Change in prostaglandin signaling during sickness syndrome hyperalgesia after ovariectomy in female rats

Highlights

• Prostaglandin(PG)-induced hyperalgesia in cycling and ovariectomized female rats was evaluated.

• PG-induced hyperalgesia was different in cycling and ovariectomized female rats.

• PG-induced signaling mechanism in cycling females involves cAMP-Epac.

• PG-induced signaling mechanism in ovariectomized females involves cAMP-PKA.

Abstract

The present study investigated hyperalgesia during sickness syndrome in female rats. Hyperalgesia was induced by an intraperitoneal injection of lipopolysaccharide (LPS) or an intracerebroventricular injection of prostaglandin E₂ (PGE₂). No differences were found in basal mechanical and thermal thresholds or in LPS-induced hyperalgesia in sham-operated animals in the diestrus or proestrus phase or in ovariectomized (OVX) animals. However, higher levels of PGE₂ where found in the cerebrospinal fluid of OVX animals compared to sham-operated females. Intracerebroventricular injection of PGE₂ produced rapid mechanical hyperalgesia in sham-operated rats while these responses were observed at later times in OVX animals. The protein kinase A (PKA) inhibitor H-89 reduced mechanical PGE₂-induced hyperalgesia in OVX female rats, whereas no effect was observed in sham-operated animals. In contrast, the exchange protein activated by cyclic adenosine monophosphate (cAMP; Epac) inhibitor ESI-09 reduced mechanical PGE₂-induced hyperalgesia, whereas no effect was observed in OVX animals. PGE₂ also induced thermal hyperalgesia in sham-operated and OVX female rats and a similar effect of ESI-09 was observed. These results suggest that PGE₂-induced hyperalgesia that is observed during sickness syndrome has different signaling mechanisms in cycling and OVX female rats involving the activation of the cAMP-Epac or cAMP-PKA pathways, respectively.

Introduction

Most people experience several episodes of acute infectious diseases during their lifetime. Despite the numerous organisms that cause such illnesses and different body tissues that are affected, a similar set of symptoms typically occurs. These symptoms include fever, achiness, hyperalgesia, loss of appetite, and sleepiness, among other autonomic, endocrine, and behavioral changes that are an adaptive response to fight the infection. This condition is called sickness syndrome [1]. Sex has emerged as an important factor in the incidence and progression of diseases that are associated with the immune system, particularly inflammation, with important implications for developing better therapeutic approaches [2]. The recognition that pain perception depends on a dynamic balance between inhibitory and excitatory mechanisms underscores the issue of individual differences in pain, in which men and women do not experience pain equally [3]. Sexual dimorphism in pain has been extensively reported. Although the underlying causes of such differences remain incompletely understood, sex hormones are certainly involved [4]. Therefore, menopause may represent a different phase of pain perception in females, with unique mechanisms.

Lipopolysaccharide (LPS) is a component of the outer membrane of Gram-negative bacteria. It is a commonly identified infectious pathogen-associated molecular pattern [5,6] and binds to Toll-like receptor 4, which leads to the activation of an inflammatory cascade. This inflammatory cascade can also affect the central nervous system and increase the expression, among other proteins, of cyclooxygenase (COX)-2 and microsomal prostaglandin E synthase-1, which will result in an increased production of prostaglandins in the brain [[7], [8], [9]]. Lipopolysaccharide exposure can produce all signs and symptoms of sickness syndrome, thus making systemic LPS administration a model for studying this condition [10].

Inflammatory symptoms in sickness syndrome, such as fever and hyperalgesia, are commonly treated with nonsteroidal antiinflammatory drugs (NSAIDs). These drugs reduce the production of prostanoids by blocking both COX-1 and COX-2. Among these, prostaglandin E₂ (PGE₂) is the most studied in pain processing [11]. PGE₂ promotes an increase in intracellular cyclic adenosine monophosphate (cAMP) and causes hyperalgesia, which can be blocked by the inactive cyclic adenosine 3′5′-monophosphate analog Rp-cAMP [12]. cAMP is synthesized from adenosine triphosphate by membrane-located enzymes. It acts by activating cAMP-dependent protein kinase A (PKA) to directly phosphorylate target proteins [13,14] or through actions on cyclic nucleotide-gated ion channels [15]. Additionally, cAMP can activate exchange protein directly activated by cAMP (Epac), leading to hyperalgesia [16].

The aim of the present study was to investigate mechanical and thermal hyperalgesia during sickness syndrome in cycling and ovariectomized (OVX) female rats and the possible influence of the estrous cycle on this condition. We also evaluated the role of PKA and Epac in mediating hyperalgesia that was induced by PGE₂ during sickness syndrome in OVX and cycling sham-operated female rats.