Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin

Autoradiographic visualisation of [3H]5-carboxamidotryptamine binding sites in the guinea pig and rat brain

We have investigated the distribution of [3H]5-carboxamidotryptamine ([3H]5-CT) binding sites by in vitro autoradiography on sections of guinea-pig and rat brain. In saturation studies, the ligand recognised a saturable, homogeneous population of binding sites with an affinity ranging from 0.19-0.45 nM depending on the region. The labelling pattern was heterogeneous, and the displacement pattern with different competing drugs selective for different 5-HT receptor subtypes was complex. [3H]5-CT appeared to label 5-HT1B/5-HT1D sites in the substantia nigra, globus pallidus and caudate/putamen, as the binding in these regions was displaced by the 5-HT1B/1D receptor selective agents sumatriptan, CP-122,288 and GR-127,935. In the hippocampus and lateral septum, the very dense [3H]5-CT binding was displaced with high affinity by the 5-HT1A receptor selective agonist 8-hydroxy-dipropylaminotetralin ((+/-)-8-OH-DPAT), dihydroergotamine and 5-HT. In contrast the affinity of the 5-HT1 receptor antagonists spiperone and methiothepine was much lower than their previously published potency at 5-HT1A receptors. The affinity of agonists, taken together with the fact that the distribution of these [3H]5-CT sites overlaps that of [3H]8-OH-DPAT binding sites in serial sections, suggest that these sites correspond to 5-HT1A receptors. Their atypical properties deserve further investigations. While [3H]5-CT binding at 5-HT1B/1D sites and these atypical 5-HT1A sites was inhibited by the GTP analogue 5'-beta, gamma-imidotriphosphate, [3H]5-CT binding in the superficial cortical layers and in midline thalamic nuclei was insensitive to this agent. It was however displaced by low concentrations of spiperone, clozapine and methiothepine, but not by sumatriptan, CP-122,288, GR-127,935 or dihydroergotamine. This binding profile is similar to that of 5-HT7 receptors, while the spatial distribution of these sites matches the known distribution of 5-HT7 messenger RNA. We did not find evidence of [3H]5-CT labelling to 5-HT5 receptors, in spite of their reported high affinity for this ligand. It is concluded that [3H]5-CT, in the presence of selective blockers, can be used to investigate the properties of 5-HT1A, 5-HT1B/1D and 5-HT7 receptors in the rodent brain, although further studies are required to explain the atypical features of [3H]5-CT binding in 5-HT1A receptors containing regions.

5-HT1A receptor agonists: recent developments and controversial issues

During the last decade, serotonin (5-HT)1A receptors have been a major target for neurobiological research and drug development. 5-HT1A receptors have been cloned and a variety of selective agonists, such as the aminotetraline 8-OH-DPAT and the pyrimidinylpiperazine ipsapirone, have become available. Demonstrations of apparent intrinsic activity of these ligands at 5-HT1A receptors, however, depend highly on the particular assay system. This may be due to the possible existence of receptor subtypes and to assay (or brain region)-dependent differences in receptor reserve and the nature of receptor-effector coupling. Nevertheless, the apparent intrinsic activity of 8-OH-DPAT seems to be higher (although possibly not yet maximal) than that of the pyrimidinylpiperazines. In the brain, 5-HT1A receptors are located presynaptically as somatodendritic receptors on 5-HT neurons and postsynaptically in particular limbic and cortical regions. Although it is generally accepted that presynaptic 5-HT1A receptors control 5-HT neuronal activity, recent evidence suggests an additional role of postsynaptic 5-HT1A receptors in cortex as part of a negative feedback loop. Anxiolytic and antidepressive properties of selective 5-HT1A receptor agonists have now been confirmed by clinical studies. Although it is well established that the latter properties depend on the agonistic activity of these compounds, the optimal level of intrinsic activity is still a matter of debate and may be dependent on the clinical indication. Such compounds may also have antiaggressive effects, and possibly anticraving effects (manifested by their alcohol intake-reducing effects in dependent animals), but the specificity of these so-called anti-impulsivity effects is still controversial and not yet tested clinically. Anticataleptic, antiemetic and neuroprotective properties have been demonstrated in different species. Behavioral studies on the mechanisms underlying the anxiolytic and antidepressive effects have examined the relative contribution of pre- and postsynaptic 5-HT1A receptors by means of local cerebral application and lesion techniques. Most evidence points towards a critical involvement of presynaptic receptors in the anxiolytic effects of 5-HT1A receptor agonists (although a possible contribution of postsynaptic receptors cannot be excluded). With regard to the antidepressive properties, a case can be made for the reverse; i.e., a strong involvement of postsynaptic receptors and a questionable contribution of presynaptic receptors. However, as the therapeutic effects of those 5-HT1A receptor (partial) agonists which have been tested clinically require repeated administration, attention has been directed increasingly towards chronic studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of the human 5-hydroxytryptamine1A receptor in Sf9 cells. Reconstitution of a coupled phenotype by co-expression of mammalian G protein subunits

The possibility that Spodoptera frugiperda (Sf9) cells can provide an intact cell setting for reconstitution of the human 5-hydroxytryptamine1A (5-HT1A) receptor with mammalian G protein subunits was explored. The 5-HT1A receptor was found to assume an uncoupled phenotype when expressed alone in Sf9 cells at relatively high levels (5-34 pmol of receptor/mg of membrane protein), i.e. agonist-binding to the receptor was characterized by a relatively high Kd and an insensitivity to GTP. Co-expression of the receptor with members of the alpha i "family" together with various combinations of beta 1 and gamma subunits increased the affinity for agonists to that observed for the coupled form of receptor in mammalian cells, concomitant with conferrance of guanosine 5'-(beta,gamma-imino)triphosphate sensitivity. The agonists employed were [3H]8-hydroxy-N,N-dipropyl-2-aminotetralin ([3H]8-OH-DPAT) and [125I]R(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl) amino]tetralin ([125I]8-OH-PIPAT). The binding of an antagonist, [125I]4-(2'-methoxyphenyl)-1-[2'-[N-(2"- pyridinyl)-p-iodobenzamido]ethyl]piperazine ([125I]p-MPPI), was unaffected by co-expression of G protein subunits. Both alpha and beta gamma subunits were required for optimal coupling. No differences were evident among alpha i1, alpha i2, alpha i3, alpha o, and alpha z when expressed with beta 1 gamma 2 in this regard, nor among most permutations of beta 1 gamma subunits when expressed with alpha i1 (beta 1 gamma 2 approximately beta 1 gamma 3 approximately beta 1 gamma 5 > beta 1 gamma 1). Alpha s and alpha q expressed with beta 1 gamma 2 did not participate in coupling. These data support the conclusion that normal interactions between a mammalian receptor and a select array of G proteins can be established in intact Sf9 cells, and extend previous observations of 5-HT1A receptor coupling to G(o) and the pertussis toxin-insensitive G protein Gz.

Quantitative autoradiographic characterisation of the binding of [3H]WAY-100635, a selective 5-HT1A receptor antagonist

The binding characteristics of [3H]WAY-100635 ([O-methyl 3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride), a potent and selective 5-HT1A antagonist radioligand, were examined in the rat brain using in vitro quantitative receptor autoradiography. The regional distribution of specific [3H]WAY-100635 binding sites was heterogeneous and demonstrated a strong correlation with that of [3H]8-OH-DPAT binding. The highest concentrations of [3H]WAY-100635-labelled sites were found in the lateral septal areas, dorsal raphe n., entorhinal cortex and the hippocampal formation (CA1, CA3 and dentate gyrus). Scatchard transformation of saturation isotherms revealed saturable [3H]WAY-100635 binding sites of high-affinity: in the hippocampal formation, Kd was approximately 1 nM and Bmax ranged between 187 and 243 fmol/mg tissue wet weight, in the entorhinal cortex, Kd = 0.44 nM and Bmax = 194 fmol/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., Kd = 0.52 nM and Bmax = 157 fmol/mg tissue wet weight. The affinity of [3H]WAY-100635 for the 5-HT1A binding site tended to be higher in the dorsal raphe n. and entorhinal cortex compared with that of the hippocampal formation. In contrast, the binding affinity of [3H]8-OH-DPAT in the hippocampal formation was between 1.1 and 2.3 nM and the Bmax was 137 to 183 fmoles/mg tissue wet weight; in the entorhinal cortex, Kd = 3.2 nM and Bmax = 141 fmoles/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., Kd = 3.4 nM and Bmax = 163 fmol/mg tissue wet weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of [O-methyl-3H]WAY-100635 as an in vivo radioligand for 5-HT1A receptors in rat brain

N-(2-(4-(2-Methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridyl)cyclohexanecarboxamide trihydrochloride (WAY-100635) is a new, potent and selective 5-HT1A receptor antagonist. We have evaluated radiolabelled WAY-100635 as a prospective radioligand for positron emission tomography (PET) by studying biodistribution in rat ex vivo. After intravenous injection, [O-methyl-3H]WAY-100635 cleared rapidly from plasma but was retained in brain. Specific binding was quantified from kinetic studies, using a reference-tissue compartment model, fitting for binding potential (k3/k4). The regional variation in binding potential correlated with the known distribution of 5-HT1A receptors. Saturation studies gave Bmax values in vivo that were consistent with those reported in vitro. At 60 min after injection, the ratio of radioactivity in 5-HT1A receptor-rich regions (e.g. septum, entorhinal cortex and hippocampus) to that in cerebellum reached approximately 16. Pre-dosing the rats with WAY-100635 (2 mg/kg) reduced this ratio to one, whereas similar pre-dosing with citalopram (5-HT uptake site inhibitor), prazosin (alpha 1A-adrenoceptor antagonist) or idazoxan (alpha 2-adrenoceptor antagonist) caused little or no reduction. Substantial (77%) blockade of [3H]WAY-100635 binding was achieved with the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the partial agonists, ipsapirone and buspirone. Thus, the properties of WAY-100635 are such that, when labelled with carbon-11, it could provide a radioligand suitable for clinical and pharmacological investigations of central 5-HT1A receptors in man using PET.