Antidepressant-and cocaine-sensitive serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) dictate clearance of extracellular 5-HT after release. To explore protein kinase C-mediated SERT regulation, we generated a stable human SERT (hSERT)-expressing cell line (293-hSERT) and evaluated modulation of 5-HT activity via studies of 5-HT flux, hSERTmediated currents under voltage clamp, and surface distribution of SERT protein. 293-hSERT cells exhibit saturable, highaffinity, and antidepressant-sensitive 5-HT uptake as well as hSERT-dependent whole-cell currents. In these cells, the protein kinase C activator ␤-PMA caused a time-dependent reduction in 5-HT uptake capacity (V max ) after acute application and a reduction in SERT-mediated currents. Effects of ␤-PMA were mimicked by the phorbol ester ␤-PDBu, were not observed with the inactive ␣-isomers, and could be blocked by treatment of cells with the protein kinase C inhibitor staurosporine. Biotinylation/immunoblot analyses showed that activity reductions are paralleled by a staurosporine-sensitive loss of surface SERT protein. These data indicate that altered surface abundance, rather than reduced catalytic transport efficiency, mediates acute PKC-dependent modulation of 5-HT uptake.

1. HeLa cells were infected with recombinant vaccinia virus containing the T7 RNA polymerase gene and transfected with the cDNA for a rat GABA transporter, GAT1, cloned downstream of a T7 RNA polymerase promoter. Six to sixteen hours after transfection, whole-cell recording with a voltage ramp in the range -90 to 50 mV revealed GABAinduced currents (approximately -100 pA at -60 mV in 100 uM GABA, 16 h after transfection at room temperature). No GABA-induced currents were observed in parental HeLa cells or in mock-transfected cells. 2. GABA-induced currents were suppressed by extracellular perfusion with GABA-free solutions or addition of GATI inhibitors SKF89976-A or SKF100330-A. At fixed voltage the GABA dependence of the inward current fitted the Michaelis-Menten equation with aHill coefficient, n, near unity and an equilibrium constant, Km, near 3 /tM. The Na+ dependence of the inward currents fitted the Michaelis-Menten equation with n t 2 and Km ; 10 mM. The constants n and Km for GABA and Na+ were independent of voltage in the range -90 to -30 mV. 3. GABA-induced currents reverse direction in the range 5-10 mV. The implication of this result is that GAT1 can mediate electrogenic (electrophoretic) influx or efflux of GABA depending on the membrane voltage. The presence of an outward current in our experiments is consistent with radioactive-labelled flux data from resealed vesicle studies. However, it is inconsistent with frog oocyte expression experiments using the same clone. In oocytes, GAT1 generates no outward current in a similar voltage range. Smaller intracellular volume or higher turnover rates in the mammalian expression system may explain the outward currents. 4. External GABA induces inward current, and internal GABA induces outward current.However, in cells initially devoid of internal GABA, external GABA can also facilitate an outward current. This GATi -mediated outward current occurs only after applying negative potentials to the cell. These data are consistent with the concept that negative potentials drive GABA and Nae into the cell, which then leads to electrogenic efflux through GATI at positive voltages. 5. Assuming coupled transport, we estimate the number of transporters, N, times the turnover rate, r, to be Nr t 109 s-' under nominal conditions ( V = -60 mV, 30 /M GABA, 130 mm Nae and room temperature). This indicates either very high levels of expression (0104 um-2), assuming published turnover rates (-1 0 s-1), or turnover rates that are significantly greater than previously reported. As an alternative, a channel may exist in the GAT1 protein that is gated by GABA and Nae and blocked by GAT1 antagonists. The channel mode of conduction would exist in addition to the coupled, fixed-stoichiometry transporter mode of conduction.

The measurement of receptor occupancy (RO) using positron emission tomography (PET) has been instrumental in guiding discovery and development of CNS directed therapeutics. We and others have investigated muscarinic acetylcholine receptor 4 (M4) positive allosteric modulators (PAMs) for the treatment of symptoms associated with neuropsychiatric disorders. In this article, we describe the synthesis, in vitro, and in vivo characterization of a series of central pyridinerelated M4 PAMs that can be conveniently radiolabeled with carbon-11 as PET tracers for the in vivo imaging of an allosteric binding site of the M4 receptor. We first demonstrated its feasibility by mapping the receptor distribution in mouse brain and confirming that a lead molecule 1 binds selectively to the receptor only in the presence of the orthosteric agonist carbachol. Through a competitive binding affinity assay and a number of physiochemical properties filters, several related compounds were identified as candidates for in vivo evaluation. These candidates were then radiolabeled with 11 C and studied in vivo in rhesus monkeys. This research eventually led to the discovery of the clinical radiotracer candidate [ 11 C]MK-6884.

Herein we describe the discovery and optimization of a new series of 2,3‐disubstituted and 2,3,6‐trisubstituted muscarinic acetylcholine receptor 4 (M4) positive allosteric modulators (PAMs). Iterative libraries enabled rapid exploration of one‐dimensional structure–activity relationships (SAR) and identification of potency‐enhancing heterocycle and N‐alkyl pyrazole substituents. Further optimization led to identification of the potent, receptor‐subtype‐selective, brain‐penetrant tool compound 24 (7‐[3‐[1‐[(1‐fluorocyclopentyl)methyl]pyrazol‐4‐yl]‐6‐methyl‐2‐pyridyl]‐3‐methoxycinnoline). It is efficacious in preclinical assays that are predictive of antipsychotic effects, producing dose‐dependent reversal of amphetamine‐induced hyperlocomotion in rats and mice, but not in M4 knockout mice. Cholinergic‐related adverse effects observed in rats treated with 24 at unbound plasma concentrations more than 3‐fold higher than an efficacious dose in the hyperlocomotion assay were fewer and less severe than those observed in rats treated with the nonselective M4 agonist xanomeline, suggesting a receptor‐subtype‐selective PAM has the potential for an improved safety profile.

The detection of HIV-1 proviral DNA in children born to seropositive mothers was studied using the polymerase chain reaction with either a radioactive electrophoretic method or a noval procedure that employs colorimetric microwell visualization. Peripheral blood mononuclear cell lysates from 18 HIV-1 infected children and 28 uninfected subjects were assayed for a 142 bp fragment of DNA from the gag region of HIV-1 using the primer pair SK145-431. Detection of amplified DNA was carried out by hybridization with a radiolabeled SK102 probe, or with a tagged SK102 probe permitting colorimetric detection. The radioactive detection procedure demonstrated 100% specificity and correlated with the serological results. The assay was more sensitive than the p24 antigen test, but two false negative results were obtained. One was from a sample taken at 2 weeks, an age at which undetectable provirus levels were reported in almost all HIV-1 infected newborns. The second was probably due to a low copy number of proviral DNA, as positive results were obtained in all other (6) samples from this child. Comparative analysis in a limited number of specimens of radioactive and colorimetric detection following PCR revealed 100% specificity and comparable sensitivity with 4 discordant results. The results show that PCR is the best method for early diagnosis of HIV-1 infection in pediatric subjects. The study also demonstrated the value of a colorimetric detection method for PCR products. This colorimetric microwell plate procedure may prove a useful technique in routine diagnosis of HIV-1 infection in children.