Does aging alter the molecular substrate of ionotropic neurotransmitter receptors in the rostral ventral lateral medulla? - A short communication

Abstract

Aging alters sympathetic nervous system (SNS) regulation, although central mechanisms are not well understood. In young rats the rostral ventral lateral medulla (RVLM) is critically involved in central SNS regulation and RVLM neuronal activity is mediated by a balance of excitatory and inhibitory ionotropic neurotransmitters and receptors, providing the foundation for hypothesizing that with advanced age the molecular substrate of RVLM ionotropic receptors is characterized by upregulated excitatory and downregulated inhibitory receptor subunits. This hypothesis was tested by comparing the relative mRNA expression and protein concentration of RVLM excitatory (NMDA and AMPA) and inhibitory (GABA and glycinergic) ionotropic neurotransmitter receptor subunits in young and aged Fischer (F344) rats. Brains were removed from anesthetized rats and the RVLM-containing area was micropunched and extracted RNA and protein were subsequently used for TaqMan qRT-PCR gene expression and quantitative ELISA analyses. Bilateral chemical inactivation of RVLM neurons and peripheral ganglionic blockade on visceral sympathetic nerve discharge (SND) was determined in additional experiments. The relative gene expression of RVLM NMDA and AMPA glutamate-gated receptor subunits and protein concentration of select receptor subunits did not differ between young and aged rats, and there were no age-related differences in the expression of RVLM ionotropic GABA_A and Gly receptors, or of protein concentration of select GABA_A subunits. RVLM muscimol microinjections significantly reduced visceral SND by 70±2% in aged F344 rats. Collectively these findings from this short communication support a functional role for the RVLM in regulation of sympathetic nerve outflow in aged rats, but provide no evidence for an ionotropic RVLM receptor-centric framework explaining age-associated changes in SNS regulation.

Keywords: Aging; Neurotransmitter receptors; RVLM; Sympathetic nervous system regulation.

Figure 1

Relative changes in gene expression of RVLM ionotropic NMDA (Grin1, Grin2a, Gri2b, Grin2c), AMPA (Gria1, Gria2, Gria3, Gria4), GABA_Aα (Gabra1, Gabra2, Gabra3, Gabra4, Gabra5), GABA_Aβ (Gabrb1, Gabrb2, Gabrb3), GABA_Aγ (Gabrg1, Gabrg2), and glycinergic (Glra1, Glrb) receptor subunits between aged (22–24 months; n=8) and young (3–4 months; n=8) F344 rats. Dots show relative fold change comparisons analyzed using the 2^−ΔΔCt method. Values above 0 indicate that gene expression is higher in aged compared with young rats, whereas values below 0 indicate that gene expression is lower in aged compared with young rats. Dotted lines depict 1.5 fold changes in gene expression. Data are represented as Log2 transformed fold change ± SE.

Figure 2

Protein quantification as determined by sandwich ELISA for (A) NMDA receptor subunits (Grin1, Grin2a, Grin2c) and (B) GABA_A receptor subunits (Gabra1, Gabra2, Gabrg2), in the RVLM of aged (22–24 months; n=8) and young (3–4 months; n=8) F344 rats. Bars show protein concentration of respective subunits in young (closed bars) and aged (open bars) rats. Data are represented as mean ± SD.