Importance of bilateral sympathetic innervation on cerebral blood flow autoregulation in the thalamus

Effect of sympathetic denervation on rabbit choroidal blood flow

In this study we demonstrate the existence of sympathetic innervation and compare the effect of unilateral or bilateral superior cervical sympathectomy on albino rabbit choroidal blood flow (CBF) during changes in perfusion pressure (PP). Forty albino rabbits weighing between 2.0 and 3.0 kg were randomly divided into three groups. The bilateral sympathectomy group (group S) included 10 rabbits (20 eyes) that received bilateral sympathectomy 1 week prior to the study. The unilateral sympathectomy group (group U) included 20 rabbits (20 eyes) that received unilateral sympathectomy 1 week prior to the study. Only the eyes ipsilateral to sympathectomy were utilized. The other 10 rabbits (20 eyes) served as controls (group N), each received the same procedure as the experimental groups except that the superior cervical ganglion (SCG) was preserved. The blood cell flux (PF), velocity (V), and concentration of moving blood cells (CMBC) were recorded simultaneously by means of a laser Doppler flowmeter (Perimed PF4001), while the intraocular pressure was increased linearly with a syringe pump. When the PP decreased steadily, the PF, V, and CMBC remained constant until PP <55 mm Hg, then decreased proportionally to the PP. When the PP decreased from 75 to 0 mm Hg, the PF, V and CMBC decreased from 100 to 6.87 +/- 0.97%, 8.44 +/- 0.92%, and 18.67 +/- 0.91% in group N, to 18.56 +/- 1.62%, 19.30 +/- 1.84%, and 38.02 +/- 9.10% in group U, and to 18.38 +/- 2.89%, 16.78 +/- 1.48%, and 34.58 +/- 4.42% in group S. The changes in PF, V, and CMBC were similar in groups S and U. Both group S and U had higher PF, V, and CMBC values than group N at comparable PPs below 55 mm Hg. These results indicate that the SCG plays a role in CBF regulation. Both unilateral and bilateral sympathectomy led to a higher PF in both groups S and U rabbits, indicating increased CBF while PP decreased gradually. This suggests that the rabbit choroid does not receive crossed innervation. The plateau response in each curve demonstrates the presence of autoregulation. This autoregulation was unchanged by either unilateral or bilateral sympathetic denervation.

Influence of cerebrovascular sympathetic, parasympathetic, and sensory nerves on autoregulation and spontaneous vasomotion

The effect of removal of cerebrovascular sympathetic, parasympathetic or sensory nerve on brain cortical blood flow and spontaneous vasomotion during changes in systemic blood pressure was studied by laser-Doppler flowmetry in anaesthetized rats. Selective section of sympathetic fibres along the internal carotid artery markedly affected the ability to autoregulate, as measured in microvessels of the middle cerebral arterial territory. Removal of the parasympathetic nerves tended to reduce the ability to autoregulate, whereas no significant influence was found after sensory denervation. Following the denervations, spontaneous vasomotion was not significantly affected in frequency or amplitude.

Inhibition of angiotensin-converting enzyme modulates the autoregulation of regional cerebral blood flow in hypertensive rats

The inhibition of angiotensin-converting enzyme activities is considered to favorably modulate the hemodynamics of the brain. We designed the present study to examine the effects of angiotensin-converting enzyme inhibitors on regional differences in the lower limits of cerebral blood flow autoregulation in spontaneously hypertensive rats. Angiotensin-converting enzyme inhibitors (either 10 mg/kg captopril or SQ 29,852 in saline) were intravenously injected 15 minutes before hemorrhagic hypotension was induced. Cerebral blood flows to the parietal cortex and thalamus were simultaneously measured by hydrogen clearance. Both captopril and SQ 29,852 significantly decreased mean arterial pressure by 14 to 18 mm Hg and also reduced calculated cerebral vascular resistance by 11% to 15% of resting values, which resulted in a well-maintained cerebral blood flow. The lower limits of autoregulation were 76 +/- 2% (mean +/- SEM) and 77 +/- 2% of resting values in the cortex and thalamus, respectively, in control rats. Administration of either captopril or SQ 29,852 significantly reduced the lower limits to 65 +/- 3% (P < .01 versus control) and 67 +/- 2% (P < .05), respectively, in the cortex, which were slightly but always larger than the 71 +/- 3% and 71 +/- 2% reduction, respectively, in the thalamus. The inhibition of angiotensin-converting enzyme activities thus may be more protective against acute hypotension for cerebral microcirculation in the cortex than in the thalamus.

Effect of thromboxane synthetase inhibitor on cerebral circulation and metabolism during experimental cerebral ischemia in spontaneously hypertensive rats

The protective effect of thromboxane synthetase inhibitor, OKY-046, on brain ischemia was studied in spontaneously hypertensive rats. Cerebral ischemia was developed by bilateral carotid artery ligation (BCL) for 1 or 3 h and thereafter, circulation was restored for 15 min. OKY-046, 5 or 30 mg/kg, or saline as control was administered i.v. before BCL. Neither blood pressure nor blood gases were altered by OKY-046 or saline injection. During BCL, cerebral cortical blood flow was reduced to 25 and 15% of the resting value at 30 and 60 min, respectively, and these changes were not different among the groups. In rats with ischemia longer than 1 h, the blood flow was well preserved by OKY-046, 30 mg/kg, to 10-17% of the resting level, thus significantly higher than that (less than 5%) in non-treated rats. After 15 min recirculation, the supratentorial lactate level was lower and adenosine triphosphate (ATP) was higher in OKY-046-treated rats than in the saline-treated ischemic rats. Plasma thromboxane B2 was increased markedly in 1 h ischemic-reperfused rats without treatment and the increase was almost completely inhibited by OKY-046. In contrast, 6-keto-prostaglandin F1 alpha was increased 8.5-fold after ischemia and the increase was not affected by the treatment. OKY-046 seems to have an antiischemic effect on acutely induced cerebral ischemia. Selective inhibition of thromboxane A2 production and an inversely high level of prostaglandin I2 may be an important contribution to protection of the microcirculation during ischemia and preservation of ischemic cerebral metabolism.

Intracranial trajectories of sympathetic nerve fibers originating in the superior cervical ganglion in the rat: WGA-HRP anterograde labeling study

Intracranial trajectories of sympathetic nerve fibers originating in the superior cervical ganglion (SCG) in the rat were investigated by means of anterograde labeling following the injection of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the unilateral SCG. The trajectory of the sympathetic fiber innervating the pineal gland and its continuing structures was found advancing along the abducent nerve, through the cavernous plexus, then along the trochlear nerve. Labeled sympathetic fibers showed two patterns of distribution in the blood vessels on the basal surface of the brain. The sympathetic fibers originating in the unilateral SCG were intermingled with those fibers from the contralateral SCG in the pineal gland, its continuing structures and the choroid plexus of the third ventricle as well as in the cerebral blood vessels.