Involvement of the midbrain tectum in the unconditioned fear promoted by morphine withdrawal

The midbrain tectum structures, dorsal periaqueductal gray (dPAG) and inferior colliculus (IC), are involved in the organization of fear and anxiety states during the exposure to dangerous stimuli. Since opiate withdrawal is associated with increased anxiety in both humans and animals, this study aimed to investigate the possible sensitization of the neural substrates of fear in the midbrain tectum and its influence on the morphine withdrawal-induced anxiety. For the production of drug withdrawal, rats received morphine injections (10 mg/kg; s.c.) twice daily during 10 days. Forty-eight hours after the interruption of the chronic treatment, independent groups were probed in the elevated plus-maze and open-field tests. Additional groups of animals were implanted with a bipolar electrode into the dPAG or the IC and submitted to the electrical stimulation of these structures for the determination of the freezing and escape thresholds after 48 h of withdrawal. Our results showed that the morphine withdrawal promoted clear-cut levels of anxiety without the somatic signs of opiate withdrawal. Moreover, morphine-withdrawn rats had an increase in the reactivity to the electrical stimulation of the dPAG and the IC. These findings suggest that the increased anxiety induced by morphine withdrawal is associated with the sensitization of the neural substrates of fear in the dPAG and the IC. So, the present results give support to the hypothesis that withdrawal from chronic treatment with morphine leads to fear states possibly engendered by activation of the dPAG and IC, regardless of the production of somatic symptoms.