Surgical Analysis of the Preoptico-Tuberal Pathway Controlling Ovulatory Release of Gonadotropins in the Rat

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Abstract
According to the current concept there is a dual neural control of the adenohypophysial gonadotropin secretion in the female rat. The 1st level is assumed to be located in the medial basal tuberal region.. This controls tonic discharge of gonadotropin secretion, and this in turn is controlled by a second level which is responsible for initiating the ovulatory surge of gonadotropins. The 2nd level appears to be located in or to operate through the septal-preoptic area. Electrochemical stimulation applied to this area can induce ovulation in the pentobarbital-blocked rat. This study was designed to investigate the nature of the neural pathway connecting these 2 levels by the method of hypothalamic transection performed with a small knife. Unilateral transection, extending between the 0.3 and 1.4 mm parasagittal planes in rostral or caudal suprachiasmatic (SCh) planes, blocked the ovulation-inducing effect of lipsilateral pre-optic (POA) stimulation (10 [mu]amp D.C., 60 sec, steel concentric electrode, applied immediately following transection), but not the effect of contralateral stimulation. If the cut failed to reach these limits or cut down to only 1.5 mm above the cranial floor, stimuli escaping through the gap evoked LH [luteinizing hormone] secretion. The larger the gap, the greater was the amount of LH released, supposedly because more of the medial basal tuberal region was stimulated. Either midsagittal section, dividing the hypothalamic retrochiasmatic area into right and left halves, or ipsilateral transection through the middle of the ventromedial nucleus (mid-VMN), failed to prevent the ovulation-inducing effect of the POA stimulation. However, when these cuts were combined to separate the rostral quadrant from the other tuberal areas, POA stimulation weeks later failed to cause the adenohypophysis to release a full quota of LH. Bilateral SCh transection extending 1.4 mm laterally blocked spontaneous ovulation as well as ovulation in response to preoptic stimulation. Animals thus treated developed anovulatory persistent-estrous syndromes, but could be induced to ovulate by LH injection. The above evidence supports previous evidence that the preopticotuberal pathway concerned with ovulation exists as a diffuse neuronal system. In the frontal SCh plane it is dispersed in the ventral part of the area extending between the 0.3 and 1.4 mm parasagittal planes. Either directly or via interneurons, its elements serve equivalent loci on both ipsilateral and contralateral sides of the hypophysiotropic area on the basis of "point-to-point" relationships. In addition, it was found that the extensive injury caused by either bilateral VMN transection or unilateral VMN transection combined with midsagittal section could cause the adenohypophysis of the pentobarbital-blocked rats to release an ovulation quota of LH. The means by which this was accomplished remain for further investigation.