Fibroblast growth factor 13-mediated regulation of medium spiny neuron excitability and cocaine self-administration

Abstract

Cocaine use disorder (CUD) is a prevalent neuropsychiatric disorder with few existing treatments. Thus, there is an unmet need for the identification of new pharmacological targets for CUD. Previous studies using environmental enrichment versus isolation paradigms have found that the latter induces increased cocaine self-administration with correlative increases in the excitability of medium spiny neurons (MSN) of the nucleus accumbens shell (NAcSh). Expanding upon these findings, we sought in the present investigation to elucidate molecular determinants of these phenomena. To that end, we first employed a secondary transcriptomic analysis and found that cocaine self-administration differentially regulates mRNA for fibroblast growth factor 13 (FGF13), which codes for a prominent auxiliary protein of the voltage-gated Na⁺ (Na_v) channel, in the NAcSh of environmentally enriched rats (i.e., resilient behavioral phenotype) compared to environmentally isolated rats (susceptible phenotype). Based upon this finding, we used in vivo genetic silencing to study the causal functional and behavioral consequences of knocking down FGF13 in the NAcSh. Functional studies revealed that knockdown of FGF13 in the NAcSh augmented excitability of MSNs by increasing the activity of Na_v channels. These electrophysiological changes were concomitant with a decrease in cocaine demand elasticity (i.e., susceptible phenotype). Taken together, these data support FGF13 as being protective against cocaine self-administration, which positions it well as a pharmacological target for CUD.

Keywords: cocaine use disorder; fibroblast growth factor 13; medium spiny neurons; nucleus accumbens shell; voltage-gated Na+ channel.

Figure 1

Cocaine differentially regulates FGF13 mRNA levels in environmentally enriched versus environmentally isolated conditions. Bar graph comparing FGF13 mRNA levels in the NAcSh of either environmentally enriched or isolated rats that self-administered saline or cocaine. A significant main effect of cocaine (shown) and an interaction effect (cocaine × housing effect) were found. Data are mean ± SEM (n = 7–8 rats/group). Significance was assessed with a likelihood ratio F-test (^**p < 0.01).

Figure 2

FGF13 regulates the I_NaT and I_NaP of MSNs of the NAcSh. (A) Schematic of AAV injection site in the NAcSh and experimental timeline. (B) Representative traces of sodium current elicited by MSNs expressing AAV-shCTRL-GFP or AAV-shFGF13-GFP in response to the depicted voltage-clamp protocol. (C) Current–voltage relationship of the I_NaT elicited by MSNs belonging to the experimental groups described in (B). (D) Comparison of the peak I_NaT density of MSNs belonging to the indicated experimental group. (E) Comparison of the I_NaP density at the −20 mV voltage command for the indicated experimental groups. (F) Comparison of the voltage-dependences of activation and steady-state inactivation of I_NaT elicited by MSNs of the experimental groups described in (B). (G,H) Comparison of V_1/2 of activation (G) and steady-state inactivation (H) of I_NaT of MSNs belonging to the indicated experimental groups. Data are mean ± SEM (n = 5 cells/group; slices from N = 2 rats/group). Significance was assessed using a student’s t-test (ns, not significant; ^*p < 0.05). For detailed statistical information, refer to Supplementary Table S1.

Figure 3

FGF13 regulates the excitability of MSNs of the NAcSh. (A) Staining of FGF13 at the AIS. Representative image of DAPI (blue), AnkyrinG (green) and FGF13 (red) at the AIS in the NAcSh with overlay and zoom to AIS. Scale bars indicate 5 μm and 2 μm. (B) Representative traces of evoked APs of MSNs expressing AAV-shCTRL-GFP or AAV-shFGF13-GFP in response to the depicted current-clamp protocol. (C) Comparison of the IFF of MSNs expressing AAV-shCTRL-GFP (black) or AAV-shFGF13-GFP (blue) over a range of injected current stimuli. (D–H) Comparison of the V_thr for AP initiation (D), maximum rise of the AP upstroke (E), maximum decay of the AP downstroke (F), RMP (G), and R_in (H) between the indicated experimental groups. Data are mean ± SEM (n = 10 cells/group; slices from N = 3 rats/group). Statistical significance was assessed using a student’s t-test (ns, not significant; ^*p < 0.05). For detailed statistical information, refer to Supplementary Table S2.

Figure 4

In vivo genetic silencing of FGF13 in the NAcSh decreases cocaine demand elasticity. (A) Timeline for stereotaxic surgery, catheter implantation, and subsequent cocaine self-administration testing. (B) Cocaine demand function across a range of cocaine prices (number of responses for 1 mg/kg of cocaine) for rats belonging to the indicated experimental groups. (C) Cocaine demand elasticity (i.e., slope) derived from (B) for the indicated experimental groups. Data are mean ± SEM (n = 9 rats/group). Statistical significance was assessed using a student’s t-test (^*p < 0.05).