Modulation of glutamate release by nicotinic receptors in layer V of the murine prefrontal cortex

By regulating the neocortical excitability, nicotinic acetylcholine receptors (nAChRs) control vigilance and cognition. In rodents, the neocortex mainly expresses homomeric a7 and heteromeric a4ß2 nAChRs. These are expressed in both pre- and postsynaptic locations and mediate classical synaptic transmission as well as slower paracrine effects. We have studied the contribution of heteromeric nAChRs to the control of glutamate (GLU) release in layer V of the murine prefrontal cortex (PFC). Tonic application of 5 µM nicotine more than doubled the frequency of spontaneous glutamatergic excitatory postsynaptic currents recorded on pyramidal neurons in acute brain slices. The effect of nicotine was inhibited by 1 µM dihydro-ß-erythroidine (DHßE, which blocks a4-containing receptors), but not by 10 nM methyllicaconitine (MLA, which blocks a7-containing receptors). We next studied the association of a4 with different populations of glutamatergic terminals, in both PFC and somatosensory cortex. The GLU transporter type 1 (VGLUT1) mostly labels the intrinsic glutamatergic terminals or cortical afferents, whereas the type 2 transporter VGLUT2 tends to label the thalamic afferents. Immunofluorescence showed that a4 was expressed in both VGLUT1 and VGLUT2 terminals and colocalization was considerably stronger in the PFC. Expression of the a4, VGLUT1 and VGLUT2 was also tested by immunoblots, which confirmed the overall higher expression of these proteins in the PFC compared to the somatosensory cortex. Hence, in PFC, a4-containing heteromeric nAChRs are expressed in both intrinsic and extrinsic glutamatergic terminals and regulate GLU release in the presence of steady agonist levels.