Dopamine Promotes Motor Cortex Plasticity and Motor Skill Learning via PLC Activation

Abstract

Dopaminergic neurons in the ventral tegmental area, the major midbrain nucleus projecting to the motor cortex, play a key role in motor skill learning and motor cortex synaptic plasticity. Dopamine D1 and D2 receptor antagonists exert parallel effects in the motor system: they impair motor skill learning and reduce long-term potentiation. Traditionally, D1 and D2 receptor modulate adenylyl cyclase activity and cyclic adenosine monophosphate accumulation in opposite directions via different G-proteins and bidirectionally modulate protein kinase A (PKA), leading to distinct physiological and behavioral effects. Here we show that D1 and D2 receptor activity influences motor skill acquisition and long term synaptic potentiation via phospholipase C (PLC) activation in rat primary motor cortex. Learning a new forelimb reaching task is severely impaired in the presence of PLC, but not PKA-inhibitor. Similarly, long term potentiation in motor cortex, a mechanism involved in motor skill learning, is reduced when PLC is inhibited but remains unaffected by the PKA inhibitor. Skill learning deficits and reduced synaptic plasticity caused by dopamine antagonists are prevented by co-administration of a PLC agonist. These results provide evidence for a role of intracellular PLC signaling in motor skill learning and associated cortical synaptic plasticity, challenging the traditional view of bidirectional modulation of PKA by D1 and D2 receptors. These findings reveal a novel and important action of dopamine in motor cortex that might be a future target for selective therapeutic interventions to support learning and recovery of movement resulting from injury and disease.

Fig 1. Experimental design.

(a) Timeline for experiments shown in Fig 2A using continuous drug release via minipump (red bar) during motor skill training. (b) Timeline for experiments shown in Fig 2B, Fig 3A and 3B using acute drug injections on day 2 and 3 of motor skill training (red arrows). (c) Timeline for experiments shown in Fig 2 using continuous drug release via minipump (red bar) after successful acquisition of the motor task.

Fig 2. PLC inhibitor impairs motor skill acquisition and the potential for M1 synaptic plasticity.

(a) Motor skill acquisition was assessed in rats that received continuous intracortical injection (grey bar) of PLC inhibitor U-73122 (blue), PKA inhibitor H-89 (green) and inactive PLC inhibitor U-73343 (black) into the M1 forelimb area via osmotic minipumps. The success rate and the latency, a measure of motivation, were impaired in the presence of PLC but not PKA inhibitor. Error bars indicate SEM. (b) Temporary application of PLC and PKA inhibitors directly to M1 on training day 2 & 3 (time of steepest learning) reveal the same effect as with continuous application. Arrows indicate local injection days. The inset illustrates the rate of learning from day 1–2, day 3–4, and day 4–5 indicating a lack of improvement from day 2–3. (c) Maximum synaptic strength (LTP saturation) by repeated induction of LTP (multiple arrows) in layer II/III horizontal connections in brain slices. Peak amplitudes were assessed in the M1 forelimb area in ACSF (dark blue) and in the presence of PLC inhibitor U-73122 (light blue). Each FP trace (insets) represents an average of 10 individual responses at times indicated by numbers. (d) Maximum LTP in the presence of PKA inhibitor is not significantly different from control LTP. Arrows indicate multipe LTP attempts. (e) Summary histogram of LTP. Grey: Control; green: PKA inhibition; blue: PLC inhibition.

Fig 3. PLC activation is not necessary for the execution of an already acquired task and does not affect baseline synaptic transmission.

(a) Continuous intracortical infusion of PLC inhibitor (grey bar) for 5 days following successful acquisition of the skilled reaching task did not affect success rate or intertrial latency. Insets illustrate mean±SEM before (day 6–8) and following (day 9–13) treatment with PLC inhibitor indication a lack of change. Error bars indicate SEM. (b) Amplitudes and shapes of extracellular field potentials (FP) in layer II/III horizontal connections of the M1 forelimb area remained unchanged in the presence of PLC inhibitor. Traces are examples of extracellular FPs at baseline stimulation intensity before (control, dark blue) and after inhibitor application (light blue).

Fig 4. PLC activation prevents DA antagonist-induced learning deficits.

(a) Co-administration of the PLC agonist m-3M3FBS and D1 receptor antagonist SCH 23390 (red) prevents SCH 23390 (orange) induced deficit in the success rate (left) compared to vehicle injected control (black). Intertrial latencies remained unaffected regardless of drug or drug combination injected (right). (b) D2 receptor antagonist raclopride (yellow) impaired skill acquisition. This impairment was prevented by PLC agonist co-administration (red). PLC agonist alone (green) had no effect on learning as compared to controls (black). Intertrial latencies remain unaffected by all treatments (right). Error bars indicate SEM. Arrows indicate days of local injections.

Fig 5. Activation of intracellular PLC pathway rescues the LTP impairment caused by DA receptor block.

(a) Maximum synaptic strength of horizontal layer II/III connections, measured by saturating LTP with multiple attempts of TBS (arrows) in raclopride (D2antag) co-administered with m-3m3fbs (PLCactivator, red) resulted in normal amounts of LTP (left). m-3m3fbs alone did not affect LTP saturation (green). The histogram (right) illustrates LTP saturation for control condition (grey), PLC agonist (green), PLC antagonist (blue), D2 antagonist (yellow). D1 and D2 antagonist co-applied with the PLC agonist (red) illustrates the rescue of D1 and D2 block. (b) FP amplitudes were not affected by co-administration of D2 antagonist and PLC agonist or by PLC agonist alone (left). The histogram (right) illustrates group data of synaptic strength for all the different conditions (color code as in a). Values are calculated relative to the baseline recordings before drug application.