Hodological characterization of the septum in anuran amphibians: I. Afferent connections.

On the basis of Nissl-stained sections, we subdivided the septum of the gray treefrog Hyla versicolor in the lateral, central, and medial septal complex. The afferent projections of the different septal nuclei were studied by combined retrograde and anterograde tracing with biotin ethylendiamine (Neurobiotin). The central and medial septal complex receives direct input from regions of the olfactory bulb and from all other limbic structures of the telencephalon (e.g., amygdalar regions, nucleus accumbens), whereas projections to the lateral septal complex are absent or less extensive. The medial pallium projects to all septal nuclei. In the diencephalon, the anterior thalamic nucleus provides the main ascending input to all subnuclei of the anuran septum, which can be interpreted as a limbic/associative pathway. The ventromedial thalamic nucleus projects to the medial and lateral septal complex and may thereby transmit multisensory information to the limbic system. Anterior preoptic nucleus, suprachiasmatic nucleus, and hypothalamic nuclei innervate the central and lateral septal complex. Only the nuclei of the central septal complex receive input from the brainstem. Noteworthy is the relatively strong projection from the nucleus raphe to the central septal complex, but not to the other septal nuclei.

Hodological characterization of the septum in anuran amphibians: II. Efferent connections.

The efferent connections of the septum of the gray treefrog Hyla versicolor were studied by combining anterograde and retrograde tracing with biotin ethylendiamine (Neurobiotin). The lateral septal complex projects mainly to the medial pallium, limbic regions (e.g., amygdala and nucleus accumbens), and hypothalamic areas but also to sensory nuclei in the diencephalon and midbrain. The central septal complex strongly innervates the medial pallium, limbic, and hypothalamic areas but also specific sensory (including olfactory) regions. The medial septal complex sends major projections to all olfactory nuclei and a weaker projection to the hypothalamus. Our results indicate that all septal nuclei may modify the animal's internal state via efferents to limbic and hypothalamic areas. Via projections to the medial pallium, lateral and central septal complexes may be involved in learning processes as well. Because of their connections to specific sensory areas, all septal areas are in a position to influence sensory processing. Furthermore, our data suggest that both the postolfactory eminence and the bed nucleus of the pallial commissure are not part of the septal complex, rather, the postolfactory eminence seems to be comparable to the mammalian primary olfactory cortex, whereas the bed nucleus may be analogous to the mammalian subfornical organ.

Dorsal striatopallidal system in anurans.

The dorsal striatopallidal system of tetrapods consists of the dorsal striatum (caudate-putamen in mammals) and the dorsal pallidum. Although the existence of striatal and pallidal structures has been well documented in anuran amphibians, the exact boundaries of these structures have so far been a matter of debate. To delineate precisely the dorsal striatopallidal system of anurans, we used quantitative analysis of leucine-enkephalin immunohistochemistry (in Bombina orientalis, Discoglossus pictus, Xenopus laevis, and Hyla versicolor), retrograde neurobiotin tracing studies (injections in the central and ventromedial thalamic nuclei in H. versicolor), and double-labeling tracing studies (injections in the lateral forebrain bundle and the caudal striatum in B. orientalis). Immunohistochemistry revealed that enkephalin-positive neurons are located mainly in the rostral and intermediate striatum. Neurobiotin tracing studies demonstrated that neurons projecting to the central and ventromedial thalamic nuclei are found in the intermediate and caudal striatum. Double-labeling studies revealed that the population of neurons in the rostral and intermediate striatum innervating the caudal striatum is separated from neurons projecting into the lateral forebrain bundle. Neurons that project to both the caudal striatum and the lateral forebrain bundle are found only in the dorsal part of the intermediate striatum. Taken together, our results suggest that the rostral striatum of anurans is homologous to the striatum proper of mammals, whereas the caudal striatum is comparable to the dorsal pallidum. The intermediate striatum represents a transition area between the two structures.