Cardiovascular responses to microinjections of GABA or anesthetics into the rostral ventrolateral medulla of conscious and anesthetized rats

Abstract

The rostral ventrolateral medulla (RVLM) contains neurons involved in tonic and reflex control of arterial pressure. We describe the effects of gamma-aminobutyric acid (GABA) and anesthetics injected into the RVLM of conscious and urethane (1.2 g/kg, iv) anesthetized Wistar rats (300-350 g). In conscious rats, bilateral microinjection of GABA (50 nmol/200 nl) induced a small but significant decrease in blood pressure (from 130 ± 3.6 to 110 ± 5.6 mmHg, N = 7). A similar response was observed with sodium pentobarbital microinjection (24 nmol/200 nl). However, in the same animals, the fall in blood pressure induced by GABA (from 121 ± 8.9 to 76 ± 8.8 mmHg, N = 7) or pentobarbital (from 118 ± 4.5 to 57 ± 11.3 mmHg, N = 6) was significantly increased after urethane anesthesia. In contrast, there was no difference between conscious (from 117 ± 4.1 to 92 ± 5.9 mmHg, N = 7) and anesthetized rats (from 123 ± 6.9 to 87 ± 8.7 mmHg, N = 7) when lidocaine (34 nmol/200 nl) was microinjected into the RVLM. The heart rate variations were not consistent and only eventually reached significance in conscious or anesthetized rats. The right position of pipettes was confirmed by histology and glutamate microinjection into the RVLM. These findings suggest that in conscious animals the RVLM, in association with the other sympathetic premotor neurons, is responsible for the maintenance of sympathetic vasomotor tone during bilateral RVLM inhibition. Activity of one or more of these premotor neurons outside the RVLM can compensate for the effects of RVLM inhibition. In addition, the effects of lidocaine suggest that fibers passing through the RVLM are involved in the maintenance of blood pressure in conscious animals during RVLM inhibition.

Urethane inhibits the GABAergic neurotransmission in the nucleus of the solitary tract of rat brain stem slices

“…In conscious rats, microinjection of GABA in RVLM induced a small decrease in blood pressure. However, in the same animals, the fall in blood pressure was greater after urethane anesthesia (15). Either GABA A receptor antagonists (bicuculline) or GABA B receptor antagonists (2-hydroxysaclofen or CGP-35348) microinjected in the RVLM produced a pressor response in urethane-anesthetized rats (1,5), suggesting that, at least in the RVLM, urethane is not producing inhibition of the tonic activity of GABAergic neurons projecting to this region.…”

Generalized arousal of mammalian central nervous system

“…Because they are distributed loosely along the ventral and medial borders of each side of the brainstem (interrupted by such specifically dedicated neuronal groups as the facial nucleus and inferior olive), I envision them as a phylogenetically ancient "crescent" of arousal-related neurons. Neurons in this region of the medullary reticular formation can respond to a diverse array of arousing inputs (Van Bockstaele et al, 1989) including, for example, changes in carbon dioxide tension or blood pressure in the service of cardiovascular and respiratory regulation (de-Oliveira et al, 1996;Lacerda et al, 2003;Schreihofer and Guyenet, 2003), painful stimuli Mason, 1998, 1999), and sexually relevant inputs (Marson and McKenna, 1990;Hubscher and Johnson, 1996;Yanagimoto et al, 1996;McKenna, 2001) from the pelvic nerve. Outputs from the nucleus paragigantocellularis and from the rostral ventrolateral medulla (RVLM), for example, have obvious import for arousal through their projections to locus coeruleus and other sites (Guyenet and Young, 1987;Huangfu et al, 1992;Johnson et al, 2002).…”

Cardiovascular, respiratory and metabolic responses to apnea induced by atlanto‐occipital intrathecal lidocaine injection in anesthetized horses

“…In both conscious and urethane‐anesthetized rats, microinjections of lidocaine into the rostroventrolateral medulla (RVLM), a region with neurons involved in tonic and reflex control of blood pressure, decreased blood pressure by approximately 25–30 mmHg (Lacerda et al. ). A decrease of this magnitude in the horses of the present study is likely to have contributed to but does not fully explain the fall in blood pressure observed during hypoxemia.…”