TARP mediation of accelerated and more regular locus coeruleus network bursting in neonatal rat brain slices.

Transmembrane AMPA receptor (AMPAR) regulatory proteins (TARP) increase neuronal excitability. However, it is unknown how TARP affect rhythmic neural network activity. Here we studied TARP effects on local field potential (LFP) bursting, membrane potential and cytosolic Ca2+ (Cai) in locus coeruleus neurons of newborn rat brain slices. LFP bursting was not affected by the unselective competitive ionotropic glutamate receptor antagonist kynurenic acid (2.5 mM). TARP-AMPAR complex activation with 25 µM CNQX accelerated LFP rhythm 2.2-fold and decreased its irregularity score from 63 to 9. Neuronal spiking was correspondingly 2.3-fold accelerated in association with a 2-5 mV depolarization and a modest Cai rise whereas Cai was unchanged in neighboring astrocytes. After blocking rhythmic activities with tetrodotoxin (1 µM), CNQX caused a 5-8 mV depolarization and also the Cai rise persisted. In tetrodotoxin, both responses were abolished by the non-competitive AMPAR antagonist GYKI 53655 (25 µM) which also reversed stimulatory CNQX effects in control solution. The CNQX-evoked Cai rise was blocked by the L-type voltage-activated Ca2+ channel inhibitor nifedipine (100 µM). The findings show that ionotropic glutamate receptor-independent neonatal locus coeruleus network bursting is accelerated and becomes more regular by activating a TARP-AMPAR complex. The associated depolarization-evoked L-type Ca2+ channel-mediated neuronal Cai rise may be pivotal to regulate locus coeruleus activity in cooperation with SK-type K+ channels. In summary, this is the first demonstration of TARP-mediated stimulation of neural network bursting. We hypothesize that TARP-AMPAR stimulation of rhythmic locus coeruleus output serves to fine-tune its control of multiple brain functions thus comprising a target for drug discovery.

α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor activation protects against phencyclidine-induced caspase-3 activity by activating voltage-gated calcium channels.

Phencyclidine (PCP) is a noncompetitive, open channel blocker of the N-methyl-D-aspartate (NMDA) receptor-ion channel complex. When administered to immature animals, it is known to cause apoptotic neurodegeneration in several regions, and this is followed by olanzapine-sensitive, schizophrenia-like behaviors in late adolescence and adulthood. Clarification of its mechanism of action could yield data that would help to inform the treatment of schizophrenia. In our initial experiments, we found that α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) inhibited PCP-induced apoptosis in organotypic neonatal rat brain slices in a concentration-dependent and 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive manner. Calcium signaling pathways are widely implicated in apoptosis, and PCP prevents calcium influx through NMDA receptor channels. We therefore hypothesized that AMPA could protect against this effect by activation of voltage-dependent calcium channels (VDCCs). In support of this hypothesis, pretreatment with the calcium channel blocker cadmium chloride eliminated AMPA-mediated protection against PCP. Furthermore, the L-type VDCC inhibitor nifedipine (10 µM) fully abrogated the effects of AMPA, suggesting that L-type VDCCs are required for AMPA-mediated protection against PCP-induced neurotoxicity. Whereas the P/Q-type inhibitor ω-agatoxin TK (200 nM) reduced AMPA protection by 51.7%, the N-type VDCC inhibitor ω-conotoxin (2 µM) had no effect. Decreased AMPA-mediated protection following cotreatment with K252a, a TrkB inhibitor, suggests that brain-derived neurotrophic factor signaling plays an important role. By analogy, these results suggest that activation of L-type, and to a lesser extent P/Q-type, VDCCs might be advantageous in treating conditions associated with diminished NMDAergic activity during early development.