Anion transport blockers inhibit dl‐2‐amino‐4‐phosphonobutyrate responses induced by quisqualate in the rat cerebral cortex

Abstract

1 Depolarizing responses to DL-2-amino-4-phosphonobutyrate (AP4) and related amino acids have been studied in the rat cerebral cortex slice following the application of quisqualate (Quis). 2 Before exposure to Quis, 500 AM DL-AP4 had little or no effect. However, following a single application of 40 jAM Quis for 2 min, DL-AP4 produced depolarizing responses. With repeated applications of DL-AP4, there was a decline in response amplitude. A second application of Quis restored the depolarizing potency of DL-AP4 to a level above that for the first DL-AP4 response after the first Quis application. With a sequence of alternate applications of Quis and DL-AP4, the amplitude of DL-AP4 responses became maximal after the second Quis application. Responses to DL-AP4 could also be induced by the application of 1 gM Quis for 60 min, but were smaller in amplitude.3 Responses to the normally inactive amino acids L-cysteine (Cys), L-cystathionine (CTN) and L-aaminoadipate (AA) were also induced once Quis was applied. These responses were also maximized after a second application of Quis, except those to L-Cys, which failed to reach a plateau after three Quis applications. 4 The co-application of DL-AP4 with the first Quis application depressed the subsequent mean DL-AP4 response by 47%. Re-application of Quis restored the amplitude of DL-AP4 responses to levels comparable to control. L-a-AA also suppressed the induction of DL-AP4 responses, when co-applied with the first Quis exposure, reducing mean response amplitude by 98%. Unlike DL-AP4, however, the effect with L-x-AA persisted so that DL-AP4 responses were significantly suppressed compared to control, even after further applications of Quis. 5 The effects of the anion transport blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetoamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) on the induction process and the DL-AP4 responses themselves were examined. DIDS (100 jLM) significantly inhibited the DL-AP4 responses, and to a lesser extent the induction of the responses by 40 AM Quis (2 min), while SITS (300 AM) only inhibited the DL-AP4 responses. However, the induction of responses by 1 gM Quis (60 min) was significantly affected by this concentration of SITS.6 DIDS (100 jAM) had no effect on responses to x-amino-3-hydroxy-5-methyl4-isoxazoleproprionate (AMPA), but selectively potentiated those to Quis. Examination of the full concentration-response curve for Quis revealed that, while the Rmax remained constant, the Hill slope was increased and the EC50 was decreased in the presence of DIDS. SITS (300 jAM), however, antagonized responses to AMPA, and had little effect on responses to Quis except at the highest concentration of Quis tested (20 jAM), where a potentiation was observed, suggesting that it is a non-NMDA receptor antagonist. 7 These observations indicate that the production of depolarizing responses to a number of amino acids, including DL-AP4, in the cerebral cortex is mediated via an anion transport mechanism sensitive to...

Dibutyryl‐cAMP (dbcAMP) up‐regulates astrocytic chloride‐dependent l‐[3H]glutamate transport and expression of both system xc− subunits

“…Compounds such as SITS, DIDS, l ‐α‐aminoadipate, l ‐2‐amino‐6‐phosphonohexanoate, homocysteate and cystine have all been shown to interact with this process (Harris et al . 1987; Whittemore and Koerner 1989; Turner 1993; Ishida and Shinozaki 1999; Chase et al . 2001).…”