Simultaneous assay of muscarinic and beta-adrenergic receptors using a double isotope technique

5-HT(1A) agonist potential of pindolol: electrophysiologic studies in the dorsal raphe nucleus and hippocampus

BACKGROUND

The ability of pindolol to block 5-HT(1A) autoreceptors on serotonin-containing neurons in the raphe nuclei is thought to underlie the clinical reports of enhanced efficacy and rate of improvement in depressed patients treated with pindolol/selective serotonin reuptake inhibitor (SSRI) combinations. Selectivity for somatodendritic 5-HT(1A) autoreceptors is a crucial requirement, as blockade of postsynaptic 5-HT(1A) sites may jeopardize the therapeutic response. Previous investigators have probed the effects of pindolol on serotonergic dorsal raphe cell firing in animal species; here we confirm their findings and extend them to include observations on postsynaptic 5-HT(1A) receptors in the hippocampus.

METHODS

Extracellular single-unit recordings were made in rats using standard electrophysiologic techniques. Firing rates of serotonin-containing neurons in the dorsal raphe nucleus and CA3 hippocampal pyramidal neurons were monitored and the effects of pindolol given alone or in combination with an SSRI (fluoxetine) or a 5-HT(1A) antagonist (WAY-100,635) were determined.

RESULTS

Pindolol inhibited the firing rates of serotonergic dorsal raphe neurons in a dose-dependent manner. Recovery to baseline firing rates was gradual, but this inhibition could be acutely reversed by WAY-100,635. A range of pindolol doses failed to block the inhibitory effects of fluoxetine on dorsal raphe cell firing. In the hippocampus, pindolol also inhibited cell firing as a function of dose, although these effects were insensitive to WAY-100,635 treatment.

CONCLUSIONS

The ability of pindolol to inhibit serotonergic dorsal raphe cell firing is indicative of its agonist potential and is consistent with previous studies. The lack of observable antagonism of the SSRI-induced slowing of raphe unit activity casts doubt on the suitability of this mechanism of action to account for the positive findings in clinical studies utilizing pindolol/SSRI combinations. The 5-HT(1A)-independent inhibition of hippocampal CA3 cell firing by pindolol suggests that this compound invokes multiple pharmacologic actions, all of which need to be assimilated into any proposed mechanism of action.

Beta adrenoceptor binding and induced relaxation in airway smooth muscle from patients with chronic airflow obstruction

Beta adrenoceptor function in central airway smooth muscle of patients with chronic airflow obstruction was investigated by radioligand binding studies and isoprenaline relaxation experiments. Receptor characteristics were determined in tracheal smooth muscle preparations obtained at necropsy from 12 patients and in bronchial tissue obtained at thoracototomy from 21 patients with chronic airflow obstruction. Receptor characteristics were compared with those obtained in airway tissue preparations from 65 control subjects without chronic airflow obstruction. The number of beta adrenoceptors, their binding affinity for the radioligand [125I]-(-)-cyanopindolol, and the tissue binding characteristics of isoprenaline were similar in tissue from patients with chronic airflow obstruction and from control subjects. Isoprenaline induced relaxation of tracheal smooth muscle without precontraction by methacholine showed slightly (though not significantly) less sensitivity to isoprenaline in patients with chronic airflow obstruction than in control subjects (mean (SEM) pD2--the negative logarithm of the concentration producing 50% relaxation--6.32 (0.16) v 6.62 (0.15)). The same pattern of pD2 values was found in segmental bronchial strips without precontraction by methacholine (chronic airflow obstruction 6.55 (0.27), control 7.14 (0.12)). Isoprenaline relaxation in segmental bronchial strips when contracted maximally was significantly less in the patients with airflow obstruction than in the control subjects (pD2 value 5.99 (0.18) v 6.45 (0.07)). These results suggest that beta adrenoceptors in airway smooth muscle of patients with chronic airflow obstruction are not abnormal in number or in binding affinity but that there is less effective coupling between components of the relaxant system distal to the beta adrenoceptor. The possibility that the reduced isoprenaline sensitivity is a consequence of previous bronchodilator treatment cannot be excluded.

Beta-adrenoceptors in human tracheal smooth muscle: characteristics of binding and relaxation

Specific binding of [125I]-(-)-cyanopindolol to human tracheal smooth muscle membranes was saturable, stereo-selective and of high affinity (Kd = 5.3 +/- 0.9 pmol/l and RT = 78 +/- 7 fmol/g tissue). The beta 1-selective antagonists atenolol and LK 203-030 inhibited specific [125I]-(-)-cyanopindolol binding according to a one binding site model with low affinity in nearly all subjects, pointing to a homogeneous beta 2-adrenoceptor population. In one subject using LK 203-030 a small beta 1-adrenoceptor subpopulation could be demonstrated. The beta-mimetics isoprenaline, fenoterol, salbutamol and terbutaline recognized high and low affinity agonist binding sites. Isoprenaline's pKH- and pKL-values for the high and low affinity sites were 8.0 +/- 0.2 and 5.9 +/- 0.3 respectively. In functional experiments isoprenaline relaxed tracheal smooth muscle strips having intrinsic tone with a pD2-value of 6.63 +/- 0.19.

Characterization of the muscarinic receptor in human tracheal smooth muscle

Muscarinic receptors in human tracheal smooth muscle were characterized by radioligand binding and functional studies. Specific [3H]-(-)-quinuclidinylbenzilate ([3H]-(-)-QNB) binding to tracheal smooth muscle membranes was reversible, stereoselective and of high affinity (Kd = 47 +/- 4 pmol/l; RT = 920 +/- 120 fmol/g tissue). Inhibition of specific [3H]-(-)-QNB binding by the M-1 selective antagonist pirenzepine was found to occur at relative high concentrations classifying the muscarinic receptor population as belonging to the M-2 subclass. Inhibition of specific [3H]-(-)-QNB binding by muscarinic agonists revealed the presence of high and low affinity sites in nearly equal proportions. 5'-Guanylylimidodiphosphate converted high affinity sites into low affinity sites although its effect was minimal. Log dose-contraction curves of methacholine had Hill coefficients of 1.10 +/- 0.04 with pD2-values of 6.75 +/- 0.02. Inhibition of specific [3H]-(-)-QNB binding by methacholine, however, was best described by a two binding site model with pKi-values considerably lower. The difference between these affinity values points to the presence of substantial receptor reserve.