Corticotropin-releasing factor and systemic capsaicin-sensitive afferents are involved in abdominal surgery-induced Fos expression in the paraventricular nucleus of the hypothalamus

Abstract

We previously reported that abdominal surgery induces Fos expression in specific hypothalamic and medullary nuclei and also causes gastric stasis. The gastric ileus is reduced by systemic capsaicin and abolished by central injection of corticotropin-releasing factor (CRF) antagonist. We studied the influence of systemic capsaicin and intracerebroventricular (i.c.v.) injection of the CRF antagonist, α-helical CRF_9–41, on Fos expression in the brain 1 h after abdominal surgery in conscious rats using immunocytochemical detection. In control groups (vehicle s.c. or i.c.v.), abdominal surgery (laparotomy with cecal manipulation) performed under 7–8 min of enflurane anesthesia induced Fos staining in neurons of the spinal trigeminal, C1/A1 group, ventrolateral medulla, central amygdala, parabrachial nucleus, cuneate nucleus, nucleus tractus solitarii (NTS), paraventricular nucleus of the hypothalamus (PVN) and supraoptic nucleus (SON). Capsaicin (125 mg/kg s.c., 2 weeks before) or α-helical CRF_9–41 (50 μg i.c.v., before surgery) reduced the number of Fos-positive cells by 50% in the PVN while not modifying the number of Fos-labelled cells in the other nuclei. These results indicate that capsaicin-sensitive primary afferents and brain CRF receptors are part of the pathways and biochemical coding through which abdominal surgery activates PVN neurons 1 h post surgery.

Introduction

Fos is a nuclear phosphoprotein product of the immediate–early gene c-fos. Together with c-jun, c-fos regulates the transcription of genes having an AP-1 consensus element and mediates the genomic response to extracellular neuronal stimuli 50, 51, 64. In addition, the induction of Fos protein is increasingly used to identify activated neurons that participate in uni- and multi-synaptic pathways in the central nervous system 24, 50, 51. Numerous stressful stimuli of somato-cutaneous or visceral origin and anesthetics induce Fos expression in a discrete population of spinal and brain neurons and activate hypothalamic corticotropin-releasing factor (CRF) containing neurons 15, 20, 27, 35, 36, 46. We recently defined the pattern of Fos expression in the brain induced 1–3 h after abdominal surgery performed in rats under short enflurane anesthesia 6, 10. In particular, Fos-positive neurons were observed in the nucleus tractus solitarii (NTS) recognized to integrate somato- and viscero-visceral reflexes and the paraventricular nucleus of the hypothalamus (PVN), supraoptic nucleus (SON) and central amygdala known to participate in the endocrine and autonomic responses to stress 45, 52, 55, 65, 66, 70. However, the neuronal pathways and biochemical events whereby abdominal surgery activates these brain nuclei are still unknown.

Compelling studies established that capsaicin-sensitive splanchnic primary afferent fibers and activation of supraspinal CRF receptors play a role in mediating abdominal surgery-induced gastric stasis in rats 6, 7, 33, 34, 57. Systemic capsaicin or capsaicin applied on the celiac/superior mesenteric ganglia, known to alter selectively unmyelinated primary afferent terminals [32], reduced the delay in gastric emptying induced by abdominal surgery 6, 7, 31, 33, 34, 57, 67. Moreover, the CRF antagonists, α-helical CRF_9–41, [d-Phe¹², Nle^21,38, C^α-MeLeu³⁷] and astressin 26, 31, 47, 59, injected intracisternally prevented the delay in gastric emptying induced by abdominal surgery in rats 6, 31, 44, 67. These functional observations suggest that capsaicin-sensitive afferents and CRF receptors may modulate the activity of a subset of neurons in the brain, particularly those which play a role in mediating postoperative gastric ileus in rats. In support of this possibility, there is evidence that the central injection of CRF induced Fos expression in specific hypothalamic, limbic, pontine and medullary nuclei 3, 5, 19, 37, 53. In addition, the CRF antagonist injected intracerebroventricularly (i.c.v.) prevents the activation of locus coeruleus and PVN neurons induced by hemodynamic or psychological (restraint) stress 12, 22, 38, 69. Systemic capsaicin was also found to attenuate Fos expression in the spinal cord and brain neurons in response to chemical nociceptive visceral stimuli 8, 11. Therefore, in the present study we investigated whether capsaicin pretreatment and α-helical CRF_9–41 injected i.c.v. influence the activation of specific brain neurons induced by abdominal surgery performed under short anesthesia in rats.