The kappa-opioid antagonist GNTI reduces U50,488-, DAMGO-, and deprivation-induced feeding, but not butorphanol- and neuropeptide Y-induced feeding in rats.

Antagonists selective for either kappa- [e.g. nor-binaltorphimine (nor-BNI)] and mu- (e.g. beta-funaltrexamine) opioid receptors have previously been shown to reduce both kappa- and mu-opioid-induced feeding. In the present studies, the anorectic effects of GNTI, a newly synthesized antagonist selective for kappa-opioid receptors, were studied in rats. GNTI (0.032-0.32 nmol; i.c.v.), administered 15 min prior to food access, reduced feeding induced by the kappa-opioid agonist U50,488 (producing a 70% maximal decrease), the mu-opioid agonist DAMGO (90% maximal decrease), and 24 h acute food deprivation (60% maximal decrease). GNTI did not reduce the orexigenic effects of butorphanol, an agonist that binds to both kappa- and mu-opioid receptors, and neuropeptide Y (NPY). Taken together, these results suggest that GNTI is a potent anorectic agent and opioid antagonist in rats. Like nor-BNI, GNTI reduced feeding induced by both kappa- and mu-opioid agonists. However, unlike nor-BNI, GNTI did not alter the orexigenic effects of butorphanol or NPY. Given the selectivity of GNTI and its effectiveness in several of the present experiments, its potency, and its short duration of action compared to nor-BNI, GNTI may serve to be a useful tool to study behavioral effects mediated by kappa-opioid receptors.

Sucrose consumption increases naloxone-induced c-Fos immunoreactivity in limbic forebrain.

Opioids have long been known to have an important role in feeding behavior, particularly related to the rewarding aspects of food. Considerable behavioral evidence suggests that sucrose consumption induces endogenous opioid release, affecting feeding behavior as well as other opioid-mediated behaviors, such as analgesia, dependence, and withdrawal. In the present study, rats were given access to a 10% sucrose solution or water for 3 wk, then they were injected with 10 mg/kg naloxone or saline. Brains were subsequently analyzed for c-Fos immunoreactivity (c-Fos-IR) in limbic and autonomic regions in the forebrain and hindbrain. Main effects of sucrose consumption or naloxone injection were seen in several areas, but a significant interaction was seen only in the central nucleus of the amygdala and in the lateral division of the periaqueductal gray. In the central nucleus of the amygdala, naloxone administration to those rats drinking water significantly increased c-Fos-IR, an effect that was significantly enhanced by sucrose consumption, suggesting an upregulation of endogenous opioid tone in this area. The data from this study indicate that the central nucleus of the amygdala has a key role in the integration of gustatory, hedonic, and autonomic signals as they relate to sucrose consumption, if not to food intake regulation in general. Furthermore, the data from this study lend further support to the hypothesis that sucrose consumption induces the release of endogenous opioids.

Discriminative stimulus effects of a centrally administered, delta-opioid peptide (D-Pen2-D-Pen5-enkephalin) in pigeons.

The present study assessed the discriminative stimulus effects of the delta-opioid agonist [D-Pen2-D-Pen5]enkephalin (DPDPE) in pigeons. Food-restricted pigeons were trained to discriminate between i.c.v injections of 100 micrograms [D-Pen2-D-Pen5]enkephalin (DPDPE) and saline in a two-key operant procedure; acquisition of discriminative control was rapid (14-28 daily sessions). [D-Ser2, Leu5, Thr6]enkephalin (DSLET) and [D-Ala2]deltorphin II, peptides selective for delta-opioid receptors, produced discriminative stimulus effects similar to DPDPE, and were approximately equipotent to DPDPE. The non-peptidic, delta-opioid agonist BW373U86 (0.032-100 mg/kg, i.m.) partially generalized to DPDPE. The kappa-opioid agonist U69,593 (0.01-1 mg/kg, i.m.), and the mu-opioid agonists, DAMGO (0.1-3.2 micrograms, i.c.v) and morphine (1-10 mg/kg, i.m.), did not produce discriminative stimulus effects similar to DPDPE, up to doses that markedly decreased response rates. Naltrindole (0.1 mg/kg. i.m.), an antagonist selective for delta-opioid receptors, produced approximately a 30-fold reduction in the potency of DPDPE. DPDPE's discriminative stimulus effect in pigeons appears to be mediated through a delta-opioid receptor; this effect may provide a procedure for assessing delta-opioid receptor function in vivo.

Nitric oxide synthase inhibitors produce phencyclidine-like behavioral effects in pigeons.

The present study assessed the ability of nitric oxide synthase (NOS) inhibitors to produce PCP-like behavioral effects in pigeons. Food-restricted pigeons were trained to discriminate between PCP (1.0 mg/kg, i.m.) from saline in a two-key operant procedure. NOS inhibitors 7-nitroindazole (7-NI) and N omega-nitro-L-arginine methyl ester (L-NAME) produced PCP-like discriminative stimulus effects. 7-NI (17.8 mg/kg, i.m.) completely generalized to PCP. L-NAME (320-1000 mg/kg) produced partial generalization to PCP. D-NAME, the enantiomer of L-NAME, did not produce PCP-appropriate behavior. L-NAME was approximately 200-times more potent i.c.v., but did not fully generalize to PCP. Both NOS inhibitors were effective in producing catalepsy, which is an effect commonly produced by competitive and uncompetitive NMDA antagonists. 7-NI (32 mg/kg) produced catalepsy in all subjects, whereas L-NAME (3200 mg/kg) produced catalepsy in 50% of the subjects, D-NAME did not produce catalepsy. Pretreatment with L-arginine (32-3200 mg/kg) prevented the PCP-like discriminative stimulus and cataleptic effects of 7-NI (17.8-32 mg/kg), demonstrating that 7-NI produced PCP-like effects through blockade of NO synthesis. The current studies reveal that NOS inhibitors induced two behavioral actions, discriminative stimulus effects and catalepsy, that are very selective for NMDA antagonists in pigeons.

Nor-binaltorphimine: an ultra-long acting kappa-opioid antagonist in pigeons.

This study assessed the ability of naltrexone and nor-binaltorphimine (NBNI) to antagonize the rate-decreasing effects of opioid agonists. Food-restricted pigeons were trained to peck a lit key under a fixed-ratio (FR) 20 schedule of food reinforcement. Bremazocine, a kappa-opioid agonist, decreased food-reinforced responding (ED(50) = 0.02mg/kg), and naltrexone (5.6mg/kg) reduced the potency of bremazocine six-fold. The effect of naltrexone lasted less than 24h. A single injection of NBNI (1mg/kg) was given to four pigeons, and the time course of antagonism of the rate-decreasing effects of bremazocine was measured. One hour after NBNI was given, it was ineffective. Eight days later, NBNI produced a five-fold reduction in the potency of bremazocine. Between 12 and 20 days after NBNI, it reduced the potency of bremazocine 14-fold. NBNI continued to antagonize bremazocine for 11 weeks. Smaller doses of NBNI (0.001-0.1mg/kg) were ineffective. The effect of NBNI was not due to tolerance to bremazocine, since tolerance failed to develop to bremazocine administered repeatedly. NBNI (1mg/kg) did not antagonize the response rate-decreasing effects of morphine, a mu-opioid agonist, or BW373U86, a delta-opioid agonist. NBNI was an effective and extremely long-lasting kappa-opioid antagonist in the pigeon. The duration of action of NBNI is among the longest yet described in any species.

Effects of neuropeptide Y, insulin, 2-deoxyglucose, and food deprivation on food-motivated behavior.

The current study demonstrates the ability of neuropeptide Y (NPY) to increase break points under a progressive ratio 1 (PR1) reinforcement schedule. An initial response resulted in delivery of a food reinforcer (45 mg pellet) under the PR1, and an additional response was required for each successive reinforcer. The break point, the number of responses emitted to obtain the last reinforcer, is considered a measure of reinforcing efficacy or motivational strength of the food reinforcer. NPY (0.3-10 micrograms) significantly increased break point to levels comparable to those produced by 36-48 h of food deprivation. Although insulin (3-8 U/kg) and 2-deoxyglucose (150-250 mg/kg) also increased food intake, neither increased break points to levels produced by NPY or food deprivation. These data suggest that NPY may change the value of food in ways that cannot be accounted for by changes in insulin, glucose levels or intracellular glucoprivation. These results emphasize that simply measuring the amount of freely available food eaten is not a fully adequate measure of the strength of the feeding behavior.

Discriminative stimulus effects of combinations of drug and visual stimuli in pigeons.

Six pigeons were studied to determine whether the brightness of a houselight interacted with the stimuli produced by methadone, and whether the nature of the interaction depended on the order of training of the two discriminations. Three pigeons were trained to peck the right key after methadone (2.0mg/kg) and the left key after saline, when the houselight was dim. The effects of a range of methadone doses (0.5, 1.0, 2.0mg/kg and saline) were tested. Three other pigeons were trained, in the absence of drug, to peck the right key when the houselight was dim and the left key when the houselight was bright. The effects of a range of houselight intensities were tested. Then, for both groups, right-key pecks were reinforced in the presence of methadone and the dim houselight, and left-key pecks were reinforced in the presence of saline and the bright houselight. Methadone doses were tested in the presence of both houselight brightnesses used in training. All pigeons pecked the methadone-appropriate key after high doses of methadone, regardless of houselight intensity. All pigeons trained to discriminate houselight brightness first, and one of the pigeons trained to discriminate methadone first, pecked according to the houselight condition when saline and lower doses were tested. In the other pigeon trained to discriminate methadone first, pecking was more related to drug dose. These data show that a drug stimulus can compete with external stimuli for behavioral control, that a drug stimulus can assume control over behavior originally controlled by external stimuli, and that discriminations based on external stimuli may be retained when saline or low doses of drug are administered.

[Leu31,Pro34]neuropeptide Y (NPY), but not NPY 20-36, produces discriminative stimulus effects similar to NPY and induces food intake.

Rats were trained to discriminate between an intracerebroventricular injection of 1.15 nmol of Neuropeptide Y (NPY) and a sham injection. Rats rapidly learned to press the appropriate lever during training. NPY's discriminative stimulus effects were compared to those of saline, and 1.15-3.45 nmol [Leu31,Pro34]NPY, a Y1 receptor agonist and NPY 20-36, Y2 receptor agonist. [Leu31,Pro34]NPY resulted in NPY-appropriate responding, whereas saline and NPY 20-36 did not. [Leu31,Pro34]NPY also increased food intake, but NPY 20-36 did not. This suggests that NPY's discriminative stimulus and orexigenic effects involve the Y1, but not the Y2, receptor.

Effects of neuropeptide Y on food-reinforced behavior in satiated rats.

The effect of NPY on behavior and food intake of food-satiated rats was examined under three different food availability conditions. Food was available during times when rats normally do not eat under either a fixed-ratio or fixed-interval reinforcement schedule, or it was freely available in the bottom of the cage (FF). Forty responses were required for each 45-mg food pellet under the ratio schedule (FR 40) and for the first response to occur 15 s after the previous reinforcement under the interval schedule (FI 15"). NPY (5 micrograms) significantly increased food intake under all conditions and increased food-reinforced responses under the FR and FI schedules. NPY's effect on food intake was greatest when food was freely available and least for rats working under the schedule requiring the most effort (FR 40). Food intake peaked after 3 days under repeated daily administration of NPY. Under free food access and under the fixed-interval schedule, eating and/or responding occurred almost immediately following the onset of the initial 4-h session under NPY. However, during the first session following NPY administration under the FR, rats emitted few responses during the first 2 h of the session. The onset of robust responding under the FR schedule began earlier with each successive daily administration of NPY. These data show NPY substantially increases food-maintained behavior and is a potent inducer of food intake even under conditions where considerable effort is required to obtain food. Further, the conditions under which food is made available can dramatically alter NPY's effect on the temporal pattern of food-maintained responding, feeding, and latency to eat.

The discriminative stimulus effects of neuropeptide Y.

Neuropeptide Y (NPY), an endogenous peptide which strongly induces food intake, is demonstrated to have discriminative stimulus properties when administered intracerebroventricularly. Rats rapidly learned to press the appropriate lever during training. NPY discrimination was dose-dependent. NPY's discriminative stimulus properties were compared to those of two doses of Peptide YY (PYY) and 24 and 48 h of food deprivation, conditions which also increase feeding. Both doses of PYY generalized to NPY, supporting previous findings that PYY has effects similar to NPY. Although food deprivation increases feeding in a manner similar to NPY, food deprivation did not result in NPY-appropriate responding.