Locus coeruleus neurons are resistant to dysfunction and degeneration by maintaining free ubiquitin levels although total ubiquitin levels decrease upon disruption of polyubiquitin gene Ubb.

Previously, we demonstrated that disruption of polyubiquitin gene Ubb leads to hypothalamic neurodegeneration and metabolic abnormalities associated with hypothalamic dysfunction. However, we cannot exclude the possibility that defects in other brain regions where Ubb is highly expressed may also contribute to the phenotypes exhibited by Ubb(-/-) mice. Upon searching for such brain regions, we identified a region in the brainstem called the locus coeruleus where both polyubiquitin genes Ubb and Ubc were highly expressed. In contrast to other brain regions, Ubc was significantly upregulated in the locus coeruleus of Ubb(-/-) mice presumably to compensate for loss of Ubb, and this upregulation was sufficient to maintain levels of free Ub, but not total Ub, in the locus coeruleus. However, in the hypothalamus of Ubb(-/-) mice, both free and total Ub levels significantly decreased. This discrepancy resulted in completely different phenotypic outcomes between the two different brain regions. While we have reported dysfunction and degeneration of hypothalamic neurons in adult Ubb(-/-) mice, there were no signs of functional impairment or degeneration in the locus coeruleus neurons, suggesting that the maintenance of free Ub above threshold levels could be an important mechanism for neuronal protection. Accordingly, we propose that, upon stress induced by disruption of Ubb, neuronal vulnerability may be determined based on the ability of neurons or neighboring cells to maintain free Ub levels for the protection of neuronal function and survival.

Gabrb3 gene deficient mice exhibit increased risk assessment behavior, hypotonia and expansion of the plexus of locus coeruleus dendrites.

Gabrb3 gene deficient (gabrb3(-/-)) mice, control littermates (gabrb3(+/+)) and their progenitor strains C57Bl/6J and 129/SvJ were assessed for changes in the morphology of the main noradrenergic nuclei, the locus coeruleus (LC) and LC-associated behaviors including anxiety and muscle tone. While the area defined by the cell bodies of the LC was found not to differ between gabrb3(-/-) mice and controls, the pericoerulear dendritic zone of the LC was found to be significantly enlarged in gabrb3(-/-) mice. Relative to controls, gabrb3(-/-) mice were also found to be hypotonic, as was indicated by poor performance on the wire hanging task. Gabrb3(-/-) mice also exhibited a significant increase in stretch-attend posturing, a form of risk assessment behavior associated with anxiety. However, in the plus maze, a commonly used behavioral test for assessing anxiety, no significant difference was observed between gabrb3(-/-) and control mice. Lastly, relative to controls, gabrb3(-/-) mice exhibited significantly less marble burying behavior, a method commonly used to assess obsessive-compulsive behavior. However, the poor marble burying performance of the gabrb3(-/-) mice could be associated with the hypotonic condition exhibited by these mice. In conclusion, the results of this study indicate that the gabrb3 gene contributes to LC noradrenergic dendrite development with the disruption of this gene in mice resulting in an enlarged plexus of LC dendrites with a concurrent reduction in muscle tone and marble burying behavior, an increase in risk assessment behavior but no change in the plus maze parameters that are commonly used for assessing anxiety.

Abnormal postnatal ontogeny of the locus coeruleus in the epileptic mutant mouse quaking.

The tonic-clonic convulsions of the quaking mutant mice have been shown to be associated with the hyperplasia of the nucleus locus coeruleus, the origin of most brain noradrenergic neurons. In the present study, the postnatal ontogeny of the locus coeruleus has been studied by tyrosine hydroxylase immunolabeling in the mutant mice quaking and their controls at postnatal days 1, 30 and 90. In the control mice, the number of immunoreactive neuronal cell bodies increased significantly in the rostral half of the locus coeruleus between birth and postnatal day 30, while it decreased significantly in the caudal half between birth and adulthood. Thus, during postnatal maturation, the distribution of locus coeruleus neurons was shifted in the rostral direction. In the quaking mutant mice, while the increase of immunolabeling between birth and postnatal day 30 was observed in the rostral half of the locus coeruleus, no diminution could be found in the caudal half between birth and adulthood. As a result, the rostral shift of tyrosine hydroxylase immunoreactivity was not observed. Consequently, in adult mice, the caudal part of the mutants locus coeruleus appeared to contain significantly more neurons than the corresponding region in the controls. These results indicate that the hyperplasia of the locus coeruleus of the quaking mice that we had previously reported results from an alteration of the postnatal maturation of this nucleus. This developmental abnormality might be a primary determinant of the inherited epilepsy of the quaking mutant mice.

Morphological deficits in noradrenergic neurons in GEPR-9s stem from abnormalities in both the locus coeruleus and its target tissues.

The epileptic condition of the genetically epilepsy-prone rat (GEPR) appears to be caused partially by deficiencies in the locus coeruleus (LC) innervation of the superior colliculus (SC). Previous studies provide quantitative documentation of noradrenergic morphological deficits in the moderately epileptic GEPR-3. The present findings extend these studies by applying cell culture methodology to assessments of the severely epileptic GEPR-9. Our data show that total neurite length, the number of neurite branch points per cell, the cross-sectional area of cell bodies, and the cell perimeter are deficient in noradrenergic neurons in LC + SC cocultures derived exclusively from GEPR-9s compared to analogous cocultures obtained solely from nonepileptic control rats. Partial restoration of LC neuron morphology toward normal occurs when the GEPR-9 SC component of the coculture is replaced with nonepileptic control SC. Finally, when the GEPR-9 SC is cocultured with the control LC, a partial morphological deficit occurs in the otherwise normal noradrenergic neurons. However, the magnitude of this deficit is less than that observed in noradrenergic neurons of the GEPR-9 LC cocultured with the control SC. These data support the hypothesis that the developmental deficiencies of noradrenergic neurons of the GEPR-9 are derived from two sources, the LC and its target tissue, in this case, the SC. Also, intrinsic abnormalities of the LC appear to make a more pronounced contribution to the noradrenergic deficits than do those which reside in the SC.

Defects in sensory and autonomic ganglia and absence of locus coeruleus in mice deficient for the homeobox gene Phox2a.

Phox2a is a vertebrate homeodomain protein expressed in subsets of differentiating neurons. Here, we show that it is essential for proper development of the locus coeruleus, a subset of sympathetic and parasympathetic ganglia and the VIIth, IXth, and Xth cranial sensory ganglia. In the sensory ganglia, we have identified two differentiation blocks in Phox2a-/- mice. First, the transient expression of dopamine-beta-hydroxylase in neuroblasts is abolished, providing evidence that Phox2a controls noradrenergic traits in vivo. Second, the expression of the GDNF receptor subunit Ret is dramatically reduced, and there is a massive increase in apoptosis of ganglion cells, which are known to depend on GDNF in vivo. Therefore, Phox2a appears to regulate conventional differentiation traits and the ability of neurons to respond to essential survival factors.