Activation of metabotropic glutamate 5 (mGlu5) receptors induces spontaneous excitatory synaptic currents in layer V pyramidal cells of the rat prefrontal cortex

Serotonin and norepinephrine, in addition to direct postsynaptic excitatory effects, also enhances glutamate release onto layer V pyramidal cells of the prefrontal cortex/neocortex via Gq/11-coupled 5-hydroxytryptamine2A (5-HT2A) and α1-adrenergic receptors, respectively. Therefore, the present study was designed to test whether a metabotropic glutamate (mGlu) receptor subtype also coupled to Gq/11-proteins, the mGlu5 receptor, also induces EPSCs when recording from layer V cortical pyramidal cells of the rat medial prefrontal cortex (mPFC). The mGlu1/5 receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induces EPSCs at a similar frequency as a near-maximally effective 5-HT concentration. The mGlu5 receptor negative allosteric modulator 2-methyl-6-(phenylethynyl)pyridine (MPEP, 300 nM) potently blocked DHPG-induced EPSCs. Previous work has suggested that activation of 5-HT2A and OX2 receptors induces glutamate release, while mGlu2, mGlu4, and mGlu8 receptor activation suppress transmitter release from thalamocortical terminals. Taken together with past results, these findings suggest that mGlu2, mGlu4, mGlu5 and mGlu8 may all act to modulate glutamate release from afferents impinging on layer V pyramidal cells of the mPFC. These findings further suggest that monoamines, neuropeptides and glutamate itself all enhance the excitability of prefrontal cortical output cells indirectly via modulation of glutamate release.