Catherine A. Priest

Hypothalamic pro-opiomelanocortin mRNA is reduced by fasting in ob/ob and db/db mice, but is stimulated by leptin

Reduction in the activity of the α-melanocyte-stimulating hormone (α-MSH) system causes obesity, and infusions of α-MSH can produce satiety, raising the possibility that α-MSH may mediate physiological satiety signals. Since α-MSH is coded for by the pro-opiomelanocortin (POMC) gene, we examined if POMC gene expression would be inhibited by fasting in normal mice or in models of obesity characterized by leptin insufficiency (ob/ob) or leptin insensitivity (db/db). In wild-type mice, hypothalamic POMC mRNA was decreased >60% after a 2-day fast and was positively correlated with leptin mRNA. Similarly, compared with controls, POMC mRNA was decreased by at least 60% in both db/db and ob/ob mice. POMC mRNA was negatively correlated with both neuropeptide Y (NPY) and melanin-concentrating hormone (MCH) mRNA. Finally, treatment of both male and female ob/ob mice with leptin stimulated hypothalamic POMC mRNA by about threefold. These results suggest that impairment in production, processing, or responsiveness to α-MSH may be a common feature of obesity and that hypothalamic POMC neurons, stimulated by leptin, may constitute a link between leptin and the melanocortin system.

Anorectic estrogen mimics leptin's effect on the rewiring of melanocortin cells and Stat3 signaling in obese animals

Metabolic hormones, such as leptin, alter the input organization of hypothalamic circuits, resulting in increased pro-opiomelanocortin (POMC) tone, followed by decreased food intake and adiposity. The gonadal steroid estradiol can also reduce appetite and adiposity, and it influences synaptic plasticity. Here we report that estradiol (E2) triggers a robust increase in the number of excitatory inputs to POMC neurons in the arcuate nucleus of wild-type rats and mice. This rearrangement of synapses in the arcuate nucleus is leptin independent because it also occurred in leptin-deficient (ob/ob) and leptin receptor–deficient (db/db) mice, and was paralleled by decreased food intake and body weight gain as well as increased energy expenditure. However, estrogen-induced decrease in body weight was dependent on Stat3 activation in the brain. These observations support the notion that synaptic plasticity of arcuate nucleus feeding circuits is an inherent element in body weight regulation and offer alternative approaches to reducing adiposity under conditions of failed leptin receptor signaling.

Targeted Deletion of the Vgf Gene Indicates that the Encoded Secretory Peptide Precursor Plays a Novel Role in the Regulation of Energy Balance

To determine the function of VGF, a secreted polypeptide that is synthesized by neurons, is abundant in the hypothalamus, and is regulated in the brain by electrical activity, injury, and the circadian clock, we generated knockout mice lacking Vgf. Homozygous mutants are small, hypermetabolic, hyperactive, and infertile, with markedly reduced leptin levels and fat stores and altered hypothalamic proopiomelanocortin (POMC), neuropeptide Y (NPY), and agouti-related peptide (AGRP) expression. Furthermore, VGF mRNA synthesis is induced in the hypothalamic arcuate nuclei of fasted normal mice. VGF therefore plays a critical role in the regulation of energy homeostasis, suggesting that the study of lean VGF mutant mice may provide insight into wasting disorders and, moreover, that pharmacological antagonism of VGF action(s) might constitute the basis for treatment of obesity.

Functional circuitry involved in the regulation of whisker movements

Neuroanatomical tract‐tracing methods were used to identify the oligosynaptic circuitry by which the whisker representation of the motor cortex (wMCx) influences the facial motoneurons that control whisking activity (wFMNs). Injections of the retrograde tracer cholera toxin subunit B into physiologically identified wFMNs in the lateral facial nucleus resulted in dense, bilateral labeling throughout the brainstem reticular formation and in the ambiguus nucleus as well as predominantly ipsilateral labeling in the paralemniscal, pedunculopontine tegmental, Kölliker‐Fuse, and parabrachial nuclei. In addition, neurons in the following midbrain regions projected to the wFMNs: superior colliculus, red nucleus, periaqueductal gray, mesencephalon, pons, and several nuclei involved in oculomotor behaviors. Injections of the anterograde tracer biotinylated dextran amine into the wMCx revealed direct projections to the brainstem reticular formation as well as multiple brainstem and midbrain structures shown to project to the wFMNs. Regions in which retrograde labeling and anterograde labeling overlap most extensively include the brainstem parvocellular, gigantocellular, intermediate, and medullary (dorsal and ventral) reticular formations; ambiguus nucleus; and midbrain superior colliculus and deep mesencephalic nucleus. Other regions that contain less dense regions of combined anterograde and retrograde labeling include the following nuclei: the interstitial nucleus of medial longitudinal fasciculus, the pontine reticular formation, and the lateral periaqueductal gray. Premotoneurons that receive dense inputs from the wMCx are likely to be important mediators of cortical regulation of whisker movements and may be a key component in a central pattern generator involved in the generation of rhythmic whisking activity. J. Comp. Neurol. 442:266–276, 2002.

Estrogen and Tamoxifen Differentially Regulate Beta-Endorphin and cFos Expression and Neuronal Colocalization in the Arcuate Nucleus of the Rat

Estrogen regulates hypothalamic gene expression, synthesis and release of the endogenous opioid peptide β-endorphin (βEND), although a consensus estrogen response element sequence has not been identified in the rat proopiomelanocortin (POMC) gene. POMC gene expression is also regulated by the activation of AP-1 promoter elements, which are known to be estrogen sensitive. The present studies examine whether estrogen modulates the hypothalamic POMC system through a non-classical mechanism involving AP-1 binding proteins such as cFos. Immunohistochemical double-labeling for βEND and cFos was used and immunoreactive (-ir) populations were quantified in the arcuate nucleus and periarcuate area across time using unbiased stereological methods. Ovariectomized rats were injected with 50 µg estradiol (E2), 500 µg tamoxifen citrate (TAM) or both (E2+TAM) and were perfused 1, 2, 4 or 48 h later. E2 rapidly increased numbers of cFos-ir, βEND-ir and doubly-labeled cells after 4 h, and the number of βEND-ir cells remained high 48 h later, suggesting that the stimulatory effects of cFos on POMC in the hypothalamus persist after the cFos signal decays. Treatment with TAM alone did not affect the numbers of immunoreactive cells, although E2+TAM blocked the E2-mediated induction in all immunoreactive populations. Similar effects were seen at the transcriptional level. E2 increased hypothalamic POMC mRNA after 4 h, while TAM treatment or coadministration of E2+TAM did not significantly change the levels of POMC mRNA. Cellular colocalization of βEND-ir and cFos-ir supports a possible intracellular co-regulation of these peptides by an estrogen-dependent mechanism within a subset of hypothalamic neurons. It does not, however, appear that E2 acts directly through an AP-1 site within the POMC gene.

Gap junction proteins in inhibitory neurons of the adult barrel neocortex

Recent studies indicate that electrical coupling among cortical neurons may persist throughout development; electrophysiological recordings made in cortical slices from young rats reveal that numerous GABAergic neurons are electrically coupled. To determine whether these in vitro findings reflect an inhibitory neural circuit that could be functionally relevant in vivo in adult rodents, we sought to identify whether inhibitory, parvalbumin-containing neurons of the mature cortex express gap junction proteins. Immunohistochemistry was used to examine the laminar distribution of the gap junction-forming proteins connexin 32 (Cx32), connexin 36 (Cx36) and connexin 43 (Cx43) in the somatosensory cortex of the adult mouse. Double labeling immunofluorescence identified Cx32, Cx36 and Cx43 in cortical neurons that were immunoreactive (-ir) for the neuronal markers neurofilament 145 kDa and neuronal nuclei (NeuN). Parvalbumin-ir neurons throughout the cortical laminae were labeled with Cx32-ir, Cx36-ir and Cx43-ir. Stereological methods were used to quantify the extent of parvalbumin colocalization with connexins. Analysis indicated that approximately 40% of parvalbumin-ir neurons were double labeled with either Cx32-ir or Cx43-ir, and approximately 50% of parvalbumin-ir neurons were double labeled with Cx36. These findings establish an anatomical substrate for widespread electrical coupling of neurons in somatosensory cortex and suggest that gap junctions among inhibitory interneurons may persist into adulthood, providing an important mechanism for neuronal communication.Recent studies indicate that electrical coupling among cortical neurons may persist throughout development; electrophysiological recordings made in cortical slices from young rats reveal that numerous GABAergic neurons are electrically coupled. To determine whether these in vitro findings reflect an inhibitory neural circuit that could be functionally relevant in vivo in adult rodents, we sought to identify whether inhibitory, parvalbumin-containing neurons of the mature cortex express gap junction proteins. Immunohistochemistry was used to examine the laminar distribution of the gap junction-forming proteins connexin 32 (Cx32), connexin 36 (Cx36) and connexin 43 (Cx43) in the somatosensory cortex of the adult mouse. Double labeling immunofluorescence identified Cx32, Cx36 and Cx43 in cortical neurons that were immunoreactive (-ir) for the neuronal markers neurofilament 145 kDa and neuronal nuclei (NeuN). Parvalbumin-ir neurons throughout the cortical laminae were labeled with Cx32-ir, Cx36-ir and Cx43-ir. Stereological methods were used to quantify the extent of parvalbumin colocalization with connexins. Analysis indicated that approximately 40% of parvalbumin-ir neurons were double labeled with either Cx32-ir or Cx43-ir, and approximately 50% of parvalbumin-ir neurons were double labeled with Cx36. These findings establish an anatomical substrate for widespread electrical coupling of neurons in somatosensory cortex and suggest that gap junctions among inhibitory interneurons may persist into adulthood, providing an important mechanism for neuronal communication.