Association between the low threshold calcium spike and activation of NMDA receptors in guinea-pig substantia nigra pars compacta neurons

Lateral Habenular Burst Firing as a Target of the Rapid Antidepressant Effects of Ketamine

The revolutionary discovery of the rapid antidepressant ketamine has been a milestone in psychiatry field in the last half century. Unlike conventional antidepressants that often take weeks to months to show efficacy, ketamine causes rapid antidepressant effects, emerging as early as within 1h after administration. However, how ketamine improves mood symptoms so quickly has remained elusive. Here, we first introduce the historical background of ketamine as a rapid antidepressant. We then discuss current hypotheses underlying ketamine's rapid antidepressant effects, with a focus on our latest discovery that ketamine silences NMDAR-dependent burst firing in the 'anti-reward center', the lateral habenula. While ketamine may act on many brain regions, we argue that its rapid antidepressant effects are critically dependent on ketamine's action in the lateral habenula, with this brain region acting as a primary site of action (or one among a few primary nodes). This molecular-, cellular-, and circuit-based mechanism advances our understanding of the etiology of depression and suggests a new conceptual framework for the rapid antidepressant effects of ketamine.

Ketamine blocks bursting in the lateral habenula to rapidly relieve depression

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine has attracted enormous interest in mental health research owing to its rapid antidepressant actions, but its mechanism of action has remained elusive. Here we show that blockade of NMDAR-dependent bursting activity in the 'anti-reward center', the lateral habenula (LHb), mediates the rapid antidepressant actions of ketamine in rat and mouse models of depression. LHb neurons show a significant increase in burst activity and theta-band synchronization in depressive-like animals, which is reversed by ketamine. Burst-evoking photostimulation of LHb drives behavioural despair and anhedonia. Pharmacology and modelling experiments reveal that LHb bursting requires both NMDARs and low-voltage-sensitive T-type calcium channels (T-VSCCs). Furthermore, local blockade of NMDAR or T-VSCCs in the LHb is sufficient to induce rapid antidepressant effects. Our results suggest a simple model whereby ketamine quickly elevates mood by blocking NMDAR-dependent bursting activity of LHb neurons to disinhibit downstream monoaminergic reward centres, and provide a framework for developing new rapid-acting antidepressants.

Bioactivity of a peptide derived from acetylcholinesterase: electrophysiological characterization in guinea-pig hippocampus

Acetylcholinesterase is well known to have noncholinergic functions. Only recently, however, has the salient part been identified of the molecule responsible for these nonclassical actions, a peptide of 14 amino acids towards the C-terminus of acetylcholinesterase. The aim of this study was to test the bioactivity of this 'acetylcholinesterase-peptide' using intracellular recordings in guinea-pig hippocampal slices. In the presence of tetrodotoxin, acetylcholinesterase-peptide alone affected neither the membrane potential nor the input resistance of CA1 neurons; however, a modulatory action was observed, as a concentration-dependent decrease of N-methyl-d-aspartic acid-induced depolarization. When calcium potentials were elicited by a depolarizing current pulse, application of acetylcholinesterase-peptide increased or reduced the degree of calcium spike firing in, respectively, the presence or absence of the N-methyl-d-aspartic acid antagonist d(-)-2-amino-5-phosphonopentanoic acid. In contrast, a peptide derived from the equivalent region of butyrylcholinesterase, which also hydrolyses acetylcholine, had no effect. In conclusion, acetylcholinesterase-peptide has a selective bioactivity in the hippocampus; it could thus offer new ways of targeting mechanisms of calcium-induced neurotoxicity.

Two distinct types of repetitive bursting activity mediated by NMDA in hypothalamic neurons in vitro

Hypothalamic magnocellular dorsal nucleus neurons were recorded from adult guinea pig brain slices with the whole-cell patch-clamp technique to determine the effects of N-methyl-D-aspartate (NMDA) applied in the bath or by iontophoresis. In a majority of cells (59 of 77, 76.6%), rhythmic bursting discharges were evoked by specific activation of NMDA receptors when the membrane was more negative than -60 mV. This endogenous rhythmic activity was resistant to tetrodotoxin. It was suppressed by removal of extracellular Mg2+, indicating the involvement of the voltage-dependent block of the NMDA channel by Mg2+. Application of thapsigargin showed that rhythmic activity did not depend on the release of Ca2+ from reticulum stores. Blockers of Ca2+ conductances Ni2+ and nifedipine had no effects on the bursts. Their repolarization did not involve the activation of a strophantidin- or ouabain-sensitive pump, but partly depended on an apamine-sensitive Ca2+-dependent K+current. In a small subset of cells (9 of 69, 13%), specific activation of NMDA receptors induced another type of bursting activity which consisted of repetitive low-threshold spikes sustaining bursts of action potentials. Rhythmic low-threshold spikes subsisted in the presence of tetrodotoxin but were suppressed by Ni2+. Increasing the amount of NMDA brought about a switch from the rhythmic low-threshold spike burst firing to the rhythmic bursting activity observed for the majority of cells. The present data show for the first time that NMDA receptor activation can induce two independent rhythmic bursting behaviours in the same neuron, probably depending on the strength of the glutamatergic drive.