We thank Ron Dotsch for making his code available and Steven McNair for help with participant recruitment. This research was supported by Biotechnology and Biological Sciences Research Council (BBSRC) grant BB/J018929/1 to P.G.S., G.A.R., and N.v.R. and by a BBSRC DTP (WestBio) scholarship to K.J. “
“Tuberomamillary nucleus (TMN) neurons expressing the histidine decarboxylase (hdc) gene are the sole neuronal source of histamine [ 13, 14 and 15]. The hdc gene shows haploinsufficiency: a 2-fold

decrease in hdc mRNA levels halves the brain learn more content of histamine in mice [ 16 and 17], and in humans, having only one functional hdc allele produces a type of Tourette syndrome [ 16]. Thus, modest changes in hdc transcript levels in TMN neurons can change the amount of histamine released and influence behavior. Changes in hdc mRNA levels also seem to occur in the normal daily cycle. hdc mRNA levels in human hypothalamus are 1.6-fold higher for daytime deaths [ 18], and HDC enzyme activity and histamine levels in rat brain vary with selleck compound time of day [ 19, 20 and 21]. In agreement with these data, immunocytochemical staining for HDC protein in

mouse TMN neurons was stronger at zeitgeber time (ZT)18 (night, mid-lights off, the period when the animals are more active) than at ZT6 (day, mid-lights on) (3.5 ± 0.19 versus 1 ± 0.09 arbitrary units [AUs]; unpaired two-tailed t test, p < 0.001) ( Figures 1A and 1B). In control mice there was also a 1.5-fold variation in hdc transcript levels over 24 hr (unpaired two-tailed, t test, p < 0.05):

hdc mRNA was highest at the start of the night (ZT12), declined during the night, and increased during the day ( Figure 1C). By contrast, transcripts encoding the enzyme that inactivates histamine, histamine N-methyltransferase (HNMT), did not show daily variation in the TMN area ( Figure 1C). This daily variation in HDC expression Phloretin could indicate a clock-like mechanism in histaminergic neurons. Indeed, histaminergic neurons express the core clock protein BMAL1 ( Figure 1D). (BMAL1 antisera specificity was confirmed by the absence of staining in suprachiasmatic nucleus [SCN] sections from BMAL1 global knockout brains [ Figure S1A available online].) We crossed HDC-Cre mice [ 22] with animals containing a floxed Bmal1 gene [ 23] ( Figure S1B). The resulting HDC-ΔBmal1 mice were similar to littermate controls in weight (control weight, 26.6 ± 0.6 g, n = 5; HDC-ΔBmal1 weight, 27 ± 0.7 g, n = 5; unpaired two-tailed t test, p = 0.34) and seemed healthy. All the HDC-positive cells lost BMAL1 ( Figure 1D). In our characterization of the HDC-Cre mice, we found that transient developmental expression of the hdc gene produced recombination in several additional places, in particular the dorsal lateral geniculate (DLG) thalamic nucleus, the ventral medial (VM) hypothalamic nucleus, and Purkinje neurons [ 22].