The distribution of Met-enkephalin like immunoreactivity in the brain of Apteronotus leptorhynchus, with emphasis on the electrosensory system.

The distribution of Met-enkephalin-like immunoreactivity in the brain of the electric fish, Apteronotus leptorhynchus was analysed by immunohistochemistry. The majority of Met-enkephalin immunoreactive neurons were found in the hypothalamus. Dense Met-enkephalin immunoreactive fiber plexuses were seen in the hypothalamus and ventral forebrain. In the dorsal telencephalon an olfacto-recipient region (ventral subdivision of the dorsolateral forebrain) was specifically and densely innervated. Regions of the brain known to be involved in electrocommunication also received a substantial innervation by Met-enkephalin-like immunoreactive fibers. This distribution of immunoreactive fibers in the brain of this gymnotiform fish indicates that Met-enkephalin may be generally involved in the regulation of sensory, neuroendocrine and reproductive functions and specifically in the regulation of electrocommunication.

Anatomical organization of the hypophysiotrophic systems in the electric fish, Apteronotus leptorhynchus.

The organization of afferents to the pituitary was investigated by applying DiI crystals to the pituitary or pituitary stalk of the gymnotiform electric fish, Apteronotus leptorhynchus. Most hypophysiotrophic cells were found in the hypothalamus and were distributed throughout its rostrocaudal extent: nucleus preopticus periventricularis, pars anterior and posterior; suprachiasmatic nucleus; anterior, dorsal, ventral, lateral, and caudal hypothalamic nuclei; and nucleus tuberis lateralis, pars anterior and posterior. In addition a small number of retrogradely labeled cells were found in the ventral telencephalon (area ventralis, pars ventralis) and, most surprisingly, in a thalamic nucleus (nucleus centralis posterioris). The nucleus preopticus periventricularis pars posterior and the anterior hypothalamic nucleus appear to correspond to the parvicellular and magnocellular divisions of the nucleus preopticus of other teleosts. Integration of these results with immunohistochemical localization of monoamines and neuropeptides in the apteronotid brain suggests many homologies between the hypophysiotrophic nuclei of teleosts and other vertebrates, including mammals. Apteronotus communicates electrically during agonistic and sexual interactions. There are numerous anatomical links between the hypophysiotrophic systems and the brain areas related to electrocommunication.

The distribution of excitatory amino acid binding sites in the brain of an electric fish, Apteronotus leptorhynchus.

The distribution of three types of exitatory amino acid receptors was examined in the brain of a high frequency weakly electric fish, Apteronotus leptorhynchus, by localizing the binding sites of ligands selective for mammalian kainic acid (KA), quisqualate (AMPA) and N-methyl-D-aspartate (NMDA) receptors. All three binding sites were densest within the forebrain and in certain hypothalamic nuclei (nucleus tuberis anterior, inferior lobe). The core of the dorsal forebrain (dorsal centralis) had a very high density of NMDA binding sites and only moderate levels of AMPA and KA binding sites, while this was reversed for the dorsolateral forebrain. The AMPA and NMDA binding sites were found throughout the brain while KA binding sites were relatively restricted and were absent from most of the brainstem. The cerebellar molecular layer contained a very high density of KA and AMPA binding sites but almost no NMDA binding sites; the granular layer had a low density of AMPA and NMDA binding sites but was lacking in KA binding sites. All three types of binding sites were found within the electromotor system (nucleus electrosensorius and prepacemaker nucleus) at sites where the iontophoresis of glutamate causes species-specific behaviours. KA binding sites were found at only two sites along the electrosensory afferent pathways: (1) in the molecular layer of the electrosensory lateral line lobe, associated with a feedback pathway emanating from granule cells of the overlying cerebellum, and (2) in the lateral nucleus praeminentialis dorsalis, associated with a descending pathway emanating from the torus semicircularis. NMDA and AMPA binding sites are found throughout the electrosensory pathways. Within the electrosensory lateral line lobe the NMDA binding sites were predominantly associated with the feedback pathways terminating in its molecular layer and not with the deep neuropil layer containing primary electroreceptor afferents.

Hypophysiotropic neurons in the goldfish hypothalamus demonstrated by retrograde transport of horseradish peroxidase.

The horseradish-peroxidase (HRP) technique was used to visualize the cell bodies of axons projecting to the goldfish pituitary. Following intravenous injections of HRP, HRP reaction products were observed in axons of the rostral pars distalis, proximal pars distalis, neurointermediate lobe, pituitary stalk and in axons coursing from the pituitary into the hypothalamus. HRP-labelled cells in the brain were localized in two regions only - the nucleus preopticus (NPO) pars magnocellularis and pars parvocellularis, and the nucleus lateralis tuberis (NLT) of the hypothalamus. These observations suggest that the NPO and NLT are the source of the neurosecretory innervation of the goldfish pituitary.