The nucleus isthmi pars magnocellularis (Imc) and pars parvocellularis (Ipc) influence the receptive field structure of neurons in the optic tectum (TeO). To understand better the anatomical substrate of isthmotectal interactions, neuronal morphology and connections of Imc were examined in chicks (Gallus gallus). Cholera toxin B injection into TeO demonstrated a coarse topographical projection from TeO upon Imc. Retrogradely labeled neurons were scattered throughout Imc and in low density within the zone of anterogradely labeled terminals, suggesting a heterotopic projection from Imc upon TeO. This organization differed from the precise homotopic reciprocal connections of Ipc and the nucleus isthmi pars semilunaris (SLu) with TeO. By using slice preparations, extracellular biotinylated dextran amine injections demonstrated a dense projection from most neurons in Imc upon both Ipc and SLu. Intracellular filling of Imc neurons with biocytin revealed two cell types. The most common, Imc‐Is, formed a widely ramifying axonal field in both Ipc and SLu, without obvious topography. A less frequently observed cell type, Imc‐Te, formed a widely ramifying terminal field in layers 10–12 of TeO. No neurons were found to project upon both Ipc/SLu and TeO. Both types possessed local axon collaterals and flat dendritic fields oriented parallel to the long axis of Imc. Imc neurons contain glutamic acid decarboxylase, which is consistent with Imc participating in center‐surround or other wide‐field inhibitory isthmotectal interactions. The laminar and columnar pattern of isthmotectal terminals also suggests a means of interacting with multiple tectofugal pathways, including the stratified subpopulations of tectorotundal neurons participating in motion detection. J. Comp. Neurol. 469:275–297, 2004. © 2003 Wiley‐Liss, Inc.

The retinofugal pathways in the California ground squirrel, Spermophilus beecheyi, were mapped after intravitreal injections of cholera toxin B-subunit. The results of the current study are consistent with work in other mammals and provide new details relevant to the organization and evolution of the visual system. All retinorecipient nuclei received bilateral input, with a contralateral predominance. The suprachiasmatic nucleus is heavily innervated, and sparse terminals were noted in other hypothalamic areas. In addition to the interstitial, medial, lateral, and dorsal terminal nuclei, a few fibers of the accessory optic tract may enter the ventral lateral geniculate and the nucleus of the optic tract, though this innervation may not derive from the same ganglion cells innervating the accessory optic nuclei. Retinal terminals are found in the intergeniculate leaflet and the "dorsal cap" of the ventral lateral geniculate. Retinal fibers pass rostrally from the dorsal cap toward the anterodorsal thalamus, confirming a projection described in the tree shrew and monkeys. Retinal termination patterns in the dorsal lateral geniculate reveal a hexilaminate organization of alternating ipsilateral and contralateral input. Variations in terminal morphology suggest that sublayers receive input from distinct ganglion cell types and that laminar comparisons can be made with primates. Finally, terminal patterns in the superior colliculus reveal a dense, highly ordered columnar organization supporting functional properties of tectal receptive fields. All the visual structures in the ground squirrel are large and well differentiated, making the sciurid visual system an accessible rodent model for comparing visual processing with that in other diurnal vertebrates.

Three days of male and female cohabitation dramatically reduces the density of vasopressin-immunoreactive (AVP-ir) fibers in the lateral septum and lateral habenular nucleus of male, but not of female prairie voles. Here we tested whether this reduction is associated with changes in AVP messenger RNA (mRNA) expression in the bed nucleus of the stria terminalis (BST), the presumed source of these fibers, and with changes in testosterone levels, which may influence AVP biosynthesis in the BST. In addition, we tested whether similar changes can be found in meadow voles, which unlike prairie voles do not dramatically change their social behaviors after mating. In both species, males showed more AVP mRNA-labeled cells in the BST and more grains per labeled cell than females. In prairie vole males, cohabitation increased the number of AVP mRNA labeled BST cells and the density of grains per labeled cells. It also raised plasma testosterone levels. No changes were found in prairie vole females nor in meadow voles of either sex. The changes in prairie vole males suggest that cohabitation stimulates AVP biosynthesis. The previously observed decrease in AVP-ir fiber density in the lateral septum and lateral habenular nucleus may therefore reflect increased synaptic release of AVP, which may contribute to mating-induced changes in social behaviors in prairie vole males.