High affinity and low affinity heteromeric nicotinic acetylcholine receptors at central synapses

Abstract

Most neuronal heteromeric nicotinic receptors seem able to adopt two different stochiometries depending on the ratio of α and β subunits. In recombinant receptors these two stoichiometries have been associated with different affinities to ACh, but it is not known which stoichiometry is present at nicotinic synapses in the nervous system. One possible clue to this identification is the speed of decay of the synaptic currents. In many ionotropic receptors this speed has been linked to the dissociation rate of the transmitter, which is itself related to its affinity. On this basis we propose that, at the synapse between motoneuron and Renshaw cells, the heteromeric nicotinic receptors are mostly low affinity receptors and suggest that, in contrast, the very slow decay of some synaptic currents recorded in other parts of the brain signs the presence of high affinity receptors rather than volume transmission.

Figure 1. The heteromeric nicotinic current at the MN–RC synapse

A, schematic representation of the experimental conditions. The ventral rootlet that exits from the spinal cord slice is stimulated by the mean of a suction electrode. Five shocks at an internal frequency of 100 Hz or a single shock are used to elicit antidromic action potentials in the rootlet. The RC is recorded with a patch-electrode and its identification is confirmed by the presence of a monosynaptic inward current in response to the antidromic stimulation. The bath application of antagonists of glutamate receptors (NBQX, 2 μm, and APV, 50 μm) and of the homomeric α7 nicotinic receptor antagonist methyllycaconitine (MLA, 10 nm) allows the isolation of the heteromeric nicotinic synaptic current. After a repetitive stimulation, the tail of the heteromeric current is well described by a bi-exponential decay. In control conditions (not shown), the amplitude of the slow component (A_s) was 55% of that of the fast component (A_f) and the time constants were τ_f = 18.8 ms and τ_s = 87 ms. Blocking the electrical coupling between the RCs with meclofenamic acid (MFA, 50 μm) accelerated the decay of the current by removing the current spreading from neighbouring cells. In presence of MFA the amplitude of the slow component was 41% of that of the fast component and the time constants were τ_f = 12 ms and τ_s = 74 ms. B, synaptic current in response to a single antidromic stimulation at low intensity (same cell, in presence of MFA). The two components are well resolved with time constants τ_f = 11 ms and τ_s = 66.6 ms. The ratio of their amplitudes (A_s and A_f) is down to 14%. C, a spontaneous EPSC, with the same characteristics as miniature events (same cell, in presence of MFA). The slow component is hardly resolved and the decay of the current is well described with a single exponential whose time constant τ = 10.7 ms matches τ_f of the synaptic current.

Figure 2. Distribution of the postsynaptic heteromeric nAChRs at the MN–RC synapse

A, diagram of the two subunit stoichiometries in a ‘generic’ nAChR assuming that what has been proposed for α4β2 subunits can be extended to assemblies of other α and β subunits. LS receptors differ from HS receptors by the presence of an additional binding site. B, hypothesis 1: LS receptors account for the fast current, HS receptors (sub-synaptic or extra-synaptic) account for the slow current. C, hypothesis 2: only LS receptors are present, and account for both components of the EPSC.