Expression of AMPA receptor subunits (GluR1-GluR4) in gonadotrophin-releasing hormone neurones of young and middle-aged persistently oestrous rats during the steroid-induced luteinising hormone surge.

Glutamate provides excitatory input to gonadotrophin-releasing hormone (GnRH) neurones and elicits a response indicative of AMPA receptors. To determine if and which AMPA subunits are expressed by GnRH neurones, we conducted triple-label immunohistochemistry and confocal analyses on tissue obtained at 08.00, 12.00, 16.00 and 20.00 h from young and middle-aged, persistently oestrous (MA-PE) rats that were ovariectomised and primed with oestrogen and progesterone to induce a luteinising hormone (LH) surge. Each AMPA subunit was found in GnRH neurones, but in different patterns across the diurnal cycle, which were influenced by age. GluR1 expression increased earlier in young rats and the percentage of Fos-positive GnRH neurones expressing GluR1 rose significantly and was sustained from 12.00-16.00 h. GluR1 expression was delayed in MA-PE rats and the percentage of Fos-positive GnRH neurones expressing GluR1 peaked at 20.00 h. GluR2 expression in GnRH neurones did not change over time and was not affected by age; however, the percentage of Fos-positive GnRH neurones expressing GluR2 increased earlier and was sustained from 08.00-16.00 h in young rats whereas, in MA-PE rats, this percentage peaked at 20.00 h. GluR3 expression also increased earlier in young rats and peaked at 12.00 h but was delayed in MA-PE rats and peaked at 20.00 h. The number of Fos-positive GnRH neurones that coexpressed GluR3 peaked at 12.00 h in young rats but showed little change from 12.00-20.00 h in MA-PE rats. GluR4 expression was maintained at higher levels at 08.00 and 12.00 h in young rats; although the percentage of Fos-positive GnRH neurones expressing GluR4 peaked at 12.00 h in young rats, it showed little change in MA-PE rats. In summary, our data show that a higher proportion of Fos-positive GnRH neurones coexpressed AMPA receptor subunits in young rats and the expression, particularly of GluR1 and GluR2, was increased and sustained throughout the surge, whereas GluR3 and GluR4 expression peaked just before. In MA-PE rats, the rate of expression of GluR subunits and Fos in GnRH neurones was altered in a manner that may explain the delay and attenuation of the LH surge.