Using Down syndrome as a model for complex trait analysis, we sought to identify loci from chromosome 21q22.2 which, when present in an extra dose, contribute to learning abnormalities. We generated low-copy-number transgenic mice, containing four different yeast artificial chromosomes (YACs) that together cover approximately 2 megabases (Mb) of contiguous DNA from 21q22.2. We subjected independent lines derived from each of these YAC transgenes to a series of behavioural and learning assays. Two of the four YACs caused defects in learning and memory in the transgenic animals, while the other two YACs had no effect. The most severe defects were caused by a 570-kb YAC; the interval responsible for these defects was narrowed to a 180-kb critical region as a consequence of YAC fragmentation. This region contains the human homologue of a Drosophila gene, minibrain, and strongly implicates it in learning defects associated with Down syndrome.

Recently a cDNA clone, vanilloid receptor subtype-1 (VR1), was isolated and found to encode an ion channel that is activated by both capsaicin, the pain producing compound in chili peppers, and by noxious thermal stimuli. Subsequently, two related cDNAs have been isolated, a stretch inactivating channel with mechanosensitive properties and a vanilloid receptor-like protein that is responsive to high temperatures (52-53°C). Here, we report the isolation of a vanilloid receptor 5-splice variant (VR.5sv) which differs from VR1 by elimination of the majority of the intracellular N-terminal domain and ankyrin repeat elements. Both VR.5sv and VR1 mRNA were shown to be expressed in tissues reportedly responsive to capsaicin including dorsal root ganglion, brain, and peripheral blood mononuclear cells. Functional expression of VR.5sv in Xenopus oocytes and mammalian cells showed no sensitivity to capsaicin, the potent vanilloid resiniferatoxin, hydrogen ions (pH 6.2), or noxious thermal stimuli (50°C). Since VR.5sv is otherwise identical to VR1 throughout its transmembrane spanning domains and C-terminal region, these results support the hypothesis that the N-terminal intracellular domain is essential for the formation of functional receptors activated by vanilloid compounds and noxious thermal stimuli.Capsaicin, the pungent main ingredient of hot chili peppers, selectively activates the peripheral termini of small diameter sensory neurons (nociceptors) and evokes the sensation of burning pain (1). The activation of nociceptors by capsaicin and other vanilloids is thought to be mediated by their binding to and activation of a ligand-gated ion channel (2, 3). Recently a functional capsaicin receptor termed vanilloid receptor, subtype-1 (VR1), 1 was cloned (4). The inferred amino acid sequence of VR1 predicts an ion channel subunit with six transmembrane-spanning segments flanked by large intracellular N-terminal and C-terminal domains (4). Thus far, VR1 has been reported to have a pattern of mRNA expression by in situ hybridization and immunohistochemical studies that is restricted to small diameter dorsal root and trigeminal and vagal sensory neurons (4 -6). In addition to its response to vanilloids, at least three additional functions for VR1 have been demonstrated as follows: the transduction of noxious thermal stimuli, its potentiation or activation by hydrogen ions (low pH) (4, 6), and most recently its activation by anandamide, an endogenous ligand of the cannabinoid receptor (7). The response properties of individual nociceptive neurons to these stimuli are, however, more complex than those responses characterized from the expression of VR1 in either Xenopus oocytes or transfected HEK293 cells (8 -14). One explanation for this heterogeneity is that individual nociceptors express multiple vanilloid receptor subtypes (10,11,14). Thus far, evidence for molecular heterogeneity of vanilloid receptors includes the isolation of cDNAs encoding a stretch-inactivating channel (15) and a vanilloid receptor-like protein, whic...

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