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Neurons of the vertebrate retina possess receptors for many neurotransmitters. Particularly interesting is a new type of GABA receptor (GABA rho) that, in contrast to GABAA and GABAB receptors, shows very little desensitization, is not blocked by bicuculline, and is not activated by baclofen. Homomeric human GABA rho 1 receptors were expressed in Xenopus oocytes. In addition to GABA, and related agonists, GABA rho 1 receptors respond to glycine (Gly) and beta-alanine (beta-Ala) by generating Cl- currents that do not desensitize and are resistant to bicuculline. The half-maximal concentrations for Gly and beta-Ala currents were 14.2 +/- 1.3 mM and 0.66 +/- 0.11 mM respectively. The current responses to Gly and beta-Ala were blocked by picrotoxin and TBPS. The cross-sensitivity of GABA rho 1 receptors to Gly and beta-Ala may play a role in retinal physiology.

The effects of Ca2+ ions on 3H-RO 5-4864 binding to the peripheral benzodiazepine receptor were examined. Preincubation of rat kidney membranes with Ca2+ at 37 degrees C produced a dose-dependent inhibition of 3H-RO 5-4864 binding. No inhibition was observed in membranes preincubated at 0 degrees C. The effect of Ca2+ was competitive in nature and was fully reversed by the addition of EGTA. At 1 mM, the maximal effect was achieved with CaCl2, whereas CoCl2 and CdCl2 had lesser effects. No other divalent cation salts examined decreased 3H-RO 5-4864 binding to rat kidney membranes. Collectively, these data demonstrate that the affinity of 3H-RO 5-4864 binding to rat kidney membranes is regulated by Ca2+ and suggest the presence of cation recognition binding sites coupled to the peripheral benzodiazepine receptor.

The effects of chronic benzodiazepine (BZD) treatment on rat peripheral-type benzodiazepine receptors (PBR) were studied. Five days treatment with the PBR ligands RO 5-4864 or diazepam (DZ) up-regulates kidney PBR. In contrast clonazepam, a specific central-type BZD receptor ligand, did not alter 3H-RO 5-4864 binding. Fourteen days administration of DZ produced an up-regulation of kidney and heart PBR and a down-regulation of testicular PBR. The results suggest that chronic BZD exposure differentially regulates PBR in peripheral organs.

We evaluated the effect of acute and chronic diazepam administration on testicular peripheral type benzodiazepine receptors (PBZD-R), serum testosterone and LH levels and the "in vitro" androgen production in response to Ro 5-4864, a PBZD-R agonist. The chronic diazepam treatment induced a significant fall in plasma testosterone concentration while LH levels remained unchanged. The number of PBZD-R was reduced by 37% and low concentrations (10(-8)-10(-6) M) of Ro 5-4864 failed to stimulate "in vitro" androgen production. The acute diazepam administration caused a significant increase in plasma testosterone levels while no changes were observed in LH concentrations and testicular PBZD-R. These results further suggest a modulatory role of PBZD-R on testicular steroidogenic activity.

The binding of 3H-RO 5-4864 to the peripheral-type benzodiazepine receptors (PBZDR) in rat testicular interstitial cells (TIC) was characterized. The binding was saturable, reversible and showed a single high-affinity (Kd = 5.02 +/- 0.86 nM) class of binding sites. The maximal binding capacity (Bmax) in crude mitochondrial fractions (77.6 +/- 9.1 pmol/mg protein) represents the highest density of PBZDR in tissues thus far studied. In comparison with the crude mitochondrial fraction the subcellular fractionation of TIC revealed a 2-fold enrichment of 3H-RO 5-4864 binding sites to the purified mitochondria (Bmax = 140 +/- 23 pmol/mg protein). The ability of various drugs to displace 3H-RO 5-4864 from TIC binding sites was examined and the inhibition constants (Ki) for RO 5-4864, PK 11195, diazepam and flunitrazepam were 3.5, 4.4, 159, and 353 nM, respectively, whereas clonazepam and RO 15-1788 were inefficient in displacing 3H-RO 5-4864 (Ki greater than 10 microM). This pharmacological profile is characteristic of PBZDR described in other tissues. In conclusion, rat TIC possess a very high concentration of PBZDR primarily associated with mitochondrial membranes.