The release of pituitary LH and sprouting of LH-releasing hormone (LHRH)-containing neurons after anterior hypothalamic deafferentation

The chronology of changes in plasma luteinizing hormone (LH) and the distribution of immunoreactive neuronal processes containing LH releasing hormone (LHRH-ir) were studied in the female rat after surgical interruption of anterior neural connections of the mediobasal hypothalamus (MBH). Spontaneous LH surges on the afternoon of proestrus and LH release of proestrus and LH release after estradiol benzoate (EB) followed 48 h later by progesterone (P) administration were studied in ovariectomized (OVX) rats. The maximum increase in plasma concentrations (Δ maxLH) after EBP was calculated for each rat at several intervals over 140 days. Control animals given EBP at monthly intervals after OVX had comparably largeΔ maxLH surges during the first few months of study. However, a gradual decline in controlΔ maxLH followed becoming significant 3 months after the start of the experiment. In contrast, frontal cuts (FC), which interrupted anterior MBH connections, produced an abrupt decrease inΔ maxLH surges after EBP to 11% of preoperative levels. However, during subsequent EBP trials, there was a gradual improvement in LH surges to about 50% of preoperative levels over 100 days. In some cases, individual improvement became equal to preoperative LH surge levels, in others there was no recovery. Examination of LHRH-ir nerve growth responses after FC suggested that sprouting by these peptide-containing neuronal process may have contributed to the functional improvements observed.