Interactions between the lateral hypothalamus and the periaqueductal gray

Anatomical and physiological experiments were conducted to characterize the interactions between the lateral hypothalamus (LH) and the periaqueductal gray (PAG) and to determine the role of neurotensin in their interaction. Anatomical studies using injection of Phaseolus vulgaris leucoagglutinin into the LH showed an extensive projection to the ventromedial and the ventrolateral PAG and a less dense projection to the medial and dorsal parts of this region. Physiological experiments were performed on both deeply and lightly anesthetized animals. Electrical stimulation of the LH caused excitation of PAG cells with an onset latency of approximately 14 msec. There was a strong correlation between the response of PAG cells to electrical stimulation and injection of glutamic acid into the LH. Electrical or chemical stimulation of the LH produced an increase in tail flick latency in the lightly anesthetized animals that outlasted the stimulation period. There was a strong correlation between the response of PAG cells to electrical and chemical stimulation of the LH and their response to pressure-injected neurotensin. It is concluded that an excitatory projection from the LH to PAG exists which may involve neurotensin. This pathway may be involved in the analgesia produced by LH stimulation.