A series of photomicrographs illustrating lesions of the VLPO (A–D) and control lesions (E–I) by ibotenic acid injection.A, The VLPO cluster is visualized by Nissl staining as a pyramidal-shaped cell cluster along the base of the brain, just lateral to the optic chiasm. The triangularcountingbox used to quantify neurons in the VLPO cluster in Nissl preparations is shown. B, The VLPO cluster stands out as an aggregation of c-fos-immunoreactive neurons after sleep. The boxes in Brepresent those used to count cells in the VLPO cluster and the medial (m)- and dorsal (d)-extended VLPO. C, D, After bilateral ibotenic acid lesions, it is possible to quantify the remaining neurons in the VLPO cluster by Nissl staining (C; dashed lines represent borders of lesions) or by Fos immunostaining (D; thearrowheads indicate Fos-positive cells in the VLPO cluster) in animals that are perfused during the height of the sleep cycle, between 10:00 and 12:00 noon. E–I, A lesion of the ventromedial preoptic area (VMPO; E) did little if any damage to the VLPO, and lesions dorsal to the VLPO (F) or larger lesions in the medial preoptic area (H) left the VLPO cluster nearly intact, as shown by higher magnification views of the areasincluded in the boxes (G,I, respectively).