Convergent, not serial, striatal and pallidal circuits regulate opioid-induced food intake

Abstract

Mu opioid receptor (MOR) signaling in the nucleus accumbens (NAcc) elicits marked increases in the consumption of palatable tastants. However, the mechanism and circuitry underlying this effect are not fully understood. Multiple downstream target regions have been implicated in mediating this effect but the role of the ventral pallidum (VP), a primary target of NAcc efferents, has not been well defined. To probe the mechanisms underlying increased consumption, we identified behavioral changes in rats' licking patterns following NAcc MOR stimulation. Because the temporal structure of licking reflects the physiological substrates modulating consumption, these measures provide a useful tool in dissecting the cause of increased consumption following NAcc MOR stimulation. Next, we used a combination of pharmacological inactivation and lesions to define the role of the VP in hyperphagia following infusion of the MOR-specific agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) into the NAcc. In agreement with previous studies, results from lick microstructure analysis suggest that NAcc MOR stimulation augments intake through a palatability-driven mechanism. Our results also demonstrate an important role for the VP in normal feeding behavior: pharmacological inactivation of the VP suppresses baseline and NAcc DAMGO-induced consumption. However, this interaction does not occur through a serial circuit requiring direct projections from the NAcc to the VP. Rather, our results indicate that NAcc and VP circuits converge on a common downstream target that regulates food intake.

Figure 1

Pharmacological inactivation of the VP blocks feeding potentiated by NAcc DAMGO, but also reduces baseline consumption. (A) Consumption of high fat chow (mean ± SEM) was elevated by NAcc DAMGO relative to control. Infusion of the GABA agonist muscimol blocked this effect, but also had strong effects on baseline levels of consumption, which were significantly reduced. NAcc DAMGO concentration, 250 ng/μl; VP muscimol concentration, 100 ng/μl. Asterisks indicated significant posthoc differences (P<0.001) from control (saline in NAcc and VP). (B) Timeline of drug effects. Shaded bar shows high fat chow consumption in the first hour; open bars show consumption in the second hour. N = 14 rats for data in Figures 1–2.

Figure 2

A four-fold lower dose of VP muscimol reduces baseline consumption. (A) As for the higher muscimol dose shown in Figure 1, VP inactivation with a lower dose of muscimol attenuated NAcc DAMGO effects, but also significantly reduced baseline feeding. NAcc DAMGO concentration, 250 ng/μl; VP muscimol concentration, 25 ng/μl. Asterisks indicated significant posthoc differences (P<0.05) from control (saline in NAcc and VP). (B) Time course of feeding. Low dose muscimol had the most pronounced effects in the first hour, with some recovery of feeding in the second hour.

Figure 3

NAcc DAMGO elevates sucrose consumption. (A) Stimulation of NAcc MORs increased mean (± SEM) 0.15 M sucrose consumption significantly. However, DAMGO effects were blocked with concurrent pharmacological inactivation of the VP. Both doses of muscimol infused into the VP had strong effects on baseline consumption of sucrose. NAcc DAMGO dose = 50 ng/μl; low VP muscimol dose = 25 ng/μl; high VP muscimol dose = 100 ng/μl. Asterisks indicated significant posthoc differences (P<0.05) from control (saline in NAcc and VP). (B) Time course of feeding. Solid lines indicate NAcc saline infusion; broken lines indicate NAcc DAMGO infusion. Colors indicate VP infusion: green = saline, black = low muscimol dose, red = high muscimol dose. Note that NAcc DAMGO (with VP saline infusion) caused a transient suppression of sucrose consumption (first ~45 minutes), followed by elevated rates of intake thereafter. N = 22 rats for data shown in Figures 3 – 7.

Figure 4

VP muscimol disrupts lick microstructure. (A) Mean (±SEM) interlick intervals occurring during consummatory bursts are shown. Relative to the control treatment (saline in both NAcc and VP), the mean interlick interval was elevated for NAcc DAMGO + VP muscimol treatment and NAcc saline + VP low muscimol treatment (asterisks, p<0.05). Overall, there was a significant elevation in ILIs comparing muscimol treatments to control saline (collapsing across DAMGO treatments, which had no effect). (B) Binned interlick interval times. Muscimol treatment shifted ILIs toward higher values. The percentage of ILIs occurring in the 0–250 ms bin was significantly decreased for muscimol treatments relative to saline control, and significantly elevated in the 251–500 ms bin (asterisks, p<0.05).

Figure 5

Meal parameters. (A) NAcc DAMGO increased meal duration, while VP inactivation dramatically decreased meal duration and blocked DAMGO effects. (B) Meal size closely tracked meal duration, with DAMGO induced increases and muscimol induced decreases. (C) Muscimol treatment significantly decreased the number of meals consumed relative to saline treatment. (D) Muscimol inactivation of the VP increased latency to meal initiation. (Inset) Muscimol infusion blocked consumption completely in a subset of rats. The x axis shows time of meal initiation in the session; the y axis shows the percentage of all sessions for each drug condition. Note that low muscimol and muscimol treatments blocked all licking in 16% and 32% of sessions, respectively. For A–D, asterisks indicate significant difference (p<0.05) relative to control condition (NAcc saline + VP saline).

Figure 6

Burst parameters. (A) Surprisingly, NAcc DAMGO infusion had no effect on burst duration, a correlate of palatability. VP inactivation decreased burst duration. (B) VP inactivation strongly reduced the number of bursts initiated. NAcc DAMGO infusion had no significant effects on the number of bursts. (C) Pause duration was not significantly altered by any treatment condition. For each graph, asterisks indicate significant difference (p<0.05) relative to control condition (NAcc saline + VP saline). (D) Though mean burst size was not elevated after NAcc DAMGO infusion, there was a significant positive correlation between DAMGO induced increases in consumption (x-axis, relative to saline condition) and increased burst duration (y-axis). Thus, DAMGO-induced consumption occurred in large part through longer bursts.

Figure 7

(A) NAcc DAMGO does not alter the rate at which licking declines. Suppression of satiation is associated a decreased rate of declining intake in later stages of consumption. DAMGO caused a transient suppression of intake relative to saline control (first 30 minutes); however, the rate at which licking declined thereafter was nearly identical for saline and DAMGO conditions. (B) Locomotor effects of VP muscimol. The low dose of muscimol (25 ng/μl) did not alter the magnitude nor pattern of locomotion tested in an open field (p≫0.05 compared to saline). The high dose of muscimol (100 ng/μl) attenuated the initial locomotor response, but elevated locomotion thereafter. Neither dose of muscimol significantly altered rearing.

Figure 8

VP function is not required for NAcc DAMGO induced hyperphagia. (A) Bilateral infusion of NAcc DAMGO (250 ng/μl) more than doubled high fat chow consumption in sated rats (left two bars; n = 6 rats). Similarly elevated levels of consumption occurred after unilateral infusion of DAMGO in either hemisphere (“left” and “right”). Asterisks indicate significant differences (p<0.05) relative to control condition (bilateral saline). (B) Bilateral VP inactivation blocked NAcc DAMGO effects. This effect could arise through serial (i) or convergent (ii) connectivity. If neural signaling caused by NAcc DAMGO were relayed through an obligatory synapse in the VP, lesion of the VP ipsilateral to NAcc DAMGO infusion should block the increase in feeding (iii). On the other hand, if NAcc and VP efferents converge downstream, the VP ipsilateral to NAcc DAMGO infusion may not be required for MOR-stimulated increases in consumption (iv). (C) NAcc DAMGO ipsilateral and contralateral to VP lesion was equally effective in increasing food intake (n = 4 rats). Asterisks indicate significant difference (p<0.001) relative to control condition (ipsilateral saline). These data support the circuit diagram shown in (B-iv).

Figure 9

Cannula and lesion placements. (A) NAcc cannulae placements were largely confined to the shell region, approaching the border between shell and core in some places. Anterioposterior position (relative to bregma) is shown to the right of each section. (B) VP cannulae placements. (C) VP lesions were large, extending from just ventral to the anterior commissure to below the VP’s ventral border in some cases. Black region indicates minimum area ablated; gray indicates maximum lesion area.