Cellular distribution of immunoreactive somatostatin in the basal forebrain of horses and ponies

Abstract

Summary In other experimental models, somatostatin (SRIF) helps coordinate central nervous system (CNS) activity associated with the response to stress. It also plays an important role in the effects of exercise, stress and bacterial infections on growth hormone (GH) release and it may interact with specific SRIF receptors found on lymphocytes. Pituitary GH is a potent immunomodulatory hor- mone which is important to normal function of the thymus and a wide range of immune defense mechanisms. This experiment mapped the distribution of immunoreactive (ir) SRIF in the basal forebrain of horses and ponies. Results are compared to those for other species in order to determine if the anatomical substrate for stress-dependant SRIF effects may be similar to that described for other animal models. Immersion fixed brain tissues were obtained from animals submitted for euthanasia and/or necropsy. Alternate sections were stained for ir-SRIF using characterized polyclonal SRIF antibody and the peroxidase-anti-peroxidase technique, as previously described. Distribution of SRIF cells and fibers was mapped on projection drawings following light microscopic examination. Tissues in this analysis included cell groups at the border between the telencephalon and the rostral diencephalon, the hypothalamus, caudate nucleus (n) subthalamus and thalamus. At rostral levels, ir-SRIF was widely dispersed throughout medial tissues. Caudally, pronounced immunoreactivity was found in ventromedial tissues. Reactive perikarya were localized in the caudate n, olfactory tuberculum, n of the lamina terminalis, rostral preoptic n, suprachiasmatic n, periventricular n, supraoptic n, paraventricular n, arcuate n and ventromedial n. Numerous SRIF fibers extended into the lateral septum, all preoptic n, paraventricular n, rostral hypothalamic n, dorsomedial n, ventromedial n and mammillary n. Reactive fibers were also found in midline thalamic n, perifomical areas, the zona incerta and the habenular n. This distribution is similar to that described in most vertebrates. It reflects the role of SRIF in multiple neurocrine interactions as well as its neuroendocrine activity. Results from this experiment suggest that equine SRIF is likely to participate in neural and endocrine pathways similar to that for other mammalian species. Similar localization patterns also suggest that the coordination of SRIF cell functions within the equine CNS may be comparable to that in other species. This suggests that studies in other species are likely to facilitate the characterization of control mechanisms for SRIF secretion in the equine species.