Presynaptic stimulation of dopaminergic CNS structures in sheep as a mechanism of immobilising action of Immobyl (fentanyl + azaperone).

The immobilising action of Immobyl (fentanyl : azaperone, 5:1) in six sheep has been analysed on the basis of the changes in dopamine, noradrenalin, adrenalin, homovanillic acid and 5-hydroxytryptamine concentrations in the corpus striatum, frontal motor cortex, pons, cerebellum and lumbosacral spinal cord as compared to the control animals which were given saline. Forty minutes after intramuscular injection of an immobilising dose (0.19 mg [kg bodyweight]-1), Immobyl caused a significant decrease in dopamine, noradrenalin and 5-hydroxytryptamine concentrations and a similarly large decrease in homovanillic acid concentration (47 per cent) in the corpus striatum with a simultaneous but insignificant increase in the concentrations of these substances in the frontal motor cortex region. In Immobyl immobilisation, sheep showed a significant increase in dopamine concentration with an equally significant decrease in homovanillic acid concentrations in the lumbosacral part of the spinal cord. It is suggested that fentanyl stimulates the presynaptic dopamine receptors in the corpus striatum in sheep, significantly decreasing synthesis and release of dopamine and noradrenalin and intensifying an inhibitory effect of the corpus striatum on locomotor activity and thus causes the immobilisation of the animal.