Final answer:
The first ion targeted by optogenetical activation in inducing aggression in mice is likely sodium (Na+), given its role in depolarizing neurons upon glutamate receptor activation.
Step-by-step explanation:
The first target of regulation by optogenetical activation of neural circuits in the hypothalamus to induce aggressive behavior in mice is likely a positively charged ion. Given the information provided about glutamate-mediated excitatory neurotransmission and the role of ligand-gated cation channels, the first ion to be targeted for regulation in this scenario could be sodium (Na+). The activation of postsynaptic glutamate receptors leads to the influx of Na+ and subsequent depolarization of the postsynaptic cell, which is integral to the propagation of nerve impulses that can culminate in specific behaviors such as aggression.