Organum cavum pre-lamina terminalis, an undescribed brain organ in rats: morphology and physiology.

Morphological as well as physiological findings reported here point to the existence of a previously undescribed brain organ we refer to as the organum cavum pre-lamina terminalis (OCPLT). The organ is a hollow, sagittal, forebrain structure consisting of a vertical interhemispheric horn that overlies the anterior aspect of the lamina terminalis and a horizontal horn that lies in the septum below the corpus callosum. This horizontal horn was previously referred to as the cavum septi pellucidi, a vestigial anatomic curiosity. The narrow cavities of the horizontal and vertical horns join under the genu of the corpus callosum and communicate freely. The two horns do not communicate with the cerebral ventricular system or the subarachnoid spaces. Microinjections of greater than or equal to 0.2 ng angiotensin II into all regions of the cavity resulted in significant drinking, whereas injections into areas surrounding the cavities were ineffective. These data show that the organ is a discrete brain entity and displays high sensitivity to angiotensin II. These findings suggest that the OCPLT is another brain organ with significant effects on fluid and electrolyte homeostasis and perhaps other brain functions.