In an experiment you observed that norepinephrine binds to their specific plasma membrane receptor. The receptor to which norepinephrine binds and elicits its functional response is generally known as beta-adrenergic receptor. Propranolol is a drug that blocks beta-adrenergic receptors. Norepinephrine and beta-adrenergic receptor interaction finally led to a functional response, such as inhibition of neuronal sodium-potassium ATPase. This neuronal preparation was devoid of nucleus. In another experiment, thyroid hormone also caused inhibition of neuronal sodium-potassium ATPase. Thyroid hormone and its specific receptor interaction also demonstrated increase in membrane adenylate cyclase specific activity and increase in cytosolic cAMP concentration in the neuronal fraction which is devoid of nucleus. Characterize the receptor system activated by thyroid hormones in this experiment and support your answer. Note that the neuronal preparation used for the experiment is devoid of nucleus, but contains membrane preparation.