflow and blood pressure, whereas the alpha(2B)-subtype increases blood pressure. Other biological functions are regulated by synergistic alpha(2)-receptor subtypes. The inhibitory presynaptic feedback loop that tightly regulates neurotransmitter release from adrenergic nerves also requires two receptor subtypes, alpha(2A) and alpha(2C). Similarly, nociception is controlled at several levels by one of the three alpha(2)-receptor subtypes. Further investigation of the specific function of alpha(2)-subtypes will greatly enhance our understanding of the relevance of closely related receptor proteins and point out novel therapeutic strategies for subtype-selective drug development.

Hypotensive and sedative effects of clonidine injected into the rostral ventrolateral medulla of conscious rats

We examined the effects of clonidine injected unilaterally into the rostral ventrolateral medulla (RVLM) of conscious, unrestrained rats. We also examined whether the local alpha(2)-adrenoceptor mechanism contributed to the action of clonidine injected into the RVLM. Injection of clonidine but not vehicle solution significantly decreased the mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) in conscious, unrestrained rats as well as in propofol-anesthetized rats. The frequency of natural behavior was significantly lower after clonidine injection than after vehicle injection. The depressor and sympathoinhibitory responses were significantly larger in the propofol-anesthetized rats than in the conscious rats. Coinjection of a selective alpha(2)-adrenoceptor antagonist, 2-methoxyidazoxan, with clonidine into the RVLM significantly attenuated the depressor, bradycardiac, sympathoinhibitory, and sedative effects of clonidine injected alone. In conclusion, clonidine injected into the RVLM decreased MAP, HR, and RSNA and caused sedation in conscious, unrestrained rats. The action of clonidine in the RVLM was at least partly mediated by alpha(2)-adrenoceptor mechanisms.