The adenosine analog, 5′-N-ethylcarboxamidoadenosine, exerts mixed agonist actions on cardiorespiratory parameters in the intact but not decerebrate rat following microinjections into the nucleus tractus solitarius

A limited occipital craniotomy was conducted on intact and decerebrate urethane-anesthetized, spontaneously breathing rats to expose the caudal medulla in the region of the obex. Microinjections of5′-N-ethylcar☐amidoadenosine (NECA), a metabolically stable adenosine analog which exhibits mixed agonist properties for adenosine receptor subtypes, were made into the medial region of the caudal nucleus tractus solitarius (NTS) at the level of the caudal tip of the area postrema, an area of the NTS in which there is known to be a functional co-existence of cardiovascular and respiratory-related neuronal elements. Cardiorespiratory responses were subsequently recorded for a 30-min test period. In the intact rat, microinjections of NECA produced significant dose-related reductions in respiratory rate which were accompanied by dose-dependent increases in tidal volume and these pronounced effects on respiration persisted throughout the test period. On the other hand, microinjections of NECA into this region of the NTS of the intact rat elicited complex, bi-directional cardiovascular responses, producing hypotension (at lower doses) and pressor responses (at higher doses) in addition to bradycardia (at lower doses). In an effort to examine the functional interactions between the NTS and forebrain structures involved in cardiorespiratory control, microinjections of NECA in the identical dose range were made into the same NTS sites of a separate group of urethane-anesthetized, spontaneously breathing rats in which reciprocal connections between forebrain areas and the brainstem had been disrupted by acute supracollicular decerebration. A simulating electrode, placed in the paraventricular nucleus of the hypothalamus (PVH), was used to confirm complete transection during the experiment and to ascertain the integrity of reciprocal connections between the brainstem and rostral brain regions involved in cardiorespiratory control. Although decerebration at the supracollicular level negligibly affected basal cardiorespiratory parameters, microinjections of NECA into the NTS revealed dramatic differences in the cardiovascular response patterns between intact and decerebrate rats. Whereas cardiovascular responses elicited by microinjections into the NTS were significantly affected by supracollicular decerebration, respiratory responses were highly similar for both intact and decerebrate animals. Indeed, repeated measures MANOVA indicated that there were no significant differences in the time-related or dose-related responses in the depression of respiration between decerebrate and intact rats following NECA microinjections. In contrast, MANOVA indicated that the effects of NECA microinjections on blood pressure and heart rate showed significant dose-related differences between decerebrate and intact rats. In fact, one-way ANOVA indicated that NECA microinjections exerted no significant dose-related effects on blood pressure or heart rate at any time point in the decerebrate rats whereas highly significant dose-related responses in blood pressure and heart rate were elicited in the intact rats. Thus, the results indicate that disruption of rostral projections to the NTS via supracollicular decerebration significantly affects NTS-mediated cardiovascular responses elicited by NECA but not the respiratory responses, suggesting that the respiratory effects of NECA may be mediated by different intrinsic mechanisms in the NTS than are the cardiovascular effects of NECA. Indeed, the data suggest that the respiratory depressant effects of NECA in both intact and decerebrate rats are likely mediated by a single population of adenosine receptors. In contrast, NECA may influence cardiovascular response patterns by activating more than one population of adenosine receptor subtypes in the NTS of intact, but not decerebrate animals, presumably by modulating neurotransmitter release from the vast network of nerve terminals projecting to the NTS from rostral brain regions. Finally, the methods for acute supracollicular decerebration and the use of the PVH stimulating electrode in combination with a limited occipital craniotomy, provide a suitable preparation to develop experimental strategies for studying brainstem cardiorespiratory regulatory mechanisms wherein equivalent experimental manipulations can be conducted under both intact and decerebrate conditions.