We examined the synchronous-asynchronous spiking behavior of weakly coupled two neurons using an analytic theory called "phase-reduction method". By modeling neurons involved in the thalamocortical loop, we show how the form of phase-response function depends on ion channels specific to each neuron model. Next, we investigate the synchronous-asynchronous spiking behavior of a pair of identical neurons interconnected by a gap junction or a reciprocal inhibitory synapse.