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

LSD is a serotonergic hallucinogen with high affinity for multiple serotonin receptor subtypes, notably 5-HT1A and 5-HT2A. Earlier research indicates that long-term treatment with conventional antidepressants tends to downregulate 5-HT2A receptors (particularly in frontal cortex) while increasing hippocampal 5-HT1A responsiveness, and that repeated—but not acute—administration of LSD selectively downregulates 5-HT2A without affecting 5-HT1A. This pattern raises the possibility that repeated LSD might ‘‘re-balance’’ postsynaptic 5-HT signalling in a manner analogous to established antidepressants. Buchborn and colleagues set out to test this idea using an animal model that specifically responds to repeated antidepressant treatment: bilateral olfactory bulbectomy in rats. The study aimed to determine whether repeated LSD administration (subchronic dosing) would produce antidepressant-like effects on a conditioned active avoidance (pole-jumping) task and whether it would normalise forebrain 5-HT1A/5-HT2 signalling. Because LSD is not selective, the investigators also measured broader monoaminergic signalling (beta-adrenergic, dopamine, noradrenaline) to assess specificity. Behavioural testing and receptor assays (radioligand binding and [35S]-GTP-γ-S functional coupling) were the principal outcome measures.

Male Wistar rats (average ~400 g) were housed under standard laboratory conditions. At seven weeks of age animals underwent either bilateral olfactory bulbectomy or sham surgery; completeness of ablation was checked after behavioural experiments. Bulbectomy followed standard stereotactic aspiration of the olfactory bulbs, with sham animals receiving identical surgical handling except for bulb removal. Lysergide [(R,R)-tartrate] was administered subcutaneously at 0.13 mg/kg once daily for 11 days. Treatment began five days before behavioural testing and continued until 24 h before decapitation; injections were given two hours after each test session to minimise acute interference with learning. Control animals received saline. Assignment to the four experimental groups (sham/saline, sham/LSD, bulb/saline, bulb/LSD) was randomised. Behavioural outcome was assessed in the one-way active avoidance (pole-jumping) paradigm. Testing occurred eight weeks after surgery on five consecutive days (10 trials per day). Each trial presented an 80 dB buzzer conditioned stimulus (CS) followed from the fourth second by an overlapping electrical foot shock (unconditioned stimulus, US; 0.2–0.4 mA adjusted per animal). Trials lasted up to 20 s and were terminated if the rat jumped onto a pole; measures recorded were successful escapes (≤20 s) and avoidances (conditioned responses ≤4 s). Neurochemical assays were performed 24 h after the last treatment on dissected frontal cortex and hippocampus. 5-HT2A receptor density was estimated by ketanserin-sensitive [3H]spiroperidol binding on membrane preparations and expressed as fmol per mg protein. Functional receptor signalling was assessed by agonist-stimulated [35S]-GTP-γ-S binding, a measure of G-protein coupling; tissue was incubated with selective agonists (alpha-methylserotonin for 5-HT2, 8-OH-DPAT for 5-HT1A, isoprenaline for β-adrenergic, plus serotonin, dopamine and noradrenaline) and results expressed as Emax (% stimulation over basal). Neurochemical determinations were performed at least in duplicate; neurochemical sample sizes reported as n=4–6 per group for mean+SEM values. Statistical analysis of avoidance learning used a two-factor ANOVA with repeated measures (mixed model) to test time and group effects, followed by pairwise contrasts. Intergroup comparisons for specifically bound radioactivity used planned nonparametric Mann–Whitney U-tests. Significance was set at p<0.05.

Behavioural results: Bulbectomised rats showed marked deficits in active avoidance learning in the pole-jumping task, consistent with previous reports. Repeated LSD treatment largely reversed these avoidance-learning deficits in the bulbectomised group, whereas sham-operated rats did not show performance benefits from LSD. The extracted text does not provide the detailed ANOVA statistics or exact group n for the behavioural measures. 5-HT2A receptor binding ([3H]spiroperidol, ketanserin-sensitive): In the hippocampus, bulbectomy produced a trend towards increased ketanserin-sensitive binding compared with sham/saline (sham/saline vs bulb/saline: U=4, p=0.095). Repeated LSD partially counteracted this trend; the direct comparison between bulb/saline and bulb/LSD fell short of significance (U=6, p=0.063), and there was no significant difference between sham/saline and bulb/LSD (U=11, p=0.46). In sham animals, repeated LSD did not affect hippocampal ketanserin-sensitive binding (sham/saline vs sham/LSD: U=11, p=0.46). In the frontal cortex, bulbectomy had no significant effect on ketanserin-sensitive binding (sham/saline vs bulb/saline: U=9, p=0.27), but LSD induced a significant increase in frontal ketanserin-sensitive binding in sham animals (sham/saline vs sham/LSD: U=0, p=0.002). [35S]-GTP-γ-S binding (agonist-stimulated G-protein coupling): The hippocampal decrease in alpha-methylserotonin (alpha-MS) stimulated [35S]-GTP-γ-S binding seen after bulbectomy was reversed by subchronic LSD (bulb/saline vs bulb/LSD: U=6, p=0.032). By contrast, in sham-operated animals LSD caused a desensitisation of alpha-MS stimulated hippocampal [35S]-GTP-γ-S binding (sham/saline vs sham/LSD: U=3, p=0.0015). Other bulbectomy-associated changes included a hippocampal decrease in isoprenaline- and noradrenaline-stimulated signalling (sham/saline vs bulb/saline: U=0, p=0.004; U=2, p=0.057, respectively); these effects were not reversed by LSD. Frontocortical increases in alpha-MS, 8-OH-DPAT and isoprenaline stimulated [35S]-GTP-γ-S binding associated with bulbectomy were reported but were not normalised by LSD. Hippocampal signalling stimulated by 8-OH-DPAT (5-HT1A), serotonin and dopamine showed no significant influence of bulbectomy or of the interaction with LSD (all reported as non-significant). Finally, in the frontal cortex of sham animals, LSD led to sensitisation of multiple receptor signalling measures, including 5-HT2 (sham/saline vs sham/LSD: U=3.5, p=0.022). The neurochemical results are reported as mean±SEM with n=4–6 for the assays; several comparisons showed only trends rather than formal significance. Some fragments in the extracted results text were incomplete, and the precise numerical values for behavioural ANOVA and some neurochemical group means were not provided in the extraction.

Buchborn and colleagues interpret their findings as evidence that repeated LSD produces an antidepressant-like effect in the olfactory bulbectomy model, because LSD largely reversed the bulbectomy-induced avoidance-learning deficits and selectively normalised a hippocampal 5-HT2-related signalling abnormality. They emphasise that the behavioural benefit was specific to bulbectomised rats and absent in sham-operated animals, arguing that this specificity supports an antidepressant-like action rather than a general cognitive enhancer or acute mood effect. The investigators link the biochemical and behavioural data by proposing a re-balancing of hippocampal 5-HT2 versus 5-HT1A signalling as a plausible mechanism. Specifically, bulbectomy-associated desensitisation of hippocampal alpha-MS (a proxy for 5-HT2) stimulated G-protein coupling was restored by LSD, whereas hippocampal 5-HT1A signalling was not significantly altered by bulbectomy or by LSD in the present assays; this pattern is taken to indicate selective engagement of 5-HT2-related pathways. The authors additionally discuss how such receptor regulation could interact with hippocampal neurogenesis and brain-derived neurotrophic factor (BDNF) signalling: they speculate that LSD, by re-sensitising 5-HT2 signalling while activating 5-HT1A, might recalibrate opposing trophic influences and thereby improve the neuronal turnover or stress integration needed for avoidance learning. The authors label this mechanistic model as speculative and in need of further study. Several important nuances are highlighted. Receptor regulation by hallucinogens can be complex and region-specific, varying with dosage, regimen, animal strain or behavioural context; accordingly, LSD increased frontocortical receptor signalling in sham animals without producing behavioural effects, which the investigators attribute to the absence of a pre-existing pathological bias in those rats and to the timing of dosing (two hours after learning sessions). They acknowledge that LSD is not selective and binds multiple monoamine receptors; however, because overall dopamine signalling was not affected and because LSD normalised hippocampal 5-HT2 but not beta-adrenergic signalling, the authors focus discussion on 5-HT2/5-HT1A balance. Suggested directions for future research include co-application of selective antagonists with LSD, combining selective 5-HT1A and 5-HT2A agents, or use of selective dual agonists to more precisely dissect receptor contributions, though practical and interpretative complications are recognised. Finally, the authors situate the findings in a clinical and psychotherapeutic context, noting that hallucinogens are unlikely to be straightforward acute antidepressants but may assist psychotherapy by facilitating access to emotion-salient cognitions and by correcting mood-relevant cognitive biases via receptor regulation. They caution that the mechanistic links and translational relevance require further investigation and replication. The extracted text does not present additional formal limitations such as exact sample sizes for behavioural groups or power calculations, beyond acknowledging the need for more targeted receptor-manipulation studies.