Repeated lysergic acid diethylamide in an animal model of depression: Normalisation of learning behaviour and hippocampal serotonin 5-HT2 signalling

This study (2014) found that repeated LSD administration to rats exhibits an anti-depressive effect in the animals, which the authors discuss in terms of a rebalancing of neurological signaling.

Abstract

“A re-balance of postsynaptic serotonin (5-HT) receptor signalling, with an increase in 5-HT_1A and a decrease in 5-HT_2A signalling, is a final common pathway multiple antidepressants share. Given that the 5-HT_1A/2A agonist lysergic acid diethylamide (LSD), when repeatedly applied, selectively downregulates 5-HT_2A, but not 5-HT_1A receptors, one might expect LSD to similarly re-balance the postsynaptic 5-HT signalling. Challenging this idea, we use an animal model of depression specifically responding to repeated antidepressant treatment (olfactory bulbectomy), and test the antidepressant-like properties of repeated LSD treatment (0.13 mg/kg/d, 11 d). In line with former findings, we observe that bulbectomised rats show marked deficits in active avoidance learning. These deficits, similarly as we earlier noted with imipramine, are largely reversed by repeated LSD administration. Additionally, bulbectomised rats exhibit distinct anomalies of monoamine receptor signalling in hippocampus and/or frontal cortex; from these, only the hippocampal decrease in 5-HT₂ related [³⁵S]-GTP-gamma-S binding is normalised by LSD. Importantly, the sham-operated rats do not profit from LSD, and exhibit reduced hippocampal 5-HT₂ signalling. As behavioural deficits after bulbectomy respond to agents classified as antidepressants only, we conclude that the effect of LSD in this model can be considered antidepressant-like, and discuss it in terms of a re-balance of hippocampal 5-HT₂/5-HT_1A signalling.”

Authors: Tobias Buchborn, Helmut Schröder, Volker Höllt & Gisela Grecksch

Summary

Introduction

Lysergic acid diethylamide (LSD) is a serotonergic hallucinogen that induces profound alterations of the human consciousness. It is a suitable ligand for a variety of monoaminergic, notably serotonin (5-HT) receptors, and might be of medical value in the context of anxiety neuroses and/or depressive reactions.

The hypothalamic-pituitary-adrenal axis and the hippocampal neurogenesis and/or cell proliferation are controlled by 5-HT signalling. Long-term treatment with diverse-class antidepressants downregulates 5-HT2A receptors in the frontal cortex and increases the responsiveness of hippocampal 5-HT1A receptors.

We used an animal model of depression to test the antidepressant-like properties of repeated LSD treatment. The results showed that LSD normalised hippocampal 5-HT2/5-HT1A signalling, and that sham-operated rats did not profit from LSD.

LSD fails to exert antidepressant-like effects in an animal model of depression that responds to one-time antidepressant application. Therefore, an animal model responding to repeated antidepressant treatment might be of more validity. Olfactory bulbectomy induces stress-associated hyperlocomotion and avoidance learning deficits in rats, which reliably ameliorate in response to drugs specified as antidepressants. These deficits may be used as a proxy for depressive-like cognition biases and their responsiveness to 5-HT-related action of antidepressant-class drugs.

We repeatedly apply LSD to bulbectomised rats and investigate its effect on avoidance learning, forebrain 5-HT1A/5-HT2 signalling and beta, overall 5-HT, dopamine and noradrenaline signalling.

Animals and housing

For experiments, male Wistar rats were used. They were housed in groups of five each cage, and were given standard food pellets and tap water ad libitum.

Bilateral olfactory bulbectomy

Rats were bulbectomised at the age of seven weeks and the cavities were filled with haemostatic sponges and the skin was closed by tissue adhesive. The extent and adequacy of the surgical ablation were assessed after decapitation.

Behavioural experiments

Rats received lysergide[(R,R)-tartrate]-anhydrate (0.13 mg/kg, subcutaneous, dissolved in isotonic saline, 10 ml/ kg) for 11 days, once every 24 h. LSD was administered two hours after each test session to avoid interference from the acute effects of LSD.

Eight weeks after surgery, on the sixth day of subchronic treatment, rats had to learn to actively avoid electrical foot stimuli by jumping onto a pole. They were allowed five minutes for exploration of the test apparatus, on the following days only one minute was granted.

24-hour after the last treatment, rats were decapitated, brain regions of interest were removed and frozen in liquid nitrogen, and the membranes were homogenised, washed in Tris buffer, and resuspended in incubation buffer. The membrane fraction was collected on GF/A grade glass-fibre filters, and the specific binding was calculated.

[35S]-GTP-gamma-S binding to 5-HT1A/2, dopamine, and (beta) adrenergic receptors was measured in the presence and absence of the relevant agonist. The amount of [35S]-GTP-gamma-S bound to the receptor was corrected for unspecific binding, and expressed as % stimulation over basal specific binding.

Statistical analysis

A two-factor ANOVA with repeated measures on one factor was conducted to assess the main effects and interaction of time and group in avoidance learning, followed by pairwise contrast analysis.

Behavioural experiments

The omnibus F-test revealed that sham-operated rats showed good progress in learning instrumental and conditioned avoidance behaviour, while LSD-treated bulbectomised rats failed to achieve the level of performance shown by the sham-operated controls.

Bulbectomy slightly increased ketanserin-sensitive [3H]spiroperidol binding in hippocampus, but repeated LSD treatment partially counteracted this increase. Sham-operated animals had no effect on ketanserin-sensitive [3H]spiroperidol binding, but repeated LSD treatment induced a significant increase in frontal cortex binding.

Bulbectomy led to a significant reduction in alpha-MS stimulated guanine nucleotide exchange in the hippocampus, which was reversed by subchronic LSD. However, LSD did not reverse the desensitisation of alpha-MS stimulated [35S]-GTP-gamma-S binding in the sham-operated animals.

Discussion

Serotonergic hallucinogens might be of assistance in the treatment of neurotic-type depression or emotional distress associated with advanced stages of cancer, but should not be conceptualised as acute mood-enhancers or antidepressants in a literal sense. Here, we compared the effects of hallucinogens on avoidance learning in bulbectomised rats with those of imipramine or trazodone. We found that hallucinogens largely reversed the deficit in avoidance learning, and that this effect was strengthened by the fact that LSD helped bulbectomised, but not sham-operated, rats.

Bulbectomised rats exhibit various anomalies of monoamine receptor signalling, with 5-HT1A, 5-HT2 and beta signalling being sensitised in the frontal cortex, and the latter two being desensitised in the hippocampus. LSD normalises hippocampal 5-HT2 signalling, but not 5-HT1A signalling.

LSD has high affinity for 5-HT1A and 2A receptors, but it also binds to beta and D4 receptors, complementing 5-HT2A in LSD’s behavioural profile. LSD normalises hippocampal 5-HT2 but not 5-HT1A signalling, and this may explain why LSD affects avoidance learning. Bulbectomy leads to deficient hippocampal neurogenesis and an upregulation of brain-derived neurotrophic factor (BDNF). LSD might be able to re-balance the anti-BDNF effect of 5-HT2A against the neurotrophic effect of 5-HT1A receptors, allowing the stress-integration system of bulbectomised rats to more effectively meet the demands of avoidance learning.

In the frontal cortex and hippocampus of sham-operated rats, LSD significantly increases 5-HT2(A) binding parameters, whereas in bulbectomised animals 5-HT2(A) binding parameters are decreased. This contrasts with the notion that LSD selectively downregulates 5-HT2A receptors. The fact that LSD did not affect the avoidance learning of the sham rats, underlines that the application scheme was well chosen, and that the LSD-induced changes of their neurochemistry might rather be unspecific and unrelated to the processes involved in avoidance learning.

Repeated LSD treatment reverses depressive-like avoidance learning deficits in bulbectomised rats, possibly by re-balancing hippocampal 5-HT2 (vs 5-HT1A) signalling.