1-5-Hydroxytryptophan-induced flat body posture in the rat: antagonism by ritanserin and potentiation after 5,7-dihydroxytryptamine

The long-term effect of maternal dietary protein restriction on 5-HT1A receptor function and behavioral responses to stress in adulthood

Maternal nutrition impacts fetal development, and may play a role in determining resilience to stress and vulnerability to stress-precipitated psychiatric disorders, such as anxiety and depression. In this study, we examined the effect of a reduction in maternal dietary protein during pregnancy on the brain neurochemistry and behavior of offspring. We focused specifically on the serotonin system, the 5-HT1A receptor and the responsivity of offspring as adults to stress. Dams were fed either a low protein diet (10% protein by weight) or isocaloric control diet (20% protein by weight). The low protein diet did not alter maternal food intake and body weight, or litter size and the average birth weight of male or female littermates. 5-HT1A receptor function, as measured by quantitative autoradiography of 8-OH-DPAT (1 μM)-stimulated [35S]GTPγS binding, was markedly reduced in hippocampus of weanling female, but not male offspring (postnatal day, PND 21) of dams fed the low protein diet. The number of serotonergic cell bodies in the rostral raphe, and 5-HT metabolism in the limbic system of weanling offspring was not altered by maternal low protein diet. The deficit in hippocampal 5-HT1A receptor function observed in weanling female offspring persisted into adulthood (PND 112), and was accompanied by an increased sensitivity to stress, specifically increased immobility during a 15-minute forced swim challenge and increased anorexia following 30-minute restraint (PND 97-100). The present work begins to uncover important future directions for understanding the early developmental origins of resilience to stress, and factors that may put individuals at greater risk for stress-related psychiatric disorders.

Directly Observable Behavioral Effects of Lorcaserin in Rats

(1R)-8-chloro-1-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine (lorcaserin) is approved by the United States Food and Drug Administration for treating obesity, and its therapeutic effects are thought to result from agonist activity at serotonin (5-HT)2C receptors. Lorcaserin has affinity for other 5-HT receptor subtypes, although its activity at those subtypes is not fully described. The current study compared the behavioral effects of lorcaserin (0.0032-32.0 mg/kg) to the effects of other 5-HT receptor selective agonists in rats (n = 8). The 5-HT2C receptor selective agonist 1-(3-chlorophenyl)piperazine (mCPP, 0.032-1.0 mg/kg) and lorcaserin induced yawning which was attenuated by the 5-HT2C receptor selective antagonist 6-chloro-5-methyl-N-(6-[(2-methylpyridin-3-yl)oxy]pydidin-3-yl)indoline-1-carboxamide (1.0 mg/kg). The 5-HT2A receptor selective agonist 2,5-dimethoxy-4-methylamphetamine (0.1-3.2 mg/kg) induced head twitching, which was attenuated by the 5-HT2A receptor selective antagonist R-(+)-2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol] (MDL 100907, 0.01 mg/kg), lorcaserin (3.2 mg/kg), and mCPP (3.2 mg/kg). In rats pretreated with MDL 100907 (1.0 mg/kg), lorcaserin also induced head twitching. At larger doses, lorcaserin produced forepaw treading, which was attenuated by the 5-HT1A receptor selective antagonist N-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridyl)cyclohexanecarboxamide (0.178 mg/kg). While the behavioral effects of lorcaserin in rats are consistent with it having agonist activity at 5-HT2C receptors, these data suggest that at larger doses it also has agonist activity at 5-HT2A and possibly 5-HT1A receptors. Mounting evidence suggests that 5-HT2C receptor agonists might be effective for treating drug abuse. A more complete description of the activity of lorcaserin at 5-HT receptor subtypes will facilitate a better understanding of the mechanisms that mediate its therapeutic effects.

Animal models of the serotonin syndrome: a systematic review

The serotonin syndrome (SS) is a potentially life-threatening disorder in humans which is induced by ingestion of an overdose or by combination of two or more serotonin (5-HT)-enhancing drugs. In animals, acute administration of direct and indirect 5-HT agonists also leads to a set of behavioral and autonomic responses. In the current review, we provide an overview of the existing versions of the animal model of the SS. With a focus on studies in rats and mice, we analyze the frequency of behavioral and autonomic responses following administration of 5-HT-enhancing drugs and direct 5-HT agonists administered alone or in combination, and we briefly discuss the receptor mediation of these responses. Considering species differences, we identify a distinct set of behavioral and autonomic responses that are consistently observed following administration of direct and indirect 5-HT agonists. Finally, we discuss the importance of a standardized assessment of SS responses in rodents and the utility of animal models of the SS in translational studies, and provide suggestions for future research.

Food restriction and streptozotocin treatment decrease 5-HT1A and 5-HT2A receptor-mediated behavioral effects in rats

Food restriction and hypoinsulinemia can affect the synthesis, turnover, and receptor function of serotonin (5-HT) in brain. This study explored the effects of food restriction and streptozotocin treatment on behavioral effects related to 5-HT1A (+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) and 5-HT2A [(+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)] receptor activation. Lower lip retraction and flat body posture (8-OH-DPAT) and head twitching (DOI) were measured in rats during free feeding, food restriction, after treatment with streptozotocin, and finally after insulin replacement. 8-OH-DPAT induced lower lip retraction and flat body posture whereas DOI induced head twitching. One week of food restriction (10 g/day) decreased 8-OH-DPAT-induced lower lip retraction, 8-OH-DPAT-induced flat body posture, and DOI-induced head twitching. Subsequently, 1 week of free access to food restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. Finally, 1 week after streptozotocin, 8-OH-DPAT-induced flat body posture and DOI-induced head twitching were markedly reduced whereas 8-OH-DPAT-induced lower lip retraction was unchanged. One week of insulin replacement restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. These results show that modest food restriction or experimentally induced diabetes can profoundly affect sensitivity to drugs acting at 5-HT1A or 5-HT2A receptors; these results could be relevant to understanding the comorbidity of depression and diabetes.

Behavioral effects of dipropyltryptamine in rats: evidence for 5-HT1A and 5-HT2A agonist activity

These studies investigated the role of serotonin-1A (5-HT1A) and 5-HT2A receptors in the behavioral effects of dipropyltryptamine (DPT). Eight rats discriminated 0.56 mg/kg 2,5-dimethoxy-4-methylamphetamine (DOM) from saline and responded under a fixed ratio 5 schedule of food presentation; 12 other rats were used for observational studies. DOM and DPT increased responding on the DOM lever with 3.2 mg/kg DPT producing greater than 95% responding on the DOM lever; this effect of DPT was antagonized by the 5-HT2A receptor antagonist MDL100907. In another study, the 5-HT1A and 5-HT7 receptor agonist 8-OH-DPAT produced lower-lip retraction and, at larger doses, flat body posture; DPT alone produced flat body posture and not lower-lip retraction; MDL100907 alone did not produce either effect. Pretreatment with DPT blocked 8-OH-DPAT-elicited lower-lip retraction, suggesting antagonist activity of DPT at 5-HT1A receptors; however, in the presence of MDL100907 DPT produced not only flat body posture but also lower-lip retraction, suggesting that agonist activity of DPT at 5-HT2A receptors masked agonist activity at 5-HT1A receptors. Lower-lip retraction and flat body posture by DPT in the presence of MDL100907 were attenuated by the 5-HT1A receptor antagonist WAY100635. These findings suggest that DPT has agonist activity at 5-HT1A and 5-HT2A receptors and that effects at 5-HT2A receptors mask effects at 5-HT1A receptors.

The effect of the repeated administration of the compound 3,4-methylenedioxymethamphetamine on the response of rats to the 5-HT2A,C receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)

In this study, we examined the effect of the 5-HT2A,C receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on locomotor activity and the head twitch response in rats by the repeated (twice daily for 4 days) subcutaneous administration of 0.9% saline (1 ml/kg) or (+/-)-3,4-Methylenedioxymethamphetamine (MDMA) (20 mg/kg), a compound that selectively destroys serotonergic nerve terminals. There was no difference between the MDMA- and saline-treated rats regarding the (+/-)-DOI-induced (0.625, 1.25 or 2.5 mg/kg, i.p.) increase and decrease in the head twitch response and horizontal locomotor activity, respectively. Overall, repeated MDMA does not alter the behavioral response of rats to (+/-)-DOI.

Distribution of the 5-hydroxytryptamine2C receptor protein in adult rat brain and spinal cord determined using a receptor-directed antibody: effect of 5,7-dihydroxytryptamine

A synthetic peptide, corresponding to the N-terminal decapeptide (+Y11C12) of the rat 5-hydroxytryptamine2C (5-HT2C) receptor protein was used to produce a sheep polyclonal antiserum. Western blot analysis showed that the resultant antibody G241 recognised two membrane proteins, one (55 kDa) approximating the molecular mass of the 5-HT2C receptor (52 kDa) and a second (63 kDa), which may be a glycosylated form of the receptor protein. HEK 293 cells transfected with human 5-HT2C cDNA displayed intense cell surface immunoreactivity with the 5-HT2C antiserum, which was completely prevented by incubating the antibody with the synthetic 5-HT2C peptide (10 microM), whilst neither non-immune serum nor untransfected cells displayed any immunoreactivity. A radioimmunoassay was developed to quantify the regional distribution of 5-HT2C-like immunoreactivity (LI) in the adult rat brain. The choroid plexus contained five-fold higher levels of 5-HT2C-LI than any brain region but high levels were found in the frontal cortex, septum, hypothalamus, and striatum, intermediate levels in the thalamus and midbrain, and lower levels in brainstem, cerebellum, and spinal cord. In rat cortical membranes, the B(max) value from [3H]-mesulergine binding was ten-fold lower than 5-HT2C-LI levels determined by radioimmunoassay, which may reflect measurement of internalised receptor protein by radioimmunoassay which is not detected with conventional 5-HT2C ligands. Ten days after depletion of 5-HT with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), there was a significant increase in 5-HT2C-LI in the choroid plexus and the ventral cervical spinal cord, suggesting that receptors therein are located post-synaptic to destroyed serotonergic nerve terminals. In contrast, the significant reduction in 5-HT2C-LI observed in the midbrain, brainstem, and dorsal thoracic spinal cord following 5,7-DHT implies that 5-HT2C receptors may be located on 5-HT nerve terminals in these regions.

Behaviors induced by 5-hydroxytryptophan in neonatal, preweaning, postweaning, and adult Sprague-Dawley rats

The behaviors induced by the 5-hydroxytryptamine (5-HT) precursor 5-hydroxytryptophan (5-HTP) has been called the "5-HT (serotonin) syndrome." These behaviors and others identified in rat pups were observed following administration of 5-HTP (300 mg/kg, SC) on postnatal (PN) days 3, 14, and 28 and in adult rats. Certain 5-HT syndrome behaviors and other uniquely neonatal behaviors were present in PN3 pups treated with vehicle. 5-HTP-treated PN3 pups showed increased head-shakes, rollovers, vocalizations, and forepaw treading and decreased hindlimb abduction. No 5-HT syndrome or neonatal behaviors were present at PN14 or PN28 or in adults treated with vehicle. 5-HTP administered at PN14 stimulated circling, forepaw treading, and resting tremor; at PN28, stimulated head-shakes and resting tremor; and in adults produced only head-shakes. To determine if prior exposure to 5-HTP affected the sensitivity of 5-HT receptor subtypes, the 5-HT1A agonist (+/-)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and the 5-HT2/1C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were administered to all rats as adults. 8-OH-DPAT (1 mg/kg, SC) produced flattened body posture unaffected by prior exposure to 5-HTP. Head-shakes induced by DOI (5mg/kg, IP) were decreased by prior exposure to 5-HTP at PN3 and adult, but increased by preexposure at PN28. Thus, serotonergic neural systems are implicated in some behaviors of neonates. The developmental patterns suggest changes in the sensitivity to these systems. Further, lasting changes in 5-HT2/1C receptor sensitivity occur due to exposure to 5-HTP.

Behavioral pharmacology of antagonists at 5-HT2/5-HT1C receptors

The possible implication of 5-HT2 receptors in CNS disorders such as schizophrenia, anxiety and depression suggests that 5-HT2 antagonists may be useful in the treatment of these disorders. The present review examines behavioral procedures used to characterize 5-HT2 antagonist properties of compounds and behavioral models of clinical activity in which 5-HT2 antagonists have been reported to be active. The pharmacological profile of 5-HT2 receptors in part resembles that of 5-HT1C receptors. Responses that have been proposed to involve the activation of 5-HT1C receptors are examined for their usefulness to detect 5-HT1C antagonist properties of compounds; these responses would help to differentiate 5-HT2 from 5-HT1C antagonist activity.

Inhibition of 5-hydroxytryptamine neuronal activity by the 5-HT agonist, DOI

Systemic, intra-raphe and microiontophoretic administration of the 5-hydroxytryptamine (5-HT)1C/5-HT2 agonist (1-(2,5-dimethoxy-4-iodophenyl)-2- aminopropane (DOI) inhibited the firing of 5-HT neurones in the dorsal raphe. DOI administered systemically and directly into the raphe also decreased the extracellular concentration of 5-hydroxytryptamine (5-HT) in the frontal cortex. In contrast, the administration of DOI directly into the frontal cortex did not significantly alter the concentration of frontal cortical extracellular 5-HT. The reduction of the firing rate of 5-HT neurons in the dorsal raphe and extracellular 5-HT concentration in the frontal cortex induced by systemic administration of DOI could not be blocked by the 5-HT2 antagonist ketanserin, ritanserin (5-HT2/5-HT1C antagonist) or the putative 5-HT1A antagonist, pindolol. These results suggest that the inhibition of 5-HT neuronal firing seen with administration of DOI is mediated via an action within the dorsal raphe and at least in close proximity to the 5-HT neurone cell bodies. The decrease in frontal cortical extracellular concentration of 5-HT release was not due to a direct action in the frontal cortex itself and may possibly be as a result of the decrease in the firing rate of the 5-HT neurones in the dorsal raphe. The mechanism of action of DOI to produce these effects is, however, unclear and warrants further investigation.

Postsynaptic 5-HT1 receptors and offensive aggression in rats: a combined behavioural and autoradiographic study with eltoprazine

The present study was designed to assess whether the antiaggressive effects of eltoprazine are mediated via presynaptic and/or postsynaptic 5-HT1 receptors. We describe the effects of central 5-HT depletion 1) on the behaviour of resident TMD-S3 rats in a territorial situation, 2) on the efficacy of eltoprazine to inhibit offensive aggression, and 3) on the 5-HT1A, 5-HT1B and 5-HT1C receptor binding in brains of rats previously used in behavioural studies. Male resident rats were given combined 5,7-dihydroxytryptamine (5,7-DHT) injections into the dorsal and median raphe nuclei. Two to four weeks after the lesions, rats were confronted with an intruder Wiser rat in their home cage for a 10-min period. The 5,7-DHT treatment resulted in a modest reduction of offensive behaviour, while having no effects on other social and nonsocial behaviours. Oral administration of eltoprazine (1 mg/kg) specifically reduced offensive aggression in both sham- and 5,7-DHT-lesioned animals, leaving social interest and exploration intact or even increasing it. A low dose (0.3 mg/kg) of eltoprazine did not affect the behavioural repertoire of sham-operated rats, whereas this dose significantly reduced offense behaviours in the 5,7-DHT-lesioned residents. Quantitative autoradiographic studies 5 weeks after 5,7-DHT treatment revealed a significant increase in radioligand binding to 5-HT1A, 5-HT1B and 5-HT1C sites in many brain regions studied, except for the raphe nuclei where [3H]8-OH-DPAT binding to 5-HT1A sites was markedly reduced. The concentrations of 5-HT and 5-HIAA in frontal cortex were reduced to approximately 10% of controls. The results indicate that serotonin has a stimulatory rather than an inhibitory influence on offensive aggressive behaviour. Central 5-HT depletion does not prevent the antiaggressive effects of eltoprazine, indicating a role for postsynaptic 5-HT1 receptors in the modulation of offensive aggression. The 5,7-DHT-induced overall upregulation of 5-HT1A, 5-HT1B and 5-HT1C binding sites suggests that these three receptor subtypes receive a tonic serotonergic influence. It is conceivable that this postsynaptic 5-HT1 receptor supersensitivity is reflected by the increased efficacy of eltoprazine to inhibit offensive aggression.