Comparison of the effects of 2,5-dimethoxy-4-iodoamphetamine and D-amphetamine on the ability of rats to discriminate the durations and intensities of light stimuli.

Rats' ability to discriminate durations is disrupted by the monoamine-releasing agent D-amphetamine and the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI). It is unknown whether this effect is specific for temporal discrimination or reflects general disruption of stimulus control. This experiment addressed this question by comparing the effects of D-amphetamine and DOI on temporal discrimination and discrimination along a nontemporal dimension, light intensity. Twelve rats responded on a schedule in which a light (intensity 22 cd/m) was presented for t seconds (2.5-47.5 s), after which levers A and B were presented. Responses on A were reinforced when t was less than 25 s, and responses on B were reinforced when t was greater than 25 s. Twelve rats responded on a similar schedule in which a light of intensity i (3.6-128.5 cd/m) was presented for 25 s. Responses on A were reinforced when i was less than 22 cd/m, and responses on B were reinforced when i was greater than 22 cd/m. Logistic functions were fitted and psychophysical parameters estimated [T50, I50 (central tendency of temporal or light-intensity discrimination); Weber fraction (relative discriminative precision)]. D-Amphetamine (0.2-0.8 mg/kg) increased the Weber fraction for temporal and light-intensity discrimination; DOI (0.625-0.25 mg/kg) increased it for temporal discrimination only. Both drugs increased T50; neither altered I50. D-Amphetamine and DOI have similar effects on temporal discrimination but different effects on light-intensity discrimination. The increase in T50 may reflect the impairment of sustained attention during prolonged stimulus presentation.

Effects of social isolation rearing on the limbic brain: a combined behavioral and magnetic resonance imaging volumetry study in rats.

Rearing rats in social isolation from weaning induces robust behavioral and neurobiological alterations resembling some of the core symptoms of schizophrenia, such as reduction in prepulse inhibition of acoustic startle (PPI) and locomotor hyperactivity in a novel arena. The aim of this study was to investigate whether social isolation rearing induces volumetric remodeling of the limbic system, and to probe for anatomical structure-behavioral interrelations. Isolation- (n=8) and group-reared (n=8) rats were examined by magnetic resonance (MR) volumetry using high-resolution T2-weighted imaging at 7 T. Volumes of medial prefrontal cortex (mPFC), anterior cingulate cortex (ACC), retrosplenial cortex (RSC) and hippocampal formation were compared between groups and with behavioral measures, i.e. PPI and locomotor activity in a novel arena. Isolation rearing induced locomotor hyperactivity and impaired PPI compared with group-housed rats. The right mPFC was significantly reduced (5.4%) in isolation-reared compared with group-reared rats, with a similar trend on the left side (5.2%). mPFC volumes changes were unrelated to behavioral abnormalities. No significant volume changes were observed in ACC, RSC or hippocampal formation. Hippocampal volumes were associated with the magnitude of PPI response in control but not in isolation-reared rats. Rearing rats in social isolation induced remodeling of the limbic brain with selective prefrontal cortex volume loss. In addition, a dissociation of the interrelation between hippocampal volume and PPI was noted in the isolation-reared rats. Taken together, limbic morphometry is sensitive to the effects of social isolation rearing but did not reveal direct brain-behavior interrelations, calling for more detailed circuitry analysis.

Impact of early exposure to a cafeteria diet on prefrontal cortex monoamines and novel object recognition in adolescent rats.

The prefrontal cortex (PFC) undergoes protracted postnatal development such that its structure and behavioural function may be profoundly altered by environmental factors. Here we investigate the effect of lactational dietary manipulations on novel object recognition (NOR) learning and PFC monoamine neurotransmitter metabolism in early adolescent rats. To this end, Wistar rat dams were fed a high caloric cafeteria diet (CD) during lactation and resultant 24-26 day old offspring exposed to NOR testing and simultaneous PFC dopamine and serotonin metabolism measurement. In the second NOR choice trial where one familiar and one novel object were presented controls explored the novel preferentially to the familiar object both after a 5 min (P < 0.001) or 30 min (P < 0.05) inter-trial intervals (ITI). By contrast, offspring from dams fed on lactational CD failed to show any significant preference for the novel object at either time point. Compared with chow fed controls, their average exploration ratio of the novel object was lower after the 5 min ITI (P < 0.05). Following a 60 min ITI, neither CD nor control offspring showed a preference for the novel object. PFC dopamine metabolism was significantly reduced in the CD group (P < 0.001), whereas serotonin metabolism was increased (P < 0.001). These results suggest that an obesogenic lactational diet can have a detrimental impact on cognition in adolescent offspring associated with aberrant PFC serotonin and dopamine metabolism.

Tolerance to the effect of 2,5-dimethoxy-4-iodoamphetamine (DOI) on free-operant timing behaviour: interaction between behavioural and pharmacological mechanisms.

RATIONALE: The psychostimulant d-amphetamine, the D(2/3) dopamine receptor agonist quinpirole and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behaviour. There is evidence that tolerance develops to the effects of psychostimulants on timing performance during chronic treatment; this tolerance is generally attributed to behavioural adaptation rather than to pharmacological desensitisation. There have been no previous investigations of tolerance to the effect of DOI on free-operant timing behaviour. OBJECTIVE: To demonstrate tolerance to DOI's effect on timing performance and to examine the nature of this tolerance. MATERIALS AND METHODS: Rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 80-s trials in which reinforcement was provided intermittently for responding on A in the first half and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 8-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices (T (50) [time corresponding to %B = 50]; Weber fraction). RESULTS: In experiment 1, DOI (0.25 mg kg(-1)) reduced T (50) compared to vehicle; tolerance to this effect was seen after repeated daily treatments with DOI if the rats were exposed to behavioural training during the period of treatment but not if the repeated treatments took place during a 'holiday' from behavioural training. In experiment 2, repeated treatment with DOI resulted in tolerance to the effect of DOI on T (50) and cross-tolerance to the effect of d-amphetamine (0.4 mg kg(-1)), but no cross-tolerance was seen to the effect of quinpirole (0.08 mg kg(-1)). CONCLUSIONS: The results indicate that behavioural adaptation is involved in the development of tolerance to DOI's effect on timing. The finding of cross-tolerance to d-amphetamine but not to quinpirole suggests that the reduction of T (50) in the free-operant psychophysical procedure may be brought about by two distinct pharmacological mechanisms, one activated by DOI and d-amphetamine, and the other by quinpirole.

Evidence for the sensitivity of operant timing behaviour to stimulation of D1 dopamine receptors.

RATIONALE: Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Previous studies using the fixed-interval peak procedure implicated D(2)-like dopamine receptors in these effects. However, recent findings suggest that d-amphetamine alters timing performance on the free-operant psychophysical procedure via D(1)-like receptors. It is not known whether this effect of d-amphetamine is mimicked by direct D(1)-like receptor stimulation. OBJECTIVE: The effects of a D(1)-like receptor agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine (SKF-81297) on performance on the free-operant psychophysical procedure and the interaction between SKF-81297 and a D(1)-like receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566) and a D(2)-like receptor antagonist haloperidol, were examined. MATERIALS AND METHODS: Rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data (percent responding on B [%B] vs time from trial onset [t]) under each treatment condition, and quantitative indices of timing (T(50) [value of t corresponding to %B = 50] and the Weber fraction [(T(75)-T(25))/2T(50); T(25) and T(75) are values of t corresponding to %B = 25 and %B = 75] were compared among treatments. RESULTS: SKF-81297 (0.8 mg kg(-1)) reduced T(50); this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) but not by haloperidol (0.05, 0.1 mg kg(-1)). CONCLUSIONS: Stimulation of D(1)-like dopamine receptors affects performance in the free-operant psychophysical procedure.

Effect of quinpirole on timing behaviour in the free-operant psychophysical procedure: evidence for the involvement of D2 dopamine receptors.

RATIONALE: Operant timing behaviour is sensitive to dopaminergic manipulations. It has been proposed that this effect is mediated principally by D(2)-like dopamine receptors. However, we recently found that the effect of d-amphetamine on timing in the free-operant psychophysical procedure was mediated by D(1)-like dopamine receptors. It has not been established whether stimulation of D(2)-like receptors affects timing in this schedule. OBJECTIVE: To examine the effects of a D(2)-like receptor agonist quinpirole on second-range timing and the ability of dopamine receptor antagonists to reverse quinpirole's effects. MATERIALS AND METHODS: Rats responded on two levers (A and B) under a free-operant psychophysical schedule in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rates [percent responding on B (%B) vs time (t)] under each treatment; quantitative timing indices [T (50) (value of t when %B = 50) and Weber fraction] were compared among treatments. RESULTS: Quinpirole (0.04, 0.08 mg kg(-1)) reduced T (50). This effect was attenuated by D(2)-like receptor antagonists haloperidol (0.05, 0.1 mg kg(-1)), eticlopride (0.04, 0.08 mg kg(-1)) and sulpiride (30, 60 mg kg(-1)), but not by the D(3) receptor-preferring antagonist nafadotride (0.5, 1 mg kg(-1)), the D(4) receptor antagonist L-745870 (1, 3 mg kg(-1)) or the D(1)-like receptor antagonist SKF-83566 (0.015 mg kg(-1)). CONCLUSIONS: Results suggest that quinpirole reduced T (50) via an action at D(2) receptors. D(1)-like and D(2)-like receptors may mediate behaviourally similar but pharmacologically distinct effects on timing behaviour.

Effects of d-amphetamine and DOI (2,5-dimethoxy-4-iodoamphetamine) on timing behavior: interaction between D1 and 5-HT2A receptors.

RATIONALE: The dopamine-releasing agent d-amphetamine and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behavior. The selective D(1) dopamine receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), but not the D(2) dopamine receptor antagonist haloperidol, can antagonize the effect of d-amphetamine, and the selective 5-HT(2A) receptor antagonist (+/-)2,3-dimethoxyphenyl-1-(2-(4-piperidine)-methanol (MDL-100907) can antagonize the effect of DOI. However, it is not known whether the effect of d-amphetamine can be reversed by MDL-100907 and the effect of DOI by dopamine receptor antagonists. OBJECTIVE: The objective of this work is to examine the interactions of d-amphetamine and DOI with MDL-100907, SKF-83566, and haloperidol on timing performance. MATERIALS AND METHODS: Rats (n = 12-15 per experiment) were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices [T (50) (time corresponding to %B = 50); Weber fraction]. Rats were treated systemically with d-amphetamine or DOI, alone and in combination with haloperidol, SKF-83566, or MDL-100907. RESULTS: d-Amphetamine (0.4 mg kg(-1)) reduced T (50) compared to vehicle; this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) and MDL-100907 (0.5 mg kg(-1)), but not by haloperidol (0.05, 0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced T (50); this effect was reversed by MDL-100907 (0.5 mg kg(-1)), but not by SKF-83566 (0.03 mg kg(-1)) or haloperidol (0.05 mg kg(-1)). CONCLUSIONS: The results suggest that both 5-HT(2A) and D(1) receptors, but not D(2) receptors, are involved in d-amphetamine's effect on timing behavior in the free-operant psychophysical procedure. DOI's effect on timing is mediated by 5-HT(2A) receptors, but neither D(1) nor D(2) receptors are involved in this effect.

Evidence for a role of D1 dopamine receptors in d-amphetamine's effect on timing behaviour in the free-operant psychophysical procedure.

RATIONALE: Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Studies using the fixed-interval peak procedure implicated D2 dopamine receptors in these effects. Less is known about the effects of dopaminergic manipulations on temporal differentiation in other timing schedules. OBJECTIVE: To examine the effects of a D1 antagonist,8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), and a D2 antagonist, haloperidol, on performance on the free-operant psychophysical procedure, and the ability of these antagonists to reverse the effects of the catecholamine-releasing agent, d-amphetamine on performance. The antagonists' ability to reverse d-amphetamine-induced hyperlocomotion was also examined. MATERIALS AND METHODS: Rats responded on two levers (A and B) under a free-operant psychophysical schedule, in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rate data (percent responding on B [%B] vs time [t]) in each treatment condition, and quantitative timing indices [T50 (value of t corresponding to %B=50) and Weber fraction] were compared among treatments. Effects of the treatments on locomotion were measured in a separate experiment. RESULTS: SKF-83566 (0.015, 0.03, 0.06 mg kg(-1)) did not affect timing performance. Haloperidol (0.025, 0.05 mg kg(-1)) had no effect; a higher dose (0.1 mg kg(-1)) reduced T (50). d-Amphetamine (0.4 mg kg(-1)) reduced T50; this effect was antagonised by SKF-83566 but not by haloperidol. Both antagonists reduced d-amphetamine-induced hyperlocomotion. CONCLUSIONS: The results suggest that d-amphetamine's effect on performance in the free-operant psychophysical procedure is mediated by D1 rather than D2 receptors.

Effects of 5-HT1A and 5-HT2A receptor stimulation on temporal differentiation performance in the fixed-interval peak procedure.

We examined the effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) on performance on the fixed-interval peak procedure, and the sensitivity of these effects to 5-HT1A and 5-HT2A receptor antagonists (N-[2-(4-[2-methoxyphenyl]-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide [WAY-100635] and ketanserin). Rats were trained to press a lever for food reinforcement in 50 min sessions consisting of 32 trials in which the lever was continuously available, separated by 10 s inter-trial intervals. In 16 trials, reinforcement was delivered following the first response after 30 s had elapsed since trial onset (fixed-interval 30 s). In 16 randomly interposed (peak/probe) trials, reinforcement was omitted, and the lever remained in the operant chamber for 120 s. Response rate in probe trials was plotted against time from trial onset. Time to peak response rate (t(peak)) and the Weber fraction were derived from modified Gaussian curves fitted to each rat's data. 8-OH-DPAT (0.05 mg kg(-1)) reduced t(peak) and increased the Weber fraction; the effect on t(peak) was antagonized by WAY-100635 (0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced t(peak) and increased the Weber fraction; the reduction of t(peak) was antagonized by ketanserin (2 mg kg(-1)). Stimulation of 5-HT1A and 5-HT2A receptors alters temporal differentiation in qualitatively similar ways.

Genetic knockout and pharmacological blockade studies of the 5-HT7 receptor suggest therapeutic potential in depression.

The affinity of several antidepressant and antipsychotic drugs for the 5-HT7 receptor and its CNS distribution suggest potential in the treatment of psychiatric diseases. However, there is little direct evidence of receptor function in vivo to support this. We therefore evaluated 5-HT7 receptors as a potential drug target by generating and assessing a 5-HT7 receptor knockout mouse. No difference in assays sensitive to potential psychotic or anxiety states was observed between the 5-HT7 receptor knockout mice and wild type controls. However, in the Porsolt swim test, 5-HT7 receptor knockout mice showed a significant decrease in immobility compared to controls, a phenotype similar to antidepressant treated mice. Intriguingly, treatment of wild types with SB-258719, a selective 5-HT7 receptor antagonist, did not produce a significant decrease in immobility unless animals were tested in the dark (or active) cycle, rather than the light, adding to the body of evidence suggesting a circadian influence on receptor function. Extracellular recordings from hypothalamic slices showed that circadian rhythm phase shifts to 8-OH-DPAT are attenuated in the 5-HT7 receptor KO mice also indicating a role for the receptor in the regulation of circadian rhythms. These pharmacological and genetic knockout studies provide the first direct evidence that 5-HT7 receptor antagonists should be investigated for efficacy in the treatment of depression.

Involvement of 5-HT receptors in the regulation of food intake in Siberian hamsters.

The Siberian hamster provides a physiological model for understanding the hypothalamic control of energy metabolism as it undergoes annual photoperiod-regulated cycles of body weight (i.e. fattening in summer, and catabolism of fat stores in winter). As a first step to investigate whether enhanced serotonergic (5-HT) tone might underlie the catabolic processes in short days, we investigated whether serotonergic stimulation can produce catabolic actions in fat hamsters housed in long days. Acute treatment with the serotonin reuptake inhibitor (+/-) fenfluramine (8 mg/kg, i.p.) produced a prolonged, dose-dependent reduction in food intake in both photoperiods. Behavioural observations and radiotelemetry analyses revealed that this anorectic effect of fenfluramine was associated with short-term increases in locomotor activity and in core body temperature. In a subsequent series of studies, hamsters were pretreated with the 5-HT2C receptor antagonist SB242084 (4 mg/kg, i.p.). This 5-HT2C receptor antagonist completely blocked the anorectic actions of fenfluramine, but did not decrease the hyperthermia or hyperlocomotion induced by fenfluramine; thus, the anorectic actions of fenfluramine probably reflect actions via the 5-HT2C receptor. Consistent with these observations, treatment of hamsters with the 5-HT2C receptor agonist VER 3323 (10 mg/kg, i.p.) or the 5-HT1B/2C receptor agonist mCPP (3 mg/kg, i.p.) reduced food intake. The response to manipulation of serotonergic pathways was not affected by the ambient photoperiod in any of these studies. We conclude that the anorectic actions of fenfluramine are not an indirect consequence of serotonergic actions on arousal pathways, and that its actions on feeding in the Siberian hamster are most likely to be mediated by the 5-HT2C receptor.

Effects of quipazine and m-chlorophenylbiguanide (m-CPBG) on temporal differentiation: evidence for the involvement of 5-HT2A but not 5-HT3 receptors in interval timing behaviour.

RATIONALE: Temporal differentiation refers to animals' ability to regulate their behaviour during an ongoing interval. Striatal dopaminergic mechanisms are purported to be involved in temporal differentiation, and recent evidence also implicates 5-hydroxytryptaminergic (5-HTergic) mechanisms, possibly mediated by 5-HT(2A) receptors. There is evidence that 5-HT(3) receptors contribute to the regulation of dopamine release in the basal ganglia; however, it is not known whether 5-HT(3) receptor stimulation can influence temporal differentiation. OBJECTIVE: We examined the effects of a selective 5-HT(3) receptor agonist m-CPBG, a mixed 5-HT(2A/3) receptor agonist quipazine, and selective 5-HT(3) and 5-HT(2A) receptor antagonists (MDL-72222 and ketanserin, respectively) on temporal differentiation in a free-operant psychophysical procedure. METHODS: Twenty-four rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement (0.6 M: , 50 microl) was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data [percent responding on B (%B) vs time from trial onset (t)], and quantitative indices of timing performance [T (50) (value of t corresponding to %B=50), Weber fraction, and mean time of switching from A to B, S (50)] were derived. RESULTS: Quipazine (0.5, 1, and 2 mg kg(-1)) altered timing performance, dose-dependently reducing T (50) and S (50); m-CPBG (2.5, 5, and 10 mg kg(-1)) had no significant effect. The effect of quipazine was antagonized by ketanserin (2 mg kg(-1)), but not by MDL-72222 (1 mg kg(-1)). CONCLUSIONS: The present results provide no evidence for the involvement of 5-HT(3) receptors in temporal differentiation and indicate that the effect of quipazine on performance was mediated by 5-HT(2A) receptor stimulation. The results are consistent with previous evidence for the involvement of 5-HT(2A) receptors in interval timing behaviour.

Thyrotropin-releasing hormone (TRH)-like immunoreactivity in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata with special emphasis on the bulbospinal tract.

The distribution of thyrotropin-releasing hormone (TRH)-like immunoreactivity (LI) has been studied in the grey monkey (Macaca fascicularis) spinal cord and medulla oblongata by the use of indirect immunofluorescence and the peroxidase-antiperoxidase (PAP) technique. Furthermore, double-labeling experiments were performed in order to study colocalization of 5-hydroxytryptamine (5-HT)- and substance P-LI. A dense innervation of TRH-immunoreactive (IR) varicose fibers was found in the ventral horn motor nuclei, in the region surrounding the central canal, in the intermediolateral cell column, and in the dorsal horn laminae II and III. In addition, cell bodies harboring TRH-LI were found in the dorsal horn laminae II-IV. In the ventral horn, many of the large cell bodies and their proximal dendrites were totally encapsulated by TRH-IR fibers. From double-labeled sections a high degree of coexistence could be established between TRH-/5-HT-LI, TRH-/substance P-LI, and 5-HT-/substance P-LI in fibers in the motor nuclei; as a consequence, a large proportion of these fibers should harbor TRH-/5-HT-/substance P-LI. A coexistence between TRH-/5-HT-LI could also be demonstrated in the intermediolateral cell column. However, no unequivocal coexistence could be found between TRH-/substance P-LI and 5-HT-/substance P-LI in this region. In the dorsal horn, no clear coexistence could be encountered for any of the above indicated combinations. Electron microscopic analysis of material from the lumbar lateral motor nucleus demonstrated TRH-IR terminals making synapses with large cell bodies and dendrites. In addition, contacts lacking synaptic specializations could also be verified. In the medulla oblongata, with the use of the PAP technique, a large number of cell bodies containing TRH-LI were encountered in the midline raphe nuclei and in nucleus reticularis lateralis. A similar distribution pattern could be found for 5-HT-LI, but no cell bodies containing substance P-LI could be seen in these regions. Chemical analysis of specimens from cervical, thoracic, and lumbar spinal cord revealed higher concentrations of TRH- and 5-HT-LI in the ventral quadrants, whereas substance P-LI dominated in the dorsal quadrants. Thus, the concentrations of TRH-, 5-HT-, and substance P-LI was in accordance with the observed regional variation in density of IR-fibers and varicosities found in the spinal cord. We have shown that TRH-LI has a distribution in the monkey spinal cord and medulla oblongata similar to that previously demonstrated in other species.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced social interaction following 3,4-methylenedioxymethamphetamine is not associated with enhanced 5-HT 2C receptor responsivity.

This study examined the long-term change in serotonergic (5-hydroxytryptamine, 5-HT) neuronal function and 5-HT(2C) receptor agonist-induced behaviour following treatment of young rats with 3,4-methylenedioxymethamphetamine (MDMA). On post-natal day (PND) 28, Lister-hooded rats received either MDMA (15 mg/kg i.p.) or saline (1 ml/kg i.p.) twice daily for 3 days. On PND 50 social interaction was assessed between treatment-matched pairs of rats derived from separate litters. The effect of either the 5-HT(2C) receptor agonist, m-chlorophenylpiperazine (m-CPP, 2.5 or 1 mg/kg i.p., respectively) or saline was examined on open-field exploration (PND 52) and elevated plus-maze behaviour (PND 56). Acutely, MDMA produced hyperlocomotion and hypothermia compared with saline injection (p<0.001). Following 20 days abstinence, social interaction was decreased by 26% (p<0.05) in MDMA pre-treated rats compared with saline controls, without any change in locomotion. There was no difference in open-field or elevated plus-maze behaviour between pre-treatment groups. m-CPP caused hypolocomotion in the open-field and decreased both the percentage entries into, and time spent in, the open arms of the elevated plus-maze to a comparable extent in MDMA and saline pre-treated rats. Hippocampal and frontal cortical 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were significantly reduced in MDMA pre-treated rats, without any change in [(3)H]paroxetine binding or plasma corticosterone levels. These data suggest that the MDMA-induced reduction in social interaction is not mediated via alteration of 5-HT(2C) receptor function.

Involvement of catecholaminergic neurones and alpha-adrenoceptors in the wet-dog shake and forepaw licking behaviour produced by the intrathecal injection of an analogue of thyrotrophin-releasing hormone (CG 3509).

Intrathecal injection of the analogue of TRH, CG 3509, into conscious rats produced dose-related wet-dog shakes and forepaw licking, which showed a bell-shaped relationship of intensity to dose. Pretreatment with alpha-MPT intraperitoneally, markedly reduced levels of noradrenaline and dopamine in the spinal cord and brainstem and attenuated both CG 3509-induced responses, while intrathecal treatment with DSP4 selectively reduced noradrenaline in the spinal cord without affecting either behaviour. Since denervation supersensitivity may develop following treatment with DSP4, these results are not inconsistent with a proposal that bulbospinal noradrenergic neurones modulate the behaviour induced by CG 3509. Wet-dog shakes and forepaw licking induced by CG 3509 were reduced by pretreatment with phenoxybenzamine or prazosin, suggesting that a tonic noradrenergic pathway may facilitate both behavioural responses through alpha 1-adrenoceptors. Methoxamine, combined with CG 3509 partially attenuated the wet dog shake behaviour, but methoxamine produced marked hindlimb jerking which might physiologically antagonise wet-dog shakes. Concomitant administration of clonidine and CG 3509 potently reduced wet-dog shakes in a dose-related manner but did not significantly affect forepaw licking, while idazoxan did not significantly affect either response. The latter findings imply that alpha 2-adrenoceptors play different roles in modulating the two behavioural responses and the possible synaptic location of the receptors is discussed. Taken together these results suggest that CG 3509 may release noradrenaline from bulbospinal neurones regulating motor function.

Comparative behavioural and biochemical effects of repeated intrathecal administration of thyrotrophin-releasing hormone (TRH) or two analogues of TRH in adult rats.

The effect of repeated intrathecal injection of thyrotrophin-releasing hormone (TRH) and two analogues of TRH, C-terminally modified RX 77638 and N-terminally modified CG 3509, were examined on behavioural (wet-dog shakes and forepaw licking) and biochemical markers for spinal motoneurones, bulbospinal raphe nerve terminals and the pituitary-thyroid axis in rats. Saline (10 microliters washed in with 15 microliters), TRH (20 micrograms), RX 77368 (2 micrograms) or CG 3509 (2 micrograms) were administered intrathecally (twice daily for 3 or 5 days), after which levels of plasma-free thyroxine and thyroid-stimulating hormone (TSH) were measured and the dorsal and ventral portions of the thoracolumbar spinal cord, brainstem and hypothalamus were assayed for TRH- and calcitonin gene-related peptide (CGRP)-like immunoreactivity, levels of indoleamines and the activity of choline acetyltransferase (ChAT). Behavioural tolerance developed rapidly with consecutive injections of RX 77368, such that wet-dog shakes were significantly reduced and forepaw-licking tended to be decreased by the third intrathecal injection. Five, but not 3, days of administration of RX 77368 selectively elevated levels of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid in the ventral spinal cord, where these substances are principally located in bulbospinal raphe nerve terminals. The time course of the change in indoleamines suggests that administration of TRH peptides elevated the synthesis, rather than the release, of 5-HT from these nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of 5-HT2B receptors in the medial amygdala causes anxiolysis in the social interaction test in the rat.

In a recent study, we reported the presence of neurones expressing 5-HT2B receptor protein in the medial amygdaloid nucleus of the adult rat brain. In the present study, bilateral micro-injection of the 5-HT2B receptor agonist 1-[5-(2-thienylmethoxy)-1H-3-indolyl]propan-2-amine hydrochloride (BW 723C86, 0.09 and 0.93 nmol, 5 min pretest) into the medial amygdaloid nuclei increased the total interaction time of a pair of male rats in the social interaction test, to a comparable extent to chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) without altering locomotor activity; indicative of anxiolytic activity. The increase in social interaction was prevented by pretreatment with the 5-HT2C/2B receptor antagonist N-(1-methyl-5-indoyl)-N'-(3-pyridyl) urea hydrochloride (SB 200646A, at 2 but not 1 mg/kg p.o., 1 hr pretest), which did not alter behaviour when given alone. Intra-amygdala BW 723C86 (0.09, 0.31 and 0.93 nmol, 5 min pretest) did not significantly alter the number of punished responses made when the same rats were examined seven days later in a Vogel punished drinking test, although chlordiazepoxide (5 mg/kg p.o., 1 hr pretest) produced the expected anxiolytic profile. The results are consistent with the proposal that activation of 5-HT2B receptors in the medial amygdala induces anxiolysis in the social interaction model but has little effect on behaviour in a punished conflict model of anxiety. These data suggest that serotonergic neurotransmission in this nucleus may selectively affect specific kinds of anxiety generated by different animal models.

Immunohistochemical localisation of the 5-HT2C receptor protein in the rat CNS.

5-HT2C receptor mRNA has a widespread distribution in the human and rat CNS but the absence of a specific high affinity ligand has made autoradiographic localisation of the receptor difficult. By raising polyclonal antibodies against the rat 5-HT2C receptor protein this study reports the immunohistochemical distribution of this receptor in the rat CNS. A sephadex purified 5-HT2C antiserum visualised a single immunopositive band (54 kDa) in Western blots of membranes prepared from several rat brain regions and caused intense membrane immunofluorescence in HEK 293 cells transfected with h5-HT2C cDNA, which were both attenuated by incubation with the antigenic peptide sequence (200-300 microM). 5-HT2C-like immunoreactivity was located on neurones throughout the CNS. The most abundant 5-HT2C-like immunoreactive cell bodies were in the anterior olfactory nucleus, medial and intercalated amygdaloid nuclei, hippocampus layers CA1 to CA3, laterodorsal and lateral geniculate thalamic nuclei, caudate-putamen and several areas of the cortex (including piriform and frontal), consistent with this receptor being located postsynaptic to serotonergic neurones. Immunopositive neurones were also found in the dorsal raphé, suggesting that 5-HT2C receptors may be on some serotonergic neurones. The overall distribution of 5-HT2C-like immunoreactivity complements previous findings with conventional radioligands and agrees well with reported levels of 5-HT2C receptor mRNA.

Modification of 5-HT2 receptor mediated behaviour in the rat by oleamide and the role of cannabinoid receptors.

Oleamide (cis-9,10-octadecenoamide) is an endogenous brain lipid which has been suggested to induce sleep in experimental animals. The mechanism of action is unclear but shares many of the characteristics of endogenous cannabinoids such as anandamide and has been shown to enhance in vitro responses to 5-HT and GABA. In the present study we investigated the effects of oleamide on two motor behaviours, back muscle contractions (BMC) and wet-dog shakes (WDS) induced in rats by treatment with the 5-HT2 receptor agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride). We then examined the potential involvement of CB1 cannabinoid receptors in the responses to oleamide and the mechanism of interaction between CB1 and 5-HT2 receptors. Oleamide and the cannabinoid receptor agonist HU210 (6aR)-trans-3-(1,1-dimethylheptyl)6a,7,10,10a-tetrahydro-1-h ydroxy-6,6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol) produced a hypolocomotion which was prevented by the CB1 antagonist SR141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-me thyl-1H-pyrazole-3-carboxamide hydrochloride). Despite having no effect alone, oleamide and HU210 potentiated BMC induced by treatment with DOI. SR141716A alone did not affect the response to DOI but it blocked the potentiations caused by oleamide or HU210. WDS were unaffected by oleamide and slightly reduced by HU210. In vitro, oleamide and HU210 enhanced the high affinity binding of 5-HT to 5-HT2 receptors on rat cerebral cortex membranes labelled with 3H-ketanserin. Neither agent, however, altered 5-HT-stimulated phosphoinositide hydrolysis in rat cerebral cortex slices. Oleamide occupied CB1 cannabinoid receptors on rat brain membranes labelled with 3H-CP55940 with an IC50 of 10 microM. The data presented are consistent with oleamide acting via a cannabinoid recognition site to enhance 5-HT2 receptor function in vivo. The mechanism of the modulation is still unclear but it does not appear to involve a potentiation of 5-HT2 receptor-stimulated phosphoinositide hydrolysis.

A role for 5-ht6 receptors in retention of spatial learning in the Morris water maze.

This study investigates the effect of intracerebroventricular administration of a 5-ht6 antisense oligonucleotide (AO) complementary to bases 1-18 of the rat 5-ht6 cDNA initiation sequence (Mol. Pharmacol. 43 (1993) 320) (1.5 microg twice daily for six days) and i.p. injection of a selective 5-ht6 receptor antagonist Ro 04-6790 (10 or 30 mg/kg once daily for three days) on acquisition and retention in the Morris water maze. Neither the 5-ht6 AO (which reduced cortical [3H]-LSD binding sites by 10-16%) nor Ro 04-6790 affected acquisition, but both enhanced retention of the learned platform position such that rats spent significantly longer searching the trained platform position than any other area during the probe tests. Furthermore, neither AO nor Ro 04-6790 had any effect on the time taken to reach a raised visible platform, indicating that visual acuity was unimpaired. In addition, AO reduced both food consumption and body weight and the later effect was also seen following Ro 04-6790, suggesting a role for the 5-ht6 receptor in the regulation of feeding. Hence, while the underlying mechanism remains unclear, enhanced retention of spatial learning following both AO and 5-ht6 antagonist administration strongly indicate a role for this receptor in memory processes.