Restoring mood balance in depression: ketamine reverses deficit in dopamine-dependent synaptic plasticity

Abstract

Background: One of the most novel and exciting findings in major depressive disorder research over the last decade is the discovery of the fast-acting and long-lasting antidepressant effects of ketamine. Indeed, the therapeutic effects of classic antidepressants, such as selective serotonin reuptake inhibitors, require a month or longer to be expressed, with about a third of major depressive disorder patients resistant to treatment. Clinical studies have shown that a low dose of ketamine exhibits fast-acting relatively sustained antidepressant action, even in treatment-resistant patients. However, the mechanisms of ketamine action at a systems level remain unclear.

Methods: Wistar-Kyoto rats were exposed to inescapable, uncontrollable footshocks. To evaluate learned helplessness behavior, we used an active avoidance task in a shuttle box equipped with an electrical grid floor. After helplessness assessment, we performed in vivo electrophysiological recordings first from ventral tegmental area dopaminergic (DA) neurons and second from accumbens neurons responsive to fimbria stimulation. Ketamine was injected and tested on helpless behavior and electrophysiological recordings.

Results: We show that ketamine is able to restore the integrity of a network by acting on the DA system and restoring synaptic dysfunction observed in stress-induced depression. We show that part of the antidepressant effect of ketamine is via the DA system. Indeed, injection of ketamine restores a decreased dopamine neuron population activity, as well as synaptic plasticity (long-term potentiation) in the hippocampus-accumbens pathway, via, in part, activation of D1 receptors.

Conclusions: This work provides a unique systems perspective on the mechanisms of ketamine on a disrupted limbic system.

Keywords: Dopamine; ketamine; learned helplessness; nucleus accumbens; synaptic plasticity; ventral tegmental area.

Figure 1. Learned helplessness is reversed by repeated injections of ketamine

A. Experimental timeline B. Helplessness paradigm. C. Number of failures to escape across 3 consecutive days of escapable shocks session (left). Data for the escapable session on day 4 are summarized in bar graphs (right). Rats fall into two groups: those showing escape (triangles, non-helpless; circles, no-shock) and those failing to escape (squares, helpless). Ketamine (red) causes helpless rats to show escape behavior. D) Latency to escape across 3 consecutive days of escapable shocks session, showing results consistent with escape failures (left). Data for the escapable session on day 4 are summarized in bar graphs (right). Red, purple and brown represents data for ketamine. e) There was no difference in locomotor activity measured in both sides of the shuttle box during the escapable shock session on day 4 in helpless rats following saline or ketamine 20 minutes or 2 hours after(striped bar) the injection. ** p<0.01; ***p<0.001. Error bars represent SEM. NH: non-helpless rats H: helpless rats

Figure 2. Helpless rats show selective decrease in dopamine neuron population activity that is reversed by repeated injections of ketamine

A) Number of spontaneously active DA neurons per electrode track. Only helpless rats showed a 50% decrease in number of active DA neurons (white bar) which was reversed by ketamine, 20 minutes post injection (red bar). Two hours after the injection, a significant increase in the population activity of non-helpless and helpless rats is observd (striped bar). In contrast, there was no change in firing rate (B) and burst firing (C) between helpless and non-helpless rats; ketamine administration 20 minutes prior recordings produced its typical increase in DA neuron rate and bursting in both groups, which is not seen 2 hours after the injection (striped bar). (D) Helpless rats show a decreased number of spontaneously active DA neurons located in the central (C) but not in the medial and lateral tracks in the VTA (white squares) compared to controls (black squares), non-helpless rats, and helpless rats treated with ketamine 20 minutes before recordings. Ketamine two hours post-injection induces an increase in population activity in the medial tracks. *p<0.05, ***p<0.001, error bars are ± SEM. Red and brown bars represent data with injection of ketamine.

Figure 3. Acute injection of ketamine restores sustainably escape behavior and VTA DA activity

A. Number of failures (left) and latency (right) to escape across 2 consecutive days of escapable shocks session. Injection of ketamine (red) after the first escapable session induces a significant decrease in the number of failures and the latency to escape (non-helpless rats: triangle; helpless rats: square). B. Number of spontaneously active DA neurons per electrode track (top). Helpless rats showed a 50% decrease in number of active DA neurons and a decrease in firing rate (middle) which was reversed by ketamine, 24h post injection (red bar). No change in the bursting activity (bottom) was observed between non-helpless, helpless rats, and helpless rats treated with ketamine. *p<0.05, **p<0.01, ***p<0.001, error bars are ± SEM; NH: non-helpless rats; H: helpless rats

Figure 4. Shell and core segments of the nucleus accumbens show different responses to HFS of fimbria in helpless vs non-helpless rats

A) Schematic of recording and stimulating electrode placements. B) Extracellular recording trace showing a representative example of the increased fimbria-evoked spike probability recorded from a NAc neuron in a control animal, 10 min after high frequency stimulation. Twenty overlaid consecutive traces are shown with the numbers demonstrating the number of evoked spikes for 20 stimulations. c) HFS of the fimbria produced LTP in control rats (black squares) but produced LTD in helpless rats (red squares). D) Recording electrode placements in the NAc of home cage control (black squares) and helpless rats (red squares) animals shown as coronal sections of the rat brain, taken from Paxinos and Watson (53). E) HFS of the fimbria produced LTP in the accumbens shell in non-helpless and no-shock rats (top) but produced LTD in the accumbens core (bottom). Plots show mean percent change (± SEM) in fimbria-evoked spike probability, normalized to the baseline. F) Recording electrode placements in the NAc of non-helpless (triangles) and helpless rats (circles) animals, shown as coronal sections of the rat brain, taken from Paxinos and Watson (53). *p<0.05; ***p<0.001; arrow indicates the time of stimulation vSub: ventral subiculum of the hippocampus; NAc: nucleus accumbens; HFS: high-frequency stimulation

Figure 5. Ketamine restores long-term potentiation in the hippocampus-accumbens pathway of helpless rats that depends on D1 receptors

Left a) injection of ketamine (i.p. 5mg/kg) in helpless animals restored fimbria HFS-induced LTP in the accumbens shell following high frequency stimulation of the fimbria and b) infusion of the D1 antagonist SCH23390 in the NAc (0.5 μg/0.5 μl) prevented ketamine (i.p. 5mg/kg) from restoring LTP in the fimbria-NAc pathway. c) Ketamine did not affect HFS-induced LTD in rats that did not show behavioral improvement. Plots show mean percent change (±SEM) in fimbria-evoked spike probability, normalized to baseline. Right. Recording electrode sites shown as coronal sections of the rat brain taken from Paxinos and Watson (53). *p<0.05, **p<0.01. HFS: high-frequency stimulation

Figure 6. Summarizing schematic

Schematic summarizing the population activity in the VTA (left) and effect of HFS on the vSub-NAc shell and core (right). (a) In home cage control rats, HFS of the vSub produces LTP in the NAc core and shell (right), (b) following inescapable shock, rats that did not show helplessness showed HFS-induced LTD in the vSub-NAc core projection, with the vSub-shell projection showing normal HFS-induced LTP (right) and a DA neuron population activity comparable to control rats. (c) In contrast, in helpless rats, the vSub-shell and -core pathways shows LTD in response to HFS (right), which corresponds to a decrease in DA neuron population activity (left). (d) Following injection of ketamine, both DA neuron activity (left) and vSub-NAc LTP (right) is restored in both core and shell regions in helpless rats. Thin arrow, LTD, thick arrow, LTP. VP: ventral pallidum; VTA: ventral tegmental area; vSub : ventral subiculum of the hippocampus; HFS: High frequency stimulation