Morphine and shuttlebox self-stimulation in the rat: tolerance studies

Area of Science:

• Neuroscience
• Pharmacology
• Behavioral Science

Background:

• Electrical brain stimulation is used to study reward pathways.
• Morphine is known for its analgesic properties and potential for abuse.

Purpose of the Study:

• To investigate the effects of morphine on self-stimulation behavior in rats.
• To explore potential differences in mechanisms underlying morphine's effects on behavior and analgesia.

Main Methods:

• Rats were trained to self-administer lateral hypothalamic electrical stimulation.
• Morphine was administered at doses of 10 mg/kg and 20 mg/kg.
• ON and OFF times of stimulation were recorded to assess behavioral changes.

Main Results:

• 10 mg/kg morphine doubled ON times without affecting OFF times; no tolerance developed.
• 20 mg/kg morphine increased ON times 4-5 fold and also increased OFF times.
• Tolerance partially developed to the OFF time increase but not to the ON time increase.

Conclusions:

• Morphine's effect on self-stimulation ON time is dissociable from its analgesic action.
• Separate neurological structures and receptor systems likely mediate the ON and OFF time effects of morphine.
• The lack of tolerance to the ON time increase suggests a robust, distinct mechanism.