Steven J. Cooper

Hyperphagia induced by direct administration of midazolam into the parabrachial nucleus of the rat.

Benzodiazepine receptor agonists increase food intake in many different species, yet there has been little investigation of the central site of actions of these drugs on ingestive behaviour. In the present experiments, direct administration of the benzodiazepine receptor agonist midazolam (3-30 micrograms/microliter) into the parabrachial nucleus of the pons significantly increased the consumption of a wet mash diet and a 3% sucrose solution in adult non-deprived rats. The hyperphagic response was blocked by pre-treatment with the selective benzodiazepine receptor antagonist flumazenil. Injection of midazolam into the parabrachial nucleus had no effect on locomotor activity, despite the fact that in the same animals an increase in mash intake was observed following intra-parabrachial midazolam. These data suggest that benzodiazepine receptors located in the parabrachial nucleus may be an important site of action for the effects of benzodiazepines specifically on ingestive behaviour.

The putative dopamine D3 agonist, 7-OH-DPAT, reduces dopamine release in the nucleus accumbens and electrical self-stimulation to the ventral tegmentum

The present experiments were designed to test further the idea that 7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin), a putative dopamine (DA) D3 agonist, has effects at DA autoreceptors to reduce intracranial DA levels and to reduce behaviours that are DA-dependent. Rats were trained to respond on a self-stimulation protocol for electrical stimulation to the ventral tegmental area (VTA). Each press of a lever delivered a 0.5 s train of square wave, 1.5 ms duration, 100 Hz, 90-120 mA stimulation. Systemic administration of 7-OH-DPAT at 0.01-0.3 mg/kg i.p., quickly dose-dependently reduced responding. Electrical stimulation using similar parameters to those that supported self-stimulation were then applied to the VTA of anaesthetized rats. Fast cyclic voltammetry (FCV) revealed that this stimulation released DA in the nucleus accumbens (NAC). 7-OH-DPAT i.p. (0.1-3.0 mg/kg) quickly and potently reduced the size of the DA-generated voltammetric signal. This effect of 0.3 mg/kg 7-OH-DPAT was not blocked by sulpiride (60 mg/kg, i.p.) a D2-specific antagonist that may preferentially block D2 autoreceptors. These data are discussed with reference to the possibility that 7-OH-DPAT reduces the release of dopamine in the NAC, at D3, but not at D2, autoreceptors and that this in turn may reduce the rewarding effect of VTA stimulation.

Evidence for early opioid modulation of licking responses to sucrose and Intralipid: a microstructural analysis in the rat

Abstract The behavioural mechanisms underlying the effects of the opioid antagonist naloxone (0.3–3u2005mg/kg IP), and the opioid agonists morphine (0.3–3u2005mg/kg SC), and U-50, 488H (0.3–3u2005mg/kg SC) on ingestive behaviour were investigated using a microstructural analysis of licking patterns for sucrose solutions and Intralipid (fat emulsions) in a brief contact test. Naloxone dose-dependently decreased the total number of licks and the number of bouts for sucrose and Intralipid, but did not affect mean bout duration. Morphine dose-dependently increased the total number of licks and the number of bouts for both test fluids. For Intralipid but not for sucrose drinking, morphine actually decreased mean bout duration. U-50, 488H significantly affected total licks, although the dose-effect relationship showed an inverted U-shaped function. There was a dose-dependent increase in mean bout duration following administration of U-50, 488H and an increase in bout number, although only the lowest dose differed significantly from the control condition. The results show that microstructural analysis can distinguish between the effects of naloxone, morphine and U-50, 488H on licking behaviour and indicate that selective opioid receptor subtypes may be differentially involved in ingestive processes.

Effects of benzodiazepine receptor ligands on the ingestion of sucrose, intralipid, and maltodextrin: an investigation using a microstructural analysis of licking behavior in a brief contact test.

Microstructural analysis of licking behavior in the rat was conducted (a) to describe in detail the characteristics of benzodiazepine-induced changes in ingestion and (b) to determine if the changes are consistent with an alteration in palatability. The effects of the benzodiazepine receptor (BZR) agonist midazolam (0.3-3 mg/kg), and the partial inverse agonist Ro 15-4513 (0.3-3 mg/kg), on licking for several concentrations of sucrose, Intralipid, and maltodextrin in a brief contact test were investigated. Midazolam increased the total number of licks for all 3 fluids; conversely, Ro 15-4513 decreased the total number of licks. Midazolam increased mean bout duration for sucrose and maltodextrin drinking and there was a trend toward a similar effect with Intralipid drinking. Ro 15-4513 reduced mean bout duration for all 3 test fluids. These data are discussed in terms of bidirectional changes in fluid palatability by drug actions at BZRs.

Benzodiazepines and palatability: Taste reactivity in normal ingestion

The taste reactivity (TR) test was devised as a method to obtain behavioural data in response to gustatory stimuli in neurologically impaired rats, incapable of voluntary feeding. Sapid solutions were infused through surgically implanted intraoral cannulae. Facial and motor responses corresponded well to known hedonic and aversive properties of tastes (e.g., sweet, bitter). TR testing has since proved effective as an adjunct to intake-based methods, in the psychopharmacology of ingestion in the normal rat. We developed a nonsurgical modification of the TR test, in which intact rats sampled stimuli voluntarily. The benzodiazepine receptor agonist midazolam (3.0 mg/kg, IP) was administered to rats first trained to consume a sweet 3% sucrose solution, and later tested with access to a bitter 0.01% quinine solution. Response were videotaped, and TR measures were scored during periods of noningestion using a frame-by-frame playback. Treatment increased ingestion and facilitated ingestive responses in accordance with published data for cannulated rats. Results support a two-component view of response palatability, in which treatment alters feeding motivation, increasing positive palatability and facilitating ingestion of both palatable and unpalatable stimuli.

7-OH-DPAT injected into the accumbens reduces locomotion and sucrose ingestion: D3 autoreceptor-mediated effects?

7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) injected bilaterally in the nucleus accumbens (NAC) resulted in profound, noncatatonic, dose-dependent (0.3-3 mg total dose) hypolocomotion but without inducing yawning. It also decreased intake of a highly preferred 3% sucrose solution (1 microgram total dose). Systemic injection of 7-OH-DPAT (0.1-3.0 mg/kg, i.p.) similarly induced hypolocomotion while failing to induce yawning. In none of these studies did rats show any signs of hyperlocomotion or any stereotyped responses normally associated with D2 or mixed D1/D2 receptor stimulation. These data suggest that hypolocomotion elicited by 7-OH-DPAT in the NAC may be mediated at the D3 receptor as distinct from the D2 dopamine receptor. We discuss the possibility that the behavioural effects we observed are mediated at D3 autoreceptors.

Midazolam-induced rapid changes in licking behaviour: evidence for involvement of endogenous opioid peptides

Abstract The role of endogenous opioid peptides in the effects of midazolam on ingestive behaviour was investigated using a detailed analysis of licking behaviour in the rat. Midazolam (1.8u2005mg/kg IP) was administered in combination with either flumazenil (10 and 20u2005mg/kg IP) or naloxone (0.1 and 0.3u2005mg/kg IP). The effect on licking patterns during 60-s exposures to a range of concentrations of a fat emulsion (Intralipid) was then recorded. Midazolam significantly increased the total number of licks for Intralipid by increasing the mean bout duration. This effect is consistent with the proposal that benzodiazepines enhance palatability. Flumazenil and naloxone were ineffective when administered alone, but both drugs blocked the effect of midazolam on total number of licks by selectively attenuating mean bout duration. Midazolam also produced a significant decrease in the intrabout lick rate, probably due to the muscle relaxant effects of this drug. This decrease in the intrabout lick rate was reversed by pretreatment with flumazenil but not by naloxone. The results suggest that endogenous opioids may be important for the palatability effects of midazolam, but may not be involved in the muscle relaxant effects of this drug.

Increased food intake following injection of the benzodiazepine receptor agonist midazolam into the IVth ventricle

Despite a prolonged period of research with benzodiazepines, the central site(s) of action for the hyperphagic effects of these compounds remains to be determined. The aim of the present studies was to examine the effect of direct administration of the benzodiazepine receptor agonist midazolam into the IVth ventricle on ingestive behavior in nondeprived rats. In Experiment 1, microinjection of midazolam (3 and 30 micrograms/microliter) into the IVth ventricle was sufficient to increase consumption of a palatable mash. In Experiment 2, the hyperphagic effect was blocked by systemic administration of the selective benzodiazepine receptor antagonist flumazenil (20 mg/kg). The results indicate that a brainstem site of action may be important for the effects of benzodiazepine receptor agonists on ingestive behavior.

Cholecystokinin Modulation of Serotonergic Control of Feeding Behavior

The study of satiety in the control of feeding behavior is concerned with innate and learned mechanisms for the inhibition of feeding as a result of food ingestion. In terms of central mechanisms underlying cessation of feeding, considerable attention has been paid to the neurotransmitter, serotonin (5-hydroxytryptamine, 5-HT). A great deal of pharmacological and biochemical evidence is consistent with the view that heightened serotonergic activity underlies the induction of satiety.*2 In the light of the discovery and classification of multiple serotonin receptor subtypes? considerable attention has been devoted to the issue of the receptor subtype(s> that may be involved in serotonin-related reductions in food consumption.*2 A recent review of the rapid advances in this area suggests that 5-HTle receptors may be involved in the process of satiation and that 5-HT2A receptors may be linked to stress-induced anorexia? As yet, a role for 5-HTzc receptors (formerly designated 5-HTlc receptors) awaits convincing pharmacological c~nfirmation.~ Thus, increased release of 5-HT, arising in connection with the ingestion, digestion, and absorption of food, may constitute an important neurochemical link that switches off feeding behavior and induces a state of satiety. A number of 5-HT-receptor subtypes may be involved, including the 5-HTls, 5-HTzA, and possibly 5-HTzc. However, another 5-HT-receptor subtype plays quite a different role in the control of feeding behavior. 5-HTIA receptors are located on dendrites and cell bodies of central 5-HT neuron^,,^ and stimulation of these receptors with selective 5-HTIA receptor agonists leads to a marked inhibition of central serotonergic activity. It follows that ~ H T I A receptor agonists like 8-OH-DPAT (8-hydroxy-2-di-n-propylamino)Tetralin@ produce an overconsumption of food (hyperphagia), due to the inhibition of the endogenous satiety system mediated by serotonergic activity. This important prediction has been confirmed8 and the result replicated many times?. Moreover, the hyperphagic effect of 8-OH-DPAT and other 5-HTIA agonists can be blocked by selective S-HTEA receptor antagonists, such as WAY-100135.

Effects of the benzodiazepine receptor inverse agonist Ro 15-4513 on the ingestion of sucrose and sodium saccharin solutions : A microstructural analysis of licking behavior

The effects of the benzodiazepine receptor partial inverse agonist Ro 15-4513 on consumption of either a 1% sucrose solution or a 0.1% sodium saccharin solution in nondeprived male rats was examined. A video-recording approach was adopted in which licks were counted in a frame-by-frame analysis. Ro 15-4513 (1-10 mg/kg) caused a significant decrease in the intake of both sucrose and saccharin solutions that was associated with a reduction in the initial rate of licking. There was a decrease in the total duration of drinking, total licks, and number of bouts for both sucrose and saccharin. For sucrose, mean bout duration was significantly reduced, although this was not so for saccharin. Intrabout lick rate, the latency to engage in drinking, and the postdrinking time were not affected for either sucrose or saccharin. These data are consistent with previous evidence that strongly suggests that benzodiazepines influence palatability.