5-HT1D-like receptors inhibit the release of endogenously formed [3H]GABA in human, but not in rabbit, neocortex

5-HT1B receptors in the basolateral amygdaloid nucleus regulate anxiety-like behaviors through AC-PKA signal pathway in a rat model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is commonly accompanied with anxiety, multiple studies indicate that the basolateral amygdaloid nucleus (BLA) is closely related to modulation of anxiety and expresses serotonin_1B (5-HT_1B) receptors, however, effects of BLA 5-HT_1B receptors on anxiety-like behaviors are unclear, particularly in PD-related anxiety.

METHODS

The open-field and elevated plus maze tests were used to examine anxiety-like behaviors. In vivo electrophysiology and microdialysis were performed to observe the firing activity of BLA neurons and GABA, glutamate, dopamine (DA) and 5-HT release in the BLA, respectively. Western blotting was used to analyze protein expression of 5-HT_1B receptors, adenylate cyclase (AC) and phosphorylated protein kinase A at threonine 197 site (p-PKA-Thr197) in the BLA.

RESULTS

Intra-BLA injection of 5-HT_1B receptor agonist CP93129 produced anxiety-like effects and antagonist SB216641 induced anxiolytic-like responses in sham-operated and 6-hydroxydopamine-lesioned rats. Further, pretreatment with AC inhibitor SQ22536 and PKA inhibitor KT5720 blocked the behavioral effects of CP93129, respectively. Intra-BLA injection of CP93129 increased the firing rate of BLA glutamate neurons and decreased GABA/glutamate ratio and DA and 5-HT levels in the BLA of sham-operated and the lesioned rats, while SB216641 induced the opposite effects. Compared with sham-operated rats, effects of CP93129 and SB216641 on behaviors, electrophysiology and microdialysis were decreased in the lesioned rats, which were associated with decreased expression of 5-HT_1B receptors, AC and p-PKA-Thr197 in the BLA.

CONCLUSION

These findings suggest that 5-HT_1B receptor-AC-PKA signal pathway in the BLA is involved in the regulation of PD-related anxiety.

The 5-HT1B receptor - a potential target for antidepressant treatment

Major depressive disorder (MDD) is the leading cause of disability worldwide. The serotonin hypothesis may be the model of MDD pathophysiology with the most support. The majority of antidepressants enhance synaptic serotonin levels quickly, while it usually takes weeks to discern MDD treatment effect. It has been hypothesized that the time lag between serotonin increase and reduction of MDD symptoms is due to downregulation of inhibitory receptors such as the serotonin 1B receptor (5-HT1BR). The research on 5-HT1BR has previously been hampered by a lack of selective ligands for the receptor. The last extensive review of 5-HT1BR in the pathophysiology of depression was published 2009, and based mainly on findings from animal studies. Since then, selective radioligands for in vivo quantification of brain 5-HT1BR binding with positron emission tomography has been developed, providing new knowledge on the role of 5-HT1BR in MDD and its treatment. The main focus of this review is the role of 5-HT1BR in relation to MDD and its treatment, although studies of 5-HT1BR in obsessive-compulsive disorder, alcohol dependence, and cocaine dependence are also reviewed. The evidence outlined range from animal models of disease, effects of 5-HT1B receptor agonists and antagonists, case-control studies of 5-HT1B receptor binding postmortem and in vivo, with positron emission tomography, to clinical studies of 5-HT1B receptor effects of established treatments for MDD. Low 5-HT1BR binding in limbic regions has been found in MDD patients. When 5-HT1BR ligands are administered to animals, 5-HT1BR agonists most consistently display antidepressant-like properties, though it is not yet clear how 5-HT1BR is best approached for optimal MDD treatment.

Human presynaptic receptors

Presynaptic receptors are sites at which transmitters, locally formed mediators or hormones inhibit or facilitate the release of a given transmitter from its axon terminals. The interest in the identification of presynaptic receptors has faded in recent years and it may therefore be justified to give an overview of their occurrence in the autonomic and central nervous system; this review will focus on presynaptic receptors in human tissues. Autoreceptors are presynaptic receptors at which a given transmitter restrains its further release, though in some instances may also increase its release. Inhibitory autoreceptors represent a typical example of a negative feedback; they are tonically activated by the respective endogenous transmitter and/or are constitutively active. Autoreceptors also play a role under pathophysiological conditions, e.g. by limiting the massive noradrenaline release occurring during congestive heart failure. They can be used for therapeutic purposes; e.g., the α₂-adrenoceptor antagonist mirtazapine is used as an antidepressant and the inverse histamine H₃ receptor agonist pitolisant has been marketed as a new drug for the treatment of narcolepsy in 2016. Heteroreceptors are presynaptic receptors at which transmitters from adjacent neurons, locally formed mediators (e.g. endocannabinoids) or hormones (e.g. adrenaline) can inhibit or facilitate transmitter release; they may be subject to an endogenous tone. The constipating effect of the sympathetic nervous system or of the antihypertensive drug clonidine is related to the activation of inhibitory α₂-adrenoceptors on postganglionic parasympathetic neurons. Part of the stimulating effect of adrenaline on the sympathetic nervous system during stress is related to its facilitatory effect on noradrenaline release via β₂-adrenoceptors.

New advances in the treatment of generalized anxiety disorder: the multimodal antidepressant vortioxetine

Generalized Anxiety Disorder (GAD) is a persistent condition characterized by chronic anxiety, exaggerated worry and tension, mainly comorbid with Major Depressive Disorder (MDD). Currently, selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors are recommended as first-line treatment of GAD. However, some patients may not respond to the treatment or discontinue due to adverse effects. Vortioxetine (VRX) is a multimodal antidepressant with a unique mechanism of action, by acting as 5-HT3A, 5-HT1D and 5-HT7 receptor antagonist, partial agonist at the 5-HT1A and 5-HT1B receptors and inhibitor at the 5-HT transporter. Preliminary clinical trials showed contrasting findings in terms of improvement of the anxiety symptomatology and/or cognitive impairment. Here, we aim to systematically review the evidence currently available on the efficacy, safety and tolerability of VRX in the treatment of GAD. The generalizability of results on the efficacy of VRX in patients with anxiety symptomatology and GAD is limited due to few and contrasting RCTs so far available. Only two studies, of which one prevention relapse trial, reported a significant improvement in anxiety symptomatology compared to three with negative findings.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

Serotonin modulation of cell excitability and of [3H]GABA and [3H]D-aspartate efflux in primary cultures of rat cortical neurons

The effects of 5-hydroxytryptamine (5-HT) on neuronal excitability, evaluated as depolarization-induced firing rate, and on amino acid release, measured as electrically-evoked [(3)H]GABA and [(3)H]d-aspartate efflux, were investigated in rat primary cortical neuronal cultures. 5-HT displayed a concentration-dependent, bimodal effect on neuronal excitability: at 3-10microM it increased excitability through 5-HT(2A) receptors, and was blocked by the selective 5-HT(2A) antagonist MDL 100907, whereas at 30-100microM it reduced excitability through 5-HT(1A) receptors, and was, in turn, blocked by the selective 5-HT(1A) antagonist WAY 100135. The electrically-evoked [(3)H]GABA efflux was concentration-dependently inhibited by 5-HT (pEC(50)=4.74) and such inhibition was prevented by WAY 100135, but not by GR 55562, a selective 5-HT(1D/B) receptor antagonist. Conversely, 5-HT concentration-dependently increased stimulus-evoked [(3)H]d-aspartate efflux (pEC(50)=4.71). The increase was facilitated by methiothepin and was reversed into inhibition by ICS 205930, a selective 5-HT(3) receptor antagonist. In the presence of ICS 205930, the inhibition induced by 5-HT was prevented by the selective 5-HT(1D/B) receptor antagonist GR 55562, but not by WAY 100135. These findings suggest that 5-HT inhibits GABA release through 5-HT(1A) receptors and exerts a dual modulation on glutamate release, mostly facilitatory (through 5-HT(3) receptors) but also inhibitory (through 5-HT(1D/B) receptors), leading to a prevalently positive modulation of the excitatory signal by amino acid neurotransmitter containing neurons.

Neuronal electrical high frequency stimulation enhances GABA outflow from human neocortical slices

Electrical high frequency stimulation of the globus pallidus internus or the subthalamic nucleus has beneficial motor effects in advanced Parkinson's disease. The mechanisms underlying these clinical results remain, however, unclear. From previous studies it is proposed that the gamma-aminobutyric acid (GABA) system is involved in the effectiveness of electrical high frequency stimulation. In these experiments, human neocortical slices were stimulated electrically (130 Hz) in vitro, and GABA outflow was measured after o-phthaldialdehyde sulphite derivatization using HPLC with electrochemical detection. Our results could demonstrate that high frequency stimulation (HFS) significantly increased basal GABA outflow in the presence of submaximal concentrations of the voltage-gated sodium channel opener veratridine. This effect could be abolished by the GABA antagonists bicuculline or picrotoxin. These results suggest that HFS has an activating effect on GABAergic neuronal terminals in human neocortical slices, depending on sodium and chloride influx. Since GABA plays a role in CNS disorders of basal ganglia, anxiety and epilepsy, its neocortical modulation by HFS may be (patho)physiologically relevant.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

High potassium-induced activation of choline-acetyltransferase in human neocortex: implications and species differences

The role of electrical and potassium (K(+))-induced depolarisation on choline-acetyltransferase (ChAT) activity in human and mouse neocortical slices was studied. When [3H]-ACh release was evoked by two K(+) stimulations in human neocortex, the mean S(2)/S(1) ratio was significantly below unity. ChAT inhibitors, like bromo-acetylcholine and ocadaic acid, raised this ratio by 79 and 63%, respectively, suggesting that the diminished S(2)/S(1) value in the absence of ChAT inhibitors reflected an increased ChAT activity at S(2) following K(+) depolarisation at S(1). When stimulated electrically, however, the S(2)/S(1) ratio in human neocortex was near unity and ocadaic acid remained without effect. In parallel experiments on mouse neocortical slices, the S(2)/S(1) ratio was near unity in both electrically or K(+)-evoked [3H]-ACh release and was not altered by ChAT inhibition. ChAT activity following K(+) depolarisation was also determined directly. ChAT activation in human neocortical slices was highest at 10 and 20mM K(+). ChAT activity in mouse neocortical tissue was not altered by K(+) depolarisation. These results suggest that in human, but not in mouse, neocortex ChAT activity may be increased due to ongoing K(+) depolarisation. This increase of ChAT activity supports a cholinergic degeneration hypothesis which has been entitled "autocannibalism" by Wurtman [TINS 15 (1992) 177].

Serotonin modifies the neuronal inhibitory responses to gamma-aminobutyric acid in the red nucleus: a microiontophoretic study in the rat

The effects of 5-hydroxytryptamine (5-HT) on the inhibitory responses evoked by gamma-aminobutyric acid (GABA) in neurons of the red nucleus (RN) were studied using a microiontophoretic technique. Extracellular unitary recordings performed in anesthetized rats demonstrated that 5-HT ejection influenced GABA-evoked inhibition in 94% of RN neurons, enhancing them in 52% and depressing them in 46% of cases. Both effects were specific and dose-dependent,although enhancements or depressions of the GABA responses were respectively inversely and directly related to the doses of 5-HT applied. The type of modulation exerted by 5-HT on the GABA responses was independent of the action of the amine on background firing. In fact, 5-HT induced an enhancement of the GABA responses in neurons mostly located in the rostral RN and a depression in those in the caudal RN. The application of 8-hydroxy-2(di-n-propylamino)tetralin, a specific 5-HT(1A) receptor agonist, enhanced GABA responses, whereas alpha-methyl-5-hydroxytryptamine, a 5-HT(2A) receptor agonist, depressed them. Both the 5-HT(2) antagonist methysergide and the 5-HT(2A) selective antagonist ketanserin were able to block partially or totally the depressive action of 5-HT on GABA responses. In contrast, the same 5-HT antagonists mimicked the enhancing action of 5-HT on the GABA responses or were ineffective. Application of bicuculline, a GABA(A) receptor antagonist, enhanced the excitatory action of 5-HT on the background firing and slightly reduced the inhibitory action. It is concluded that 5-HT is able to modulate GABA-evoked responses in RN neurons by acting on both 5-HT(1A) and 5-HT(2A) receptors. The functional significance of a serotonergic control on GABAergic inhibitory effects in RN is discussed.

The possible role of 5-HT(1B/D) receptors in psychiatric disorders and their potential as a target for therapy

Serotonin (5-hydroxytryptamine, 5-HT) is implicated in several psychiatric diseases. Is this also true for 5-HT(1B/D) receptors? These receptors are found in high density in substantia nigra, globus pallidus, striatum and basal ganglia and in other brain regions. This ubiquity makes 5-HT(1B/D) receptors responsible for many physiological and behavioural functions. This review focuses on the role of 5-HT(1B) receptors in the regulation of 5-HT release and synthesis. Microdialysis experiments performed on freely moving animals are an interesting in vivo model to study the function of the terminal 5-HT(1B) autoreceptor. Synthesis of 5-HT, estimated by the measurement of the accumulation of 5-hydroxytryptophan (5-HTP) ex vivo or in vitro, is modulated by the 5-HT(1B) autoreceptors. Many reports have shown that chronic administration with selective serotonin reuptake inhibitors leads to the desensitisation of the terminal 5-HT(1B) autoreceptors. With the help of some animal models of depression and anxiety and with some data from clinical studies it has been hypothesised that 5-HT(1B) receptors may be supersensitive in depression, anxiety and obsessive compulsive disorder. Thus, since the dysfunction of 5-HT(1B) receptors may be involved in some pathological states, particularly in the psychiatric field, these receptors represent important potential targets for drugs to treat mental diseases.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Acute stress induces a differential increase of 5-HT-moduline (LSAL) tissue content in various rat brain areas

5-HT-moduline is an endogenous cerebral tetrapeptide (LSAL) which specifically interacts as an allosteric modulator with 5-HT1B receptors controlling serotonergic activity [O. Massot, J.C. Rousselle, M.P. Fillion, B. Grimaldi, I. Cloez-Tayarani, A. Fugelli, N. Prudhomme, L. Seguin, B. Rousseau, M. Plantefol, R. Hen, G. Fillion, 5-Hydroxytryptamine-moduline, a new endogenous cerebral peptide, controls the serotonergic activity via its specific interaction with 5-hydroxytryptamine1B/1D receptors, Mol. Pharmacol. 50 (1996) 752-762; J.C. Rousselle, O. Massot, M. Delepierre, E. Zifa, G. Fillion, Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic1B receptor subtypes, J. Biol. Chem. 271 (1996) 726-735; J.C. Rousselle, M. Plantefol, M.P. Fillion, O. Massot, P.J. Pauwels, G. Fillion, Specific interaction of 5-HT-moduline with human 5-HT1b as well as 5-HT1d receptors expressed in transfected cultured cells, Naunyn-Schmiedeberg's Arch. Pharmacol. 358 (1998) 279-286]. Cerebral tissue contents of 5-HT-moduline were determined in various rat brain areas after an acute restraint stress, and after repetition of this stress, to examine whether or not mechanisms involving this peptide could be affected by stress situations. The measurement of the peptide was carried out using specific polyclonal antibodies [B. Grimaldi, M.P. Fillion, A. Bonnin, J.C. Rousselle, O. Massot, G. Fillion, Immunocytochemical localization of neurons expressing 5-HT-moduline in the mouse brain, Neuropharmacology 36 (1997) 1079-1087] in a dot-ELISA (enzyme-linked-immunosorbent assay) assay in cortex, hippocampus, hypothalamus, substantia nigra, striatum and in adrenal glands. Tissue contents of 5-HT-moduline progressively and transiently increased in most studied brain regions and reached a maximal value 20 min after the beginning of the restraint stress. The increase in 5-HT-moduline tissue contents represented 323% of the value observed in unstressed control animals in the cortex, 207% in the hippocampus, 149% in the hypothalamus and 156% in the substantia nigra. Thereafter, the peptide content of the latter tissues diminished during the last 20 min of restraint and returned to control values within 1 h after the end of the stress period. The striatum did not show any significant variation of 5-HT-moduline content during restraint stress. In adrenal glands, the 5-HT-moduline content rapidly decreased (60% of controls) after the beginning of the restraint stress, the effect of this stress being progressively less pronounced, still representing 80% of controls after 40 min. Repetition of the restraint stress daily for 3 weeks totally abolished the effect of the stress on variations of 5-HT-moduline tissue content in all the studied brain regions. These results show that an acute restraint stress induces a rapid and significant increase in the amount of 5-HT-moduline contained in various brain areas. This phenomenon is likely to be related to the stress-induced 5-HT1B receptor desensitization which was previously demonstrated.

Pharmacological diversity between native human 5-HT1B and 5-HT1D receptors sited on different neurons and involved in different functions

The releases of [3H]5-hydroxytryptamine ([3H]5-HT) and of endogenous glutamic acid and their modulation through presynaptic h5-HT1B autoreceptors and h5-HT1D heteroreceptors have been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery. The inhibition by 5-HT of the K+ (15 mM)-evoked overflow of [3H]5-HT was antagonized by the 5-HT1B/5-HT1D receptor ligand GR 127935, which was ineffective on its own; this drug was previously found to behave as a full agonist at the h5-HT1D heteroreceptor regulating glutamate release. The recently proposed selective h5-HT1B receptor ligand SB-224289 also prevented the effect of 5-HT at the autoreceptor, being inactive on its own; in contrast, SB-224289, at 1 microM, was unable to interact with the h5-HT1D heteroreceptor. The inhibitory effect of 5-HT on the K+-evoked overflow of glutamate was antagonized by the h5-HT1D receptor ligand BRL-15572; added in the absence of 5-HT the compound was without effect. BRL-15572 (1 microM) was unable to modify the effect of 5-HT at the autoreceptor regulating [3H]5-HT release. The selective 5-HT1A receptor antagonist (+)-WAY 100135, previously found to be an agonist at the h5-HT1D heteroreceptor regulating glutamate release, could not interact with the h5-HT1B autoreceptor when added at 1 microM. It is concluded that native h5-HT1B and h5-HT1D receptors exhibit a hitherto unexpected pharmacological diversity.