Zinc Potentiates Neuronal Nicotinic Receptors by Increasing Burst Duration

Abstract

Micromolar zinc potentiates neuronal nicotinic acetylcholine receptors (nAChRs) in a subtype-dependent manner. Zinc potentiates receptor function even at saturating agonist concentrations, without altering the receptor desensitization rate. Potentiation could occur through an increase in the number of available receptors, an increase in single-channel current amplitude, or an increase in single-channel open probability. To distinguish among these possibilities, we examined rat neuronal nAChRs expressed in Xenopus oocytes. Blockade of a large fraction of ACh activated alpha4beta4 or alpha4beta2 receptors by the open channel blocker hexamethonium failed to change the extent of potentiation by zinc, suggesting that zinc does not change the number of available receptors. The single-channel amplitudes of ACh (1 microM) activated alpha4beta4 receptors in outside-out patches were similar in the absence and the presence of 100 microM zinc (3.0 +/- 0.1 and 2.9 +/- 0.1 pA, respectively). To determine the effect of zinc on single-channel open probability, we examined alpha4beta4 receptors in cell-attached patches. The open probability at 100 nM ACh (0.011 +/- 0.002) was increased 4.5-fold by 100 microM zinc (0.050 +/- 0.008), accounting for most of the potentiation observed at the whole cell level. The increase in open probability was due to an increase in burst duration, which increased from 207 +/- 38 ms in the absence of zinc to 830 +/- 189 ms in the presence of zinc. Our results suggest that potentiation of neuronal nAChRs by zinc is due to a stabilization of the bursting states of the receptor.

Morantel Allosterically Enhances Channel Gating of Neuronal Nicotinic Acetylcholine α3β2 Receptors

“…All told, experiments on individual patches lasted 30 Ϫ 45 min, with no detectable rundown. This is in contrast to some reports of nAChR single-channel recordings (e.g., Papke and Heinemann, 1991;Hsiao et al, 2008) and may be due to the low ACh concentrations used and the low inherent activity of ␣3␤2 receptors.…”

“…We previously hypothesized that levamisole potentiates nAChRs by binding to alternate, noncanonical subunit interfaces based on the conclusion that it does not compete with agonist and on the logical parallel to the benzodiazepine modulators of GABA A receptors (Levandoski et al, 2003). This idea is further supported by the demonstration that zinc potentiates nAChRs by binding to alternate subunit interfaces and increasing burst duration (Hsiao et al, 2006(Hsiao et al, , 2008, and indirectly by the demonstration that noncompetitive inhibitors bind to subunit interfaces in ACh-binding protein (Hansen and Taylor, 2007). We are pursuing the hypothesis that morantel binds to the noncanonical ␣/␤ interface of ␣3␤2 receptors using chemical modification of cysteines substituted in these regions.…”

Number and Locations of Agonist Binding Sites Required to Activate Homomeric Cys-Loop Receptors

“…Thus, a third agonist binding site allows positive modulation by the same neurotransmitter and resembles allosteric modulation observed in heteromeric receptors. In heteromeric receptors, the third site allows potentiation by heterotropic regulatory ligands, such as benzodiazepines for GABA A receptors (Cromer et al, 2002) or zinc for neuronal nicotinic receptors (Hsiao et al, 2008). Moreover, the arrangement of GABA and benzodiazepine binding sites in GABA A receptors (Cromer et al, 2002) is analogous to that shown here for functionally relevant binding sites in ␣7-5HT 3A receptors.…”

“…Homomeric receptors are the simplest structural class of Cys-loop receptors and contain five identical agonist binding sites. Most present-day Cys-loop receptors are heteromeric and contain two agonist binding sites, whereas a third site may bind heterotropic ligands (Cromer et al, 2002;Hsiao et al, 2008).…”

Targeting α4β2 Nicotinic Acetylcholine Receptors in Central Nervous System Disorders: Perspectives on Positive Allosteric Modulation as a Therapeutic Approach

“…This point is acknowledged by the authors of this review; however, for readability in this review, NS9283 will be termed a PAM. Ubiquitous ion in the central nervous system [90,91] identified a4b2 PAMs, it is currently not clear how modulation of desensitization versus deactivation will translate into behavioural effects, but it is likely that differential modulation of receptor kinetics will impact central synaptic transmission and thereby their in vivo efficacy. This is further discussed below.…”