Three distinct binding sites for [3H]-prazosin in the rat cerebral cortex

Abstract

1. The putative alpha 1-adrenoceptor subtypes of rat cerebral cortex membranes were characterized in binding. 2. Specific binding of [3H]-prazosin was saturable between 20-5000 pm. Scatchard plots of the binding data were non-linear, indicating the presence of two distinct affinity sites for prazosin (pKD, high = 10.18, Rhigh = 308 fmol mg-1 protein; pKD, low = 8.96, Rlow = 221 fmol mg-1 protein). 3. In the membranes pretreated with chlorethylclonidine (CEC) two affinity sites for prazosin were also observed: the affinities were similar to those without CEC pretreatment, but the maximum numbers of binding sites were reduced by CEC pretreatment to 23 and 62% for prazosin-high (Rhigh) and low affinity sites (Rlow), respectively. 4. The prazosin-high affinity sites were further subdivided into two subclasses by WB4101(2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane) and phentolamine; the low affinity sites for WB4101 and phentolamine were more potently inactivated by CEC as compared with the high affinity sites. On the other hand, prazosin, HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)- amino )-propyl)benzeneacetonitrile fumarate) and yohimbine inhibited [3H]-prazosin binding to prazosin-high affinity sites monophasically. 5. In addition to the high affinity sites, the prazosin-low affinity sites were labelled at high concentrations of [3H]-prazosin. Thus, prazosin and WB4101 showed shallow displacement curves. On the other hand, HV723 and yohimbine did not discriminate between prazosin-high and low affinity sites. 6. Two distinct alpha 1-adrenoceptor subclassifications have been recently proposed (alpha 1A, alpha 1B subtypes and alpha 1H, alpha 1L, alpha 1N subtypes). 5. In both motoneurones and dorsal root fibres, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) effectively depressed the depolarization induced by kainoids, and neither 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1- phosphonic acid (CPP) nor picrotoxin blocked or affected the depolarization, but there were some differences in pharmacological potencies of glutamate antagonists between both preparations.6. MFPA, HFPA and acromelic acids should provide valuable pharmacological tools for analysis of physiological functions of excitatory amino acids, in particular, as specific agonists for some subtypes of kainate receptors.