Inhibitory and Facilitatory Areas of the Ventral Midbrain Mediating Release of Corticotropin in the Cat*

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Abstract
To examine the role of the ventral midbrain in the control of release of ACTH, 92 sites were stimulated electrically in the mesencephalon of 15 cats anesthetized with chloralose/urethane. Responses of arterial pressure could not account for change of release of ACTH. Three active areas were identified. First, in a dorsal facilitatory area that includes the dorsal longitudinal fasciculus, electrical stimulation led to changes in ACTH of +106, +117, and +90 pg/ml at 1.5, 3.5 and 6.5 min, respectively (P < 0.05). Second, in a more ventral inhibitory area that includes the mammillary peduncle, electrical stimulation led to changes in ACTH of -63, -72, and -47 pg/ml, respectively (P < 0.05). Third, in a ventral facilitatory area that includes the ventral tegmental area of Tsai, electrical stimulationled to changes in ACTH of +57, +56, and +59 pg/ml, respectively (P < 0.01). The inhibitory and facilitatory areas of the ventral midbrain appeared to be continuous, respectively, with the inhibitory and facilitatory areas mediating control of ACTH in the dorsal rostral pons and in the hypothalamus. Anatomical evidence indicates projections from these ACTH-active areas of the midbrain and of the pons to ACTH-active areas of the hypothalamus. The present results suggest that the midbrain areas identified may represent pathways from ACTH-active areas of the pons to the hypothalamus.