Altered hippocampal synaptic function in transgenic mice with increased astrocyte expression of CCL2 after withdrawal from chronic alcohol

Abstract

CNS actions of the chemokine CCL2 are thought to play a role in a variety of conditions that can have detrimental consequences to CNS function, including alcohol use disorders. We used transgenic mice that express elevated levels of CCL2 in the CNS (CCL2-tg) and their non-transgenic (non-tg) littermate control mice to investigate long-term consequences of CCL2/alcohol/withdrawal interactions on hippocampal synaptic function, including excitatory synaptic transmission, somatic excitability, and synaptic plasticity. Two alcohol exposure paradigms were tested, a two-bottle choice alcohol (ethanol) drinking protocol (2BC drinking) and a chronic intermittent alcohol (ethanol) (CIE/2BC) protocol. Electrophysiological measurements of hippocampal function were made ex vivo, starting ∼0.6 months after termination of alcohol exposure. Both alcohol exposure/withdrawal paradigms resulted in CCL2-dependent interactions that altered the effects of alcohol on synaptic function. The synaptic alterations differed for the two alcohol exposure paradigms. The 2BC drinking/withdrawal treatment had no apparent long-term consequences on synaptic responses and long-term potentiation (LTP) in hippocampal slices from non-tg mice, whereas synaptic transmission was reduced but LTP was enhanced in hippocampal slices from CCL2-tg mice. In contrast, the CIE/2BC/withdrawal treatment enhanced synaptic transmission but reduced LTP in the non-tg hippocampus, whereas there were no apparent long-term consequences to synaptic transmission and LTP in hippocampus from CCL2-tg mice, although somatic excitability was enhanced. These results support the idea that alcohol-induced CCL2 production can modulate the effects of alcohol exposure/withdrawal on synaptic function and indicate that CCL2/alcohol interactions can vary depending on the alcohol exposure/withdrawal protocol used.

Keywords: Chronic intermittent alcohol exposure; Hippocampus; Memory and learning; Synaptic plasticity; Synaptic transmission; Two-bottle choice alcohol drinking.

Figure 1

Diagram showing CIE/2BC treatment paradigm.

Figure 2

Synaptic responses were depressed in hippocampus from non-dependent CCL2-tg mice. (A) CCL2 levels in hippocampus from mice used in behavioral studies measured at the time of sacrifice for electrophysiological studies. (B1,C1) Graphs showing I/O relationships for fEPSP in hippocampus from non-dependent and alcohol naïve non-tg (B1) and CCL2-tg mice (C1). (B2,C2) Graphs showing I/O relationships for PS in hippocampus from non-dependent and alcohol naïve non-tg (B2) and CCL2-tg mice (C2). Inserts show sample recordings. The top recording (black) is from a naïve mouse and the bottom recording (red) is from an alcohol-treated mouse. *significant difference from control values for the same genotype (Repeated Measures ANOVA).

Figure 3

Synaptic responses were altered in hippocampus from dependent mice. (A1,B1) Graphs showing I/O relationships for fEPSP in hippocampus from dependent (CIE) non-tg (A1) and CCL2-tg mice (B1) and their respective controls (cont). (A2,B2) Graphs showing I/O relationships for PS in hippocampus from dependent non-tg (A1) and CCL2-tg mice (B1) and their respective controls. Inserts show sample recordings. The top recording (black) is from a naïve mouse and the bottom recording (red) is from an alcohol-treated mouse *significant difference from control values for the same genotype (Repeated Measures ANOVA).

Figure 4

PPR of the PS is differentially altered in hippocampus from non-dependent (2BC drinking) and dependent mice (CIE). Left panels shows sample recordings. The black recording is the response to the first stimulation of the paired pulses and the red recording is the response to the second stimulation of the pair. The interpulse interval for the sample recording was 10 ms. Right panels show graphs of mean values. (A) PPR was greater than 1 in hippocampus from non-dependent non-tg and CCL2-tg mice. PPR was increased at the 10 ms timepoint in hippocampus from non-dependent CCL2-tg mice, indicating that 2BC drinking reduced inhibition. (B) PPR was increased in hippocampus from dependent non-tg mice at both time intervals tested, reflecting reduced inhibitory influences. In contrast, PPR was reduced in hippocampus from dependent CCL2-tg mice at both 10 and 20 ms intervals, indicating increased inhibitory influences. *significant difference from control values for the same genotype (unpaired t-test).

Figure 5

Genotypic differences in synaptic plasticity elicited by TBS observed in hippocampus from non-dependent (2BC) and dependent (CIE) mice. Left panels show sample recordings for baseline (base), PTP, and LTP under the various experimental conditions. Right panels show graphs of mean values. (A1,2) Enhancement of the fEPSP by TBS in hippocampus from non-dependent non-tg (A1) and CCL2-tg (A2) mice. Enhancement of the fEPSP was similar in hippocampus from non-dependent and naïve non-tg mice, but was greater in hippocampus from the non-dependent CCL2-tg mice compared to naïve CCL2-tg mice. (B1,2) Enhancement of the fEPSP by TBS in hippocampus from dependent non-tg (B1) and CCL2-tg (B2) mice. STP and LTP were reduced in dependent non-tg mice compared to controls. There was no change observed in dependent CCL2-tg mice compared to 2BC controls. TBS was applied at the 0 time point (arrow) of the graphs. Insert graphs show mean values for PTP, STP, and LTP. *significant difference from the respective control values for the same genotype (unpaired t-test).