A critical role for D1 receptors in the 5-HT1A-mediated facilitation of in vivo acetylcholine release in rat frontal cortex

Vortioxetine as an analgesic in preclinical inflammatory pain models: Mechanism of action

Vortioxetine is a novel atypical antidepressant with multimodal activity that has recently demonstrated efficacy against neuropathic pain. There is no published data about its analgesic properties in models characterized by peripheral inflammation and consequent pain pathway sensitization, nor data on its mechanism of antinociceptive action. This study aimed to investigate vortioxetine's antinociceptive/antihyperalgesic effects in trigeminal, visceral, and somatic inflammatory pain models, and provide evidence on its mechanism of action in the modulation of trigeminal nociception. Vortioxetine's effects on the nociceptive behavior in orofacial formalin test (OFT) and acetic acid-writhing test in mice and on mechanical hyperalgesia in carrageenan-induced paw inflammation in rats were examined following peroral single administration. The involvement of serotonergic/adrenergic/cholinergic/cannabinoid/adenosine receptors was evaluated in OFT by intraperitoneally treating mice with an appropriate antagonist immediately after vortioxetine application. We used antagonists of 5-HT_1B/1D serotonergic (GR 127935), α₁ -adrenergic (prazosin), α₂ -adrenergic (yohimbine), β₁ -adrenergic (metoprolol), muscarinic (atropine), α₇ nicotinic (methyllycaconitine), CB₁ /CB₂ cannabinoid (AM251 and AM630), and adenosine A₁ (DPCPX) receptors. Vortioxetine dose-dependently reduced pain behavior in OFT and acetic acid writhing test, as well as inflammatory hyperalgesia in paw pressure test. All examined antagonists except prazosin dose-dependently inhibited vortioxetine's antinociceptive effects. In conclusion, vortioxetine exerted analgesic efficacy in trigeminal, visceral, and somatic inflammatory pain. The effect is at least in part mediated by 5-HT_1B/1D serotonergic, α₂ /β₁ -adrenergic, muscarinic and nicotinic cholinergic, CB₁ /CB₂ cannabinoid, and adenosine A₁ receptors. These findings contribute to better understanding of the analgesic effect of vortioxetine and suggest its potential usefulness for inflammatory pain treatment.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

5-HT1A parital agonism and 5-HT7 antagonism restore episodic memory in subchronic phencyclidine-treated mice: role of brain glutamate, dopamine, acetylcholine and GABA

RATIONALE

The effect of atypical antipsychotic drugs (AAPDs), e.g., lurasidone, to improve cognitive impairment associated with schizophrenia (CIAS), has been suggested to be due, in part, to enhancing release of dopamine (DA), acetylcholine (ACh), and glutamate (Glu) in cortex and hippocampus.

RESULTS

The present study found acute lurasidone reversed the cognitive deficit in novel object recognition (NOR) in subchronic (sc) phencyclidine (PCP)-treated mice, an animal model for CIAS. This effect of lurasidone was blocked by pretreatment with the 5-HT_1AR antagonist, WAY-100635, or the 5-HT₇R agonist, AS 19. Lurasidone significantly increased medial prefrontal cortex (mPFC) ACh, DA, and Glu efflux, all of which were blocked by WAY-100635, with similar effects in the dorsal striatum (dSTR), except for the absence of an effect on Glu increase. AS 19 inhibited Glu, but not DA efflux, in the dSTR. The selective 5-HT₇R antagonist, SB-26970, increased mPFC DA, 5-HT, Glu, and, importantly, also GABA efflux and striatal DA, NE, 5-HT, and Glu efflux, indicating tonic inhibition of the release of these neurotransmitters by 5-HT₇R stimulation. These results provide new evidence that GABA release in the mPFC is tonically inhibited by 5-HT₇R stimulation and suggest that a selective 5-HT₇R antagonist might be clinically useful to enhance cortical GABAergic release. All SB-269970 effects were blocked by AS 19 or WAY-100635, suggesting 5-HT_1AR agonism is necessary for the release of these neurotransmitters by SB-269970. Lurasidone increased ACh, DA, and NE but not Glu efflux in mPFC and dSTR DA and Glu efflux in 5-HT₇ KO mice.

CONCLUSION

We conclude that lurasidone-induced Glu efflux in mPFC requires 5-HT₇R antagonism while its effects on cortical ACh and DA efflux are mainly due to 5-HT_1AR stimulation.

Modes and nodes explain the mechanism of action of vortioxetine, a multimodal agent (MMA): actions at serotonin receptors may enhance downstream release of four pro-cognitive neurotransmitters

Vortioxetine is an antidepressant with multiple pharmacologic modes of action that enhance release of dopamine, norepinephrine, acetylcholine, and histamine.

Modes and nodes explain the mechanism of action of vortioxetine, a multimodal agent (MMA): blocking 5HT3 receptors enhances release of serotonin, norepinephrine, and acetylcholine

Vortioxetine is an antidepressant with multiple pharmacologic modes of action at targets where serotonin neurons connect with other neurons. 5HT3 receptor antagonism is one of these actions, and this leads to increased release of norepinephrine (NE), acetylcholine (ACh), and serotonin (5HT) within various brain circuits.

Vortioxetine (Lu AA21004), a novel multimodal antidepressant, enhances memory in rats

The serotonergic system plays an important role in cognitive functions via various 5-HT receptors. Vortioxetine (Lu AA21004) in development as a novel multimodal antidepressant is a 5-HT3, 5-HT7 and 5-HT1D receptor antagonist, a 5-HT1B receptor partial agonist, a 5-HT1A receptor agonist and a 5-HT transporter (5-HTT) inhibitor in vitro. Preclinical studies suggest that 5-HT3 and 5-HT7 receptor antagonism as well as 5-HT1A receptor agonism may have a positive impact on cognitive functions including memory. Thus vortioxetine may potentially enhance memory. We investigated preclinical effects of vortioxetine (1-10mg/kg administered subcutaneously [s.c.]) on memory in behavioral tests, and on cortical neurotransmitter levels considered important in rat memory function. Contextual fear conditioning and novel object recognition tests were applied to assess memory in rats. Microdialysis studies were conducted to measure extracellular neurotransmitter levels in the rat medial prefrontal cortex. Vortioxetine administered 1h before or immediately after acquisition of contextual fear conditioning led to an increase in freezing time during the retention test. This mnemonic effect was not related to changes in pain sensitivity as measured in the hotplate test. Rats treated with vortioxetine 1h before training spent more time exploring the novel object in the novel object recognition test. In microdialysis studies of the rat medial prefrontal cortex, vortioxetine increased extracellular levels of acetylcholine and histamine. In conclusion, vortioxetine enhanced contextual and episodic memory in rat behavioral models. Further demonstration of its potential effect on memory functions in clinical settings is warranted.

The selective 5-HT(1A) receptor antagonist NAD-299 increases acetylcholine release but not extracellular glutamate levels in the frontal cortex and hippocampus of awake rat

The effects of the HT(1A) receptor antagonist NAD-299 on extracellular acetylcholine (ACh) and glutamate (Glu) levels in the frontal cortex (FC) and ventral hippocampus (HPC) of the awake rats were investigated by the use of in vivo microdialysis. Systemic administration of NAD-299 (0.3; 1 and 3micromol/kg s.c.) caused a dose-dependent increase in ACh levels in FC and HPC (peak value of 209% and 221%, respectively) and this effect was comparable to that induced by donepezil (2.63micromol/kg s.c.). Moreover, the ACh levels in the FC increased even after repeated (14days) treatment with NAD-299 and when NAD-299 was injected locally into the nucleus basalis magnocellularis or perfused through the microdialysis probe implanted in the cortex. In contrast, NAD-299 failed to alter the extracellular levels of glutamate after systemic (3micromol/kg s.c.) or local (100microM) administration. The present data support the hypothesis that cholinergic transmission in cortico-limbic regions can be enhanced via blockade of postsynaptic 5-HT(1A) receptors, which may underlie the proposed cognitive enhancing properties of NAD-299 in models characterized by cholinergic deficit.

Buspirone-induced changes in the serotonergic and non-serotonergic cells in the dorsal raphe nucleus of rats

Buspirone, a 5-HT (5-hydroxytryptamine, serotonin)(1A) partial agonist, is being used as an anxiolytic drug. The mechanism of action is explained by an effect on the 5-HT system. The main source of 5-HT in the forebrain is the dorsal raphe nucleus (DRN). However, there are also other populations of non-5-HT neurons in the DRN. Here, we investigated the effect of acute and chronic buspirone treatments on the 5-HT and non-5-HT cells, the neuronal nitric oxide synthase (nNOS) and tyrosine hydroxylase (TH) cells, in the DRN. Rats received either an acute or chronic administration of buspirone or saline. Hereafter, the brains were processed for 5-HT, nNOS, and TH immunohistochemistry. We found that acute and chronic buspirone treatments significantly lowered the mean optical density of nNOS in the DRN as compared to controls. Meanwhile only the chronic buspirone treatment reduced the mean density of 5-HT and TH immunoreactivity but not the acute buspirone as compared to saline treated animals. Our findings suggest that buspirone treatment affects not only the intracellular content of 5-HT but also nNOS and TH. Therefore, the cellular effect of buspirone is more complex than thought.

F15063, a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties: influence on neuronal firing and neurotransmitter release

F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)-ethyl]-(3-cyclopenten-1-yl-benzyl)-amine) is a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties. Herein, we compared its effects on rat ventral tegmental area dopamine and dorsal raphe serotonin electrical activity with those of the dopamine D2 receptor partial agonist/5-HT1A receptor agonist, SSR181507. Further, we investigated the modulation of extracellular dopamine and noradrenaline in the medial prefrontal cortex and serotonin in the hippocampus of freely moving rats by F15063 using in vivo microdialysis. In the ventral tegmental area, F15063 (200-700 microg/kg, i.v.) did not alter the electrical activity of dopamine neurons whereas SSR181507 (250-1000 microg/kg, i.v.) partially inhibited it, consistent with dopamine D2 receptor partial agonism. Both compounds reduced the inhibition of firing rate induced by the full agonist apomorphine. In the dorsal raphe, both ligands suppressed firing activity, consistent with agonism at 5-HT1A autoreceptors, although SSR181507 (25-75 microg/kg, i.v.) was more potent than F15063 (100-300 microg/kg, i.v.). F15063 (0.63-40 mg/kg, i.p.) dose-dependently increased dopamine levels in the prefrontal cortex and decreased hippocampal 5-HT. These effects were reversed by the selective 5-HT1A receptor antagonist WAY100635 (0.16 mg/kg, s.c.), indicating that they were mediated by 5-HT1A receptors (at post- and pre-synaptic levels, respectively). In the medial prefrontal cortex, noradrenaline levels were moderately but significantly increased by F15063 at 2.5 mg/kg. In conclusion, whereas SSR181507 exhibits (partial) agonism at dopamine D2 and 5-HT1A receptors, F15063 blocks dopamine D2-like receptors whilst activating 5-HT1A receptors. Such a profile distinguishes F15063 from SSR181507 and currently available antipsychotic drugs.

Bilobalide modulates serotonin-controlled behaviors in the nematode Caenorhabditis elegans

Background

Dysfunctions in the serotonergic system have been implicated in several neurological disorders such as depression. Elderly individuals who have been diagnosed with clinical depression show elevated cases of neurodegenerative diseases. This has led to suggestions that modulating the serotonin (5-HT) system could provide an alternative method to current therapies for alleviating these pathologies. The neuroprotective effects of bilobalide in vitro have been documented. We aim to determine whether bilobalide affects the 5-HT system in the nematode C. elegans. The wild type worms, as well as well-characterized 5-HT mutants, were fed with bilobalide in a range of concentrations, and several 5-HT controlled behaviors were tested.

Results

We observed that bilobalide significantly inhibited 5-HT-controlled egg-laying behavior in a dose-dependent manner, which was blocked in the 5-HT receptor mutants (ser-4, mod-1), but not in the 5-HT transporter (mod-5) or synthesis (tph-1) mutants. Bilobalide also potentiated a 5-HT-controlled, experience-dependent locomotory behavior, termed the enhanced slowing response in the wild type animals. However, this effect was fully blocked in 5-HT receptor mod-1 and dopamine defective cat-2 mutants, but only partially blocked in ser-4 mutants. We also demonstrated that acetylcholine transmission was inhibited in a transgenic C. elegans strain that constitutively expresses Aβ, and bilobalide did not significantly affect this inhibition.

Conclusion

These results suggest that bilobalide may modulate specific 5-HT receptor subtypes, which involves interplay with dopamine transmission. Additional studies for the function of bilobalide in neurotransmitter systems could aid in our understanding of its neuroprotective properties.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Pro-cognitive effects of 5-HT6 receptor antagonists in the social recognition procedure in rats: implication of the frontal cortex

RATIONALE

5-HT6 receptor antagonists improve cognitive processes in rodents. However, their site(s) of action remains unexplored and their influence upon social memory has been little investigated.

OBJECTIVES

We examined the influence of 5-HT6 receptor ligands upon social memory in rats by use of systemic or local administration into the frontal cortex (FCX), striatum, or nucleus basalis magnocellularis (NBM).

MATERIALS AND METHODS

The social recognition test is based upon the ability of an adult rat to recognize a younger conspecific during the second of two 5-min sessions. In a procedure without an inter-session interval, the actions of drugs alone and the ability to reverse "amnesia" induced by the muscarinic antagonist, scopolamine (1.25 mg/kg, s.c.), were examined. The potential promnesic effect of drugs was also investigated in another procedure where a spontaneous deficit of recognition was induced by a 120-min inter-session interval.

RESULTS

The 5-HT6 receptor agonist, WAY-181187 (10.0 mg/kg, i.p.), significantly impaired social recognition. This effect was abolished by the 5-HT6 receptor antagonists, SB-271046 (20.0 mg/kg, i.p.) and SB-258585 (10.0 mg/kg, i.p.). These agents also abolished scopolamine-induced amnesia (10.0 and 2.5 mg/kg, i.p., respectively) and reversed the delay-induced deficit (10.0-20.0 and 2.5-10.0 mg/kg, i.p., respectively). WAY-181187 into the FCX significantly impaired social recognition (0.16-0.63 microg/side). Conversely, SB-271046 into the FCX (2.5-5.0 microg/side), but neither into the striatum nor the NBM, significantly reversed spontaneous deficit.

CONCLUSION

These results indicate that 5-HT6 receptors modulate social recognition by actions in the FCX and underpin their pertinence as targets for the treatment of psychiatric disorders in which cognitive function is compromised.

Role of postsynaptic serotonin1A receptors in risperidone-induced increase in acetylcholine release in rat prefrontal cortex

Most atypical antipsychotic drugs increase acetylcholine release in the prefrontal cortex, but the detailed mechanism is still unknown. The present study examined the role of serotonin (5-HT)1A receptors in risperidone-induced increases in acetylcholine release in rat prefrontal cortex. Systemic administration of risperidone at doses of 1 and 2 mg/kg increased acetylcholine release in the prefrontal cortex in a dose-dependent manner. This increase was antagonized by systemic administration of high doses (1 and 3 mg/kg) of N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635), a 5-HT1A receptor antagonist/dopamine D4 receptor agonist, but not by a low dose (0.1 mg/kg) of the antagonist which antagonizes preferentially presynaptic 5-HT1A autoreceptors. Furthermore, local application of WAY100635 into the prefrontal cortex also attenuated risperidone-induced increases in acetylcholine release. WAY100635 alone did not affect acetylcholine release in the prefrontal cortex. On the other hand, local application of risperidone (3 and 10 microM), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (1 and 10 microM), and the dopamine D4 receptor antagonist 3-(4-(4-iodophenyl)piperazine-1-yl)methyl-1H-pyrrolo[2,3-b]pyridine (1 and 10 microM) into the cortex did not affect acetylcholine release in the prefrontal cortex. These results suggest that risperidone increases acetylcholine release in the prefrontal cortex through a complex mechanism which is enhanced by prefrontal 5-HT1A receptor activation.

Effects of divalproex and atypical antipsychotic drugs on dopamine and acetylcholine efflux in rat hippocampus and prefrontal cortex

Mood stabilizers (e.g., valproic acid) and antipsychotic drugs (APDs) are commonly co-administered in the treatment of bipolar disorder and schizophrenia. The basis for any synergism between these classes of drugs in either group of disorders has been little studied. Previous studies have shown that atypical APDs (e.g., clozapine) preferentially increases dopamine (DA) and acetylcholine (ACh) efflux in rat medial prefrontal cortex (mPFC) and hippocampus (HIP), both of which have been suggested to contribute to their ability to improve cognition in patients with schizophrenia. We have recently reported that the anticonvulsant mood stabilizers (AMS), valproic acid, carbamazepine, and zonisamide, but not lithium, also preferentially increase DA efflux in the rat mPFC, and that, at subthreshold doses, the AMS also augment the ability of the atypical APDs clozapine and risperidone to increase DA but not ACh efflux in the mPFC. The present study examined the ability of divalproex (DVX), which is chemically related to valproic acid, to enhance DA and ACh efflux in the HIP and to augment the effect of atypical APDs on ACh efflux in the HIP and mPFC. DVX, 500 mg/kg, significantly increased DA and ACh efflux in the HIP, and DA, but not ACh, efflux in the mPFC, whereas a lower dose of DVX, 50 mg/kg, had no effect on DA or ACh in either region. However, DVX, 50 mg/kg, combined with the atypical APDs olanzapine (1.0 mg/kg) or aripiprazole (0.3 mg/kg) significantly potentiated the effect of both APDs on DA, but not ACh efflux in the HIP and mPFC. Pretreatment of olanzapine or aripiprazole with the selective serotonin 5-HT(1A) antagonist, WAY100635 (1.0 mg/kg) partially but significantly blocked the effect of the combination of DVX, 50 mg/kg, and olanzapine or aripiprazole, on DA efflux in both the HIP and mPFC. WAY100635 did not affect the ability of the combination of olanzapine or aripiprazole and DVX to enhance ACh efflux in the HIP or mPFC. Subchronic administration of the combination of DVX, 50 mg/kg, and risperidone, produced significantly greater increases in DA and ACh efflux in the mPFC, but these increases were not significantly different from those following the acute administration of the combination of risperidone and DVX. These results provide further evidence that the AMS, DVX, augments the ability of atypical APDs to increase DA or ACh efflux in either the HIP or mPFC or both. The clinical significance of this potentiation for the beneficial clinical effects of this combination of agents and the differences between AMS in this regard warrants further study.

Presynaptic serotonergic modulation of 5-HT and acetylcholine release in the hippocampus and the cortex of 5-HT1B-receptor knockout mice

Lesioning of serotonergic afferents increases hippocampal ACh release and attenuates memory deficits produced by cholinergic lesions. Improved memory performance described in 5-HT1B-knockout (KO) mice might thus be due to a weaker 5-HT1B-mediated inhibitory influence of 5-HT on hippocampal ACh release. The selective delay-dependent impairment of working memory observed in these KO mice suggests, however, that cortical regions also participate in task performance, possibly via indirect influences of 5-HT on ACh release. To provide neuropharmacological support for these hypotheses we measured evoked ACh and 5-HT release in hippocampal and cortical slices of wild-type (WT) and 5-HT1B KO mice. Superfused slices (preincubated with [3H]choline or [3H]5-HT) were electrically stimulated in the absence or presence of 5-HT1B receptor ligands. In hippocampus and cortex, 5-HT1B agonists decreased and antagonists increased 5-HT release in WT, but not in 5-HT1B KO mice. In 5-HT1B KO mice, 5-HT release was enhanced in both structures, while ACh release (in nCi) was reduced. ACh release was inhibited by 5-HT1B agonists in hippocampal (not cortical) slices of WT but not of 5-HT1B KO mice. Our data (i) confirm the absence of autoinhibition of 5-HT release in 5-HT1B-KO mice, (ii) demonstrate a reduced release of ACh, and the absence of 5-HT1B-receptor-mediated inhibition of ACh release, in the hippocampus and cortex of 5-HT1B-KO mice, and (iii) are compatible with an indirect role of cortical ACh in the working memory impairment observed in these KO mice.

Effect of a serotonin depleting regimen of 3,4-methylenedioxymethamphetamine (MDMA) on the subsequent stimulation of acetylcholine release in the rat prefrontal cortex

The amphetamine analog 3,4-methylenedioxymethamphetamine (MDMA) is considered to be selectively neurotoxic to serotonergic nerve terminals. Although the long term effects of MDMA on serotonin (5-HT) terminals have been well studied, other potential neurochemical consequences associated with MDMA-induced 5-HT depletion have been less well investigated. In view of the cognitive impairments in human MDMA abusers and the role of acetylcholine (ACh) in learning and memory, it was of interest to determine the influence of a 5-HT depleting regimen of MDMA on subsequent stimulation of ACh release in the prefrontal cortex (PFC). Male rats received vehicle or MDMA (10 mg/kg, i.p. every 2 h for four injections) and underwent in vivo microdialysis 7 days later to assess the subsequent drug- (e.g., MDMA, 5-HT1A agonist) or stress- (e.g., tail pinch, presence of an intruder rat) induced stimulation of ACh release. The increase in the extracellular concentration of ACh in the PFC produced by MDMA (10 mg/kg, i.p.) was significantly less in rats previously exposed to the neurotoxic regimen of MDMA than that in control animals. In contrast, there was no difference in the magnitude of the stimulation of cortical ACh release elicited by the 5-HT1A agonist, 8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT, 0.3mg/kg, s.c.), tail pinch (30 min) or the presence of an intruder rat (40 min) between control animals and animals previously exposed to a neurotoxic regimen of MDMA. These results suggest that although MDMA-induced 5-HT depletion diminishes subsequent MDMA-induced ACh release, there is little impact on cortical ACh release elicited by the stress of pain or the novelty of an environmental intruder.

Contrasting Contribution of 5-Hydroxytryptamine 1A Receptor Activation to Neurochemical Profile of Novel Antipsychotics: Frontocortical Dopamine and Hippocampal Serotonin Release in Rat Brain

Several novel antipsychotics, such as aripiprazole, bifeprunox, SSR181507 [(3-exo)-8-benzoyl-N-(((2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl)methyl)-8-azabicyclo(3.2.1)octane-3-methanamine], and SLV313 [1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine], activate serotonin 5-hydroxytryptamine (5-HT)1A receptors. Such activity is associated with enhanced treatment of negative symptoms and cognitive deficits, which may be mediated by modulation of cerebral dopamine and serotonin levels. We employed microdialysis coupled to high pressure liquid chromatography with electrochemical detection to examine 5-HT1A receptor activation in the modulation of extracellular dopamine in medial prefrontal cortex and serotonin in hippocampus of freely moving rats. The above compounds were compared with drugs that have less interaction with 5-HT1A receptors (clozapine, nemonapride, ziprasidone, olanzapine, risperidone, and haloperidol). Hippocampal 5-HT was decreased by bifeprunox, SSR181507, SLV313, sarizotan, and nemonapride, effects similar to those seen with the 5-HT1A agonist, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)8-OH-DPAT], consistent with activation of 5-HT1A autoreceptors. These decreases were reversed by the selective 5-HT1A antagonist, WAY100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide]. In contrast, haloperidol, risperidone, clozapine, olanzapine, ziprasidone, and aripiprazole did not significantly modify hippocampal serotonin levels. In medial prefrontal cortex, dopamine levels were increased by SSR181507, SLV313, sarizotan, and (+)8-OH-DPAT. These effects were reversed by WAY100635, indicating mediation by 5-HT1A receptors. In contrast, the increases in dopamine levels induced by clozapine, risperidone, olanzapine, and ziprasidone were not blocked by WAY100635, consistent with predominant influence of other mechanisms in the actions of these drugs. Haloperidol, nemonapride, and the D2 partial agonists, aripiprazole and bifeprunox, did not significantly alter dopamine release. Taken together, these data demonstrate the diverse contribution of 5-HT1A receptor activation to the profile of antipsychotics and suggest that novel drugs selectively targeting D2 and 5-HT1A receptors may present distinctive therapeutic properties.

Age-dependent effects of serotonin-1A receptor gene deletion in spatial learning abilities in mice

The serotonin (5-hydroxytryptamine, 5-HT) receptor 1A is involved in many physiological functions, including the regulation of learning and memory by acting either as an autoreceptor located on 5-HT neurons (raphe nuclei) or as a heteroreceptor on non-5-HT neurons, mainly in the hippocampal formation. To investigate whether the effects of 5-HT via 5-HT1A receptors on learning are age-sensitive, we evaluated the performance of young-adult (3 months old) and aged (22 months old) 5-HT1A knockout (KO) mice and their homologous wild types (WT) in the hippocampal-dependent spatial reference memory version of the Morris water maze. We demonstrated that young-adult 5-HT1AKO mice exhibit an impairment in learning and retention of the spatial task, as compared to WT mice, without showing any sign of change in their sensori-motor and locomotor abilities or motivation. This genotype effect does not persist during aging. In fact, aged 5-HT1AKO mice seem to be slightly facilitated during the early stages of learning. These results are consistent with a possible prevalence of 5-HT1A raphe functions in learning and memory abilities of young-adult animals, since the effects of the mutation on mice performance (impairment) are opposite to those found after intra-raphe injection of 5-HT1A agonists (facilitation), and with data showing increased activity of 5-HT neurons in 5-HT1AKO mice. The reduced effect of the mutation in aged animals possibly reflects the lower efficacy of autoreceptors due to aging and/or a prevalence of hippocampal heteroreceptors.

Clozapine increases both acetylcholine and dopamine release in rat ventral hippocampus: role of 5-HT1A receptor agonism

Atypical antipsychotic drugs (APDs) such as clozapine, but not the typical APD haloperidol, improve some aspects of cognition in schizophrenia. This advantage has been attributed, in part, to the ability of the atypical APDs to markedly increase acetylcholine (ACh) and dopamine (DA) release in rat medial prefrontal cortex (mPFC), while producing a minimal effect in the nucleus accumbens (NAC) or striatum. The atypical APD-induced preferential release of DA, but not ACh, in the mPFC is partially inhibited by the selective 5-HT(1A) antagonist WAY100635. However, little is known about these effects of atypical APDs in the ventral hippocampus (vHIP), another possible site of action of atypical APDs with regard to cognitive enhancement. The present study demonstrates that clozapine (10 mg/kg) comparably increases both ACh and DA release in the vHIP and mPFC. The increases in DA, but not ACh, release in both regions were partially attenuated by WAY100635 (0.2 mg/kg), which had no effect by itself on the release of either neurotransmitter in either region. Tetrodotoxin (TTX; 1 microM), a Na(+) channel blocker, in the perfusion medium, eliminated the clozapine (10 mg/kg)-induced ACh and DA release in the vHIP, indicating their neuronal origin. Haloperidol produced a slight increase in ACh release in the vHIP at 1 mg/kg, and DA release in the mPFC at 0.1 mg/kg. In conclusion, clozapine increases ACh and DA release in the vHIP and mPFC, whereas haloperidol has minimal effects on the release of these two neurotransmitters in either region. These differences may contribute, at least in part, to the superior ability of clozapine, compared to haloperidol, to improve cognition in schizophrenia. 5-HT(1A) agonism is important to the ability of clozapine and perhaps other atypical APDs to increase DA, but not ACh, release in the vHIP, as well as the mPFC. The role of hippocampus in the cognitive effects of atypical APDs warrants more intensive study.

Psychopharmacological approaches to modulating attention in the five-choice serial reaction time task: implications for schizophrenia

RATIONALE

In schizophrenia, attentional disturbance is a core feature which may not only accompany the disorder, but may precede the onset of psychiatric symptoms.

OBJECTIVES

The five-choice serial reaction time task (5CSRTT) is a test of visuo-spatial attention that has been used extensively in rats for measuring the effects of systemic and central neurochemical manipulations on various aspects of attentional performance, including selective attention, vigilance and executive control. These findings are relevant to our understanding of the neural systems that may be compromised in patients with schizophrenia.

METHODS

The 5CSRTT is conducted in an operant chamber that has multiple response locations, in which brief visual stimuli can be presented randomly. Performance is maintained using food reinforcers to criterion levels of accuracy. Various aspects of performance are measured, including attentional accuracy and premature responding, especially under different attentional challenges.

RESULTS

The effects of systemic and intra-cerebral infusions of selective dopamine, serotonin and cholinergic receptor agents on the 5CSRTT are reviewed with a view to identifying attention-enhancing effects that may be relevant to the treatment of cognitive deficits in schizophrenia. In addition, some novel agents such as modafinil and histamine receptor agents are also considered. Examining the effects of selective neurochemical lesions helped define the neural locus of attentional effects. Similarly, findings from microdialysis studies helped identify the extracellular changes in neurotransmitters and their metabolites in freely moving rats during performance of the 5CSRTT.

CONCLUSIONS

The monoaminergic and cholinergic systems have independent but complementary roles in attentional function, as measured by the 5CSRTT. These functions are predominantly under the control of the prefrontal cortex and striatum. These conclusions are considered in the context of their application towards therapeutic approaches for attentional disturbances that are typically observed in schizophrenic patients.

Aripiprazole, a novel antipsychotic drug, preferentially increases dopamine release in the prefrontal cortex and hippocampus in rat brain

Aripiprazole,7-(4-[4-(2,3-dichlorophenyl)-1-piperazinyl]butyloxy)-3,4-dihydro-carbostycil (OPC-14597), a novel atypical antipsychotic drug, is a dopamine D2 receptor partial agonist with functional 5-HT2A receptor antagonist, and 5-HT1A receptor partial agonist properties as well. Other atypical antipsychotic drugs, e.g. clozapine, but not typical antipsychotic drugs, e.g. haloperidol, produce significant increases in dopamine and acetylcholine release in the medial prefrontal cortex in rats, effects believed to be related to the ability to improve cognitive function. The increase in the medial prefrontal cortex dopamine release by the atypical antipsychotic drugs has been shown to be partially inhibited by N-[2[4-)2-methoxyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635), a selective 5-HT1A receptor antagonist. Aripiprazole, 0.1 and 0.3 mg/kg, significantly increased dopamine release in the hippocampus. Moreover, aripiprazole, 0.3 mg/kg, slightly but significantly increased dopamine release in the medial prefrontal cortex but not in the nucleus accumbens. These increases were significantly inhibited by WAY100635. By contrast, aripiprazole, 3.0 mg/kg and 10 mg/kg, significantly decreased dopamine release in the nucleus accumbens but not the medical prefrontal cortex. However, aripiprazole 10 mg/kg significantly decreased dopamine release in the both regions. Aripiprazole had no effect on acetylcholine release in the medial prefrontal cortex, hippocampus, or nucleus accumbens at any dose, except for 3.0 mg/kg, which decreased acetylcholine release in the nucleus accumbens only. Aripiprazole, 0.3 mg/kg, transiently potentiated haloperidol (0.1 mg/kg)-induced dopamine release in the medial prefrontal cortex but inhibited that in the nucleus accumbens. The present study demonstrated that aripiprazole, at low doses of 0.1 and 0.3 mg/kg, increases dopamine release in the medial prefrontal cortex and hippocampus. It also suggests that the function of both the medial prefrontal cortex and hippocampus may contribute to the ability of aripiprazole to improve negative symptom and cognition.

The serotonin1A receptor partial agonist S15535 [4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine] enhances cholinergic transmission and cognitive function in rodents: a combined neurochemical and behavioral analysis

These studies examined the influence of the selective 5-hydroxytryptamine (serotonin) (5-HT)(1A) receptor partial agonist S15535 [4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine] upon cholinergic transmission and cognitive function in rodents. In the absence of acetylcholinesterase inhibitors, S15535 dose-dependently (0.04-5.0 mg/kg s.c.) elevated dialysis levels of acetylcholine in the frontal cortex and dorsal hippocampus of freely moving rats. In the cortex, the selective 5-HT(1A) receptor antagonist WAY100,635 [(N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide) fumarate] dose-dependently (0.0025-0.63) blocked this action of S15535. By contrast, in dorsal hippocampus, WAY100,635 mimicked the induction of acetylcholine release by S15535. In a social recognition paradigm, S15535 dose-dependently (0.16-10.0) improved retention, an action blocked by WAY100,635 (0.16), which was ineffective alone. Furthermore, S15535 dose-dependently (0.04-2.5) and WAY100,635 reversibly abolished amnesic properties of the muscarinic antagonist scopolamine (0.63) in this procedure. Cognitive deficits provoked by scopolamine in autoshaping and Morris water-maze procedures were likewise blocked by S15535 at doses of 0.63 to 10.0 and 0.16 to 2.5, respectively. In a two-platform spatial discrimination task, in which S15535 similarly abrogates cognitive deficits elicited by scopolamine, injection of S15535 (1.0 and 10.0 microg) into dorsal hippocampus blocked amnesic effects of the 5-HT(1A) agonist 8-hydroxy-2-dipropylaminotetralin (0.5 microg). Finally, S15535 (0.16-0.63) improved performance in a spatial, delayed nonmatching to sample model in mice, and in an operant delayed nonmatching to sample model in old rats, S15535 (1.25-5.0 mg/kg p.o.) increased response accuracy and reduced latency to respond. In conclusion, S15535 reinforces frontocortical and hippocampal release of acetylcholine and displays a broad-based pattern of procognitive properties. Its actions involve both blockade of postsynaptic 5-HT(1A) receptors and engagement of 5-HT(1A) autoreceptors.

p-Chloroamphetamine blocks physostigmine-induced memory enhancement in rats with unilateral nucleus basalis lesions

The present experiment examined whether p-chloroamphetamine (PCA), a serotonergic releasing/depleting agent, would block the memory-enhancing effect of physostigmine in rats with N-methyl-D-aspartic acid (NMDA)-induced unilateral lesions of the nucleus basalis of Meynert (uni-nbM). Six groups of subjects with uni-nbM lesions in addition to an isolated sham-operated control group were included. Subjects were trained and tested 72 h later on a one-trial passive avoidance task. Thirty minutes before training, rats with uni-nbM lesions were injected with either 1.0 or 5.0 mg/kg PCA or saline. Immediately after training, approximately half the subjects in each group were injected with either saline or 0.06 mg/kg physostigmine. Animals in the sham group received saline injections. Saline-injected animals with uni-nbM lesions performed poorly at test, a deficit that was reversed with physostigmine. Pretraining injections of PCA blocked physostigmine's memory-enhancing effect, although motor impairment during training may have contributed to decrements in test performance in animals injected with 5.0 mg/kg. Subjects were killed about 10 days later and their frontal cortices examined for choline acetyltransferase (ChAT). Results from the neurochemical analysis revealed that the lesion decreased ChAT levels and that the injection of 1.0 mg/kg PCA exaggerated this lesion-induced depletion. Implications for the interaction between acetylcholine and serotonin are discussed.

The 5-HT 1A receptor antagonist WAY 100635 improves rats performance in different models of amnesia evaluated by the object recognition task

The effects of the 5-HT(1A) receptor antagonist WAY 100635 on recognition memory were investigated in two different amnestic models in the rat by using the object recognition task. WAY 100635 at 1 mg/kg, but not at 0.3 mg/kg, counteracted scopolamine-induced performance deficits in the acquisition version of this behavioral paradigm. At the same dose, WAY 100635 antagonized extinction of recognition memory in the normal rat, suggesting that it affected acquisition, storage and retrieval of information. These results support and extend prior findings that interactions between the serotonergic and cholinergic systems are relevant to cognition and indicate that WAY 100635 modulates different aspects of recognition memory.

Intra-prefrontal 8-OH-DPAT and M100907 improve visuospatial attention and decrease impulsivity on the five-choice serial reaction time task in rats

RATIONALE

The central serotonergic systems are a major target for drugs used to treat neuropsychiatric disorders such as depression and schizophrenia in which disruption of frontal cortex function has been implicated. However, it is not known precisely how serotonin (5-HT) modulates the medial prefrontal cortex (mPFC) to affect cognitive function and behaviour.

OBJECTIVE

To investigate the roles of 5-HT(1A) and 5-HT(2A) receptors in mPFC on performance of the five-choice serial reaction time task (5CSRT), which assesses visuospatial attention, impulsivity and motivational processes.

METHODS

Following training on the 5CSRT, rats were implanted with bilateral guide cannulae aimed at the mPFC. Rats received intra-mPFC infusions of either 8-OH-DPAT (10, 30 and 100 ng) or M100907 (30, 100 and 300 ng) according to a Latin square design.

RESULTS

Both 8-OH-DPAT and M100907 selectively enhanced accuracy of target detection. When the stimulus duration was shortened, infusions of 8-OH-DPAT continued to improve accuracy, whereas M100907 decreased premature responding and omissions, thus partly dissociating the effects of these two compounds. Similar effects were obtained following systemic administration of M100907 and 8-OH-DPAT. The effects of 8-OH-DPAT were blocked by the 5-HT(1A) antagonist WAY 100635, at a dose that itself had no significant effects on behaviour.

CONCLUSIONS

These results indicate that modulation of 5-HT function within the mPFC via distinct receptors can enhance performance on the 5CSRT. These findings suggest a mechanism by which serotonergic agents improve cognitive function, which may be relevant to their therapeutic benefit in the treatment of neuropsychiatric disorders.

Reversal of visual attention dysfunction after AMPA lesions of the nucleus basalis magnocellularis (NBM) by the cholinesterase inhibitor donepezil and by a 5-HT1A receptor antagonist WAY 100635

RATIONALE

Degeneration of the cholinergic magnocellular neurons in the basal forebrain and their cortical projections is a major feature of the neuropathology of Alzheimer's disease (AD). In addition to memory dysfunction, attentional functions are also impaired in AD.

OBJECTIVE

We investigated the extent to which the cholinesterase inhibitor donepezil reversed the attentional performance deficit in nucleus basalis magnocellularis (NBM) lesioned rats. We also examined the effects of a selective and potent 5-HT(1A) receptor antagonist, WAY 100635, on the attentional deficit of NBM lesioned rats.

METHODS

We injected alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) into the NBM to selectively destroy cholinergic neurons projecting to the neocortex. Attentional functions were examined using the 5-CSRT task, in which hungry rats were required to locate brief visual targets presented randomly in one of five locations in a specially designed chamber. RESULTS. AMPA lesions of the NBM caused marked reductions in choline acetyltransferase activity (ChAT) ranging from 30 to 46% in medial areas of the cortex (medial-frontal and cingulate) and from 58 to 72% in more lateral areas (anterior-dorso-lateral and parietal). AMPA lesioned rats made fewer correct responses (choice accuracy), longer latency to correct response and an increase in the number of premature and perseverative responses. These impairments showed some recovery over the next 12 weeks. Reducing the duration of the visual stimulus reinstated the impairments in choice accuracy. The anticholinesterase inhibitor donepezil at 1.0 mg/kg but not 0.5 mg/kg reversed the impairments in choice accuracy and correct response latency. The premature and perseverative over-responding of AMPA lesioned rats remained unchanged. A dose of 0.1 mg/kg WAY 100635 to AMPA-lesioned rats improved their choice accuracy but did not shorten correct response latencies. The number of premature responses was reduced by WAY 100635 but perseverative over-responding was not affected.

CONCLUSIONS

The attentional impairments induced due to cortical cholinergic dysfunction may be ameliorated by cholinergic treatments such as cholinesterase inhibitors. In addition, 5-HT(1A) receptors and the cortical cholinergic system exert balanced opposition in regulating attentional performance in the rat. Blockade of 5-HT(1A) receptors may be useful to treat some aspects of attentional dysfunction in AD.

Cerebral Cholinergic Neurotransmission in Protein and Tryptophan-Restricted Adult Rats

To evaluate the effects of protein and tryptophan restriction on cholinergic neurotransmission in terms of choline acetyltransferase (CAT) activity and its expression as well as muscarinic receptors number at cerebral cortex and hippocampus, Wistar rats were raised on a chronically protein and tryptophan restricted diets with 8% protein based on either Purina chow or corn. There was a significant decrease in both body and cerebral weight in the restricted animals compared with the control group fed with 23% protein diet. In cerebral cortex CAT activity and its expression were significantly increased in corn fed (C) versus protein restricted (HP) and control (T) animals, without no significant changes in muscarinic receptors number. On the other hand, hippocampus CAT activity and its expression were significantly lesser and muscarinic receptors significantly increased in C group and CAT activity in HP group was significantly increased without significant changes in muscarinic receptors related to T group. These results indicate that a reduction in 5-HT disponibility on brain induced by tryptophan restricted diet during development affects clearly the cholinergic system modifying probably the short term memory and learning.

5-HT(1A) and 5-HT(2A) receptors minimally contribute to clozapine-induced acetylcholine release in rat medial prefrontal cortex

The atypical antipsychotic drugs (APDs) clozapine, olanzapine, risperidone, and ziprasidone preferentially increase dopamine (DA) release in rat medial prefrontal cortex (mPFC). These effects have been shown to depend upon potent 5-HT(2A) relative to weak D(2) antagonism, and 5-HT(1A) agonism as well. Atypical APDs also increase acetylcholine (ACh) release in the mPFC, but not the nucleus accumbens (NAC) or striatum (STR), whereas typical APDs such as haloperidol, S(-)-sulpiride and thioridazine do not produce either effect in the mPFC. This study examined the role of 5-HT(1A) agonism, 5-HT(2A) and D(2) antagonism, and the combination thereof, in the ability of clozapine to increase ACh release in rat mPFC. R(+)-8-OH-DPAT (0.2 mg/kg), a 5-HT(1A) agonist, WAY100635 (0.2-0.5 mg/kg), a 5-HT(1A) antagonist, and DOI (0.6-2.5 mg/kg), a 5-HT(2A/2C) agonist, increased ACh release in the mPFC, whereas M100907 (0.03-1 mg/kg), a 5-HT(2A) antagonist, did not. DOI (2.5 mg/kg) and M100907 (0.1 mg/kg) had no effect on ACh release in the NAC or STR. WAY100635 and M100907 inhibited the ability of R(+)-8-OH-DPAT and DOI, respectively, to increase ACh release in the mPFC. WAY100635, which inhibits clozapine-induced DA release in the mPFC, failed to inhibit clozapine (20 mg/kg)-induced ACh release in that region. Similarly, the combination of M100907 and haloperidol (0.1 mg/kg), which enhances DA release in the mPFC, failed to increase ACh release in that region. These results suggest that 5-HT(1A) agonism and 5-HT(2A) antagonism, as well as DA release, contribute minimally to the ability of clozapine, and perhaps other atypical APDs, to increase ACh release in the mPFC.

Effects of estrogen on acetylcholine release in frontal cortex of female rats: involvement of serotonergic neuronal systems

The effects of estrogen on cortically projecting cholinergic neurons were investigated using in vivo microdialysis to measure cortical basal acetylcholine (ACh) levels and serotonin (5-HT)-stimulated ACh release in frontal cortex of freely moving Wistar female rats. Bilateral ovariectomy (OVX) or sham operations were performed under anesthesia. Immediately after surgery, each OVX animal was subcutaneously implanted with pellet containing 0.1/0.5 mg of 17beta-estradiol (E(2)) or a vehicle. Nineteen days later, a transverse microdialysis probe was stereotaxically implanted in the frontal cortex (AP: +2.7 mm, DV: -2.5 mm relative to bregma). Two days later (21 days after beginning of estrogen treatment), in vivo microdialysis experimentation was conducted. Serum E(2) levels of animals with 0.1 and 0.5 mg-pellets were equivalent to those levels during diestrous and proestrous, respectively. Although the replacement of different amounts of E(2) produced significant changes in body weight, it failed to affect basal ACh levels in the frontal cortex. Systemically administered serotonin releasing agent, fenfluramine, significantly increased cortical ACh release in all animal groups. The fenfluramine's ability to increase ACh release was potentiated by E(2) replacement with a 0.5 mg-pellet. E(2)-induced enhancement was also observed when the selective 5-HT(1A) agonist, 8-hydroxy-2-(di-n-propylamino) tetralin, but not the 5-HT(2A/2C) agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, was administered. Therefore, the effect of estrogen on 5-HT-stimulated ACh release might be exerted partly via 5-HT(1A) receptors, and not via 5-HT(2) receptors. These results suggest that the positive effects of estrogen on cognitive functions might be mediated through the ACh-5-HT interactions.

Prefrontocortical serotonin depletion results in plastic changes of prefrontocortical pyramidal neurons, underlying a greater efficiency of short-term memory

The prefrontal cortex activity is involved in organizing the short-term memory. Although the involvement of serotonin for an appropriate performance in learning and memory tests is well known, its role is still unclear; as is the cellular basis of short-term memory behavioral performance. Sprague-Dawley rats were stereotactically injected with 1 microg/microl of 5, 7-dihydroxitryptamine to cause a lesion to the dorsal raphe nucleus. Sham-operated or intact rats were also studied as control groups. Before surgery and 20 days post-operatively, each animal was placed in the Biel maze for five consecutive trials. In the pre-treatment test, all three groups decreased significantly the number of errors beginning with the fourth trial. The same occurred in the post-treatment test, except for the experimental group, whose animals committed less errors beginning with the second trial. After behavioral testing, the dorsomedial prefrontal cerebral cortex was dissected out, and the Golgi study of the third-layer pyramidal neurons revealed that the length of both the apical and the basilar dendrites was smaller than that of controls, and that the apical and oblique dendrites had a greater spine density. A major proportion of thin spines was also seen on the basilar and oblique dendrites, and more stubby spines were seen on the apical dendrite. Serotonin depletion in the prefrontal cerebral cortex resulted in cytoarchitectural alterations of the prefrontocortical pyramidal neurons, which may be underlying partially the greater efficiency observed in the short-term memory behavioral performance.

5-HT(1A) and 5-HT(2A) serotonin receptor turnover in adult rat offspring prenatally exposed to cocaine

This study investigated the effects of prenatal exposure to cocaine on the intracellular kinetics (i.e. rate constant of receptor production and degradation) that govern the maintenance and regulation of cortical 5-HT(1A) and 5-HT(2A) receptor densities in offspring. Adult male rat offspring, prenatally exposed to saline or (-) cocaine (15 mg/kg, s.c., b.i.d, from gestational day 13 through 20), were injected with either vehicle or the irreversible receptor antagonist, EEDQ (10 mg/kg, s.c.), and sacrificed at various post-injection times to monitor the recovery of receptor densities in cerebral cortex. In both saline and cocaine exposed offspring, initial EEDQ-induced reductions (>80%) in 5-HT(1A) and 5-HT(2A) receptor densities were followed by a time-dependent repopulation that reached steady state ([B(max)](ss)) densities comparable to non-EEDQ treated controls by day 10 post-treatment. Calculation of 5-HT(1A) receptor kinetic parameters indicated that prenatal exposure to cocaine did not significantly alter: (1) the receptor production rate (saline: 0.809 fmol/mg protein/h; cocaine: 0.724 fmol/mg protein/h), (2) the receptor degradation rate constant (saline: 0.0063 h(-1); cocaine: 0.0062 h(-1)) or (3) the half-life (t(1/2)) of receptor repopulation (saline: 109.2 h; cocaine: 111.5 h). Similarly, 5-HT(2A) receptor rate constants for production (1. 550 fmol/mg protein/h) and degradation (0.0061 h(-1)) and consequently, t(1/2) (113.2 h), were not significantly altered by prenatal exposure to cocaine. These data suggest that within homogenates of cerebral cortex, prenatal exposure to cocaine did not alter the overall intracellular processes that underlie receptor production or degradation and determine steady state densities of 5-HT(1A) or 5-HT(2A) receptors.

Effect of 5-HT(1A) receptor ligands on Fos-like immunoreactivity in rat brain: evidence for activation of noradrenergic transmission

This study investigated the effects of 8-OH-DPAT and various other 5-HT(1A) receptor agonists on brain noradrenergic transmission using Fos-like immunoreactivity (Fos-LI) as a marker of neural activation. Administration of 8-OH-DPAT (0.1 and 1 mg/kg) induced a marked and dose-related increase in the number of cells positive for Fos-LI in the locus coeruleus (LC), the main source of noradrenergic projections to the forebrain. This effect was also induced by the non-selective, partial 5-HT(1A) receptor agonist buspirone (10 mg/kg). The effect of both 8-OH-DPAT (0.1 mg/kg) and buspirone (10 mg/kg) on Fos-LI in the LC was blocked by pretreatment with the selective 5-HT(1A) receptor antagonist WAY 100635 (1 mg/kg). The active S(-)-enantiomer of the partial 5-HT(1A) receptor agonist (+/-)-MDL 75005EF (1 mg/kg) also induced the expression of Fos-LI in the LC, whereas the inactive R(+)-enantiomer of (+/-)-MDL 73005EF at the same dose did not. In addition to the LC, 8-OH-DPAT (0.1 mg/kg) also induced a marked increase in Fos-LI in various forebrain areas including the medial prefrontal cortex (infralimbic and cingulate cortical areas). More detailed analysis of the Fos response to 8-OH-DPAT in the medial prefrontal cortex revealed that the effect was attenuated by pretreatment with a combination of the beta(1)- and beta(2)-adrenoceptor antagonists ICI 118551 (4 mg/kg) and metoprolol (4 mg/kg), but not the alpha(1)-adrenoceptor antagonist prazosin (5 mg/kg). Taken together, the present findings provide immunocytochemical evidence that 5-HT(1A) receptor agonists activate noradrenergic neurones in the LC and that this leads to increased noradrenergic transmission at postsynaptic sites in the forebrain (specifically medial prefrontal cortex).

Enhancement of cortical and hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by BAY x 3702 in freely moving rats

Involvement of serotonin (5-HT) in the regulation of cholinergic neuronal activity by modulation of acetylcholine (ACh) release has been reported for various regions of the brain. Cortical and hippocampal cholinergic neurotransmission is of particular importance in the mechanisms of attention as well as learning and memory. In the present study, we investigated the effect of R-(-)-2-[4-[(chroman-2-yl-methyl)-amino-butyl]-1,1-dioxo-benzo[d]++ +isothiazolone hydrochloride (BAY x 3702), a novel, high-affinity 5-HT1A receptor agonist, on ACh release in the cerebral cortex and hippocampus of freely moving rats using an in vivo microdialysis technique. Acetylcholine efflux from the cortex and hippocampus was measured every 30 m using a sensitive and specific radioimmunoassay and was stable for at least 5 h. The ACh efflux from the cortex and hippocampus was increased significantly by BAY x 3702 (0.3 mg/kg, i.p.) compared with saline administration. WAY-100635 (0.6 mg/kg, s.c.), a selective 5-HT1A receptor antagonist, eliminated the augmentation of ACh efflux induced by BAY x 3702 in both brain regions. These results demonstrate that stimulation by BAY x 3702 enhanced ACh release in both the cortex and hippocampus through 5-HT1A receptor-mediated pathways.

5-HT1A receptors mediate inhibition of ethanol-induced ascorbic acid release in rat striatum studied by microdialysis

Our previous study showed that the serotonergic system was involved in the ethanol-induced striatal ascorbic acid release in rat. In the present study, the 5-HT1A agonists and antagonists were used to analyze the possible mechanism of ethanol-induced ascorbic acid release in rat striatum. The results showed that ethanol (3.0 g/kg, i.p.) significantly increased striatal ascorbic acid release. Buspirone (5.0 mg/kg, s.c.), a partial agonist of 5-HT1A receptors, and 8-OH-DPAT (0.5 mg/kg, s.c.), a selective agonist of 5-HT1A receptors, showed no effect on basal ascorbic acid release in striatum, but both drugs significantly antagonized the ascorbic acid release induced by ethanol in striatum. WAY 100635 (0.5 mg/kg, s.c.), a selective antagonist of 5-HT1A receptors, affecting neither the basal nor the ethanol-induced ascorbic acid release per se, antagonized the suppressing effect of 8-OH-DPAT on ethanol-induced ascorbic acid release in striatum. This study gives the first evidence that activation of 5-HT1A receptors is involved in ethanol-induced ascorbic acid release in rat striatum.

Effect of WAY-100135 on the hippocampal acetylcholine release potentiated by 8-OH-DPAT, a serotonin1A receptor agonist, in normal and p-chlorophenylalanine-treated rats as measured by in vivo microdialysis

The mechanisms involved in the enhancement of acetylcholine (ACh) release in the rat hippocampus by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a serotonin (5-HT)1A receptor agonist, were investigated using in vivo microdialysis. Administration of p-chlorophenylalanine (PCPA, 300 mg/kg, i.p.), a tryptophan hydroxylase inhibitor, 3 days before the dialysis experiments reduced the hippocampal 5-HT content to 30% of that in saline-treated rats, but did not affect basal ACh release in the hippocampus. 8-OH-DPAT administered systemically (0.5 mg/kg, s.c.) or applied locally (30 microM) into the hippocampus through the dialysis probe significantly enhanced the release of ACh in the hippocampus of PCPA-treated rats to the same degree as that in saline-treated rats. Pretreatment with (+)WAY-100135 (5 mg/kg, i.p.), a selective 5-HT1A receptor antagonist, completely eliminated the enhancement of ACh release induced by locally applied 8-OH-DPAT, but only partially reduced the effects induced by systemically administered 8-OH-DPAT, in both groups of rats. Systemically administered 8-OH-DPAT induced hyperlocomotion in the both saline- and PCPA-treated rats, but this was not eliminated by (+)WAY-100135. 8-OH-DPAT applied locally into the hippocampus did not elicit hyperlocomotion in either group of rats. These results suggest that the modification of endogenous 5-HT release via the 5-HT1A autoreceptor is not involved in the 8-OH-DPAT-induced increase of hippocampal ACh release, and that the increase of ACh release induced by locally applied 8-OH-DPAT involves mainly hippocampal postsynaptic 5-HT1A receptor stimulation. In addition, a possibility that subtypes of 5-HT receptors other than the 5-HT1A receptor, probably 5-HT7 receptor in the septum as well as postsynaptic 5-HT1A receptor in the hippocampus, are involved in the increased hippocampal ACh release induced by systemically administered 8-OH-DPAT is discussed.

Extracellular serotonin changes in VLM during muscle contraction: effects of 5-HT1A-receptor activation

This study determined whether muscle contraction causes an increase in extracellular levels of serotonin (5-HT) in the rostral (rVLM) or caudal ventrolateral medulla (cVLM) in anesthetized rats. Muscle contraction, evoked by tibial nerve stimulation, increased mean arterial blood pressure (MAP) by 27 +/- 4 mmHg (n = 8). In addition, 5-HT levels in the rVLM were elevated by 65 +/- 9% during the contraction (n = 8). Results were similar over two repeated contractions. In contrast, muscle contraction increased MAP, but not 5-HT, levels in the cVLM (n = 6). Tibial nerve stimulation after muscle paralysis had no effect on either MAP or 5-HT levels in both rVLM and cVLM. Microdialysis of a 5-HT1A agonist, 8-OH-DPAT (10 mM), into the rVLM for 30 min (n = 6) blunted the MAP change and reduced 5-HT release during contraction. Administration of NAN-190, a 5-HT1A antagonist, into the rVLM had no effect on 5-HT release and cardiovascular responses during muscle contraction and blocked the changes in 5-HT, MAP, and heart rate to static contraction after subsequent microdialysis of 8-OH-DPAT. Results demonstrate that 5-HT levels in the rVLM increase during muscle contraction and that 5-HT1A-receptor activation in the rVLM blunts MAP response to muscle contraction via a decrease in the extracellular concentration of 5-HT.

MKC-242, a novel 5-HT1A receptor agonist, facilitates cortical acetylcholine release by a mechanism different from that of 8-OH-DPAT in awake rats

We have previously reported that 5-¿3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy¿-1,3-be nzodioxole (MKC-242), a potent and selective serotonin (5-HT)1A receptor agonist, exerts anxiolytic- and antidepressant-like effects in animal models and that the antidepressant-like effect may be mediated by postsynaptic 5-HT1A receptors. The present study, using a microdialysis technique, was undertaken to characterize in vivo the effect of MKC-242 on cholinergic neurons. Subcutaneous injection of MKC-242 (0.5-1.0 mg/kg), like the typical 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), increased extracellular acetylcholine (ACh) levels in the rat cerebral cortex. The increase in ACh release by MKC-242 was also observed in the hippocampus. The effect of MKC-242 on cortical ACh release was attenuated by pretreatment with the 5-HT1A receptor antagonists (10 mg/kg, s.c.) propranolol and N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropana mide. The increase in cortical ACh release by MKC-242 was blocked by lesion of serotonergic neurons with 5,7-dihydroxytryptamine, whereas that by 8-OH-DPAT was not. Lesion of noradrenergic neurons with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine did not affect the MKC-242-induced increase in ACh release. These results suggest that systemic injection of MKC-242 facilitates in vivo ACh release via an activation of somadendritic 5-HT1A autoreceptors, and that MKC-242 and 8-OH-DPAT affect cholinergic neurons in the rat cerebral cortex via different mechanisms.

Modulation of hippocampal acetylcholine release after fimbria-fornix lesions and septal transplantation in rats

Female Long-Evans rats sustained electrolytic lesions of the fimbria and the dorsal fornix causing a partial lesion of the septohippocampal pathway. Two weeks later, the rats received intra-hippocampal grafts of fetal septal cell suspensions. Nine to twelve months later, the release of acetylcholine (ACh) in the hippocampus of sham-operated, lesion-only and grafted rats was measured by microdialysis. The extent of cholinergic (re)innervation was determined by acetylcholinesterase (AChE) staining and densitometry. In both lesion-only and grafted rats, the ratio of ACh release to AChE staining intensity was increased as compared to sham-operated rats, indicating a loss of endogenous inhibitory mechanisms. Scopolamine (0.5 mg/kg i.p.), a muscarinic antagonist, increased ACh release in all treatment groups. 8-OH-DPAT (0.5 mg/kg s.c.), an agonist at serotonergic 5HT1A-receptors, induced an increase of hippocampal ACh release in sham-operated rats. This effect was lost in lesion-only rats, but was fully restored by neuronal grafting. As 8-OH-DPAT influences hippocampal ACh release by a postsynaptic action, this finding indicates that the host brain exerts a serotonergic influence on the grafted cholinergic neurons.

Contrasting effects of systemic and intracerebral infusions of the 5-HT1A receptor agonist 8-OH-DPAT on spatial short-term working memory in rats

The present study compared the effects of systemic 8-OH-DPAT (0.05, 0.1 and 1.0 mg/kg) with intra-raphe and intra-hippocampal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10, 30 100 ng) on delayed non-matching-to-position (DNMP) performance in rats. The highest dose of 8-OH-DPAT administered systemically impaired DNMP performance in a delay-independent manner, increased premature responding and increased response bias. Infusions of 8-OH-DPAT (100 ng) into the median raphe nucleus improved performance accuracy, independent of delay whilst having no effect on any other response measure. Infusions of 8-OH-DPAT into the dorsal raphe nucleus had no effect on performance at any dose tested. Infusions of 8-OH-DPAT into the dorsal hippocampus produced a small impairment in performance which was also independent of delay. However, this decrement in performance accuracy was not accompanied by any changes in other response measures. These findings demonstrate a dissociation between the effects of stimulation of pre- and post-synaptic 5-HT1A receptors on performance of a DNMP task although the changes in performance cannot be accounted for by changes in mnemonic function.

8-OH-DPAT-induced release of hippocampal noradrenaline in vivo: evidence for a role of both 5-HT1A and dopamine D1 receptors

Here we investigate the effects of the novel selective 5-HT1A receptor antagonist, N-[2-[4-(2 methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl cyclo-hexanecarboxamide (WAY 100635), and the dopamine D1 receptor antagonist, R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin++ +-7-ol (SCH 23390), on the increase in extracellular noradrenaline in rat hippocampus induced by the 5-HT1A receptor agonist, 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT). 8-OH-DPAT (0.1 and 1 mg/kg s.c.) caused a dose-related increase in extracellular noradrenaline. WAY 100635 (0.3 and 1 mg/kg s.c.) did not block the release of noradrenaline induced by the higher dose of 8-OH-DPAT (1 mg/kg s.c.) but abolished the response to the lower dose (0.1 mg/kg s.c.). When administered alone, WAY 100635 (0.3 and 1 mg/kg s.c.) had no effect on extracellular noradrenaline. The postsynaptically mediated 5-HT behavioural syndrome induced by the higher dose of 8-OH-DPAT, in contrast to the increase in noradrenaline, was completely blocked by WAY 100635 (0.3 mg/kg s.c.). Finally, the noradrenaline response to 8-OH-DPAT (0.1 mg/kg s.c.) was blocked by SCH 23390 (0.5 mg/kg s.c.). Our data confirm that noradrenaline can be released by activation of 5-HT1A receptors but show that these receptors are not tonically activated, and may be more sensitive to stimulation than classical postsynaptic 5-HT1a receptors. A role for the dopamine D1 receptor in the noradrenaline response to 8-OH-DPAT is also suggested.