Locomotor activating effects of cocaine and scopolamine combinations in rats: isobolographic analysis

Application of dose-addition analyses to characterize the abuse-related effects of drug mixtures

Polysubstance use makes up a majority of drug use, yet relatively few studies investigate the abuse-related effects of drug mixtures. Dose-addition analyses provide a rigorous and quantitative method to determine the nature of the interaction (i.e., supraadditive, additive, or subadditive) between two or more drugs. As briefly reviewed here, studies in rhesus monkeys have applied dose-addition analyses to group level data to characterize the nature of the interaction between the reinforcing effects of stimulants and opioids (e.g., mixtures of cocaine + heroin). Building upon these foundational studies, more recent work has applied dose-addition analyses to better understand the nature of the interaction between caffeine and illicit stimulants such as MDPV and methamphetamine in rats. In addition to utilizing a variety of operant procedures, including drug discrimination, drug self-administration, and drug-primed reinstatement, these studies have incorporated potency and effectiveness ratios as a method for both statistical analysis and visualization of departures from additivity at both the group and individual subject level. As such, dose-addition analyses represent a powerful and underutilized approach to quantify the nature of drug-drug interactions that can be applied to a variety of abuse-related endpoints in order to better understand the behavioral pharmacology of polysubstance use.

Combinations of a full and partial agonist: Experimental evidence of curved isoboles

Concentration addition as a classic null model for toxicology and pharmacology is based on Loewe's mathematical formulation and the linearity of the isoboles. Novel mathematical models, however, propose curved isoboles in certain conditions. This article aims to test the hypothesis of the curvature of isoboles in experimental measurements. With the assumption of linear isoboles, a partial agonist acts as an antagonist above its maximal effect level. The isoboles automatically convert to a positive slope. For curved isoboles, a partial agonist acts as an antagonist at higher effect levels than its maximal effect alone. The discrepancies between effect levels were studied with an estrogen receptor binding assay (BMAEREluc/ERα) using a mixture of 17β-estradiol and fulvestrant as a partial agonist. A mixture of 17β-estradiol and fulvestrant acts as a partial agonist and causes the diminishing of the effect level of 17β-estradiol at a significantly higher level than the maximal effect of their partial-agonistic dose-response curve. Measured, elevated effect levels were well predicted by the mathematical model. Nonlinear isoboles may change our understanding and definition of synergism or antagonism and prompt further attention in receptor theory.

Scopolamine Impairs Spatial Information Recorded With "Miniscope" Calcium Imaging in Hippocampal Place Cells

The hippocampus and associated cholinergic inputs have important roles in spatial memory in rodents. Muscarinic acetylcholine receptors (mAChRs) are involved in the communication of cholinergic signals and regulate spatial memory. They have been found to impact the memory encoding process, but the effect on memory retrieval is controversial. Previous studies report that scopolamine (a non-selective antagonist of mAChR) induces cognitive deficits on animals, resulting in impaired memory encoding, but the effect on memory retrieval is less certain. We tested the effects of blocking mAChRs on hippocampal network activity and neural ensembles that had previously encoded spatial information. The activity of hundreds of neurons in mouse hippocampal CA1 was recorded using calcium imaging with a miniaturised fluorescent microscope and properties of place cells and neuronal ensemble behaviour in a linear track environment were observed. We found that the decoding accuracy and the stability of spatial representation revealed by hippocampal neural ensemble were significantly reduced after the administration of scopolamine. Several other parameters, including neural firing rate, total number of active neurons, place cell number and spatial information content were affected. Similar results were also observed in a simulated hippocampal network model. This study enhances the understanding of the function of mAChRs on spatial memory impairment.

Understanding Central Nervous System Effects of Deliriant Hallucinogenic Drugs through Experimental Animal Models

Hallucinogenic drugs potently alter human behavior and have a millennia-long history of use for medicinal and religious purposes. Interest is rapidly growing in their potential as CNS modulators and therapeutic agents for brain conditions. Antimuscarinic cholinergic drugs, such as atropine and scopolamine, induce characteristic hyperactivity and dream-like hallucinations and form a separate group of hallucinogens known as "deliriants". Although atropine and scopolamine are relatively well-studied drugs in cholinergic physiology, deliriants represent the least-studied class of hallucinogens in terms of their behavioral and neurological phenotypes. As such, novel approaches and new model organisms are needed to investigate the CNS effects of these compounds. Here, we comprehensively evaluate the preclinical effects of deliriant hallucinogens in various animal models, their mechanisms of action, and potential interplay with other signaling pathways. We also parallel experimental and clinical findings on deliriant agents and outline future directions of translational research in this field.

Effects of muscarinic M1 and M4 acetylcholine receptor stimulation on extinction and reinstatement of cocaine seeking in male mice, independent of extinction learning

RATIONALE

Stimulating muscarinic M₁/M₄ receptors can blunt reinforcing and other effects of cocaine. A hallmark of addiction is continued drug seeking/craving after abstinence and relapse.

OBJECTIVES

We tested whether stimulating M₁ and/or M₄ receptors could facilitate extinction of cocaine seeking, and whether this was mediated via memory consolidation.

METHODS

Experimentally naïve C57BL/6J mice were allowed to acquire self-administration of intravenous cocaine (1 mg/kg/infusion) under a fixed-ratio 1 schedule of reinforcement. Then, saline was substituted for cocaine until responding extinguished to ≤30% of cocaine-reinforced responding. Immediately after each extinction session, mice received saline, the M₁/M₄ receptor-preferring agonist xanomeline, the M₁ receptor-selective allosteric agonist VU0357017, the M₄ receptor-selective positive allosteric modulator VU0152100, or VU0357017 + VU0152100. In additional experiments, xanomeline was administered delayed after the session or in the home cage before extinction training began. In the latter group, reinstatement of responding by a 10-mg/kg cocaine injection was also tested.

RESULTS

Stimulating M₁ + M₄ receptors significantly expedited extinction from 17.2 sessions to 8.3 using xanomeline or 7.8 using VU0357017 + VU0152100. VU0357017 alone and VU0152100 alone did not significantly modify rates of extinction (12.6 and 14.6 sessions). The effect of xanomeline was fully preserved when administered delayed after or unpaired from extinction sessions (7.5 and 6.4 sessions). Xanomeline-treated mice showed no cocaine-induced reinstatement.

CONCLUSIONS

These findings show that M₁/M₄ receptor stimulation can decrease cocaine seeking in mice. The effect lasted beyond treatment duration and was not dependent upon extinction learning. This suggests that M₁/M₄ receptor stimulation modulated or reversed some neurochemical effects of cocaine exposure.

Effects of muscarinic receptor antagonists on cocaine discrimination in wild-type mice and in muscarinic receptor M1, M2, and M4 receptor knockout mice

Muscarinic M₁/M₄ receptor stimulation can reduce abuse-related effects of cocaine and may represent avenues for treating cocaine addiction. Muscarinic antagonists can mimic and enhance effects of cocaine, including discriminative stimulus (S^D) effects, but the receptor subtypes mediating those effects are not known. A better understanding of the complex cocaine/muscarinic interactions is needed to evaluate and develop potential muscarinic-based medications. Here, knockout mice lacking M₁, M₂, or M₄ receptors (M₁^-/-, M₂^-/-, M₄^-/-), as well as control wild-type mice and outbred Swiss-Webster mice, were trained to discriminate 10mg/kg cocaine from saline. Muscarinic receptor antagonists with no subtype selectivity (scopolamine), or preferential affinity at the M₁, M₂, or M₄ subtype (telenzepine, trihexyphenidyl; methoctramine, AQ-RA 741; tropicamide) were tested alone and in combination with cocaine. In intact animals, antagonists with high affinity at M₁/M₄ receptors partially substituted for cocaine and increased the S^D effect of cocaine, while M₂-preferring antagonists did not substitute, and reduced the S^D effect of cocaine. The cocaine-like effects of scopolamine were absent in M₁^-/- mice. The cocaine S^D attenuating effects of methoctramine were absent in M₂^-/- mice and almost absent in M₁^-/- mice. The findings indicate that the cocaine-like S^D effects of muscarinic antagonists are primarily mediated through M₁ receptors, with a minor contribution of M₄ receptors. The data also support our previous findings that stimulation of M₁ receptors and M₄ receptors can each attenuate the S^D effect of cocaine, and show that this can also be achieved by blocking M₂ autoreceptors, likely via increased acetylcholine release.