Role of hypothalamic prostaglandin production and signalling in regulating energy balance during sickness

Abstract

Under normal physiological conditions, hypothalamus maintains energy homeostasis, through a neuronal network that produces orexigenic and anorexigenic neuropeptides such as NPY and POMC respectively. During sickness, inflammatory mediators released by the host immune system alter the interplay of these neuronal systems, leading to a state of negative energy balance. Prostaglandins (PG) primarily of the E2 type play a significant role in altering these changes in energy balance within the hypothalamus. Production of PG depends on the expression of cyclooxygenase (COX) enzymes, of which there are 2 types. PGE2 binds to 4 receptor subtypes termed EP1-4, expressed throughout the brain. The following thesis project aims to characterize the role of COX-2 dependent PG production and signalling during sickness induced states of negative energy balance, with a focus on hypothalamic neuropeptide systems as key mediators. We hypothesized that during sickness induced-inflammatory state, energy balance is regulated by the modulation of PGE2 production and signaling through hypothalamic neuropeptide systems. The present project also aims to elucidate the distribution of EP receptors and its region-specific role in regulating energy balance with an emphasis on EP3 and EP4 distribution within hypothalamic regions known to regulate energy balance. Furthermore we will also attempt to understand the immunosuppressive effects of PGE2 on hypothalamic microglial activity. C57/bl6 mice were pretreated with a diet inclusive of the COX-2 specific inhibitor, celecoxib (50mg/day/kg of b.w) and injected with either saline or LPS (100µg/kg of b.w) and perfused 4 hrs after for gene expression analysis. The indices of energy balance (food intake, locomotor activity, 02 consumption, C02 Expiration) were measured 24hrs before and after the LPS/Veh injections. Similarly, Mice implanted with intracerebroventricular cannula were injected with PGE2 (100nml/animal) or saline, and perfused 4hrs post injections for gene expression analysis. Indices of energy balance were also monitored 24hrs before and after injections. Pretreatment with celecoxib attenuated LPS induced fever, hypophagia, and delayed decreases in RER and activity, suggesting COX-2 is involved in regulating delayed inflammatory signaling in response to LPS. Similar decrease in indices of energy balance were observed in mice given central infusions of PGE2, confirming the critical role of PGE2 in regulating energy balance during sickness. In situ hybridizations and immunohistochemistry revealed that COX-2 dependent PGE2 is capable of regulating key hypothalamic neural systems (POMC/NPY) in order to drive the changes in energy balance. In addition COX-2 inhibition attenuated LPS-induced increase in EP4 receptor within Arc and PVN, suggesting that COX-2 derived PGE2 is the primary regulator of this receptor. Furthermore our results suggest that in acute inflammation, PGE2 is likely to induce microglial activity, while at rest endogenous PGE2 may inhibit glial activity, suggesting a dose dependent immunosuppressive function of PGE2. In conclusion, the studies described in this thesis, highlights the critical role of COX-2 derived PGE2 in energy balance regulation during acute sickness along with the possible neural circuitry, and receptors involved. Further investigations are required to elucidate the specific targets of PGE2 and identify the receptors and neuronal populations involved in regulating energy balance.