Localization by kainic acid lesions of neurones transmitting the carotid chemoreceptor stimulus for respiration in rat.

Abstract

SUMMARY1. An attempt has been made to test the hypothesis that in the nucleus of the tractus solitarius (NTS) in the rat, the most caudal region of synaptic terminals of the carotid sinus nerve, just caudal to the obex, represents mainly the site of synapse of chemoreceptor fibres from the carotid body.2. Under halothane anaesthesia, the neurotoxin kainic acid was used to lesion this region and a second region, immediately rostral to obex, where terminals are thought to arise mainly from baroreceptor fibres of the carotid sinus nerve.3. Measurements based on the distribution of fluorescent dye co-injected with the kainic acid showed that the two groups of 100 nl microinjections were centred 0-82 mm apart and that the injectate spread through mean distances of 0 57 mm (caudal microinjections) and 0-52 mm (rostral microinjections). Nissl staining was used to determine cellular degeneration. The caudal lesions mostly involved ventrolateral and commissural subnuclei of NTS and the rostral lesions involved lateral and dorsolateral subnuclei.4. Ventilatory sensitivity to hypoxia was tested under light halothane anaesthesia, 1 day after lesioning. To enhance the responses, the contralateral carotid sinus nerve was sectioned prior to experiments. Caudal lesions reduced the ventilatory response to inspired oxygen (20-9-9-6 % 02) by a mean of 67 % and rostral lesions by 18 % of the effect produced by carotid sinus nerve section on that side. Subsequent section of the carotid sinus nerve on the side of the NTS lesion confirmed that caudal lesions produced effects comparable to those of carotid body denervation; rostral lesions did not.5. These results strongly support the hypothesis that chemoreceptor and baroreceptor afferent fibres in the carotid sinus nerve synapse at substantially separable sites in the nucleus of the tractus solitarius. The identification of the site in NTS caudal to the obex as the principal site of carotid chemoreceptor synapses places them close to but not upon respiratory premotor neurones of the same nucleus.4-2

Carotid sinus nerve section abolishes NMDA evoked respiratory effects in anaesthetised rats

“…Among multiple neurotransmitters, which coexist in the carotid bodies and take part in the chemotransmission, high concentration of l-glutamate abounds but is not being released from the glomus cells (Torrealba et al, 1996). Analysis of the published data does provide evidence that glutamate is released from the primary afferent neurones of the peripheral chemoreceptors relaying to the nucleus tractus solitarii (Lewis et al, 1987;Houseley and Sinclair, 1998). Further, it has been evidenced, that NMDA receptors present on glutaminergic pathways of the bulbospinal neurones, play an important role in the central processing of the peripheral chemoreceptive information (Dogaš et al, 1995).…”

Effects of Acute and Chronic Starvation on Central and Peripheral Noradrenaline Turnover, Blood Pressure and Heart Rate in the Rat

“…The A2 cell group, a discrete region located in the nucleus tractus solitarius (NTS), was subdivided topographically into two areas. The most rostral A2 region was located ahead of the obex, while the caudal A2 region was behind the obex (Housley & Sinclair, 1988).…”

“…The most notable central noradrenergic region involved in cardiovascular regulation is the NTS including the A2 region, which receives both baro-and chemoreceptor afferents that are topographically distinct (Housley & Sinclair, 1988). Indeed, the chemosensory fibres project to the region caudal to the obex (caudal A2), while the baroreceptor fibres project to an area rostral to the obex (rostral A2).…”

Ventilatory acclimatization to chronic hypoxia: relationship to noradrenaline metabolism in the rat solitary complex.

“…Results from studies using various techniques, such as electrophysiological stimulations, discrete central lesioning and anterograde tracing, have established that afferent chemosensitive fibres contained in the carotid sinus nerve project to the solitary complex (Donoghue, Felder, Jordan & Spyer, 1984;Housley, Martin-Body, Dawson & Sinclair, 1987). It is generally believed that the region located immediately caudal to the obex within the solitary complex represents the main target site of carotid body chemoreceptor afferent fibres, which then relay to other sites involved in respiratory and cardiovascular control (Trzebski, 1983;Donoghue, Felder, Gilbey, Jordan & Spyer, 1985;Onai, Saji & Miura, 1987;Housley & Sinclair, 1988;Finley & Katz, 1992;Mifflin, 1992). Ventilatory acclimatization to hypoxia in most species studied is characterized by a time-dependent hyperventilation, involving increases in both respiratory frequency and tidal volume followed by a persistent and slow decrease in hyperventilation upon restoration of normoxic conditions (Olson & Dempsey, 1978;Smith et al 1986).…”