Effect of chronic activation of 5-HT3 receptors on 5-HT3, 5-HT1A and 5-HT2A receptors functional activity and expression of key genes of the brain serotonin system

Among serotonin (5-HT) receptors, the 5-HT₃ receptor is the only ligand-gated ion-channel. Little is known about the interaction between the 5-HT₃ receptor and other 5-HT receptors and influence of 5-HT₃ chronic activation on other 5-HT receptors and the expression of key genes of 5-HT system. Chronic activation of 5-HT₃ receptor with intracerebroventricularly administrated selective agonist 1-(3-chlorophenyl)biguanide hydrochloride (m-CPBG) (14 days, 40 nmol, i.c.v.) produced significant desensitization of 5-HT₃ and 5-HT_1A receptors. The hypothermic responses produced by acute administration of selective agonist of 5-HT₃ receptor (m-CPBG, 40 nmol, i.c.v.) or selective agonist of 5-HT_1A receptor (8-hydroxy-2-(di-n-propylamino)tetralin) (8-OH-DPAT, 1 mg/kg, i.p.) was significantly lower in m-CPBG treated mice compared with the mice of control groups. Chronic m-CPBG administration failed to induce any significant change in the 5-HT_2A receptor functional activity and in the expression of the gene encoding 5-HT_2A receptor. Chronic activation of 5-HT₃ receptor produced no considerable effect on the expression on 5-HT₃, 5-HT_1A, and 5-HT transporter (5-HTT) and tryptophan hydroxylase-2 (TPH-2) genes – the key genes of brain 5-HT system, in the midbrain, frontal cortex and hippocampus. In conclusion, chronic activation of ionotropic 5-HT₃ receptor produced significant desensitization of 5-HT₃ and postsynaptic 5-HT_1A receptors but caused no considerable changes in the expression of key genes of the brain 5-HT system.     

Central 5-HT3 receptor-induced hypothermia in mice: Interstrain differences and comparison with hypothermia mediated via 5-HT1A receptor

The selective agonist of serotonin 5-HT₃ receptor 1-(3-chlorophenyl)biguanide hydrochloride (m-CPBG) administered intracerebroventricularly (40, 80 or 160 nmol) produced long-lasting dose-dependent hypothermic response in AKR/2J mice. m-CPBG (160 nmol i.c.v.) induced profound hypothermia (delta t = −4 °C) that lasted up to 7 h. m-CPBG (40 nmol i.c.v.)-induced hypothermia was attenuated by 5-HT₃ receptor antagonist ondansetron pretreatment. At the same time, intraperitoneal administration of m-CPBG in a wide range of doses (0.5, 1.0, 5.0 or 10.0 mg/kg) did not affect the body temperature. These findings indicate: (1) the implication of central, rather than peripheral 5-HT₃ receptor in the thermoregulation; (2) the inability of m-CPBG to cross blood–brain barrier in mice. The comparison of brain 5-HT₃-induced hypothermic reaction in six inbred mouse strains (DBA/2J, CBA/Lac, C57BL/6, BALB/c, ICR, AKR/J) was performed and two highly sensitive to m-CPBG strains (CBA/Lac and C57BL/6) were found. In the same six mouse strains the functional activity of 5-HT_1A receptor was studied. The comparison of hypothermic reactions produced by 5-HT_1A receptor agonist 8-OH-DPAT (1.0 mg/kg i.p.) and m-CPBG revealed significant correlation between 5-HT₃ and 5-HT_1A-induced hypothermia in five out of six investigated mouse strains. 5-HT_1A receptor antagonist p-MPPI pretreatment (1 mg/kg i.p.) diminished hypothermia produced by centrally administered m-CPBG (40 nmol i.c.v.). The data suggest the cross-talk between 5-HT_1A and 5-HT₃ receptors in the mechanism of 5-HT-related hypothermia.     

Autoradiographic mapping of serotonin 5-HT1A, 5-HT1D, 5-HT2A and 5-HT3 receptors in the aged human spinal cord

Quantitative autoradiography with selective radioligands was used to establish the respective distribution of serotonin 5-HT_1A, 5-HT_1D, 5-HT_2A and 5-HT₃ receptors at the cervical, thoracic and lumbar levels of the spinal cord from subjects who died at 81–94 years. A high density of 5-HT_1A receptors, labeled by [³H]8-hydroxy-2-(di-n-propylamino)tetralin ([³H]8-OH-DPAT), was found in the superficial layers of the dorsal horn, with a significant enrichment ( 20%) in the lumbar vs. the thoracic and cervical segments. In contrast, only very low specific labeling by [³H]8-OH-DPAT (i.e. less than 10% of that measured in the dorsal horn), was detected in the ventral horn. 5-HT_1D sites labeled by [¹²⁵I]serotonin-O-carboxymethyl-glycyl-iodo-tyrosinamide ([¹²⁵I]GTI) were also mainly located within the superficial layers of the dorsal horn, but no difference in their relative density was noted at the three levels of the spinal cord examined. 5-HT_2A sites labeled by [³H]ketanserin were found in the dorsal horn of the cervical segments but no specific binding of this radioligand could be detected at any other level of the spinal cord of such aged subjects. Finally, a high density of [³H]S-zacopride-labeled 5-HT₃ receptors was noted especially in the most superficial layer (lamina I) of the dorsal horn at all segments examined. These data provide anatomical support for a role of spinal serotonin especially in nociception processing.     

Changes in 5-HT2A-mediated behavior and 5-HT2A- and 5-HT1A receptor binding and expression in conditional brain-derived neurotrophic factor knock-out mice

Changes in brain-derived neurotrophic factor (BDNF) expression have been implicated in the etiology of psychiatric disorders. To investigate pathological mechanisms elicited by perturbed BDNF signaling, we examined mutant mice with central depletion of BDNF (BDNF^2L/2LCk-cre). A severe impairment specific for the serotonin 2A receptor (5-HT_2AR) in prefrontal cortex was described previously in these mice. This is of much interest, as 5-HT_2ARs have been linked to neuropsychiatric disorders and anxiety-related behavior. Here we further characterized the serotonin receptor alterations triggered by BDNF depletion. 5-HT_2A ([³H]-MDL100907) and 5-HT_1A ([³H]-WAY100635) receptor autoradiography revealed site-specific alterations in BDNF mutant mice. They exhibited lower 5-HT_2A receptor binding in frontal cortex but increased binding in hippocampus. Additionally, 5-HT_1A receptor binding was decreased in hippocampus of BDNF mutants, but unchanged in frontal cortex. Molecular analysis indicated corresponding changes in 5-HT_2A and 5-HT_1A mRNA expression but normal 5-HT_2C content in these brain regions in BDNF^2L/2LCk-cre mice. We investigated whether the reduction in frontal 5-HT_2AR binding was reflected in reduced functional output in two 5-HT_2A-receptor mediated behavioral tests, the head-twitch response (HTR) and the ear-scratch response (ESR). BDNF^2L/2LCk-cre mutants treated with the 5-HT_2A receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) showed a clearly diminished ESR but no differences in HTR compared to wildtypes. These findings illustrate the context-dependent effects of deficient BDNF signaling on the 5-HT receptor system and 5-HT_2A-receptor functional output.     

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats

This study assessed the possible antinociceptive role of peripheral 5-HT₁ receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 μl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT₁ receptor agonists R(+)-UH-301 (5-HT_1A; 0.1–3 μg/paw), CGS-12066A (5-HT_1B; 0.01–0.3 μg/paw), GR46611 (5-HT_1B/1D; 0.3–10 μg/paw), BRL54443 (5-HT_1E/1F; 3–300 μg/paw) or LY344864 (5-HT_1F; 3–300 μg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 μg/paw) was significantly reduced by WAY-100635 (30–100 μg/paw; a 5-HT_1A receptor antagonist). Moreover, the antagonists GR55562 (30–100 μg/paw; 5-HT_1B/D) or SB224289 (30–100 μg/paw; 5-HT_1B) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 μg/paw) whereas GR55562 (30–100 μg/paw) or BRL15572 (30–100 μg/paw, 5-HT_1D) reduced the antinociceptive effect of GR46611 (0.3 μg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 μg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_1F and, probably, 5-HT_1E receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT₁ receptor agonists could be a therapeutic strategy to reduce inflammatory pain.     

In vivo modulation of the firing activity of putative slow- and fast-spiking interneurons in the medial prefrontal cortex by 5-HT3 receptors in 6-hydroxydopamine-induced Parkinsonian rats

In the present study, we examined changes in the firing rate and firing pattern of putative slow-spiking (SS) and fast-spiking (FS) interneurons in medial prefrontal cortex (mPFC) and the effect of 5-hydroxytryptamine-3 (5-HT₃) receptor agonist SR 57227A on the neuronal firing in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc) by using extracellular recording. The lesion of the SNc in rats decreased the firing rate of FS interneurons and the firing pattern of both SS and FS interneurons changed towards a more burst-firing. Systemic administration of SR 57227A (40–640 μg/kg, i.v.) increased the firing rate of SS interneurons, and decreased FS interneurons in sham-operated and the lesioned rats, respectively. The doses producing excitation or inhibition in the lesioned rats were higher than sham-operated rats. The local application of SR 57227A (0.01 μg) in mPFC excited SS interneurons, and inhibited FS interneurons in sham-operated rats, while having no effects on firing rate in the lesioned rats. Systemic administration of GABA_A receptor antagonist bicuculline (2 mg/kg, i.v.) excited FS interneurons in sham-operated rats, whereas bicuculline did not change the activity of FS interneurons in the lesioned rats. Our findings indicate that the putative SS and FS interneurons activity is modulated through activation of 5-HT₃ receptor by direct or indirect action, and the lesion of the SNc leads to changes in firing activity of the SS and FS interneurons and decreased response of these interneurons to SR 57227A, suggesting dysfunction and/or down-regulation of 5-HT₃ receptor on interneurons in the 6-hydroxydopamine-lesioned rats.     

Enhancement of agonist binding to 5-HT1A receptors in rat brain membranes by millimolar Mn

Manganese in millimolar concentration caused increase in specific binding of [³H]8-OH-DPAT to rat hippocampal membranes up to 44% in comparison with experiments in the presence of Mg²⁺, while no significant differences were found in rat cortical membranes. Similar increase in high-affinity agonist binding sites by Mn²⁺ was found in displacement curves of 8-OH-DPAT, where antagonist [³H]WAY100635 was used as reporter ligand. The removal of bivalent ions with EDTA caused full loss of high-affinity binding of agonists, but not for antagonists. Therefore it was hypothesized, that the effect of Mn²⁺- and Mg²⁺-ions was modulated through their action on different G-proteins. Results showed that efficient coupling of G-protein and 5-HT_1A receptors is crucial to modify Mg²⁺ and Mn²⁺ effects, whereas Mn²⁺ is more potent stabilizer of agonist high-affinity binding, especially when GTPγS is present. Using Sf9 cells as model system, we have shown that G_i1 proteins are required to modulate Mn²⁺-dependent high-affinity agonist binding to 5-HT_1A receptors, but further studies are necessary to find the cofacors of Mn²⁺ modulation to signal transduction.     

Anticataleptic effects of 5-HT1B receptors in the globus pallidus

The globus pallidus occupies an important position in the indirect pathway of the basal ganglia. Being a monoamine neurotransmitter, 5-HT is involved in mediating many physiological functions and pathophysiological processes in several movement disorders. Morphological studies have revealed that the globus pallidus receives serotonergic innervation arising from the raphe nuclei, mainly the dorsal raphe nucleus. A high level of 5-HT and 5-HT_1B receptors were detected in the globus pallidus. In the present study, bilateral microinjection of 5-HT or 5-HT_1B receptor agonist, CP-93129, into the globus pallidus significantly alleviated the symptoms of rigidity caused by haloperidol. To further elucidate 5-HT_1B receptor-induced anticatalepsy, in vivo extracellular recordings were performed to examine the effects of 5-HT_1B receptor activation on the firing activity of the globus pallidus neurons under the presence of haloperidol. Micro-pressure ejection of 5-HT or CP-93129 increased the spontaneous firing rate of the pallidal neurons. Furthermore, by using immunohistochemistry, positive staining of 5-HT_1B receptor was observed in the globus pallidus neurons. Taken together, the present findings provide evidence that activation of 5-HT_1B receptor may exert anticataleptic effects by increasing the activity of pallidal neurons.     

Chronic passive exposure to aggression decreases D2 and 5-HT1B receptor densities

It has been recently reported that passive exposure to aggression induces aggressive behavior in a rodent model. However, it remains unclear whether this response is correlated with neurochemical changes that correspond either to stress-induced aggression or non-stressed, learned aggression. Stress-induced aggression has been shown to result in increased brain dopamine D₂ receptor and serum corticosterone levels. In contrast, learned aggression is probably associated with reward deficiency syndrome, characterized by low dopamine D₂ receptor levels, without stress effects (i.e., high corticosterone levels). We hypothesized that chronic passive exposure to aggression would produce learned aggression, represented by low levels of dopamine D₂ receptor binding but normal levels of stress hormone. The present study additionally focused on serum testosterone and serotonin 5-HT_1B receptor density that has been associated with aggression/reward circuits. Hormonal results indicated that there were no differences between the “observer” rats that had been passively exposed to aggression and non-aggression for 10 min/day for 23 consecutive days. However, receptor binding autoradiography identified lower densities of dopamine D₂ receptors in the cortical-accumbal regions (shell of the nucleus accumbens and cingulate and motor cortices) and lower 5-HT_1B receptor densities in the tegmental regions (ventral tegmental area, substantia nigra pars compacta, and periaqueductal gray) among observers exposed to aggression, compared to controls. Changes in dopamine D₂ receptor densities due to chronic exposure to aggression do not resemble those patterns reported for stress-induced aggressive behavior. Our evidence suggests that the development of aggressive behavior among passive observers occurs through a learned, and not a stress-induced, mechanism.     

Effects of the 5-HT4 receptor agonist RS67333 and paroxetine on hippocampal extracellular 5-HT levels

The 5-HT₄ receptor modulates activity of serotonergic neurons and is a new potential target for antidepressant treatment. This microdialysis study evaluated the effect of the 5-HT₄ receptor agonist, RS67333, on extracellular serotonin (5-hydroxytryptamine, 5-HT) and 5-HIAA levels in rat ventral hippocampus during chloral hydrate anaesthesia, and explored the ability of RS67333 to augment the effect of the selective serotonin reuptake inhibitor paroxetine. The effect of RS67333 was examined after acute and subchronic (3 days) administration. Acute RS67333 (1.5 mg/kg i.v.) had no effect on extracellular 5-HT or 5-HIAA levels, while acute paroxetine (0.5 mg/kg i.v.) increased 5-HT levels by 299 ± 16% and decreased 5-HIAA levels by 25 ± 4%. Administration of RS67333 80 min after paroxetine caused an additional transient increase in 5-HT levels (to 398 ± 52% of baseline). Subchronic RS67333 administration (1.5 mg/kg i.p.) increased basal 5-HT levels by 73 ± 15% and decreased 5-HIAA levels by 27 ± 13%. In conclusion, the 5-HT₄ receptor agonist RS67333 augmented the acute effect of paroxetine on extracellular 5-HT levels in the ventral hippocampus, and after 3 days increased basal hippocampal 5-HT levels.     

Cloning of serotonin 5-HT1 receptor subtypes from the chimpanzee, gorilla and Rhesus monkey and their agonist-induced guanosine 5′γS triphosphate binding

5-HT₁ receptor subtypes (_1B, _1D and _1F) have been implicated in migraine pathophysiology and their ligands have been examined for pharmacological actions in various experimental animal models. Considerable divergences exist, however, in their primary sequences between experimental animals and human, and additional models closer to human, such as non-human primates seem to be useful for migraine research. Earlier, we cloned the 5-HT_1D, and here 5-HT_1B and 5-HT_1F receptors from the chimpanzee, gorilla and Rhesus monkey, via polymerase chain reactions with their genomic DNAs and primers designed from the corresponding human receptors. Direct sequencing of PCR products showed that the 5-HT_1B receptors from the chimpanzee, gorilla and monkey differ from the human receptor by 0, 1 and 7 residues, respectively while 5-HT_1F receptors differ by 0, 3 and 10 residues, respectively. These divergent residues are mostly conservatively substituted and also largely confined to the N-terminal region and the 3rd intracellular loop, away from transmembrane segments and intracellular loops near membrane which are critical for ligand binding and G protein coupling. The chimpanzee 5-HT_1D, 5-HT_1B and monkey 5-HT_1F receptors, as heterologously expressed in human embryonic kidney 293 cells, showed robust agonist-induced guanosine 5′gamma ³⁵S triphosphate (GTPγ³⁵S) binding through activation of G proteins containing Gγ_i subunits. Moreover, pronounced inhibition of basal GTPγ³⁵S binding by methiothepin (an antagonist), representing constitutively active receptors, was observed with only 5-HT_1D. Overall, ligand binding and GTPγ³⁵S binding profiles for these primate receptors are comparable to those for the human receptors and validate non-human primates as useful models for human migraine research.     

Colocalization of 5-HT1F receptor and calcitonin gene-related peptide in rat vestibular nuclei

The aim of this study was to determine whether calcitonin gene-related peptide (CGRP) colocalizes with 5-HT_1F receptor in rat vestibular nuclei using a double immunohistochemical staining procedure. The frequent co-occurrence of migraine and balance disorders suggests a pathophysiologic link between the two. However, the mechanism of migrainous vertigo has not been elucidated, though serotonin (5-HT) and its receptors are believed to involve in the pathogenesis of migrainous vertigo. Furthermore, 5-HT_1F receptor agonists and CGRP receptor antagonists have recently attracted attention as potential treatments for migraine, and CGRP release from trigeminal neurons has been associated with migraine. This study demonstrates the colocalization of 5-HT_1F receptor and CGRP in the rat vestibular nuclei, which suggests that 5-HT_1F receptor regulates the release of CGRP from vestibular nuclei. This finding indicates that 5-HT_1F receptor agonists may ameliorate migrainous vertigo by attenuating elevated levels of CGRP release from vestibular nuclei.     

LP-211 is a brain penetrant selective agonist for the serotonin 5-HT7 receptor

We have determined the pharmacological profile of the new serotonin 5-HT₇ receptor agonist N-(4-cyanophenylmethyl)-4-(2-diphenyl)-1-piperazinehexanamide (LP-211). Radioligand binding assays were performed on a panel of 5-HT receptor subtypes. The compound was also evaluated in vivo by examining its effect on body temperature regulation in mice lacking the 5-HT₇ receptor (5-HT₇^−/−) and their 5-HT₇^+/+ sibling controls. Disposition studies were performed in mice of both genotypes. It was found that LP-211 was brain penetrant and underwent metabolic degradation to 1-(2-diphenyl)piperazine (RA-7). In vitro binding assays revealed that RA-7 possessed higher 5-HT₇ receptor affinity than LP-211 and a better selectivity profile over a panel of 5-HT receptor subtypes. In vivo it was demonstrated that LP-211, and to a lesser degree RA-7, induced hypothermia in 5-HT₇^+/+ but not in 5-HT₇^−/− mice. Our results suggest that LP-211 can be used as a 5-HT₇ receptor agonist in vivo.     

Negative crosstalk between M1 and M2 muscarinic autoreceptors involves endogenous adenosine activating A1 receptors at the rat motor endplate

At the rat motor nerve terminals, activation of muscarinic M₁ receptors negatively modulates the activity of inhibitory muscarinic M₂ receptors. The present work was designed to investigate if the negative crosstalk between muscarinic M₁ and M₂ autoreceptors involved endogenous adenosine tonically activating A₁ receptors on phrenic motor nerve terminals. The experiments were performed on rat phrenic nerve-hemidiaphragm preparations loaded with [³H]-choline (2.5 μCi/ml). Selective activation of muscarinic M₁ and adenosine A₁ receptors with 4-(N-[3-clorophenyl]-carbamoyloxy)-2-butyryltrimethylammonium (McN-A-343, 3 μM) and R-N⁶-phenylisopropyladenosine (R-PIA, 100 nM), respectively, significantly attenuated inhibition of evoked [³H]-ACh release induced by muscarinic M₂ receptor activation with oxotremorine (10 μM). Attenuation of the inhibitory effect of oxotremorine (10 μM) by R-PIA (100 nM) was detected even in the presence of pirenzepine (1 nM) blocking M₁ autoreceptors, suggesting that suppression of M₂-inhibiton by A₁ receptor activation is independent on muscarinic M₁ receptor activity. Conversely, the negative crosstalk between M₁ and M₂ autoreceptors seems to involve endogenous adenosine tonically activating A₁ receptors. This was suggested, since attenuation of the inhibitory effect of oxotremorine (10 μM) by McN-A-343 (3 μM) was suppressed by the A₁ receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (2.5 nM), and by reducing extracellular adenosine with adenosine deaminase (0.5 U/mL) or with the adenosine transport blocker, S-(p-nitrobenzyl)-6-thioinosine (NBTI, 10 μM). The results suggest that the negative crosstalk between muscarinic M₁ and M₂ autoreceptors involves endogenous adenosine outflow via NBTI-sensitive (es) nucleoside transport system channelling to the activation of presynaptic inhibitory A₁ receptors at the rat motor endplate.     

5-HT activates vagal afferent cell bodies in vivo: role of 5-HT2 and 5-HT3 receptors

Occipital artery (OA) injections of 5-HT elicit pronounced reductions in heart rate and mean arterial blood pressure (MAP) in urethane-anesthetized rats by activation of vagal afferent cell bodies in the ipsilateral nodose ganglion. In contrast, internal carotid artery (ICA) and i.v. injections elicit similar cardiovascular responses by activation of peripheral vagal afferent terminals. The aim of this study was to examine the roles of 5-HT3 and 5-HT2 receptors in the 5-HT-induced activation of vagal afferent cell bodies and peripheral afferent terminals in urethane-anesthetized rats. OA, ICA and i.v. injections of 5-HT elicited dose-dependent reductions in heart rate and MAP that were virtually abolished after i.v. administration of the 5-HT3 receptor antagonists, MDL 7222 or ICS 205-930. The responses elicited by the OA injections of 5-HT were markedly diminished after i.v. injection of the 5-HT2 receptor antagonists, xylamidine or ketanserin, whereas the responses elicited by i.v. or ICA injections of 5-HT were not affected. The present findings suggest that (1) 5-HT3 and 5-HT2 receptor antagonists gain ready access to nodose ganglion cells upon i.v. administration, and (2) functional 5-HT3 and 5-HT2 receptors exist on the cell bodies of vagal afferent neurons mediating the cardiovascular responses elicited by OA injections of 5-HT. These findings also support a wealth of evidence that 5-HT3 receptors exist on the peripheral terminals of vagal afferents, and although they do not discount the possibility that 5-HT2 receptors exist on peripheral vagal afferent terminals, it appears that activation of these receptors does not have pronounced effects on 5-HT3 receptor activity on terminals that mediate the hemodynamic responses to 5-HT.     

5-HT2A receptor levels increase in MPTP-lesioned macaques treated chronically with L-DOPA

Long-term L-3,4-dihydroxyphenylalanine (L-DOPA) treatment in Parkinson's disease (PD) is associated with motor complications such as dyskinesia. There are clear functional interactions between dopaminergic and serotonergic type 2A receptors (5-HT_2A)-mediated neurotransmission. Moreover, 5-HT_2A receptor antagonists can reduce L-DOPA-induced dyskinesia (LID). We hypothesized that enhanced 5-HT_2A-mediated neurotransmission may be involved in the genesis of L-DOPA-induced dyskinesia. Radioligand binding autoradiography, using [³H]-ketanserin, was performed to define 5-HT_2A receptor levels in brain tissue from macaques: 6 normal; 5 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned, parkinsonian macaques, without exposure to L-DOPA; 6 MPTP-lesioned, parkinsonian macaques, receiving a single administration of L-DOPA, and exhibiting no dyskinesia; and 6 MPTP-lesioned, parkinsonian, macaques chronically treated with L-DOPA, and exhibiting dyskinesia. 5-HT_2A receptor binding was increased in the caudate, putamen, and middle layers of the motor cortex in chronically L-DOPA-treated animals, by 50%, 50%, and 45% respectively, compared with normal macaques. 5-HT_2A binding was not significantly altered in parkinsonian, untreated, or parkinsonian, single treatment, nondyskinetic macaques, compared with normal. These data provide an anatomical basis for mechanisms to explain the efficacy of 5-HT_2A antagonists against dyskinesia.     

Stimulation of basolateral amygdaloid serotonin 5-HT2C receptors promotes the induction of long-term potentiation in the dentate gyrus of anesthetized rats

We have previously found that the induction of hippocampal long-term potentiation (LTP) is modulated by neuron activities in the basolateral amygdala (BLA). However, little is known about what neurotransmitter system in the BLA contributes to modulation of hippocampal LTP. In the present study, we investigated possible involvement of BLA serotonergic system in the induction of LTP at the perforant path (PP)-dentate gyrus (DG) granule cell synapses of anesthetized rats. The induction of PP-DG LTP was significantly inhibited by intra-BLA injection of the 5-HT₂ receptor antagonist cinanserin (25–50 nmol), but not by intra-BLA injection of the 5-HT_1,7 receptor antagonist methiothepin (50 nmol), the 5-HT₃ receptor antagonist ondansetron (50 nmol) or the 5-HT₄ receptor antagonist RS23597-190 (100 nmol). In addition, intra-BLA injection of the 5-HT_2C receptor agonist MK212 (50 nmol) facilitated the induction of PP-DG LTP. These results suggest that the induction of PP-DG LTP is promoted by activation of 5-HT_2C receptors in the BLA.     

Role of 5-HT3 receptors in the mechanisms of central pain syndrome

Electrophysiological and behavioral studies showed that spinal 5-HT₃ receptors are involved in the regulation of pain sensitivity in rats. Intrathecal administration of the 5-HT₃ receptor antagonist tropine (200 g) produced allodynia, reduced the threshold, decreased the latency, and increased the number of spikes in the late component of the nociceptive flexion reflex. Intrathecal administration of 5-HT₃ receptor agonist quipazine (200 mg) abolished nociceptive flexion reflex and alleviated spinal pain syndrome produced by impairment of GABAergic inhibition in the lumbar spinal segments. Our results indicate that spinal 5-HT₃ receptors are involved in the modulation of pain sensitivity: activation of these receptors inhibits nociceptive reactions, while blockade of 5-HT₃ receptors potentiates the nociceptive response via modulation of excitability of GABAergic interneurons.