Pharmacological profile of the alpha4beta2 nicotinic acetylcholine receptor partial agonist varenicline, an effective smoking cessation aid

The preclinical pharmacology of the alpha4beta2 nicotinic acetylcholine receptor (nAChR) partial agonist varenicline, a novel smoking cessation agent is described. Varenicline binds with subnanomolar affinity only to alpha4beta2 nAChRs and in vitro functional patch clamp studies in HEK cells expressing nAChRs show that varenicline is a partial agonist with 45% of nicotine's maximal efficacy at alpha4beta2 nAChRs. In neurochemical models varenicline has significantly lower (40-60%) efficacy than nicotine in stimulating [(3)H]-dopamine release from rat brain slices in vitro and in increasing dopamine release from rat nucleus accumbens in vivo, while it is more potent than nicotine. In addition, when combined with nicotine, varenicline effectively attenuates the nicotine-induced dopamine release to the level of the effect of varenicline alone, consistent with partial agonism. Finally, varenicline reduces nicotine self-administration in rats and supports lower self-administration break points than nicotine. These data suggest that varenicline can reproduce to some extent the subjective effects of smoking by partially activating alpha4beta2 nAChRs, while preventing full activation of these receptors by nicotine. Based on these findings, varenicline was advanced into clinical development and recently shown to be an effective and safe aid for smoking cessation treatment.

(3R,4S)-3-[4-(4-fluorophenyl)-4-hydroxypiperidin-1-yl]chroman-4,7-diol: a conformationally restricted analogue of the NR2B subtype-selective NMDA antagonist (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)- 1-propanol

(1S,2S)-1-(4-Hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP-101,606, 1) is a recently described antagonist of N-methyl-D-aspartate (NMDA) receptors containing the NR2B subunit. In the present study, the optimal orientation of compounds of this structural type for their receptor was explored. Tethering of the pendent methyl group of 1 to the phenolic aromatic ring via an oxygen atom prevents rotation about the central portion of the molecule. Several of the new chromanol compounds have high affinity for the racemic [3H]CP-101,606 binding site on the NMDA receptor and protect against glutamate toxicity in cultured hippocampal neurons. The new ring caused a change in the stereochemical preference of the receptor-cis (erythro) compounds had better affinity for the receptor than the trans isomers. Computational studies suggest that steric interactions between the pendent methyl group and the phenol ring in the acyclic series determine which structures can best fit the receptor. The chromanol analogue, (3R,4S)-3-[4-(4-fluorophenyl)-4-hydroxypiperidin-1- yl]chroman-4,7-diol (12a, CP-283,097), was found to possess potency and selectivity comparable to CP-101,606. Thus 12a is a new tool to explore the function of the NR2B-containing NMDA receptors.

Barium permeability of neuronal nicotinic receptor alpha 7 expressed in Xenopus oocytes

The rat alpha 7 neuronal nicotinic acetylcholine receptor was expressed and studied in Xenopus oocytes. The magnitude and reversal potential of instantaneous whole cell currents were examined in solutions containing varying concentrations of either calcium or barium, and in the presence or absence of the intracellular calcium chelator BAPTA. In external barium, application of nicotine elicits an inwardly rectifying response; in calcium the response is larger and has a linear IV relation. Pretreatment of oocytes with BAPTA-AM could not prevent activation of calcium-dependent chloride channels in external Ringer containing calcium. Using an extended GHK equation, the permeability ratio PBa/PNa of the alpha 7 receptor was determined to be about 17. Our results suggest that alpha 7 nicotinic receptors are highly permeable to divalent cations.

Neuronal nicotinic acetylcholine receptor currents in phaeochromocytoma (PC12) cells: dual mechanisms of rectification

1. Neuronal nicotinic acetylcholine receptor (nAChR) currents in PC12 cells were studied using single-channel and whole-cell gigaohm seal voltage-clamp techniques. Nicotinic AChR agonists were applied using external pipettes. 2. The single-channel conductance of neuronal nAChRs in outside-out patches under Mg(2+)-free ionic conditions was 48 pS. In the absence of internal Mg2+ single-channel currents (outside-out patches) did not rectify. 3. Whole-cell nAChR currents recorded with normal internal solution (lacking Mg2+ chelators as described below) were strongly inwardly rectifying. Current vs. voltage relations showed a sharp inflexion near 0 mV, and outward current was never observed. 4. Extensive dialysis with internal solution containing EDTA and Na2-ATP, chelators of intracellular Mg2+ (Mgi2+), relieved rectification of instantaneous nAChR currents during voltage jumps from negative potentials. The instantaneous I-V relation became linear with voltage, in agreement with the expectation from single-channel measurements made under similar ionic conditions. 5. Agonist-induced currents recorded under Mgi(2+)-free conditions relaxed towards zero current during depolarizing voltage steps. Rectification of ACh-induced currents at the steady state could be described by a Boltzmann relation, with one-half of channels available at +3.4 mV and an effective gating charge of -0.97. Channel availability was approximately 95% at the resting potential. Opening and closing relaxations under Mgi(2+)-free conditions could be fitted by single exponential functions whose time constants were weakly voltage dependent. 6. A model of rectification was constructed incorporating two voltage-dependent processes: block by Mgi2+ and intrinsic channel gating. The I-V relations predicted by this model for both normal and Mgi(2+)-free conditions were in good agreement with the experimental data. We suggest that rectification of nAChR currents in these cells is due to concurrent activity of these two voltage-dependent processes. The relative contributions of these two mechanisms are frequency dependent, with block by Mgi2+ dominant for fast events and intrinsic channel gating more important at the steady state.

Calcium permeability of neuronal nicotinic acetylcholine receptor channels in PC12 cells

The conductance properties of neuronal nicotinic acetylcholine receptors (neuronal nAChR channels) in PC12 cells were studied using single channel and whole cell gigaohm seal voltage clamp techniques. Acetylcholine receptor agonists were applied using external pipettes. Neuronal nAChR channels were selective for monovalent cations, and were impermeable to anions. Based on measurements of shifts in reversal potential with changes in external Na and K ion concentrations and evaluation of the GHK constant field relation, PK/PNa was 1.45 (ignoring Ca permeability). PCa/PK was 1.75 based on the shift in reversal potential seen when raising the external Ca concentration from 2 to 50 mM (holding the external Na ion concentration constant), and determining the best fit to the extended GHK constant field relation using ionic activities. This value is larger than that determined for the muscle-type nAChR channel. Evaluation of the extended GHK current equations using these permeability coefficients indicated that at subthreshold voltages approximately 5% of total current through neuronal nAChR channels will be carried by Ca ions. Thus, these channels may be important mediators of activity-dependent Ca influx in the nervous system.

A quantitative description of excitatory amino acid neurotransmitter responses on cultured embryonic Xenopus spinal neurons

We have performed a quantitative analysis of excitatory amino acid neurotransmitter receptors on cultured embryonic Xenopus spinal neurons using the whole-cell patch-clamp technique. Neuroblasts and underlying mesodermal cells isolated from spinal regions of neural plate-stage embryos were placed into dissociated cell culture, and responses were studied soon after the appearance of neurites on embryonic neurons. Glutamate (Glu) receptors were separated into two general classes based on responses to the characteristic agonists quisqualate (Quis), kainate (Ka) and N-methyl-D-aspartate (NMDA); these were NMDA receptors (those activated by NMDA) and non-NMDA receptors (those activated by Ka and Quis). Half-maximal responses to Glu and other agonists on NMDA and non-NMDA receptors were determined from Hill analysis of dose response relations. The order of sensitivities observed was: GluNMDA (ED50 = 5.1 microM) greater than Glunon-NMDA (ED50 = 28 microM), and for Glu receptor agonists, Quis (ED50 = 1.5 microM) greater than NMDA (ED50 = 41 microM) greater than Ka (ED50 = 58 microM). The order of response amplitudes recorded at concentrations near the appropriate ED50s was GluNMDA greater than Glunon-NMDA, and Ka greater than NMDA greater than Quis. A 10-fold decrease in external [Na+] shifted the reversal potentials for Glunon-NMDA, Ka, and Quis to more negative voltages. Increasing external [Ca2+] shifted the reversal potential for NMDA responses to more positive potentials, an observation consistent with Ca2+ permeation of the embryonic NMDA-activated channel. NMDA-evoked currents could not be recorded in nominally glycine (Gly)-free media. Addition of Gly to external solutions potentiated NMDA responses (ED50 = 644 nM). NMDA responses were blocked by DL-2-amino-5-phosphonovaleric acid (APV; ED50 = 1.9 microM) and by Mg2+ at negative potentials. In their sensitivities to agonists and antagonists, and ionic dependences, amino acid neurotransmitter responses on embryonic Xenopus neurons closely resembled those previously observed for mature Xenopus and mammalian central neurons. The GluNMDA receptors present on these immature neurons were sufficiently sensitive to be activated by endogenous concentrations of extracellular Glu, suggesting a possible role for receptor activation in modulating early neural development.

NMDA receptor activation by residual glutamate in glutamine preparations: a cautionary note regarding weak NMDA receptor agonists

During an investigation of excitatory amino acids on cultured embryonic Xenopus neurons, we observed that commercial preparations of glutamine had weak agonist activity on NMDA-type glutamate receptors. Threshold responses were observed at 100 microM glutamine, and the dose-response relation did not show inflection or saturation at concentrations of up to 10 mM. However, NMDA receptor activation induced by glutamine probably represented activity of residual glutamate because: (a) recrystallization of glutamine reduced residual glutamate levels (measured by HPLC analysis) and NMDA receptor activation by comparable amounts; and (b) glutamate at concentrations close to those predicted to be present in glutamine preparations elicited currents of similar amplitudes. Our data indicate that residual glutamate at levels of less than 0.05% are sufficient to confound studies of weak NMDA receptor agonists.

Charybdotoxin blocks voltage-gated K+ channels in human and murine T lymphocytes

A variety of scorpion venoms and purified toxins were tested for effects on ion channels in human T lymphocytes, a human T leukemia cell line (Jurkat), and murine thymocytes, using the whole-cell patch-clamp method. Nanomolar concentrations of charbdotoxin (CTX), a purified peptide component of Leiurus quinquestriatus venom known to block Ca2+-activated K+ channels from muscle, blocked "type n" voltage-gated K+ channels in human T lymphoid cells. The Na+ channels occasionally expressed in these cells were unaffected by the toxin. From the time course of development and removal of K+ channel block we determined the rates of CTX binding and unbinding. CTX blocks K+ channels in Jurkat cells with a Kd value between 0.5 and 1.5 nM. Of the three types of voltage-gated K+ channels present in murine thymocytes, types n and n' are blocked by CTX at nanomolar concentrations. The third variety of K+ channels, "type l," is unaffected by CTX. Noxiustoxin (NTX), a purified toxin from Centruroides noxius known to block Ca2+-activated K+ channels, also blocked type n K+ channels with a high degree of potency (Kd = 0.2 nM). In addition, several types of crude scorpion venoms from the genera Androctonus, Buthus, Centruroides, and Pandinus blocked type n channels. We conclude that CTX and NTX are not specific for Ca2+ activated K+ channels and that purified scorpion toxins will provide useful probes of voltage-gated K+ channels in T lymphocytes. The existence of high-affinity sites for scorpion toxin binding may help to classify structurally related K+ channels and provide a useful tool for their biochemical purification.