Background Serotonin 1A receptors (5-HT1A) are implicated in major depressive disorder (MDD). We previously reported higher 5-HT1A binding potential (BPF=Bavail/KD) in antidepressant naïve MDD subjects compared to controls while other studies report lower BPND(BPND=fNDBavail/KD). Discrepancies can be related to differences in study population or methodology. We sought to replicate our findings in a novel cohort and determine whether choice of reference region and outcome measure could explain discrepancies. Methods Nine new controls and 22 new not recently medicated (4 years, NRM) MDD subjects underwent positron emission tomography with [11C]WAY100635. BPF and BPND were determined using a metabolite and free fraction corrected arterial input function. BPND was also determined using cerebellar gray matter (CGM) and cerebellar white matter (CWM) reference regions as input functions. Results BPF is higher in the new NRM cohort (p=0.037) compared to new controls, which is comparable to the effect in the previous cohort (p=0.04). We combined the cohorts to examine the effects of the reference region and outcome measure. In the combined cohort, BPF is higher in the NRM compared to controls (p=0.0001). Neither BPND using CWM (p=0.86) nor VT (p=0.374) differs between groups. When CGM is used, the NRM group has lower 5HT1A BPND compared with controls (p=0.03). CGM VT is higher in NRM compared to controls (p=0.007). Conclusions Choice of reference region and outcome measure can produce different 5-HT1A findings in MDD. Higher 5-HT1A BPF in MDD was found with the method with fewest assumptions about nonspecific binding and a reference region without receptors.

Objective Pharmacotherapy to rapidly relieve suicidal ideation in depression may reduce suicide risk. Rapid reduction in suicidal thoughts after ketamine treatment has mostly been studied in patients with low levels of suicidal ideation. Method This randomized clinical trial tested the effect of adjunctive sub-anesthetic intravenous ketamine on clinically significant suicidal ideation in major depressive disorder (MDD). Adults (N=80) with current MDD and score ≥4 on the Scale for Suicidal Ideation (SSI), of whom 54% (N=43) were taking antidepressant medication, were randomized to ketamine or midazolam infusion. The primary outcome was Day 1 SSI score (24 hours post-infusion). Other outcomes included global depression and adverse effects. Results Reduction of SSI score was 4.96 points greater after ketamine compared with midazolam at Day 1 (95% confidence interval (CI)=2.33 to 7.59; p=0.0003; Cohen’s d=0.75). Proportion of responders (≥50% reduction in SSI) at Day 1 was 55% after ketamine and 30% after midazolam (OR=2.85 (95% CI=1.14 to 7.15); p=0.0237; NNT=4.00). Improvement in the Profile of Mood States (POMS) depression subscale was greater at Day 1 compared with midazolam treatment (Estimate=7.65 (95% CI=1.36 to 13.94), df=75, t=2.42, p=0.0178), and this effect mediated 33.6% of ketamine’s effect on SSI score. Side effects were short-lived. Benefit was sustained for up to six weeks with clinical pharmacotherapy. Conclusions Adjunctive ketamine demonstrated greater reduction of clinically significant suicidal ideation in depressed patients within 24 hours compared to midazolam, partially independent of antidepressant effect. Research is needed to understand ketamine’s mechanism of action and to develop safe, longer-term treatment.

Different brain regions correlate with discrete symptom components that compose the overall syndrome of major depression. Future studies should extend knowledge about specific regional networks by identifying responsible neurotransmitters related to specific psychopathologic components of mood disorders.

The serotonin (5-hydroxytryptamine, or 5-HT) type 1A receptor (5-HT1AR) is implicated in the pathophysiology of numerous neuropsychiatric disorders. We have published the initial evaluation and reproducibility in vivo of [O-methyl-11C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5 (2H,4H)dione (11C-CUMI-101), a novel 5-HT1A agonist radiotracer, in Papio anubis. Here, we report the optimal modeling parameters of 11C-CUMI-101 for human PET studies. Methods PET scans were obtained for 7 adult human volunteers. 11C-CUMI-101 was injected as an intravenous bolus, and emission data were collected for 120 min in 3-dimensional mode. We evaluated 10 different models using metabolite-corrected arterial input functions or reference region approaches and several outcome measures. Results When using binding potential (BPF = Bavail/KD [total available receptor concentration divided by the equilibrium dissociation constant]) as the outcome measure, the likelihood estimation in the graphical analysis (LEGA) model performed slightly better than the other methods evaluated at full scan duration. The average test–retest percentage difference was 9.90% ± 5.60%. When using BPND (BPND = fnd × Bavail/KD; BPND equals the product of BPF and fnd [free fraction in the nondisplaceable compartment]), the simplified reference tissue method (SRTM) achieved the lowest percentage difference and smallest bias when compared with nondisplaceable binding potential obtained from LEGA using the metabolite-corrected plasma input function (r2 = 0.99; slope = 0.92). The time–stability analysis indicates that a 120-min scan is sufficient for the stable estimation of outcome measures. Voxel results were comparable to region-of-interest–based analysis, with higher spatial resolution. Conclusion On the basis of its measurable and stable free fraction, high affinity and selectivity, good blood–brain barrier permeability, and plasma and brain kinetics, 11C-CUMI-101 is suitable for the imaging of high-affinity 5-HT1A binding in humans.

The NMDA receptor antagonist ketamine can improve major depressive disorder (MDD) within hours. To evaluate the putative role of glutamatergic and GABAergic systems in ketamine’s antidepressant action, medial prefrontal cortical (mPFC) levels of glutamate + glutamine (Glx) and γ-aminobutyric acid (GABA) were measured before, during, and after ketamine administration using proton magnetic resonance spectroscopy. Ketamine (0.5 mg/kg i.v.) was administered to eleven depressed patients with MDD. Glx and GABA mPFC responses were measured as ratios relative to unsuppressed voxel tissue water (W) successfully in 8/11 patients. Ten of 11 patients remitted (50% reduction in 24-item Hamilton Depression Rating Scale and total ≤ 10) within 230 minutes of commencing ketamine. mPFC Glx/W and GABA/W peaked at 37.8%±7.5% and 38.0%±9.1% above baseline in ~26 minutes. Mean areas under the curve (AUC) for Glx/W (p = 0.025) and GABA/W (p = 0.005) increased and correlated (r = 0.796; p=0.018). Clinical improvement correlated with 90-minute norketamine concentration (df=6, r=−0.78, p=0.023), but no other measures. Rapid increases in Glx and GABA in MDD following ketamine administration support the postulated antidepressant role of glutamate and for the first time raises the question of GABA’s role in the antidepressant action of ketamine. These data support the hypothesis1 that ketamine administration may cause an initial increase in glutamate that potentially activates mammalian target of rapamycin (mTOR) pathway via AMPA receptors, since ketamine blocks NMDA receptors. The role of the contemporaneous surge in GABA remains to be determined.2

The study demonstrated feasibility. Suicidal thoughts were lower after ketamine than after midazolam at a trend level of significance, likely due to the small pilot sample. Memory improvement and BDNF are promising biomarkers. Replication is needed in an adequately powered full-scale trial.

Background Chronic selective serotonin reuptake inhibitor (SSRI) administration to rodents desensitizes or downregulates raphe 5-HT1A autoreceptors. We previously found elevated 5-HT1A binding in antidepressant-naïve and not recently-medicated major depressive disorder, and now report the effect of SSRI treatment on 5-HT1A autoreceptors in depressed patients. Methods 5-HT1A binding (BPF) was quantified in medication-free subjects using PET with [11C]-WAY-100635 before and after treatment of major depressive disorder (MDD) with an SSRI for 5 to 9 weeks (mean 47±8 days). 19 subjects without recent history of antidepressant pharmacotherapy completed both [11C]WAY-100635 PET scans with a metabolite-corrected arterial input function and depression severity was rated before and after the treatment course. Results 5-HT1A autoreceptor BPF in the raphe was reduced 18% on SSRI treatment (df=1,18; F=5.12; p=0.036). However, the degree of reduction in 5-HT1A autoreceptor BPF was unrelated to improvement in depression (df=1,16; F=1.27; p=0.276). Conclusion Downregulation of 5-HT1A autoreceptor binding by SSRI treatment of major depression is consistent with animal studies. This may be a necessary but insufficient requirement for clinical response to SSRIs. A PET agonist ligand that binds selectively to the high affinity conformation of this receptor can determine whether SSRIs also cause desensitization of the autoreceptor as reported by some rodent studies, and whether that effect may be related to clinical response.

Purpose Metabotropic glutamate receptor subtype 5 (mGluR5) dysfunction has been implicated in several disorders. [11C]ABP688, a Positron Emission Tomography (PET) ligand targeting mGluR5, could be a valuable tool in the development of novel therapeutics for these disorders by establishing in vivo drug occupancy. Due to safety concerns in humans, these studies may be performed in nonhuman primates. Therefore, in vivo characterization of [11C]ABP688 in nonhuman primates is essential. Methods Test/retest studies were performed in Papio anubis to compare modeling approaches and determine the optimal reference region. The mGluR5-specific antagonist 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) was then used in test/block studies, in which ligand binding was measured before and after MTEP administration. Test/block data were analyzed both by calculating changes in binding and using a graphical approach, which allowed estimation of both MTEP occupancy and nonspecific binding. Results Test/retest results, which have not been previously reported for [11C]ABP688, indicated that [11C]ABP688 variability is low using an unconstrained two-tissue compartment model. The most appropriate, though not ideal, reference region was found to be the grey matter of the cerebellum. Using these optimal modeling techniques on the test/block data, ~90% occupancy was estimated by the graphical approach. Conclusions These studies are the first to demonstrate the specificity of [11C]ABP688 for mGluR5 with in vivo PET in nonhuman primates. The results indicate that, in baboons, occupancy of mGluR5 is detectable by in vivo PET, a useful finding for proceeding to human studies, or performing further baboon studies, quantifying the in vivo occupancy of novel therapeutics targeting mGluR5.