BMY 7378: Partial agonist at spinal cord 5-HT1A receptors

Recent data indicate that BMY 7378 demonstrates high affinity, selectivity and low intrinsic activity at hippocampal 5-HT_1A receptors, suggesting that BMY 7378 may represent the first selective 5-HT_1A functional antagonist. The present study examined the agonist and antagonist properties of BMY 7378 at spinal cord 5-HT_1A receptors. In electrophysiological studies, iontophoretic administration of either the 5-HT_1A agonist 8-OH-DPAT (43.8 ± 5.4 nA) or BMY 7378 (46.3 ± 5.2 nA) significantly inhibited the firing rate of wide-dynamic-range dorsal horn units indicating that BMY 7378 demonstrates significant intrinsic activity at spinal cord 5-HT_1A receptors. Concomitant BMY 7378 and 8-OH-DPAT administration identified no BMY 7378 ejection current (20–100 nA) which antagonized the 8-OH-DPAT induced inhibition of dorsal horn unit activity. In behavioral studies in the spinal rat, 8-OH-DPAT increased the animals' sensitivity to noxious levels of mechanical stimulation (ED₅₀ = 269 ± 24 nmol/kg) as did BMY 7378 (ED₅₀ = 295 ± 70 nmol/kg) with no statistically significant difference in the maximal response (Y_max) observed. Concomitant BMY 7378 and 8-OH-DPAT administration identified no BMY 7378 dose (10–1100 nmol/kg) which blocked the hyperalgesic effect of 8-OH-DPAT, rather, each drug produced similar effects which were additive. Further, the 5-HT_1A agonist effects of BMY 7378 were blocked by the 5-HT_1A antagonist, spiperone. Therefore, both the electrophysiologic and reflex data indicate that BMY 7378 possesses significant intrinsic activity at spinal cord 5-HT_1A receptors, and like 8-OH-DPAT is a partial agonist at these receptors. Differences in BMY 7378 intrinsic activity at spinal cord as opposed to hippocampal 5-HT_1A receptors are discussed in terms of regional differences in G-proteins coupled to 5-HT_1A receptors in these two CNS regions.     

Substitution of 5-HT1A Receptor Signaling by a Light-activated G Protein-coupled Receptor

Understanding serotonergic (5-HT) signaling is critical for understanding human physiology, behavior, and neuropsychiatric disease. 5-HT mediates its actions via ionotropic and metabotropic 5-HT receptors. The 5-HT_1A receptor is a metabotropic G protein-coupled receptor linked to the G_i/o signaling pathway and has been specifically implicated in the pathogenesis of depression and anxiety. To understand and precisely control 5-HT_1A signaling, we created a light-activated G protein-coupled receptor that targets into 5-HT_1A receptor domains and substitutes for endogenous 5-HT_1A receptors. To induce 5-HT_1A-like targeting, vertebrate rhodopsin was tagged with the C-terminal domain (CT) of 5-HT_1A (Rh-CT_5-HT1A). Rh-CT_5-HT1A activates G protein-coupled inward rectifying K⁺ channels in response to light and causes membrane hyperpolarization in hippocampal neurons, similar to the agonist-induced responses of the 5-HT_1A receptor. The intracellular distribution of Rh-CT_5-HT1A resembles that of the 5-HT_1A receptor; Rh-CT_5-HT1A localizes to somatodendritic sites and is efficiently trafficked to distal dendritic processes. Additionally, neuronal expression of Rh-CT_5-HT1A, but not Rh, decreases 5-HT_1A agonist sensitivity, suggesting that Rh-CT_5-HT1A and 5-HT_1A receptors compete to interact with the same trafficking machinery. Finally, Rh-CT_5-HT1A is able to rescue 5-HT_1A signaling of 5-HT_1A KO mice in cultured neurons and in slices of the dorsal raphe showing that Rh-CT_5-HT1A is able to functionally compensate for native 5-HT_1A. Thus, as an optogenetic tool, Rh-CT_5-HT1A has the potential to directly correlate in vivo 5-HT_1A signaling with 5-HT neuron activity and behavior in both normal animals and animal models of neuropsychiatric disease.     

Dissociation of the plasticity of 5-HT1A sites and 5-HT transporter sites

To study the early effects of neonatal 5,7-dihydroxytryptamine lesions on 5-hydroxytryptamine_1A (5-HT_1A) receptors, we measured regional [³H]8-OH-DPAT-labeled 5-HT_1A sites in binding assays and compared them to our previous studies of [³H]paroxetine-labeled 5-HT transporter sites during the first month in the same rats. While there were significant time- and dose-dependent effects of 5,7-DHT on 5-HT transporter sites, there were no significant changes in 5-HT_1A sites in cortex, hippocampus, diencephalon, brainstem, cerebellum, or spinal cord. 5,7-DHT lesions also did not alter the K_i of Gpp(NH)p at brainstem 5-HT_1A sites or the K_i of 5-HT in cortex or brainstem in the presence or absence of GTPS or Gpp(NH)p. There were significant regional differences between the density of 5-HT_1A sites and 5-HT transporter sites. The ontogeny of brainstem 5-HT_1A sites was a pattern of increases until three weeks postnatal, and 5,7-DHT lesions did not alter the ontogeny of 5-HT_1A sites. These data suggest differential plasticity of 5-HT_1A and 5-HT transporter binding sites during the first month after neonatal 5,7-DHT lesions.     

Characterization of three serotonin receptors from the small white butterfly,Pieris rapae

Serotonin (5-hydroxytryptamine, 5-HT) plays a key role in modulating diverse physiological processes and behaviors in both protostomes and deuterostomes. These functions are mediated through the binding of serotonin to its receptors, which are recognized as potential insecticide targets. We investigated the sequence, pharmacology and tissue distribution of three 5-HT receptors (Piera5-HT_1A, Piera5-HT_1B, Piera5-HT₇) from the small white butterfly Pieris rapae, an important pest of cultivated cabbages and other mustard family crops. Activation of Piera5-HT_1A or Piera5-HT_1B by 5-HT inhibited the production of cAMP in a dose-dependent manner. Stimulation of Piera5-HT₇ with 5-HT increased cAMP level significantly. Surprisingly, with the exception of 5-methoxytryptamine, agonists including α-methylserotonin, 8-Hydroxy-DPAT and 5-carboxamidotryptamine activated these receptors poorly. The results are consistent with previous findings in Manduca sexta. All three receptors were blocked by methiothepin, but ketanserin and yohimbine were not effective. The selective mammalian 5-HT receptor antagonists SB 216641 and SB 269970 displayed potent inhibition effects on Piera5-HT_1B and Piera5-HT₇ respectively. The results we achieved here indicate that the pharmacological properties of Lepidoptera 5-HT receptors are quite different from those in other insects and vertebrates and may contribute to development of new selective pesticides. This study offers important information on three 5-HT receptors from P. rapae that will facilitate further analysis of the functions of 5-HT receptors in insects.     

PKCδ Knockout Mice Are Protected from Dextromethorphan-Induced Serotonergic Behaviors in Mice: Involvements of Downregulation of 5-HT<Subscript>1A</Subscript> Receptor and Upregulation of Nrf2-Dependent GSH Synthesis

We investigated whether a specific serotonin (5-HT) receptor-mediated mechanism was involved in dextromethorphan (DM)-induced serotonergic behaviors. We firstly observed that the activation of 5-HT_1A receptor, but not 5-HT_2A receptor, contributed to DM-induced serotonergic behaviors in mice. We aimed to determine whether the upregulation of 5-HT_1A receptor induced by DM facilitates the specific induction of certain PKC isoform, because previous reports suggested that 5-HT_1A receptor activates protein kinase C (PKC). A high dose of DM (80 mg/kg, i.p.) induced a selective induction of PKCδ out of PKCα, PKCβI, PKCβII, PKCξ, and PKCδ in the hypothalamus of wild-type (WT) mice. More importantly, 5-HT_1A receptor co-immunoprecipitated PKCδ in the presence of DM. Consistently, rottlerin, a pharmacological inhibitor of PKCδ, or PKCδ knockout significantly protected against increases in 5-HT_1A receptor gene expression, 5-HT turnover rate, and serotonergic behaviors induced by DM. Treatment with DM resulted in an initial increase in nuclear factor erythroid-2-related factor 2 (Nrf2) nuclear translocation and DNA-binding activity, γ-glutamylcysteine (GCL) mRNA expression, and glutathione (GSH) level. This compensative induction was further potentiated by rottlerin or PKCδ knockout. However, GCL mRNA and GSH/GSSG levels were decreased 6 and 12 h post-DM. These decreases were attenuated by PKCδ inhibition. Our results suggest that interaction between 5-HT_1A receptor and PKCδ is critical for inducing DM-induced serotonergic behaviors and that inhibition of PKCδ attenuates the serotonergic behaviors via downregulation of 5-HT_1A receptor and upregulation of Nrf2-dependent GSH synthesis.     

Alterations In Serotonin Receptors in the Neostriatum of Weaver Mutant Mice

Mice that carry the autosomal recessive gene weaver show a distinctive loss of nigrostriatal dopamine innervation, with the greatest deficits in the dorsal caudate-putamen and almost complete sparing in the nucleus accumbens and ventral caudate. In addition to loss of dopamine in this model, it has recently been shown that markers of serotonin (5-hydroxytryptamine, 5-HT) innervation including 5-HT content, synaptosomal uptake of [³H]5-HT and [³H]citalopram binding were elevated in the dorsal neostriatum of the weaver mutant mouse. Using quantitative autoradiography of specific ligands for dopamine and 5-HT uptake sites as well as serotonin 5-HT₁ and 5-HT_2A receptors, we found an increased density of 5-HT uptake sites and 5-HT₁ receptors restricted to the dorsal portion of the neostriatum of the weaver mouse. In contrast, 5-HT_2A receptors were increased in both the dorsal and ventral portions of the rostral neostriatum as well as the nucleus accumbens. The behavioural and functional relevance of these receptor changes is unclear, although, adaptations in 5-HT may play a role in certain aspects of spontaneous behaviour in the weaver mutant mouse.     

Molecular Design of Novel Ligands for 5-HT1A Receptors

Abstract

Serotonin (5-HT) is a potent bioactive substance known to function through a number of different receptor types and subtypes. In our attempt to develop new agents that would interact selectively at certain 5-HT receptors, especially the 5-HT_1A subtype, 8-hydroxy-2-di-n-propylamino tetralin (8-OH-DPAT) served as a template for the design of novel agents sharing aspects of the pharmacophore of 8-OH-DPAT and 5-HT. 5-HT contains no center of asymmetry, and 8-OH-DPAT shows only very modest stereospecificity for 5-HT_1A receptors. To develop agents having enhanced potency and selectivity for the 5-HT_1A site, several ring systems offering enhanced conformational rigidity which approximate the oxygen to nitrogen interatomic distances of 8-OH-DPAT and (to a lesser extent) 5-HT were synthesized. Exemplary ring systems include the 8-alkoxy-hexahydroindeno[1,2-c]pyrrole, 5-alkoxy-hexahydro-1H-indeno-[2,1-c]pyridine, and 9-alkoxy-hexahydro-1H-benz[e]isoindole systems. These couformationally restricted molecules demonstrated moderate stereospecificity in their interaction with the 5-HT_1A binding site, which was enhanced in compounds with larger nitrogen substituents. Appropriate choice of such derivatives led to highly potent compounds selective for 5-HT_1A sites compared with their activity at other 5-HT and/or adrenergic receptors. The pharmacological profile of compounds which appear to act as agonists at 5-HT_1A receptors in the central nervous system to lower blood pressure in animal models of hypertension is presented     

Serotonin2 Receptors in the Nucleus Tractus Solitarius: Characterization and Role in the Baroreceptor Reflex Arc

1. There is a general agreement concerning the key role of the baroreceptor reflex in blood pressure homeostasis. It is also well accepted that baroreceptor afferent messages are first integrated within the nucleus tractus solitarius (NTS) and that an excitatory amino acid, probably glutamate, is the principal neurotransmitter of corresponding afferents fibers. However, important points concerning the processing of baroreceptor messages within the NTS remain to be clarified, in particular the possible modulatory role of other neuroactive substances at this particular level in the medulla oblongata.2. In this context, the present review focuses on serotonin, and the possible facilitatory influence of NTS serotonergic afferents and receptors on the baroreceptor reflex arc. Relevant pharmacological, electrophysiological, immunohistochemical, and biochemical data, are presented and discussed. They can be summarized as follows.3. The selective destruction of the nodose ganglion-NTS serotonergic pathway produces a long-term increase in blood pressure variability, similar to that caused by baroreceptor denervation.4. Microinjection of picomolar doses of 5-HT into the NTS elicits the typical responses of baroreceptor activation.5. The cardiovascular effects elicited by local microinjections of specific agonists and antagonists into the NTS of intact rats and of animals that underwent nodose ganglionectomy indicate that the baroreceptor-like effects of locally administered 5-HT are mediated by the activation of postsynaptic 5-HT₂ receptors.6. The medullary pathways which mediate NTS 5-HT₂ receptor-evoked responses are similar to those involved in the baroreceptor reflex arc.7. Pharmacological and electrophysiological studies suggest that the cardiovascular effects of intra-NTS 5-HT involve the 5-HT_2A receptor subtype expressed by NTS barosensitive neurons that receive polysynaptic vagal afferents.8. Intra-NTS microinjection of a subthreshold dose of DOI, a 5-HT₂ receptor agonist, which, on its own, does not produce any cardiovascular changes, significantly enhances the bradycardiac component of the baroreflex.9. Altogether, the data summarized above show that, in the NTS, 5-HT acting at 5-HT_2A receptors exerts a facilitatory influence on the baroreceptor reflex, especially on the cardiac component of this reflex.10. Convergent pharmacological and electrophysiological data indicate that, in the NTS, functional interactions between NMDA- and 5-HT_2A-receptors coexpressed by the same neurons probably underlie the facilitatory influence of 5-HT upon the baroreceptor reflex.11. Under physiological conditions, the 5-HT_2A receptor-mediated facilitatory modulation of the cardiovagal component of the baroreflex might be triggered by 5-HT released from nodose ganglion-NTS serotoninergic afferent neurons and/or for serotoninergic projections originating in raphe nuclei. The latter possibility might notably occur during recovery after physical exercise and/or during the freezing reaction in stressed animals.     

Antidepressant-Like Activity of YL-0919: A Novel Combined Selective Serotonin Reuptake Inhibitor and 5-HT1A Receptor Agonist

It has been suggested that drugs combining activities of selective serotonin reuptake inhibitor and 5-HT_1A receptor agonist may form a novel strategy for higher therapeutic efficacy of antidepressant. The present study aimed to examine the pharmacology of YL-0919, a novel synthetic compound with combined high affinity and selectivity for serotonin transporter and 5-HT_1A receptors. We performed in vitro binding and function assays and in vivo behavioral tests to assess the pharmacological properties and antidepressant-like efficacy of YL-0919. YL-0919 displayed high affinity in vitro to both 5-HT_1A receptor and 5-HT transporter prepared from rat cortical tissue. It exerted an inhibitory effect on forskolin-stimulated cAMP formation and potently inhibited 5-HT uptake in both rat cortical synaptosomes and recombinant cells. After acute p.o. administration, very low doses of YL-0919 reduced the immobility time in tail suspension test and forced swimming test in mice and rats, with no significant effect on locomotor activity in open field test. Furthermore, WAY-100635 (a selective 5-HT_1A receptor antagonist, 0.3 mg/kg) significantly blocked the effect of YL-0919 in tail suspension test and forced swimming test. In addition, chronic YL-0919 treatment significantly reversed the depressive-like behaviors in chronically stressed rats. These findings suggest that YL-0919, a novel structure compound, exerts dual effect on the serotonergic system, as both 5-HT_1A receptor agonist and 5-HT uptake blocker, showing remarkable antidepressant effects in animal models. Therefore, YL-0919 may be used as a new option for the treatment of major depressive disorder.     

The influence of isoflurane on the synaptic activity of 5-hydroxytryptamine

The effects of isoflurane on uptake of 5-hydroxytryptamine(serotonin; 5-HT) by rat brain synaptosomes and binding of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and ketanserin to 5-HT_1A and 5-HT₂ receptors were examined. Isoflurane caused a concentration-dependent decrease in synaptosomal 5-HT uptake that was kinetically defined as non-competitive; exposure to isoflurane decreased V_max but had no effect on the apparent K_m. Removal of the drug from the reaction mixture resulted in the return of 5-HT accumulation rates to control levels. Isoflurane inhibited 8-OH-DPAT binding to hippocampal membranes by up to 27±6% at 4.5 mM, but did not significantly affect ketanserin binding to 5-HT₂ receptors. These findings suggest that presynaptic actions are more important than postsynaptic actions in the modulation of serotonergic neutrotransmission by isoflurane.     

Flesinoxan challenge suggests that chronic treatment with paroxetine in rats does not desensitize receptors controlling 5-HT synthesis

It has been proposed that the desensitization of 5-HT_1A (5-hydroxytryptamine; serotonin) receptors following chronic therapy with selective serotonin reuptake inhibitors (SSRIs) is necessary for their therapeutic efficacy. Stimulation of the 5-HT_1A receptors decreases serotonin (5-HT) synthesis and release, but it is not clear if the receptors are fully desensitized following chronic SSRI treatment. The main objective of this study was evaluation of ability of 5-HT_1A receptors to modulate 5-HT synthesis after 14-day paroxetine treatment. 5-HT_1A receptor sensitivity following chronic administration of the SSRI paroxetine was assessed by the ability of an acute challenge with the 5-HT_1A agonist, flesinoxan, to modulate 5-HT synthesis in the rat brain. The rates of 5-HT synthesis were measured using the α-[¹⁴C]methyl-l-tryptophan autoradiographic method. The rats were treated for 2 weeks with paroxetine (10 mg/(kg day), s.c., delivered by osmotic minipump). After this treatment, the rats received an acute challenge with flesinoxan (5 mg/kg, i.p.), while the control rats were injected with the vehicle. Forty minutes following the flesinoxan injection, the tracer, α-[¹⁴C]methyl-l-tryptophan, was injected over 2 min. 5-HT synthesis rates were calculated from autoradiographically measured tissue tracer concentrations and plasma time–activity curves. The results demonstrated that the acute flesinoxan challenge produced a significant decrease in 5-HT synthesis rates throughout the rat brain. The greatest decrease was observed in the ventral hippocampus, somatosensory cortex and the ascending serotonergic cell bodies. In comparison with data reported on an acute challenge with flesinoxan in naïve rats (rats without any other treatment), the results presented here suggest a greater effect of flesinoxan on synthesis reduction in rats chronically treated with paroxetine. The results also suggest that the 5-HT receptors were not fully desensitized by paroxetine treatment, and that the stimulation of 5-HT_1A receptors with an agonist is still capable of reducing 5-HT synthesis.     

Involvement of 5-HT<Subscript>2A</Subscript> receptors in genetic mechanisms of autoregulation of brain 5-HT system

The brain serotonin (5-HT) system has been implicated in the pathophysiology of anxiety, depression, drug addiction, and schizophrenia. 5-HT_2A receptors are involved in the mechanisms of stressinduced psychopathology and impulsive behavior. In this work, we investigated the role of 5-HT_2A receptors in the autoregulation of the brain 5-HT system. Chronic treatment with DOI, a 5-HT_2A receptor agonist (1.0 mg/kg, i.p./14 days), produced a considerable decrease in the number of 5-HT_2A receptor-mediated head twitches in AKR/J mice, indicating the desensitization of 5-HT_2A receptors. Chronic DOI treatment did not affect the expression of the 5-HT_2A receptor gene in the midbrain, hippocampus and frontal cortex. At the same time, an increase in the expression of the gene encoding a key enzyme of 5-HT synthesis, tryptophan hydroxylase-2 (TPH-2), accompanied with an increase in TPH-2 activity and 5-HT levels, and decreased expression of the serotonin transporter (5-HTT) gene were observed in the midbrain of DOI-treated mice. These results provide new evidence of receptor-gene cross-talk in the brain 5-HT system and implication 5-HT_2A receptors in the autoregulation of the brain 5-HT system.     

Flesinoxan challenge suggests that chronic treatment with paroxetine in rats does not desensitize receptors controlling 5-HT synthesis

It has been proposed that the desensitization of 5-HT_1A (5-hydroxytryptamine; serotonin) receptors following chronic therapy with selective serotonin reuptake inhibitors (SSRIs) is necessary for their therapeutic efficacy. Stimulation of the 5-HT_1A receptors decreases serotonin (5-HT) synthesis and release, but it is not clear if the receptors are fully desensitized following chronic SSRI treatment. The main objective of this study was evaluation of ability of 5-HT_1A receptors to modulate 5-HT synthesis after 14-day paroxetine treatment. 5-HT_1A receptor sensitivity following chronic administration of the SSRI paroxetine was assessed by the ability of an acute challenge with the 5-HT_1A agonist, flesinoxan, to modulate 5-HT synthesis in the rat brain. The rates of 5-HT synthesis were measured using the α-[¹⁴C]methyl-l-tryptophan autoradiographic method. The rats were treated for 2 weeks with paroxetine (10 mg/(kg day), s.c., delivered by osmotic minipump). After this treatment, the rats received an acute challenge with flesinoxan (5 mg/kg, i.p.), while the control rats were injected with the vehicle. Forty minutes following the flesinoxan injection, the tracer, α-[¹⁴C]methyl-l-tryptophan, was injected over 2 min. 5-HT synthesis rates were calculated from autoradiographically measured tissue tracer concentrations and plasma time–activity curves. The results demonstrated that the acute flesinoxan challenge produced a significant decrease in 5-HT synthesis rates throughout the rat brain. The greatest decrease was observed in the ventral hippocampus, somatosensory cortex and the ascending serotonergic cell bodies. In comparison with data reported on an acute challenge with flesinoxan in naïve rats (rats without any other treatment), the results presented here suggest a greater effect of flesinoxan on synthesis reduction in rats chronically treated with paroxetine. The results also suggest that the 5-HT receptors were not fully desensitized by paroxetine treatment, and that the stimulation of 5-HT_1A receptors with an agonist is still capable of reducing 5-HT synthesis.     

Modulation of Antagonist Binding to Serotonin1A Receptors from Bovine Hippocampus by Metal Ions

1. The serotonin_1A(5-HT_1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins. They appear to be involved in various behavioral and cognitive functions. Although specific 5-HT_1Aagonists have been discovered more than a decade back, the development of selective 5-HT_1Aantagonists has been achieved only recently.2. We have examined the modulation of the specific antagonist [³H]p-MPPF binding to 5-HT_1Areceptors from bovine hippocampal membranes by monovalent and divalent metal ions. Our results show that the antagonist binding to 5-HT_1Areceptors is inhibited by both monovalent and divalent cations in a concentration-dependent manner. This is accompanied by a concomitant reduction in binding affinity.3. Our results also show that the specific antagonist p-MPPF binds to all available receptors in the bovine hippocampal membrane irrespective of their state of G-protein coupling and other serotonergic ligands such as 5-HT and OH-DPAT effectively compete with the specific antagonist [³H]p-MPPF.4. These results are relevant to ongoing analyses of the overall modulation of ligand binding in G-protein-coupled seven transmembrane domain receptors.     

5-HT receptors on identified Lymnaea neurones in culture. Pharmacological characterization of 5-HT1A receptors

The ability of the selective 5-HT_1A receptor agonist R(+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT) to bind with 5-HT receptor(s) on cultured, identified neurones in Lymnaea stagnalis was examined. The identified neurones studied were from the buccal ganglia and the serotonin-containing cerebral giant cells (CGCs). 5-HT and its agonists were applied from puffer pipettes, whilst 5-HT antagonists were applied in the bathing medium. At 10⁻³ M, the 5-HT_1A agonist, always produced paroxysmal depolarizing shifts (PDS) while at a lower concentration (10⁻⁴ M), it always mimicked the effects of 10⁻³ M 5-HT. 8-OH-DPAT (10⁻⁴ M) and 5-HT 10⁻³ M produced dose-dependent increases in the responses they evoked. At 10⁻⁴ M, the 5-HT₃ receptor agonist 1-(m-chlorophenyl)-biguanide hydrochloride (m-CPBG), failed to hyperpolarize most of the neurones hyperpolarized by 5-HT. At 10⁻⁴ M, the antagonists ketanserin (5-HT₂), MDL 72222 (5-HT₃), and pindobind-5-HT_1A (5-HT_1A) consistently abolished spike generation ii spontaneously active neurones. Both ketanserin and MDL 72222 failed to block the actions of 8-OH-DPAT and only partially blocked those of 5-HT, but pindobind-5-HT_1A completely, but reversibly,blocked the 8-OH-DPAT effects while greatly reducing those of 5-HT. These results suggest that 5-HT_1A receptor subtypes might be involved in the hyperpolarizing responses of the CGCs and their follower motor neurones in the buccal ganglia of Lymnaea stagnalis to 5-HT. The presence of 5-HT_1A receptors on these neurones can be considered to correspond with those found in mammals because their pharmacological responses resemble those of mammalian 5-HT_1A receptors.     

Serotonin in pain and analgesia

The purpose of this article is to summarize recent findings on the role of serotonin in pain processing in the peripheral nervous system. Serotonin (5-hydroxtryptamine [5-HT]) is present in central and peripheral serotonergic neurons, it is released from platelets and mast cells after tissue injury, and it exerts algesic and analgesic effects depending on the site of action and the receptor subtype. After nerve injury, the 5-HT content in the lesioned nerve increases. 5-HT receptors of the 5-HT₃ and 5-HT_2A subtype are present on C-fibers. 5-HT, acting in combination with other inflammatory mediators, may ectopically excite and sensitize afferent nerve fibers, thus contributing to peripheral sensitization and hyperalgesia in inflammation and nerve injury.     

Prefrontal Serotonergic Denervation Induces Increase in the Density of 5-HT<Subscript>2A</Subscript> Receptors in Adult Rat Prefrontal Cortex

The 5-HTergic system and particularly 5-HT_2A receptors have been involved in prefrontal cognitive functions, but the underlying mechanisms by which the serotonin (5-HT) system modulates these processes are still unclear. In this work, the effects of prefrontal 5-HTergic denervation on the density and expression levels of 5-HT_2A receptors were evaluated by immunohistochemical and molecular biology studies in the prefrontal cortex (PFC). The [³H]-Ketanserin binding study revealed an increase in the B_max, along with no change in the binding affinity (K_D) for 5-HT_2A receptors. The increase in PFC of 5-HT_2A receptor density in response to denervation was accompanied by increase in 5-HT_2A receptor mRNA and protein levels. This increase in the number of 5-HT_2A receptors may be interpreted as an adaptive plastic change, i.e., hypersensitivity; resulting from the selective pharmacological lesion of the raphe-proceeding 5-HTergic fibers to the PFC. Based on previous evidence, this could be strongly related to the abnormal expression of short-term memory.     

Pharmacological characterization of the 5-hydroxytryptamine receptor coupled to adenylyl cyclase stimulation in human brain

Recently, a 5-hydroxytryptamine (5-HT) receptor has been described, whose pharmacology was distinct from that of the already known serotonergic receptors, so that it has been called 5-HT₄. Because the lack of a high affinity radioligand, the identification of this receptor depends entirely on functional pharmacological analysis. Its stimulation leads to an increase in cyclic AMP accumulation in mouse embryo colliculi neurons, in guinea pig hippocampus and in human heart. We studied the effect of two indoleamines, 5-HT and 5-methoxytryptamine (5-MeO-T), and a benzimidazolone derivative, BIMU 8, in stimulating basal adenylyl cyclase activity in human frontal cortex, and characterized the receptor subtype involved. In membranes prepared from this tissue, 5-HT, 5-MeO-T and BIMU 8 dose-dependently stimulated (13–25 %) the basal enzyme activity (220 pmoles cyclic AMP/min/mg protein). 5-MeO-T behaved as a full agonist, BIMU 8 elicited about 60 % of the maximal 5-HT effect. The selective 5-HT_1A agonist 8-OH-DPAT, was devoid of any stimulating activity. ICS 205–930, a low affinity 5-HT₄ receptor antagonist, completely reversed the effect of all three agonists at high concentrations. Therefore, the present data are consistent with the 5-HT-mediated stimulation of adenylyl cyclase in human frontal cortex resulting by the activation of a 5-HT₄ receptor subtype.     

Distribution of serotonin (5-HT) and its receptors in the insect brain with focus on the mushroom bodies: lessons from Drosophila melanogaster and Apis mellifera

The biogenic amine serotonin (5-hydroxytryptamine, 5-HT) plays a key role in regulating and modulating various physiological and behavioral processes in both protostomes and deuterostomes. The specific functions of serotonin are mediated by its binding to and subsequent activation of membrane receptors. The vast majority of these receptors belong to the superfamily of G-protein-coupled receptors. We report here the in vivo expression pattern of a recently characterized 5-HT₁ receptor of the honeybee Apis mellifera (Am5-HT_1A) in the mushroom bodies. In addition, we summarize current knowledge on the distribution of serotonin and serotonin receptor subtypes in the brain and specifically in the mushroom bodies of the fruit fly Drosophila melanogaster and the honeybee. Functional studies in these two species have shown that serotonergic signaling participates in various behaviors including aggression, sleep, circadian rhythms, responses to visual stimuli, and associative learning. The molecular, pharmacological, and functional properties of identified 5-HT receptor subtypes from A. mellifera and D. melanogaster will also be summarized in this review.     

Divergent actions of serotonin receptor activation during fictive swimming in frog embryos

We have investigated the pharmacology underlying locomotor system responses to serotonin (5-HT) in embryos of the frog, Rana temporaria, to provide a comparison to studies in embryos of its close relative, Xenopus laevis. Our findings suggest that two divergent mechanisms underlie the modulation of locomotion by 5-HT in Rana. Bath-applied 5-HT or 5-carboxamidotyptamine, a 5-HT_1,5A,7 receptor agonist, can modulate fictive swimming in a dose-dependent manner, increasing burst durations and cycle periods. However, activation of 5-HT_1,7 receptors with R8-OHDPAT or 8-OHDPAT fails to mimic 5-HT, and in some cases exerts exactly the opposite response; decreasing burst durations and cycle periods. Elevating endogenous 5-HT levels by blocking re-uptake with clomipramine transiently increases burst durations. The receptors involved in this endogenous response include 5-HT_1A receptors, as in Xenopus, but also 5-HT₇ receptors. However, like the 8-OHDPAT enantiomers, prolonged re-uptake inhibition can result in a motor response in the opposite direction to exogenous 5-HT. This effect is not reversed by 5-HT_1A and/or 5-HT₇ receptor antagonism, implicating 5-HT_1B/1D receptors. Remarkably, antagonism of these receptors using methiothepin unmasks a dose-dependent response to clomipramine, reminiscent of exogenous 5-HT. Our data suggest that 5-HT_1A,7 and 5-HT_1B/1D receptors act as gain-setters of burst durations, whilst 5-HT_5A receptors are involved in the effects of bath-applied 5-HT on locomotion.