Spike timing-dependent plasticity at GABAergic synapses in the ventral tegmental area

Abstract

Persistent changes in excitatory and inhibitory synaptic strengths to the ventral tegmental area (VTA) dopamine (DA) neurons in response to addictive drugs may underlie the transition from casual to compulsive drug use. While an enormous amount of work has been done in the area of glutamatergic plasticity of the VTA, little is known regarding the learning rules governing GABAergic plasticity in the VTA. Spike timing-dependent plasticity, STDP, has attracted considerable attention primarily due to its potential roles in processing and storage of information in the brain and there is emerging evidence for the existence of STDP at inhibitory synapses. We therefore used whole-cell recordings in rat midbrain slices to investigate whether near-coincident pre- and postsynaptic firing induces a lasting change in synaptic efficacy of VTA GABAergic synapses. We found that a Hebbian form of STDP including long-term potentiation (LTP) and long-term depression (LTD) can be induced at GABAergic synapses onto VTA DA neurons and relies on the precise temporal order of pre- and postsynaptic spiking. Importantly, GABAergic STDP is heterosynaptic (NMDA receptor dependent): triggered by correlated activities of the presynaptic glutamatergic input and postsynaptic DA cells. GABAergic STDP is postsynaptic and has an associative component since pre- or postsynaptic spiking per se did not induce STDP. STDP of GABAergic synapses in the VTA provides physiologically relevant forms of inhibitory plasticity that may underlie natural reinforcement of reward-related behaviours. Moreover, this form of inhibitory plasticity may mediate some of the reinforcing, aversive and addictive properties of drugs of abuse.

Figure 1. VTA DA neurons express STD-LTP_GABA in response to a pre-post spiking (positive timing) protocol

A, sample bursts of the pre-post spiking protocol (with a delay of +15 ms) for induction of STD-LTP_GABA. B and C, single experiments showing induction of STD-LTP_GABA recorded in I_h⁽⁺⁾ (presumably DA) neurons using KCl or caesium gluconate-filled pipettes. At the arrow, STD-LTP_GABA was induced. Insets: averaged IPSCs before and 25 min after STDP protocol. In this and all figures, ten consecutive traces from each condition were averaged for illustration as inset. Calibration: 100 pA, 25 ms. D, averaged experiments from I_h⁽⁺⁾ neurons (filled circles) exhibiting STD-LTP_GABA in response to the pre-post pairing protocol using either KCl or caesium gluconate-filled pipettes. VTA DA neurons express STD-LTP_GABA (140.54 ± 5.7% of pre-STDP values, F_10.45,73.19= 3.546, P < 0.001, n= 12). All I_h⁽⁺⁾ cells recorded with either KCl or caesium gluconate that received the pre-post STDP protocol are included for the means in this graph. E, no changes in PPR were detected after induction of STD-LTP_GABA (97.13 ± 4.5% of pre-STDP values, F_5.66,39.23= 1.597, P= 0.177, n= 12). F, no changes in CV were detected after induction of STD-LTP_GABA (101.19 ± 7.0% of pre-STDP values, F_3.16,25.29= 1.522, P= 0.232, n= 12). Values shown throughout figures are the mean ± SEM.

Figure 2. VTA DA neurons exhibit STD-LTD_GABA in response to a post-pre spiking (negative timing) protocol

A, sample bursts of the post-pre spiking protocol (with a delay of −5 ms) for induction of STD-LTD_GABA. B and C, single experiments showing induction of STD-LTD_GABA recorded in I_h⁽⁺⁾ neurons using KCl or caesium gluconate-filled pipettes. At the arrow, STD-LTD_GABA was induced. Insets: averaged IPSCs before and 25 min after STDP protocol. Calibration: 100 pA, 25 ms. D, averaged experiments from I_h⁽⁺⁾ neurons (open circles) exhibiting STD-LTD_GABA in response to the post-pre pairing protocol using either KCl or caesium gluconate-filled pipettes. VTA DA neurons also express STD-LTD_GABA (77.43 ± 1.5% of pre-STDP values, F_4.88,48.83= 4.368, P < 0.001, n= 17). All I_h⁽⁺⁾ cells recorded with either KCl or caesium gluconate that received the post-pre STDP protocol are included for the means in this graph. E, no changes in PPR were detected after induction of STD-LTD_GABA (103.52 ± 3.4% of pre-STDP values, F_2.13,17.07= 0.836, P= 0.457, n= 17). F, no changes in CV were detected after induction of STD-LTD_GABA (110.39 ± 6.0% of pre-STDP values, F_6.71,33.57= 1.135, P= 0.356, n= 17).

Figure 3. Induction of STDP requires NMDAR activation

A and C, single experiments showing induction of STD-LTP_GABA and STD-LTD_GABA in DA cells in the presence of 50 μm APV (half-filled symbols). Insets: averaged IPSCs before and 25 min after STDP protocol. Calibration: 100 pA, 25 ms. B, averaged control STD-LTP_GABA experiments (filled symbols) or APV experiments (half-filled symbols). Bath-applied APV blocks the induction of STD-LTP_GABA whereas control cells exhibit STD-LTP_GABA (control cells, 146.61 ± 10% of pre-STDP values, F_8.18,40.93= 5.608, P < 0.0001; APV cells, 94.26 ± 2% of pre-STDP values, F_3.16,18.99= 0.377, P= 0.781). D, averaged control STD-LTD_GABA experiments (open symbols) or APV experiments (half-filled symbols). Bath-applied APV also blocks the induction of STD-LTD_GABA whereas control cells exhibit STD-LTD_GABA (control cells, 70.5 ± 4% of pre-STDP values, F_4.1,74.2= 6.012, P < 0.0001; APV cells, 96.85 ± 4% of pre-STDP values, F_13.01,52.04= 0.725, P= 0.731).

Figure 4. Pre-/postsynaptic spiking alone does not induce STDP at GABAergic synapses in VTA DA neurons

A and C, sample experiments of IPSCs recorded from DA neurons that only received bursts of presynaptic stimulations of the STDP protocol (pre-only, filled square symbols) or bursts of postsynaptic depolarizing steps of the STDP protocol (post-only, open square symbols). Insets: averaged IPSCs before and 25 min after pre-/post-spiking protocol. Calibration: 100 pA, 25 ms. B and D, averaged experiments illustrating the absence of GABAergic STDP in response to pre-/postsynaptic spiking (pre-only cells, filled square symbols, 93 ± 3% of baseline values before pre-spiking protocol, F_16.46,50.28= 1.309, P= 0.227; post-only cells, open square symbols, 96.7 ± 1% of baseline values before post-spiking protocol, F_23.06,46.13= 0.809, P= 0.703). Pairing of pre- and postsynaptic spiking is necessary for successful induction of GABAergic STDP.

Figure 5. VTA DA neurons exhibit STD-LTD_GABA in response to a complex spike-pairing protocol

A, sample bursts of the complex STDP protocol (triplets of pre-post pairs with +5 ms delay separated by 15 ms intervals). B, single experiment showing induction of STD-LTD_GABA recorded in I_h⁽⁺⁾ neuron using KCl-filled pipette. At the arrow, STD-LTD_GABA was induced. Insets: averaged IPSCs before and 25 min after STDP protocol. Calibration: 100 pA, 25 ms. C, averaged experiments from I_h⁽⁺⁾ neurons (filled triangle symbols) exhibiting STD-LTD_GABA in response to the complex spike pairing protocol using KCl-filled pipettes. VTA DA neurons also express STD-LTD_GABA in response to a complex spiking protocol (80.03 ± 1% of pre-STDP values, F_3.073,18.440= 7.754, P= 0.0014, n= 10). D, no changes in PPR were detected after induction of STD-LTD_GABA (99.93 ± 3.1% of pre-STDP values, F_4.788,28.727= 1.337, P= 0.278, n= 10). E, no changes in CV were detected after induction of STD-LTD_GABA (95.056 ± 3.0% of pre-STDP values, F_4.94,19.79= 1.522, P= 0.754, n= 10).