Mesencephalic central gray: Locus of morphine and electrical stimulation induced tail erection

Abstract

Previous ablation studies suggested that morphine most likely acts on the mesencephalic central gray producing tail erection. A microinjection technique, therefore, was used in the present investigation to further localize morphine induced tail erection (MITE) in the mesencephalon. Morphine, 1 to 10 μg, administered through chronically implanted injection cannula into the mesencephalic central gray elicited dose dependent tail erection in conscious mice equal to that produced by morphine, 20 to 100 mg/kg SC. In contrast, morphine 20 μg administered intracerebrally into areas outside the central gray in the mesencephalon did not produce any appreciable tail elevation. In addition, focal electrical stimulation was used to evaluate the physiological role of the mesencephalon in tail erection of the conscious mouse. Only stimulation of the mesencephalic central gray produced dose dependent tail erection. Stimulation of other brain areas never produced a tail response. Pretreatment with haloperidol abolished tail erection produced either by intracerebrak morphine or by electrical stimulation. Tail elevation elicited by subcutaneous morphine was also antagonized by haloperidol administered into the mesencephalic central gray. Results indicate that the mesencephalic central gray is the locus of MITE in the central nervous system. By analogy with the site and mechanism of morphine action, it is proposed that focal stimulation activates a locomotor system concentrated in the periaqueductal region which elicits tail elevation by transmitting impulses through the spinal cord.