Nicotine's attenuation of body weight involves the perifornical hypothalamus

Abstract

Previously we showed that intermittent administration of nicotine (NIC) in the dark phase decreased food intake and body weight and this could be blocked when the NIC receptor antagonist mecamylamine was infused into the fourth ventricle. Catecholaminergic neurons adjacent to the fourth ventricle contain NIC receptors and directly innervate the perifornical hypothalamus (PFH) which has been shown to be involved in regulation of feeding. This study explored whether NIC regulates feeding behavior by modulating catecholaminergic input to the PFH. Epinephrine and norepinephrine neuronal input was ablated within the PFH by infusion of 6-hydroxydopamine hydrobromide (6-OHDA), while bupropion was infused to protect dopaminergic neurons. After recovery of body weights to pre-surgery levels, food intake, meal size, meal number and body weight were measured after intermittent NIC injections. The results showed the PFH lesioned animals did not exhibit the typical prolonged drop in food intake, meal size and body weight normally associated with NIC administration. High performance liquid chromatography analyses demonstrated that compared to control rats, 6-OHDA administration significantly reduced PFH norepinephrine and epinephrine levels, but not dopamine levels. These results are consistent with NIC reducing food intake in part by acting through catecholaminergic neurons within or extending through the PFH.

Figure 1

Panel A is a schematic drawing of parasagittal rat brain showing phenylethanolamine N-methyltransferase (PNMT) positive fibers (black lines and arrows) originating from PNMT positive cells (dots) in the C1, C2, and C3 regions extending into the hypothalamus (HY). PNMT fibers from the C1, C2, and C3 fasciculate to form part of the medial forebrain bundle (thick black line). Panel B shows a coronal section at the level of the paraventricular hypothalamic nucleus (PVN). The PVN extends anterior-posterior −1.3 to −2.3 mm from Bregma. Panel C shows a coronal section at the level of the dorsomedial hypothalamic nucleus (DMN). The DMN extends anterior-posterior −2.5 to −3.6 from Bregma. Thick straight lines in panels B and C indicate cannula placement (arrowhead). Density of PNMT fibers staining was arbitrarily graded as +3 most intense, +2 less intense and +1 least intense as described in (Hokfelt et al., 1984). PNMT fibers in cross section are indicated by plus signs (axons cross) and PNMT fibers longitudinally visualized (see key =axons long.) were marked as thick wavy lines. F= fornix, FMT= fasciculus mamillothalamicus and PFH= perifornical hypothalamic area. Redrawn with permission from Elsevier Publishing.

Figure 2

Changes in tissue content of catecholamines measured in the perifornical hypothalamus (PFH) after lesions (LESION) were produced by infusing 4 μg of 6-OHDA (0.2 μl) per site into the PFH bilaterally in two anterior-posterior locations, one at the level of the PVN and the other at the level of the DMN. Sham lesioned animals (SHAM) had the same amount of vehicle infused. Values are given as the amount of catecholamine (pg) in an mg of brain tissue. Comparing SHAM (n = 8 and LESION (n -= 10) groups an asterisk indicates p<0.05. Means ± SEM. See Fig. 1 legend for abbreviations.

Figure 3

Mean group 24 hour food intake. Food intake, in grams, was recorded for one (−1) and two days (−2) before nicotine injection and 14 days post injection in sham lesion and i.p. injected with saline (SHAM/SALINE, n = 6); sham lesioned and i.p. injected with nicotine (SHAM/NICOTINE, n = 6); PFH lesioned and i.p. injected with saline (LESIONED/SALINE, n = 8) and PFH lesioned and i.p. injected with nicotine (LESIONED/NICOTINE, n = 10) animals. Nicotine (1.4 mg/kg/day, free base.) or a similar volume of 0.9% saline was given i.p. in four equal increments throughout the dark phase. Values on the X-axis are represent days before (−2, −1) or after nicotine injection (1, 2, etc.). Lettering indicates a significant difference between groups of at least p<0.05; a = SHAM/SALINE vs SHAM/NICOTINE; b = SHAM/SALINE vs LESION/NICOTINE; c = SHAM/NICOTINE vs LESION/NICOTINE, d = LESION/SALINE vs SHAM/NICOTINE. See Fig. 1 and 2 legends for abbreviations and explanations.

Figure 4

Mean group 24 hour meal size. Lettering indicates a significant difference between groups of at least p<0.05; a = SHAM/SALINE vs SHAM/NICOTINE; b = SHAM/SALINE vs LESION/SALINE; c = LESION/SALINE vs LESION/NICOTINE; d = SHAM/NICOTINE vs LESION/NICOTINE. See Fig. 1 and 2 legends for abbreviations and explanations.

Figure 5

Mean group 24 hour meal number. Lettering indicates a significant difference between groups of at least p<0.05; b = SHAM/SALINE vs LESION/SALINE; c = LESION/SALINE vs LESION/NICOTINE; d = SHAM/NICOTINE vs LESION/NICOTINE. See Fig. 1 and 2 legends for abbreviations and explanations.

Figure 6

Mean group change in body weight (in g) for non-lesioned rats (SHAM) and lesioned rats (LESION). The change in body weight (GRAMS DIFFERENCE) on the 14 days post injection was calculated by subtracting the body weight of that day from the body weight of that animal one day before NIC injection (−1). Lettering indicates a significant difference between groups of at least p<0.05; a = SHAM/SALINE vs SHAM/NICOTINE; b = SHAM/SALINE vs LESION/NICOTINE, c = LESION/SALINE v LESION/NICOTINE; c = SHAM/NICOTINE vs LESION/NICOTINE. See Fig. 1 and 2 legends for abbreviations and explanations.