Human platelet 5-HT2 receptor binding sites re-evaluated: A criticism of recurrent techniques

Summary The human platelet 5-HT₂ receptor may resemble a peripheral model of central 5-HT₂ binding sites and has been linked to changes in 5-HT₂ receptor function in depression. Therefore, evaluation of the human platelet 5-HT₂ binding characteristics is important. Comparing [³H]ketanserin and [³H]LSD as ligands clearly indicated [³H]LSD as ligand of choice for binding studies dealing with the human platelet 5-HT₂ receptor. [³H]LSD binding was specific, saturable, and depended upon incubation time, protein concentration and previous handling of tissue, i.e., use of fresh or frozen tissue. In contrast, studies with [³H]ketanserin were unsatisfactory. Although mean receptor densities and affinities have been relatively constant between individuals and over time in healthy subjects with [³H]LSD, examination of the individual data showed considerable variations within single subjects. Thus, K_D ranged between 0.50 and 0.68 nM, and B_max was in the range of 64.9 to 97.1 fmol/mg protein in healthy individual subjects. Therefore, we recommend [³H]LSD as ligand of choice to study platelet 5-HT₂ receptor binding in humans. Furthermore, repeated measurement of individual data over time should be interpreted cautiously, especially when data from depressed patients are under examination.     

Selective heterologous regulation of 5-HT1A receptor-stimulated 35S GTPgammaS binding in the anterior cingulate cortex as a result of 5-HT2 receptor activation

Previous studies have shown that administration of the 5-HT(2) receptor agonist DOI to rats results in the heterologous desensitization of 5-HT(1A) receptor-mediated behavioral and neuroendocrine responses [Neuropsychopharmacology 19 (1998) 354; J. Neurosci. 21 (2001) 7919]. We hypothesized that the basis for these changes in 5-HT(1A) receptor function may involve changes in the capacity of the 5-HT(1A) receptor to activate G proteins. We examined the effect of chronic administration of DOI on the regulation of 5-HT(1A) receptor function at the level of receptor-G protein interaction using quantitative autoradiography of [(35)S]GTPgammaS binding stimulated by the 5-HT(1A) receptor agonist (+/-)8-OH-DPAT (1 microM). Repeated administration of DOI (1 mg/kg, s.c. once daily for 8 days) resulted in a marked down-regulation in 5-HT(2A) binding sites, as labeled by the antagonist radioligand [(3)H]ketanserin, throughout the cerebral cortex. Chronic DOI treatment also resulted in a significant and selective attenuation of 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in the anterior cingulate cortex (vehicle-treated: 74+/-7.7% above basal; DOI-treated: 43+/-4.6% above basal). Interestingly, 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding was not altered in the dorsal or median raphe, or in the limbic structures and other cortical regions examined. The decrease in 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in anterior cingulate cortex was not due to a decrease in 5-HT(1A) receptor number, indicating that the capacity of the 5-HT(1A) receptor to activate G proteins is attenuated in this cortical area following repeated DOI treatment. The heterologous regulation of 5-HT(1A) receptor function by chronic 5-HT(2) receptor activation in the anterior cingulate cortex raises interesting questions as to how the regulatory interaction between these serotonin receptor subtypes influences cognition, memory and emotion.     

Urethane reduces contraction to 5-hydroxytryptamine (5-HT) and enhances the action of the 5-HT antagonist ketanserin on the rat thoracic aortic ring

Summary The general anesthetic urethane (ethyl carbamate) is widely used in electrophysiological in vivo experiments. However, its pharmacological effects are poorly understood. Here, the effects of urethane on in vitro contractile responses of the rat thoracic aortic ring preparation were investigated. Bath application of 5-HT produced a concentration-dependent contractile response (EC₅₀=4.3 × 10^–6M). Urethane (11.2 mM=1mg/ml) shifted the concentration — response curve (CRC) for 5-HT to the right (EC₅₀=1.7 × 10^–5M) and decreased the maximal contraction by 30.8%. The CRC for NA (EC₅₀=7.2 × 10^–9M) was also shifted to the right by urethane (EC₅₀=1.4 × 10^–8M), but the shift of the 5-HT-CRC was twice that of the NA-CRC (3.95 vs. 1.95). The CRC to KCl was shifted rightwards only slightly by urethane (ratio 1.27) and the maximal contraction to KCl was not affected. The CRC to replacement of CaCl₂ (0.1 – 10 mM) to KCl-depolarized vessels in a Ca²⁺ -free Krebs solution was unaffected by urethane. Ketanserin (10^–9M) antagonized the contraction to 5-HT, and a combination of ketanserin and urethane was markedly more effective than either drug alone, decreasing the maximal contraction by 58%. Antagonism of NA contraction by prazosin (5×10^–8M) was not increased by addition of urethane.The urethane dose used here approximates blood and brain concentrations required to produce anesthetic effects in mammals. It is possible that reductions in 5-HT transmission and, to a lesser extent, in NA transmission, but not blockade of Ca²⁺ or K⁺ channels, may contribute to the anesthetic effect of urethane. In addition, the action of the selective 5-HT₂ antagonist ketanserin is clearly altered by urethane. These findings are important to consider when urethane is used for in vivo neurophysiological investigations, particularly when 5-HT mechanisms are involved.     

Tachyphylaxis to 5-HT3-receptor-mediated activation of vagal afferents is prevented by co-activation of 5-HT2 receptors

Functional studies have provided evidence that 5-HT(3) ion-channel receptors (5-HT(3)Rs) on vagal cardiopulmonary afferents mediating the Bezold-Jarisch reflex (BJR) rapidly desensitize upon repeated exposure to selective 5-HT(3)R agonists. G-protein-coupled 5-HT(2) receptors (5-HT(2)Rs) also exist on vagal afferents, although activation of these receptors does not elicit the BJR. However, there is in vivo evidence that 5-HT(2)Rs may regulate the activity of 5-HT(3)Rs. The aim of this study was to determine whether co-activation of 5-HT(2)Rs prevents desensitization of 5-HT(3)Rs mediating the BJR in conscious rats. The principal findings were that (1) tachyphylaxis rapidly developed to the BJR-mediated hemodynamic responses elicited by successive injections of 5-HT(3)R agonists and (2) co-injection of the selective 5-HT(2)R agonist, alpha-methyl-5-HT, prevented tachyphylaxis to the BJR-mediated hemodynamic responses elicited by the 5-HT(3)R agonists. Additional studies provided evidence that (1) tachyphylaxis to the 5-HT(3)R agonists was not due to impairment of the central or efferent processing of the BJR, and (2) the pressor responses elicited by alpha-methyl-5-HT were not responsible for preventing tachyphylaxis to the BJR reflex responses elicited by 5-HT(3)R agonists. These results suggest that the loss of response to 5-HT(3)R agonists is due to desensitization of 5-HT(3)Rs on vagal afferents mediating the BJR and that co-activation of 5-HT(2)Rs prevents the desensitization of these 5-HT(3)Rs.     

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. II. Serotonin-2 receptors

The distribution of serotonin-2 (5-HT2) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with four ligands: [3H]ketanserin, [3H]mesulergine, [3H]LSD and [3H]spiperone, which are reported to show high affinity for 5-HT2 receptors. Co-incubation with increasing concentrations of several well-known 5-HT2-selective drugs, such as pirenperone, cinanserin and ketanserin, resulted in an inhibition of the binding of the four 3H-labeled ligands to the same areas. However, all of them recognized, in addition to 5-HT2 sites, other populations of binding sites. Receptor densities were quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Our results reveal a heterogeneous distribution of 5-HT2 receptor densities in the rat brain. Very high concentrations were localized in the claustrum, olfactory tubercle and layer IV of the neocortex. The anterior olfactory nucleus, piriform cortex and layer I of neocortex were also rich in 5-HT2 receptors. Intermediate concentrations of receptors were found in caudate putamen, nucleus accumbens, layer V of neocortex, ventral dentate gyrus and mammillary bodies. Areas containing only low concentrations of receptors included the thalamus, hippocampus, brainstem, medulla, cerebellum and spinal cord. The specificity of the different ligands used is discussed in terms of the other populations of sites recognized by them. The distribution of 5-HT2 receptors here reported is discussed in correlation with (a) the known distribution of serotoninergic terminals, (b) the specific anatomical systems and (c) the central effects reported to be mediated by 5-HT2-selective drugs.     

The effects of manipulation of presynaptic 5-HT nerve terminals on postsynaptic 5-HT1 and 5-HT2 binding sites of the rat brain

Summary The effects of long-term treatment of rats with alaproclate and amiflamine on the number and kinetics of 5-HT₁ and 5-HT₂ binding sites were investigated usingin vitro receptor binding techniques. Some other studies have reported down-regulatory effects of alaproclate and amiflamine on 5-HT₂ binding sites in certain regions of the rat forebrain, but no such effects could be detected in the present study. Induction of a high-affinity binding site for³H-5-HT after long-term antidepressant treatment, as has been reported elsewhere, was not obtained in the present study. The results are compared to the effects obtained by treatment of rats with para-chloroarnphetamine (PCA), which depletes the presynaptic neurons of monoamines. These different types of treatment do not cause any change in the binding properties of the specific 5-HT binding sites. It is thus concluded that such manipulations of the presynaptic 5-HT neurons do not affect the postsynaptic 5-HT₁ and 5-HT₂ binding sites.     

Differential effects of the 5-HT2 receptor antagonist on the anti-immobility effects of noradrenaline and serotonin reuptake inhibitors in the forced swimming test

The effects of the 5-HT(2) receptor antagonist, LY 53857 on the effects of noradrenaline and serotonin reuptake inhibitors were investigated using the forced swimming test. LY 53857 enhanced anti-immobility effects of clomipramine and maprotiline, which can inhibit reuptake of noradrenaline. However, LY 53857 did not affect the immobility time of mice treated with the selective serotonin reuptake inhibitors (SSRIs) fluoxetine and fluvoxamine. These results suggest that antagonism of the 5-HT(2) receptor leads to potentiation of the antidepressant effects of noradrenaline reuptake inhibitors but not SSRIs and that LY 53857 may modify the activity of noradrenergic neurons.     

Acute selective serotonin reuptake inhibitors increase conditioned fear expression: blockade with a 5-HT(2C) receptor antagonist.

BACKGROUND: Selective serotonin reuptake inhibitors (SSRIs) effectively treat various anxiety disorders, although symptoms of anxiety are often exacerbated during early stages of treatment. We previously reported that acute treatment with the SSRI citalopram enhances the acquisition of auditory fear conditioning, which is consistent with the initial anxiogenic effects reported clinically. Here, we extend our findings by assessing the effects of acute SSRI treatment on the expression of previously acquired conditioned fear. METHODS: Rats underwent fear conditioning drug-free. Tone-evoked fear responses were tested after drug treatment the following day. This protocol more closely resembles the clinical setting than pre-conditioning treatment, because it evaluates effects of treatment on a pre-existing fear rather than on the formation of a new fear memory. RESULTS: A single pre-testing injection of the SSRIs citalopram or fluoxetine significantly increased fear expression. There was no effect of the antidepressant tianeptine or the norepinephrine reuptake inhibitor tomoxetine, indicating that this effect is specific to SSRIs. The SSRI-induced enhancement in fear expression was not blocked by tropisetron, a 5-HT(3) receptor antagonist, but was blocked by SB 242084, a specific 5-HT(2C) receptor antagonist. CONCLUSIONS: Enhanced activation of 5-HT(2C) receptors might be a mechanism for the anxiogenic effects of SSRIs observed initially during treatment.     

Imaging cortical dopamine D1 receptors using [11C]NNC112 and ketanserin blockade of the 5-HT2A receptors

[¹¹C]NNC112 (8-chloro-7-hydroxy-3-methyl-5-(7-benzofuranyl)-2,3,4,5-tetrahydro-IH-3-benzazepine), a selective positron-emission tomography (PET) ligand for the D₁ receptor (R) over the 5-HT_2A R in vitro, has shown lower selectivity in vivo, hampering measurement of D₁ R in the cortex. [¹¹C]NNC112 PET and intravenous (i.v) ketanserin challenge were used to (1) confirm the previous findings of [¹¹C]NNC112 in vivo D₁ R selectivity, and (2) develop a feasible methodology for imaging cortical D₁ R without contamination by 5-HT_2A R. Seven healthy volunteers underwent [¹¹C]NNC112 PET scans at baseline and after a 5-HT_2A R-blocking dose of ketanserin (0.15 mg/kg, i.v.). Percent BP_ND change between the post-ketanserin and baseline scans was calculated. Irrespective of the quantification method used, ketanserin pretreatment led to significant decrease of BP_ND in the cortical (∼30%) and limbic regions (∼20%) but not in the striatum, which contains a much lower amount of 5-HT_2A R. Therefore, ketanserin allows D₁ R signal to be detected by [¹¹C]NNC112 PET without significant 5-HT_2A R contamination. These data confirm the presence of a significant 5-HT_2A R contribution to cortical [¹¹C]NNC112 signal, and call for caution in the interpretation of published [¹¹C]NNC112 PET findings on cortical D₁ R in humans. In the absence of more selective ligands, [¹¹C]NNC112 PET with ketanserin can be used for cortical D₁ R imaging in vivo.     

Involvement of 5-HT2 receptors in the LSD- and L-5-HTP-induced suppression of lordotic behavior in the female rat

Summary Copulatory behavior in the ovariectomized rat, the lordotic response (L. R.), was induced by estrogen followed by progesterone. L. R. is inhibited by lysergic acid diethylamide (LSD) (0.05 mg/kg) and by Levo-5-hydroxytryptophan (L-5-HTP) (2.5 mg/kg). The effects of the putative 5-HT antagonists lisuride, metergoline, methysergide, mianserin, cinanserin, cyproheptadine, pirenperone and altanserin on the LSD-induced inhibition of L. R. were tested. Lisuride, metergoline, methysergide and mianserin were found to have no LSD-blocking effect. In contrast, cinanserin, cyproheptadine and pirenperone acted antagonistically to LSD, within a critical dose range. The selective 5-hydroxytryptamine₂ (5-HT₂) receptor antagonist altanserin effectively prevented the LSD-induced inhibition of L. R., and the doses required (0.05–0.20 mg/kg) indicated a comparatively high antagonistic potency. In addition altanserin (0.2 mg/kg) effectively prevented the lordosis inhibitory effect induced by L-5-HTP (2.5 mg/kg), after pretreatment with pargyline and RO4-4602. It is suggested that the suppression of copulatory behavior caused by LSD and L-5-HTP is mediated by 5-HT₂ receptors.     

Pre- and post-synaptic effects of the 5-HT3 agonist 2-Methyl-5-HT on the 5-HT system in the rat brain

Microiontophoretic applications of 5-HT and of the 5-HT₃ agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA₁ and CA₃) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT₃ antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT_1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT_1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT_1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA₃ pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT₃/5-HT₄ antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyld-HT on the electrically-evoked release of [³H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT₃ receptors, but rather by 5-HT_1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT₃ receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [³H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT₃ receptors. © 1995 Wiley-Liss, Inc.     

Characterization and quantitative autoradiography of [H]tryptamine binding sites in rat brain

[³H]tryptamine binds with high affinity (K_d = 9.1nM, B_max= 54fmol/mg wet wt.) to tissue sections of rat brain. The binding occurs rapidly and is reversible. Low concentrations of the β-carbolines harmaline (IC₅₀ = 25nM) and tetrahydronorharman (tetrahydro-β-carboline), IC₅₀ = 50nM) inhibit [³H]tryptamine binding. Serotonin (5-HT, IC₅₀ = 2600nM) as well as the 5-HT receptor antagonists methysergide and metergoline displace [³H]tryptamine at much higher concentrations from brain slices. The distribution of [³H]tryptamine binding sites in section of rat brain has been analyzed by quantitative autoradiography. The highest density of binding sites is found in the nucleus (n.) interpreduncularis, a slightly lower one in the locus coeruleus. Moderately labelled are the n. accumbens septi, n. septi lateralis, n. medalis habenulae, n. tractus olfactorii lateralis, the central region of the amydgala, n. caudatsu/putamen, n. reuniens and the hippocampal formation. A low density of binding sites is detected in the cerebral cortex and the subiculum. Even less binding sites are found in the n. dorsalis raphe and the substantia nigra. The pattern of distribution of [³H]tryptamine binding sites differs from that of [³H]5-HT(5-HT₁), [³H]ketanserin (5-HT₂) as well as [³H]imipramine binding sites. These data suggest unique tryptamine binding sites.     

Atypical in vitro and in vivo binding of [3H]S-14506 to brain 5-HT1A receptors

Summary The tritiated derivative of the potent 5-HT_1A receptor agonist S-14506 {1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)piperazine} was tested for its capacity to selectively label the serotonin 5-HT_1A receptors both in vitro in the rat and the mouse brain, and in vivo in the mouse. In vitro studies showed that the pharmacological profile and the distribution of [³H]S-14506 specific binding sites (Kd=0.15 nM) in different brain regions matched perfectly those of the prototypical 5-HT_1A receptor ligand [³H]8-OH-DPAT. However, in the three regions examined (hippocampus, septum, cerebral cortex), the density of [³H]S-14506 specific binding sites was significantly higher (+ 66–90%) than that found with [³H]8-OH-DPAT. Whereas the specific binding of [³H]8-OH-DPAT was markedly reduced by GTP and Gpp(NH)p and increased by Mn²⁺, that of [³H]S-14506 was essentially unaffected by these compounds. In addition, the alkylating agent N-ethylmaleimide was much less potent to inhibit the specific binding of [³H]S-14506 than that of [³H]8-OH-DPAT. Measurement of in vivo accumulation of tritium one hour after i.v. injection of [³H]S-14506 to mice revealed marked regional differences, with about 2.5 times more radioactivity in the hippocampus than in the cerebellum. Pretreatment with 5-HT_1A receptor ligands prevented tritium accumulation in the hippocampus but not in the cerebellum. Autoradiograms from brain sections of injected mice confirmed the specific in vivo labeling of 5-HT_1A receptors by [³H]S-14506, therefore suggesting further developments with derivatives of this molecule for positron emission tomography in vivo in man.     

5-HT1A and 5-HT2 receptors differentially regulate the excitability of 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro

We have used intracellular recording techniques to examine the effects of 5-hydroxytryptamine (5-HT, serotonin) on 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro. Bath-applied 5-HT (30–300 μM) had two opposing effects on the membrane excitability of these cells, reflecting the activation of distinct 5-HT receptor subtypes. As demonstrated previously in the rat, 5-HT evoked a hyperpolarization and inhibition of 5-HT neurones, which appeared to involve the activation of an inwardly rectifying K⁺ conductance. This hyperpolarizing response was blocked by the 5-HT_1A receptor-selective antagonist WAY-100635 (30–100 nM). In the presence of WAY-100635, 5-HT induced a previously unreported depolarizing, excitatory response of these cells, which was often associated with an increase in the apparent input resistance of the neurone, likely due to the suppression of a K⁺ conductance. Like the hyperpolarizing response to 5-HT, this depolarization could be recorded in the presence of the Na⁺ channel blocker tetrodotoxin. In addition, the response was not significantly attenuated by the α₁-adrenoceptor antagonist prazosin (500 nM), indicating that it is not due to the release of noradrenaline, or to the direct activation of α₁-adrenoceptors by 5-HT. The 5-HT₃ receptor antagonist granisetron (1 μM) and the 5-HT₄ receptor antagonist SB 204070 (100 nM) failed to reduce the depolarizing response to 5-HT; however, ketanserin (100 nM), mesulergine (100 nM) and lysergic acid diethylamide (1 μM) significantly reduced or abolished the depolarization, indicating that this effect of 5-HT is mediated by 5-HT₂ receptors.     

Role of 5-HT3 and 5-HT2C receptors located within the medial amygdala in the control of salt intake in sodium-depleted rats

In the present study, we investigated the role of 5-HT(3) and 5-HT(2C) receptors located within the medial amygdala (MeA) in the control of water and salt intake in sodium-depleted rats. Pharmacological activation of 5-HT(3) receptors located in the medial amygdala by the selective 5-HT(3) receptor agonist m-CPBG significantly reduced salt intake in sodium-depleted rats, an effect that is reverted by pretreatment with the selective 5-HT(3) receptor antagonist ondansetron. In addition, the injection of ondansetron alone into the medial amygdala had no effect on salt intake in sodium-depleted and in sodium-repleted rats. Pharmacological stimulation of 5-HT(2C) receptors located in the medial amygdala by the selective 5-HT(2C) receptor agonist m-CPP failed to modify salt intake in sodium-depleted rats, whereas the blockade of these receptors by the selective 5-HT(2C) receptor antagonist SDZ SER 082 significantly reduced salt intake in this same group of animals. These results lead to the conclusion that the pharmacological activation of 5-HT(3) receptors located within the MeA inhibits salt intake in sodium-depleted rats and that, in this same brain region, the functional integrity of 5-HT(2C) receptors is required to achieve the full expression of sodium appetite in sodium-depleted rats.     

[3H]Paroxetine and [3H]citalopram as markers of the human brain 5-HT uptake site: a comparison study

Summary The binding of [3H]paroxetine and [3H]citalopram to the human brain serotonin (5-HT) uptake site has been characterized and compared. Our results reveal that the binding exclusively involved with the 5-HT uptake site is identical for both [3H]ligands. The selective 5-HT uptake inhibitor citalopram displays the highest affinity for this uptake site, as compared with the affinities obtained for desipramine and norzimeldine, which is in accordance with their respective blockage of 5-HT uptake. Similar Bmax values were obtained for both radioligands in the brain regions studied, indicating their binding to the same presynaptic membrane protein. Together these findings suggest that both [3H]paroxetine and [3H]citalopram are good markers of the 5-HT transporter as both bind selectively and with high affinity to the serotonin uptake sites. However, the higher affinity of [3H]paroxetine confirms that this compound is the best radioligand for the 5-HT uptake site available today.     

5-HT agonist-induced phase-advances of the circadian pacemaker are diminished by chronic antidepressant drug treatment

Serotonin (5-HT) and its agonists alter the timing of the circadian pacemaker. Previous research has shown that when they are injected 4 h before or after the onset of wheel-running, they phase-advance or delay, respectively, the timing of the pacemaker. Because serotonergic interventions alter 5-HT receptor number in the hypothalamus, we asked whether chronic treatment with an antidepressant drug (AD) that modifies serotonergic function could alter the phase-shifting effects of the 5-HT agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT). Hamsters were treated chronically with the monoamine oxidase inhibitor (MAOI), clorgyline, and then injected with 8-OH-DPAT or vehicle (VEH) either 4 h before or after the onset of wheel-running. MAOI treatment decreased the magnitude of both 8-OH-DPAT- and VEH-induced phase advances, but not the magnitude of 8-OH-DPAT-induced phase-delays. The results indicate that 8-OH-DPAT-induced phase-advances and delays are functionally distinct with regard to adaptive changes during chronic AD treatment.     

Characterization of specific binding sites labeled by [H]LSD in coronal sections of paraformaldehyde-fixed rat brain

Specific, high-affinity binding of [³H]lysergic acid diethylamide (LSD) in coronal sections of paraformaldehyde-fixed rat brain is described. Intracardiac perfusion of paraformaldehyde selectively altered 40% of the total binding sites normally labeled by [³H]LSD in unfixed sections. Competition by unlabeled LSD and serotonin (5-HT) was not altered by the fixation procedure. Competition by spiperone, however, revealed that the fixation procedure preferentially altered sites for which spiperone has high affinity. This technique should facilitate the combination of neuroanatomical techniques such as radioautography and immunocytochemistry.     

5-HT2C receptors inhibit and 5-HT1A receptors activate the generation of spike-wave discharges in a genetic rat model of absence epilepsy

The present study was conducted to investigate the role of 5-HT(2C) and 5-HT(1A) receptors in the generation of spike-wave discharges (SWD) in the genetic absence epilepsy model Wistar Albino Glaxo rats from Rijswijk, Netherlands (WAG/Rij rats). We have determined the effects of the 5-HT(2C) receptor preferring agonist m-chlorophenyl-piperazine (m-CPP), the selective 5-HT(2C) receptor antagonist SB-242084, the selective 5-HT(1A) receptor antagonist WAY-100635, two selective serotonin re-uptake inhibitors (SSRI, fluoxetine and citalopram) and their combinations in this model. The 5-HT(2C) agonist m-CPP caused marked, dose-dependent decreases in the cumulative duration and number of SWD administered either intraperitoneally (0.9 and 2.5 mg/kg) or intracerebroventricularly (0.05 and 0.1 mg/kg). Treatment with SB-242084 (0.2 mg/kg, ip) alone failed to cause any significant change in SWD compared to vehicle. Pretreatment with SB-242084 (0.2 mg/kg, ip) eliminated the effects of m-CPP on SWD. Fluoxetine (5.0 mg/kg, ip) alone caused moderate increase in SWD. After pretreatment with SB-242084, the effect of fluoxetine was significantly enhanced. The combination of SB-242084 and citalopram (2.5 mg/kg, ip) caused a similar effect, namely an increase in SWD. In contrast, pretreatment with WAY-100635 significantly attenuated the effect of fluoxetine. In conclusion, these results indicate that the increase in endogenous 5-HT produces a dual effect on SWD; the inhibition of epileptiform activity is mediated by 5-HT(2C), the activation by 5-HT(1A) receptors.