How do tranquilizing agents selectively inhibit conditioned avoidance responding?

Area of Science:

• Neuroscience
• Behavioral Pharmacology
• Animal Models

Background:

• Tranquilizing agents often inhibit conditioned avoidance responses (CAR) at doses that do not affect escape responses (ER).
• This selective action has been hypothesized to result from differences in the response strengths of CAR and ER.

Purpose of the Study:

• To investigate the hypothesis that the selective inhibition of CAR by tranquilizing agents is due to differential response strengths.
• To determine if drugs differentially affect CAR and ER based on shock intensity and response probability.

Main Methods:

• Rats were trained to perform CAR and ER in an operant chamber, with shock intensity adjusted to equalize response probability and latency.
• The effects of chlorpromazine, clonidine, diazepam, and morphine on CAR and ER were assessed at varying shock intensities.

Main Results:

• Drugs that reduced CAR by 30%-50% did not significantly affect high-shock ER (ER on CAR trials where no CAR occurred).
• These same drug doses interfered with low-shock ER to the same extent as CAR.
• A small differential effect of chlorpromazine on CAR and low-shock ER across trials suggested potentially different underlying mechanisms.

Conclusions:

• The selective blockade of CAR by tranquilizing drugs in this paradigm is primarily attributed to differential strengths between CAR and ER.
• These findings support the view that tranquilizing drugs impair avoidance behavior by hindering motor response initiation, rather than affecting associative processes or emotional reactions.
• However, subtle differential effects suggest that distinct mechanisms might underlie the suppression of CAR and low-shock ER.