Anesthesia alters NO-mediated functional hyperemia.

Many properties of nitric oxide, NO, (localization, diffusiveness, half-life, vasodilatory affects) have supported its potential role in mediating the link between local cerebral activity and blood flow. However, evidence that both supports and refutes a role for NO in functional hyperemia have been presented. The present study employed multiple nitric oxide synthase inhibitors, two anesthetic regimes and laser-Doppler flowmetry to test the hypothesis that NO is critically involved in mediating the functional hyperemic response within rodent whisker-barrel cortex (WBC). In urethane anesthetized animals, functional hyperemic responses were obtained both before and after 1 mg/kg atropine infusion, 30 mg/kg i.v. L-NAME (N-Nitro-L-arginine methylester) infusion, 30 mg/kg L-NA (N-Nitro-L-arginine) infusion or 25 mg/kg 7-NI (7-nitroindazole). L-NAME was also tested in a group of animals pretreated with halothane before urethane anesthesia. Neither the magnitude of the blood flow response nor its time course was altered by NO blockade or atropine administration when compared to pre-infusion controls in urethane anesthetized rats. In contrast, animals that were pretreated with halothane exhibited a 33% inhibition of functional hyperemia after L-NAME administration. Taken together, these data do not support a primary role for NO in rat WBC functional hyperemia and suggest that previous reports of inhibition may have been secondary to the anesthesia employed.

Regional cerebral blood flow responses to variable frequency whisker stimulation: an autoradiographic analysis.

Activation of the rat primary somatosensory barrel field (S1BF) is a commonly used model to study the mechanisms of evoked coupled cortical blood flow changes. However, the relationship between these blood flow changes and variable whisker movement has not been completely characterized. We have previously shown that in urethane anesthetized rats, the magnitude of laser-Doppler measured cortical blood flow changes increase linearly with the frequency of full pad whisker movement over the physiological range of 1.5 to 10.5 s. To further test the hypothesis that local cortical blood flow increases with frequency of whisker movement and underlying neuronal activity, regional cerebral blood flow (rCBF) was determined autoradiographically in seven urethane anesthetized SD rats. Selected rows of whiskers (rows C, D, E) were stimulated at 3 s on the right side of the rat's face and simultaneously at 10 s on the left side for 2 min prior to radioactive tracer administration. Subregions of somatosensory cortex were identified with the aid of thionin and cytochrome oxidase stained sections. Mean rCBF (ml/100 g/min) for S1BF were: S1BF [0 s] left cortex, 146+/-13; S1BF [0 s] right cortex, 158+/-15; S1BF[3 s], 160+/-13; S1BF [10 s] 178+/-14. In both stimulated and nonstimulated regions, the profile of blood flow increased across cortex laminae, peaking in layer IV and decreasing through deeper layers. Maximal blood flow increases elicited by whisker movement occurred in cortical layers I-IV. These data support the hypothesis that whisker movement elicited rCBF changes are input frequency dependent and are most pronounced in cortical layers I though IV. These data provide a strong framework in which to study the mechanisms of neuronal activity-blood flow coupling.

Laser-Doppler flowmetry utilizing a thinned skull cranial window preparation and automated stimulation.

For several decades, cranial windows have been used to investigate questions relating to cerebral blood flow and its regulation. In general, these techniques have utilized either 'open' cranial windows for the direct observation of the intracranial vasculature, or 'closed' cranial windows in which the skull and dura are removed and replaced with a clear seal, such as a coverslip. Here we describe a method of studying blood flow responses elicited by the physiological stimulus of whisker movement while using a 'thinned skull' cranial window created over the rat whisker-barrel cortex. This method employing an automated whisker stimulator coupled with laser-Doppler flowmetry focused through the thinned skull cranial window, is less invasive than other cranial window techniques, and allows for the study of the effects of stimulation parameters and systemically administered compounds on whisker movement elicited blood flow responses. Automated whisker stimulation and data collection also allow for precise temporal averaging of laser-Doppler measured responses, leading to increased precision in determining the true shape of the evoked blood flow response pattern.

Blood flow increases linearly in rat somatosensory cortex with increased whisker movement frequency.

It has long been known that the level of neuronal activity is correlated to the level of localized blood flow. Despite the importance of functional hyperemia in the brain, the relationship between blood flow and electrical activity has not been clearly demonstrated parametrically in a single region of cerebral cortex. We investigated both the magnitude and temporal characteristics of the blood flow response in somatosensory cortex while varying the frequencies of whisker movement. The full whisker pad on one side of the rat's face was repeatedly moved for 13 s at frequencies of 1.5, 2, 3, 4, 6, 8, and 10.5 Hz, and the resulting changes in blood flow were quantified using Laser-Doppler flowmetry (LDF). The magnitude of the blood flow response increased linearly with increasing frequency while the temporal parameters of time to half maximal value and time to return halfway to baseline after stimulus termination did not vary. Baseline blood flow levels were elevated by breathing rats on a 5% CO2 mixture. No significant alteration in the LDF plateau response to whisker movement was observed compared to normal air, suggesting sustained vasodilation reserve capacity remained after CO2-induced vasodilation. These data demonstrate linear blood flow responses to presumptive linear increases in neuronal activity with sufficient vascular reserve capacity to overcome moderate CO2-induced dilation, and support the use of blood flow changes in neuroimaging studies. They provide a framework to study the neurobiological signal transduction mechanisms coupling neuronal electrical activity with regional alterations in blood flow.

Metabolic clearance and secretion rates of porcine growth hormone in genetically lean and obese swine.

The metabolic clearance rate (MCR) and the secretion rate (SR) of porcine growth hormone (pGH) have been examined in swine rendered genetically either lean or obese after 18 generations of selection for or against backfat thickness. At 15 weeks of age (when the muscle:fat ratio was greater than 1) the mean half-life (t1/2), MCR, and SR, for the obese, control, and lean swine were: t1/2 = 7.4, 8.9, and 9.8 min; MCR = 341, 279, and 158 ml/min; SR = 907, 802, and 520 ng/min, respectively. At 90 kg body weight (when muscle:fat ratio was less than 1, and the age was about 30 weeks) the data for obese, control, and lean swine were: t1/2 = 11.3, 12.0, and 11.7 min; MCR =305, 280, and 336 ml/min; SR= 535, 626, and 932 ng/min, respectively. The t1/2, MCR, and SR were not significantly different among the obese, control, and lean swine at either 15 weeks or 90 kg body weight. Comparing the two stages of development, the younger swine (15 weeks of age) had a shorter t1/2 (P less than .01), and secreted and cleared more pGH on a per kg body weight basis (P less than .05) than the older swine (90 kg bodyweight, about 30 weeks of age). However, the results suggest that the selection of swine for either leanness or fatness for 18 generations did not alter the MCR and SR of pGH. In addition, the differences observed between the younger and older swine suggest that GH is cleared at a more rapid rate and more GH is available per unit of mass in the younger animals.