Effects of the 5-HT1A receptor agonist 8-OH-DPAT on operant food intake in food-deprived pigs

The role of serotonin in modulating common waxbill behaviour

Abstract

Serotonin or 5-hydroxytryptamine (5-HT) is a monoaminergic neurotransmitter that is known to influence behaviour in various animal species. Its actions, however, are complex and not well-understood yet. Here, we tested whether and how two 5-HT receptor agonists and a 5-HT receptor antagonist influence behaviour in common waxbills (Estrilda astrild), focusing on aggression, movement and feeding. We applied acute administration of either 8-OH-DPAT (a 5-HT1A receptor agonist), fluoxetine (a selective serotonin reuptake inhibitor; SSRI) or WAY 100,635 (a 5-HT1A receptor antagonist), and then quantified behaviour in the context of competition for food. Waxbills treated with the SSRI fluoxetine showed an overall decrease of aggressive behaviour, activity and feeding, while we found no significant effects of treatment with the other serotonergic enhancer (8-OH-DPAT) or with the antagonist WAY 100,635. Since both 8-OH-DPAT and WAY 100,635 act mainly on 5-HT1A receptor pathways, while fluoxetine more generally affects 5-HT pathways, our results suggest that receptors other than 5-HT1A are important for serotonergic modulation of waxbill behaviour.

Significance statement

The serotonergic system is of interest for current behavioural research due to its influence on a range of behaviours, including aggression, affiliative behaviour, feeding and locomotion in various species. There are, however, numerous discrepancies regarding the behavioural effects of serotonin across studies. We used acute pharmacological manipulations of the serotonergic system in common waxbills, using two serotonin enhancers (8-OH-DPAT and fluoxetine) and a serotonin blocker (WAY 100,635). Behavioural effects of these pharmacological manipulations on aggressiveness, movement and feeding, during tests of competition over food, indicated an anxiogenic-like effect of fluoxetine, but not of 8-OH-DPAT and WAY 100,635. This suggests a distinct role for different serotonergic pathways on waxbill behaviour.

A systematic investigation of the differential roles for ventral tegmentum serotonin 1- and 2-type receptors on food intake in the rat

Central serotonin (5-HT) pathways are known to influence feeding and other ingestive behaviors. Although the ventral tegmentum is important for promoting the seeking and consumption of food and drugs of abuse, the roles of 5-HT receptor subtypes in this region on food intake have yet to be comprehensively examined. In these experiments, food restricted rats were given 2-h access to rat chow; separate groups of non-restricted animals had similar access to a sweetened fat diet. Feeding and locomotor activity were monitored following ventral tegmentum stimulation or blockade of 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, or 5-HT2C receptors. 5-HT1A receptor stimulation transiently inhibited rearing behavior and chow intake in food-restricted rats, and had a biphasic effect on non-restricted rats offered the palatable diet. 5-HT1B receptor agonism transiently inhibited feeding in restricted animals, but did not affect intake of non-restricted rats. In contrast, 5-HT1B receptor antagonism decreased palatable feeding. Although stimulation of ventral tegmental 5-HT2B receptors with BW723C86 did not affect hunger-driven food intake, it significantly affected palatable feeding, with a trend for an increasing intake at 2.0µg/side but not at 5.0µg/side. Antagonism of the same receptor modestly but significantly inhibited feeding of the palatable diet at 5.0µg/side ketanserin. Neither stimulation nor blockade of 5-HT2A or 5-HT2C receptors caused prolonged effects on intake or locomotion. These data suggest that serotonin's effects on feeding within the ventral tegmentum depend upon the specific receptor targeted, as well as whether intake is motivated by food restriction or the palatable nature of the offered diet.

Selective Lifelong Destruction of Brain Monoaminergic Nerves Through Perinatal DSP-4 Treatment

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) is a highly selective neurotoxin for noradrenergic projections originating from the locus coeruleus (LC). The outcome of the systemic DSP-4 treatment of newborn rats is an alteration in postnatal development of the noradrenergic system, involving the permanent denervation of distal noradrenergic projection areas (neocortex, hippocampus, spinal cord), accompanied by noradrenergic hyperinnervation in regions proximal to the LC cell bodies (cerebellum, pons-medulla). DSP-4 is well tolerated by developing rats and does not increase the mortality rate. Permanent noradrenergic denervation in the cerebral cortex and spinal cord is present at all developmental stages, although this effect is more pronounced in rats treated with DSP-4 at an early age, i.e., up to postnatal day 5 (PND 5). Notably, regional hyperinnervation is a hallmark of neonatal DSP-4 treatment, which is not observed after either prenatal or adult DSP-4 application. In contrast to robust biochemical changes in the brain, DSP-4 treatment of newborn rats has a marginal effect on arousal and cognition functions assessed in adulthood, and these processes are critically influenced by the action of the noradrenergic neurotransmitter, norepinephrine (NE). Conversely, neonatal DSP-4 does not significantly affect 5-hydroxytryptamine (serotonin; 5-HT), dopamine (DA), gamma-aminobutyric acid (GABA), and histamine levels in brain. However, as a consequence of altering the functional efficacy of 5-HT1A, 5-HT1B, DA, and GABA receptors, these neurotransmitter systems are profoundly affected in adulthood. Thus, the noradrenergic lesion obtained with neonatal DSP-4 treatment represents a unique neurobiological technique for exploring the interplay between various neuronal phenotypes and examining the pathomechanism of neurodevelopmental disorders.

The role of central 5-HT(2C) and NMDA receptors on LPS-induced feeding behavior in chickens

In mammals, LPS regulate feeding primarily through the 5-HT(1A) and 5-HT(2c) receptors within the brain. However, the central effect of 5-HT(1A) and 5-HT(2c) on LPS-induced feeding behavior has not been studied in non-mammalian species. Also, the role of glutamatergic system in LPS-induced anorexia has never been examined in either mammalian or non-mammalian species. Therefore, in this study, we examined the role of serotonergic and glutamatergic systems on LPS-induced anorexia in chickens. Food intake was measured in chickens after centrally administered lipopolysaccharide (LPS) (20 ng) (0 h), followed by intracerebroventricular (ICV) injection of the 5-HT(1A) autoreceptor agonist (8-OH-DPAT, 61 nmol), 5-HT(2c) receptor antagonist (SB 242084, 30 nm), and NMDA receptor antagonist (DL-AP5, 5 nm) at the onset of anorexia (4 h). In the following experiments, we used DL-AP5 before 5-HT (10 μg) and SB242084 before glutamate (300 nm) for evaluation of the interaction between 5-HTergic and glutamatergic systems on food intake. The results of this study showed that SB 242084 and DL-AP5 significantly attenuated food intake reduction caused by LPS (P < 0.05) but 8-OH-DPAT had no effect. In addition, 5-HT-induced anorexia was significantly attenuated by DL-AP5 pretreatment (P < 0.05), while SB 242084 had no effect on glutamate-induced hypophagia. These results indicated that 5-HT and glutamate (via 5-HT(2c) and NMDA receptor, respectively) dependently regulate LPS-induced hypophagia in chickens.

A serotoninergic basis for hyperphagic eating changes in Alzheimer's disease

Hyperphagia and associated eating changes occur frequently in Alzheimer's disease (AD) and lead to considerable morbidity. However, the neurochemical basis for these neuropsychiatric behaviours is at present unclear. In this study, we measured serotonin transporters, 5-HT(1A), 5-HT(2A), and 5-HT(4) receptors using radioligand binding assays in the postmortem temporal cortex of a cohort of controls and AD patients longitudinally assessed for hyperphagia. We found significant decreases in 5-HT(4) receptor densities in the hyperphagic, but not normophagic, AD group. Our data suggest that 5-HT(4) receptor deficits may be a specific neurochemical correlate of hyperphagia, and point to the potential pharmacotherapeutic utility of 5-HT(4) agonists for these behaviours in AD.

Hypophagic and dipsogenic effect of the 5-HT1A receptor agonist 8-OH-DPAT in broiler chickens

The effects of the 5-HT(1A) receptor agonist 8-OH-DPAT on food and water intake in male broiler chickens were investigated. The injection of 25 or 50 microg/kg of 8-OH-DPAT 15 min before refeeding in fasted animals produced a decrease in food intake. No effect was observed in drinking. The injection of 25 or 50 microg/kg of the 8-OH-DPAT 60 min after the start of refeeding did not produce any significant modification in food intake. No effect on drinking was recorded. The agonist 8-OH-DPAT injected 15 min before water presentation in water-deprived chickens, produced an increased drinking 60 min after the presentation of water. No effect on food intake was observed. The results show that the effect on food intake of the agonist 8-OH-DPAT in fasted-refed broiler chickens was similar to those observed in mammals and layer-strain chickens. However, the agonist did not alter significantly the food intake when the broilers were fed 60 min before the injection. These results are contrary to the observed effects in mammals and in layer-strain chickens. Probably, the selection for rapid growth rate in broilers causes modifications in the feeding control pattern. The comparison between broilers and layers strain may be a useful tool to elucidate the complex mechanisms involved in food and water intake regulation in chickens.

The effects of metergoline and 8-OH-DPAT injections into arcuate nucleus and lateral hypothalamic area on feeding in female rats during the estrous cycle

The present study examined the effects of local injections of metergoline (MET, an antagonist of 5-HT1/2 receptors, 2 and 20 nmol) and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, selective 5-HT1A receptor agonist, 0.6 and 6 nmol) into the arcuate nucleus (ARC) and the lateral hypothalamus (LH), on ingestive and non-ingestive behaviors of female rats. These effects were examined during the diurnal periods of diestrus and estrus in rats adapted to eat a wet mash diet (enriched with 10% sucrose) during 1h for 3 consecutive days at the recording chamber. The results showed that 8-OH-DPAT injected into the LH significantly reduced food intake at all doses and both cycle stages, while in the ARC these treatments evoked hypophagia only at the highest 8-OH-DPAT dose and only at the estrous phase. MET administered into the ARC (at all doses) failed to affect food intake during both estrous stages. On the other hand, food intake decreased after injection of both doses of MET into the LH of rats during estrous and diestrus phases. In estrus stage, injections of the higher dose of 8-OH-DPAT into the ARC and into the LH decreased the duration of feeding. Latency to start feeding, drinking, and non-ingestive behaviors were not affected by 8-OH-DPAT or MET treatments in the ARC or the LH in both cycle phases. These results indicated that 5-HT1A receptors participate in the serotonergic control of feeding-related mechanisms located at the ARC and the LH. These feeding-related serotonergic circuits in both areas are possibly affected by ovarian hormones that could increase sensitivity of ARC neurons to the hypophagic effects of 8-OH-DPAT or increase the efficacy of satiety signals that terminate feeding. In addition, the present data indicated that serotonergic inputs do not exert a tonic inhibitory activity on the ARC and the LH feeding-related circuits.

A comparison of the effects of the 5HT1A antagonists MM-77 and WAY-100635 on the mouse isolated vasa deferentia

1. Experiments were carried out to characterize the possible adrenergic properties of the 5-HT(1A) antagonists WAY 100635 and MM-77 using the mouse isolated vasa deferentia preparation. 2. When vasa deferentia were preincubated for 10 min in the presence of MM-77 (10(-8)-10(-6) m) or WAY100635 (10(-8)-7 x 10(-7) m), a concentration-dependent inhibition of the contractile response to submaximal electrical field stimulation (10 Hz, 50 V, 50 ms) was observed with pIC(50) values of 7.05 +/- 0.01 and 6.85 +/- 0.1 respectively. 3. MM-77 (10(-8)-10(-6) m) antagonized the contractile responses of the vasa deferentia to phenylephrine (PE) (10(-6)-10(-3) m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean rhoA(2) value of 6.81 +/- 0.084. The mean slope was 1.42 +/- 0.22. 4. WAY100635 (10(-8)-10(-6) m) antagonized the contractile responses of the vasa deferentia to PE (10(-6)-10(-3) m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean rhoA(2) value of 7.05 +/- 0.08. The mean slope was 0.97 +/- 0.1. 5. The results suggest that while WAY100635 acts as a competitive antagonist at alpha(1)-adrenoceptors, MM-77 displays non-competitive antagonist characteristics at this receptor subtype. 6. These results may have important implications for the use of these compounds as 5-HT(1A) receptor antagonists in in vivo studies.

Behavioral effects of 8-OH-DPAT injections into pontine and mesencephalic areas containing 5-HT-immunoreactive perikarya in the pigeon

This study examined the distribution of 5-HT-immunoreactive perikarya (5-HT-IRp) and the effects of local injections of 8-OH-DPAT into 5-HT-IRp-containing pontine and mesencephalic regions on feeding and drinking behaviors in free-feeding pigeons. When infused into the midline 5-HT-IRp-containing areas, 8-OH-DPAT (6.1 nmol) reliably elicited drinking and, to a lesser extent, feeding responses during the first hour after injection. These responses were significantly higher than the ingestive indexes observed (1) after vehicle (ascorbic acid 0.1%, 200 nl) injections at the same sites and (2) after 8-OH-DPAT injections into adjacent sites devoid of 5-HT-IRp. Increases in drinking were proportionally higher than those observed in feeding and a significant negative correlation was observed between water and food after midline 8-OH-DPAT injections. Similar dipsogenic responses were observed after injections of different 8-OH-DPAT doses (0.6, 2.0, and 6.1 nmol). Pretreatment with local injections of p-MPPI (an antagonist of 5-HT1A receptors) attenuated the ingestive responses evoked by 8-OH-DPAT injections. Injections of 8-OH-DPAT into lateral 5-HT-IRp-containing sites evoked only inconsistent and weak ingestive responses. These results indicate that 5-HT1A receptor-mediated circuits located in the midline superior raphe system of the pigeon may play an important role in mechanisms controlling water intake, similar to that observed in mammals.

Ingestive behaviors and metabolic fuels after central injections of 5-HT1A and 5-HT1D/1B receptors agonists in the pigeon

The effects of intracerebroventricular injections of 8-OH-DPAT (a 5-HT1A agonist; 3, 15 or 30 nmol) or GR46611 (a 5-HT1B/1D agonist; 3, 15 or 30 nmol) on feeding, drinking, preening and sleep-like behaviors were investigated in free-feeding (FF) pigeons. The effects of these 5-HT agonists on blood glucose and free fatty acids levels were also examined. Injections of 8-OH-DPAT evoked intense lipolytic and dipsogenic effects, but failed to affect feeding, non-ingestive behaviors and glycemic levels. On the other hand, GR46611 evoked significant increases in food intake (at the higher dose), as well as lipolytic and hyperglycemic effects, but left drinking and other non-ingestive behaviors unchanged. These effects are opposed to those found in rodents, and may be associated with the diverse, species-specific nature and distribution of these receptors, underscoring the need to examine the functional aspects of the 5-HT1 receptor family in a more extensive range of non-rodent species.

5-HT1A receptor-mediated regulation of mitogen-activated protein kinase phosphorylation in rat brain

Mitogen-activated protein kinases (MAPKs), a family of signal transduction mediators important in a host of cellular activities, include the extracellular signal-regulated kinases Erk1 and Erk2. We determined whether 5-HT(1A) receptors activate Erk1/2 in rat brain in vivo, as they do in recombinant cell lines. In contrast to the effect in cells, the 5-HT(1A) receptor agonist 8-hydroxy-N,N-diproylaminotetralin (8-OH-DPAT) dose- and time-dependently decreased basal levels of phosphorylated Erk1/2 (phospho-Erk1/2) in rat hippocampus (ED(50) approximately 0.1 mg/kg, maximum approximately 90%) without altering total Erk1/2. The effects were kinase-specific, as 8-OH-DPAT did not modify phosphorylated or total levels of the MAPKs c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 MAPK. Moreover, 8-OH-DPAT did not modify phospho-Erk1/2 in striatum or frontal cortex. The effect of 8-OH-DPAT was blocked by pretreatment with the selective 5-HT(1A) receptor antagonists N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635), 1-(2-methoxyphenyl)-4-(4-[2-phthalimido]butyl)piperazine (NAN-190) and 4-fluoro-N-(2-[4-(2-methoxyphenyl)1-piperazinyl]ethyl)-N-(2-pyridinyl)benzamide dihydrochloride (p-MPPF), but not by the weak partial agonist/antagonist 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8-azaspiro(4.5)decane-7,9-dione dihydrochloride (BMY 7378). Other 5-HT(1A) receptor agonists (buspirone, gepirone and ipsapirone) also reduced phospho-Erk1/2 levels in hippocampus. 8-OH-DPAT also reduced the levels of the upstream activator of Erk1/2, phosphorylated extracellular signal-regulated kinase kinase (phospho-MEK1/2), and at least one potential downstream target, the nuclear transcription factor phospho-Elk-1. The region- and kinase-specific effects suggest that the Erk1/2 signal transduction cascade is likely an important differential mediator of 5-HT(1A) receptor-regulated events in the central nervous system.