Development of gonadotropin-releasing hormone (GnRH) neuron regulation in the female rat.

1. After reaching its final destination the GnRH neuronal network develops under the influence of both excitatory and inhibitory inputs. 2. In the first 2 weeks of life, the immaturity of the GnRH neuronal system is reflected in sporadic unsynchronized bursts of the decapeptide, which determine the pattern of serum gonadotropin levels observed in female rats: high FSH levels and transient bursts of LH. The main inhibitory neuronal systems that operate in this period are the opioid and dopaminergic systems. A decrease in their inhibitory effectiveness may not be sufficient correctly to activate and synchronize the GnRH neuronal system. 3. There is a concomitant increase in excitatory inputs, mainly noradrenaline, excitatory amino acids, and NPY, which increase the synthesis and release of GnRH at the beginning of the juvenile period and participate in the coupling of GnRH neural activity to the ongoing rhythmic activity of a hypothalamic circadian oscillator. 4. The morphological changes of GnRH neurons which take place during the third and fourth weeks of life, and which are probably related to increasing estradiol levels, reflects the increasing complexity of the GnRH neuronal network, which establishes synaptic contacts to enable the expression of pulsatility and of the positive feedback of estradiol, both necessary components for the occurrence of puberty.