br Acknowledgements We thank Dr James Ritchie and his dedica
Acknowledgements We thank Dr. James Ritchie and his dedicated staff (Deptartment of Pathology, Emory University) for performing measures of serum VPA levels; Sumitomo Pharmaceuticals (Osaka, Japan) for the generous donation of DOPS, which is required for breeding Dbh −/− mice; Pfizer (Sandwich, Kent, UK) for their generous donation of reboxetine; Z. Donaldson, P. Kirkland, and M. Marino for technical assistance. This project was supported by the Epilepsy Foundation.
Introduction Norepinephrine is a neurotransmitter that signals through alpha-1, alpha-2 and beta adrenergic receptors (α1-AR, α2-AR and β-AR) (Bylund et al., 1994). These are G-protein-coupled receptors whose signaling is important for the regulation of diverse processes within the mature and developing 3065 (Morris et al., 1983, Lau et al., 1990, Nutt et al., 1997, Troadec et al., 2001). Recently, investigation of noradrenergic regulation of CNS function has been facilitated by the development of mice with a genomic deletion of dopamine-β-hydroxylase (DBH), the enzyme necessary for conversion of dopamine to norepinephrine (Thomas et al., 1995). Mice with a homozygous deletion of DBH (Dbh−/−) have a selective and complete absence of norepinephrine throughout postnatal development and into adulthood, whereas heterozygotes (Dbh+/−) have normal central norepinephrine levels (Thomas et al., 1995, Thomas et al., 1998, Bourdelat-Parks et al., 2005). Prenatally adrenergic agonists as well as the norepinephrine precursor, L-3,4-dihydroxyphenylserine, are provided to pregnant dams via drinking water to assure survival of the Dbh−/− mice (Thomas et al., 1995). Behaviorally, Dbh−/− mice manifest a variety of anomalies that point to the importance of norepinephrine in brain function. These include deficits in active-avoidance learning (Thomas and Palmiter, 1997), a failure to respond to anti-depressants (Cryan et al., 2004), a sensitization to amphetamine (Weinshenker et al., 2002b) and an increased susceptibility to seizure-inducing stimuli (Szot et al., 1999). Despite the in-depth characterization of behavior in Dbh−/− mice, little is known about the effects of this deletion on noradrenergic receptor expression, the natural target of norepinephrine. This information will be helpful in interpreting the differing responses of Dbh+/− and Dbh−/− mice to noradrenergic agents and it will provide data relevant to the role of norepinephrine in regulating the developmental expression of noradrenergic receptors in the postnatal period. In the current studies, we examined differences in the expression of α1-AR, α2-AR and β-AR within the brains of the Dbh+/− and Dbh−/− genotypes, a model of specific and non-traumatic norepinephrine elimination throughout postnatal development and into adulthood (Thomas et al., 1995).
Discussion Many studies have suggested that norepinephrine can play an important role in regulating development of the CNS (Felten et al., 1982, Blue and Parnavelas, 1982, Parnavelas and Blue, 1982). This regulation would be coordinated via norepinephrine\'s signaling through α1-AR, α2-AR and β-AR (Bylund et al., 1994). Each of these receptors has at least three subtypes and the subtypes possess distinct pharmacological profiles and differential distributions within the central nervous system (Bylund et al., 1994) (Zhong and Minneman, 1999, 7388/id; Nicholas et al., 1996, 5798/id; McCune et al., 1993, 5413/id; Rainbow et al., 1984, 6706/id). There are also changes in adrenergic receptor distribution during development, but few have examined receptor subtypes (Jones et al., 1985, Goffinet et al., 1986, Winzer-Serhan and Leslie, 1997, Winzer-Serhan et al., 1997a, Winzer-Serhan et al., 1997b, Happe et al., 2004). Because adrenergic receptors are the physiological target of norepinephrine, it is reasonable to expect that loss of norepinephrine during postnatal development would lead to changes in these receptors. We examined this possibility using mice lacking DBH, the final enzymatic step in the synthetic pathway for norepinephrine. Although it is necessary for the survival of Dbh−/− embryos to provide adrenergic receptor stimulation prenatally by providing receptor agonists and DOPS in the drinking water of pregnant dams (see Experimental procedures), these mice completely lack norepinephrine from birth forward (Thomas et al., 1995). In these studies, we compared the Dbh−/− mice to Dbh+/− mice, which have been shown to have normal central levels of norepinephrine (Thomas et al., 1995, Thomas et al., 1998, Bourdelat-Parks et al., 2005). These animals also have normal localization and density of noradrenergic terminals as indicated by the distribution and density of the norepinephrine transporter (Weinshenker et al., 2002b), a highly specific marker for norepinephrine neurons. This indicates the changes found cannot be ascribed to changes in distribution of noradrenergic terminals.