European Journal of Pharmacology, 66 (1980) 133--136 @)Elsevier/North-Holland Biomedical tress Short communication 'BRAIN-SPECIFIC' BENZODIAZEPINE RECEPTORS ARE LOCALIZED IN THE INNER PLEXIFORM LAYER OF RAT RETINA PHIL SKOLNICK 1, STEVEN PAUL 2, MARTIN ZATZ 3 and ROBERT ESKAY 3 1 Laboratory of Bioorganic Chemistry, NIAMDD; 2 Clinical Psychobiology Branch, NIMH; s Laboratory of Clinical Science, NIMH, National Institutes of Health, Bethesda, MD 20205, U.S.A. Received 1 May 1980, accepted 7 July 1980 133 P. SKOLNICK, S. PAUL, M. ZATZ and R. ESKAY, 'Brain-specific' benzodiazepine receptors are localized in the innerplexiform layer of rat retina, European J. Pharmacol. 66 (1980) 133--136. Saturable, high affinity binding sites for [3H]diazepam have recently been characterized in rat brain. A signifi- cant component of this binding was determined to occur to a population of receptors which is pharmacologically similar to receptors present in mammalian brain and spinal cord. Neonatal treatment of rats with monosodium glutamate (MSG), intraocular administration of kainic acid, and the use of pink-eyed, tan-hooded (PETH) rats with a genetic retinopathy (degeneration of photoreceptor cells) suggests that these 'brain-type' receptors are highly localized on neuronal elements of the inner plexiform layer and/or ganglion cells. Benzodiazepine receptors Retina MSG Kainic acid PETH rat 1. Introduction The presence of high affinity, saturable and stereospecific binding sites for the benzodia- zepines has been demonstrated in brain and spinal cord of a number of mammalian species, including man (cf. Tallman et al., 1980). Although [3H]benzodiazepines (such as diazepam) bind with a relatively high affin- ity to peripheral tissues and cultured cells of neural origin both in vivo and in vitro (Wil- liamson et al., 1978; Syapin and Skolnick, 1979), the binding to brain and spinal cord is biochemically distinct from that of other tis- sues (cf. Syapin and Skolnick, 1979). The good correlations obtained between the in vitro potenties of a series of benzodiazepines in displacing [3H]diazepam binding from brain membranes and their corresponding pharmacologic potenties suggests that these central binding sites (in contrast to peripheral tissues) are pharmacological receptors. We (Paul et al., 1980) have recently demonstrated the presence of benzodiazepine receptors in homogenates of rat retina. Using appropriate pharmacologic probes, both a high affinity 'brain-specific' receptor and a lower affinity, 'kidney' (peripheral) type binding site were resolved in this tissue. We now report that 'brain-specific' benzodiazepine receptors are localized on neuronal elements of the inner plexiform layer. 2. Materials and methods PETH (Pink-Eyed Tan-Hooded) and con- trol Osborne-Mendel (male) rats were ob- tained from the Veterinary Resources Branch, NIH. Neonatal Zivic-Miller (male) rats were treated with monosodium glutamate (MSG), 4 mg/g (i.p.) on alternate days through the first ten days postpartum. Neonatal Controls received an equal volume of vehicle. Adult rats were injected with a solution of 60 nmol of kainic acid in a volume of 2.5 pl (0.9% NaC1 neutralized to pH 7.4) directly into the vitreous humor. Control animals received an