Response of Plasma Adrenocorticotropin to Injections of L-Glutamate or Norepinephrine in the Dorsal Rostral Pons of Cats*

Abstract

Previously, electrical stimulation of several nuclei in the dorsal rostral pons of the cat was shown to modulate the release of ACTH. However, the stimulation may have activated fibers of passage. To determine if there are specific groups of neurons within the pons that modulate plasma ACTH when stimulated with glutamate, 30 cats were prepared acutely under chloralose anesthesia. Microinjections of several agents were made at each of 2 sites in the pons of each cat. ACTH was measured by RIA. Injections of 150 mM L-glutamate (100 nl/min.2 min) elicited increases in arterial pressure that were related to the loci of injection. The responses did not decrease significantly when the rate and volume of the injection were reduced by half. Eight sites in a lateral area that extended rostrally from the parabracheal nucleus elicited a significant pressor response that was greater in duration and magnitude than a second significant pressor response that was obtained from 18 sites in a medial area that included the rostral locus coeruleus. Pressor responses did not occur when 0.1 mM norepinephrine or vehicle was substituted for L-glutamate in any area. The larger, but not the smaller, dose of L-glutamate elicited changes in plasma ACTH that were related to the loci of injection. Eight sites in a caudal area that included the ventral locus coeruleus and was within the medial pressor area elicited a significant increase in ACTH. Seven sites in an area rostral to the ventral locus coeruleus that included the rostromedial locus subcoeruleus elicited a significant decrease in plasma ACTH. ACTH responses were not observed when 0.1 mM norepinephrine or vehicle was substituted for L-glutamate in any area. In conclusion, the ventral locus coeruleus and the rostrally adjacent locus subcoeruleus contain neurons with receptors for L-glutamate that can modulate plasma ACTH and arterial pressure independently of a second pathway that includes the parabracheal region and influences arterial pressure. Because the neurons in the coeruleus are known to respond to hemodynamic input, they may participate in the hemodynamic control of ACTH release.