5-HT1A, 5-HT1B and 5-HT2 receptor agonists induce differential behavioral responses in preweanling rat pups

Profiling of behavioral effects evoked by ketamine and the role of 5HT2 and D2 receptors in ketamine-induced locomotor sensitization in mice

Ketamine has addictive potential, a troublesome fact due to its promising use as a therapeutic drug. An important phenomenon associated with drug addiction is behavioral sensitization, usually characterized as augmented locomotion. However, other behaviors may also be susceptible to sensitization, and/or interfere with locomotor activity. Thus, this study drew a comprehensive behavioral 'profiling' in an animal model of repeated administration of ketamine. Adult Swiss mice received single daily ketamine injections (30 or 50 mg/Kg, i.p.), which were followed by open field testing for 7 days (acquisition period, ACQ). A ketamine challenge (sensitization test, ST) was carried out after a 5-day withdrawal. Locomotion, rearing, grooming, rotation and falling were assessed during ACQ and ST. All behaviors were affected from the first ACQ day onwards, with no indication of competition between locomotion and the other behaviors. Only locomotion in response to 30 mg/Kg of ketamine both escalated during ACQ and expressed increased levels at ST, evidencing development and expression of locomotor sensitization. Considering the involvement of serotonin 5HT₍₂₎ and dopamine D₍₂₎ receptors on addiction mechanisms, we further tested the involvement of these receptors in ketamine-induced sensitization. Ketanserin (5HT₂ antagonist, 3 mg/Kg, s.c.) prevented ketamine-evoked development of locomotor sensitization. However, ketanserin pretreatment during ACQ failed to inhibit its expression during ST. Raclopride (D₂ antagonist, 0.5 mg/Kg, s.c.) evoked less robust reductions in locomotion but prevented the development of ketamine-evoked sensitization. Pretreatment during ACQ further inhibited the expression of sensitization during ST. These results indicate that a partial overlap in serotonergic and dopaminergic mechanisms underlies ketamine-induced locomotor sensitization.

Lasting neurobehavioral abnormalities in rats after neonatal activation of serotonin 1A and 1B receptors: possible mechanisms for serotonin dysfunction in autistic spectrum disorders

RATIONALE

Perinatal exposure of rats to selective serotonin reuptake inhibitors (SSRIs) produces sensory and social abnormalities paralleling those seen in autistic spectrum disorders (ASDs). However, the possible mechanism(s) by which this exposure produces behavioral abnormalities is unclear.

OBJECTIVE

We hypothesized that the lasting effects of neonatal SSRI exposure are a consequence of abnormal stimulation of 5-HT1A and/or 5-HT1B receptors during brain development. We examined whether such stimulation would result in lasting sensory and social deficits in rats in a manner similar to SSRIs using both direct agonist stimulation of receptors as well as selective antagonism of these receptors during SSRI exposure.

METHODS

Male and female rat pups were treated from postnatal days 8 to 21. In Experiment 1, pups received citalopram (20 mg/kg/day), saline, (±)-8-hydroxy-dipropylaminotetralin hydrobromide (8-OH-DPAT; 0.5 mg/kg/day) or 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]-quinoxaline dimaleate (CGS-12066B; 10 mg/kg/day). In Experiment 2, a separate cohort of pups received citalopram (20 mg/kg/day), or saline which was combined with either N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclo-hexanecarboxamide maleate (WAY-100635; 0.6 mg/kg/day) or N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1-1'-biphenyl-4-carboxamide (GR-127935; 6 mg/kg/day) or vehicle. Rats were then tested in paradigms designed to assess sensory and social response behaviors at different time points during development.

RESULTS

Direct and indirect neonatal stimulation of 5-HT1A or 5-HT1B receptors disrupts sensory processing, produces neophobia, increases stereotypic activity, and impairs social interactions in manner analogous to that observed in ASD.

CONCLUSION

Increased stimulation of 5-HT1A and 5-HT1B receptors plays a significant role in the production of lasting social and sensory deficits in adult animals exposed as neonates to SSRIs.

Evidence that the deficit in sexual behavior in adult rats neonatally exposed to citalopram is a consequence of 5-HT1 receptor stimulation during development

Neonatal (postnatal days 8-21) exposure of rats to the selective serotonin reuptake inhibitor (SSRI), citalopram, results in persistent changes in behavior including decreased sexual activity in adult animals. We hypothesized that this effect was a consequence of abnormal stimulation of 5-HT(1A) and/or 5-HT(1B) receptors as a result of increased synaptic availability of serotonin during a critical period of development. We examined whether neonatal exposure to a 5-HT(1A) (8OH-DPAT) or a 5-HT(1B) (CGS 12066B) receptor agonist can mimic the effect of neonatal exposure to citalopram on adult sexual behavior. Results showed that neonatal treatment with 5-HT(1B) receptor agonist robustly impaired sexual behavior similar to the effect of citalopram, whereas exposure to 5-HT(1A) receptor agonist only moderately influenced male sexual activity in adult animals. These data support the hypothesis that stimulation of serotonin autoreceptors during development contributes to the adult sexual deficit in rats neonatally exposed to citalopram.

Low dose effects in psychopharmacology: ontogenetic considerations

Low doses of psychoactive drugs often elicit a behavioral profile opposite to that observed following administration of more substantial doses. Our laboratory has observed that these effects are often age-specific in rats. For instance, whereas moderate to high doses of the dopamine agonist apomorphine increase locomotion, suppressed locomotor activity is seen following low dose exposure, with this low dose effect not emerging consistently until adolescence. A somewhat earlier emergence of a low dose "paradoxical" effect is seen with the 5HT(1a) receptor agonist, 8-OH-DPAT, with late preweanling, but not neonatal, rats showing increases in ingestive behavior at low doses but suppression at higher doses. In contrast to these ontogenetic increases in expression of low dose drug effects, low dose facilitation of social behavior is seen following ethanol only in adolescent rats and not their mature counterparts, although suppression of social interactions at higher doses is seen at both ages. This hormesis-like low dose stimulation appears related in part to overcompensation, with brief social suppression preceding the subsequent stimulation response, and also bears a number of ontogenetic similarities to acute tolerance, a well characterized, rapidly emerging adaptation to ethanol. Implications of these and other ontogenetic findings for studies of hormesis are discussed.

5-HT(1A) receptor subsensitivity in infancy and supersensitivity in adulthood in an animal model of depression

Flinders Sensitive Line (FSL) rats, a genetic animal model of depression, exhibit 5-HT abnormalities including an increased sensitivity to serotonergic agonists. Studies of 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT)-induced hypothermia have shown 5-HT(1A) receptor supersensitivity in adult FSL rats. The aims of the present study were to: (a) further characterize the 5-HT(1A) system in adult FSL rats by assessing their behavioral responses to 8-OH-DPAT (the 'serotonergic syndrome'-a pattern of stereotypic behavior on a number of parameters). (b) Examine behavioral and hypothermic responses of neonatal FSL pups to 8-OH-DPAT. In adult FSL rats the 'serotonergic syndrome' was measured (Exp. 1) and in 1-day-old pups body temperature and ultrasonic vocalizations (USV) were also examined (Exp. 2) following administration (s.c. and i.p., respectively) of 8-OH-DPAT (1 mg/kg). Overall, adult FSL rats exhibited a significant supersensitive behavioral response to 8-OH-DPAT compared to Sprague-Dawley controls. Under baseline conditions, body weight, USV and rectal temperature were significantly lower in FSL pups compared to controls. In contrast to the adults, 1-day-old FSL pups exhibited an overall pattern of subsensitive stereotypic response to the agonist. In conclusion, an abnormal pattern of response to 8-OH-DPAT was observable in FSL rats, as early as postnatal day 1. This may contribute to a better understanding of the involvement of the serotonergic system in depression, and lead to pharmacological interventions aiming to modulate 5-HT regulation.

Cholinergic/serotonergic interactions in hypothermia: implications for rat models of depression

This article reviews published reports and presents new evidence that support a number of commonalties between lines of rats selectively bred for differences in cholinergic (muscarinic) and serotonergic (5-HT1A) sensitivity. The Flinders Sensitive Line (FSL) rat, a genetic animal model of depression derived for cholinergic supersensitivity, is more sensitive to both cholinergic and serotonergic agonists, and exhibits exaggerated immobility in the forced swim test relative to the control, Flinders Resistant Line (FRL), rat. Similar exaggerated responses are seen in a line of rats recently selected for increased sensitivity to the 5-HT1A agonist, 8-OH-DPAT (High DPAT Sensitive--HDS), relative to lines selectively bred for either low (Low DPAT Sensitive--LDS) or random (Random DPAT Sensitive--RDS) sensitivity to 8-OH-DPAT. For both the FSL and HDS rats, their exaggerated immobility in the forced swim test is reduced following chronic treatment with antidepressants. The present studies examined further the interaction between cholinergic and serotonergic systems in the above lines. Supersensitive hypothermic responses to 8-OH-DPAT were observed very early (postnatal day 18) in FSL rats, suggesting that both muscarinic and serotonergic supersensitivity are inherent characteristics of these rats. Scopolamine, a muscarinic antagonist, completely blocked the hypothermic effects of the muscarinic agonist oxotremorine in FSL and FRL rats, but had no effect on the hypothermic responses to 8-OH-DPAT, suggesting an independence of muscarinic and 5-HT1A systems. On the other hand, genetic selection of genetically heterogeneous rats for differential hypothermic responses to the muscarinic agonist oxotremorine were accompanied by differential hypothermic responses to 8-OH-DPAT, suggesting an interaction between muscarinic and 5-HT1A systems. Overall, these studies argue for an inherent interaction between muscarinic and 5-HT1A systems, which probably occurs beyond the postsynaptic receptors, possibly at the level of G proteins.

Role of the 5-HT1A receptor in development of the neonatal rat brain: preliminary behavioral studies

Serotonin exerts an influence on the prenatal development of rat brain. However, later developmental times may be more applicable to the understanding of the role of serotonin in human developmental disorders. Therefore, the current study was undertaken to gain preliminary information on the postnatal effects of serotonin on rat brain development. As the 5-HT1A receptor has been shown to be involved in much of the developmental functions of serotonin, an agonist for this receptor, 8-hydroxy-DPAT (8-OH-DPAT), was used. Neonatal rat pups at three ages (postnatal days, PNDs) 3-10, 10-17 or 17-24) were injected daily with 1 mg/kg 8-OH-DPAT and evaluated for behavioral consequences. The youngest group showed accelerated incisor eruption and eye-opening, a possible consequence of 5-HT1A receptor interactions with epidermal growth factor (EGF). Behaviorally, the animals were more anxious. Animals treated from PND 10-17, showed no change in craniofacial development but showed greater behavioral maturity in measures of spontaneous alternation and activity in the open field. The oldest animals (PND 17-24) showed no behavioral alterations, suggesting that this time length is beyond the critical period for serotonin's influence in brain development.

Neonatal organizational effects of the 5-HT2 and 5-HT1A subsystems on adult behavior in the rat

Males, females, neonatally androgenized females, and neonatally castrated males were treated over the second week of life with 0.25 mg/kg of either the 5-HT2 agonist 1-(2,5-dimethoxy-3-iodophenyl)-2-aminopropane HCl (DOI), the 5-HT2 antagonist ritanserin (Rit), the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), or the 5-HT1A antagonist WAY100135 (WAY). Exploration, anxiety, sociosexual preferences, and sexual behavior were measured in adulthood. Agents acting on 5-HT1A receptors do not appear to affect organization of any of the behavioral systems studied. DOI increased exploratory activity but in females only, which suggests that testosterone antagonizes the stimulatory effect of 5-HT2 activity on exploration. Neonatal ritanserin selectively reduced anxiety in females, and DOI had a similar effect in androgenized females. This indicates that neonatal 5-HT2 activity is anxiogenic in normal females, anxiolytic in androgenized females, and has no effect on anxiety in males. Males and androgenized females both showed a preference for the female teaser that was abolished by the 5-HT2 agonist, DOI. These results point out that 5-HT2 activity selectively suppresses heterosexual preference induced in the presence of neonatal testosterone. DOI also reduced both male sexual behavior in males and female sexual behavior in androgenized females. Thus, the 5-HT2 system antagonizes the action of testosterone in stimulating heterosexual orientation and sexual activity, and this is independent of genetic sex.

The 5-HT1A agonist 8-OH-DPAT increases attachment maintenance but decreases suckling-related intake in 17-18-day-old rat pups

Deprived and nondeprived preweanling (17-18 days of age) Sprague-Dawley rat pups were injected with 0, 0.03, 0.06, 0.1, or 0.5 mg/kg of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and observed in a suckling test using a milk replete anesthetized dam, with milk let-downs being intermittently precipitated via IV infusions of oxytocin. In experiment 1, the 0.5 mg/kg dose of 8-OH-DPAT was observed to increase the proportion of nondeprived animals which attached to a nipple; no dose effect was seen in deprived animals, who generally all attached. Deprived pups given the 0.5-mg/kg dose exhibited a lower frequency of nipple disattachment/reattachment following milk let-downs and had significantly lower percent body weight gains when compared with saline controls. In experiment 2a, the 0.5-mg/kg dose of 8-OH-DPAT was observed to decrease the overall incidence of nipple disattachment/reattachment as well as to suppress nipple shifting per se in both deprived and nondeprived 17-18-day-old rat pups; this dose also suppressed body weight gains in both the deprived and nondeprived pups. The suppression in weight gain by 8-OH-DPAT does not appear to be primarily related to a drug-induced reduction in nipple shifting. In experiment 2b, where pups were given access to only one nipple, an 8-OH-DPAT-related reduction in body weight gain was still evident. These experiments, which demonstrate that attachment maintenance and suckling ingestion are altered in opposite ways by 8-OH-DPAT, provide strong evidence that these two suckling-related phenomena are subject to different physiological controls.

Effects of the serotonin antagonist amesergide on reproduction in female rats

Amesergide, a serotonin (5-HT2) antagonist intended to treat depression, was administered orally to female CD rats (20/group) at doses of 0, 3, 10, or 30 mg/kg to evaluate effects on mating, fertility, litter size, live birth index (100 x total liveborn progeny/litter size), progeny survival, and weight gain of each litter. The treatment period extended from two weeks prior to mating through postpartum day 21 to cover possible effects of estrous cycle, mating, gestation, and postpartum events. Twenty additional female rats were given 30 mg/kg through gestation day 18, after which they received the acacia vehicle (recovery group). All females were allowed to deliver naturally and rear their progeny. On postpartum day 8, progeny in the control, 30 mg/kg and 30 mg/kg recovery groups were removed from dams for 4 h. Progeny were weighed as litters, returned to the dams for a 1-h nursing period, and then weighed again to provide an indication of milk intake. Mating and fertility, using the present study design, were not affected by treatment with amesergide. No effects were observed on litter size, live birth index, or progeny survival. In contrast, treatment with amesergide throughout gestation and lactation produced a significant dose-related depression in progeny body weight gains. However, when treatment was discontinued after day 18 of gestation (30 mg/kg recovery group), progeny body weight gains did not differ from those of the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent changes in serotonergic modulation of yawning in the rat

Serotonin (5-HT) effects on physostigmine (PHY)-induced yawning were studied in LY Sprague-Dawley rats by injecting Lu 10 171 (citalopram), a specific 5-HT uptake blocker, and two antagonists--methiothepine and ritanserin--which differ slightly in the selectivity of their actions on different 5-HT receptor subtypes. Infant and young rats show significant increases in PHY-induced yawning when preinjected with citalopram (5-10 mg/kg). Two-month-old animals show this effect only with 10 mg/kg. With adult animals (3-5 months old), the effect is the opposite: Yawning decreases. The facilitory effect in infant and young rats was counteracted by methiothepine but not by ritanserin, suggesting that it is mediated through 5-HT1A or 5-HT1B receptor subtypes. The inhibitory effect of citalopram in adult rats was unmodified by the two antagonists used, leaving open the possibility that it is mediated by 5-HT3 receptors.

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.

Low doses of the 5-HT1A receptor agonist 8-OH-DPAT increase ingestive behavior in late preweanling and postweanling, but not neonatal rat pups

Late preweanling (postnatal day 17-18; P17-18) and postweanling (P28-29) Sprague-Dawley rat pups were tested in the presence of both mash and liquid diets following s.c. administration of 0, 0.03, 0.06, 0.1 or 0.5 mg/kg of the 5-HT1A receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT). Preweanling pups exhibited both a low dose stimulatory and high dose depressant effect of 8-OH-DPAT on mash-related ingestive behaviors, whereas postweanling rat pups were less sensitive to 8-OH-DPAT, exhibiting only an increase in mash ingestion within the dose range examined. In contrast, when neonatal (P3-4) rat pups were tested with milk across a dose range of 0.001-1.0 mg/kg 8-OH-DPAT, no increases in ingestion were seen, with lower doses in this range having no detectable effects and higher doses decreasing ingestion and mouthing. To the extent that low doses of 8-OH-DPAT increase feeding via preferential stimulation of 5-HT1A autoreceptors, these data suggest that neonates may lack functional 5-HT1A autoreceptors, with these receptors maturing by the late preweanling period.