DiI tracing of the hypothalamic projection systems during perinatal development

The goals of this article are to suggest a basic wiring diagram for the motor neural network that controls motivated behavior, and to provide a model for the organization of cerebral hemisphere inputs to this network. Cerebral projections mediate voluntary regulation of a behavior control column in the ventromedial upper brainstem that includes (from rostral to caudal) the medial preoptic, anterior hypothalamic, descending paraventricular, ventromedial, and premammillary nuclei, the mammillary body, and finally the substantia nigra and ventral tegmental area. The rostral segment of this column is involved in controlling ingestive (eating and drinking) and social (defensive and reproductive) behaviors, whereas the caudal segment is involved in controlling general exploratory or foraging behaviors (with locomotor and orienting components) that are required for obtaining any particular goal object. Virtually all parts of the cerebral hemispheres contribute to a triple descending projection - with cortical excitatory, striatal inhibitory, and pallidal disinhibitory components - to specific parts of the behavior control column. The functional dynamics of this circuitry remain to be established.

The organization of lateral septal connections has been re-examined with respect to its newly defined subdivisions, using anterograde (PHAL) and retrograde (fluorogold) axonal tracer methods. The results confirm that progressively more ventral transverse bands in the hippocampus (defined by the orientation of the trisynaptic circuit) innervate progressively more ventral, transversely oriented sheets in the lateral septum. In addition, hippocampal field CA3 projects selectively to the caudal part of the lateral septal nucleus, which occupies topologically lateral regions of the transverse sheets, whereas field CA1 and the subiculum project selectively to the rostral and ventral parts of the lateral septal nucleus, which occupy topologically medial regions of the transverse sheets. Finally, the evidence suggests that progressively more ventral hippocampal bands innervate progressively thicker lateral septal sheets. In contrast, ascending inputs to the lateral septum appear to define at least 20 vertically oriented bands or subdivisions arranged orthogonal to the hippocampal input (Risold, P.Y. and Swanson, L.W., Chemoarchitecture of the rat lateral septal nucleus, Brain Res. Rev., 24 (1997) 91-113). Hypothalamic nuclei forming parts of behavior-specific subsystems share bidirectional connections with specific subdivisions of the lateral septal nucleus (especially the rostral part), suggesting that specific domains in the hippocampus may influence specific hypothalamic behavioral systems. In contrast, the caudal part of the lateral septal nucleus projects to the lateral hypothalamus and to the supramammillary nucleus, which projects back to the hippocampus and receives its major inputs from brainstem cell groups thought to regulate behavioral state. The neural system mediating defensive behavior shows these features rather clearly, and what is known about its organization is discussed in some detail.

The fluorescent carbocyanine dyes dil and diO have an extensive history of use in cell biology, but their use as neuronal tracers is relatively recent. We found in 1985 that these molecules were excellent retrograde and anterograde tracers in the developing nervous system. We went on to show that these dyes were retained in neurons placed in culture, that they initially labelled the processes as well as the cell bodies of cultured neurons, and that they were seemingly non-toxic. We suggested that the major mechanism of translocation for these molecules was lateral diffusion in the membrane, rather than fast axonal transport. This suggestion was recently confirmed in a striking manner by Godement et al., when they showed that these dyes can be used to label axonal projections in fixed tissues. Labelling with carbocyanine dyes has already allowed several exciting advances in developmental neurobiology. In this article we review the properties of carbocyanine dyes and point out some of their uses and advantages.