Efferent microvascular responses to electrical stimulation of the area postrema in rats

Efferent projection from the rostral ventrolateral medulla to the area postrema in rats

The rostral ventrolateral medulla (RVLM) is a region of the brain primarily involved in cardiovascular control. It receives information from several areas of the brainstem, among which the area postrema (AP) and the nucleus of the solitary tract (NTS). The medial subnuclei of the solitary tract (TS) project towards the RVLM, providing cardiopulmonary information, and the AP serves information about circulatory hormones. Although the efferent pathways are well known, it is not the case for the connections from the RVLM towards the AP and the NTS. The present study was designed to examine the efferent connections from the RVLM onto the dorsal structures of the medulla: quantitatively by means of anatomical techniques, and functionally by means of electrophysiological techniques. Morphologically, Biocytin or Biotinylated dextran amine microinjections into the RVLM were followed by labelling of many fibres running towards the bulbar dorsomedial structures, with some pathways lying in the AP itself, or located in its caudal vicinity. Conversely, when microinjections of Fast Blue (FB) were made into the AP, FB-labelled cells could be observed within the RVLM. Electrophysiologically, single shock stimulation carried on AP allowed identification of axonal fibres issuing from somata located into the cardiovascular neuronal pool in the RVLM. From these results, we can assume: (1) the existence of dense efferent projection from RVLM to aspects of the dorsal vagal complex, including the AP and, among this dense projection, (2) the existence of some fibres terminating in, or crossing through the AP, and identified as conveying baroreceptor-related information, in the rat.

Nicotine raises the influx of permeable solutes across the rat blood-brain barrier with little or no capillary recruitment

Nicotine (1.75 mg/kg s.c.) was administered to rats to raise local CBF (lCBF) in various parts of the brain, test the capillary recruitment hypothesis, and determine the effects of this increase in lCBF on local solute uptake by brain. lCBF as well as the local influx rate constants (K1) and permeability-surface area (PS) products of [14C]antipyrine and [14C]-3-O-methyl-D-glucose (3OMG) were estimated by quantitative autoradiography in 44 brain areas. For this testing, the finding of significantly increased PS products supports the capillary recruitment hypothesis. In 17 of 44 areas, nicotine treatment increased lCBF by 30-150%, K1 of antipyrine by 7-40%, K1 of 3OMG by 5-27%, PS product of antipyrine by 0.20% (mean 7%), and PS product of 3OMG by 0-23% (mean 8%). Nicotine had no effect on blood flow or influx in the remaining 27 areas. The increases in lCBF and K1 of antipyrine were significant, whereas those in K1 of 3OMG and in PS for both antipyrine and 3OMG were not statistically significant. The lack of significant changes in PS products implies that in brain areas where nicotine increased blood flow: (a) essentially no additional capillaries were recruited and (b) blood flow within brain capillary beds rises by elevating linear velocity. The K1 results indicate that the flow increase generated by nicotine will greatly raise the influx and washout rates of highly permeable materials, modestly elevate those of moderately permeable substances, and negligibly change those of solutes with extraction fractions of < 0.2, thereby preserving the barrier function of the blood-brain barrier.