The hallucinogen d-lysergic diethylamide (LSD) decreases dopamine firing activity through 5-HT1A, D2 and TAAR1 receptors

LSD is a prototypical hallucinogen synthesised in 1938 that produces profound alterations of consciousness and psychotic-like phenomena. Previous work has primarily linked its psychotropic properties to serotonergic actions, notably partial agonism at 5-HT2A and agonism/partial agonism at 5-HT1A receptors, and in-vivo studies have shown LSD suppresses firing of dorsal raphe nucleus (DRN) 5-HT neurons while increasing cortical neuronal activity. In-vitro evidence also indicates LSD binds to dopamine (DA) receptors, especially D2, and to the trace-amine associated receptor 1 (TAAR1), but in-vivo interactions with mesolimbic DA neurons of the ventral tegmental area (VTA) and the role of TAAR1 had not been explored. De Gregorio and colleagues set out to characterise LSD's in-vivo effects on VTA DA neurons, to compare those effects with LSD's actions on DRN 5-HT neurons, and to test whether 5-HT1A, D2 and TAAR1 receptors mediate LSD's influence on VTA DA firing. The investigators hypothesised a pleiotropic mechanism involving both serotonergic and dopaminergic systems, and designed electrophysiological experiments in rats to probe dose-dependent effects and receptor involvement.

Adult male Sprague Dawley rats (300–330 g) were used. Animals were anaesthetised with chloral hydrate (400 mg/kg, i.p.; supplemental 100 mg/kg doses as needed) and placed in a stereotaxic apparatus for extracellular single-unit recordings. Single-barrelled glass micropipettes were used to record spontaneous activity of individual neurons in the DRN (putative 5-HT neurons) and VTA (putative DA neurons); neuronal identity was established by established electrophysiological criteria (firing rate, waveform and regularity for 5-HT; wide action potential, slow firing rate and waveform for DA). Burst activity for DA neurons was defined by interspike interval criteria (initial ISI ≤ 80 ms and maximum ISI 160 ms) and quantified as number of bursts per 200 s and percentage of spikes in bursts. Drugs were administered intravenously via a tail-vein catheter. The main test compound was cumulative doses of LSD; the extracted text reports dose ranges as "low" (5-20 g/kg, i.v.) and "high" (30-150 g/kg, i.v.), noting that the extraction uses the unit "g/kg". Receptor antagonists and probes included haloperidol (Halo, D2 antagonist), MDL 100,907 (5-HT2A antagonist), WAY-100,635 (5-HT1A antagonist), EPPTB (TAAR1 antagonist), apomorphine (Apo, tested as a D2 agonist probe), and PCPA for serotonin depletion (350 mg/kg, i.p., administered 48 h and 24 h before recordings). Only one neuron per rat was tested. Baseline firing was recorded for at least five minutes before drug administration; drugs were given sequentially every five minutes according to the experimental plan. The extracted methods include detailed equipment, electrode preparation and histological marking of recording sites to confirm placements. Data analysis treated neuronal responses as percentage change from vehicle (vehicle = 100%) and used Student t-tests, one-way ANOVA or two-way repeated-measures ANOVA with Bonferroni post hoc tests where appropriate; significance was set at P ≤ 0.05. ED50 values for LSD dose–response curves were estimated by non-linear regression after log-transforming doses.

Low-dose effects on DRN and VTA: Cumulative low doses of LSD (reported as 5–20 g/kg, i.v.) produced a dose-dependent decrease of DRN 5-HT firing. In four DRN neurons, 10 g/kg significantly reduced activity (P = 0.007) and 20 g/kg abolished firing (P < 0.001); the ED50 for DRN inhibition was 13.13 g/kg. The same low-dose range did not alter spontaneous or burst firing of VTA DA neurons (no significant change reported). Receptor blockade of low-dose effects: Pretreatment with haloperidol (50 g/kg, i.v.) prevented the inhibitory effects of low-dose LSD on DRN 5-HT firing (interaction and main-effect statistics reported, P < 0.001) while haloperidol alone did not affect 5-HT firing. Similarly, the 5-HT2A antagonist MDL 100,907 (200 g/kg, i.v.) blocked the suppressive effect of low doses of LSD on DRN 5-HT neurons. The 5-HT1A agonist 8-OH-DPAT (5–10 g/kg, i.v.) was able to silence DRN 5-HT neurons when administered after LSD, supporting a multi-receptorial mechanism for LSD's action on 5-HT cells. High-dose effects on VTA DA neurons: Increasing LSD doses (reported as 30–120 g/kg cumulative) produced a dose-dependent decrease of VTA DA firing in six recorded neurons. Significant suppression occurred at 60 and 90 g/kg (P < 0.001), and 120 g/kg completely silenced DA firing (P < 0.001). The ED50 for VTA inhibition was 71.80 g/kg. Burst-firing parameters (number of bursts per 200 s and percentage of spikes in bursts) were also significantly reduced by high doses. Role of serotonin depletion: Pre-treatment with PCPA (serotonin depletion > 89% at the VTA by the protocol used) did not alter the overall inhibitory effect of high-dose LSD on VTA DA firing: LSD reduced DA firing in both PCPA-treated and control animals (no interaction; P = 0.38), and ED50 values were not significantly different (59.24 g/kg in PCPA-treated v. 71.80 g/kg in controls). However, in 5-HT depleted rats a low dose of LSD (30 g/kg) decreased the number of DA bursts, an effect not seen in controls, indicating an influence of 5-HT tone on LSD's effects on burst firing. Effects of receptor antagonists on high-dose inhibition: Haloperidol pretreatment prevented the inhibitory effects of high-dose LSD on VTA DA firing and could reinstate firing when administered after LSD; statistical tests showed significant interactions and main effects (P < 0.001). Notably, in PCPA-pretreated animals haloperidol did not restore DA firing after LSD, suggesting haloperidol's action may depend on intact 5-HT signalling. WAY-100,635 (500 g/kg, i.v.; 5-HT1A antagonist) similarly blocked LSD-induced suppression of VTA DA firing; in WAY-100,635-pretreated neurons apomorphine still silenced DA cells, indicating the 5-HT1A blockade prevented LSD effects independently of D2 receptor capacity. TAAR1 involvement: Pretreatment with the TAAR1 antagonist EPPTB (5 mg/kg, i.v.) increased baseline VTA DA firing (single-injection effect P = 0.002, n = 7) and completely blocked the inhibitory effects of cumulative high-dose LSD on DA activity. EPPTB pretreatment also prevented apomorphine-induced changes in DA neurons in these experiments, suggesting a functional interaction between TAAR1 and D2-mediated mechanisms. Across these manipulations, burst-firing measures were variably affected; EPPTB showed a trend to increase bursts but cumulative LSD after EPPTB left burst parameters overall unchanged.

De Gregorio and colleagues interpret their findings as evidence that LSD exerts dose-dependent, biphasic electrophysiological effects: low doses primarily suppress DRN 5-HT neurons via a mechanism involving 5-HT2A and D2 receptors, whereas higher doses inhibit VTA DA neurons through a multi-receptorial mechanism engaging D2, 5-HT1A and TAAR1 receptors. They highlight that this study provides the first in-vivo electrophysiological indication of TAAR1 involvement in LSD's action on DA neurons. The investigators note that the higher potency of LSD at 5-HT versus DA neurons is consistent with prior literature and with behavioural data describing two temporal phases of LSD effects, an early serotonergic-mediated ‘‘psychedelic’’ phase and a later dopaminergic-mediated ‘‘paranoid’’ or psychotic-like phase. The result that serotonin depletion (PCPA) did not abolish LSD's suppression of DA firing supports a 5-HT-independent component for the high-dose DA effects, although depletion did sensitize burst-related responses and rendered haloperidol ineffective at reversing LSD in depleted animals; the authors suggest this may relate to mechanisms of neuroleptic-resistant psychosis and call for further research. Regarding TAAR1, the authors discuss evidence that TAAR1 is a modulator of aminergic systems and that TAAR1 antagonism (EPPTB) both increased basal VTA DA firing and blocked LSD-induced inhibition, implying a role for TAAR1 in mediating LSD's dopaminergic effects. They also point to cross-talk between TAAR1 and D2 receptors, citing that EPPTB prevented apomorphine effects in their preparation, and suggest further work is required to determine the precise TAAR1–D2 interaction. The authors acknowledge that further behavioural pharmacology and mechanistic studies are needed to clarify D2 involvement and the therapeutic implications, and they note that distinctions between TAAR1 antagonists and partial agonists require additional investigation. Overall, they position their findings as supporting a pleiotropic mechanism for LSD and as motivating exploration of combined receptor antagonism for drug-induced psychosis.

The study concludes that LSD operates via a pleiotropic, dose-dependent mechanism: at low doses it primarily affects the serotonergic system interacting with 5-HT2A and D2 receptors, whereas at higher doses it influences the dopaminergic system via 5-HT1A, D2 and TAAR1 receptors. The authors remark that this biphasic electrophysiological profile parallels biphasic psychotropic effects reported in humans, and they propose that combined antagonism at 5-HT1A, D2 and TAAR1 could represent a novel approach to manage drug-induced psychosis.