Dissociation between rat hippocampal CA1 and dentate gyrus cells in their response to corticosterone: effects on calcium channel protein and current

Abstract

Stress and corticosterone affect, via glucocorticoid receptors, cellular physiology in the rodent brain. A well-documented example concerns corticosteroid effects on high-voltage activated (L type) calcium currents in the hippocampal CA1 area. We tested whether corticosterone also affects calcium currents in another hippocampal area that highly expresses glucocorticoid receptors, i.e. the dentate gyrus (DG). Remarkably, corticosterone (100 nm, given for 20 min, 1-4.5 hr before recording) did not change high-voltage activated calcium currents in the DG, whereas currents in the CA1 area of the same rats were increased. Follow-up studies revealed that no apparent dissociation between the two areas was observed with respect to transcriptional regulation of calcium channel subunits; thus, in both areas corticosterone increased mRNA levels of the calcium channel-beta4 but not the (alpha) Ca(v)1.2 subunit. At the protein level, however, beta4 and Ca(v)1.2 levels were significantly up-regulated by corticosterone in the CA1 but not the DG area. These data suggest that stress-induced elevations in the level of corticosterone result in a regionally differentiated physiological response that is not simply determined by the glucocorticoid receptor distribution and that the observed regional differentiation may be caused by a gene involved in the translational machinery or in mechanisms regulating mRNA or protein stability.

Figure 1

Differential effects of corticosterone on calcium currents in CA1 and DG cells. A, Total calcium current was evoked with the voltage protocol shown on top. A 3-sec hyperpolarizing prepulse was given to activate all VDCCs. When stepping directly from holding potential with the voltage protocol shown below, only part (HVA) of the currents are activated. Results presented here focus on the latter protocol. B, Typical HVA current (evoked by a voltage step to 0 mV) indicating how the peak and sustained part of each trace were determined. Current density is shown as a function of voltage for HVA calcium currents in CA1 pyramidal cells (C) and DG cells (D) incubated 1–4.5 h earlier with vehicle solution (VEH) or corticosterone (CORT; 100 nm). Here only values in the range from −50 to 0 mV are depicted because currents evoked by more negative steps were negligible. Each point represents the average ± sem. Data were obtained in n cells and N animals: CA1/vehicle, n = 8 (n = 5); CA1/corticosterone, n = 5 (n = 4); DG/vehicle, n = 15 (n = 7); DG/corticosterone, n = 15 (n = 7).

Figure 2

Calcium channel subunit Ca_v1.2 mRNA expression in the hippocampal subfields CA1, CA3, and DG. Expression (gray value in arbitrary units) was significantly decreased by an injection with vehicle solution (VEH) when compared with naive, whereas injection with a high dose of corticosterone (CORT) had no additional effect. On the right, hippocampal Ca_v1.2 mRNA hybridization signals are shown for a naive, vehicle-, and corticosterone-injected animal. Asterisk indicates significant difference between the groups (P < 0.05). These were based on n = 8 animals in the naive and vehicle-treated group and n = 7 in the corticosterone-injected group. Data of this study on the comparison between vehicle- and CORT-treated rats in CA1 were reported earlier (54). AU, Arbitrary units.

Figure 3

Calcium channel subunit β4 mRNA expression in the hippocampal subfields CA1, CA3, and DG. Expression was significantly increased in all hippocampal subfields by an injection with corticosterone (CORT) solution when compared with naive animals. Injection with vehicle solution (VEH) had no significant effect. On the right, β4 mRNA hybridization signals in the hippocampus are shown for a naive, vehicle-, and corticosterone-injected animal. Asterisk indicates significant difference between the groups (P < 0.05). Based on n = 8 animals in the naive and vehicle-treated group and n = 7 in the corticosterone-injected group. AU, Arbitrary units.

Figure 4

Calcium channel subunit β4 protein expression in CA1 area (A) and DG (B). Protein level is expressed as percent change in corticosterone-incubated (cort) compared with vehicle-incubated (veh) slices. Immunosignal values were corrected for loading differences as determined by immunoblotting for GAPDH. P2 indicates pellet of the second spin, which is enriched for heavy membrane fragments. P3 indicates pellet of the third spin, which mainly contains light membrane fragments. Asterisk indicates significant difference from vehicle (P < 0.05). Below the graphs, typical examples of β4 expression are shown for each experimental group. Based on (paired) observations from n = 11 samples (each sample from a different animal) of CA1 tissue and n = 9 samples of DG tissue, for both the P2 and P3 fractions.

Figure 5

Calcium channel subunit Ca_v1.2 protein expression in CA1 area (A) and DG (B). Protein level is expressed as percent change in corticosterone-incubated (cort) compared with vehicle-incubated (veh) slices. Immunosignals were corrected for loading differences as determined by immunoblotting for Tfr. P2 is enriched for heavy and P3 for light membrane fragments. Asterisk indicates significant difference from vehicle (*, P < 0.05; **, P < 0.01). Below the graphs, typical examples of Ca_v1.2 expression are shown for each experimental group. These were based on (paired) observations from n = 11 samples (each sample from a different animal) of CA1 tissue and n = 10 samples of DG tissue for the P2 fraction and n = 6 and 7, respectively, for the P3 fraction. In the current experiment we used a polyclonal rabbit antibody against Ca_v1.2 (55).