Effects of acute and repeated treatment with serotonin 5-HT2A receptor agonist hallucinogens on intracranial self-stimulation in rats

This rodent study evaluated the acute effects of LSD on intracranial self-stimulation (ICSS), an operant conditioning reward measure. It found that acute LSD treatment did not alter its own ICSS depressant and the likelihood of methamphetamine abuse effects.

Abstract

“The prototype 5-HT2A receptor agonist hallucinogens LSD, mescaline, and psilocybin are classified as Schedule 1 drugs of abuse by the U.S. Drug Enforcement Administration. Accumulating clinical evidence has also suggested that acute or repeated “microdosing” with these drugs may have utility for treatment of some mental health disorders, including drug abuse and depression. The goal of the present study was to evaluate LSD, mescaline, and psilocybin effects on intracranial self-stimulation (ICSS), a procedure that has been used to evaluate abuse-related effects of other classes of abused drugs. Effects of repeated LSD were also examined to evaluate potential changes in its own effects on ICSS or changes in effects produced by the abused psychostimulant methamphetamine or the prodepressant kappa opioid receptor (KOR) agonist U69,593. Male Sprague–Dawley rats were implanted with microelectrodes targeting the medial forebrain bundle and trained to respond under a “frequency-rate” ICSS procedure, in which many drugs of abuse increase (or “facilitate”) ICSS. In acute dose-effect and time-course studies, evidence for abuse-related ICSS facilitation was weak and inconsistent; the predominant effect of all 3 drugs was dose- and time-dependent ICSS depression. Repeated LSD treatment failed to alter either its own ICSS depressant effects or the abuse-related effects of methamphetamine; however, repeated LSD did attenuate ICSS depression by U69,593. These results extend those of previous preclinical studies to suggest weak expression of abuse-related effects by 5-HT2A agonist hallucinogens and provide supportive evidence for therapeutic effects of repeated LSD dosing to attenuate KOR-mediated depressant effects but not abuse potential of psychostimulants.“

Authors: Farhana Sakloth, Elizabeth Leggett, Megan J. Moerke, E. Andrew Townsend, Matthew L. Banks & S. Stevens Negus

Summary

LSD, mescaline, and psilocybin were studied for their effects on intracranial self-stimulation. The results suggest that these drugs have weak abuse-related effects, but that repeated LSD treatment may have therapeutic effects to attenuate KOR-mediated depressant effects but not abuse potential of psychostimulants.

INTRODUCTION

The prototype serotonergic hallucinogens LSD, mescaline, and psilocybin are all tryptamines with distinct molecular scaffolds but share activity as serotonin 2A (5-HT2A) receptor agonists. All three compounds are categorized by the United States Drug Enforcement Administration as Schedule 1 drugs with high abuse liability. Although none of these compounds is currently approved for clinical use, a growing body of evidence suggests that they may have utility for treatment of some neurological or psychiatric disorders.

Although LSD, mescaline, and psilocybin are categorized as Schedule I drugs by the DEA, they fail to produce robust signals in preclinical procedures used to assess abuse potential, including intracranial selfstimulation (ICSS).

The goal of this study was to compare the effects of LSD, mescaline, and psilocybin on ICSS in rats using a “frequency-rate” procedure. Additionally, the effects of the abused monoamine releaser methamphetamine and prodepressant kappa opioid receptor agonist U69,593 were also evaluated in rats treated with repeated vehicle or LSD.

Rats were anesthetized with isoflurane and had a stainless steel electrode implanted in the left medial forebrain bundle at the level of the lateral hypothalamus. Ketoprofen was administered intraperitoneally immediately and 24 hours post-surgery.

Studies were conducted in operant conditioning chambers equipped with a response lever, three stimulus lights above the lever, a house light, and an ICSS stimulator.

Rats were trained to press a lever under a fixed-ratio 1 (FR 1) schedule for brain stimulation delivered via the intracranial electrode. The stimulation frequency and amplitude were fixed at 126 Hz and 150 A, respectively, during initial 60-min training sessions. Frequency manipulations were introduced during 30-min behavioral sessions consisting of three 10-min components. Each component consisted of 10 1-min frequency trials, with the frequency decreasing in 0.05 log increments from 2.2 log Hz to 1.75 log Hz. ICSS training was considered complete when frequency-rate curves were not statistically different over three consecutive days.

Separate groups of rats were used for acute dose-effect and time-course studies with LSD, mescaline, and psilocybin. Test sessions consisted of three baseline components followed by a 15-min time-out period and then by two test components.

Rats were given low, medium, and high doses of LSD, mescaline, and psilocybin. They were then given time-course tests that consisted of three baseline ICSS components followed by injections of saline vehicle or a test-drug dose and then by pairs of test components.

Twenty-seven drug-nave rats were divided into four groups for repeated treatment with saline, 0.01, 0.1, or 0.32 mg/kg/day LSD. The effects of repeated treatments were evaluated using a nine-day protocol. Cumulative-dosing test sessions consisted of three baseline components followed by five LSD injections administered IP at 30-min intervals. Methamphetamine and U69,593 cumulative-dosing test sessions were conducted on Days 8 and 9, and were identical to those with LSD.

Data analysis

To normalize the data, the reinforcement rate in stimulations/trial during each frequency trial was converted into the percent maximum control rate (% MCR). The mean baseline and test frequency-rate curves were then averaged within each rat and across rats, and analyzed using two-way repeated measures analysis of variance. ICSS performance was summarized by determining the average number of stimulations delivered across all 10 frequency trials for each component and analyzing the results by one-way ANOVA.

Data from the study of repeated LSD administration were analyzed using a similar approach, except that baseline MCR and total stimulations were calculated from the three-day Predrug Baseline components conducted before any LSD administration.

Lysergic acid diethylamide, mescaline HCl, psilocybin, and U69,593 HCl were obtained from the National Institute on Drug Abuse drug supply program.

Dose-effect and time-course studies with acute LSD, mescaline and psilocybin dosing

LSD, mescaline, and psilocybin produce weak and inconsistent evidence of ICSS facilitation at lower doses and more consistent ICSS depression at higher doses.

Figure 1 shows the effects of LSD, mescaline, and psilocybin on ICSS frequency-rate curves and total stimulations per component. LSD produced biphasic effects with low doses tending to increase ICSS while higher doses decreased ICSS. Mescaline and psilocybin only decreased ICSS at the highest dose, and these effects also significantly decreased the number of stimulations per component.

Figure 2 shows that acute treatment with low and high doses of LSD, mescaline, and psilocybin failed to alter ICSS at any time, and that no dose of any drug produced significant effects on ICSS after 180 or 300 min.

Effects 7-day repeated LSD treatment

The mean frequency-rate curves of 27 rats used in repeated LSD studies were not different from the baseline curves determined in any group. Figure 3 shows that cumulative dosing of LSD, methamphetamine, and U69,593 produced dose-dependent ICSS depression in all groups, whereas cumulative dosing of methamphetamine produced dose-dependent ICSS facilitation in all groups. LSD treatment attenuated effects of all three drugs, but none of these effects met criteria for statistical significance.

DISCUSSION

In a study, LSD, mescaline, and psilocybin produced weak and inconsistent evidence for abuse-related ICSS facilitation, but reliable and robust ICSS depression. Repeated LSD treatment did not alter ICSS depression or facilitation.

LSD, mescaline and psilocybin produced primarily ICSS depression in initial dose-effect and time-course studies, and failed to produce ICSS facilitation in cumulative-dosing studies. This is consistent with previous findings that acute dosing with 5-HT2A agonist hallucinogens fails to produce reliable evidence of abuse-related facilitation in ICSS procedures.

The present study extended previous findings by also evaluating effects of repeated daily LSD treatment. These repeated-dosing studies failed to produce evidence for an abuse-related effect of LSD, and thus failed to provide evidence for a pattern of drug use called “microdosing”.

Subhallucinogenic drug doses at intervals of a day or two are reputed to produce sustained anxiolytic and/or antidepressant effects, but ICSS procedures are unreliable to unmask abuse-related ICSS facilitation by 5-HT2A agonist hallucinogens.

The weak evidence for ICSS facilitation by 5-HT2A agonist hallucinogens is consistent with their legal status as DEA Schedule 1 drugs of abuse, and the failure of these drugs to produce reliable ICSS facilitation in this study is consistent with clinical observation.

After a week of repeated daily LSD treatment, rats did not develop tolerance to the ICSS-depressant effects, but tolerance did develop to the rate-decreasing effects of 0.13 mg/kg LSD after eight days, but not to the slightly higher dose of 0.195 mg/kg LSD after 10 days.

Treatment with hallucinogens has been proposed to produce therapeutic effects for some mental health disorders. The present study evaluated the effects of repeated LSD treatment on abuse-related ICSS facilitation produced by the monoamine releaser methamphetamine and anhedonia-related ICSS depression produced by the kappa opioid receptor agonist U69,593.

Two limitations of the present study warrant mention: first, effects in females may differ; second, the potency and duration of ICSS effects may differ in different serotonergic drugs.

ICSS has been less extensively validated to predict drug effectiveness to treat drug abuse or depression than to predict abuse potential. Consequently, clinical practice would likely use hallucinogens in conjunction with other treatment modalities. Additional studies may be necessary to detect therapeutic effects produced by repeated LSD or other hallucinogens. For example, the sample size in this study was sufficient to detect effects of repeated LSD on subsequent effects of LSD itself and of U69,593, but may have been underpowered to detect effects on methamphetamine. In this study, repeated dosing of LSD was used to mimic microdosing, but the lowest dose was selected in part because it was the smallest.

Repeated LSD dosing at 0.01 mg/kg/day attenuated U69,593-induced depression, but not at lower doses. This suggests that microdosing LSD does not alleviate depression symptoms that might be mediated by activation of endogenous kappa-opioid receptor signaling.

Figure 1.

Acute LSD, mescaline, and psilocybin administration resulted in significantly increased or decreased ICSS rates as determined by two-way ANOVA followed by Holm-Sidak post hoc test, p 0.05. The main effect of LSD dose was 35.81, the main effect of mescaline dose was 4.578, the main effect of psilocybin dose was 15.51, and the main effect of dose approached significance.

Figure 2.

LSD, mescaline, and psilocybin produced different effects on the ICSS in rats. The effects were measured using the Ordinates method and the Filled symbols represent statistically different rates after drug administration. Significant main effects of time and frequency time interaction were observed; no significant main effects of time or frequency time interaction were observed; and no significant main effects of time or frequency time interaction were observed.

Figure 3.

Results show that repeated treatment for 7 consecutive days with saline, 0.01 mg/kg/day LSD, 0.1 mg/kg/day LSD, or 0.32 mg/kg/day LSD significantly decreased reinforcement rates as determined by two-way ANOVA followed by Holm-Sidak post hoc test. Significant main effects of LSD, methamphetamine, frequency-dose interaction, U69,593 dose, and frequency-dose interaction were observed. Main effects of U69,593 dose, frequency-dose interaction, and main effect of methamphetamine were observed.

Figure 4.

A comparison of the effects produced by LSD, methamphetamine, and U69,593 after repeated treatment with saline or LSD shows that LSD attenuates ICSS depression by 0.56 mg/kg U69,593, but there is no significant main effect of LSD treatment group for any drug.