Characterization of an Inhibitory Receptor in Rat Hippocampus: A Microiontophoretic Study Using Con‐formationally Restricted Amino Acid Analogues

Abstract

I Pyramidal cells in rat hippocampus were used to study the molecular dimensions of a receptor for inhibitory amino acids in the central nervous system. The inhibitory potencies of y-aminobutyrate (GABA), ,B-alanine and glycine were compared by standard microiontophoretic techniques. Subsequently, rigid cyclopentane and cyclohexane amino acid analogues were applied by iontophoresis and their relative efficacies were compared with those of the naturally occurring amino acids. 2 GABA was the most effective of the small aliphatic amino acids in producing inhibition of the firing of hippocampal pyramidal neurones. 13-Alanine was less effective and glycine was the least effective. GABA-induced inhibition was antagonized by concurrent iontophoresis of picrotoxin or bicuculline, whereas strychnine did not antagonize GABA inhibition. 3 The ability of the series of substituted aminocyclopentane and aminocyclohexane carboxylic acids to produce inhibition of pyramidal cells was a direct function of the separation of amino and carboxylic acid groups. In both series of the cyclic amino acids the most potent inhibition was demonstrated when the spatial separation was similar to that of the extended GABA molecule (4.74 A). Additionally, the inhibition of hippocampal pyramidal cells by (±)-cis-3-amino-cyclopentanecarboxylic acid, like that produced by GABA, could be blocked by simultaneous application of picrotoxin or bicuculline, but not by strychnine. 4 The present results suggest that the physiologically active conformation of GABA is the fully extended molecule, and additionally indicate that one dimension of the postsynaptic receptor site is within the range of 4.2 to 4.8 angstr6ms.

THE EFFECT OF CONFORMATIONALLY RESTRICTED AMINO ACID ANALOGUES ON THE FROG SPINAL CORD in vitro

“…Interestingly, Segal et al (1975) found the cyclohexane analogues to be weakly active on hippocampal neurones. Furthermore, these compounds had little effect on root potentials which are thought to involve the release of neutral amino acids (Barker & Nicoll, 1972;Davidoff, 1972;Barker et al, 1975a,b) suggesting that they neither compete for neutral amino acid receptors nor uptake sites.…”

“…especially aminocyclopentane carboxylic acids in such a study have been discussed (Segal et al, 1975). It should be kept in mind that the dual action of many of the compounds may have introduced small errors in relative potency, but this problem was minimized by appropriate adjustment of amino acid concentrations.…”

“…The fact that these amino acids can exist in numerous molecular conformations in aqueous solution have led investigators to use a number of analogues, including conformationally restricted anlogues to determine the conformation of GABA which is physiologically active on a variety of preparations (McGeer, McGeer & McLennan, 1961;Beart, Curtis & Johnston, 1971;Bowery & Brown, 1974;Walker, Azuza, Kerkut & Woodruff, 1975;Segal, Sims & Smissman, 1975;Johnston, Curtis, Beart, Game, McCulloch & Twitchin, 1975;Krogsgaard-Larsen, Johnston, Curtis, Game & McCulloch, 1975;Bowery & Jones, 1976). These investigations have all led to the conclusion that it is the extended form of GABA that is physiologically active.…”

Stereospecific Synthesis of New 4-Amino-1,2,3-cyclohexanetricarboxylic Acids and 4-Amino-1,3-cyclohexanedicarboxylic Acids

“…CHCl 3 (200 ml) and water (100 ml) were added to the residue, and the whole was stirred for a few minutes, after which the insoluble material was filtered out using Hyflo Super-Cel 4 aqueous solution (142 ml) were mixed and then vigorously stirred at room temperature for 2 h. The AcOEt layer was separated, and the aqueous layer was extracted with AcOEt (25 mlϫ3). Isopropyl alcohol (1 ml) was added to the combined AcOEt layers, and the solution was left to stand for 1 h. The precipitated RuO 2 was filtered off, and the solution was dried over anhydrous Na 2 12 mmol) in AcOEt (24 ml), RuO 2 · xH 2 O (71 mg), and a 10% NaIO 4 aqueous solution (71 ml) were mixed and then vigorously stirred at room temperature for 3.5 h. The AcOEt layer was separated, and the aqueous layer was extracted with AcOEt (13 mlϫ3). Isopropyl alcohol (0.5 ml) was added to the combined AcOEt layers, and the solution was left to stand for 1 h. The precipitated RuO 2 was filtered off, and the solution was dried over anhydrous Na 2 SO 4 , then concentrated under reduced pressure.…”

Crystal Structures of (1<i>S</i>,2<i>R</i>)-2-benzamidocyclohexane-1-calboxylic acid and [(1<i>S</i>,2<i>R</i>)-2-(benzylamino)cyclohexyl]methanol

“…One of the title compounds, (1S,2R)-2-(benzamido)cyclohexanecarboxylic acid, BACO (I), was used as such a chiral resolving agent, 1 but it was originally synthesized in stereochemical studies, 2 and its ciscyclicamino acid is used as an intermediate for an analog of GABA. 3,4 It was considered that (I) and its reduction product, (1S,2R)-2-(benzylamino)cyclohexylmethanol, BACH (II), 5 recognized the amine/amino-alcohol and carboxy groups in diastereomer salts, respectively, but the structures have not been cleared. In this study, the crystal structures of (I) and (II) were determined and correlated with their chemical properties.…”