Octopamine Sensitivity of the Blood-brain Barrier of an Insect

Octopaminergic sensitivity of the glia that form the blood—brain barrier of the cockroach was examined using microelectrode recordings of the potential developed across this barrier layer, the perineurium. Exposure to 10~moll~ DL-octopamine had no significant effect upon the changes in potential induced by substitution of Tris for external sodium. Treatment with concentrations ranging from 10~moll~' down to 5 X 10~ mol 1~' enhanced the resting level of the potential after 5 min, with an accompanying depression of the magnitude of potassium-induced steps. At 10~moll~', there was no significant effect upon the resting level, but there was a reduction in potassium sensitivity. In a study of the effect of other neurohormonal factors, it was found that DL-synephrine could depress the response to potassium but was less effective than octopamine. In contrast, carbamylcholine, histamine and some compounds related to octopamine enhanced the potassium sensitivity. The effect of octopamine was completely blocked by phentolamine, and partially blocked by propranolol. It is concluded that the perineurium possesses an octopamine receptor that could respond to octopamine circulating in the haemolymph, and which appears to mediate a reduction in the potassium sensitivity of the basolateral membrane of these glia.