Cardiovascular Actions of Endomorphin-2 in the Nucleus Tractus Solitarius

Cardiovascular effects of microinjections of endomorphin-2 into the medial subnucleus of the nucleus tractus solitarius (mNTS) were studied in adult, male, urethane-anesthetized, Wistar rats. Microinjections of endomorphin-2 into the mNTS elicited depressor and bradycardic responses via mu-opioid receptors. Similar microinjections of endomorphin-2 attenuated the carotid sinus and aortic baroreflex responses. Endomorphin-2 inhibits GABAergic mNTS neurons via mu-opioid receptors, causing disinhibition which results in excitation of secondary mNTS neurons. This effect causes depressor and bradycardic responses. Endomorphin-2 also inhibits glutamate release from baroreceptor terminals via mu-opioid receptors located on these terminals and attenuates baroreflex responses.