[3H]A-585539 [(1S,4S)-2,2-dimethyl-5-(6-phenylpyridazin-3-yl)-5-aza-2-azoniabicyclo[2.2.1]heptane], a novel high-affinity alpha7 neuronal nicotinic receptor agonist: radioligand binding characterization to rat and human brain.

Receptor binding was characterized for [(3)H](1S,4S)-2,2-dimethyl-5-(6-phenylpyridazin-3-yl)-5-aza-2-azoniabicyclo[2.2.1]heptane ([(3)H]A-585539), a selective high-affinity alpha7 nicotinic acetylcholine receptor (nAChR) agonist with rapid kinetics, low nonspecific binding, and high specific activity. At 4 degrees C, the association was monophasic and rapid (t((1/2)) = 8.0 min); dissociation was slower (t((1/2)) = 64.2 min). The K(d) in rat brain at 4 degrees C was 0.063 nM, whereas at 22 and 37 degrees C, the K(d) values were 0.188 and 0.95 nM, respectively. In contrast, the B(max) (34 fmol/mg protein) was unaffected by temperature. In human cortex, [(3)H]A-585539 bound with a K(d) of 0.066 nM and a B(max) of 5.8 fmol/mg protein at 4 degrees C, whereas under similar conditions, specific [(3)H]methyllycaconitine ([(3)H]MLA) binding was not measurable. A number of agonist and antagonist nAChR ligands displaced binding to rat brain membranes with rank order of affinity similar to that for [(3)H]MLA, and in general, a 5 to 10-fold higher affinity was observed for [(3)H]A-585539 binding. There was also a good correlation of K(i) values between [(3)H]A-585539 binding to rat brain and human cortex. The use of a alpha7/5-hydroxytryptamine type-3 chimera revealed that the N-terminal domain of alpha7 nAChR was sufficient to faithfully reproduce the pharmacology of [(3)H]A-585539 binding. Autoradiographic studies comparing [(3)H]A-585539 and [(125)I]alpha-bungarotoxin revealed a similar pattern of labeling in the rat. In summary, [(3)H]A-585539 was shown to have excellent binding characteristics in rat and human brain and represents the first high-affinity alpha7 agonist radioligand with utility in the characterization of this important nAChR subtype that is targeted toward ameliorating cognitive deficits underlying neuropsychiatric and neurodegenerative disorders.

A-186253, a specific antagonist of the alpha 4 beta 2 nAChRs: its properties and potential to study brain nicotinic acetylcholine receptors.

Imaging the living brain and the distribution of the ligand gated channels that participate in the neurotransmission is one of the challenges that is hoped to bring new insights for the treatment of neurological diseases. Herein, we probed a new nicotinic derivative, A-186253 as a potential molecule to discriminate with high resolution the different neuronal nicotinic receptor subtypes that are expressed in distinct brain areas. Binding with a high affinity of 440 pM at the major brain alpha4beta2 receptor subtype and presenting an excellent safety margin, properties of the A-186253 were thoroughly evaluated. While autoradiography confirmed its specificity for the alpha4beta2 subtype, functional investigations revealed for short exposures a broader spectrum of action at receptors including the ganglionic alpha3beta4 and the homomeric alpha7 subtypes. Specificity was, however, observed at alpha4beta2 when receptors were exposed for several minutes with low concentration of the A-186253. In view of these promising results, the A-186253 was radiolabeled and tested in positron emission tomography on rats and pigs. Despite the high selectivity observed in vitro, the A-186253 displayed a complex binding profile and little displacement by the agonist cytisine. While the A-186253 can be valuable to discriminate receptor subtypes, improvements of this molecule must be brought for in vivo measurements.

Up-regulation of vasopressin and angiotensin II receptors in the thalamus and brainstem of inbred polydipsic mice.

Vasopressin (AVP), angiotensin II (Ang II) and oxytocin (OT) receptors were mapped in the brain of inbred polydipsic mice of the STR/N strain by quantitative in vitro autoradiography and receptor binding levels, compared with those found in control non-polydipsic mice of the ICR strain. A remarkable difference was evidenced in the thalamic paraventricular nucleus where AVP receptor binding was 7- to 10-fold higher in polydipsic mice than in control mice. Another disparity was observed in the hypothalamic paraventricular nucleus, which contained AVP binding sites in the control mice, but was unlabelled in the polydipsic animals. Ang II receptor binding was reduced in the hypothalamic paraventricular nucleus of the polydipsic mice, whereas it was abundant in the brainstem region, encompassing area postrema and the nucleus of the solitary tract. The distribution and amount of OT receptor binding were similar in the polydipsic and control mice. Strain-related differences of AVP and Ang II receptor binding were observed both in male and female animals. A sex-related difference was seen only for OT receptor binding in the hypothalamic ventromedial nucleus, where labelling was less intense in males than in females of both strains. Altogether, our results support the view that central AVP and Ang II systems are involved in the mechanisms responsible for polydipsia in STR/N mice.