Inhibition of the firing activity of locus coeruleus neurons by EP3 receptors in rat brain slices

A. Nazabal, A. Mendiguren, J. Pineda. Department of Pharmacology, Faculty of Medicine and Nursing, University of the Basque Country, Leioa, SPAIN.

Introduction: Prostaglandin E2 (PGE2) is involved in inflammation and other physiological functions. PGE2 receptors (EP) are members of the G protein-coupled receptor family and comprise four subtypes: EP2 and EP4 (coupled to Gs proteins), EP1 (coupled to Gq proteins) and EP3 (coupled to Gi/o proteins). The synthetic enzyme for PGE2 COX-2 has been described to be constitutively present in the brain1. However, the function of prostanoid system in the brain is not yet well understood. The locus coeruleus (LC), the main noradrenergic nucleus of the brain, has high expression of the EP3 receptor2. Thus, our aim was to characterise pharmacologically the EP3 receptors in LC neurons by electrophysiological recordings in rat brain slices.

Method: Adult male Sprague-Dawley rats were anaesthetised with chloral hydrate (400 mg/kg i.p.) and decapitated. Brain pontine slices (600 µm) were obtained as previously described3. Single-unit extracellular recordings of LC neurons were carried out in vitro. We performed concentration-effect curves for different agonists of the EP3 receptor until a maximal effect was reached, including sulprostone (EP1/EP3 agonist, 0.3-80 nM), the endogenous PGE2 (0.3 nM-1.28 µM) and the PGE1analogue misoprostol (0.3-320 nM). The involvement of EP3 receptor was tested by perfusing the selective EP3 receptor antagonist L-798,106 (3-10 µM), and compared to selective antagonists of EP2 receptors (PF-04418948, 10 µM) and EP4 receptors (L-161982, 10 µM). Data was analysed by one-way ANOVA followed by Dunnett's post hoc test.

Results: Increasing concentrations of sulprostone completely inhibited the firing rate of LC cells, with the EC50 being 15 nM (n = 9). The EP3 receptor antagonist L-798,106 induced a rightward shift (>8 fold) in the concentration-effect curve for sulprostone (n = 6, p < 0.001), but neither PF-04418948 nor L-161982 caused similar changes in the sulprostone effect (n = 4). Likewise, perfusion with PGE2 or misoprostol induced a concentration-dependent inhibition of the neuronal activity of LC cells (EC50 = 51 and 112 nM; n = 5 and n = 5; respectively). In both cases, only the EP3 antagonist L-798,106 caused a rightward shift (>8 fold) in the concentration-effect curves for the prostanoid agonists (n = 5, p < 0.001).

Conclusion: LC neurons are regulated in an inhibitory manner by the prostanoid system, likely through the EP3 receptor.

References: 1. Hétu PO and Riendeau D (2005). Biochem J 391: 561-566. 2. Ek M et al. (2000). J Comp Neurol 428: 5-20. 3. Pablos P et al. (2015). Neuropharmacology 99: 422-431.