Rostrocaudal somatotopy in the neural connections between the lateral hypothalamus and the dorsal periaqueductal gray of the rat brain.

1. The lateral hypothalamus (LH) and the dorsal periaqueductal gray area (dPAG) are two important brain structures involved in central cardiovascular control. 2. In the present study, we searched for possible rostrocaudal somatotopy in the neural connections from the three subdivisions of the LH (anterior-LHa; tuberal-LHt and posterior-LHp) to the different rostrocaudal portions of the dPAG. 3. The bidirectional neuronal tracer biotinylated-dextran-amine (BDA) was microinjected into different rostrocaudal coordinates of the dPAG (AP 3.4-2.7 mm) of male Wistar rats. One week later, animals were sacrificed and brain slices were processed and analyzed to detect neuronal efferent projections from the LH to the dPAG. 4. Neuronal cell body staining was observed along all the rostrocaudal axis of the LH when BDA was microinjected in more rostral dPAG coordinates. When the BDA was microinjected into more caudal dPAG regions, labeled neurons were observed only in the caudal portion of the LH. 5. Efferent projections from the LHa were directed only to the rostral portion of the dPAG. Projections from the rostral and medial portions of the LHt were also directed to the rostral dPAG, whereas both rostral and caudal dPAG received projections from the caudal portion of the LHt. Efferent projections from the anterior portion of the LHp were directed to both rostral and caudal dPAG, whereas projections from the caudal LHp were only directed to the rostral portion of the dPAG.6. The results suggest a somatotopic correlation in LH projections to the dPAG with main connections to the rostral dPAG, which are efferent from the three divisions of the LH. More caudal regions of the dPAG received afferents only from posterior sites in the LH. 7. Moreover, the results point out to extensive and complex neural somatotopic projections from all LH subdivisions to different rostrocaudal portions of the dPAG, reinforcing the idea of significant functional interactions between the brain structures.