Ligand binding to transiently accessible sites: Mechanisms for varying apparent binding rates†
Many biological processes are characterized by models where ligand has continuous access to binding sites. Use-dependent processes such as ion channel blockade appear to represent a class of processes where binding site access is transient and is dependent on channel conformation (state). (Alternatively, the existence of a proper conformation for binding might be transient.) We assume binding takes place only with a site that is accessible or in a bindable conformation. For ion channels, channel conformation responds to local chemical or electrical stimulation. The stimulus amplitude determines the mixture of channels with accessible and inaccessible sites. When conformation equilibrium is achieved rapidly in relation to ligand binding, then ligand binding can be considered the result of an apparent binding rate determined by the true binding rate as modified by the fraction of accessible sites. With repetitive stimulation, the continuous access model can be extended to a setting where apparent binding rates repetitively switch in response to the stimulus induced variation in mixture of site states. The resulting theoretical description provides a means for relating equilibrium binding parameters with those obtained under conditions of repetitive stimulation. In particular: (1) binding is piecewise exponential; (2) the envelope of binding measured at each pulse in a train varies exponentially with an apparent rate linearly dependent on the mixture specific rates; and (3) the steady state degree of binding site saturation is linearly dependent on the mixture specific equilibria.
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Cited by (21)
Rate-dependent effects of state-specific sodium channel blockers in cardiac tissue: Insights from idealized models
2023, Journal of Theoretical BiologyA common side effect of pharmaceutical drugs is an increased propensity for cardiac arrhythmias. Many drugs bind to cardiac ion-channels in a state-specific manner, which alters the ionic conductances in complicated ways, making it difficult to identify the mechanisms underlying pro-arrhythmic drug effects. To better understand the fundamental mechanisms underlying the diverse effects of state-dependent sodium (Na+) channel blockers on cellular excitability, we consider two canonical motifs of drug-ion-channel interactions and compare the effects of Na+ channel blockers on the rate-dependence of peak upstroke velocity, conduction velocity, and vulnerable window size. In the literature, both motifs are referred to as “guarded receptor,” but here we distinguish between state-specific binding that does not alter channel gating (referred to here as “guarded receptor”) and state-specific binding that blocks certain gating transitions (“gate immobilization”). For each drug binding motif, we consider drugs that bind to the inactivated state and drugs that bind to the non-inactivated state of the Na+ channel. Exploiting the idealized nature of the canonical binding motifs, we identify the fundamental mechanisms underlying the effects on excitability of the various binding interactions. Specifically, we derive the voltage-dependence of the drug binding time constants and the equilibrium fractions of channels bound to drug, and we then derive a formula that incorporates these time constants and equilibrium fractions to elucidate the fundamental mechanisms. In the case of charged drug, we find that drugs that bind to inactivated channels exhibit greater rate-dependence than drugs that bind to non-inactivated channels. For neutral drugs, the effects of guarded receptor interactions are rate-independent, and we describe a novel mechanism for reverse rate-dependence resulting from neutral drug binding to non-inactivated channels via the gate immobilization motif.
The rewarding properties of NMDA and MK-801 (dizocilpine) as indexed by the conditioned place preference paradigm
1999, Pharmacology Biochemistry and BehaviorN-Methyl-d-aspartate (NMDA) ([R]-2-[Methylamino]succinic acid) is a specific excitatory amino acid. Two experiments were conducted to determine the rewarding properties of this compound using the conditioned place preference paradigm. In the first experiment, 40 male Sprague–Dawley rats received place preference conditioning for a 4 day period. The conditioned place preference apparatus consisted of two chambers with distinct visual and tactile cues, separated by a removable door. On days 2 and 4, rats were systemically administered NMDA (1.0, 15.0, and 30.0 mg/kg) paired with one chamber. On days 3 and 5, rats were systemically administered saline paired with the other chamber. Day 6 was the test day, and the rat was allowed free run of the entire apparatus in a drug-free state. Time spent in each side of the apparatus was computer recorded. NMDA produced a significant increase in the amount of time spent on the side previously paired with drug for 15.0 and 30.0, but not 1.0 mg/kg NMDA. In the second experiment, systemic administration of NMDA (30.0 mg/kg) paired with the noncompetitive NMDA receptor antagonist, MK-801 (0.5 mg/kg), resulted in neither place preference nor place aversion.
Frequency-dependent processes: a model for short-term memory
1992, Journal of Statistical Planning and InferenceMany biological respomses are dependent on the frequency of excitation. Frequency-dependent processes are a class of ligand binding processes where the degree of binding site saturation may be dependent on the cumulative duration of ligand release or receptor access. Here we derive several results that aid in designing experiments in the study of frequency dependent processes. Specifically, we show that for activity dependent memory processes, the steady state degree of binding site saturation is dependent on the frequency of stimulation as well as the uptake and recovery rates of the ligand. Based on this formalism, one could predict the ‘quality’ of short term memory to be directly related to the number of training sessions and inversely related to the interval between training sessions. A simulation of a Hodgkin-Huxley membrane demonstrates frequency dependent increases of action potential duration in response to progressive reduction in potassium conductance, consistent with experimental observations. These results suggest that action potential duration may be a general measure of ‘memory’ associated with a specific neutral pathway.
Lidocaine blockade of continuously and transiently accessible sites in cardiac sodium channels
1991, Journal of Molecular and Cellular CardiologyLidocaine binds to sodium channels in a voltage dependent manner where depolarization enhances block and hyperpolarization relieves block. Voltage-clamp studies demonstrate that there are two components of block: one involving interaction with a binding site that is accessible for the duration of a depolarizing clamp (continuous access or availability) and one involving interaction with a site that is transiently available or accessible during transitions between polarized and depolarized potentials. Here we report results demonstrating two distinct voltage dependencies of blockade. The voltage dependence of block of the transiently accessible site is similar to that of channel activation and exhibits a maximal binding rate of 1.37 × 106/m/s and an unbinding rate of 39.5/s at −30 mV. Blockade of the sustained site exhibits a voltage dependence similar to inactivation with a maximal binding rate of 3.59 × 104/m/s and an unbinding rate of 0.678/s at −30 mV. Recovery from blockade acquired by either process is voltage dependent and proportional to exp(−0.037Vm). Drug induced shifts in channel availability and transient site block are accurately predicted from kinetic rates estimated from frequency dependent protocols.
A radiohistochemical measure of [<sup>3</sup>H]TCP binding to the activated NMDA-receptor-gated ion channel in rat brain
1990, Brain ResearchTheN-methyl-d-aspartate (NMDA) subtype of excitatory amino acid receptor is linked to an ion channel that is blocked by the phencyclidine analogN-(1-[thienyl]cyclohexyl)piperidine (TCP). Previous studies have shown that NMDA and glycine act together to increase the access of [3H]TCP to its binding site, presumably by increasing channel opening; NMDA/glycine-enhanced [3H]TCP binding performed under non-equilibrium conditions thereby serves as a dynamic molecular marker of channel activation. In this study we tested whether NMDA and glycine regulate [3H]TCP binding in slide-mounted brain sections. Striking activation of the NMDA ion channel was observed in neo- and allocortex; dentate gyrus and strata radiatum and oriens of CA1 and CA3 of hippocampal formation; and certain amygdaloid nuclei (basomedial, basolateral, and cortical). Other nuclei of the amygdala (central, posterolateral, and medial), basal ganglia, and numerous regions within the diencephalon, brainstem and cerebellum showed relatively little activation of the NMDA ion channel. Our demonstration of enriched NMDA/glycine-stimulated [3H]TCP binding in stratum radiatum of hippocampal region CA1 but not in cerebellar granule cell layer correlates with electrophysiologic studies that showed NMDA channel ion flux in CA1 but not in cerebellar granule cell layer in adult rats. These data demonstrate that NMDA and glycine regulation of [3H]TCP binding can be quantified with a radiohistochemical method to provide a regional measure of the activated NMDA-receptor-gated ion channel. This technique is a powerful tool for functional analysis of the NMDA receptor/channel complex in both physiologic and pathologic states.
Allosteric Modulation of N-Methyl-D-Aspartate Receptors
1990, Advances in PharmacologyThis chapter discusses the allosteric modulation of N–methyl-D-aspartute (NMDA) receptors. It focuses on the NMDA-preferring glutamate receptor. NMDA receptors are involved in a wide range of physiological and pathophysiological events, and this has resulted in a great deal of interest in the development of tools to probe and manipulate NMDA receptor function. Moreover, many studies have demonstrated that the activity of the NMDA receptor can be modulated by a wide range of endogenous agents. The purpose of this chapter, therefore, is to examine the actions of the known modulators of the receptor in terms of their binding sites, and to try to determine how these binding sites interact with each other. By understanding the interactions between the known modulators of this receptor it may be possible to comprehend better how the receptor may operate in different circumstances, and it may also be possible to target more accurately therapeutic agents to interrupt specific NMDA receptor-mediated events.
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This work was supported in part by the following grants: RR01693, HL32994 and HL32708 from the National Institutes of Health, and a grant from the American Heart Association.