An AMPA glutamatergic receptor activation-nitric oxide synthesis step signals transsynaptic apoptosis in limbic cortex

We have previously shown that pyramidal neurons engaged in cortico-cortical connectivity in limbic cortex are vulnerable to denervation lesions, i.e. relay pyramidal neurons in layer II of piriform cortex undergo transsynaptic apoptosis after lesions interrupting their inputs from the olfactory bulb (bulbotomies). At least one trigger of this transsynaptic degenerative phenomenon is the activation of inhibitory interneurons in layer I, which are induced to upregulate neuronal nitric oxide synthase (nNOS) and release NO. Thus, we have demonstrated that cortical interneurons play an essential role in transducing injury to apoptotic signaling that selectively targets pyramidal neurons. In the present study, we confirm the role of nNOS with pharmacological inhibition of a significant ∼30% of transsynaptic apoptosis with the selective nNOS inhibitor BRNI at optimal doses. Outcomes were studied both at the histological and molecular level using DNA blots. We also show that the first-generation competitive non-NMDA (AMPA) antagonist NBQX ameliorates transsynaptic apoptosis by the same margin of difference as BRNI and it also reduces nNOS activation as indicated by a significant decrease in NADPH diaphorase histochemical activity in layer I of piriform cortex. Our findings confirm the role of nNOS activation/NO release in transsynaptic apoptosis and show that glutamatergic agonism at AMPA sites also plays a significant role. In addition, our data suggest that AMPA agonism may occur upstream to nNOS upregulation in inhibitory interneurons of layer I. In concert, our findings indicate that transsynaptic neuronal degeneration in limbic cortex involves complex AMPA-glutamatergic and nitrinergic signaling events. An AMPA-mediated upregulation of nNOS and release of NO by inhibitory interneurons may play a prominent role in this type of injury.