Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats

Abstract

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

Keywords: LHb; early life stress; glutamatergic synaptic transmission; lateral habenula; morphine self-administration.

Figure 1.

MD increased E/I ratios and spontaneous activity of LHb neurons. A) Summary of E/I ratios of LHb neurons obtained from non-MD (n=21/4) and MD rats (n=12/3) with individual data points and representative traces of evoked EPSCs (black, recorded at −55 mV holding potential) and IPSCs (red, recorded at +10 mV holding potential) in response to electrical stimulation of inputs via the stria medullaris. B) Relative AMPAR/GABA_AR components of synaptic currents normalized to the total EPSC+IPSC amplitude for each cell evoked in response to the same stimulus intensity. C-D) Pie charts of voltage-clamp cell-attached recordings (C, V=0 mV, non-MD, n=38/4; MD, n=46/5) and current-clamp whole-cell recordings (D, I=0 pA, non-MD, n=27/4; MD, n=37/5) of spontaneous neuronal activity across non-MD and MD rats. Comparison of the percent distributions of silent (black), tonic (blue), or bursting (red) LHb neurons showed a significant increase in tonic (both C-D) and bursting (only in D) LHb neuronal activity following MD. E-F) Representative traces of E) voltage-clamp cell-attached recordings and F) current-clamp whole-cell recordings of silent (black), and examples of low- and high-frequency tonic (blue) and bursting (red) activities of LHb neurons. *p<0.05, **p<0.01 by unpaired Welch’s t-tests or Chi squared tests. n represents the number of recorded cells/rats, and matching symbols in scatter plots denote cells analyzed from a single rat.

Figure 2.

MD decreased morphine intake during FR3 MSA and sucrose preference in the sucrose preference test. A) Average number of drug-paired lever presses, B) inactive lever presses, C) drug infusions, D) locomotor activity, and E) defecation during FR1 and FR3 SSA (black) and MSA (0.3 mg/kg per infusion of morphine, red) sessions. MD significantly decreased drug-paired lever presses and morphine intake in FR3 sessions of MSA. In both non-MD and MD rats, MSA induced locomotor sensitization and constipation (n=5–9/group). F) MD rats consumed less sucrose solution compared to the control non-MD counterparts in the sucrose preference test (n=10/group). *p<0.05, **p<0.01 by 3-way Repeated Measures ANOVA or unpaired Welch’s t tests. n represents the number rats.

Figure 3.

Cumulative hourly time course of final FR1 and first FR3 SSA/MSA sessions. Time course of hourly A) drug-paired lever presses, B) number of infusions, and C) locomotor activity during day 6 (FR1) and day 7 (FR3) of the 4h sessions of SSA (black) and MSA (0.3 mg/kg per infusion of morphine, red) (n=5–9/group). *p<0.05 for interaction effect for MDxMSA, +p<0.05 for post-hoc tests of interaction effect for MDxMSAxtime by 3-way Repeated Measures ANOVA. n represents the number of rats.

Figure 4.

Effects of intravenous SSA or MSA 24h after the last morphine intake at LHb glutamatergic synapses from non-MD and MD rats. A) Representative AMPAR-mediated mEPSC traces from non-MD and MD rats (calibration bars, 20pA/1 s), average and cumulative probability plots of mEPSC B) amplitude, C) charge transfer (area under the curve), D) decay time constants (Tau) and E) frequency (inter-event interval) in non-MD and MD rats (non-MD+SSA: n = 25/5, MD+SSA: n=15/3, non-MD+MSA: n=15/4, MD+MSA: n=24/4). MD potentiated glutamatergic synapses onto LHb neurons but this potentiation was significantly decreased 24h following MSA in MD rats. MSA changed the kinetics of AMPAR mEPSCs in non-MD rats. ^#p=0.06, *p<0.05, **p<0.01, ****p<0.0001 by 2-Way ANOVA or Kolmogorov–Smirnov tests. n represents the number of recorded cells/rats, and matching symbols in scatter plots denote cells analyzed from a single rat.