Hippocampal α5-GABAA Receptors Modulate Dopamine Neuron Activity in the Rat Ventral Tegmental Area.

Background: Aberrant dopamine neuron activity is attributable to hyperactivity in hippocampal subfields driving a pathological increase in dopamine neuron activity, which is positively correlated with psychosis in humans. Evidence indicates that hippocampal hyperactivity is due to loss of intrinsic GABAergic (gamma-aminobutyric acidergic) inhibition. We have previously demonstrated that hippocampal GABAergic neurotransmission can be modulated by targeting α5-GABAA receptors, which are preferentially expressed in hippocampal regions. Positive and negative allosteric modulators of α5-GABAA receptors (α5-PAMs and α5-NAMs) elicit effects on hippocampal-dependent behaviors. We posited that the selective manipulation of hippocampal inhibition, using α5-PAMs or α5-NAMs, would modulate dopamine activity in control rats. Further, α5-PAMs would reverse aberrant dopamine neuron activity in a rodent model with schizophrenia-related pathophysiologies (methylazoxymethanol acetate [MAM] model). Methods: We performed in vivo extracellular recordings of ventral tegmental area dopamine neurons in anesthetized rats to compare the effects of two novel, selective α5-PAMs (GL-II-73, MP-III-022), a nonselective α-PAM (midazolam), and two selective α5-NAMs (L-655,708, TB 21007) in control and MAM-treated male Sprague Dawley rats (n = 5-9). Results: Systemic or intracranial administration of selective α5-GABAA receptor modulators regulated dopamine activity. Specifically, both α5-NAMs increased dopamine neuron activity in control rats, whereas GL-II-73, MP-III-022, and L-655,708 attenuated aberrant dopamine neuron activity in MAM-treated rats, an effect mediated by the ventral hippocampus. Conclusions: This study demonstrated that α5-GABAA receptor modulation can regulate dopamine neuron activity under control or abnormal activity, providing additional evidence that α5-PAMs and α5-NAMs may have therapeutic applications in psychosis and other psychiatric diseases where aberrant hippocampal activity is present.

Positive Allosteric Modulation of α5-GABAA Receptors Reverses Stress-Induced Alterations in Dopamine System Function and Prepulse Inhibition of Startle.

BACKGROUND: Up to 64% of patients diagnosed with posttraumatic stress disorder (PTSD) experience psychosis, likely attributable to aberrant dopamine neuron activity. We have previously demonstrated that positive allosteric modulators of α5-GABAARs can selectively decrease hippocampal activity and reverse psychosis-like physiological and behavioral alterations in a rodent model used to study schizophrenia; however, whether this approach translates to a PTSD model remains to be elucidated. METHODS: We utilized a 2-day inescapable foot shock (IS) procedure to induce stress-related pathophysiology in male Sprague-Dawley rats. We evaluated the effects of intra-ventral hippocampus (vHipp) administration GL-II-73, an α5-GABAAR, or viral overexpression of the α5 subunit, using in vivo electrophysiology and behavioral measures in control and IS-treated rats. RESULTS: IS significantly increased ventral tegmental area dopamine neuron population activity, or the number of dopamine neurons firing spontaneously (n = 6; P = .016), consistent with observation in multiple rodent models used to study psychosis. IS also induced deficits in sensorimotor gating, as measured by reduced prepulse inhibition of startle (n = 12; P = .039). Interestingly, intra-vHipp administration of GL-II-73 completely reversed IS-induced increases in dopamine neuron population activity (n = 6; P = .024) and deficits in prepulse inhibition (n = 8; P = .025), whereas viral overexpression of the α5 subunit in the vHipp was not effective. CONCLUSIONS: Our results demonstrate that pharmacological intervention augmenting α5-GABAAR function, but not α5 overexpression in itself, can reverse stress-induced deficits related to PTSD in a rodent model, providing a potential site of therapeutic intervention to treat comorbid psychosis in PTSD.

Analgesic Effects of Oxycodone in Combination With Risperidone or Ziprasidone: Results From a Pilot Randomized Controlled Trial in Healthy Volunteers.

Background and Objectives: Patients taking opioids are at risk of developing dependence and possibly abuse. Given the role of the mesolimbic dopamine system in opioid reward, blocking dopamine D2 receptors should limit the abuse liability of opioid analgesics. This pilot study evaluates the analgesic efficacy of oxycodone combined with an atypical antipsychotic (dopamine D2 receptor antagonist). Methods: A randomized, double-blind, within-subjects, controlled trial in healthy volunteers was conducted at UT Health SA Pain Clinic. Fifteen volunteers with previous medical exposure to opioids were enrolled. Risperidone (2 mg) or ziprasidone (80 mg) in combination with oxycodone (5, 10, 15 mg) was administered. Pain intensity using the cold pressor test, Current Opioid Misuse Measure (COMM), Addiction Research Center Inventory (ARCI, opioid subscale), Drug likability with drug effects questionnaire (DEQ) were assessed. Results: Oxycodone produced dose dependent increases in thermal analgesia on the cold pressor test that was significant at 10 and 15 mg (t = 3.087, P = 0.017). The combination did not significantly alter thermal analgesia. There was no significant effect of the combination on the ARCI or the POMS. Discussion and Conclusion: The combination of an atypical antipsychotic with oxycodone does not alter analgesic response or increase the incidence of adverse effects when compared to oxycodone alone. Such information is critical for the development of drug combinations for the treatment of pain and provide the foundation for future studies of abuse potential in drug users. Scientific Significance: This intervention in chronic pain patients is unique because it utilizes FDA approved drugs in combination to reduce abuse liability. The first step, and aim of this study, is to confirm the drug combination does not interfere with analgesic efficacy. The next step is to examine the combination in recreational drug users to assess the potential to block the euphoric effects of oxycodone. Ultimately, if this combination is effective, this approach could be beneficial in management of chronic pain.

Mechanisms associated with the antidepressant-like effects of L-655,708.

Previous research has demonstrated that selective modulation of hippocampal transmission by systemic administration of an α5-GABAA receptor negative allosteric modulator, L-655,708, reproduces the sustained antidepressant-like (AD-like) effect of R,S-ketamine in the absence of any psychotomimetic or abuse-related effects. Pharmacological, electrophysiological (whole-cell patch clamp), and behavioral approaches were used to examine the mechanisms by which L-655,708 produces plasticity within the hippocampus that accounts for its sustained AD-like effect in rats. Inhibitors of either transcription or translation prevented the sustained AD-like effect of L-655,708. Unlike R,S-ketamine, L-655,708 did not cause an increase in the phosphorylation of the receptor for BDNF, TrkB, in the ventral hippocampus (vHipp) 30 or 60 min after its administration nor did administration of the TrkB inhibitor, K252a, directly into the vHipp, block the sustained AD-like effect of L-655,708. Similar to previous results with R,S-ketamine, administration of L-655,709 increased levels of GluA1 in the mPFC and, blockade of such receptors by direct administration of NBQX into the mPFC blocked the sustained AD-like effect of L-655,708. Patch-clamp recordings of ventral CA1 pyramidal cells 24 h after a single systemic administration of L-655,708 revealed a significant increase in input resistance, which resulted in an approximately two-fold increase in action potential frequency. These experiments indicate that the sustained AD-like effects of L-655,708 require protein synthesis and plasticity of GluA1 glutamate receptors in the mPFC. The drug also caused changes in GABAA receptor gating properties in the vHipp with resultant changes in ventral CA1 that indirectly increases neuronal excitability. Such effects likely contribute to its sustained AD-like activity.

Ketamine: Leading us into the future for development of antidepressants.

Numerous randomized double-blind clinical trials have consistently shown that that a single intravenous administration of a subanesthetic dose of ketamine to treatment-resistant depressed patients significantly improved depressive symptomatology rapidly, within two hours, with the effect lasting up to seven days. Despite its very promising effects, ketamine has long been associated with potential for abuse as it can cause psychotropic side effects, such as hallucinations, false beliefs, and severe impairments in judgment and other cognitive processes. Consequently, within the last two decades preclinical research has been carried out aimed at understanding its mechanisms of action and the brain circuits involved in ketamine's antidepressant effects, both of which are discussed in this review. Furthermore, with the hippocampus being a key target for ketamine's beneficial antidepressant effects, we and others have begun to examine behavioral and neurochemical effects of drugs that act selectively on the hippocampus due to the preferential location of their receptor targets. Such drugs are negative allosteric modulators (NAMs) and positive allosteric modulator (PAM) of the α5-GABAA receptor. Such compounds are discussed within the framework of how lessons learned with ketamine point to novel classes of drugs, targeting the GABAergic system, that can recapitulate the antidepressant effects of ketamine without its adverse effects.

Vagal Nerve Stimulation for Treatment-Resistant Depression.

Major depressive disorder (MDD) is prevalent. Although standards antidepressants are more effective than placebo, up to 35% of patients do not respond to 4 or more conventional treatments and are considered to have treatment-resistant depression (TRD). Considerable effort has been devoted to trying to find effective treatments for TRD. This review focuses on vagus nerve stimulation (VNS), approved for TRD in 2005 by the Food and Drugs Administration. Stimulation is carried by bipolar electrodes on the left cervical vagus nerve, which are attached to an implanted stimulator generator. The vagus bundle contains about 80% of afferent fibers terminating in the medulla, from which there are projections to many areas of brain, including the limbic forebrain. Various types of brain imaging studies reveal widespread functional effects in brain after either acute or chronic VNS. Although more randomized control trials of VNS need to be carried out before a definitive conclusion can be reached about its efficacy, the results of open studies, carried out over period of 1 to 2 years, show much more efficacy when compared with results from treatment as usual studies. There is an increase in clinical response to VNS between 3 and 12 months, which is quite different from that seen with standard antidepressant treatment of MDD. Preclinically, VNS affects many of the same brain areas, neurotransmitters (serotonin, norepinephrine) and signal transduction mechanisms (brain-derived neurotrophic factor-tropomyosin receptor kinase B) as those found with traditional antidepressants. Nevertheless, the mechanisms by which VNS benefits patients nonresponsive to conventional antidepressants is unclear, with further research needed to clarify this.

Selective Pharmacological Augmentation of Hippocampal Activity Produces a Sustained Antidepressant-Like Response without Abuse-Related or Psychotomimetic Effects.

Background: Selective augmentation of hippocampal activity in ways similar to that caused by ketamine may have therapeutic advantages over ketamine, which has psychotomimetic and reinforcing effects likely due to effects outside the hippocampus (i.e., off-target effects). Methods: Here we evaluated the antidepressant-like response to a negative allosteric modulator of α5 subunit- containing gamma aminobutyric acid subtype A receptors, L-655,708, as these receptors are expressed to a much greater extent in the hippocampus than in other brain areas. Results: Systemic administration of L-655,708 produced a sustained antidepressant-like effect in the forced swim test that was comparable with that of ketamine and was blocked by hippocampal inactivation with lidocaine. However, in contrast to ketamine, L-655,708 did not affect prepulse inhibition of startle, nor did it maintain responding in rats trained to self-administer i.v. ketamine. Conclusion: Taken together, these findings suggest that activation of the hippocampus by L-655,708 produces an antidepressant-like effect in the absence of any psychotomimetic or abuse-related effects.

Comparison of the Antidepressant-Like Effects of Estradiol and That of Selective Serotonin Reuptake Inhibitors in Middle-Aged Ovariectomized Rats.

This study investigated the effect of age and that of the post-ovariectomy (OVX) time interval on the antidepressant (AD)-like effects of estradiol (E2) and selective serotonin reuptake inhibitors (SSRIs) in middle-aged (10 month) OVX rats (10m-OVX). Acute or chronic effects of these treatments in 10m-OVX were compared with those (1) in young adult (4-month) OVX rats (4m-OVX) or with older (14-month) OVX rats (14m-OVX), at a short time: 2 weeks post-OVX (+2w) and (2) in 10m-OVX rats after a longer times: 4 or 8 months post-OVX (+4m or +8m). Using in vivo chronoamperometry in the CA3 region of the hippocampus, E2 at 20 pmol, a dose shown previously to inhibit the serotonin transporter (SERT) in 4m-OVX, had no effect in 10m-OVX+2w. A higher dose of E2 (40 pmol) increased T80 value, a measure of serotonin or 5-hydroxytryptamine (5-HT) clearance, and also blocked the ability of fluvoxamine to increase T80. By contrast, estradiol had no effects on SERT function in 10m-OVX+4m, even at a higher dose than 40 pmol. Fluvoxamine slowed 5-HT clearance in 10m-OVX at +2w, +4m and +8m post-OVX as it did in the 4m-OVX. Using the forced swim test, 2 weeks treatment with E2 (5 µg/day), a dose shown previously to induce AD-like effects in 4m-OVX, had no effect in 10m-OVX+2w. However, a higher dose (10 µg/day) of E2 induced an AD-like effect as demonstrated by significantly increased swimming behavior and decreased immobility. This effect was not seen in 10m-OVX+4m. By contrast, significant AD-like effects were obtained in 14m-OVX+2w, thereby demonstrating that the lack of an AD effect of E2 is due to the 4-month hormone withdrawal and not to an age effect. After 2 weeks treatment with the SSRI sertraline, similar AD-like effects were obtained in 10m-OVX tested at +2w, +4m or +8m post-OVX as those found in 4m-OVX. Thus, the potency of estradiol to produce effects consistent with inhibition of the SERT was not only decreased in older rats but its effects were markedly diminished the longer hormonal depletion occurred. By contrast, the ability of SSRIs to inhibit the SERT was not affected either by age or the length of hormonal depletion.

Effects of Long-Term Treatment with Estradiol and Estrogen Receptor Subtype Agonists on Serotonergic Function in Ovariectomized Rats.

Acute estradiol treatment was reported to slow the clearance of serotonin via activation of estrogen receptors (ER)ß and/or GPR30 and to block the ability of a selective serotonin reuptake inhibitor (SSRI) to slow serotonin clearance via activation of ERα. In this study, the behavioral consequences of longer-term treatments with estradiol or ER subtype-selective agonists and/or an SSRI were examined in the forced swim test (FST). Ovariectomized rats were administered the following for 2 weeks: estradiol, ERß agonist (diarylpropionitrile, DPN), GPR30 agonist (G1), ERα agonist (PPT), and/or the SSRI sertraline. Similar to sertraline, longer-term treatment with estradiol, DPN or G1 induced an antidepressant-like effect. By contrast, PPT did not, even though it blocked the antidepressant-like effect of sertraline. Uterus weights, used as a peripheral measure of estrogenic activity, were increased by estradiol and PPT but not DPN or G1 treatment. A second part of this study investigated, using Western blot analyses in homogenates from hippocampus, whether these behavioral effects are accompanied by changes in the activation of specific signaling pathways and/or TrkB. Estradiol and G1 increased phosphorylation of Akt, ERK and TrkB. These effects were similar to those obtained after treatment with sertraline. Treatment with DPN increased phosphorylation of ERK and TrkB, but it did not alter that of Akt. Treatment with PPT increased phosphorylation of Akt and ERK without altering that of TrkB. In conclusion, activation of at least TrkB and possibly ERK may be involved in the antidepressant-like effect of estradiol, ERß and GPR30 agonists whereas Akt activation may not be necessary.

Evidence for shortening the duration of clinical trials of antidepressants and a proposed paradigm for such studies.

The model for the clinical trial of putative antidepressants is older than 50 years. Recent failures resulted in several drug companies, citing excessive costs of lengthy multiweek trials, abandoning new drug development. Collateral problems include patients being maintained on ineffective drugs for 6 to 8 weeks, increasing the pain associated with the disorder. This study proposes an alternative model for testing new drugs that both shortens the clinical trial and broadens its aims to include a profile of the new drug's specific clinical actions. This alternative model makes it possible to uncover the drug's application to treatment of other mental disorders. It is based on recent findings that onset of action and a large proportion of an effective drug's positive effects, contrary to early reports, occur within the first 2 weeks. It uses an index of the 2-week "early improvement" to predict a 6-week outcome. Measuring effects on the dimensions of the disorder determined that effective antidepressants act on mood and behavioral components and that the Hamilton and new "multivantaged" methods can provide a profile of specific drug actions distinguished from nonspecific placebo effects, at 2 weeks. This early improvement is predictive of positive outcome of 6-week trials. Because of the implications of successful 2-week trials for reducing costs, providing data on specific clinical drug actions, potentially stimulating new drug development, and reducing patient suffering from extended treatment with ineffective drugs, a large sample, prospective study designed in accord with this test trial is recommended.

Therapeutic modalities for treatment resistant depression: focus on vagal nerve stimulation and ketamine.

Treatment resistant depression (TRD) is a global health concern affecting a large proportion of depressed patients who then require novel therapeutic options. One such treatment option that has received some attention in the past several years is vagal nerve stimulation (VNS). The present review briefly describes the relevance of this treatment in the light of other existing pharmacological and non-pharmacological options. It then summarizes clinical findings with respect to the efficacy of VNS. The anatomical rationale for its efficacy and other potential mechanisms of its antidepressant effects as compared to those employed by classical antidepressant drugs are discussed. VNS has been approved in some countries and has been used for patients with TRD for quite some time. A newer, fast-acting, non-invasive pharmacological option called ketamine is currently in the limelight with reference to TRD. This drug is currently in the investigational phase but shows promise. The clinical and preclinical findings related to ketamine have also been summarized and compared with those for VNS. The role of neurotrophin factors, specifically brain derived neurotrophic factor and its receptor, in the beneficial effects of both VNS and ketamine have been highlighted. It can be concluded that both these therapeutic modalities, while effective, need further research that can reveal specific targets for intervention by novel drugs and address concerns related to side-effects, especially those seen with ketamine.

Vagal nerve stimulation reverses aberrant dopamine system function in the methylazoxymethanol acetate rodent model of schizophrenia.

Vagal nerve stimulation (VNS) is an alternative therapy for epilepsy and treatment refractory depression. Here we examine VNS as a potential therapy for the treatment of schizophrenia in the methylozoxymethanol acetate (MAM) rodent model of the disease. We have previously demonstrated that hyperactivity within ventral regions of the hippocampus (vHipp) drives the dopamine system dysregulation in this model. Moreover, by targeting the vHipp directly, we can reverse aberrant dopamine system function and associated behaviors in the MAM model. Although the central effects of VNS have not been completely delineated, positron emission topographic measurements of cerebral blood flow in humans have consistently reported that VNS stimulation induces bilateral decreases in hippocampal activity. Based on our previous observations, we performed in vivo extracellular electrophysiological recordings in MAM- and saline-treated rats to evaluate the effect of chronic (2 week) VNS treatment on the activity of putative vHipp pyramidal neurons, as well as downstream dopamine neuron activity in the ventral tegmental area. Here we demonstrate that chronic VNS was able to reverse both vHipp hyperactivity and aberrant mesolimbic dopamine neuron function in the MAM model of schizophrenia. Additionally, VNS reversed a behavioral correlate of the positive symptoms of schizophrenia. Because current therapies for schizophrenia are far from adequate, with a large number of patients discontinuing treatment due to low efficacy or intolerable side effects, it is important to explore alternative nonpharmacological treatments. These data provide the first preclinical evidence that VNS may be a possible alternative therapeutic approach for the treatment of schizophrenia.

Influence of acute or chronic administration of ovarian hormones on the effects of desipramine in the forced swim test in female rats.

RATIONALE: Gender may influence antidepressant (AD) treatment outcome. In order to address this preclinically, the potential effects of ovarian hormones on AD treatment in ovariectomized female rats were investigated. OBJECTIVES: In the first study, the effect of acute administration of estrogen and progesterone on the antidepressant-like effects of desipramine (DMI), a selective norepinephrine reuptake inhibitor (SNRI), was investigated in the forced swimming test (FST). In the second study, the effect of chronic administration of these hormones on the effects of chronically administered DMI was investigated. RESULTS: In the acute study, the hormones blocked the effects of DMI in the FST as demonstrated by the absence of either a reduction in immobility or an increase in climbing behavior in animals treated with DMI in combination with the hormones. Concentration-response experiments on hippocampal synaptosomes revealed no changes in the Km or Bmax for uptake of (3)H-NE in hormone-treated rats. In the chronic study, the antidepressant-like effects of DMI in the FST were not blocked by chronic administration of hormones. Interestingly, the hormones affected the serum concentrations of DMI. These levels were significantly higher in animals receiving 10 or 15 mg/kg/day in hormone-treated rats as compared to those with placebo. CONCLUSIONS: Acute administration of hormones blocked the effects of DMI (given three times over 24 h) in the FST. However, chronic administration of these hormones failed to block the effects of chronically administered DMI (at a dose that produces clinically relevant serum concentrations).

Signaling mechanisms involved in the acute effects of estradiol on 5-HT clearance.

Estradiol was found previously to have an antidepressant-like effect and to block the ability of selective serotonin reuptake inhibitors (SSRIs) to have an antidepressant-like effect. The antidepressant-like effect of estradiol was due to estrogen receptor ß (ERß) and/or GPR30 activation, whereas estradiol's blockade of the effect of an SSRI was mediated by ERα. This study focuses on investigating signaling pathways as well as interacting receptors associated with these two effects of estradiol. In vivo chronoamperometry was used to measure serotonin transporter (SERT) function. The effect of local application of estradiol or selective agonists for ERα (PPT) or ERß (DPN) into the CA3 region of the hippocampus of ovariectomized (OVX) rats on 5-hydroxytryptamine (5-HT) clearance as well as on the ability of fluvoxamine to slow 5-HT clearance was examined after selective blockade of signaling pathways or that of interacting receptors. Estradiol- or DPN-induced slowing of 5-HT clearance mediated by ERß was blocked after inhibition of MAPK/ERK1/2 but not of PI3K/Akt signaling pathways. This effect also involved interactions with TrkB, and IGF-1 receptors. Estradiol's or PPT's inhibition of the fluvoxamine-induced slowing of 5-HT clearance mediated by ERα, was blocked after inhibition of either MAPK/ERK1/2 or PI3K/Akt signaling pathways. This effect involved interactions with the IGF-1 receptor and with the metabotropic glutamate receptor 1, but not with TrkB. This study illustrates some of the signaling pathways required for the effects of estradiol on SERT function, and particularly shows that ER subtypes elicit different as well as common signaling pathways for their actions.

Activation of signaling pathways downstream of the brain-derived neurotrophic factor receptor, TrkB, in the rat brain by vagal nerve stimulation and antidepressant drugs.

Vagal nerve stimulation (VNS) has been approved for treatment resistant depression (TRD) by the Food and Drug Administration (FDA) since 2005. However, the cellular and molecular targets responsible for its effects are still not characterized. Previously, chronic administration of VNS to rats was found to phosphorylate tyrosine 515 on TrkB, the neurotrophin receptor, whereas traditional antidepressants did not do this. In the present study, Western blot analysis was used to characterize activation due to phosphorylation in the hippocampus of down-stream pathways linked to specific key tyrosine residues on TrkB (namely Y816 and Y515) after either acute or chronic administration of VNS and traditional antidepressant drugs. Chronic administration of VNS caused phosphorylation of effectors linked to Y 515; namely Akt, ERK and p70S6 kinase, but this was not produced by either desipramine or sertraline. All the treatments, when given chronically, caused phosphorylation of the transcription factor, CREB. Acute administration of all the treatments also caused phosphorylation of PLCγ1 but this was not maintained with chronic treatment. Further research is required to determine what role, if any, activation of down-stream targets of Y515 plays in the behavioural effects of VNS.