Reaction shift keying (RSK) is a recently proposed modulation scheme for diffusion-based molecular communication networks. For RSK, the transmitter uses different chemical reactions to generate different emission patterns to represent different symbols. The receiver front end consists of a molecular circuit and the back end uses a bank of demodulation filter for detection. The aim of this paper is to study the impact of the choice of molecular circuits on communication performance. We consider two circuits: (1) ligand-receptor binding with feedback regulation; (2) ligand-receptor with multiple binding sites. For each circuit, we derive the maximum a posteriori demodulation filter by solving a Bayesian filtering problem. We show that appropriate amount of feedback can reduce symbol error rate. We also show how the choice of measurements in the multiple multiple binding site case can affect communication performance.