Research reportInsular cortex lesions alter baroreceptor sensitivity in the urethane-anesthetized rat
Introduction
The insular cortex is an important site of autonomic representation in the rat, monkey and possibly the human 3, 8, 9, 10, 11, 12, 15, 17, 18. Within the rat insula, viscerotopic organization has been identified: gustatory and gastric mechanoreceptor inputs relay to insular sites rostral to the crossing of the anterior commissure, whereas insular regions caudal to this point receive baroreceptor and cardiac related information [3]. In addition, cardiac chronotropic organization exists within the posterior insular cortex of the rat. Sites influencing tachycardia are sited in the posterior insular cortex rostral to those from which bradycardia may be elicited [8]. Similar posterior insular regions are also involved in blood pressure control: elevations of blood pressure can be induced by both chemical and electrical stimulation of the rostral posterior insula, whereas depressor effects are elicitable from the more caudal posterior insula [15].
Recently we have demonstrated a more widespread distribution of baroreceptor-related neurons in the rat insular cortex than previously suggested by Cechetto and Saper [3]. Using extracellular recording we have characterized neurons responsive to blood pressure changes in the anterior insula rostral to the crossing of the anterior commissure [17]. Thus, baroreceptor related cells do not appear to be principally confined to the posterior insula in the rat. However, the role of these more anteriorly placed insular sites (if any) in the control of cardiovascular function is unclear.
Lateralization of cardiovascular representation at the cortical level has been indicated in humans by the studies of Zamrini et al. and by our previous stimulation experiments in epileptics undergoing neurosurgical procedures 9, 16. In these investigations, sympathetic cardiovascular expression (tachycardia and increased diastolic pressure) was suggested to be a function of right hemisphere (and particularly right insular) function. Left insular stimulation on the other hand, was noted to produce principally bradycardia and depressor effects. Our previous studies in the rat suggested a significant clustering of baroreceptor-related units within the right posterior insula [17].
The aim of this study is to extend these observations in the rat by investigating whether the anterior insula is involved in the control of baroreceptor function, and whether insular lateralization of cardiovascular control can be identified.
There has been preliminary communication of these data in abstract form [7]
Section snippets
Methods
Fifty-seven male Sprague–Dawley rats (250–350 g) were anesthetized with i.p. injections of urethane (1.5 g/kg). Catheters were inserted into the femoral artery and vein using PE-50 tubing for the arterial line and PE-20 tubing for the vein. These lines were filled with heparinized saline. The blood pressure was recorded using a Grass PT300 pressure transducer (Astro/Med Grass, West Warwick RI) connected to a Grass AC/DC strain gage amplifier. The blood pressure wave form was converted to mean
Results
Twenty lesions were placed within the right and 21 in the left insular cortex (Fig. 1). Sixteen extrainsular lesions were made, seven of which were located in the surrounding right frontoparietal cortex (Fig. 2). In addition, one lesion centered on the right caudate/putamen, one lesion centered on the right external capsule and seven lesions involved the surrounding left frontoparietal cortex (Fig. 2). The lesions were mainly confined to the anterior or posterior insular cortex or adjacent
Discussion
Our previous investigations indicated lateralization of cardiovascular function in the rat. Extracellular recordings during injections of PE and SNP showed a significant clustering of neurons responsive to baroreceptor challenge in the right posterior compared with the left posterior insula [17]. However, we also showed that many baroreceptor related units could be found in insular regions rostral to the crossing of the anterior commissure, but without any evidence of lateralization. This was
Acknowledgements
The technical help of Mr. Navin Kulshreshthra is appreciated. The study was supported by PHS grant R01NS33770-03 and the EL Wiegand Foundation.
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