Serotonin-elicited amplification of adenylate cyclase activity in hippocampal membranes from adult rat

Serotonergic synapses modulate generation of spikes from retinal ganglion cells of teleosts

Serotonin [5-hydroxytryptamine (5-HT)] is a common neurotransmitter/neuromodulator found widely in the nervous system. Cellular morphology and retinal distribution of serotonergic amacrine cells in the channel catfish (Ictalurus punctatus) retina are identified using monoclonal anti-5HT antibody. These cells receive ribbon synapses from OFF-center (hyperpolarizing) bipolar cells as well as conventional synapses with other non-serotonergic amacrine cells. Output synapses from the serotonergic cells are mainly channel onto retinal ganglion cells. Output synapses from the serotonergic cells occur as "the branched synapses" onto the ganglion cell dendrites at the dyads of the ribbon synaptic sites, and are made onto the ganglion cells, apart from the ribbon synapses. Application of serotonin receptor agonist: 5HT(1A) serotonin receptor agonist, (+)-8-hydroxy-dipropylaminotetralin [8-OH-DPAT; 1-10 muM] is also known to activate 5HT(7) serotonin receptor, coupled with activation of adenylate cyclase, generated continuous repetitive spikes from large retinal ganglion cells of the adult goldfish (Carassius auratus) in flat-mounted preparations, using amphotericin-B-perforated patch-clamp. Under control conditions of bleached retina with continuous light illumination, goldfish large retinal ganglion cells had generated only few spikes. This is the first observation of positive neuromodulation promoting retinal ganglion cell excitation in the retina. The results confirm previous reports of a serotonergic system in the mammalian retina. These results support the presence of developed postsynaptic serotonin receptors in cyprinid fish retina together with other physiological and anatomical studies, and suggest that the action of serotonin in the retina may be more important than previously believed.

Selective serotonin reuptake inhibitors: a review of its effects on intraocular pressure

The increase in serotonin (5-HT) neurotransmission is considered to be one of the most efficacious medical approach to depression and its related disorders. The selective serotonin reuptake inhibitors (SSRIs) represent the most widely antidepressive drugs utilized in the medical treatment of depressed patients. Currently available SSRIs include fluoxetine, sertraline, paroxetine, fluvoxamine, citalopram and escitalopram. The primary SSRIs pharmacological action's mechanism consists in the presynaptic inhibition on the serotonin reuptake, with an increased availability of this amine into the synaptic cleft. Serotonin produces its effects as a consequence of interactions with appropriate receptors. Seven distinct families of 5-HT receptors have been identified (5-HT(1) to 5-HT(7)), and subpopulations have been described for several of these. The interaction between serotonin and post-synaptic receptors mediates a wide range of functions. The SSRIs have a very favorable safety profile, although clinical signs of several unexpected pathologic events are often misdiagnosed, in particular, those regarding the eye. In all cases reported in the literature the angle-closure glaucoma represents the most important SSRIs-related ocular adverse event. Thus, it is not quite hazardous to hypothesize that also the other reported and unspecified visual disturbances could be attributed - at least in some cases - to IOP modifications. The knowledge of SSRIs individual tolerability, angle-closure predisposition and critical IOP could be important goals able to avoid further and more dangerous ocular side effects.

5-HT(1A) and 5-HT(7) receptor expression in the mammalian retina

Serotonin or 5-hydroxytryptamine (5-HT) is a common neurotransmitter found widely in the nervous system. Here, using RT-PCR, we have identified both the 5-HT(1A) and 5-HT(7) receptor transcripts in the rabbit retina. Furthermore, we found the same two receptors in the rat retina which was previously believed not to have a serotoninergic system. These results confirm previous reports of 5-HT(7) gene expression in retina and together with other biochemical, physiological and anatomical studies, they support the presence of multiple 5-HT receptors in the mammalian retina and suggest that the action of serotonin in the retina may be more complicated than previously believed.

Pindolol-insensitive [3H]-5-hydroxytryptamine binding in the rat hypothalamus; identity with 5-hydroxytryptamine7 receptors

Pindolol-insensitive [3H]-5-hydroxytryptamine ([3H]-5-HT) binding to rat hypothalamic membranes was pharmacologically and functionally characterized to resolve whether this procedure selectively labels 5-HT7 receptors. Consistent with a previous report, 3 microM and not 100 nM pindolol was required to occupy fully 5-HT1A and 5-HT1B receptors. Remaining [3H]-5-HT binding was saturable (KD, 1.59+/-0.21 nM; Bmax, 53.8+/-3.1 fmol x mg protein(-1)). Displacement of [3H]-5-HT with metergoline and 5-CT revealed shallow Hill slopes (<0.5) but seven other compounds had slopes >0.8 and pKi values and the rank order of affinity were significantly correlated (r = 0.81 and 0.93, respectively) with published [3H]-5-HT binding to rat recombinant 5-HT7 receptors. In the presence of pindolol, 5-HT-enhanced accumulation of [32P]-cyclic AMP was unaffected by the 5-HT4 antagonist RS39604 (0.1 microM) or the 5-ht6 antagonist Ro 04-6790 (1 microM) but significantly attenuated by mesulergine (250 nM), ritanserin (450 nM) or methiothepin (200 nM) which have high affinity for the 5-HT7 receptor. Intracerebroventricular pretreatment with the serotonergic neurotoxin 5,7-dihydroxytryptamine, 5,7-DHT, elevated the [3H]-5-HT Bmax 2 fold, indicating that the hypothalamic 5-HT7 receptor is post-synaptic to 5-HT nerve terminals and regulated by synaptic 5-HT levels. These results suggest that, in the presence of 3 microM pindolol, [3H]-5-HT selectively labels hypothalamic binding sites consistent with functional 5-HT7 receptors.

Sequence-dependent interactions between transient calcium and transmitter stimuli in activation of mammalian brain adenylyl cyclase

Recent evidence implicates Ca2+/CaM-sensitive adenylyl cyclase (AC) as a molecular coincidence detector for temporally paired stimuli during associative learning. During conditioning in Aplysia, AC is optimally activated when Ca2+ influx, the cellular signal for the conditioned stimulus (CS), precedes binding of modulatory transmitter, the cellular signal for the unconditioned stimulus (US). This sequence preference of the AC for Ca2+-before-transmitter, parallels the CS-preceding-US pairing requirement of classical conditioning. In this study, we have examined the response of AC from rat cerebellum to brief exposures to Ca2+ and to transmitter in a perfused membrane assay. We observed modest synergism between Ca2+ and transmitter in activating AC. Activation was more effective when a Ca2+ stimulus immediately preceded a transmitter stimulus than when the two stimuli were delivered in the reverse order. Thus, rat cerebellar AC displayed a sequence preference for optimal activation by paired stimuli similar to that observed in Aplysia; this sequence dependence could contribute to the CS-US sequence requirement observed in most mammalian classical conditioning paradigms.

Serotonin modulates voltage-dependent calcium current in Necturus taste cells

Necturus taste buds contain two primary cell types: taste receptor cells and basal cells. Merkel-like basal cells are a subset of basal cells that form chemical synapses with taste receptor cells and with innervating nerve fibers. Although Merkel-like basal cells cannot interact directly with taste stimuli, recent studies have shown that Merkel-like basal cells contain serotonin (5-HT), which may be released onto taste receptor cells in response to taste stimulation. With the use of whole cell voltage clamp, we examined whether focal applications of 5-HT to isolated taste receptor cells affected voltage-activated calcium current (I(Ca)). Two different effects were observed. 5-HT at 100 microM increased I(Ca) in 33% of taste receptor cells, whereas it decreased I(Ca) in 67%. Both responses used a 5-HT receptor subtype with a pharmacological profile similar to that of the 5-HT1A receptor, but the potentiation and inhibition of I(Ca) by 5-HT were mediated by two different second-messenger cascades. The results indicate that functional subtypes of taste receptor cells, earlier defined only by their sensitivity to taste stimuli, may also be defined by their response to the neurotransmitter 5-HT and suggest that 5-HT released by Merkel-like basal cells could modulate taste receptor function.

Circadian rhythm in the response of central 5-HT1A receptors to 8-OH-DPAT in rats

Circadian rhythm in the behavioral responsiveness to the selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was studied in rats. Rats were subcutaneously injected with 8-OH-DPAT at one of the following times of day: 0000, 0400, 0800, 1200, 1600, 2000 hours. The post-synaptic 5-HT1A receptor behavioral syndrome, that is, forepaw treading, head weaving, and flat body posture, were measured after the administration of 8-OH-DPAT. Circadian rhythms were found in each of the behavioral responses to 8-OH-DPAT. Peak responses were observed in the mid-dark phase (1200 hours) while the weakest responses were observed in the mid-light phase (0000 hours). In a subsequent experiment, 8-OH-DPAT was administered intracerebroventricularly during the mid-dark phase and the mid-light phase. The behavioral responses to the drug in the mid-dark phase were significantly higher than those in the mid-light phase. These results suggest that the function of central postsynaptic 5-HT1A receptor exhibits circadian rhythm.

Effects of subchronic pretreatment with D-fenfluramine or p-chloroamphetamine on [3H]inositolmonophosphate accumulation in rat cortical miniprisms

Phosphatidylinositol (PI) breakdown in rat cerebral cortex is stimulated by serotonin (5-HT), acting via 5-HT2 and possibly 5-HT3 receptors and by acetylcholine or carbachol, acting via muscarinic M1 and M3 receptors. Serotoninergic neurons have been described as tonically inhibiting cortical acetylcholine release. We studied the effects of subchronic pretreatment with high doses of D-fenfluramine (10 mg/kg, i.p., daily for 4 days), which releases 5-HT and blocks its reuptake, on 5-HT-and carbachol-stimulated PI breakdown, as measured by [3H]inositolmonophosphate ([3H]IP1) accumulation in cortical miniprisms. This pretreatment decreased 5-HT-stimulated [3H]IP1 accumulation, suggesting that a prolonged increase of 5-HT in the synaptic cleft reduces the activity of the transducing system used by postsynaptic 5-HT receptors. Carbachol-stimulated PI breakdown was unaltered by pretreatment with D-fenfluramine. Pretreatment with a single dose of p-chloroamphetamine (5 mg/kg), a serotoninergic neurotoxin, which depleted cortical 5-HT by 85%, did not change [3H]IP1 accumulation after stimulation by 5-HT or by the muscarinic agonist carbachol. Subchronic pretreatment, which depleted cortical 5-HT by 90%, decreased both 5-HT- and carbachol-stimulated [3H]IP1 accumulation. The mechanism by which p-chloroamphetamine, but not D-fenfluramine, diminishes the PI response to carbachol might involve impairment of the tonic serotoninergic inhibition of acetylcholine release.

Characterization of galanin and 5-HT1A receptor coupling to adenylyl cyclase in discrete regions of the rat brain

We have studied the coupling of galanin and 5-HT1A receptors with adenylyl cyclase in the hypothalamus, the entorhinal cortex and the hippocampus of the rat brain. Furthermore, we have evaluated the effects of simultaneous activation of galanin and 5-HT1A receptors on adenylyl cyclase activity. Galanin-(1-29) and galanin-(1-15) showed a dose-dependent inhibitory effect on forskolin-stimulated adenylyl cyclase activity in the hypothalamus and entorhinal cortex. No clear effects were observed in the hippocampus. Neither galanin-(1-29) nor galanin-(1-15) had any effect on the basal activity of adenylyl cyclase in these regions. The selective 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)-tetralin (8-OH-DPAT) induced a dose-dependent inhibition of forskolin stimulated adenylyl cyclase activity in the hippocampus and the entorhinal cortex. 5-HT induced an inhibition in the hypothalamus. In all regions the effects could be fully counteracted by methiothepin. 5-HT was shown to stimulate the basal activity of adenylyl cyclase in the hippocampus and the entorhinal cortex. The effects could be counteracted by methiothepin. When galanin-(1-29) and 5-HT/8-OH-DPAT were incubated simultaneously additive inhibitory effects, but no synergistic interactions, could be observed on the stimulated adenylyl cyclase activity. In conclusion, galanin and 5-HT1A receptors seem to be linked to different independent pools of G proteins, indicating that the previously demonstrated intramembrane interactions between galanin and 5-HT1A receptors involve a mechanism not directly related to adenylyl cyclase.

PKA mediates the effects of monoamine transmitters on the K+ current underlying the slow spike frequency adaptation in hippocampal neurons

The Ca(2+)-activated K+ current IAHP, which underlies spike frequency adaptation in cortical pyramidal cells, can be modulated by multiple transmitters and probably contributes to state control of the forebrain by ascending monoaminergic fibers. Here, we show that the modulation of this current by norepinephrine, serotonin, and histamine is mediated by protein kinase A in hippocampal CA1 neurons. Two specific protein kinase A inhibitors, Rp-cAMPS and Walsh peptide, suppressed the effects of these transmitters on IAHP and spike frequency adaptation. The effects of the cyclic AMP analog 8CPT-cAMP were also inhibited, whereas muscarinic and metabotropic glutamate receptor agonists had full effect. Intracellular application of protein kinase A catalytic subunit or a phosphatase inhibitor mimicked the effects of monoamines or 8CPT-cAMP. These results demonstrate that monoaminergic modulation of neuronal excitability in the mammalian CNS is mediated by protein phosphorylation.

Serotonin and the mammalian circadian system: I. In vitro phase shifts by serotonergic agonists and antagonists

The primary mammalian circadian clock, located in the suprachiasmatic nuclei (SCN), receives a major input from the raphe nuclei. The role of this input is largely unknown, and is the focus of this research. The SCN clock survives in vitro, where it produces a 24-hr rhythm in spontaneous neuronal activity that is sustained for at least three cycles. The sensitivity of the SCN clock to drugs can therefore be tested in vitro by determining whether various compounds alter the phase of this rhythm. We have previously shown that the nonspecific serotonin (5-HT) agonist quipazine resets the SCN clock in vitro, inducing phase advances in the daytime and phase delays at night. These results suggest that the 5-HT-ergic input from the raphe nuclei can modulate the phase of the SCN circadian clock. In this study we began by using autoradiography to determine that the SCN contain abundant 5-HT1A and 5-HT1B receptors, very few 5-HT1C and 5-HT2 receptors, and no 5-HT3 receptors. Next we investigated the ability of 5-HT-ergic agonists and antagonists to reset the clock in vitro, in order to determine what type or types of 5-HT receptor(s) are functionally linked to the SCN clock. We began by providing further evidence of 5-HT-ergic effects in the SCN. We found that 5-HT mimicked the effects of quipazine, whereas the nonspecific 5-HT antagonist metergoline blocked these effects, in both the day and night. Next we found that the 5-HT1A agonist 8-OH-DPAT, and to a lesser extent the 5-HT1A-1B agonist RU 24969, mimicked the effects of quipazine during the subjective daytime, whereas the 5-HT1A antagonist NAN-190 blocked quipazine's effects. None of the other specific agonists or antagonists we tried induced similar effects. This suggests that quipazine acts on 5-HT1A receptors in the daytime to advance the SCN clock. None of the specific agents we tried were able either to mimic or to block the actions of 5-HT or quipazine at circadian time 15. Thus, we were unable to determine the type of 5-HT receptor involved in nighttime phase delays by quipazine or 5-HT. However, since the dose-response curves for quipazine during the day and night are virtually identical, we hypothesize that the nighttime 5-HT receptor is a 5-HT1-like receptor.

5-HT-1A receptor-mediated effects of antidepressants

1. Antidepressant (AD) drugs in general induce subsensitivity of behavioural functions associated with activation of 5-HT-1a receptors in animals. 2. Electrophysiological studies in animals in general indicate increased serotonergic transmission after AD administration, mediated partly by increased functioning of post-synaptic 5-HT-1a receptors in the hippocampus. 3. Binding studies have in general shown no change in 5-HT-1a receptor number either pre-or post-synaptically, while results of second messenger studies (inhibition of adenylate cyclase) indicate subsensitivity after AD administration. 4. Human studies also indicate subsensitivity of 5-HT-1a receptors after ADs.

A cyclic AMP mechanism mediates the serotonin-induced increase in glutamic acid decarboxylase activity in the preoptic area and hypothalamus

Previous studies have shown that the injection of 5-hydroxytryptamine (5-HT) into the third ventricle of rats on the afternoon of proestrus increases glutamic acid decarboxylase (GAD) activity in the preoptic area and the hypothalamus. In the present report we examine the adenylate cyclase-cyclic AMP (cAMP) system as mediator of that effect. The increase in GAD activity induced by intraventricular injection of 5-HT was completely blocked by injecting an antiserum against cAMP into the third ventricle 30 min earlier, whereas an injection of serum from normal rabbits was ineffective. On the contrary, activation of adenylate cyclase activity by intraventricular injection of forskolin increased GAD activity, an effect that was also blocked by anti-cAMP serum. Anti-cAMP serum also lowered GAD activity in the preoptic area and hypothalamus when injected on the morning of proestrus but not when injected in the afternoon, when the values of GAD activity were already low. The results suggest that a cAMP mechanism may be involved in the changes in preoptic-area and hypothalamic GAD activity such as the rise in enzyme activity induced by intraventricular injection of 5-HT.

Serotonergic regulation of type II corticosteroid receptor binding in hippocampal cell cultures: evidence for the importance of serotonin-induced changes in cAMP levels

The development of the hypothalamic-pituitary-adrenal response to stress is profoundly altered by environmental events. One target for environmental regulation within the hypothalamic-pituitary-adrenal axis is the hippocampal type II corticosteroid (or glucocorticoid) receptor system, which mediates the negative-feedback effects of glucocorticoids on hypothalamic-pituitary-adrenal activity. Thus, adult rats handled early in life show increased hippocampal type II corticosteroid receptor density and increased sensitivity to the inhibitory effects of circulating glucocorticoids on post-stress hypothalamic-pituitary-adrenal activity. Both effects persist throughout life. The effects of handling on type II corticosteroid receptor development are, at least in part, mediated by changes in hippocampal 5-hydroxytryptamine turnover. Moreover, 5-hydroxytryptamine can regulate type II corticosteroid receptor density in cultured hippocampal cells, providing a paradigm for examining the neurochemical mechanisms by which environmental stimuli might regulate neural differentiation. In the present studies, we examined the intracellular mechanisms underlying the effects of 5-hydroxytryptamine on type II corticosteroid receptors ([3H]RU 28362 binding) in hippocampal cell cultures. cAMP, but not cGMP, levels in cultured hippocampal cells were significantly increased by the addition of 5-hydroxytryptamine to the medium. The cAMP response to 5-hydroxytryptamine was biphasic: an initial increase in cAMP levels occurred in response to nanomolar 5-hydroxytryptamine concentrations (EC50 = 7.2 nM), while a second increase was apparent at low micromolar concentrations. 5-Hydroxytryptamine also increased [3H]RU 28362 binding (EC50 = 4.5 nM) with a maximal effect at a concentration of 10 nM. There was no further increase in [3H]RU 28362 binding even with higher, micromolar concentrations of 5-hydroxytryptamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the 5-HT receptor subtypes involved in the motor behaviours produced by intrathecal administration of 5-HT agonists in rats

1. The motor behavioural effects of intrathecal injections of 5-hydroxytryptamine (5-HT) and a variety of 5-HT receptor agonists were examined in adult Wistar rats to establish; (a) which 5-HT receptor subtype/s elicit each behaviour and (b) whether these receptors are located within the spinal cord. 2. Intrathecal injection of 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) or 2,5-dimethoxy-alpha,4-dimethylbenzene ethamine hydrochloride (DOM) produced dose-related back muscle contractions (BMC) and wet dog shakes (WDS) which were both markedly attenuated by intraperitoneal pretreatment with either ritanserin (1 mg kg-1), ketanserin (0.16 mg kg-1) or mianserin (0.6 mg kg-1) indicating the involvement of 5-HT2 receptors in both these motor behaviours. Both fluoxetine (1-20 mg kg-1, i.p.) and high doses of 5-HT (50 micrograms) following fluoxetine (5 mg kg-1, i.p.) also elicited BMC, further confirming the involvement of 5-HT in this behaviour. 3. Intrathecal 5-carboxamidotryptamine (5-CT) evoked a marked wet-dog shake response without producing any BMC. Intrathecal pretreatment with 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) enhanced, while in contrast 2-methyl-5-HT pretreatment attenuated, 5-HT agonist-induced BMC without affecting WDS. These data suggest that the spinal 5-HT2 receptors mediating BMC are positively modulated by 5-HT1A but negatively influenced by 5-HT3 receptor activation and may be of a different subtype to the supra-spinal 5-HT2 receptors which elicit WDS. 4. A contrast, reciprocal forepaw treading, lateral head weaving, flat body posture and Straub-tail were evoked by 5-MeODMT, 8-OH-DPAT or 5-CT but not by DOI or DOM indicating that these behaviours were not produced by 5-HT2 receptor activation alone. Ritanserin (1 mg kg- 1, i.p.) or ketanserin (0.16mgkg-1, i.p.) pretreatment reduced the reciprocal forepaw treading induced by high intrathecal doses of either 5-MeODMT (25.pg) or 5-CT (50,ug) suggesting that this behaviour may be facilitated by 5-HT2 receptor activation. 5. Intrathecal injection of 5-HT (0.05-50pg, after systemic fluoxetine, 5mg kg 1, i.p.), or 1-(3-chlorophenyl) piperazine (mCPP) produced dose-related forepaw-licking and grooming, neither of which were attenuated by ketanserin (0.16 mgkg-1, i.p.) pretreatment suggesting these behaviours may be mediated by 5-HT1c receptors. In contrast, 2-methyl-5-HT (50 and 100pg) produced sideward tail-flicks, not evoked by any other 5-HT agonist and could therefore be mediated by spinal 5-HT3 receptor activation. 6. These data provide behavioural evidence for the existence of spinal 5-HT2 receptors which produce a novel motor behaviour, BMC. Ligand binding studies and dose-response studies with a range of selective 5-HT antagonists are required to establish whether BMC and WDS are mediated by different subtypes of 5-HT2 receptors.

Postsynaptic 5-HT1 receptors and offensive aggression in rats: a combined behavioural and autoradiographic study with eltoprazine

The present study was designed to assess whether the antiaggressive effects of eltoprazine are mediated via presynaptic and/or postsynaptic 5-HT1 receptors. We describe the effects of central 5-HT depletion 1) on the behaviour of resident TMD-S3 rats in a territorial situation, 2) on the efficacy of eltoprazine to inhibit offensive aggression, and 3) on the 5-HT1A, 5-HT1B and 5-HT1C receptor binding in brains of rats previously used in behavioural studies. Male resident rats were given combined 5,7-dihydroxytryptamine (5,7-DHT) injections into the dorsal and median raphe nuclei. Two to four weeks after the lesions, rats were confronted with an intruder Wiser rat in their home cage for a 10-min period. The 5,7-DHT treatment resulted in a modest reduction of offensive behaviour, while having no effects on other social and nonsocial behaviours. Oral administration of eltoprazine (1 mg/kg) specifically reduced offensive aggression in both sham- and 5,7-DHT-lesioned animals, leaving social interest and exploration intact or even increasing it. A low dose (0.3 mg/kg) of eltoprazine did not affect the behavioural repertoire of sham-operated rats, whereas this dose significantly reduced offense behaviours in the 5,7-DHT-lesioned residents. Quantitative autoradiographic studies 5 weeks after 5,7-DHT treatment revealed a significant increase in radioligand binding to 5-HT1A, 5-HT1B and 5-HT1C sites in many brain regions studied, except for the raphe nuclei where [3H]8-OH-DPAT binding to 5-HT1A sites was markedly reduced. The concentrations of 5-HT and 5-HIAA in frontal cortex were reduced to approximately 10% of controls. The results indicate that serotonin has a stimulatory rather than an inhibitory influence on offensive aggressive behaviour. Central 5-HT depletion does not prevent the antiaggressive effects of eltoprazine, indicating a role for postsynaptic 5-HT1 receptors in the modulation of offensive aggression. The 5,7-DHT-induced overall upregulation of 5-HT1A, 5-HT1B and 5-HT1C binding sites suggests that these three receptor subtypes receive a tonic serotonergic influence. It is conceivable that this postsynaptic 5-HT1 receptor supersensitivity is reflected by the increased efficacy of eltoprazine to inhibit offensive aggression.

Effect of subchronic antidepressants administration on serotonin-stimulated phosphoinositide hydrolysis in para-chlorophenylalanine-treated rat hippocampal slices

1. This study examines the effect of subchronic parachlorophenylalanine (PCPA) treatment upon serotonin (5-HT)-stimulated inositol monophosphate (IP-1) accumulation in rat hippocampal slices and also the effect of antidepressants upon this 5-HT response in the hippocampus from rats treated with or without concurrent administration of PCPA. 2. For high dose PCPA treatment, animals were injected intraperitoneally with 300 mg/kg daily for the first 5 days and then 100 mg/kg for 5 days, while for low dose PCPA treatment animals were injected for 10 days at a dose of 100 mg/kg. Imipramine or iprindole (15 mg/kg i.p.) was given once daily for 10 consecutive days. 3. 10-Day treatment with high dose of PCPA resulted in a significant increase in 5-HT-stimulated IP-1 accumulation, whereas low dose of PCPA had no significant effect upon the 5-HT response as compared to vehicle. 5-HT-stimulated IP-1 accumulation in rat hippocampus was not affected by subchronic treatment with imipramine or iprindole. The enhancement of the 5-HT response induced by high dose of PCPA was not attenuated by repeated antidepressants treatment.

The long-term treatment with the Ca(2+)-antagonists nifedipine, verapamil, flunarizine and with the calmodulin antagonist trifluoperazine decreases the activity of 5-HT1 receptors in rat cerebral cortex and hippocampus

1. The binding activity of 5-HT1 receptors was studied in membrane fractions from the cerebral cortex and hippocampus of male Wistar rats treated orally for 13 days with the Ca(2+)-antagonists nifedipine (20 mg/kg), verapamil (50 mg/kg) and flunarizine (10 mg/kg) and with the calmodulin antagonist trifluoperazine (3 mg/kg). 2. The binding capacity and affinity of the 5-HT1 receptors in the cerebral cortex were significantly decreased after the treatment with the Ca(2+)-antagonists nifedipine, verapamil and flunarizine. The dissociation constant (Kd) was increased after the treatment with the calmodulin antagonist trifluoperazine. 3. In the hippocampus the 5-HT1 receptor affinity and number of binding sites were significantly reduced after the treatment with all four antagonists tested--nifedipine, verapamil, flunarizine and trifluoperazine, the Kd value being increased insignificantly after the flunarizine treatment. 4. The results obtained afford the suggestion that the reduction of 5-HT1 receptor activity is at least one of the results of the well known Ca(2+)-ions mediated automodulation of 5-HT release. The data confirm the view about the great importance of Ca(2+)-ions for the regulation of membrane neurotransmitter receptor activities.

Subchronic administration of para-chlorophenylalanine enhances serotonin-stimulated phosphoinositide hydrolysis in rat hippocampal slices

Serotonin (5-HT) caused a dose-dependent accumulation of inositol monophosphate (IP-1) in rat hippocampal slices (maximal effect + 172%, EC50 = 630 nM) in the presence of LiCl. The 5-HT response was blocked potently by a non-selective 5-HT receptor antagonist metergoline and a 5-HT1C/5-HT2 receptor antagonist ritanserin. The 5-HT2 receptor antagonist ketanserin and spiperone were, however, less potent at inhibiting the 5-HT response. m-Chlorophenylpiperazine (mCPP), a 5-HT1C receptor agonist but a 5-HT2 antagonist stimulated directly PI turnover, yet mCPP inhibited 5-HT-stimulated PI response. These findings indicate that both 5-HT1C and 5-HT2 receptors are involved with a complicated interaction of each receptor in 5-HT-stimulated PI hydrolysis in rat hippocampus. 10-Day treatment with para-chlorophenyl-alanine (PCPA), a 5-HT synthesis inhibitor, resulted in a significant increase (maximal effect + 225%, EC50 = 580 nM) in 5-HT-stimulated IP-1 accumulation with no substantial change in EC50 value, which suggest that subchronic treatment with PCPA enhances the 5-HT-mediated PI hydrolysis in rat hippocampus.

Stimulation and inhibition of adenylyl cyclase by distinct 5-hydroxytryptamine receptors

5-Hydroxytryptamine (serotonin, 5-HT) stimulates basal adenylyl cyclase activity in membranes from guinea pig or rat hippocampi, but 5-HT inhibits forskolin-stimulated adenylyl cyclase activity in these same membranes. The opposing effects of 5-HT on adenylyl cyclase activity indicate that distinct 5-HT receptors, positively and negatively coupled to adenylyl cyclase, are present in these membranes. Stimulation of adenylyl cyclase activity is mediated by two distinct 5-HT receptors. The receptor with lower affinity for 5-HT, designated as RL, is apparently homologous with a 5-HT receptor present in rat collicular membranes, but it is not homologous with the stimulatory receptor characterized in neuroblastoma hybrid cell (NCB-20) membranes. The receptor with higher affinity for 5-HT is homologous with the 5-HT1A binding site. The magnitude of stimulation by 5-HT1A receptors is variable with respect to stimulation by RL and is sometimes completely absent. Inhibition of forskolin-stimulated adenylyl cyclase activity, in membranes from either rat or guinea pig hippocampus or rat cortex, is a functional correlate of the 5-HT1A binding site. This inhibitory response was used to determine the pharmacological characteristics of drugs that reportedly have high affinity for 5-HT1A binding sites, such as 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)piperazine (PAPP) and (-)pindolol. PAPP inhibited adenylyl cyclase activity in guinea pig hippocampal membranes with an EC50 value of 27 +/- 3 nM. (-)Pindolol was a partial agonist in inhibiting adenylyl cyclase activity in guinea pig and rat hippocampal membranes. Because of the low intrinsic activity of (-)pindolol, it was tested as an antagonist of the inhibition produced by 5-HT1A receptor agonists in rat hippocampal membranes. The Kb of (-)pindolol was 40 nM as measured by a Schild plot. (-)Propranolol was a simple competitive antagonist at the rat hippocampal receptor with a Kb value of 550 nM. In summary, guinea pig and rat hippocampal membranes possess two distinct populations of 5-HT receptors, a 5-HT receptor that mediates inhibition of adenylyl cyclase activity and is pharmacologically homologous with the 5-HT1A binding site, and a stimulatory receptor that appears to be homologous with the 5-HT receptor first characterized in infant rat collicular membranes.

5-Hydroxytryptamine releases adenosine and cyclic AMP from primary afferent nerve terminals in the spinal cord in vivo

5-Hydroxytryptamine (5-HT) releases a purine nucleotide, which is subsequently converted to adenosine, from primary afferent nerve terminals in vitro. This release may mediate spinal antinociception by 5-HT. In the present study, we have investigated whether release also occurs from the spinal cord in vivo using an intrathecal perfusion system in rats. Adenosine was quantitated using high performance liquid chromatography (HPLC) with fluorescence detection. Following perfusion of the spinal cord with 50 and 500 microM 5-HT, a 35-50% increase in the release of endogenous adenosine was observed. This release was completely blocked by 50 microM methysergide, and by intrathecal injection with 100 micrograms capsaicin 5-8 days prior to release experiments. Intrathecal perfusion with 50 and 500 microM 5-HT also released a nucleotide which eluted from the HPLC column at a retention time identical to that of cyclic AMP standards, and was reduced following incubation with cyclic AMP phosphodiesterase. This release of cyclic AMP also was eliminated following intrathecal pretreatment with capsaicin. In contrast to 5-HT, noradrenaline (NA, 500 microM and 5 mM) did not release adenosine or cyclic AMP from the intact spinal cord. These data demonstrate that release of nucleotide, probably cyclic AMP, and subsequent metabolism to adenosine, can be induced by 5-HT but not NA in vivo. This strengthens the hypothesis that release of adenosine from the spinal cord may mediate antinociception by intrathecal 5-HT but not NA.

5-Hydroxytryptamine and the metabolism of arachidonic acid by the lipoxygenase and cyclooxygenase of washed human platelets

During secondary aggregation, platelets release 5-hydroxytryptamine (5-HT) from their dense granule stores concurrent with arachidonic acid (AA) metabolism. To examine the hypothesis that released 5-HT has a modulatory effect on the metabolism of AA by platelets, we incubated nonaggregating washed human platelets with 5-HT in the presence of [3H]AA. Stimulation with 10(-4) M 5-HT, followed by incubation with 3 microM AA and 1 microCi [3H]AA for 5 min, resulted in a decrease in the formation of thromboxane B2 (TxB2) and 12-hydroxyheptadecatrienoic acid (HHT, P less than 0.05). The same treatment conditions and stimulation with 10(-7) to 10(-4) M 5-HT resulted in an elevation of 12-hydroxyeicosatetraenoic acid (12-HETE) formation (P less than 0.05). Treatment with the monoamine uptake inhibitor imipramine (20 microM) further increased the stimulation of 12-HETE formation observed in the presence of 10(-4) M 5-HT, suggesting that 5-HT may act at the platelet surface. A 5-HT1A receptor agonist, 8-hydroxy-dipropylaminotetralin (DPAT, 10(-6) to 10(-4) M) stimulated the formation of platelet cyclooxygenase (CO) products, whereas (+/-)1-(2,5-dimethoxy-4-iodo phenyl)-amino propane hydrochloride (DOI, 10(-6) to 10(-4) M), a 5-HT2 receptor agonist, had no significant effect on CO product formation. In addition, the 5-HT2 receptor antagonist ketanserin (10(-7) M) did not block the changes in CO or lipoxygenase metabolism induced by 5-HT. Since both DOI and DPAT stimulated 12-HETE formation whereas ketanserin was unable to reverse the 5-HT-enhanced 12-HETE formation, it seems unlikely that the stimulation of a 5-HT2 receptor is responsible for this action of 5-HT on platelets. We conclude that 5-HT depresses CO product formation while increasing 12-HETE formation through interaction with a platelet serotonergic binding site other than the 5-HT2 receptor.

Neuron-glia interactions: effect of serotonin on the astroglial expression of GFAP and of its encoding message

The trophic effect of serotonin on the glial fibrillary acidic protein (GFAP) expression was investigated in rat brainstem astrocytes in primary culture. GFAP immunolabelling decreased and gliofilaments appeared localized in the cytoplasm periphery. GFAP protein level decreased in parallel with a decrease in its encoding message. Serotonin may act as an inhibitor of GFAP expression either on the transcription or on the stability of the GFAP-mRNA.

Characterization of two distinct 5-HT receptors coupled to adenylate cyclase activation and ion current generation in NCB-20 cells

The level of cyclic AMP in NCB-20 cells was increased by serotonin (5-HT), 5-methoxytryptamine and 2-methyl-5-HT with EC50 of 0.5 +/- 0.1, 1.0 +/- 0.1, 10 +/- 0.1 microM, respectively. The 5-HT-mediated increase of cyclic AMP content was completely blocked by metergoline but unaffected by 5-HT3 antagonists, ICS 205-930, MDL 72222, quipazine and 5-HT2 antagonist, ketanserin. Putative 5-HT1A agonists (8-OH-DPAT, ipsapirone, and buspirone) and 5-HT1B agonists (TFMPP and m-CPP) affected neither basal nor forskolin-dependent cyclic AMP accumulation. Receptor binding studies suggest that NCB-20 cells are devoid of 5-HT1A and 5-HT1B receptor sites. Application of 5-HT onto NCB-20 cells resulted in membrane depolarization by an evoked inward current which displayed rapid desensitization. 5-HT-mediated current had a reversal potential around 0 mV and was potently and reversibly inhibited by ICS 205-930. Our data suggest that in NCB-20 cells the 5-HT3 receptor is involved in the generation of inward currents, while the 5-HT receptor coupled to adenylate cyclase does not seem to correspond to any of the known receptor subtypes.

Regional central serotonin-2 receptor binding and phosphoinositide turnover in rats with 5,7-dihydroxytryptamine lesions

"Denervation supersensitivity" of serotonin (5-HT) receptors has been proposed to explain the behavioral supersensitivity to 5-hydroxytryptophan (5-HTP) which develops after lesions of indoleamine neurons with 5,7-dihydroxytryptamine (5,7-DHT). To examine the possible role of receptor recognition sites and second messenger activity in supersensitivity, we measured regional 5-HT2 receptor ligand binding and 5-HT-stimulated phosphoinositide turnover in adult rats with 5,7-DHT lesions made by intracisternal injection and their saline-treated controls. In [3H]ketanserin binding studies of fresh brain tissue two weeks after 5,7-DHT injection, there were no significant changes in frontal cortex, brainstem, or spinal cord in Bmax, Kd, or nH of 5-HT2 receptors, 5,7-DHT lesions did not affect basal levels of [3H]inositol phosphate (IP) accumulation but significantly increased 5-HT-stimulated [3H]IP accumulation in the brainstem (+27%) and cortex (+23%). Because brainstem rather than cortex is involved in 5-HTP-evoked myoclonus, increased 5-HT-stimulated phosphoinositide hydrolysis in brainstem following 5,7-DHT lesions in the rat may be relevant to serotonergic behavioral supersensitivity.

Changes in 5-HT1 receptors in different brain structures of rats with isolation syndrome

1. Prolonged (3 month) individual housing of Wistar rats produced aggressive muricidal behavior in 28% of the animals. 2. Binding studies with [3H]5-HT showed that the affinity (Kd) of 5-HT1 receptors in the frontal cortex, striatum, hippocampus and hypothalamus of isolated aggressive rats was significantly decreased as compared to that in grouped rats. 3. The affinity of 5-HT1 receptors was also significantly decreased in the striatum, hippocampus and hypothalamus of isolated nonaggressive rats. 4. The number (Bmax) of 5-HT1 receptors was significantly decreased in the hippocampus and hypothalamus of isolated aggressive rats and in isolated nonaggressive rats it was decreased only in the hypothalamus. 5. The data suggest that aggressive muricidal behavior in rats with syndrome of social isolation is connected with the decreased activity of brain 5-HT1 receptors.

Evidence for the presence of serotonin receptors negatively coupled to adenylate cyclase in the rabbit iris-ciliary body

Serotonin has no obvious effect on basal cyclic AMP levels but reduces the forskolin-, isoproterenol-, and vasoactive intestinal peptide-induced stimulation of cyclic AMP levels in a dose-dependent manner. Serotonergic, cholinergic, muscarinic, alpha-adrenergic, and dopaminergic antagonists have no effect on the serotonin response. Topical application of a serotonin/pargyline solution to the living eye causes desensitisation of the serotonin response in the iris-ciliary body, an observation confirming the presence of specific serotonergic receptors linked to adenylate cyclase. The 5-HT1A [5-hydroxytryptamine (serotonin) type 1A] receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin and buspirone mimic the serotonin response in reducing the forskolin-stimulated cyclic AMP levels, as do the indole derivatives 5-methoxytryptamine, 5-hydroxtryptophan, and tryptamine. However, the ineffectiveness of the 5-HT1A agonist ipsapirone and the inability of spiroxatrine to block the serotonin response show that classical 5-HT1A receptors are not involved. The serotonin response is blocked by pertussis toxin and is insensitive to the phosphodiesterase inhibitor theophylline, which indicates the involvement of an inhibitory guanine regulatory protein in the coupling of the serotonin receptor to the adenylate cyclase catalytic unit.

Further characterization of the 5-HT receptor mediating vascular relaxation and elevation of cyclic AMP in porcine isolated vena cava

1. 5-Hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) produce both smooth muscle relaxation and elevation of tissue adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels in isolated rings of neonatal porcine vena cava. We now present studies attempting to characterize in more detail the 5-HT receptor mediating these responses. 2. Both 5-HT and 5-CT relaxed porcine isolated vena cava rings (EC50 values 200 nM and 4 nM respectively) and elevated tissue cyclic AMP levels (EC50 values 1500 nM and 16 nM respectively). For both responses 5-CT was approximately 50-100 fold more potent than 5-HT. 3. Both 5-CT-induced smooth muscle relaxation and cyclic AMP elevation were potently and specifically antagonized to a similar extent by methiothepin, methysergide and spiperone. 4. At concentrations up to 1 microM, 8-hydroxy-2-(di-n-propylamino) tetralin, buspirone, ipsapirone, n,n-dipropyl-5-CT, cyanopindolol, RU24969, ketanserin, GR38032 and GR43175 were devoid of both agonist and antagonist activity for both responses. 5. These findings suggest that the same 5-HT1-like receptor mediates both smooth muscle relaxation and elevation of cyclic AMP. This receptor is unlike any known 5-HT1 ligand binding site or adenylate cyclase-coupled 5-HT receptor in brain tissues.

Electrophysiological and biochemical characterization of the development of alpha 1-adrenergic and 5-HT1 receptors associated with dorsal raphe neurons

Electrophysiological and quantitative autoradiographic receptor binding approaches were used to characterize the postnatal development of alpha 1-adrenergic and serotonergic receptors in the midbrain dorsal raphe nucleus (DRN) in rats. Dose-response data were obtained from extracellular records of the firing of DRN neurons recorded in midbrain slices perfused with artificial cerebral spinal fluid containing varying concentrations of the drugs investigated. These data indicated that DRN neurons, at all postnatal ages, are equally sensitive to the excitatory effects of phenylephrine (an alpha 1-norepinephrine agonist) and the inhibitory effects of serotonin (5-HT). In contrast, LSD (a 5-HT agonist) was considerably more potent in neonates as compared to adults. With one exception, the autoradiographic binding data was found to be in agreement with the electrophysiological findings. There were no observed changes in the number of alpha 1-adrenergic ([3H]prazosin) binding sites at different ages suggesting that cells in this region exhibit functionally mature alpha 1-adrenergic responses at birth. Similarly, the number of [3H]LSD binding sites was found to increase in older animals consistent with the increased potency of LSD in older animals. Paradoxically, the number of [3H]5-HT sites was observed to increase with age, a finding inconsistent with the functional data obtained using 5-HT in the slice. Hypotheses for the observed discrepancy are examined.

Post-receptor-mediated increases in adenylate cyclase activity after chronic antidepressant treatment: relationship to receptor desensitization

Administration to rats of chronic electroconvulsive shock (ECS) or chronic desipramine (DMI, 10 mg/kg daily i.p. for 3 weeks) did not affect either basal or forskolin-stimulated adenylate cyclase activity in membranes prepared from the caudate nucleus. In cerebellar membranes prepared from rats which had received chronic ECS, forskolin-stimulated activity was significantly increased compared to activity in sham- or single ECS treated rats. Forskolin-stimulated adenylate cyclase was increased in both hippocampal and cerebellar membranes from rats which received chronic DMI, compared to saline-treated animals. In cerebellar membranes, increases comparable to those with forskolin were also obtained with guanylyl-5'-imidodiphosphate (GppNHp) after both treatments, while with Mn2+ ions, either alone or in the additional presence of forskolin, the changes observed were similar to those previously reported in cortical membranes. A possible mechanism for these effects was investigated by studying antidepressant-induced and in vitro desensitization of the cyclic AMP response in slice preparations from the various brain areas. In slices from caudate nucleus, chronic DMI did not alter stimulation of cyclic AMP formation by either noradrenaline or forskolin, while in cerebellar slices the noradrenaline response was significantly reduced, and in hippocampal slices both responses were reduced (heterologous desensitization). In vitro incubation of cortical slices with noradrenaline also resulted in a reduction in the response to both agents. However, in membranes prepared from the desensitized cortical slices, there was no change in the degree of activation of adenylate cyclase by either NaF or forskolin. Thus, the increase in these activities, observed in certain brain areas after chronic antidepressant treatment may not necessarily be related to beta-adrenoceptor desensitization.

Abnormal signal transduction: a hypothetical model for bipolar affective disorder

The molecular basis of bipolar affective disorder is poorly understood at this time. The episodic nature of the condition in which relatively euthymic periods of variable duration separate periods of mania and depression, and the specificity of lithium therapy suggests that a molecular target of the illness may be a system that bidirectionally influences neurotransmission and is affected by lithium. Signal transduction pathways, which are important mediators of neurotransmitter generated signals, may represent such a system because they: 1) generate second messenger molecules that stimulate neurotransmission and also mediate negative feedback mechanisms, and 2) appear to be a direct target of lithium's action on cells. In this paper, we present a model in which abnormal regulation of signal transduction could lead to the episodic accumulation of biologically active transducers or second messengers. These alterations may result in prolonged effector stimulation which may underlie mania, followed by excessive receptor desensitization, which may result in depression. Using our model we suggest a plausible hypothesis that can explain the clinical spectrum of the disorder and the therapeutic action of lithium.

Modulation of second messenger function in rat brain by in vivo alteration of receptor sensitivity: relevance to the mechanism of action of electroconvulsive therapy and antidepressants

1. The second messengers cyclic AMP and inositol triphosphate are the intracellular mediators for a number of neurotransmitters for which receptors exist on brain neurons. 2. Up- or down-regulation of these receptors in general produce corresponding changes in the associated second messenger systems. 3. Chronic administration of antidepressants including electroconvulsive shock to rats produces a number of changes in cerebral receptors, notably down-regulation of beta-adrenergic and serotonin 5-HT2 receptors and up-regulation of alpha-1 adrenergic receptors. 4. The changes in receptor number induced by such antidepressant treatments are in general accompanied by corresponding changes in the associated second messenger reactions. 5. Antidepressant administration has also been shown to induce increased post-receptor mediated adenylate cyclase activity in cortical membranes, and similar effects have also been reported in striatum after chronic administration of neuroleptics. The relevance of these effects to the mechanism of action of the drugs is discussed.

Postnatal development of 5-HT1 receptors: [3H]5-HT binding sites and 5-HT induced adenylate cyclase activations in rat brain cortex

The postnatal development of the 5-HT1 receptor system was studied in young rat brain cortex from birth to adulthood (14 successive ages). The high-affinity binding of [3H]5-HT was low at birth but developed markedly between the 8th and the 15th day postnatally. The basal adenylate cyclase activity produced 50 pmoles cAMP/mg protein/min at birth and increased from the 8th to the 15th day. 5-HT could stimulate the adenylate cyclase activity in adult rat brain cortex with two different affinity constants: Km = 1 nM and Km = 0.5 microM; these low- and high-affinity constants presumably correspond to 5-HT1A and 5-HT1non-A.non-B.non-C (5-HT1D) respectively. These two activities developed parallelly from the 14-15th to the 28th day. The 8-hydroxy-2-(di-n-propylamino-tetralin) (8-OH-DPAT)-induced activity described a curve similar to the one that corresponded to 10 microM 5-HT. These results establish that 5-HT1A and 5-HT1non-A.non-B.non-C receptors mainly develop during the synaptogenesis.

Antagonism of 5-hydroxytryptamine1A (5-HT1A) receptor-mediated modulation of adenylate cyclase activity by pindolol and propranolol isomers

The interactions of the stereoisomers of pindolol and propranolol with 5-hydroxytryptamine1A (5-HT1A) binding sites and adenylate cyclase activity were examined in rat hippocampus. (-)Pindolol and (-)propranolol displayed high affinity for 5-HT1A binding sites, and their affinities were not affected significantly by the addition of 10(-4) M GTP to the radioligand assay. The selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) decreased forskolin-stimulated adenylate cyclase activity. The (-)isomers of pindolol and propranolol did not affect basal or forskolin-stimulated activity but, at a concentration of 10(-5) M, they reversed the 8-OH-DPAT inhibition of the forskolin-stimulated cyclase activity. The (+)isomers were less potent in producing this effect. These data suggest that (-)pindolol and (-)propranolol are potent antagonists at 5-HT1A receptors in rat hippocampus.

Role of dopamine and indolamine derivatives in the regulation of the sea urchin adenylate cyclase

The adenylate cyclase of the sea urchin egg is stimulated by dopamine in the presence of GTP. The enzyme activity is strongly enhanced when Gpp (NH)p is substituted for GTP, or after cholera toxin treatment. Gramine, an indolamine derivative, brings about non-competitive inhibition of the dopamine-stimulated adenylate cyclase activity. Pertussis toxin causes an attenuation of the gramine-induced inhibition of adenylate cyclase. These results show that dopamine and indolamine derivatives partecipate in the regulation of the adenylate cyclase activity of the sea urchin egg.

5-Hydroxytryptamine stimulates two distinct adenylate cyclase activities in rat brain: high-affinity activation is related to a 5-HT1 subtype different from 5-HT1A, 5-HT1B, and 5-HT1C

5-HT binding sites of the 5-HT1 type are heterogeneous and appear to comprise several subtypes (5-HT1A, 5-HT1B and 5-HT1C); their physiological role is as yet unclear. The stimulation of adenylate cyclase induced by 5-HT has been investigated in membrane fractions prepared from rat brain cortex. Enzymatic activity was determined by measuring cAMP production with an HPLC technique. It was shown that 5-HT stimulates adenylate cyclase activity with 2 activation constants (Kact): one shows a high apparent affinity (Kact = 0.8 nM) and the other a lower apparent affinity (Kact = 0.30 microM). The latter activity, induced by micromolar concentrations of 5-HT, was inhibited by spiperone at concentrations that block 5-HT1A binding. 5-Methoxytryptamine, bufotenin, and LSD also had a stimulatory biphasic effect on adenylate cyclase activity, whereas trifluoromethylphenylpiperazine, 5-carboxyamidotryptamine, 8-hydroxy-(2-di-n-propylamino)tetralin, RU 24969 had a monophasic effect. Enzyme activation by drugs acting in the micromolar range was inhibited by spiperone (1 microM), suggesting a link between this activation and 5-HT1A sites. On the other hand, the high-affinity activation of the enzyme induced by 5-HT, 5-methoxytryptamine, bufotenin, LSD, and the activation induced by TFMPP were not inhibited by spiperone (1 microM), by propranolol (3 microM), or by mesulergine (0.1 microM), which selectively block 5-HT1A, 5-HT1B, and 5-HT1C sites. Inhibition was produced by dihydroergotamine, methysergide, cinanserin, and mianserin, but not by naloxone, phenoxybenzamine, and phentolamine. Therefore, these activations seem related to 5-HT1 receptors but not to 5-HT1A, 5-HT1B, or 5-HT1C sites. Accordingly, binding of [3H]5-HT to 5-HT1-like sites was examined in the presence of spiperone (1 microM) and propranolol (3 microM); in these conditions, a high-affinity site (KD = 3.4 nM) was indeed revealed. The relative potencies of a series of drugs that stimulate or inhibit the activation of the adenylate cyclase with a high affinity and their ability to inhibit this binding of [3H]5-HT showed a positive correlation, strongly suggesting a direct relation between this recognition site for 5-HT and the production of a second messenger (cAMP). Moreover, this potential receptor is shown to be heterogeneously distributed within the brain, and was localized postsynaptically at serotonergic synapses.

Purification and reconstitution of serotonin receptors from bovine brain

An affinity-chromatography column was used to isolate and purify 5-hydroxytryptamine (serotonin, 5-HT) receptors from bovine brain frontal cortex. The affinity ligand lysergic acid ethylamidoethylbromide was synthesized and coupled to an agarose matrix via a thioether bond. Receptors in the crude cortical membrane fragments were solubilized using 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), affinity purified, and reconstituted into lipid vesicles. [3H]5-HT binding analysis indicates a single class of high-affinity binding site (Kd, 16.9 nM) that was reconstituted. 5-Methoxytryptamine, a competitor for high-affinity serotonin sites, inhibited this binding and showed a Ki of 27.4 nM. Ketanserin, a high-affinity ligand for 5-HT2 type receptors, was ineffective in displacing [3H]5-HT binding at concentrations up to 4 microM indicating a 5-HT1 receptor as the primary receptor type isolated. The average specific activity of 359 pmol/mg in the reconstituted fractions is an enrichment of 1062-fold over crude membrane fragments. Sodium dodecyl-sulfate electrophoresis showed the presence of four proteins in the reconstituted vesicles with approximate relative Mr values of 63,000, 70,000, 81,000, and 94,000.

The signal transducing system coupled to serotonin-S2 receptors

The signal transducing system coupled to the serotonin-S2 receptor on platelets involves metabolism of inositol-containing phospholipid, elevation of intracellular free Ca2+ and activation of protein kinase C. Evidence for coupling of the serotonin-S2 receptor to the same signal transducing system in brain and smooth muscle tissue is reviewed.

Characterization of two [3H]ketanserin recognition sites in rat striatum

Two [3H]ketanserin recognition sites are present in the rat striatum. The high-affinity site (KD, 0.39 nM) is similar to the 5-hydroxytryptamine2 (5-HT2) site previously characterized by various investigators. The low-affinity site (KD, 21.8 nM) has a unique pharmacologic specificity and is preferentially localized to rat striatum and septum. Conventional 5-HT2 antagonists as well as 5-HT and 5-HT uptake inhibitors are ineffective at inhibiting [3H]-ketanserin binding to this low-affinity site. Also, chronic treatment with p-chlorophenylalanine, which depletes brain 5-HT, upregulates only the high-affinity site. Thus, in the striatum and septum, [3H]ketanserin labels a unique recognition site. This site has recently been shown to be associated with dopaminergic nerve endings and may regulate biogenic amine release.

Pharmacologically distinct actions of serotonin on single pyramidal neurones of the rat hippocampus recorded in vitro

1. The actions of serotonin (5-HT) on pyramidal cells of the CA1 region of the rat hippocampus were characterized using intracellular recording in in vitro brain slices. 2. 5-HT typically evokes a biphasic response consisting of a hyperpolarization which is followed by a longer-lasting depolarization. These effects on membrane potential are accompanied by a decrease in the calcium-activated after-hyperpolarization (a.h.p). 3. Detailed analysis using 5-HT antagonists and agonists indicates that the hyperpolarization is mediated by a 5-HT1A receptor. Spiperone is the most effective antagonist of the response and the selective 5-HT1A agonist, 8-OHDPAT, behaves as a partial agonist at this receptor. In agreement with the distribution of 5-HT1A binding sites, responses to 5-HT were most prominent in the stratum radiatum. 4. The hyperpolarizing response is associated with a decrease in input resistance, is blocked by extracellular barium and intracellular caesium, is unaffected by the chloride gradient, and its reversal potential shifts with the extracellular concentration of potassium as predicted for a response mediated by a selective increase in potassium permeability. 5. The depolarizing response and reduction in the a.h.p. could be studied in isolation by blocking the hyperpolarizing response with either pertussis toxin or spiperone. The pharmacology of these responses did not correspond to that of any of the 5-HT binding sites reported in C.N.S. tissue. Although the depolarization and blockade of the a.h.p. have the same time course it is unclear if they are mediated by the same or different receptors. 6. The depolarization most likely results from a decrease in resting potassium conductance. However, neither a blockade of the M current nor the a.h.p. current can account for the depolarization. 7. Blockade of phosphodiesterase activity by 3-isobutyl-1-methylxanthine (IBMX) did not enhance the depressant action of 5-HT on the a.h.p., making it unlikely that this action is mediated by cyclic AMP. 8. Blockade of the a.h.p. by 5-HT reduces spike frequency adaptation and counteracts the inhibitory action of 5-HT on 5-HT1A receptors. This excitatory action outlasts the hyperpolarizing action. 9. In summary 5-HT acts on at least two distinct receptors on hippocampal pyramidal cells, one coupled to the opening of potassium channels and a second coupled to a decrease in a resting potassium conductance and a decrease in the a.h.p.

Serotonergic and cholinergic stimulation of inositol phosphate formation in the rabbit retina. Evidence for the presence of serotonin and muscarinic receptors

A direct assay which involves prelabelling with [3H]inositol has been performed to characterize receptor-mediated breakdown of inositol phospholipids in the rabbit retina. In the presence of 10 mM lithium, the receptor agonists, carbachol and serotonin (5-HT), evoked an increase in the accumulation of tritiated inositol phosphates in a dose-dependent manner. A variety of 5-HT and other antagonists were used to show that at least part of the 5-HT-induced response is mediated by 5-HT2 receptor sites, adding weight to the theory that 5-HT plays a neurotransmitter role in the mammalian retina. The very high rate of carbachol-stimulated inositol phosphate accumulation observed in retinas from young animals (10 days) decreases significantly during development of the rabbit to the adult stage. In contrast 5-HT only induces significant stimulation of inositol phosphate accumulation in retinas of animals matured to at least 17 days. Separation of total accumulated inositol phosphates (after stimulation with both carbachol and 5-HT) showed that the vast majority of tritium label was associated with the monophosphate fraction.

Multiple mechanisms of serotonergic signal transduction

In this article we review serotonergic signal transduction mechanisms in the central and peripheral nervous systems and in a variety of target organs. The various classes of pharmacologically defined serotonergic receptors are coupled to three major effector systems: (1) adenylate cyclase; (2) phospholipase C mediated phosphoinositide (PI) hydrolysis and (3) ion channels (K+ and Ca++). Long term occupancy of serotonergic receptors also appears to induce alterations in mRNA and protein synthesis. For all three types of signal transduction there is evidence accumulating which suggests the involvement of guanine nucleotide regulatory proteins. Recent findings suggest that the distinct types of pharmacologically defined serotonergic receptors (5HT1A, 5HT1B, 5HT1c, 5HT2) may be coupled to one or more signal transduction systems. Thus, 5HT1 receptors may both activate and inhibit adenylate cyclase and increase K+-ion conductance in the hippocampus. 5HT2 receptors which activate PI hydrolysis in the brain, both open voltage-gated calcium channels and activate PI metabolism in certain smooth muscle preparations. Thus, each class of serotonergic receptor may be linked to one or more distinct biochemical transduction mechanisms. The possibility is raised that selective agonists and antagonists might be developed which have specific effects on a particular receptor-linked effector system.

Rat brain serotonin receptors in Xenopus oocytes are coupled by intracellular calcium to endogenous channels

Serotonin activates chloride currents in Xenopus oocytes injected with a subfraction of rat brain poly(A)+ mRNA. Patch-clamp recordings from cell-attached patches showed that serotonin, applied locally outside the patch, caused the opening of channels of approximately equal to 3 pS conductance and an average lifetime of approximately equal to 100 msec. The extrapolated reversal potential indicated that the channels are chloride-selective. Single-channel currents with similar characteristics were observed in inside-out patches from native oocytes in response to elevated calcium concentrations on the cytoplasmic side. Measurements of intracellular calcium concentration ([Ca2+]i) by fura-2 fluorescence showed approximately equal to 10-fold increases in [Ca2+]i in response to serotonin application in both normal and calcium-free Ringer solution in mRNA-injected oocytes. Little or no response to serotonin was observed in native oocytes. These results suggest that serotonin activation of receptors that are inserted into the oocyte membrane following injection of rat brain poly(A)+ mRNA can induce calcium release from intracellular stores. The increase in [Ca2+]i subsequently activates calcium-dependent chloride channels. Because calcium-dependent chloride channels and a receptor-controlled mechanism of internal calcium release have been shown to exist in native oocytes, we conclude that the newly inserted serotonin receptors utilized the endogenous second-messenger-mediated calcium release to activate endogenous calcium-dependent chloride channels.

Effects of norepinephrine and 5-hydroxytryptamine on phosphoinositide-PO4 turnover in rabbit cornea

We have investigated: (a) Phospholipid composition and phosphoinositide-PO4 turnover in rabbit cornea tissues; and (b) the effects of adrenergic and serotonergic agonists on breakdown of phosphoinositides in the rabbit cornea. The data obtained from these studies can be summarized as follows: (1) in the cornea phosphatidylcholine and phosphatidylethanolamine constitute about 55%, phosphatidylinositol (PI) 10%, and phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) comprise about 1% each of the total phospholipids; (2) incubation of cornea in 32Pi-containing medium resulted in incorporation of radioactivity in tissue phospholipids. The radioactivity was highest in PIP2 (39%), followed by PI (19%), PIP (16%) and PA (5% of the total radioactivity). When compared with stroma and endothelium, the cornea epithelium was most active in phosphoinositide metabolism; (3) addition of norepinephrine (NE) or 5-hydroxytryptamine (5-HT), 200 microM each, to 32P-labeled cornea resulted in a loss of radioactivity in PIP and PIP2 by about 12- and 20%, respectively. Concomitantly, the radioactivity in PA and PI was increased by 44- and 66%, respectively. The effects of the neurotransmitters were time- and concentration-dependent. When added to the cornea labeled with myo [3H] inositol, NE and 5-HT increased the production of labeled myo-inositol phosphates; (4) prazosin (20 microM), but not yohimbine or propranolol, blocked the effects of NE. Similarly, the effects of 5-HT were antagonized by methysergide (20 microM) and ketanserin (10 microM) but not by prazosin. These data demonstrate that NE and 5-HT stimulate phospholipase C-mediated hydrolysis of PIP2 into diacylglycerol (DG) and myo-inositol trisphosphate (IP3). Furthermore, the effects of NE and 5-HT are mediated by alpha 1-adrenergic and 5-HT2 receptors, respectively. It is suggested that IP3, by releasing Ca2+ from ER, and DG, by activating protein kinase C, may function as second-messenger molecules which may participate in agonist-induced functional responses, including chloride transport, in the cornea epithelium.