SUBSTANCE-P RECEPTORS IN BRAIN-STEM RESPIRATORY CENTERS OF THE RAT - REGULATION OF NK1 RECEPTORS BY HYPOXIA

Substance P (SP) is a key neurotransmitter involved in the brain stem integration of carotid body chemoreceptor reflexes. In this study, the characteristics and location of SP receptors in the rat brain stem an d their regulation by hypoxia were investigated using homogenate radio ligand binding and quantitative autoradiography. Specific binding of [ I-125] Bolton-Hunter SP (BHSP) to brain stem homogenates was saturable (similar to 0.3 nM) and to a single class of high-affinity sites (K-d , 0.16 nM; maximum density of binding sites, 0.43 fmol/mg wet weight t issue). The order of potency of agonists for inhibition of BHSP bindin g was SP > [Sar(9)Met(O-2)(11)]SP >> neurokinin A > septide > neurokin in B >> [Nle(10)]-neurokinin A(4-10) = senktide, and for nonpeptide an tagonists, RP 67580 > CP-96,345 >> RP 68651 = CP-96,344, consistent wi th binding to NK1 receptors. The effect of single and multiple, 5-min bouts of hypoxia (8.5% O-2/91.5% N-2) on BHSP binding was investigated using quantitative autoradiography. Binding sites were localized to t he lateral, medial and commissural nucleus of the solitary tract (NTS) , the hypoglossal nucleus, central gray and the spinal trigeminal trac t and nucleus (Sp5 and nSp5, respectively). Five min after a single bo ut of hypoxia, the density of BHSP binding sites had decreased signifi cantly (P < .05) in the medial NTS (-33%) and lateral NTS (-24%) when compared to normoxic controls. However, the normal receptor complement was restored within 60 min of the hypoxic challenge. In the Sp5, a si gnificant decrease (P < .05) in binding was observed 5 min after hypox ia which was still apparent after 60 min. In contrast, the density of BHSP binding sites in the hypoglossal nucleus decreased slowly and was significantly lower (P < .05) than normoxic controls 60 min after hyp oxia. Five min after repetitive hypoxia (3 x 5 min bouts), BHSP bindin g in the NTS was reduced by more than 40%. Studies in homogenates show ed that the affinity of SP for BHSP binding sites was not affected by repetitive hypoxia (K(d)s, normoxic, 0.27 nM; hypoxic, 0.24 nM). These data suggest that afferent input from carotid body chemoreceptors may dynamically regulate NK1 receptors in several brain stem nuclei that are intimately involved in stimulating ventilation during hypoxia, and that the time-course of receptor turnover may differ from region to r egion in the brain stem. The temporary loss of NK1 receptors in the NT S may partly explain why adequate ventilation is often not maintained during hypoxia.