Responses to microinjections of endomorphin and nociceptin into the medullary cardiovascular areas.

1. Cardiovascular effects of microinjections of nociceptin and endomorphin-2 into the following medullary areas were studied in urethane-anaesthetized rats: chemoreceptor projection site (CPS), intermediate portion of the nucleus tractus solitarius (I-NTS), caudal ventrolateral medullary depressor area (CVLM) and rostral ventrolateral medullary pressor area (RVLM). 2. Microinjections of nociceptin or endomorphin-2 (0.6 mmol/L each) into the CPS and RVLM elicited depressor and bradycardic responses, whereas similar injections into the I-NTS and CVLM elicited pressor and tachycardic responses. 3. The mechanism of cardiovascular responses to microinjections of these opioid peptides into different medullary areas involved in cardiovascular function can be postulated as follows: the direct effect of nociceptin and endomorphin-2 on neurons is usually inhibitory. Because the activation of CPS and RVLM by microinjections of L-glutamate results in pressor and tachycardic responses, inhibition of these areas by nociceptin and endomorphin-2 elicits depressor and bradycardic responses. Similarly, activation of neurons in the I-NTS and CVLM by microinjections of L-glutamate elicits depressor and bradycardic responses. Therefore, inhibition of these areas by microinjections of these opioid peptides elicits an increase in blood pressure and heart rate.