Temporal parameters of enhanced opioid reward after initial opioid exposure in rats

Endogenous opiates and behavior: 2021

This paper is the forty-fourth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2021 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonizts and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Opioid-like adverse effects of tianeptine in male rats and mice

RATIONALE

Tianeptine is a mu-opioid receptor (MOR) agonist with increasing reports of abuse in human populations. Preclinical data regarding the abuse potential and other opioid-like adverse effects of tianeptine at supratherapeutic doses are sparse.

OBJECTIVES

The present study evaluated tianeptine in a rat model of abuse potential assessment and in mouse models of motor, gastrointestinal, and respiratory adverse effects.

METHODS

Abuse potential was assessed in adult male Sprague-Dawley rats using an intracranial self-stimulation (ICSS) procedure to determine effects of acute and repeated tianeptine on responding for electrical brain stimulation. Male ICR mice were used to determine the effects of tianeptine in assays of locomotor behavior and gastrointestinal motility. Male Swiss-Webster mice were monitored for respiratory changes using whole-body plethysmography.

RESULTS

In rats, acute tianeptine produced weak and delayed evidence for abuse-related ICSS facilitation at an intermediate dose (10 mg/kg, IP) and pronounced, naltrexone-preventable ICSS depression at a higher dose (32 mg/kg, IP). Repeated 7-day tianeptine (10 and 32 mg/kg/day, IP) produced no increase in abuse-related ICSS facilitation, only modest tolerance to ICSS depression, and no evidence of physical dependence. In mice, tianeptine produced dose-dependent, naltrexone-preventable locomotor activation. Tianeptine (100 mg/kg, SC) also significantly inhibited gastrointestinal motility and produced naloxone-reversible respiratory depression.

CONCLUSIONS

Tianeptine presents as a MOR agonist with resistance to tolerance and dependence in our ICSS assay in rats, and it has lower abuse potential by this metric than many commonly abused opioids. Nonetheless, tianeptine produces MOR agonist-like acute adverse effects that include motor impairment, constipation, and respiratory depression.

Lack of effect of the nociceptin opioid peptide agonist Ro 64-6198 on pain-depressed behavior and heroin choice in rats

RATIONALE AND OBJECTIVE

One objective of the National Institutes of Health Helping to End Addiction Long-term (HEAL) initiative is to accelerate research on safer and more effective medications for both pain and opioid use disorder. Ligands that activate the nociceptin opioid peptide receptor (NOP) constitute one class of candidate drugs for both applications. The present preclinical study determined the effectiveness of the NOP agonist Ro 64-6198 to produce antinociception in a pain-depressed behavior procedure and attenuate opioid self-administration in a heroin-vs-food choice procedure.

METHODS

In Experiment 1, Adult Sprague-Dawley rats were equipped with microelectrodes and trained to respond for electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. The potency, time course, and receptor mechanism of effects produced by R0 64-6198 alone (0.32-3.2 mg/kg) on ICSS were examined, followed by evaluation of 0.32-1.0 mg/kg Ro 64-6198 effectiveness to block lactic acid-induced depression of ICSS. In Experiment 2, rats self-administered heroin under a heroin-vs-food choice procedure during a regimen of repeated, daily intraperitoneal administration of vehicle or Ro 64-6198 (1-3.2 mg/kg/day).

RESULTS

Ro 64-6198 produced dose- and time-dependent ICSS depression that was blocked by the selective NOP antagonist SB612111 but not by naltrexone. Ro 64-6198 failed to block acid-induced depression of ICSS. Repeated Ro 64-6198 pretreatment also failed to attenuate heroin-vs-food choice up to doses that significantly decreased operant behavior.

CONCLUSIONS

These results do not support the utility of Ro 64-6198 as a stand-alone medication for either acute pain or opioid use disorder.