Electrophysiological evidence for a large receptor reserve for inhibition of dorsal raphe neuronal firing by 5‐HT1A agonists

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Abstract
Previous studies [Meller et al. (1990) Mol. Pharmacol., 37:231-237] have shown that a large receptor reserve exists for the inhibition of serotonin synthesis in rat cortex and hippocampus by the 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), whereas little or no reserve exists for the lower efficacy agonists ipsapirone and BMY 7378. The current studies were undertaken to determine if the above drugs exhibit similar relative efficacies and receptor reserves in an electrophysiological model of 5-HT1A receptor activation, i. e., the inhibition of dorsal raphe cell firing. Intravenous dose-response curves were constructed in untreated control rats, or in rats which received an injection of the irreversible receptor inactivator N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ, 6 mg/kg, s. c.) 24 hours before recording. All three drugs fully inhibited dorsal raphe cell firing in control rats (ED50's: 1.5 μg/kg, 8-OH-DPAT; 30.0 μg/kg, ipsapirone; 17.5 μg/kg, BMY 7378). However, unlike effects on serotonin synthesis, EEDQ treatments caused no depression of the maximal inhibitory response for any of the agonists, although all dose-response curves were shifted to the right (ED50's: 10.1 μg/kg, 6.7-fold shift, 8-OH-DPAT; 139.9 μg/kg, 4.7-fold shift, ipsapirone; 53.8 μg/kg, 3.1-fold shift, BMY 7378). Although the order of agonist efficacies was similar for both inhibition of serotonin synthesis and dorsal raphe cell firing (8-OH-DPAT > ipsapirone > BMY 7378), a large (>50%) receptor reserve was estimated for all three drugs in this electrophysiological system. This suggests that 5-HT1A receptor populations mediating the inhibition of transmitter synthesis and neuronal firing may be differently regulated or have different receptor-effector coupling characteristics (G-proteins, effectors, and/or transduction efficiencies).