Although the predominant subtype in rat brain was a4b2, non-a4b2 binding sites were prominent in many regions. In the habenulo-peduncular system, cerebellum, substantia gelatinosa, and many medullary nuclei, a3b4-like binding accounted for more than 40% of [ 125 I]epibatidine binding, and nearly all binding in superior cervical ganglion and pineal gland. Other regions enriched in a3b4-like binding included locus ceruleus, dorsal tegmentum, subiculum and anteroventral thalamic nucleus. Regions enriched in a3b2-like binding included the habenulo-peduncular system, many visual system structures, certain geniculate nuclei, and dopaminergic regions. The combination of autoradiography using a broad spectrum radioligand in the presence of selective competitors, and data from binding to defined receptor subtypes in expression systems, allowed us to quantify the relative populations of these three subtypes.

Similar to Arabidopsis thaliana, the wild soybeans (Glycine soja) and many cultivars exhibit indeterminate stem growth specified by the shoot identity gene Dt1, the functional counterpart of Arabidopsis TERMINAL FLOWER1 (TFL1). Mutations in TFL1 and Dt1 both result in the shoot apical meristem (SAM) switching from vegetative to reproductive state to initiate terminal flowering and thus produce determinate stems. A second soybean gene (Dt2) regulating stem growth was identified, which, in the presence of Dt1, produces semideterminate plants with terminal racemes similar to those observed in determinate plants. Here, we report positional cloning and characterization of Dt2, a dominant MADS domain factor gene classified into the APETALA1/SQUAMOSA (AP1/SQUA) subfamily that includes floral meristem (FM) identity genes AP1, FUL, and CAL in Arabidopsis. Unlike AP1, whose expression is limited to FMs in which the expression of TFL1 is repressed, Dt2 appears to repress the expression of Dt1 in the SAMs to promote early conversion of the SAMs into reproductive inflorescences. Given that Dt2 is not the gene most closely related to AP1 and that semideterminacy is rarely seen in wild soybeans, Dt2 appears to be a recent gain-of-function mutation, which has modified the genetic pathways determining the stem growth habit in soybean.

Subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) are differentially sensitive to up-regulation by chronic nicotine exposure in vitro. To determine whether this occurs in animals, rats were implanted with minipumps containing saline Ϯ nicotine (6.0 mg/kg/rat/day) for 14 days. Autoradiography with [125 I]epibatidine using 3-(2(S)-azetidinylmethoxy)pyridine dihydrochloride (A-85380) or cytisine as selective competitors allowed quantitative measurement in 33 regions of 3 families of nAChR binding, with properties of ␣4␤2, ␣3␤4, and ␣3/␣6␤2. Chronic nicotine exposure caused increases of 20 to 100% for ␣4␤2-like binding in most regions surveyed. However, binding to this subtype was not increased in some regions, including habenulopeduncular structures, certain thalamic nuclei, and several brainstem regions. In 9 of 33 regions, including catecholaminergic areas and visual structures, ␣3/ ␣6␤2-like binding represented Ͼ10% of total binding. Binding to this subtype was up-regulated by nicotine in only two of these nine regions: the nucleus accumbens and superior colliculus. ␣3␤4-Like binding represented Ͼ10% of total in 15 of the 33 regions surveyed. Binding to this subtype was increased by nicotine in only 1 of these 15 regions, and actually decreased in subiculum and cerebellum. These studies yielded two principal findings. First, chronic nicotine exposure selectively up-regulates ␣4␤2-like binding, with relatively little effect on ␣3/␣6␤2-like and ␣3␤4-like binding in vivo. Second, upregulation by chronic nicotine exposure shows considerable regional variation. Differential subtype sensitivity to chronic nicotine exposure may contribute to altered pharmacological response in individuals who smoke or use nicotine replacement therapy.The actions of nicotine in brain are mediated by neuronal nicotinic acetylcholine receptors (nAChRs). These receptors are ligand-gated ion channels comprising combinations of ␣ and ␤ subunit proteins in a pentameric structure. Expression of at least seven ␣ and three ␤ subunits has been detected in mammalian brain. Despite the large number of subunits, pharmacological studies have so far found evidence for only a limited number of actual subtypes. Initially, two classes were identified by receptor binding methods: one selectively labeled by [125 I]␣-bungarotoxin, thought to consist largely of ␣7 homomers, and another class with higher affinity for acetylcholine, nicotine, and cytisine. The introduction of radiolabeled epibatidine revealed the presence of additional subclasses of nAChR not labeled by these other ligands. It is now recognized that there are several different subtypes of these so-called high-affinity nAChRs, based on selective ligands and molecular studies. The major form is believed to be ␣4␤2 (Flores et al., 1992); multiple forms may exist based on variable stoichiometry or the inclusion of additional subunits such as ␣5, although these variations have only minimal effects on ligand binding properties. A second form of highaffinity nAChR closely resemb...