Grass rat. Line drawing of every 6th section through the region of the grass rat hypothalamus that contains orexin cells. Sections are ordered from rostral (A) to caudal (L). Filled circles indicate locations where both orexin A and orexin B neurons are found, while open circles indicate orexin A neurons only. 3V: third ventricle; SCN: suprachiasmatic nucleus; OC: optic chiasm; f: fornix; mt: mammillothalamic tract. Scale bar = 500 μm.