Joseph Kelly

Chemically defined projections linking the mediobasal hypothalamus and the lateral hypothalamic area

Recent studies have identified several neuropeptide systems in the hypothalamus that are critical in the regulation of body weight. The lateral hypothalamic area (LHA) has long been considered essential in regulating food intake and body weight. Two neuropeptides, melanin‐concentrating hormone (MCH) and the orexins (ORX), are localized in the LHA and provide diffuse innervation of the neuraxis, including monosynaptic projections to the cerebral cortex and autonomic preganglionic neurons. Therefore, MCH and ORX neurons may regulate both cognitive and autonomic aspects of food intake and body weight regulation. The arcuate nucleus also is critical in the regulation of body weight, because it contains neurons that express leptin receptors, neuropeptide Y (NPY), α‐melanin‐stimulating hormone (α‐MSH), and agouti‐related peptide (AgRP). In this study, we examined the relationships of these peptidergic systems by using dual‐label immunohistochemistry or in situ hybridization in rat, mouse, and human brains. In the normal rat, mouse, and human brain, ORX and MCH neurons make up segregated populations. In addition, we found that AgRP‐ and NPY‐immunoreactive neurons are present in the medial division of the human arcuate nucleus, whereas α‐MSH‐immunoreactive neurons are found in the lateral arcuate nucleus. In humans, AgRP projections were widespread in the hypothalamus, but they were especially dense in the paraventricular nucleus and the perifornical area. Moreover, in both rat and human, MCH and ORX neurons receive innervation from NPY‐, AgRP‐, and α‐MSH‐immunoreactive fibers. Projections from populations of leptin‐responsive neurons in the mediobasal hypothalamus to MCH and ORX cells in the LHA may link peripheral metabolic cues with the cortical mantle and may play a critical role in the regulation of feeding behavior and body weight. J. Comp. Neurol. 402:442–459, 1998.

Leptin differentially regulates NPY and POMC neurons projecting to the lateral hypothalamic area

Recent studies have reinforced the view that the lateral hypothalamic area (LHA) regulates food intake and body weight. We identified leptin-sensitive neurons in the arcuate nucleus of the hypothalamus (Arc) that innervate the LHA using retrograde tracing with leptin administration. We found that retrogradely labeled cells in the Arc contained neuropeptide Y (NPY) mRNA or proopiomelanocortin (POMC) mRNA. Following leptin administration, NPY cells in the Arc did not express Fos but expressed suppressor of cytokine signaling-3 (SOCS-3) mRNA. In contrast, leptin induced both Fos and SOCS-3 expression in POMC neurons, many of which also innervated the LHA. These findings suggest that leptin directly and differentially engages NPY and POMC neurons that project to the LHA, linking circulating leptin and neurons that regulate feeding behavior and body weight homeostasis.

Leptin Activates Hypothalamic CART Neurons Projecting to the Spinal Cord

The adipocyte-derived hormone leptin decreases body weight in part by activating the sympathetic nervous system, resulting in increased thermogenesis and energy expenditure. We investigated hypothalamic pathways underlying leptins effects on stimulating the sympathetic nervous system. We found that leptin activates neurons in the retrochiasmatic area (RCA) and lateral arcuate nucleus (Arc) that innervate the thoracic spinal cord and also contain cocaine- and amphetamine-regulated transcript (CART). We also found that most CART-containing neurons in the RCA and Arc of the hypothalamus also contain proopiomelanocortin (POMC) mRNA. The finding that leptin activates CART/POMC neurons innervating sympathetic preganglionic neurons in the thoracic spinal cord suggests that this pathway may contribute to the increased thermogenesis and energy expenditure and decreased body weight observed following leptin administration.

Characterization of CART neurons in the rat and human hypothalamus

Cocaine‐ and amphetamine‐regulated transcript (CART) is a recently described neuropeptide widely expressed in the rat brain. CART mRNA and peptides are found in hypothalamic sites such as the paraventricular nucleus (PVH), the supraoptic nucleus (SON), the lateral hypothalamic area (LHA), the dorsomedial nucleus of the hypothalamus (DMH), the arcuate nucleus (Arc), the periventricular nucleus (Pe), and the ventral premammillary nucleus (PMV). Intracerebroventricular administration of recombinant CART peptide decreases food intake and CART mRNA levels in the Arc are regulated by leptin. Leptin administration induces Fos expression in hypothalamic CART neurons in the PVH, the DMH, the Arc, and the PMV. In the current study, we used double label in situ hybridization histochemistry to investigate the potential direct action of leptin on hypothalamic CART neurons and to define the chemical identity of the hypothalamic CART neurons in the rat brain. We found that CART neurons in the Arc, DMH, and PMV express long form leptin‐receptor mRNA, and the suppressor of cytokine signaling‐3 (SOCS‐3) mRNA after an acute dose of intravenous leptin. We also found that CART neurons in the parvicellular PVH, in the DMH and in the posterior Pe coexpress thyrotropin‐releasing hormone (TRH) mRNA. CART neurons in the magnocellular PVH and in the SON coexpress dynorphin (DYN), and CART cell bodies in the LHA and in the posterior Pe coexpress melanin‐concentrating hormone (MCH) and glutamic acid decarboxylase (GAD‐67) mRNA. In the Arc, a few CART neurons coexpress neurotensin (NT) mRNA. In addition, we examined the distribution of CART immunoreactivity in the human hypothalamus. We found CART cell bodies in the PVH, in the SON, in the LHA, in the Arc (infundibular nucleus) and in the DMH. We also observed CART fibers throughout the hypothalamus, in the bed nucleus of the stria terminalis, and in the amygdala. Our results indicate that leptin directly acts on CART neurons in distinct nuclei of the rat hypothalamus. Furthermore, hypothalamic CART neurons coexpress neuropeptides involved in energy homeostasis, including MCH, TRH, DYN, and NT. The distribution of CART cell bodies and fibers in the human hypothalamus indicates that CART may also play a role in the regulation of energy homeostasis in humans. J. Comp. Neurol. 432:1–19, 2001.

Chemical characterization of leptin-activated neurons in the rat brain

Leptin has profound effects on food intake, body weight, and neuroendocrine status. The lack of leptin results in hormonal and metabolic alterations and a dramatic increase in body weight. Leptin acts in the brain, especially in the hypothalamus; however, the central nervous system sites that respond to leptin have not been examined comprehensively. In this study, we explored systematically the distribution of leptin‐activated neurons throughout the rat brain. Furthermore, we investigated the chemical identity of subsets of these leptin‐activated cells. Fos‐like immunoreactivity (Fos‐IR) was investigated in the rat brain after two different doses of leptin (1.0 mg/kg and 5.0 mg/kg) at 2 hours and 6 hours after injections. The induction of Fos‐IR was observed in hypothalamic nuclei, including the paraventricular nucleus (PVH), the retrochiasmatic area (RCA), the ventromedial nucleus (VMH), the dorsomedial nucleus (DMH), the arcuate nucleus (Arc), and the ventral premammillary nucleus (PMV). In addition, leptin‐induced Fos‐IR was found in several nuclei of the brainstem, including the superior lateral and external lateral subdivisions of the parabrachial nucleus (slPB and elPB, respectively), the supragenual nucleus, and the nucleus of the solitary tract (NTS). By using double‐labeling immunohistochemistry or immunohistochemistry coupled with in situ hybridization, leptin‐activated neurons were found that contained cocaine‐ and amphetamine‐regulated transcript mRNA in several hypothalamic nuclei, including the RCA, Arc, DMH, and PMV. In the Arc and DMH, leptin‐induced Fos‐IR was observed in neurons that expressed neurotensin mRNA. Dynorphin neurons in the VMH and in the Arc also expressed Fos‐IR. In the brainstem, we found that cholecystokinin neurons in the slPB and glucagon‐like peptide‐1 neurons in the NTS were activated by leptin. We also investigated the coexpression of Fos‐IR and the long form of the leptin receptor (OBRb) mRNA. We found double‐labeled neurons surrounding the median eminence and in the RCA, Arc, VMH, DMH, and PMV. However, in brainstem sites, very little OBRb mRNA was found; thus, there were very few double‐labeled cells. These results suggest that leptin stimulates brain pathways containing neuropeptides that are involved in the regulation of energy balance, autonomic homeostasis, and neuroendocrine status. J. Comp. Neurol. 423:261–281, 2000.

Distinct Physiologic and Neuronal Responses to Decreased Leptin and Mild Hyperleptinemia

Leptin acts on specific populations of hypothalamic neurons to regulate feeding behavior, energy expenditure, and neuroendocrine function. It is not known, however, whether the same neural circuits mediate leptin action across its full biologic dose-response curve, which extends over a broad range, from low levels seen during starvation to high levels characteristic of obesity. Here, we show that the characteristic fall in leptin with fasting causes a rise in neuropeptide Y (NPY) messenger RNA (mRNA), as well as a fall in POMC and cocaine and amphetamine-regulated transcript (CART) mRNAs. Sc infusion of leptin sufficient to maintain plasma levels within the physiologic range during the fast prevents changes in the expression of these peptides, as well as changes in neuroendocrine function, demonstrating that multiple neural circuits are highly sensitive to small changes in leptin within its low physiologic range. In contrast, a modest elevation of plasma leptin above the normal fed range by constant sc in...

Neuropeptide Y Has a Central Inhibitory Action on the Hypothalamic-Pituitary-Thyroid Axis.

Recent evidence suggests that neuropeptide Y (NPY), originating in neurons in the hypothalamic arcuate nucleus, is an important mediator of the effects of leptin on the central nervous system. As these NPY neurons innervate hypophysiotropic neurons in the hypothalamic paraventricular nucleus (PVN) that produce the tripeptide, TRH, we raised the possibility that NPY may be responsible for resetting of the hypothalamic-pituitary-thyroid (HPT) axis during fasting. To test this hypothesis, the effects of intracerebroventricularly administered NPY on circulating thyroid hormone levels and proTRH messenger RNA in the PVN were studied by RIA and in situ hybridization histochemistry, respectively. NPY administration suppressed circulating levels of thyroid hormone (T(3) and T(4)) and resulted in an inappropriately normal or low TSH. These alterations were associated with a significant suppression of proTRH messenger RNA in the PVN, indicating that NPY infusion had resulted in a state of central hypothyroidism. Similar observations were made in NPY-infused animals pair fed to the vehicle-treated controls. These data are reminiscent of the effect of fasting on the thyroid axis and indicate that NPY may play a major role in the inhibition of HPT axis during fasting.

Relationship of EP1‐4 prostaglandin receptors with rat hypothalamic cell groups involved in lipopolysaccharide fever responses

The action of prostaglandin E2 (PGE2) in the preoptic area is thought to play an important role in producing fever. Pharmacologic evidence suggests that, among the four subtypes of E‐series prostaglandin (EP) receptors, i.e., EP1, EP2, EP3, and EP4, the EP1 receptor mediates fever responses. In contrast, evidence from mice with EP receptor gene deletions indicates that the EP3 receptor is required for the initial (<1 hour) fever after intravenous (i.v.) lipopolysaccharide (LPS). To investigate which subtypes of EP receptors mediate systemic infection‐induced fever, we assessed the coexpression of Fos‐like immunoreactivity (Fos‐IR) and EP1‐4 receptor mRNA in nuclei in the rat hypothalamus that have been shown to be involved in fever responses. Two hours after the administration of i.v. LPS (5 μg/kg), Fos‐IR was observed in the ventromedial preoptic nucleus, the median preoptic nucleus, and the paraventricular hypothalamic nucleus. In these nuclei, EP4 receptor mRNA was strongly expressed and the Fos‐IR intensely colocalized with EP4 receptor mRNA. Strong EP3 receptor mRNA expression was only seen within the median preoptic nucleus but Fos‐IR showed little coexpression with EP3 receptor mRNA. EP2 receptor mRNA was not seen in the PGE2 sensitive parts of the preoptic area. Although approximately half of the Fos‐immunoreactive neurons also expressed EP1 receptor mRNA, EP1 mRNA expression was weak and its distribution was so diffuse in the preoptic area that it did not represent a specific relationship. In the paraventricular nucleus, EP4 mRNA was found in most Fos‐immunoreactive neurons and levels of EP4 receptor expression increased after i.v. LPS. Our findings indicate that neurons expressing EP4 receptor are activated during LPS‐induced fever and suggest the involvement of EP4 receptors in the production of fever. J. Comp. Neurol. 428:20–32, 2000.

Activation of SOCS-3 messenger ribonucleic acid in the hypothalamus by ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) is a neurocytokine expressed in glial cells that acts on brain cells to promote gene expression, survival, and differentiation. When administered systemically, CNTF reduces food intake and body weight in rodents. Genes encoding suppressors of cytokine signaling (SOCS) are induced by cytokines that activate membrane receptors in the same class as those that are activated by CNTF. We therefore examined the ability of CNTF to induce expression of socs genes in brain and peripheral tissues of rats and mice. Peripheral CNTF administration to ob/ob mice rapidly induced SOCS-3 messenger RNA (mRNA) in hypothalamus, as determined by Northern blotting and quantitative RT-PCR, but had no effect on cytokine-inducible sequence (CIS), SOCS-1, or SOCS-2 mRNA. In situ hybridization histochemistry of hypothalamus from ob/ob mice and normal rats demonstrated that CNTF induced SOCS-3 mRNA in the arcuate nucleus (Arc). Strong hybridization signals were also detected in the ependymal lining ...