GABAB receptor subunits, R1 and R2, in brainstem catecholamine and serotonin neurons

Effects of intraoperative sodium oxybate infusion on post-operative sleep quality in patients undergoing gynecological laparoscopic surgery: A randomized clinical trial

STUDY OBJECTIVE

Post-operative sleep quality is an important factor that influences post-operative recovery. Sodium oxybate has been used to treat sleep disturbances associated with various pathological conditions. However, whether intraoperative intravenous infusion of sodium oxybate improves post-operative sleep quality is unknown. This study aimed to examine the effects of sodium oxybate on the post-operative sleep quality of patients who underwent gynecological laparoscopic surgery.

DESIGN

A single-center, prospective, two-arm, double-blinded randomized controlled trial.

SETTING

The Shengjing Hospital of China Medical University in Liaoning, China.

PATIENTS

We enrolled 180 adult patients (90 for each group) undergoing elective gynecological laparoscopic surgery, and 178 patients (89 for each group) were included in the final analysis.

INTERVENTIONS

Patients were randomly allocated in a 1:1 ratio to receive either sodium oxybate (30 mg kg-1) or an equivalent volume of saline after intubation. The patients, anesthetists, and follow-up staff were blinded to group assignment.

MEASUREMENTS

The primary outcome was sleep quality measured using the Richards-Campbell Sleep Questionnaire (RCSQ) on post-operative days (PODs) one and three. Secondary outcomes included post-operative pain measured using the visual analog scale, sleep quality at one and three months post-operatively measured using the Pittsburgh Sleep Quality Index, and factors associated with post-operative sleep quality.

MAIN RESULTS

Analysis with generalized estimating equations showed that sodium oxybate significantly improved post-operative sleep quality, as represented by increased total RCSQ scores (mean difference (95% CI); 9 (2, 16), P = 0.010) over PODs one and three. There was no difference in post-operative pain between the two groups over PODs one and three or in post-operative sleep quality over one and three months post-operatively. Age, surgery type, start time of surgery, and use of sufentanil-based patient-controlled intravenous analgesia were significantly associated with post-operative sleep quality.

CONCLUSIONS

Intraoperative sodium oxybate infusion improved post-operative sleep in patients who underwent gynecological laparoscopic surgery.

TRIAL REGISTRATION

Chinese Clinical Trial Registry, Clinical trial number: ChiCTR2200061460.

Sleep, Narcolepsy, and Sodium Oxybate

Sodium oxybate (SO) has been in use for many decades to treat narcolepsy with cataplexy. It functions as a weak GABAB agonist but also as an energy source for the brain as a result of its metabolism to succinate and as a powerful antioxidant because of its capacity to induce the formation of NADPH. Its actions at thalamic GABAB receptors can induce slow-wave activity, while its actions at GABAB receptors on monoaminergic neurons can induce or delay REM sleep. By altering the balance between monoaminergic and cholinergic neuronal activity, SO uniquely can induce and prevent cataplexy. The formation of NADPH may enhance sleep’s restorative process by accelerating the removal of the reactive oxygen species (ROS), which accumulate during wakefulness. SO improves alertness in normal subjects and in patients with narcolepsy. SO may allay severe psychological stress - an inflammatory state triggered by increased levels of ROS and characterized by cholinergic supersensitivity and monoaminergic deficiency. SO may be able to eliminate the inflammatory state and correct the cholinergic/ monoaminergic imbalance.

Structures of metabotropic GABAB receptor

GABA (γ-aminobutyric acid) stimulation of the metabotropic GABA_B receptor results in prolonged inhibition of neurotransmission that is central to brain physiology¹. GABA_B belongs to the Family C of G protein-coupled receptors (GPCRs), which operate as dimers to relay synaptic neurotransmitter signals into a cellular response through the binding and activation of heterotrimeric G proteins^2,3. GABA_B, however, is unique in its function as an obligate heterodimer in which agonist binding and G protein activation take place on distinct subunits^4,5. Here we show structures of heterodimeric and homodimeric full-length GABA_B receptors. Complemented by cellular signaling assays and atomistic simulations, the structures reveal an essential role for the GABA_B extracellular loop 2 (ECL2) in relaying structural transitions by ordering the linker connecting the extracellular ligand-binding domain to the transmembrane region. Furthermore, the ECL2 of both GABA_B subunits caps and interacts with the hydrophilic head of a phospholipid occupying the extracellular half of the transmembrane domain, thereby providing a potentially crucial link between ligand binding and the receptor core that engages G protein. These results provide a starting framework to decipher mechanistic modes of signal transduction mediated by GABA_B dimers and have important implications for rational drug design targeting these receptors.

Unique and independent role of the GABAB1 subunit in embryo implantation and uterine decidualization in mice

Embryo implantation and decidualization are crucial for successful pregnancy, which include multiple genes and signaling pathways, while the precise mechanism regarding embryo implantation and decidualization has yet to be explored. The GABA which activates GABA_A or GABA_B receptors has been found playing an important role in early pregnancy. Here we seek to investigate whether GABA_B receptors participate in embryo implantation in mice. This study first characterized the spatiotemporal expression pattern of GABA_B receptors in the uterus during the peri-implantation period and found that GABA_B1 expression was drastically upregulated in stromal cells on days 4–6, a period of embryo implantation and early stages of decidualization. Embryo delayed implantation and oil-induced decidualization models were further used to confirm that the GABA_B1 was associated with embryo implantation and decidualization. We also found estrogen or progesterone had no directly effect on expression of GABA_B1 in ovariectomized model. Because we were unable to detect significant GABA_B2 which couples with GABA_B1 to form whole GABA_B receptors, and the agonist and antagonist of whole GABA_B receptors had weak effect on the proliferation and differentiation of stromal cells as well, we excluded the possibility whole GABA_B receptors function, and concluded it should be non-classical signals of GABA_B1 involving in embryo implantation and decidualization. Future studies should focus on investigating the roles and mechanisms of GABA_B1 during embryo implantation and decidualization.

The Neurobiological Mechanisms of Gamma-Hydroxybutyrate Dependence and Withdrawal and Their Clinical Relevance: A Review

OBJECTIVE

x03B3;-Hydroxybutyrate (GHB) has gained popularity as a drug of abuse. In the Netherlands the number of patients in treatment for GHB dependence has increased sharply. Clinical presentation of GHB withdrawal can be life threatening. We aim, through this overview, to explore the neurobiological pathways causing GHB dependency and withdrawal, and their implications for treatment choices.

METHODS

In this work we review the literature discussing the findings from animal models to clinical studies focused on the neurobiological pathways of endogenous but mainly exogenous GHB.

RESULTS

Chronic abuse of GHB exerts multifarious neurotransmitter and neuromodulator effects on x03B3;-aminobutyric acid (GABA), glutamate, dopamine, serotonin, norepinephrine and cholinergic systems. Moreover, important effects on neurosteroidogenesis and oxytocin release are wielded. GHB acts mainly via a bidirectional effect on GABAB receptors (GABABR; subunits GABAB1 and GABAB2), depending on the subunit of the GIRK (G-protein-dependent ion inwardly rectifying potassium) channel involved, and an indirect effect of the cortical and limbic inputs outside the nucleus accumbens. GHB also activates a specific GHB receptor and β1-subunits of α4-GABAAR. Reversing this complex interaction of neurobiological mechanisms by the abrupt cessation of GHB use results in a withdrawal syndrome with a diversity of symptoms of different intensity, depending on the pattern of GHB abuse.

CONCLUSION

The GHB withdrawal symptoms cannot be related to a single mechanism or neurological pathway, which implies that different medication combinations are needed for treatment. A single drug class, such as benzodiazepines, gabapentin or antipsychotics, is unlikely to be sufficient to avoid life-threatening complications. Detoxification by means of titration and tapering of pharmaceutical GHB can be considered as a promising treatment that could make polypharmacy redundant.

Anatomical and ultrastructural study of PRAF2 expression in the mouse central nervous system

Prenylated Rab acceptor family, member 2 (PRAF2) is a four transmembrane domain protein of 19 kDa that is highly expressed in particular areas of mammalian brains. PRAF2 is mostly found in the endoplasmic reticulum (ER) of neurons where it plays the role of gatekeeper for the GB1 subunit of the GABA_B receptor, preventing its progression in the biosynthetic pathway in the absence of hetero-dimerization with the GB2 subunit. However, PRAF2 can interact with several receptors and immunofluorescence studies indicate that PRAF2 distribution is larger than the ER, suggesting additional biological functions. Here, we conducted an immuno-cytochemical study of PRAF2 distribution in mouse central nervous system (CNS) at anatomical, cellular and ultra-structural levels. PRAF2 appears widely expressed in various regions of mature CNS, such as the olfactory bulbs, cerebral cortex, amygdala, hippocampus, ventral tegmental area and spinal cord. Consistent with its regulatory role of GABA_B receptors, PRAF2 was particularly abundant in brain regions known to express GB1 subunits. However, other brain areas where GB1 is expressed, such as basal ganglia, thalamus and hypothalamus, contain little or no PRAF2. In these areas, GB1 subunits might reach the cell surface of neurons independently of GB2 to exert biological functions distinct from those of GABA_B receptors, or be regulated by other gatekeepers. Electron microscopy studies confirmed the localization of PRAF2 in the ER, but identified previously unappreciated localizations, in mitochondria, primary cilia and sub-synaptic region. These data indicate additional modes of GABA_B regulation in specific brain areas and new biological functions of PRAF2.

Glutamate input in the dorsal raphe nucleus as a determinant of escalated aggression in male mice

Although the dorsal raphe nucleus (DRN) has long been linked to neural control of aggression, little is known about the regulatory influences of the DRN when an animal engages in either adaptive species-typical aggressive behavior or escalated aggression. Therefore it is important to explore which neurotransmitter inputs into the DRN determine the escalation of aggression in male mice. Previously, we observed that microinjection of the GABAB receptor agonist baclofen into the DRN escalates aggressive behavior in male mice. Here, we used a serotonin (5-HT) neuron-specific GABAB receptor knock-out mouse to demonstrate that baclofen acts on nonserotonergic neurons to escalate aggression. Intra-DRN baclofen administration increased glutamate release, but did not alter GABA release, within the DRN. Microinjection of l-glutamate into the DRN escalated dose-dependently attack bites toward an intruder. In vivo microdialysis showed that glutamate release increased in the DRN during an aggressive encounter, and the level of glutamate was further increased when the animal was engaged in escalated aggressive behavior after social instigation. Finally, 5-HT release was increased within the DRN and also in the medial prefrontal cortex when animals were provoked by social instigation, and during escalated aggression after social instigation, but this increase in 5-HT release was not observed when animals were engaged in species-typical aggression. In summary, glutamate input into the DRN is enhanced during escalated aggression, which causes a phasic increase of 5-HT release from the DRN 5-HT neurons.

Efferent projections of neuropeptide Y-expressing neurons of the dorsomedial hypothalamus in chronic hyperphagic models

The dorsomedial hypothalamus (DMH) has long been implicated in feeding behavior and thermogenesis. The DMH contains orexigenic neuropeptide Y (NPY) neurons, but the role of these neurons in the control of energy homeostasis is not well understood. NPY expression in the DMH is low under normal conditions in adult rodents but is significantly increased during chronic hyperphagic conditions such as lactation and diet-induced obesity (DIO). To understand better the role of DMH-NPY neurons, we characterized the efferent projections of DMH-NPY neurons using the anterograde tracer biotinylated dextran amine (BDA) in lactating rats and DIO mice. In both models, BDA- and NPY-colabeled fibers were limited mainly to the hypothalamus, including the paraventricular nucleus of the hypothalamus (PVH), lateral hypothalamus/perifornical area (LH/PFA), and anteroventral periventricular nucleus (AVPV). Specifically in lactating rats, BDA-and NPY-colabeled axonal swellings were in close apposition to cocaine- and amphetamine-regulated transcript (CART)-expressing neurons in the PVH and AVPV. Although the DMH neurons project to the rostral raphe pallidus (rRPa), these projections did not contain NPY immunoreactivity in either the lactating rat or the DIO mouse. Instead, the majority of BDA-labeled fibers in the rRPa were orexin positive. Furthermore, DMH-NPY projections were not observed within the nucleus of the solitary tract (NTS), another brainstem site critical for the regulation of sympathetic outflow. The present data suggest that NPY expression in the DMH during chronic hyperphagic conditions plays important roles in feeding behavior and thermogenesis by modulating neuronal functions within the hypothalamus, but not in the brainstem.

Therapeutic potential of GABA(B) receptor ligands in drug addiction, anxiety, depression and other CNS disorders

Glutamate and γ-aminobutyric acid (GABA) are the major excitatory and inhibitory neurotransmitter systems, respectively in the central nervous system (CNS). Dysregulation, in any of these or both, has been implicated in various CNS disorders. GABA acts via ionotropic (GABA(A) and GABA(C) receptor) and metabotropic (GABA(B)) receptor. Dysregulation of GABAergic signaling and alteration in GABA(B) receptor expression has been implicated in various CNS disorders. Clinically, baclofen-a GABA(B) receptor agonist is available for the treatment of spasticity, dystonia etc., associated with various neurological disorders. Moreover, GABAB receptor ligands has also been suggested to be beneficial in various neuropsychiatric and neurodegenerative disorders. The present review is aimed to discuss the role of GABA(B) receptors and the possible outcomes of GABA(B) receptor modulation in CNS disorders.

Immunoreactivity for the NMDA NR1 subunit in bulbospinal catecholamine and serotonin neurons of rat ventral medulla

Bulbospinal neurons in the ventral medulla play important roles in the regulation of sympathetic outflow. Physiological evidence suggests that these neurons are activated by N-methyl-D-aspartate (NMDA) and non-NMDA subtypes of glutamate receptors. In this study, we examined bulbospinal neurons in the ventral medulla for the presence of immunoreactivity for the NMDA NR1 subunit, which is essential for NMDA receptor function. Rats received bilateral injections of cholera toxin B into the tenth thoracic spinal segment to label bulbospinal neurons. Triple immunofluorescent labeling was used to detect cholera toxin B with a blue fluorophore, NR1 with a red fluorophore, and either tyrosine hydroxylase or tryptophan hydroxylase with a green fluorophore. In the rostral ventrolateral medulla, NR1 occurred in all bulbospinal tyrosine hydroxylase-positive neurons and 96% of bulbospinal tyrosine hydroxylase-negative neurons, which were more common in sections containing the facial nucleus. In the raphe pallidus, the parapyramidal region, and the marginal layer, 98% of bulbospinal tryptophan hydroxylase-positive neurons contained NR1 immunoreactivity. NR1 was also present in all of the bulbospinal tryptophan hydroxylase-negative neurons, which comprised 20% of bulbospinal neurons in raphe pallidus and the parapyramidal region. These results show that virtually all bulbospinal tyrosine hydroxylase and non-tyrosine hydroxylase neurons in the rostral ventrolateral medulla and virtually all bulbospinal serotonin and non-serotonin neurons in raphe pallidus and the parapyramidal region express NR1, the obligatory subunit of the NMDA receptor. NMDA receptors on bulbospinal neurons in the rostral ventral medulla likely influence sympathoexcitation in normal and pathological conditions.

Cloning and characterization of GABAA α subunits and GABAB subunits in Xenopus laevis during development

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the adult nervous system, acts via two classes of receptors, the ionotropic GABA(A) and metabotropic GABA(B) receptors. During the development of the nervous system, GABA acts in a depolarizing, excitatory manner and plays an important role in various neural developmental processes including cell proliferation, migration, synapse formation, and activity-dependent differentiation. Here we describe the spatial and temporal expression patterns of the GABA(A) and GABA(B) receptors during early development of Xenopus laevis. Using in situ hybridization and qRT-PCR, GABA(A) α2 was detected as a maternal mRNA. All other α-subunits were first detected by tailbud through hatching stages. Expression of the various subunits was seen in the brain, spinal cord, cranial ganglia, olfactory epithelium, pineal, and pituitary gland. Each receptor subunit showed a distinctive, unique expression pattern, suggesting these receptors have specific functions and are regulated in a precise spatial and temporal manner.

Postnatal changes in tryptophan hydroxylase and serotonin transporter immunoreactivity in multiple brainstem nuclei of the rat: implications for a sensitive period

Previously, we found that the brainstem neuronal network in normal rats undergoes abrupt neurochemical, metabolic, and physiological changes around postnatal days (P) 12-13, a critical period when the animal's response to hypoxia is also the weakest. This has special implications for sudden infant death syndrome (SIDS), insofar as seemingly normal infants succumb to SIDS when exposed to respiratory stressors (e.g., hypoxia) during a narrow postnatal window. Because an abnormal serotonergic system has recently been implicated in SIDS, we conducted a large-scale investigation of the 5-HT-synthesizing enzyme tryptophan hydroxylase (TPH) and serotonin transporter (SERT) with semiquantitative immunohistochemistry in multiple brainstem nuclei of normal rats aged P2-21. We found that 1) TPH and SERT immunoreactivity in neurons of raphé magnus, obscurus, and pallidus and SERT in the neuropil of the pre-Bötzinger complex, nucleus ambiguus, and retrotrapezoid nucleus were high at P2-11 but decreased markedly at P12 and plateaued thereafter until P21; 2) SERT labeling in neurons of the lateral paragigantocellular nucleus (LPGi) and parapyramidal region (pPy) was high at P2-9 but fell significantly at P10, followed by a gradual decline until P21; 3) TPH labeling in neurons of the ventrolateral medullary surface was stable except for a significant fall at P12; and 4) TPH and SERT immunoreactivity in a number of other nuclei was relatively stable from P2 to P21. Thus, multiple brainstem nuclei exhibited a significant decline in TPH and SERT immunoreactivity during the critical period, suggesting that such normal development can contribute to a narrow window of vulnerability in postnatal animals.

The GABA B receptor antagonists CGP35348 and CGP52432 inhibit glycine exocytosis: study with GABA B1- and GABA B2-deficient mice

GABA(B) receptors mediate inhibition of neurotransmitter exocytosis from nerve endings. Unexpectedly, the well known GABA(B) receptor antagonist CGP35348 and, in part, the compound CGP52432, are now found to inhibit on their own the K(+)-evoked exocytosis of glycine when added at low micromolar concentrations to superfused mouse glycinergic nerve endings prelabelled with [(3)H]glycine through GLYT2 transporters. CGP35348 inhibited [(3)H]glycine release both in spinal cord and in hippocampus, but was also able to prevent the inhibitory effect of (-)-baclofen; CGP52432 exhibited intrinsic activity only in the hippocampus; in spinal cord, it behaved exclusively as a silent orthosteric antagonist by blocking the release inhibition brought about by (-)-baclofen. The intrinsic activity of CGP35348 in spinal cord was not prevented by CGP52432, indicating that CGP35348 is not a partial GABA(B) agonist in this experimental system. CGP54626, an extremely potent antagonist, exhibited only a minimal intrinsic activity. SCH50911, a GABA(B) antagonist belonging to a different chemical class, was devoid of significant activity, while phaclofen was effective only at 100-300 microM. In synaptosomes purified from the spinal cord or the hippocampus of mice lacking either the GABA(B1) (GABA(B1-/-) mice) or the GABA(B2) (GABA(B2-/-) mice) subunit, the evoked exocytosis of [(3)H]glycine was no longer inhibited by (-)-baclofen, whereas the intrinsic activity of CGP35348 and CGP52432 was not decreased. Activation of unknown sites on glycinergic terminals is likely to be involved. These unexpected effects should not be ignored when interpreting results obtained with the above GABA(B) receptor antagonists.

Narcolepsy and depression and the neurobiology of gammahydroxybutyrate

A voluminous literature describes the relationship between disturbed sleep and depression. The breakdown of sleep is one of the cardinal features of depression and often also heralds its onset. Frequent arousals, periods of wakefulness and a short sleep onset REM latency are typical polysomnographic features of depression. The short latency to REM sleep has been attributed to the combination of a monoaminergic deficiency and cholinergic supersensitivity and these irregularities have been proposed to form the biological basis of the disorder. A similar imbalance between monoaminergic and cholinergic neurotransmission has been found in narcolepsy, a condition in which frequent awakenings, periods of wakefulness and short sleep onset REM latencies are also characteristic findings during sleep. In many cases of narcolepsy, this imbalance appears to result from a deficiency of hypocretin but once established, whether in depression or narcolepsy, this disequilibrium sets the stage for the dissociation or premature appearance of REM sleep and for the dissociation of the motor inhibitory component of REM sleep or cataplexy. In the presence of this monoaminergic/cholinergic imbalance, gammahydroxybutyrate (GHB) may acutely further reduce the latency of REM sleep and induce cataplexy, in both patients with narcolepsy or depression. On the other hand, the repeated nocturnal application of GHB in patients with narcolepsy improves the continuity of sleep, prolongs the latency to REM sleep and prevents cataplexy. Evidence to date suggests that GHB may restore the normal balance between monoaminergic and cholinergic neurotransmission. As such, the repeated use of GHB at night and the stabilization of sleep over time makes GHB an effective treatment for narcolepsy and a potentially effective treatment for depression.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

The effects of electroconvulsive therapy on GABAergic function in major depressive patients

OBJECTIVES

It has been proposed that major depression is associated with a dysfunction of the gamma-aminobutyric acid (GABA) system. This study was planned to investigate whether there are any alterations in GABAergic activities in major depressive patients and, if there are, whether electroconvulsive therapy (ECT) has any effect on these changes.

METHODS

Twenty-five depressed inpatients who responded to a course of ECT and 23 healthy subjects were included in the study. Serum GABA levels were measured 2 days before and 10 minutes after the first ECT and 3 days after the last ECT, and a baclofen challenge test was performed 2 days before the first ECT and 3 days after the last ECT in the patients. The same tests were carried out only once in the control group.

RESULTS

Depressive patients had lower serum GABA levels compared with healthy individuals, and ECT caused a significant increase in these levels. The acute effect of the one-ECT procedure was a huge increase in the baseline GABA levels. Although there was no difference in the maximum alteration in growth hormone with baclofen between the patients and controls before the therapeutic ECT course, it became significantly higher in the depressive patients than in the controls after the treatment.

CONCLUSIONS

The findings of this study support the GABA deficit hypothesis of major depression because major depressive patients have lower levels of serum GABA that are increased by a completed ECT course. ECT seems to increase brain GABA levels as well as GABAB activity, and these effects may contribute to its mechanism of therapeutic effect.

Increased GABA B receptor subtype expression in the nucleus of the solitary tract of the spontaneously hypertensive rat

Expression of GABA(B) receptor messenger RNA (mRNA) in the central nervous system was compared between the spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rat. Polymerase chain reaction (PCR) revealed all the isoforms except B1e in cortex, hypothalamus, and medulla oblongata. In the nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM), the B1a-c and 1 g isoforms were present as well as B2. Real-time PCR detected significantly higher levels of B1a (p < 0.01) and B2 (p < 0.05) mRNA in the NTS of SHR compared to WKY. A significant increase in B1a expression (p < 0.05) was detected in VLM. Immunolabeling suggested presynaptic and postsynaptic expression of B1a, B1b, and B2 subtypes throughout the NTS, with significant differences in distribution patterns and labeling between subtypes and between SHR and WKY. These findings suggest that GABA(B) receptors expressed by neurones in NTS may be involved in cardiovascular regulation and that changes in GABA(B) mRNA expression levels may contribute to the hypertensive state in SHR.

GABAB receptor association with the PDZ scaffold Mupp1 alters receptor stability and function

gamma-Aminobutyric acid, type B (GABA(B)) receptors are heterodimeric G protein-coupled receptors that mediate slow inhibitory synaptic transmission in the central nervous system. To identify novel interacting partners that might regulate GABA(B) receptor (GABA(B)R) functionality, we screened the GABA(B)R2 carboxyl terminus against a recently created proteomic array of 96 distinct PDZ (PSD-95/Dlg/ZO-1 homology) domains. The screen identified three specific PDZ domains that exhibit interactions with GABA(B)R2: Mupp1 PDZ13, PAPIN PDZ1, and Erbin PDZ. Biochemical analysis confirmed that full-length Mupp1 and PAPIN interact with GABA(B)R2 in cells. Disruption of the GABA(B)R2 interaction with PDZ scaffolds by a point mutation to the carboxyl terminus of the receptor dramatically decreased receptor stability and attenuated the duration of GABA(B) receptor signaling. The effects of mutating the GABA(B)R2 carboxyl terminus on receptor stability and signaling were mimicked by small interference RNA knockdown of endogenous Mupp1. These findings reveal that GABA(B) receptor stability and signaling can be modulated via GABA(B)R2 interactions with the PDZ scaffold protein Mupp1, which may contribute to cell-specific regulation of GABA(B) receptors in the central nervous system.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Differential sensitivity to the motor and hypothermic effects of the GABA B receptor agonist baclofen in various mouse strains

RATIONALE

Comparison of different mouse strains can provide valuable information about the genetic control of behavioural and molecular phenotypes. Recent evidence has demonstrated the importance of GABA B receptors in anxiety and depression. Investigation of the phamacogenetics of GABA B receptor activation may aid in the understanding of mechanisms underlying the role of GABA B in affect.

OBJECTIVES

The aim of current study was to determine the relative sensitivity of different mouse strains to GABA B receptor agonism in two models of GABA B receptor function, namely hypothermia and motor incoordination.

METHODS

Mice each from 11 strains (BALB/cByJIco, DBA/2JIco, OF1, FVB/NIco, CD1, C3H/HeOuJIco, 129/SvPasIco, NMRI, C57BL/6JIco, A/JOlaHsd and Swiss) were trained to walk on a rotarod for 300 s. On the following day, mice received 0, 3, 6 or 12 mg/kg of L: -baclofen PO. Rectal temperature and rotarod performance were measured at 0, 1, 2 and 4 h after drug application.

RESULTS

L: -Baclofen produced a significant dose-dependent hypothermia and ataxia in most, but not all, mouse strains examined. The magnitude and duration of response was influenced by strain, with mice of the 129/SvPasIco strain showing largest hypothermic response to 12 mg/kg l-baclofen and C3H/HeOuJIco the lowest, whereas the BALB/cByJIco strain demonstrated greatest ataxic response on the rotarod, and NMRI the least. Interestingly, some strains (notably C3H/HeOuJIco) had marked differential hypothermic and ataxic responses, with minimal body temperature responses to L: -baclofen but significant ataxia on the rotarod observed.

CONCLUSION

There is differential genetic control on specific GABA B receptor populations that mediate hypothermia and ataxia. Further, these studies demonstrate that background strain is an important determinant of GABA B receptor mediated responses, and that hypothermic and ataxic responses may be influenced by independent genetic loci.

Redistribution of GABAB(1) protein and atypical GABAB responses in GABAB(2)-deficient mice

GABAB receptors mediate slow synaptic inhibition in the nervous system. In transfected cells, functional GABAB receptors are usually only observed after coexpression of GABAB(1) and GABAB(2) subunits, which established the concept of heteromerization for G-protein-coupled receptors. In the heteromeric receptor, GABAB(1) is responsible for binding of GABA, whereas GABAB(2) is necessary for surface trafficking and G-protein coupling. Consistent with these in vitro observations, the GABAB(1) subunit is also essential for all GABAB signaling in vivo. Mice lacking the GABAB(1) subunit do not exhibit detectable electrophysiological, biochemical, or behavioral responses to GABAB agonists. However, GABAB(1) exhibits a broader cellular expression pattern than GABAB(2), suggesting that GABAB(1) could be functional in the absence of GABAB(2). We now generated GABAB(2)-deficient mice to analyze whether GABAB(1) has the potential to signal without GABAB(2) in neurons. We show that GABAB(2)-/- mice suffer from spontaneous seizures, hyperalgesia, hyperlocomotor activity, and severe memory impairment, analogous to GABAB(1)-/- mice. This clearly demonstrates that the lack of heteromeric GABAB(1,2) receptors underlies these phenotypes. To our surprise and in contrast to GABAB(1)-/- mice, we still detect atypical electrophysiological GABAB responses in hippocampal slices of GABAB(2)-/- mice. Furthermore, in the absence of GABAB(2), the GABAB(1) protein relocates from distal neuronal sites to the soma and proximal dendrites. Our data suggest that association of GABAB(2) with GABAB(1) is essential for receptor localization in distal processes but is not absolutely necessary for signaling. It is therefore possible that functional GABAB receptors exist in neurons that naturally lack GABAB(2) subunits.

GABAB Receptor Antagonist-Mediated Antidepressant-Like Behavior Is Serotonin-Dependent

There is an emerging body of data purporting a role of gamma-aminobutyric acid (GABA) in the pathophysiology of mood disorders. However, the role of metabotropic GABA(B) receptors in depression is not well defined. The modified forced swim test has recently emerged as an excellent tool to assess behaviorally the role of monoamines in antidepressant action. To assess the role of GABA(B) receptors in antidepressant-related behavior, we examined a number of selective GABA(B) receptor ligands (novel positive modulators and antagonists) on behavior in the modified forced swim test. We demonstrate that the selective GABA(B) receptor antagonists CGP56433A [[3-{1-(S)-[{3-cyclohexylmethyl)hydroxy phosphinyl}-2-(S) hydroxy propyl]amino}ethyl]benzoic acid; 1-10 mg/kg] and [3-[[1-(S)-3-dichlorophenyl)ethyl]amino]-2-(S)-hydroxy-propyl]phenylmethyl-phosphinic acid hydrochloride; 3-10 mg/kg] had a similar profile to the selective serotonin reuptake inhibitor fluoxetine; they decreased immobility and increased swimming behavior. The tricyclic antidepressant desipramine decreased immobility but increased climbing behavior. In contrast, the novel GABA(B) receptor-positive modulator GS39783 (10-40 mg/kg) did not display antidepressant-like activity in the modified forced swim test. To further assess the possible interaction between GABA(B) receptor antagonism and serotonin, rats were pretreated with the tryptophan hydroxylase inhibitor para-chlorophenylalanine. 5-Hydroxytryptamine depletion (>90%) abolished the antidepressant-like behavior of CGP56433A (10 mg/kg) by attenuating the increase in swimming. Together, these data demonstrate that GABA(B) receptor antagonists via an interaction with the serotonergic system display antidepressant-like properties and therefore represent a novel approach for the treatment of depression.

Genetic and pharmacological evidence of a role for GABA(B) receptors in the modulation of anxiety- and antidepressant-like behavior

Although there is much evidence for a role of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the pathophysiology of anxiety and depression, the role of GABA(B) receptors in behavioral processes related to these disorders has not yet been fully established. GABA(B) receptors are G-protein-coupled receptors, which act as functional heterodimers made up of GABA(B(1)) and GABA(B(2)) subunits. Using recently generated GABA(B(1)) -/- mice, which lack functional GABA(B) receptors, and pharmacological tools we assessed the role of GABA(B) receptors in anxiety- and antidepressant-related behaviors. In the light-dark box, GABA(B(1)) -/- mice were more anxious than their wild-type littermates (less time spent in the light; reduced number of transitions). GABA(B(1)) -/- mice were also more anxious in the staircase test. Conversely, acute and chronic treatment with GS39783, a novel GABA(B) receptor positive modulator, decreased anxiety in the light-dark box and elevated zero maze tests for anxiety. On the other hand, GABA(B(1)) -/- mice had decreased immobility (antidepressant-like behavior) in the forced swim test (FST). These behavioral effects are unrelated to alterations in locomotor activity. In confirmation of the genetic data, acute and chronic treatment with CGP56433A, a selective GABA(B) receptor antagonist, also decreased immobility in the FST, whereas GS39783 did not alter this behavior. Taken together, these data suggest that positive modulation of the GABA(B) receptor may serve as a novel therapeutic strategy for the development of anxiolytics, whereas GABA(B) receptor antagonism may serve as a basis for the generation of novel antidepressants.