Distribution of met-enkephalin immunoreactivity in the diencephalon and the brainstem of the dog

The endogenous opioid system, in particular the enkephalins, has been impli cated in a vast array of neurological functions. The dog could be a suitabl e model for the study of complex interactions between behavioral state and regulatory physiology in which the opioid system appealed to be implicated. Moreover. opiate derivatives are currently used in veterinary clinic and s ometimes pharmacologically tested in the dog. However, there are no anatomi cal data regarding the organization of the opioid system in this species. T he present work represents the first attempt to map the distribution of Met (5)-enkephalin-like-immunoreactive (Met-enk-li) cell bodies and fibers in t he diencephalon and the brainstem of the dog. In the diencephalon, labeled cells were present in all the mid-line and intralaminar thalamic nuclei. th e lateral posterior, pulvinar and suprageniculate nuclei; the ventral nucle us of the lateral geniculate body and the medial geniculate body. Additiona lly, Met-enk-li cells were seen in every hypothalamic nucleus except in the supraoptic. Variable densities of labeled fibers were also seen in all the se nuclei except in the medial geniculate body and in most areas of the lat eral posterior and pulvinar nuclei. In the mesencephalon, positive cells we re found in the periaqueductal gray, the Edinger-Westphal and interpeduncul ar nuclei, delimited areas of the superior and inferior colliculi and the v entral tegmental area. In the rhombencephalon, labeled cells were seen in t he majority of the nuclei in the latero-dorsal pontine tegmentum, the nucle i of the lateral lemniscus, the trapezoid, vestibular medial, vestibular in ferior and cochlear nuclei, the prepositus hypoglossal, the nucleus of the solitary tract and the dorsal motor nucleus of the vagus, the infratrigemin al nucleus and the caudal part of the spinal trigeminal nucleus and in the rhombencephalic reticular formation. The distribution of fibers included ad ditionally the substantia nigra, all the trigeminal nerve nuclei, the facia l nucleus and a restricted portion of the inferior olive. These results are discussed with regard to previous reports on the distribution of Met-enk i n other species. (C) 2000 Elsevier Science B.V. All rights reserved.