Quantitative analysis of synaptic input to gonadotropin-releasing hormone neurons in normal mice and hpg mice with preoptic area grafts.

Mutant hypogonadal (hpg) mice with a truncated gene for the precursor to gonadotropin-releasing hormone (GnRH) show certain aspects of recovery of reproductive function after receiving grafts of normal preoptic area into the third ventricle. We have previously shown that GnRH neurons from within the grafts can innervate the appropriate neural-hemal target in the host. To determine if in turn these exogenously derived neurons receive a synaptic input comparable to the GnRH neurons in the normal animal we have now carried out a quantitative ultrastructural analysis to compare the synaptic input to GnRH neurons in the normal preoptic area and in the grafts. In almost all cases GnRH cells or dendrites in normal brains and within the grafts received a synaptic input. In normal animals, input to GnRH dendritic profiles was significantly greater (P less than 0.001) than to the somatic plasma membrane and this trend was also observed within the grafts though the difference was not statistically significant. In addition, no statistically significant difference was found between the input to GnRH structures within the grafts and in normal preoptic area. However, a substantial variability in input among grafted animals was evident which was not observed in normal animals. The sources of variability within the grafts are discussed and we suggest that the deficiencies and differences that exist in regulation of gonadotropin secretion among grafted hpg animals may be reflected in aberrant synaptic input.