Differential Stimulation of Inositol Phospholipid Turnover in Brain by Analogs of Oxotremorine

Abstract

Structural analogs of oxotremorine have been employed to examine the relationship between the binding of agonists to muscarinic receptors in guinea pig cerebral cortex and the enhancement of inositol lipid turnover. Large differences were observed in the ability of the analogs to stimulate inositol phospholipid turnover, as measured both by the increase in labeling of phosphatidate and phosphatidylinositol from 32Pi in a nerve-ending fraction, and by the stimulated release of labeled inositol phosphates from slices of cerebral cortex, a direct measure of inositol lipid breakdown. The quaternary N+ analogs, oxotremorine-M and its N-methylacetamide derivative, were five to thirteen times as effective as oxotremorine. In contrast, methyl substitution of the pyrrolidone ring of oxotremorine resulted in a complete loss of agonist activity. Receptor occupancy data obtained from the displacement of labeled quinuclidinyl benzilate bound to receptors in a nerve-ending fraction indicated that the more efficacious agonists interacted with at least two affinity forms of the muscarinic receptor, whereas the less effective agonists bound to a single affinity form. Dose-response curves obtained in the presence of oxotremorine-M for inositol lipid turnover in both the nerve-ending fraction and slice preparation correlated with the occupancy of a single low-affinity form of the muscarinic receptor. The results suggest that the differential abilities of analogs of oxotremorine to enhance inositol lipid turnover in brain are closely related to the extent of agonist-induced conformational change in the muscarinic receptor.

A Long-Lasting Calcium-Activated Nonselective Cationic Current Is Generated by Synaptic Stimulation or Exogenous Activation of Group I Metabotropic Glutamate Receptors in CA1 Pyramidal Neurons

“…If so, other types of metabotropic receptors linked to the I P 3 production pathway (for review, see Schoepp and Conn, 1993) should activate I CAN also. In keeping with this hypothesis and in agreement with previous data (Shen and North, 1992;Colino and Halliwell, 1993;Fraser and MacVicar, 1996), we found that activation of muscarinic receptors induced a current showing similar features to those of I CAN : under perfusion with medium A, bath application of 60 -120 M carbachol (a selective agonist of muscarinic receptors linked to IP 3 production pathway) (Fisher et al, 1983(Fisher et al, , 1984Dutar and Nicoll, 1988) generated a CAN current that was smaller but otherwise comparable to I CAN induced by 1S,3R-ACPD (n ϭ 9; Fig. 6C, Table 1).…”

Receptor Activation and Inositol Lipid Hydrolysis in Neural Tissues

“…The initial observations that activation of mAChRs in brain slices resulted in an increase in phosphoinositide turnover has since been amply confirmed in a variety of neural preparations, including nerve ending preparations, brain minces, and primary cultures of neurons and glia, as well as transformed cells of neural origin, such as neuroblastoma and astrocytoma (for review, see Fisher and Agranoff, 1985). Within the CNS, there exists a strong correlation between the density of mAChRs, as determined by radioligand binding techniques, and Bemdge et al (1982), Brown et al (1984), Fisher et al (1984), Gonzales and Crews (1984), Fisher and Bartus (1985), Gil and Wolfe (1989, Jacobson et al (1989, Lazareno et al (1983, Eva and Costa (1986),Heacocketal. (1987 Schoepp et al (1984), Johnson and Minneman (1985), Kendall et al (1989, Minneman andJohnson (1984) -Bush (1984-Bush ( , 1985.…”

“…Muscarinic agonists differ considerably in their ability to enhance phosphoinositide turnover in brain. In the cerebral cortex, a group of agonists, including ACh, carbamoylcholine, and oxotremorine-M, is more effective than several others, including pilocarpine, bethanechol, and oxotremorine (Fisher et al, 1983(Fisher et al, , 1984Gonzales and Crews, 1984;Jacobson et al, 1985). The former group of agonists have been termed "group A" agonists, and the latter, "group B."…”

“…Two lines of evidence support this statement. First, the abilities of either muscarinic or adrenergic ligands to stimulate the release of labeled inositol phosphates correlates well with their abilities to displace radiolabeled antagonists from specific membrane binding sites (Fisher et al, 1983(Fisher et al, , 1984Minneman and Johnson, 1984). Second, the loss of receptor number either in vivo following neurotoxin treatment (Fisher et al, 1981~) or in vitro following irreversible inactivation of a proportion of the muscarink, a,-adrenergic, 5-HT2-serotonergic, or H,-histaminergic receptors results in a comparable loss of receptor-stimulated phosphoinositide turnover (Johnson and Minneman, 1985; Kendall and Nahorski, 1985b;Claro et al, 1986;Fisher and Snider, 1986).…”

Relative affinities of drugs acting at cholinoceptors in displacing agonist and antagonist radioligands: the NMS/Oxo‐M ratio as an index of efficacy at cortical muscarinic receptors

“…Furthermore, whilst it is relatively easy to identify antagonists, partial agonists and full agonists it is often more difficult to distinguish differing degrees of partial agonism without detailed secondary biochemical tests e.g. phosphatidyl-inositol turnover or adenylate cyclase assays (Olianas et al, 1983;Fisher et al, 1984).…”