Selected opioids and responding for intracranial reinforcement.

Area of Science:

• Neuroscience
• Behavioral Pharmacology
• Opioid Research

Background:

• Intracranial stimulation (ICS) of the lateral hypothalamus is a model for brain reward.
• Ethylketocyclazocine (EKC) is an opioid with complex effects on behavior.

Purpose of the Study:

• To investigate the effects of EKC isomers on lever pressing for ICS.
• To determine if opioid analgesia and reward-enhancing properties are separable.

Main Methods:

• Rats with electrodes for ICS were administered racemic EKC, (+)EKC, or (-)EKC, with or without naloxone (NX).
• Lever pressing behavior for ICS was recorded and analyzed across different drug doses.

Main Results:

• Racemic EKC and (+)EKC facilitated lever pressing at specific doses.
• (-)EKC, a potent analgesic, did not facilitate pressing and often depressed it.
• Naloxone paradoxically facilitated pressing when combined with a high dose of racemic EKC, suggesting selective blockade of (-)EKC effects.

Conclusions:

• Opioid analgesia and the ability to enhance brain stimulation reward are distinct properties.
• (+)EKC enhances reward-seeking behavior, while (-)EKC does not, despite its analgesic effects.
• Opioid receptor interactions are complex and isomer-dependent, influencing reward pathways.