Lipopolysaccharide-induced leptin release is neurally controlled

Abstract

Our hypothesis is that leptin release is controlled neurohormonally. Conscious, male rats bearing indwelling, external, jugular catheters were injected with the test drug or 0.9% NaCl (saline), and blood samples were drawn thereafter to measure plasma leptin. Anesthesia decreased plasma leptin concentrations within 10 min to a minimum at 120 min, followed by a rebound at 360 min. Administration (i.v.) of lipopolysaccharide (LPS) increased plasma leptin to almost twice baseline by 120 min, and it remained on a plateau for 360 min, accompanied by increased adipocyte leptin mRNA. Anesthesia largely blunted the LPS-induced leptin release at 120 min. Isoproterenol (beta-adrenergic agonist) failed to alter plasma leptin but reduced LPS-induced leptin release significantly. Propranolol (beta-receptor antagonist) produced a significant increase in plasma leptin but had no effect on the response to LPS. Phentolamine (alpha-adrenergic receptor blocker) not only increased plasma leptin (P < 0.001), but also augmented the LPS-induced increase (P < 0.001). alpha-Bromoergocryptine (dopaminergic-2 receptor agonist) decreased plasma leptin (P < 0.01) and blunted the LPS-induced rise in plasma leptin release (P < 0.001). We conclude that leptin is at least in part controlled neurally because anesthesia decreased plasma leptin and blocked its response to LPS. The findings that phentolamine and propranolol increased plasma leptin concentrations suggest that leptin release is inhibited by the sympathetic nervous system mediated principally by alpha-adrenergic receptors because phentolamine, but not propranolol, augmented the response to LPS. Because alpha-bromoergocryptine decreased basal and LPS-induced leptin release, dopaminergic neurons may inhibit basal and LPS-induced leptin release by suppression of release of prolactin from the adenohypophysis.

Figure 1

The effect of ketamine/acepromazine/xylazine anesthesia (90 + 2 + 6 mg/ml, respectively; 0.1 ml/100 g body weight) on leptin release 15 min after injection. The number of animals (n) = 13 in both groups. In this and subsequent figures, bars or symbols represent the mean response at each time point. The vertical line above or below the bars or symbols represents 1 SEM. ***, P < 0.001 vs. saline.

Figure 2

The effect of ketamine/acepromazine/xylazine anesthesia (90 + 2 + 6 mg/ml, respectively; 0.1 ml/100 g body weight) on leptin release. The number of animals (n) = 7 in the saline-treated and anesthetized groups; n = 6 in the saline + LPS-injected rats and in the anesthetic + LPS-treated group. a, P < 0.05 vs. saline; aa, P < 0.01 vs. saline; aaa, P < 0.001 vs. saline; b, P < 0.05 vs. ketamine alone; c, P < 0.05 vs. ketamine + LPS; cc, P < 0.01 vs. ketamine + LPS.

Figure 3

The effect of 10 mg/kg i.p. isoproterenol (Iso) (agonist β-adrenergic) on leptin release. n = 8 in the saline-treated rats and in the Iso-injected group; n = 7 in the saline + LPS treated-animals and in the Iso + LPS-injected rats. aa, P < 0.01 vs. saline; aaa, P < 0.001 vs. saline; b, P < 0.05 vs. Iso alone (25); bb, P < 0.01 vs. Iso alone; c, P < 0.05 vs. Iso + LPS.

Figure 4

The effect of 10 mg/kg propranolol (β-adrenergic antagonist) on leptin release. The number of animals (n) = 8 in the saline-injected group, n = 10 in the propranolol-treated animals, n = 9 in the saline + propranolol-injected animals, and n = 13 in the propranolol + LPS-treated rats. *, P < 0.05 vs. saline.

Figure 5

The effect of 10 mg/kg i.p. phentolamine (Phent) (α-adrenergic antagonist) on leptin release 10 min after injection. n = 17 in the saline-injected group and n = 11 in the phentolamine-treated group; **, P < 0.01 vs. saline.

Figure 6

The effect of 10-mg/kg i.p. phentolamine (Phent) (α-adrenergic antagonist) on leptin release. The number of animals (n) = 7 in the saline-injected group, n = 5 in the Phent-treated animals, n = 10 in the saline + LPS-injected animals, and n = 6 in the Phent + LPS-treated rats. a, P < 0.05 vs. saline; aa, P < 0.01 vs. saline; aaa, P < 0.001 vs. saline; bbb, P < 0.001 vs. Phent alone; c, P < 0.05 vs. saline + LPS; cc, P < 0.01 vs. saline + LPS. Standard error plotted below the mean is represented by a thin line; those above the mean are represented by thick lines.

Figure 7

The effect of 5 mg/kg i.v. α-bromoergocryptine (Bromo) (dopaminergic-2 agonist) on leptin release. The values are expressed as a percentage of the initial values and were obtained 15 min after i.p. injection of saline or Bromo. The number of animals (n) = 13 in both groups. **, P < 0.01 vs. saline.

Figure 8

The effect of 5 mg/kg i.v. bromocryptine (Bromo) (dopaminergic-2 agonist) on leptin release. The number of animals (n) = 7 in the saline-treated animals and the Bromo-injected group; n = 6 in the saline + LPS-treated rats and in the Bromo + LPS-injected animals. a, P < 0.05 vs. saline; aa, P < 0.01 vs. saline; aaa, P < 0.001 vs. saline; bb, P < 0.01 vs. Bromo alone; bbb, P < 0.001 vs. Bromo alone; c, P < 0.05 vs. Bromo + LPS; cc, P < 0.01 vs. Bromo + LPS; ccc, P < 0.001 vs. Bromo + LPS.

Figure 9

(A) Representative autoradiogram showing the detection of leptin (307 bases) mRNA expressed in total fat RNA (12 μg) from control (lanes 2–4) and LPS-treated rats (lanes 5–7) and digested cyclophilin (132 bases; lanes 2–7). Lane 1 illustrates the undigested, full-length RNA probes for leptin (376 bases) and cyclophilin (244 bases). In lane 8, where no hybridization is seen, 12 μg of yeast tRNA was used. (B) Composite drawings showing the densitometric quantitation of the bands from two blots corresponding to the fat leptin mRNA isolated from control and LPS (*, P < 0.05 vs. saline).

Figure 10

For description, see Discussion. Opened arrows indicate inhibition, and solid arrows indicate stimulation. DAn, tuberoinfundibullar dopaminergic neurons; DA, dopamine; ME, median eminence; PV, portal vessels; L, lactotropes; AP, anterior pituitary; PRL, prolactin; PRL_r, PRL receptors; D2_r, DA2 receptors; Lm, leptin mRNA; LV, leptin vesicles; A, adipocyte; E, epinephrine; NE, norepinephrine; A-α_r, α-adrenergic receptors; A-β_r, β-adrenergic receptors; AM, adrenal medulla; S, sympathetic.