Muscarinic receptor changes in the gerbil thalamus during aging.

Here we studied muscarinic receptors in the gerbil thalamus at 8 different ages - from 6 to 36 months - using receptor and functional autoradiography. The pharmacological profile inhibiting [(3)H]N-methyl scopolamine ([(3)H]NMS) binding with 50 and 200 nM pirenzepine, 30 nM pFHHSiD and 100 nM AF-DX 116 revealed the predominance of the M(2) muscarinic subtype in the thalamic nuclei studied, mainly in the anteroventral, anteromedial and paraventricular thalamic nuclei. These data correlated with the highest [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTP gamma S) binding induced in these nuclei by the muscarinic agonist oxotremorine in functional autoradiographic assays. Significant aging-dependent increases in the functional response in these three nuclei were observed, but only the anteroventral and anteromedial thalamic nuclei showed aging-dependent increases in [(3)H]NMS binding. Since these nuclei exert relevant functions, in which cholinergic pathways are involved and acetylcholine release is reported to decrease during aging, we suggest that the anteroventral and anteromedial thalamic nuclei would play critical roles in the cholinergic transmission that require compensatory mechanisms during the aging process and that are not observed in other thalamic nuclei.

Differential effects on [35S]GTPgammaS binding using muscarinic agonists and antagonists in the gerbil brain.

In this work, we studied the in vitro G-protein activation induced by muscarinic agonists using [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) autoradiographic methods to characterize the M(2) and M(4) muscarinic subtypes response. Thus, we describe a detailed characterization of the increases in [(35)S]GTPgammaS binding elicited by carbachol (Cch) and oxotremorine (OXO) (binding in the presence minus binding in the absence of agonist) throughout the gerbil brain (Meriones unguiculatus). For both agonists, the strongest stimulations were found in the superficial gray layer of the superior colliculus, the anteroventral and anteromedial thalamic nuclei, the anterior paraventricular thalamic nucleus, and the caudate-putamen. The comparative study using OXO and Cch suggested that OXO is able to detect differences in the response of structures enriched in M(4) muscarinic receptors, showing a lower potency to stimulate these brain areas. Furthermore, using increasing concentrations of selective M(2) (AF-DX 116) and M(1)/M(4) (pirenzepine) antagonists to inhibit specific Cch- or OXO-induced [(35)S]GTPgammaS binding, significant differences were observed in M(2)-enriched structures but not in M(4)-enriched ones such as the caudate-putamen. These data indicate that appropriate muscarinic agonist stimulation, together with selective inhibition of this effect using functional autoradiography, can be used as a tool to unravel the M(2)- and M(4)-muscarinic subtype-mediated response.