Serotonin inhibits synaptic glutamate currents in rat nucleus accumbens neurons via presynaptic 5-HT1B receptors

Cellular effects of swim stress in the dorsal raphe nucleus

Swim stress regulates forebrain 5-hydroxytryptamine (5-HT) release in a complex manner and its effects are initiated in the serotonergic dorsal raphe nucleus (DRN). The purpose of this study was to examine the effects of swim stress on the physiology of DRN neurons in conjunction with 5-HT immunohistochemistry. Basic membrane properties, 5-HT(1A) and 5-HT(1B) receptor-mediated responses and glutamatergic excitatory postsynaptic currents (EPSCs) were measured using whole-cell patch clamp techniques. Rats were forced to swim for 15min and 24h later DRN brain slices were prepared for electrophysiology. Swim stress altered the resting membrane potential, input resistance and action potential duration of DRN neurons in a neurochemical-specific manner. Swim stress selectively elevated glutamate EPSC frequency in 5-HT DRN neurons. Swim stress non-selectively reduced EPSC amplitude in all DRN cells. Swim stress elevated the 5-HT(1B) receptor-mediated inhibition of glutamatergic synaptic activity that selectively targeted 5-HT cells. Non-5-HT DRN neurons appeared to be particularly responsive to the effects of a milder handling stress. Handling elevated EPSC frequency, reduced EPSC decay time and enhanced a 5-HT(1B) receptor-mediated inhibition of mEPSC frequency selectively in non-5-HT DRN cells. These results indicate that swim stress has both direct, i.e., changes in membrane characteristics, and indirect effects, i.e., via glutamatergic afferents, on DRN neurons. These results also indicate that there are distinct local glutamatergic afferents to neurochemically specific populations of DRN neurons, and furthermore that these distinct afferents are differentially regulated by swim stress. These cellular changes may contribute to the complex effects of swim stress on 5-HT neurotransmission and/or the behavioral changes underlying the forced swimming test model of depression.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

Cocaine increases 5-HT1B mRNA in rat nucleus accumbens shell neurons

Serotonin 5-HT(1B) receptors modulate behavioral responses to cocaine, but the effects of cocaine on endogenous 5-HT(1B) receptor expression are not known. Therefore, we examined the effect of binge cocaine administration on 5-HT1B mRNA expression in rat brain. We found that chronic, but not acute, binge cocaine exposure increased 5-HT(1B) mRNA by approximately 80% in nucleus accumbens shell and dorsal striatum. Surprisingly, 5-HT(1B) mRNA was increased in nucleus accumbens shell after chronic vehicle treatment as well, but this effect was driven by animals that were housed with cocaine-treated animals. Thus, 5-HT(1B) mRNA is upregulated by repeated exposure to cocaine and perhaps by social stress as well; both of these factors are relevant to the risk for relapse in cocaine addiction.

The anatomy of co-morbid neuropsychiatric disorders based on cortico-limbic synaptic interactions

Many brain disorders appear to involve dysfunctions of aminergic systems. Alterations in dopamine activity may underpin both schizophrenia and the establishment and maintenance of drug dependence while disruption of serotonergic signalling may be crucial in both depression and schizophrenia. The co-existence of nicotine and alcohol abuse with depression and schizophrenia is well-documented as is the particular vulnerability of adolescents. At the same time, a common group of brain structures is increasingly implicated in neuropathological studies. For example, depression may involve a lack of serotonin signalling, particularly in the prefrontal cortex, while in schizophrenia there is evidence for reduced dopamine signalling in the same brain region, co-existing with hyperactivity in the mesolimbic dopamine pathway. Increased dopamine release from the mesolimbic dopamine pathway is also a common factor of drugs of abuse. Furthermore, the control of motivational behaviour and dopamine release is apparently modified by hippocampal and amygdala activity, both brain regions showing pathological changes in schizophrenia and depression. Our work has focused on the intricate synaptic interactions of aminergic terminals and cortical and subcortical neurons in order to unravel the anatomical basis for these disorders and their treatments. We show convergence of dopamine and cortical inputs onto single neurons in the nucleus accumbens, and between different cortical inputs to individual neurons, providing a basis for the gating mechanisms attributed to these interactions. We have also examined local and extrinsic connections in the prefrontal cortex and the basis for regulation of both cortical neurons and midbrain dopamine neurons by serotonin from the raph é nucleus. Together with data concerning subcellular receptor distributions, this information provides a detailed synaptic framework for interpreting behavioural, pharmacological and physiological data and enhances our understanding of possible circuitry underlying comorbidity of disorders such as schizophrenia and depression with drug abuse, information invaluable in the introduction of enhanced therapies.

Biphasic alterations in Serotonin-1B (5-HT1B) receptor function during abstinence from extended cocaine self-administration

Alterations in 5-HT1B receptor function during cocaine abstinence were evaluated in rats given either limited- or extended access (LA and EA, respectively) to cocaine self-administration. The locomotor response to the 5-HT1B/1A agonist RU24969 was significantly reduced in cocaine-experienced animals relative to cocaine-naïve controls following 6 h of abstinence but became sensitized over the subsequent 14 days of abstinence. Both the early phase subsensitivity and later phase supersensivity to RU 24969-induced activity were greater in EA versus LA animals. Intra-nucleus accumbens administration of the 5-HT1B agonist CP 93, 129 produced significantly greater increases in dialysate dopamine levels in EA versus control animals following 14 days of abstinence. However, there was no difference between EA and cocaine-naïve control animals in the augmentation of cocaine-induced increases in nucleus accumbens DA produced by intra-VTA CP 93, 129. Collectively these findings demonstrate that 5-HT1B receptor function is persistently altered by cocaine self-administration.

Increased expression of 5-HT1B receptors in rat nucleus accumbens via virally mediated gene transfer increases voluntary alcohol consumption

Serotonin 5-HT(1B) receptors have been linked to alcoholism in humans and alcohol consumption in rodents. We hypothesize that these receptors, which are located on the axon terminals of nucleus accumbens' (NAcc) projection neurons, modulate alcohol reward mechanisms. To test this hypothesis, we measured ethanol consumption by rats that received bilateral microinjections of a viral vector producing 5-HT(1B) overexpression (HA1B/GFP). Other groups received either control (GFP-only) herpes simplex viral vectors into the medial NAcc shell or were handled briefly with no surgery. All animals were housed singly and had continuous access to water, 6% ethanol, and 12% ethanol in their home cages both before and after surgery. There were no differences in the amount or rate of weight gain, amount of food eaten, or total fluid consumed. There were also no differences in the amount of ethanol consumed between groups prior to surgery. However, after surgery, the HA1B/GFP group consumed twice as much ethanol as the other groups. The main effect of total ethanol consumption was significant (p<.05); the control groups did not differ from each other. Whereas there were no between-group differences in 6% ethanol consumption, there was a large increase in the amount of 12% ethanol consumed by the HA1B/GFP-expressing animals compared to the two control groups as well as to their own presurgery intake (p<.05). We hypothesize that increased 5-HT(1B) expression in NAcc led to either greater reward or reduced aversive effects from the 12% ethanol, thereby leading to increased voluntary ethanol consumption.

Manipulations of serotonin function in the nucleus accumbens core produce differential effects on ethanol and sucrose seeking and intake

OBJECTIVE

Behaviorally relevant stimuli, including alcohol, are processed through the nucleus accumbens/ventral tegmental area (VTA)/prefrontal cortex circuit. It is hypothesized that serotonin affects ethanol-directed behaviors by interacting with this system via projections from the dorsal raphe to the nucleus accumbens and VTA. The current studies utilized two different operant paradigms, one focusing on reinforcer seeking and one focusing on reinforcer self-administration (both with an ethanol and a sucrose solution as the reinforcer) to elucidate serotonin-specific regulation of these behaviors.

METHODS

The present experiments assessed the effects of microinjections of a serotonin1B agonist (CGS12066B) and a serotonin1A agonist (8-OH-DPAT) in the nucleus accumbens core on ethanol- and sucrose-reinforced seeking and intake. In four separate experiments, male Long-Evans rats were trained to complete a single response requirement that resulted in access to 10% ethanol or 2% sucrose for a 20-min drinking period.

RESULTS

Before microinjections, ethanol-reinforced subjects were consuming an average of 0.5-0.95 g/kg ethanol and making 50-100 responses during intermittent nonreinforced sham (no drug) sessions (sucrose groups had similar baseline response levels). In summary, findings from the four experiments showed the following: (1) manipulations of serotonin function that had effects on ethanol-reinforced responding had either no effect or less pronounced effects on sucrose-reinforced responding; (2) administration of the serotonin1B agonist decreased seeking behaviors to a greater degree than drinking behaviors; and (3) administration of the serotonin1A agonist decreased ethanol intake but not seeking with no impact at all on sucrose-reinforced behaviors.

CONCLUSIONS

Manipulations of serotonin activity in the nucleus accumbens core had little effect on sucrose-reinforced behaviors and differential effects on ethanol seeking versus intake, suggesting that this area may play a complex but selective role in the stimulus processing of external and internal alcohol-associated cues.

Involvement of 5-HT1B receptors within the ventral tegmental area in ethanol-induced increases in mesolimbic dopaminergic transmission

Evidence suggests that 5-hydroxytriptamine-1B (5-HT1B) receptors play a role in modifying ethanol's reinforcing effects and voluntary intake, and that 5-HT1B receptors within the ventral tegmental area (VTA) are involved in regulation of mesolimbic dopaminergic neuronal activity. Since increased mesolimbic dopaminergic transmission has been implicated in ethanol's reinforcing properties, this study was designed to assess the involvement of VTA 5-HT1B receptors in mediating the stimulatory effects of ethanol on VTA dopaminergic neurons. Dual-probe microdialysis was performed in freely moving adult Sprague-Dawley rats with one probe within the VTA and the other within the ipsilateral nucleus accumbens (NACC). Dopamine (DA) levels in dialysates from both areas, as the index of the activity of mesolimbic DA neurons, were measured simultaneously. The results showed that intraperitoneal injection of ethanol at the doses of 1 and 2 g/kg increased extracellular DA concentrations in both the VTA and the NACC, suggesting increased DA neuronal activity. These ethanol-induced increases of the DA release in the VTA and the NACC were significantly attenuated by intra-tegmental infusion of SB 216641 (a 5-HT(1B) receptor antagonist), but not BRL 15572 (a 5-HT(1D/1A) receptor antagonist) or WAY 100635 (a 5-HT1A receptor antagonist). Administration of ethanol at the same doses did not significantly alter extracellular levels of GABA in the VTA. The results also showed that intra-tegmental infusion of CP 94253, a 5-HT1B receptor agonist, significantly prolonged the effects of ethanol on NACC DA. The results suggest that blockade and activation of VTA 5-HT1B receptors attenuates and potentiates the neurochemical effects of ethanol, respectively, and support the suggestion that VTA 5-HT(1B) receptors may be involved in part in mediating the activating effects of ethanol on mesolimbic DA neurons.

The wake-promoting peptide orexin-B inhibits glutamatergic transmission to dorsal raphe nucleus serotonin neurons through retrograde endocannabinoid signaling

The wake-promoting neuropeptides orexins (hypocretins) play a crucial role in controlling neuronal excitability and synaptic transmission in the CNS. In this study, using whole-cell patch-clamp recordings in an acute dorsal raphe nucleus (DRN) slice preparation, we report that orexin B (Orx-B) depresses the evoked glutamate-mediated synaptic currents in DRN 5-HT neurons. The Orx-B-induced depression is accompanied by an increase in the paired-pulse ratio and the coefficient of variance, suggesting a presynaptic site of action. Orx-B also reduces the frequency but not the amplitude of miniature EPSCs, indicating that depression of glutamatergic transmission is mediated by a decrease in glutamate release. Surprisingly, the Orx-B-induced inhibition of glutamatergic transmission is abolished by postsynaptic inhibition of G-protein signaling with GDPbetaS, suggesting that this effect is signaled by postsynaptic orexin receptors and expressed presynaptically, presumably through a retrograde messenger. Interestingly, the Orx-B-induced depression of glutamate release is mimicked and occluded by the cannabinoid receptor agonist WIN 55,212-2, and is abolished by the CB1 cannabinoid receptor antagonist AM 251. These results imply that the Orx-B-induced depression of glutamatergic transmission to DRN 5-HT neurons is mediated by retrograde endocannabinoid release. Examination of downstream signaling pathways involved in this response indicates that the effect of Orx-B requires the activation of phospholipase C and DAG lipase enzymatic pathways but not a rise in postsynaptic intracellular calcium. Therefore, our findings reveal a previously unsuspected mechanism by which postsynaptic orexin receptors can modulate glutamatergic synaptic transmission to DRN 5-HT neurons.

Fast neurotransmission in the rat medial preoptic nucleus

The functional properties of neurotransmission in the medial preoptic nucleus (MPN) were studied in a brain slice preparation from young male rats. The aims were to evaluate the thin slice preparation for studying evoked synaptic responses in MPN neurons, to characterize the fast responses triggered by activation of presynaptic nerve fibers in the MPN, and to identify the involved receptor types. Presynaptic stimulation within the MPN evoked postsynaptic voltage and current responses that were blocked by 200 microM Cd2+ or by 2.0 microM tetrodotoxin and were attributed to action potential-evoked transmitter release. The relation to stimulus strength and comparison with spontaneous synaptic currents suggested that in many cases only one presynaptic nerve fiber was excited by the stimulus. Furthermore, the transmission was probabilistic in nature, with frequent failures. Thus, response probability, most likely reflecting transmitter release probability, could be evaluated in the thin slice preparation. Evoked excitatory postsynaptic currents recorded under voltage-clamp conditions were, due to kinetics, I-V relation, and pharmacological properties, attributed to AMPA/kainate receptors and NMDA receptors, whereas inhibitory currents were attributed to GABAA receptors. No responses that could be attributed to glycine or other types of primary transmitters were detected. Although serotonin (5-HT) did not appear to function as a primary transmitter, glutamate- as well as GABA-mediated transmission was suppressed by 500 microM 5-HT, with a clear reduction in response probability observed. 5-HT also reduced the frequency, but not the amplitude, of spontaneous postsynaptic currents and was therefore ascribed a presynaptic site of action.

Differential effects of intra-midbrain raphe and systemic 8-OH-DPAT on VTA self-stimulation thresholds in rats

RATIONALE

Intra-median raphe nucleus (MRN) administration of the 5-HT(1A) receptor agonist 8-OH-DPAT decreases lateral hypothalamic self-stimulation thresholds and is reported to have biphasic effects following systemic administration. These experiments attempted to extend the previous findings to mesolimbic pathway self-stimulation at ventral tegmental area (VTA) electrodes.

OBJECTIVES

This study was conducted to provide comparative data for systemic and intra-dorsal raphe nucleus (DRN) and intra-MRN effects of 8-OH-DPAT on VTA self-stimulation.

METHODS

Male Sprague-Dawley rats with VTA electrodes were trained to respond for electrical stimulation. Systemic and intra-midbrain raphe 8-OH-DPAT effects on rate-frequency thresholds were measured. Systemic administration of WAY 100635 was used to confirm 5-HT(1A) receptor mediation of 8-OH-DPAT effects.

RESULTS

8-OH-DPAT (0.003-0.3 mg kg(-1) SC) increased rate-frequency thresholds and decreased maximal response rates. WAY 100635 alone (0.0125-0.1 mg kg(-1) SC) did not alter these measures. Intra-DRN and intra-MRN 8-OH-DPAT (5.0 microg) decreased rate-frequency thresholds without altering maximal response rates. Intra-DRN 8-OH-DPAT (0.1-5.0 microg) induced a slight decrease and intra-MRN 8-OH-DPAT a slight increase in locomotor activity. WAY 100635 (0.1 mg kg(-1)) blocked effects of 8-OH-DPAT on VTA self-stimulation.

CONCLUSION

These results confirm threshold-decreasing effects of intra-MRN 8-OH-DPAT and extend this to the DRN and to VTA thresholds. Monophasic dose dependent increases in VTA thresholds following systemic 8-OH-DPAT are not equivalent to reports for hypothalamic self-stimulation. Differences between studies may be attributable to stimulation site and/or differences in threshold measurement procedures. Effects of WAY 100635 in this study indicate 5-HT(1A) receptor mediation of these 8-OH-DPAT effects.

Serotonin1B receptors in the ventral tegmental area modulate cocaine-induced increases in nucleus accumbens dopamine levels

Previous work has demonstrated that peripheral serotonin(1B) (5-HT(1B)) receptor agonist administration facilitates the behavioral and neurochemical effects of cocaine. This study used dual probe microdialysis to investigate whether activation of serotonin(1B) (5-HT(1B)) receptors in the ventral tegmental area (VTA) alters the ability of peripherally administered cocaine to elevate dopamine (DA) levels in the ipsilateral nucleus accumbens (NAcc) of drug-naive Wistar rats. Intra-VTA administration of the selective 5-HT(1B) agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo [3,2-b]pyridin-5-one dihydrochloride (CP 93,129) by reverse dialysis produced a dose-dependent (30 and 100 microM) potentiation of cocaine-induced (10 mg/kg i.p.) increases in NAcc DA efflux and concurrent cocaine-induced decreases in VTA GABA efflux. There was no effect of either local CP 93,129 or peripheral cocaine on VTA glutamate efflux. Intra-VTA administration of the 5-HT(1A/7) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT; 100 microM) did not alter cocaine-induced alterations in NAcc DA or VTA GABA, suggesting that the effects of CP 93,129 were not mediated through 5-HT(1A) receptors. Moreover, the effects of intra-VTA CP 93,129 (100 microM) on both cocaine-induced increases in NAcc DA levels and cocaine-induced decreases in VTA GABA levels were reversed by coadministration of the selective 5-HT(1B) receptor antagonist 3-[3-(dimethylamine)propyl]-4-hydroxy-N-[4-(4-pyridinyl] phenyl] benzamide dihydrochloride (GR 55562; 300 microM). In the absence of cocaine, intra-VTA CP 93,139 produced an increase in NAcc DA and decrease in VTA GABA levels. However, intra-VTA GR 55562 alone had no effect on any of our neurochemical measures. These findings indicate that activation of VTA 5-HT(1B) receptors potentiates cocaine-induced increases in NAcc DA levels by enhancing the ability of cocaine to decrease VTA GABA efflux.

Building Bridges: The Transdisciplinary Study of Craving From the Animal Laboratory to the Lamppost

This article represents the proceedings of a symposium at the 2003 Research Society on Alcoholism meeting in Ft. Lauderdale, Florida, organized and chaired by Peter M. Monti. The presentations and presenters were (1) Alcohol Seeking and Self-Administration in Rats: The Role of Serotonin Activity, by Cristine L. Czachowski; (2) Assessing Binge Drinking in Monkeys, by Kathleen A. Grant; (3) Craving and the Perception of Time, by Michael Sayette; (4) Ecological and Laboratory Assessment of Alcohol Urges and Drinking: Effects of Naltrexone, by Peter M. Monti; and (5) Discussion, by Damaris J. Rohsenow.

Elevated expression of 5-HT1B receptors in nucleus accumbens efferents sensitizes animals to cocaine

Although the effects of psychostimulants on brain dopamine systems are well recognized, the direct actions of cocaine on serotonin systems also appear to be important to its addictive properties. For example, serotonin actions at 5-HT1B receptors in the ventral tegmental area (VTA) modulate cocaine-induced dopamine release in the nucleus accumbens (NAcc) and alter the rewarding and stimulant properties of cocaine. However, the mechanisms of these effects have been unclear, because several neuron types in VTA express 5-HT1B receptors. One possibility is that 5-HT1B receptors on the terminals of GABAergic projections from NAcc to VTA inhibit local GABA release, thereby disinhibiting VTA neurons. We tested this hypothesis directly by using viral-mediated gene transfer to overexpress 5-HT1B receptors in NAcc projections to VTA. A viral vector containing either epitope hemagglutinin-tagged 5-HT1B and green fluorescent protein (HA1B-GFP) cassettes or green fluorescent protein cassette alone (GFP-only) was injected into the NAcc shell, which sends projections to the VTA. HA1B-GFP injection induced elevated expression of 5-HT1B receptors in neuronal fibers in VTA and increased cocaine-induced locomotor hyperactivity without affecting baseline locomotion. Overexpression of 5-HT1B receptors also shifted the dose-response curve for cocaine-conditioned place preference to the left, indicating alterations in the rewarding effects of cocaine. Thus, increased expression of 5-HT1B receptors in NAcc efferents, probably in the terminals of medium spiny neurons projecting to the VTA, may contribute to psychomotor sensitization and offer an important target for regulating the addictive effects of cocaine.

Pharmacological characterization of 5-HT(1B) receptor-mediated inhibition of local excitatory synaptic transmission in the CA1 region of rat hippocampus

1 In the hippocampus, axon collaterals of CA1 pyramidal cells project locally onto neighbouring CA1 pyramidal cells and interneurones, forming a local excitatory network which, in disinhibited conditions, feeds polysynaptic epscs (poly-epscs). 5-hydroxytryptamine (5-HT) has been shown to inhibit poly-epscs through activation of a presynaptic receptor. The aim of the present work was the pharmacological characterization of the 5-HT receptor involved in this 5-HT action. 2 Poly-epscs, evoked by electrical stimulation of the stratum radiatum and recorded in whole-cell voltage-clamp from CA1 pyramidal neurones, were studied in mini-slices of the CA1 region under pharmacological block of GABA(A), GABA(B), and 5-HT(1A) receptors. 3 The 5-HT(1B) receptor selective agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one dihydrochloride (CP 93129) inhibited poly-epscs (EC(50)=55 nM), an effect mimicked by the 5-HT(1B) ligands 5-carboxamidotryptamine (5-CT; EC(50)=14 nM) and methylergometrine (EC(50)=78 nM), but not by 1-(3-chlorophenyl)piperazine dihydrochloride (mCPP; 10 micro M) or 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline dimaleate (CGS 12066B; 10 micro M). 4 The effects of CP 93129 and 5-CT were blocked by the selective 5-HT(1B) receptor antagonist 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; K(B) approximately 100 nM) and by cyanopindolol (K(B)=6 nM); methiothepin (10 micro M) and dihydroergotamine (1 micro M). For both GR 55562 and methiothepin, application times of at least two hours were required in order to achieve their full antagonistic effects. 5 Our results demonstrate that 5-HT(1B) receptors are responsible for the presynaptic inhibition of neurotransmission at CA1/CA1 local excitatory synapses exerted by 5-HT.

Serotonergic control of cerebellar mossy fiber activity by modulation of signal transfer by rat pontine nuclei neurons

Serotonergic modulation of precerebellar nuclei may be crucial for the function of the entire cerebellar system. To study the effects of serotonin (5-HT) on neurons located within the pontine nuclei (PN), the main source of cerebellar mossy fibers, we performed standard intracellular recordings from PN neurons in a slice preparation of the rat pontine brain stem. Application of 5 microM 5-HT significantly altered several intrinsic membrane properties of PN neurons. First, it depolarized the somatic membrane potential by 6.5 +/- 3.5 mV and increased the apparent input resistance from 49.5 +/- 14.6 to 62.7 +/- 21.1 MOmega. Second, 5-HT altered the I-V relationship of PN neurons: it decreased the inward rectification in hyperpolarizing direction, but increased it when depolarizing currents were applied. Third, it decreased the rheobase from 0.32 +/- 0.14 to 0.24 +/- 0.14 nA without affecting the firing threshold. Finally, the amplitude of medium-duration after hyperpolarizations was reduced from -14.9 +/- 2.0 to -12.3 +/- 2.4 mV. Together, these 5-HT effects on the intrinsic membrane properties result in an increase in excitability and instantaneous firing rate. In addition, application of 5 microM 5-HT also modulated postsynaptic potentials (PSPs) evoked by electric stimulations within the cerebral peduncle. The amplitude, maximal slope, and integral of these PSPs were reduced to 46.2 +/- 23.4%, 45.7 +/- 23.7%, and 61.4 +/- 28.4% of the control value, respectively. In contrast, we found no change in the decay and voltage dependence of PSPs. To test modulatory effects on short-term synaptic facilitation, we applied pairs of electrical stimuli at intervals between 10 and 1,000 ms. 5-HT selectively enhanced the paired-pulse facilitation for interstimulus-intervals >20 ms. The alteration of paired-pulse facilitation points to a presynaptic site of action for 5-HT effects on synaptic transmission. Pharmacological experiments suggested that pre- and postsynaptic effects of 5-HT were mediated by two different kinds of 5-HT receptors: changes in intrinsic membrane properties were blocked by the 5-HT(2) receptor antagonist cinanserin while the reduction of PSPs was prevented by the 5-HT(1) receptor antagonist cyanopindolol. In conclusion, 5-HT increases the excitability of PN neurons but decreases the synaptic transmission on them. The selective enhancement of synaptic facilitation may, however, allow high-frequency inputs to effectively drive PN neurons, thus the PN may act as a high-pass filter during periods of 5-HT release.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Autoradiographic mapping of 5-HT(1B) and 5-HT(1D) receptors in the post mortem human brain using [(3)H]GR 125743

The distribution of 5-HT(1B) and 5-HT(1D) receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [(3)H]GR 125743. [(3)H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [(3)H]GR 125743 binding was observed, reflecting low densities of 5-HT(1B/1D) receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT(1D) receptors and the second using SB 224289 to inhibit 5-HT(1B) receptor binding. The autoradiograms indicated that in the human brain, the 5-HT(1B) receptor is much more abundant than the 5-HT(1D) receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [(3)H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT(1B) receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT(1D) receptor in the human brain.

Further evidence that extracellular serotonin in the rostral ventrolateral medulla modulates 5-HT(1A) receptor-mediated attenuation of exercise pressor reflex

We determined changes in extracellular levels of glutamate, serotonin (5-HT), norepinephrine (NE), and dopamine (DA) within rostral ventrolateral medulla (RVLM) during 5-HT(1A)-receptor stimulation-mediated inhibition of cardiovascular responses to static muscle contraction using anesthetized rats. In ten rats, muscle contraction significantly increased (P<0.01) mean arterial pressure (MAP) by 29+/-4 mm Hg, heart rate (HR) by 25+/-3 bpm, and glutamate levels by 4.5+/-0.8 ng/5 microl. Microdialysis of a 5-HT(1A) receptor agonist, 8-OH-DPAT (10 mM), into the RVLM for 30 min attenuated cardiovascular responses to muscle contraction and had no effect on glutamate levels. A subsequent administration of 10 mM WAY100635, a 5-HT(1A) antagonist, into the RVLM antagonized the attenuating effects of 8-OH-DPAT. In another ten rats, muscle contraction significantly increased (P<0.01) MAP and HR by 20+/-2 mmHg and 25+/-8 bpm, respectively. In addition, levels of 5-HT, NE, and DA in the RVLM significantly increased (P<0.01) by 3.6+/-0.3, 3.2+/-0.3, and 3.3+/-0.4 pg/10 microl, respectively. Administration of 8-OH-DPAT (10 mM) into the RVLM for 30 min attenuated cardiovascular responses to muscle contraction and had no effects on NE and DA levels. However, the drug significantly attenuated 5-HT levels following a muscle contraction. Microdialysis of 10 mM WAY100635 into the RVLM reversed both cardiovascular and 5-HT changes. These results suggest that stimulation of 5-HT(1A)-receptors within the RVLM attenuates cardiovascular responses to static exercise via a reduction of extracellular 5-HT concentration and most likely not through changes in glutamate, NE or DA levels.

Increased amyloid precursor protein expression and serotonergic sprouting following excitotoxic lesion of the rat magnocellular nucleus basalis: neuroprotection by Ca(2+) antagonist nimodipine

In the present study plastic neural responses to N-methyl-D-aspartate-induced excitotoxic lesions and the neuroprotective effects of the L-type voltage-dependent Ca(2+) channel antagonist nimodipine were investigated in the rat magnocellular nucleus basalis. Assessment of spontaneous behaviour in the elevated plus maze and small open-field paradigms on day 5 and day 14 post-surgery indicated anxiety and persistent hypoactivity of N-methyl-D-aspartate-lesioned rats, as compared with sham-operated controls. Nimodipine administration significantly alleviated the behavioural deficits. Quantitative histochemical analysis of acetylcholinesterase-positive fibre innervation of the somatosensory cortex and determination of the numbers of choline-acetyltransferase-positive proximal fibre branches of cholinergic projection neurons in the magnocellular nucleus basalis demonstrated a severe cholinergic deficit as a consequence of the excitotoxic lesion 14 days post-surgery. Nimodipine pre-treatment significantly attenuated the loss of cortical cholinergic innervation and preserved the functional integrity of cholinergic projection neurons in the magnocellular nucleus basalis. Double-labelling immunocytochemistry demonstrated increased amyloid precursor protein expression in shrinking and presumably apoptotic choline-acetyltransferase-positive neurons, whereas surviving cholinergic nerve cells were devoid of excessive amyloid precursor protein immunoreactivity. Moreover, as a consequence of N-methyl-D-aspartate infusion, rim-like accumulation of amyloid precursor protein-positive astrocytes was visualized in a penumbra-like zone of the excitotoxic injury. Furthermore, abundant sprouting of serotonergic projection fibres invading the damaged magnocellular nucleus basalis subdivision was demonstrated. Pharmacological blockade by the Ca(2+) antagonist nimodipine significantly attenuated both neuronal and glial amyloid precursor protein immunoreactivity and serotonergic fibre sprouting following N-methyl-D-aspartate infusion. The present data characterize plastic endogenous glial and neuronal responses in the magnocellular nucleus basalis model of acute excitotoxic brain damage. The increased amyloid precursor protein expression may indicate effective means of intrinsic neuroprotection, as secreted amyloid precursor protein isoforms are suggested to play a role in neuronal rescue following excitotoxic injury. From a pharmacological point of view, extensive sprouting of serotonergic projections in the damaged magnocellular nucleus basalis may also counteract N-methyl-D-aspartate excitotoxicity via serotonin-induced inhibition of Ca(2+) currents and membrane hyperpolarization. Hence, lesion-induced changes in spontaneous animal behaviour, such as anxiety and novelty-induced hypoactivity, may well be attributed to the considerable re-distribution of serotonergic projections in the basal forebrain. In conclusion, our present data emphasize a role of neuron-glia and neurotransmitter-system interactions in functional recovery after acute excitotoxic brain injury, and the efficacy of L-type Ca(2+) channel blockade by the selective 1,4-dihydropyridine antagonist nimodipine.

Experimental gerontology in the Nordic countries

Research in geriatric medicine developed in the Nordic countries in the 1950s, following the tradition from the United Kingdom. Quite early, longitudinal epidemiological studies of 'normal' ageing emerged. Now there are chairs in geriatric medicine at many of the medical schools. Experimental gerontology came much later, typically scattered in a variety of medical school departments. There is only one chair in gerontology (in Tampere). Two major research undertakings have emerged in recent years, the Danish Centre for Molecular Gerontology, and a cluster of research groups at the Division of Geriatrics at the Karolinska Institutet. Other research groups are found in Denmark at the universities in Aarhus, Copenhagen and Odense; in Finland at the universities in Jyväskylä, Kuopio, Tampere and Turku; and in Norway at the university in Trondheim. These activities are reviewed country-wise.

Regulation of central synaptic transmission by 5-HT(1B) auto- and heteroreceptors

Although 5-HT(1B) receptors are believed to be expressed on nerve terminals, their precise mode of action is not fully understood because of the lack of selective antagonists. The 5-HT(1B) receptor knockout mouse was used in the present investigation to assess the function of 5-HT(1B) receptors in the modulation of synaptic transmission in three areas of the central nervous system: the dorsal raphe, the ventral midbrain, and the nucleus accumbens. N-(3-Trifluoromethylphenyl)piperazine, a 5-HT(1B) receptor agonist, potently inhibited 5-HT(1A) receptor-mediated slow inhibitory postsynaptic potentials (IPSPs) in the dorsal raphe of wild-type but not knockout mice. Both synaptically released 5-HT and exogenous 5-HT caused a presynaptic inhibition that outlasted the postsynaptic hyperpolarization only in wild-type mice. In the ventral midbrain, 5-HT(1B) receptor-dependent inhibition of gamma-aminobutyric acid(B) IPSPs in dopamine neurons was present in wild-type animals and absent in knockout animals. Similar results were obtained in the nucleus accumbens measuring glutamate-mediated excitatory postsynaptic currents in medium spiny neurons. Finally, cocaine, which blocks 5-HT uptake, inhibited IPSPs in the dorsal raphe and the ventral midbrain of wild-type but not knockout mice, whereas cocaine produced comparable inhibition of excitatory postsynaptic currents in the nucleus accumbens of both types of animals. These results indicate that 5-HT(1B) receptors function as autoreceptors and heteroreceptors to exert presynaptic inhibition of transmitter release in the central nervous system. Furthermore, this study underscores the role played by presynaptic 5-HT(1B) receptors in mediating the effects of cocaine on synaptic transmission.

An antidepressant-induced decrease in the responsiveness of hippocampal neurons to group I metabotropic glutamate receptor activation

Imipramine, a serotonin and noradrenaline uptake inhibitor, is the prototypical tricyclic antidepressant. The effects of imipramine on neuronal responsiveness to the group I glutamate metabotropic (mGlu) receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) were studied ex vivo, in the CA1 area of rat hippocampus, using extracellular and intracellular recording. DHPG increased the population spike amplitude, depolarized CA1 cells and decreased the slow afterhyperpolarization. Imipramine (20 microM) administered acutely in vitro did not change the effect of DHPG on population spikes. Repeated treatment with imipramine (10 mg/kg, twice daily, for 14 days) significantly attenuated the enhancing effect of DHPG (2.5 and 5 microM) on population spikes, as well as the DHPG-induced depolarization and the decrease in the slow afterhyperpolarization. Repeated treatment with imipramine had no effect on passive or active membrane properties of CA1 pyramidal cells. The results of the time-course experiment demonstrated that the imipramine-induced decrease in the responsiveness of CA1 cells to DHPG was apparent after a 7-day treatment; there was a further decrease after 14 days of treatment to a level which was not changed by longer (21-day) administration of imipramine. The attenuation of neuronal responsiveness to DHPG induced by a 14-day treatment was still detectable 7 days after imipramine withdrawal. It is concluded that repeated treatment with imipramine induces a decrease in the responsiveness of rat CA1 hippocampal neurons to group I mGlu receptor activation with a time course which correlates with the delayed onset of the therapeutic effect of antidepressants in humans. This suggests that alterations in mGlu receptors may contribute to antidepressant efficacy.

Ultrastructural localization of the serotonin transporter in limbic and motor compartments of the nucleus accumbens

Extracellular levels of serotonin [5-hydroxytryptamine (5-HT)] in the nucleus accumbens (NAc) can influence both cognitive and motor functions involving extensive connections with the frontal cortex. The 5-HT levels reflect vesicular release and plasmalemmal reuptake through the serotonin transporter (SERT). We used electron microscopic immunocytochemistry to determine the sites for SERT activation in the limbic shell and motor-associated core of the rat NAc. Of the SERT-immunoreactive profiles in each region, >90% were serotonergic axons and axon terminals; the remainder were nonserotonergic dendrites and glia. Axonal SERT immunogold labeling was seen mainly at nonsynaptic sites on plasma membranes and often near 5-HT-containing large dense core vesicles (DCVs). SERT-labeled axonal profiles were larger and had a higher numerical density in the shell versus the core but showed no regional differences in their content of SERT immunogold particles. In contrast, immunoreactive dendrites had a lower numerical density in the shell than in the core. SERT labeling in dendrites was localized to segments of plasma membrane near synaptic contacts from unlabeled terminals and/or dendritic appositions. Our results suggest that in the NAc (1) reuptake into serotonergic axons is most efficient after exocytotic release from DCVs, and (2) increased 5-HT release without concomitant increase in SERT expression in individual axons may contribute to higher extracellular levels of serotonin in the shell versus the core. These findings also indicate that SERT may play a minor substrate-dependent role in serotonin uptake or channel activity in selective nonserotonergic neurons and glia in the NAc.

5-HT1B receptor-mediated presynaptic inhibition of retinal input to the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain, and serotonin (5-HT) can modify the response of the SCN circadian oscillator to light. 5-HT1B receptor-mediated presynaptic inhibition has been proposed as one mechanism by which 5-HT modifies retinal input to the SCN (Pickard et al., 1996). This hypothesis was tested by examining the subcellular localization of 5-HT1B receptors in the mouse SCN using electron microscopic immunocytochemical analysis with 5-HT1B receptor antibodies and whole-cell patch-clamp recordings from SCN neurons in hamster hypothalamic slices. 5-HT1B receptor immunostaining was observed associated with the plasma membrane of retinal terminals in the SCN. 1-[3-(Trifluoromethyl)phenyl]-piperazine HCl (TFMPP), a 5-HT1B receptor agonist, reduced in a dose-related manner the amplitude of glutamatergic EPSCs evoked by stimulating selectively the optic nerve. Selective 5-HT1A or 5-HT7 receptor antagonists did not block this effect. Moreover, in cells demonstrating an evoked EPSC in response to optic nerve stimulation, TFMPP had no effect on the amplitude of inward currents generated by local application of glutamate. The effect of TFMPP on light-induced phase shifts was also examined using 5-HT1B receptor knock-out mice. TFMPP inhibited behavioral responses to light in wild-type mice but was ineffective in inhibiting light-induced phase shifts in 5-HT1B receptor knock-out mice. The results indicate that 5-HT can reduce retinal input to the circadian system by acting at presynaptic 5-HT1B receptors located on retinal axons in the SCN.