Lesion of the lateral parabrachial nucleus attenuates the anorectic effect of peripheral amylin and CCK

Abstract

Amylin and CCK activate the area postrema (AP)/nucleus of the solitary tract (NTS) – lateral parabrachial nucleus (LPBN) – central amygdala (CeA) pathway. However, except for the brainstem structures the role of these nuclei for the anorectic effect of these peptides is not yet well characterized. The current study investigated the role of the LPBN in mediating the inhibitory effect of peripheral amylin and CCK on feeding behavior. Rats with electrolytic lesions in the LPBN (LPBN-X) were used in behavioral as well as in immunohistological c-Fos studies. LPBN-X significantly reduced the anorectic effect of amylin (5 μg/kg, i.p.). The effect of a higher amylin dose (10 μg/kg, i.p.) was only slightly attenuated in the LPBN-X rats. In agreement with previous studies, LPBN lesions also reduced the inhibitory effect of CCK on food intake. In the immunohistological experiments, amylin and CCK induced c-Fos expression in the AP, NTS, LPBN and CeA in the SHAM rats. Both the amylin- and CCK-induced activation of the CeA was completely abolished in the animals with a LPBN lesion. These results clearly suggest that the signal transduction pathway between the AP/NTS and CeA has been disrupted by the LPBN ablation. We conclude that the LPBN is a crucial brain site mediating the anorectic effect of amylin and CCK. Furthermore, an intact LPBN seems to be essential for the c-Fos response in the CeA induced by these peptides.

Introduction

A well-established neuronal axis regulating ingestive behavior comprises the area postrema (AP), the nucleus of the solitary tract (NTS), the lateral parabrachial nucleus (LPBN) and the central nucleus of the amygdala (CeA). Based on functional c-Fos studies, this pathway is hypothesized to mediate the satiating effect of the pancreatic hormone amylin (Rowland and Richmond, 1999, Rowland et al., 1997, Riediger et al., 2004). Cholecystokinin (CCK), which is released prandially from the endocrine cells of the small intestine, activates a similar central pathway when administered peripherally (Day et al., 1994, Li and Rowland, 1994, Li and Rowland, 1995, Mönnikes et al., 1997, Mayne et al., 1998).

Despite the similarities, there is a fundamental difference between the action of amylin and CCK at the level of their primary target sites. While the AP in the brainstem is believed to function as the sensory interface for circulating amylin (Lutz et al., 1998b, Lutz et al., 2001, Rowland and Richmond, 1999, Riediger et al., 2004), CCK at least in rats seems to act mainly in a paracrine manner on gastrointestinal vagal afferents to reduce food intake (for review see e.g. Ritter et al., 1999). The vagal afferents make their initial synapses in the dorsal vagal complex of the brainstem, involving mainly the NTS but also the AP (Shapiro and Miselis, 1985a, Shapiro and Miselis, 1985b). Thus in the ascending pathway of CCK-induced satiation, the NTS represents the primary relay station in the brain. CCK seems to modulate AP activity indirectly, through vagal inputs and through the NTS (for review see Ritter et al., 1999). The facts that vagotomy abolishes the CCK-induced c-Fos expression in the brain (Day et al., 1994, Li and Rowland, 1995) and that a specific AP lesion does not block the anorectic effect of CCK (Edwards et al., 1986) strongly support that notion. On the other hand, the anorectic effect of amylin seems to be independent of vagal or other sensory abdominal inputs (Morley et al., 1994, Lutz et al., 1994, Lutz et al., 1995, Lutz et al., 1998a). Hence, the amylin-induced activation of the NTS is believed to be secondary, due to neuronal inputs from the AP (Riediger et al., 2001, Riediger et al., 2004).

Several studies have shown that both AP and NTS neurons project to the LPBN (Norgen, 1978, Voshart and Van Der Kooy, 1981, Hermann and Rogers, 1985, Shapiro and Miselis, 1985a, Shapiro and Miselis, 1985b), and the latter to the CeA (Block et al., 1989). Since peripheral amylin and CCK both activate the LPBN and CeA (Day et al., 1994, Li and Rowland, 1994, Li and Rowland, 1995, Rowland and Richmond, 1999, Rowland et al., 1997, Mayne et al., 1998, Riediger et al., 2004) it seems plausible that these nuclei are linearly connected forming an AP/NTS–LPBN–CeA neuronal axis that processes the amylin and CCK signal. However, the role of the nuclei downstream from the AP/NTS region in the feeding-related effects of amylin and CCK is not clear. Concerning CCK, Trifunovic and Reilly, 2001, Trifunovic and Reilly, 2003 have shown that the effect of LPBN ablation (LPBN-X) on CCK-induced satiation depends on the experimental conditions, most importantly on the texture of the food. In these studies, LPBN-X blocked the effect of CCK only when solid rodent chow was presented to the rats, but not when a liquid milk diet was offered. Studies investigating the role of the LPBN in the anorectic effect of amylin are missing.

Therefore, the aim of our study was first, to investigate the role of the LPBN in mediating the anorectic effect of amylin. Second, we wanted to test whether LPBN-X blocks the satiating effect of CCK under our experimental conditions, i.e. when rats are fed powdered medium-fat diet. Furthermore, we hypothesized that the flow of afferent information is directed from the AP/NTS region via the LPBN towards the CeA. Thus, if the LPBN as the intermediate relay station is eliminated from this pathway through ablation, the amylin- and CCK-induced neuronal activation would not be transmitted to the CeA. This should result in an attenuated c-Fos expression in the CeA in amylin- or CCK-treated LPBN-lesioned animals. Further, assuming that the neuronal activation in the AP/NTS-LPBN-CeA pathway correlates with the anorectic action of amylin and CCK, the disruption of this pathway should also reduce the anorectic effect of these peptides. To test these hypotheses, we conducted feeding and c-Fos immunohistochemical studies with LPBN-lesioned and sham-operated (SHAM) rats, injected peripherally with amylin or CCK.