Journal of Neuroscience, Vol 7, 2025-2034, Copyright © 1987 by Society for Neuroscience
Heterogeneous distribution of neurotensin-like immunoreactive neurons and fibers in the midbrain periaqueductal gray of the rat
MT Shipley, JH McLean and MM Behbehani
The midbrain periaqueductal gray (PAG) has been shown to be a site where
various manipulations induce pain suppression. Recent physiological
evidence (Behbehani and Pert, 1984; Behbehani et al., 1987) suggests that
neurotensin has pronounced physiological actions in PAG and effects pain
suppression. We have performed immunohistochemical studies in order to
determine the magnitude and distribution of neurotensin-like immunoreactive
(NT-IR) cell bodies and fibers in PAG. NT-IR cell bodies were common
throughout PAG, although there were more in the caudal than the rostral
half. NT-IR neurons were much more numerous in the ventral than the dorsal
half of PAG, and some appeared to be located within the dorsal raphe
nucleus. The pattern of NT-IR fibers was analyzed with the aid of image
enhancement/analysis and densitometry. The fibers were found to be
heterogeneously distributed, being most heavily concentrated in the region
adjacent to the cerebral aqueduct in the caudal two-thirds of PAG. The
distribution of NT fibers closely matches sites where exogenously applied
NT elicits long-lasting excitation of PAG neurons (Behbehani et al., 1987).
Based on the known physiological and behavioral actions of NT in PAG, the
present anatomical results suggest that NT acts on elements located
predominantly in the medial and ventrolateral parts of PAG. Neurons
activated by NT may project directly to the nucleus raphe magnus and
adjacent ventral medulla (Behbehani and Pert, 1984) to activate neurons
that project to the spinal cord and modulate nociceptive circuits.