The medial prefrontal cortex (mPFC) serves executive cognitive functions such as decision-making that are impaired in neuropsychiatric disorders and pain. We showed previously that amygdala-driven abnormal inhibition and decreased output of mPFC pyramidal cells contribute to pain-related impaired decision-making (Ji et al., 2010). Therefore, modulating pyramidal output is desirable therapeutic goal. Targeting metabotropic glutamate receptor subtype mGluR5 has emerged as a cognitive-enhancing strategy in neuropsychiatric disorders, but synaptic and cellular actions of mGluR5 in the mPFC remain to be determined. The present study determined synaptic and cellular actions of mGluR5 to test the hypothesis that increasing mGluR5 function can enhance pyramidal cell output.
Whole-cell voltage- and current-clamp recordings were made from visually identified pyramidal neurons in layer V of the mPFC in rat brain slices. Both the prototypical mGluR5 agonist CHPG and a positive allosteric modulator (PAM) for mGluR5 (VU0360172) increased synaptically evoked spiking (E–S coupling) in mPFC pyramidal cells. The facilitatory effects of CHPG and VU0360172 were inhibited by an mGluR5 antagonist (MTEP). CHPG, but not VU0360172, increased neuronal excitability (frequency–current [F–I] function). VU0360172, but not CHPG, increased evoked excitatory synaptic currents (EPSCs) and amplitude, but not frequency, of miniature EPSCs, indicating a postsynaptic action. VU0360172, but not CHPG, decreased evoked inhibitory synaptic currents (IPSCs) through an action that involved cannabinoid receptor CB1, because a CB1 receptor antagonist (AM281) blocked the inhibitory effect of VU0360172 on synaptic inhibition. VU0360172 also increased and prolonged CB1-mediated depolarization-induced suppression of synaptic inhibition (DSI). Activation of CB1 with ACEA decreased inhibitory transmission through a presynaptic mechanism.
The results show that increasing mGluR5 function enhances mPFC output. This effect can be accomplished by increasing excitability with an orthosteric agonist (CHPG) or by increasing excitatory synaptic drive and CB1-mediated presynaptic suppression of synaptic inhibition (“dis-inhibition”) with a PAM (VU0360172). Therefore, mGluR5 may be a useful target in conditions of impaired mPFC output.
This article is part of a Special Issue entitled ‘Metabotropic Glutamate Receptors’.