Regulation of 5-HT(1A) receptor-stimulated [35S]-GtpgammaS binding as measured by quantitative autoradiography following chronic agonist administration

Abstract

1. Because changes 5-HT(1A) receptor number do not occur following repeated agonist treatment, we hypothesized that the basis for 5-HT(1A) receptor desensitization involves changes in receptor-G protein coupling. We measured the effect of repeated agonist administration on 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding in forebrain areas, (i.e. anterior cingulate cortex, lateral septum, hippocampus, entorhinal cortex), and serotonergic cell body areas, the dorsal and median raphe nuclei. 2. Following treatment of rats with (+/-)8-OH-DPAT (1 mg kg(-1), s.c.) for 7 or 14 days, 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding was significantly attenuated in both the dorsal and median raphe nuclei. 3. 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding was significantly attenuated in the CA(1) region of the hippocampus after 7, but not 14 days of 8-OH-DPAT administration. 5-HT(1A) receptor-stimulated [(35)S]-GTPgammaS binding was not altered in other forebrain areas examined. 4. The binding of [(3)H]-MPPF to 5-HT(1A) receptor sites was not altered in any brain region examined following repeated agonist administration, suggesting that the observed changes in (+/-)8-OH-DPAT-stimulated [(35)S]-GTPgammaS binding were not due to changes in 5-HT(1A) receptor number. 5. Our data indicate that in serotonergic cell body areas the regulation of presynaptic 5-HT(1A) receptor function following repeated agonist administration occurs at the level of receptor-G protein interaction. In forebrain areas, however, the regulation of postsynaptic 5-HT(1A) receptor sensitivity appears not to be at the level of receptor-G protein coupling.

Figure 1

Autoradiograms of [³⁵S]-GTPγS binding to sections of rat brain. Coronal sections at the level of (A) the lateral septum, (B) the dorsal hippocampus, and (C) the dorsal raphe nucleus, were incubated with [³⁵S]-GTPγS (80 pM). Nonspecific binding was defined in the presence of 10 μM GTPγS. The binding of [³⁵S]-GTPγS was stimulated by (±)8-OH-DPAT (1 μM).

Figure 2

Effect of repeated administration of 8-OH-DPAT on 5-HT_1A receptor-stimulated [³⁵S]-GTPγS binding in terminal field areas of serotonergic innervation. Rats were administered either saline vehicle or (±)8-OH-DPAT (1 mg kg⁻¹, once daily, s.c.) for 7 or 14 days. Coronal sections were incubated with [³⁵S]-GTPγS (80 pM). Nonspecific binding was defined in the presence of 10 μM GTPγS. [³⁵S]-GTPγS binding was stimulated by (±)8-OH-DPAT (1 μM). Specific binding of [³⁵S]-GTPγS is expressed as per cent above basal. Shown are the mean±s.e.mean saline-treated, n=16; 8-OH-DPAT-treated, n=8 per experimental group. ^*P<0.05.

Figure 3

Effect of repeated administration of 8-OH-DPAT on 5-HT_1A receptor-stimulated [³⁵S]-GTPγS binding in serotonergic cell body areas. Rats were administered either saline vehicle or 8-OH-DPAT (1 mg kg⁻¹, once daily, s.c.) for 7 or 14 days. Coronal sections were incubated with [³⁵S]-GTPγS (80 pM). Nonspecific binding was defined in the presence of 10 μM GTPγS. [³⁵S]-GTPγS binding was stimulated by 8-OH-DPAT (1 μM). Specific binding of [³⁵S]-GTPγS is expressed as per cent above basal. Shown are the mean±s.e.mean. Saline-treated, n=16; 8-OH-DPAT-treated, n=8 per experimental group. ^*P<0.05.