title: Programming of formalin-induced nociception by neonatal LPS exposure: Maintenance by peripheral and central neuroimmune activity.
abstract: The immune and nociceptive systems are shaped during the neonatal period where they undergo fine-tuning and maturation. Painful experiences during this sensitive period of development are known to produce long-lasting effects on the immune and nociceptive responses. It is less clear, however, whether inflammatory pain responses are primed by neonatal exposure to mild immunological stimuli, such as with lipopolysaccharide (LPS). Here, we examine the impact of neonatal LPS exposure on inflammatory pain responses, peripheral and hippocampal interleukin-1β (IL-1β), as well as mast cell number and degranulation in preadolescent and adult rats. Wistar rats were injected with LPS (0.05mg/kg IP, Salmonella enteritidis) or saline on postnatal days (PNDs) 3 and 5 and later subjected to the formalin test at PNDs 22 and 80-97. At both time-points, and one-hour after formalin injection, blood and hippocampus were collected for measuring circulating and central IL-1β levels using ELISA and Western blot, respectively. Paw tissue was also isolated to assess mast cell number and degree of degranulation using Toluidine Blue staining. Behavioural analyses indicate that at PND 22, LPS-challenged rats displayed enhanced flinching (p<.01) and licking (p<.01) in response to formalin injection. At PNDs 80-97, LPS-challenged rats exhibited increased flinching (p<.05), an effect observed in males only. Furthermore, neonatal LPS exposure enhanced circulating IL-1β and mast cell degranulation in preadolescent but not adult rats following formalin injection. Hippocampal IL-1β levels were increased in LPS-treated adult but not preadolescent rats in response to formalin injection. These data suggest neonatal LPS exposure produces developmentally regulated changes in formalin-induced behavioural responses, peripheral and central IL-1β levels, as well as mast cell degranulation following noxious stimulation later in life. These findings highlight the importance of immune activation during the neonatal period in shaping immune response and pain sensitivity later in life. This is of clinical relevance given the high prevalence of bacterial infection during the neonatal period, particularly in the vulnerable population of preterm infants admitted to neonatal intensive care units.
authors.affiliation: Laboratory of Neuroimmunology, School of Psychology, University of Newcastle, Newcastle, New South Wales, Australia; School of Biomedical Sciences and Pharmacy, University of Newcastle, Newcastle, New South Wales, Australia; Hunter Medical Research Institute, Newcastle, New South Wales, Australia. Electronic address: Ihssane.Zouikr@uon.edu.au.
authors.forename: Ihssane
authors.initials: I
authors.lastname: Zouikr
authors.affiliation: School of Biomedical Sciences and Pharmacy, University of Newcastle, Newcastle, New South Wales, Australia.
authors.forename: Abdulrzag F
authors.initials: AF
authors.lastname: Ahmed
authors.affiliation: School of Biomedical Sciences and Pharmacy, University of Newcastle, Newcastle, New South Wales, Australia; Hunter Medical Research Institute, Newcastle, New South Wales, Australia.
authors.forename: Jay C
authors.initials: JC
authors.lastname: Horvat
authors.affiliation: Institute of Health Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia.
authors.forename: Kenneth W
authors.initials: KW
authors.lastname: Beagley
authors.affiliation: Robinson Institute, University of Adelaide, Adelaide, South Australia, Australia.
authors.forename: Vicki L
authors.initials: VL
authors.lastname: Clifton
authors.affiliation: Laboratory of Neuroimmunology, School of Psychology, University of Newcastle, Newcastle, New South Wales, Australia.
authors.forename: Allyson
authors.initials: A
authors.lastname: Ray
authors.affiliation: School of Biomedical Sciences and Pharmacy, University of Newcastle, Newcastle, New South Wales, Australia; Hunter Medical Research Institute, Newcastle, New South Wales, Australia.
authors.forename: Rick F
authors.initials: RF
authors.lastname: Thorne
authors.affiliation: Hunter Medical Research Institute, Newcastle, New South Wales, Australia.
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Jarnicki
authors.affiliation: Hunter Medical Research Institute, Newcastle, New South Wales, Australia.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Hansbro
authors.affiliation: Laboratory of Neuroimmunology, School of Psychology, University of Newcastle, Newcastle, New South Wales, Australia.
authors.forename: Deborah M
authors.initials: DM
authors.lastname: Hodgson
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D002452:Cell Count; D015550:Cell Degranulation; D004660:Encephalitis / Q000139:chemically induced / Q000276:immunology* / Q000378:metabolism; D005260:Female; D005557:Formaldehyde; D006624:Hippocampus / Q000276:immunology / Q000378:metabolism; D007249:Inflammation / Q000139:chemically induced / Q000276:immunology / Q000378:metabolism; D053583:Interleukin-1beta / Q000097:blood / Q000276:immunology; D008070:Lipopolysaccharides; D008297:Male; D008407:Mast Cells / Q000276:immunology / Q000502:physiology; D009043:Motor Activity; D059225:Nociception / Q000502:physiology*; D010146:Pain / Q000139:chemically induced / Q000276:immunology*; D010147:Pain Measurement; D051381:Rats; D017208:Rats, Wistar
chemical_list: D053583:Interleukin-1beta; D008070:Lipopolysaccharides; D005557:Formaldehyde
keywords: Cytokines; Degranulation; Formalin test; Hippocampus; IL-1β; Inflammation; LPS; Mast cell; Neonatal immune challenge; Pain