Stimulation of Luteinizing Hormone Release after Stereotaxic Microinjection of Neurotensin into the Medial Preoptic Area of Rats

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Abstract
Neurons immunoreactive to luteinizing hormone-releasing hormone (LHRH) are localized to the medial preopticoseptal region of the rat hypothalamus, an area known to be essential for the regulation of the surge of luteinizing hormone (LH) that precedes ovulation. The effect of neurotensin (NT) on circulating LH levels following its discrete microinjection into this region was evaluated. Within 30 min after the stereotaxic microinjection of 40 ng of NT in 50 nl into the medial preoptic area (MPOA) just rostral to the organum vasculosum of the lamina terminalis in ovariectomized (OVX), anesthetized rats, plasma LH levels rose from 417 ± 36 to 923 ± 113 ng/ml (p < 0.001). There was also a significant increase (p < 0.001) in plasma LH levels following the microinjection of 50 ng of norepinephrine in 50 nl in this same site. Similarly, levels of LH increased from 48 ± 7 to 110 ± 19 ng/ml (p < 0.004) following the microinjection of NT into the rostral MPOA on the morning of proestrus in anesthetized rats. In contrast, saline, substance P, leuenkephalin and LHRH microinjected in the rostral MPOA of OVX rats had no significant effect on circulating levels of LH. The microinjection of NT 0.4 mm rostral or caudal to this area was also without effect. The presence of two binding sites with Kds of 4.2 and 29 nM and site concentrations of 0.11 and 0.68 pmol/mg in tissue homogenates of the rostral MPOA were also established. These data suggest a role for NT in the central regulation of LH release.