mGlu1 and mGlu5 modulate distinct excitatory inputs to the nucleus accumbens shell

Glutamatergic transmission in the nucleus accumbens shell (NAcSh) is a substrate for reward learning and motivation. Metabotropic glutamate (mGlu) receptors regulate NAcSh synaptic strength by inducing long-term depression (LTD). Inputs from prefrontal cortex (PFC) and medio-dorsal thalamus (MDT) drive opposing motivated behaviors yet mGlu receptor regulation of these synapses is unexplored. We examined Group I mGlu receptor regulation of PFC and MDT glutamatergic synapses onto specific populations of NAc medium spiny neurons (MSNs) using D1tdTom BAC transgenic mice and optogenetics. Synaptically evoked long-term depression (LTD) at MDT-NAcSh synapses required mGlu(5) but not mGlu(1) and was specific for D1(+) MSNs, whereas PFC LTD was expressed at both D1(+) and D1(-) MSNs and required mGlu(1) but not mGlu(5). Two weeks after five daily non-contingent cocaine exposures (15 mg/kg), LTD was attenuated at MDT-D1(+) synapses but was rescued by the mGlu(5)-positive allosteric modulator (PAM) VU0409551. These results highlight unique plasticity mechanisms regulating specific NAcSh synapses.