Diurnal changes in rodent AMPA receptors: correlation with changes in corticosterone and 5HT2C receptor levels?

Paul Chazot1, Peter Donoghue1, Mike Spedding2, Abdel Ennaceur3. 1Durham University, Durham, United Kingdom, 2Servier Laboratories Ltd, Paris, France, 3Sunderland University, Sunderland, United Kingdom.

There is accumulating evidence that AMPA receptors are implicated in the mode of action of a number of psychotherapies. We hypothesise that there maybe a connection between 5HT2C and AMPA receptors in the forebrain (particularly in the frontal cortex and hippocampus – structures implicated in psychological and cognitive effects of psychoactive agents) via TARPs and PSD95. TARPγ8 is highly expressed in both brain structures; in fact its expression closely matches the expression pattern for GluR2. We have previously reported the first evidence for a physical and functional interaction between the 5HT2C and AMPA receptor family in the forebrain, which now provides a structural framework to delineate signalling implications of this interaction (Donoghue et al., 2008). It has been previously shown that the levels of Wistar rat hippocampus 5HT2C mRNA and corticosterone changes dramatically during the day, which may have implications for the daily circadian behavioural state of these animals.

In this present study, we investigated whether AMPA receptor expression is influenced by the diurnal changes in 5HT2C and corticosterone. Male wistar strain rats (3 month old) were investigated at 8.00 and 20.00 (n = 4 separate animals each), the reported peak and trough time-points for 5HT2C expression, respectively (Holmes et al., 1997). AMPA GluR1 and GluR2 subunits and TARP γ8 protein expression were compared using quantitative immunoblotting (Chazot et al., 2002), and semi-quantitative immunohistochemistry, using validated commercial and in house antibodies, respectively. Furthermore, AMPA receptor complex expression was investigated using radioligand-autoradiography with [3H] AMPA as described previously (Payne et al., 2006).

Based on immunoblotting, no significant changes in TARP γ8 and GluR1 were detected in whole forebrain, hippocampus or cortex. However, a significant reduction of GluR2 subunit (-42 +/- 17%) in the hippocampus and [3H] AMPA receptor binding was observed in both the cortex (-62 +/- 5%) and hippocampus (-38 +/- 6%) for animals sacrificed at 20.00 in comparison to those at 08.00. These effects correspond with the chronobiology of the 5HT2C, providing further evidence for a functional link between AMPA and 5HT2C receptors in the cortico-hippocampal network. Further studies are now underway to investigate the influence of anxiety upon cortical and hippocampal TARP γ8 topology and AMPA receptor expression at these diurnal time points.

Chazot PL et al. (2002) J Neurochem. 83(5):1235-8.
Donoghue PS et al (2008) PA2 C005.
Holmes MC et al. (1997) J Neurosci.17(11):4056-65.
Payne HL, Donoghue PS et al. (2006) J Neurosci. 26(33):8600-8.

This work was funded by a BBSRC(UK)/Servier CASE award and MRC.