Effects of local application of 5-HT and 8-OH-DPAT into the dorsal and median raphe nuclei on core temperature in the rat

The core temperature of male Wistar rats was measured after local application of 5-HT (10 µg) or 8-OH-DPAT (5 µg) into the dorsal (DR) or the median raphe (MR) nuclei. The core temperature was measured by a rectal thermistor probe, 20 and 60 min after the injection procedure started. The injected volume was 0.5 µl and injections were made by means of 31G needles, at a rate of 0.33 µl min^–1. The raphe nuclei were approached at 30° in order to avoid penetration of the cerebral aqueduct or to avoid the DR with injections aimed for the MR. The application of 5-HT or the 5-HT_1A agonist 8-OH-DPAT into the DR produced a marked decrease in core temperature, whereas injections into the MR had no effect. These results demonstrate an important role for the DR in temperature regulation in the rat. The fact that the 5-HT_1A agonist 8-OH-DPAT produced a decrease in core temperature, together with the observation that administration of the 5-HT₁ antagonist (–)pindolol antagonized the 5-HT as well as the 8-OH-DPAT-induced decrease, indicates the involvement of DR 5-HT_1A receptors in rat thermoregulation.     

On the elevated plus-maze the anxiolytic-like effects of the 5-HT1A agonist, 8-OH-DPAT, but not the anxiogenic-like effects of the 5-HT1A partial agonist, buspirone, are blocked by the 5-HT1A antagonist, WAY 100635

 Nicotine has been shown to maintain intravenous self-administration behaviour in humans and laboratory animals. However, factors critical in the initiation of nicotine self administration are not well defined. In particular genetic differences and effects of pre-exposure to nicotine have not been examined. Male Sprague-Dawley or Long-Evans rats were surgically prepared with indwelling jugular catheters and 3 days later received chronic injections of nicotine (0.4 mg/kg SC) or vehicle (saline, 1 ml/kg) for 7 days in their home cage. The next day, 2-h daily test sessions were initiated, during which rats were given the opportunity to nose-poke for nicotine infusions (0.015, 0.03 or 0.06 mg/kg per infusion) under a one-response fixed-ratio (FR-1) schedule of reinforcement with a 20-s time out after each infusion. One hole was defined as active while pokes in the other hole were recorded but had no scheduled consequence. The response requirement was increased progressively to five (FR-5) over successive sessions. Both saline- and nicotine-pretreated Sprague-Dawley rats showed a preference for the active hole, while only the saline-pretreated Long-Evans rats acquired the self-administration as defined by significant differences between responding in the active versus the inactive holes. The Fisher (F344) and Lewis inbred strains also failed to acquire self-administration of nicotine under these conditions. With Sprague-Dawley and Long-Evans rats that acquired the self-administration, and showed stable levels of maintained responding for nicotine, substituting saline for the nicotine or pretreating with mecamylamine (2.0 mg/kg SC) extinguished the behaviour. When dose per infusion was varied, an inverted U-shaped dose-response curve was obtained. These results support previous reports that nicotine can serve as a reinforcer in rodents and demonstrate that environmental factors such as prior nicotine exposure or genetic factors such as rat strain can affect acquisition of nicotine self-administration. Received: 29 March 1996 / Final version: 20 August 1996     

The 5-HT1A receptor agonist, 8-OH-DPAT,preferentially activates cell body 5-HT autoreceptors in rat brain in vivo

Summary The present study was undertaken in an attempt to assess whether the effects of the potent and selective 5HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin, 8-OH-DPAT, on cerebral 5-hydroxytryptamine (5-HT) neurochemistry in vivo are mediated via 5-HT autoreceptors on the cell bodies or on the terminals, and/or via postsynaptic 5-HT receptors. To this end we determined in vivo indices of 5-HT synthesis and release/turnover rates in a number of prominent 5-HT neuronal projection areas in the CNS i) after systemic administration of 8-OH-DPAT to rats with an acute unilateral axotomy of the ascending mesencephalic monoamine neurones, or ii) after local infusion of the compound into the dorsal raphé (DRN) 5-HT cell body region of intact rats. Transection did not alter 5-HT synthesis per se, but prevented the synthesis-inhibitory effect of 8-OH-DPAT. Thus, the 5-HT synthesis-inhibiting action of 8-OH-DPAT is highly dependent upon intact impulse flow in the central 5-HT neurones. On the other hand, local DRN application of the compound (1 g) resulted in a clearcut reduction of the 5-HT synthesis and release indices measured in 5-HT terminals in, e. g., the striatum. These findings provide direct neurochemical evidence that by preferentially stimulating somatodendritic 5-HT_1A receptors, 8-OH-DPAT inhibits the 5-HT neuronal impulse flow, thereby effectuating decreased terminal 5-HT synthesis and release. Taken together, the data are consistent with the suggestion that 8-OH-DPAT acts as an agonist preferentially at cell body vs. terminal 5-HT autoreceptors, therefore also emphasizing the distinction between terminal and cell body 5-HT autoreceptors. The results obtained may have important implications for the understanding of mechanisms involved in regulating the activity of central serotoninergic neurones.Part of these data were presented at the 6th European Winter Conference on Brain Research, Avoriaz, France, March 9–15, 1985, and at the 18th Annual Meeting, Society for Neuroscience, Washington (DC), USA, Nov. 9–15, 1986 (Hjorth et al. 1986, 1987). Send offprint requests to S. Hjorth at the above address     

Strain differences in the response to the 5-HT1A receptor agonist, 8-OH-DPAT

Fischer and Sprague-Dawley ovariectomized rats were hormonally primed with estradiol benzoate (EB) and progesterone, and the ability of the 5-HT(1A) receptor agonist, (+/-) 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), to inhibit lordosis behavior was examined. Both strains showed rapid inhibition of lordosis behavior following either intraperitoneal or subcutaneous treatment with 8-OH-DPAT. Similarly, in both strains, pretreatment with EB (1 week prior to estrogen and progesterone priming) attenuated the lordosis-inhibiting effects of 8-OH-DPAT. However, Sprague-Dawley females showed a greater decline in lordosis behavior with a lower dose of 8-OH-DPAT than did Fischer females. The strain difference was present in females that had been preprimed with EB as well as in females receiving a single estrogen and progesterone priming. Moreover, strain differences were present across different priming doses of EB. Sprague-Dawley females were also more likely to show flat body posture after injection with 8-OH-DPAT so that the greater sensitivity of this strain to the 5-HT(1A) receptor agonist was not restricted to the drug's effect on lordosis behavior. These findings lead to the suggestion that, relative to Fischer rats, Sprague-Dawley females are more responsive to the 5-HT(1A) receptor agonist. Possible explanations for this strain difference are discussed.     

Effects of 5-HT1A-receptor agonist, 8-OH-DPAT, and GABAB-receptor agonist, baclofen, on lordosis in female rats with lesions in either the dorsal raphe nucleus or septum

The inhibitory effects of 8-OH-DPAT, a 5-HT1A-receptor agonist, and baclofen, a GABAB-receptor agonist, on lordosis were examined in estrogen and progesterone-treated ovariectomized rats with lesions in either the dorsal raphe nucleus (DRN) or septum and in rats with either sham lesions or no lesions. The first behavior test series was carried out 6 days after implantation of the rats with silicon tubes containing estradiol. Four hours after injection with 0.5 mg progesterone, behavioral tests were performed before and 30 min after an injection with 1 mg/kg body weight 8-OH-DPAT. As a result, the mean lordosis quotient (LQ)s were changed from 100 to less than 20 before and after the injection in all groups. These results suggest that 8-OH-DPAT acts on areas other than the DRN and the septum, leading to a decrease in lordosis. Two weeks after implantation with estradiol, the next behavioral test series was carried out after injection with progesterone. Behavioral tests were performed before and after an injection with 10 mg baclofen. The results showed that the mean LQs decreased after the injection in all groups, but the mean LQ in the DRN lesion group was higher than that in the sham groups. These results indicate that baclofen may act partially on the DRN in inhibiting lordosis in female rats.     

8-OH-DPAT increases corticosterone but not other 5-HT1A receptor-dependent responses more in females

The 5-HT1A receptor subtype agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (50-1000 micrograms/kg s.c.) dose dependently increased rat plasma corticosterone. Tube restraint for 30 min also increased plasma corticosterone; this effect was completely blocked by (-)-pindolol (1 mg/kg i.p.). Increases of corticosterone following either 8-OH-DPAT injection or restraint were significantly greater in female animals. The restraint stress-induced changes but not those due to 8-OH-DPAT were decreased by pretreatment with the tranquiliser chlordiazepoxide (10 mg/kg i.p.). In anaesthetized rats, restraint no longer significantly affected corticosterone levels but 8-OH-DPAT caused increases which (though much attenuated) were significantly greater in the females. Dose-dependent increases of plasma corticosterone also resulted on infusing 8-OH-DPAT (500-1500 ng) into the paraventricular nucleus of the hypothalamus; increases were significantly greater in the females. As mentioned in the discussion, these results may be relevant to the greater incidents of depression in women and the possible role of adrenal corticoids in the illness.     

Effect of the 5-HT1A agonist, 8-OH-DPAT on instrumental performance in rats

These experiments assessed whether reported increases in food consumption and food-reinforced instrumental performance in undeprived rats by the 5-HT_1A agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) are due to an increment in the incentive value of foods. Against this hypothesis, we found that when undeprived rats were trained to lever press for the food pellets and then allowed to consume the pellets under 8-OH-DPAT, this reexposure decreased subsequent instrumental extinction performance regardless of test drug condition relative to reexposure under vehicle. Although both food consumption and reinforced lever press performance were incremented, 8-OH-DPAT was found generally to reduce instrumental extinction performance and lever pressing during a period when the reinforcer was delivered non-contingently. Rats injected with 8-OH-DPAT were, however, more sensitive to delay of reinforcement, and increased their lever press performance at a 3-s delay but decreased performance at 6-s and 12-s delays relative to animals injected with vehicle. These results are consistent with the hypothesis that 8-OH-DPAT modifies arousal processes in a manner similar to mild stress, thereby acting both to elevate rewarded instrumental performance and to increase sensitivity to the effects of non-reward.     

Estradiol modulation of the hyperphagia induced by the 5-HT1A agonist, 8-OH-DPAT.

Ovariectomized rats were primed with sesame oil or estradiol benzoate followed 48 h later by either sesame oil or progesterone. Four hours later, rats were treated with either saline or 0.25 mg/kg 8-hydroxy-2-9(di-n-propylamino)tetralin (8-OH-DPAT). Rats were allowed to eat for 4 h after this final treatment. Animals in all hormonal conditions showed hyperphagia following 8-OH-DPAT. However, the hyperphagia was significantly attenuated by pretreatment with estradiol benzoate. There was no effect of progesterone on the hyperphagic response. These results suggest that previous findings of an estrous cycle modulation of the hyperphagic response to 8-OH-DPAT arise from the modulatory effects of estradiol, and not progesterone, during the female reproductive cycle.     

The 5-HT1A agonist 8-OH-DPAT attenuates the satiating action of cholecystokinin.

To investigate the dependence of the satiating action of cholecystokinin (CCK) on serotonergic action at central 5-HT receptors, we examined the effect of systemic pretreatment with 8-OH-DPAT (a 5-HT1A agonist that decreases central 5-HT synthesis and release via an action at somatodendritic autoreceptors in the brainstem raphe) on the suppression of food intake induced by systemic administration of cholecystokinin octapeptide (CKK-8). 8-OH-DPAT significantly attenuated the satiating action of CKK-8. This result is consistent with the hypothesis that peripherally acting CCK recruits central serotonergic processes to elicit normal satiety.     

Selective 5-HT1A receptor agonist, 8-OH-DPAT, locally administered into the dorsal raphe nucleus increased extracellular acetylcholine concentrations in the medial prefrontal cortex of conscious rats.

The effects of a selective 5-hydroxytryptamine (5-HT)1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), directly administered into the dorsal raphe nucleus (DR) on acetylcholine (ACh) release in the medial prefrontal cortex (mPFC) of freely moving rats were investigated by using a microdialysis technique. 8-OH-DPAT (1.0 and 5.0 micrograms) administered into DR significantly increased extracellular ACh concentrations in mPFC in a dose-dependent manner with a maximal increase to 215% and 237% of basal level, respectively, whereas a 0.1 microgram dose of this drug failed to exert such an increase. The present study suggests that the stimulation of somatodendritic 5-HT1A autoreceptors in DR is involved in an enhancement in ACh release in mPFC.     

The 5-HT1A agonist 8-OH-DPAT increases consumption of palatable wet mash and liquid diets in the rat

The 5-HT_1A agonist 8-OH-DPAT, at a dose of 30 g/kg, enhanced the consumption of sweetened wet mash and sweetened milk in non-deprived rats. In partially satiated rats, sensitivity to the hyperphagic effect of 8-OH-DPAT on wet mash intake was substantially increased, so that the minimally effective dose was reduced to 3 g/kg. Similarly, 8-OH-DPAT was more efficacious in increasing milk intake in satiated rats. Thus, 30 and 40 g/kg 8-OH-DPAT produced a 4-fold increase of milk intake in completely satiated rats compared to a 2-fold increase in partially satiated animals at a dose of 30 g/kg. The increased intake of liquid and wet mash diets observed after treatment with low doses of 8-OH-DPAT argues against the involvement of non-specific gnawing in the increased consumption of solid food produced by the drug. Rather, the data suggest that 8-OH-DPAT may specifically stimulate appetite by counteracting a tonic serotonergic inhibition of feeding. The present experiments also showed that 8-OH-DPAT attenuates fenfluramine-induced anorexia which is thought to depend on increased serotonergic neurotransmission.     

Dimerization of 8-OH-DPAT increases activity at serotonin 5-HT1A receptors

[³⁵S]GTPγS binding responses can be used to measure differences between the intrinsic activity of ligands at human 5-hydroxytrypamine-_1A (h 5-HT_1A) receptors expressed in recombinant cell lines. The maximal [³⁵S]GTPγS binding response to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was lower than that to 5-HT in a recombinant C6-glial membrane preparation and dependent on the GDP concentration: it was attenuated by about 60% vs 5-HT by increasing the concentration of GDP from 0.3 to 30 and 300 μM. Whereas dimerization of 8-OH-DPAT almost did not affect its potency at h 5-HT_1A receptors (pEC₅₀: 7.45 and 7.40 for 8-OH-DPAT and its dimer at 30 μM GDP), it increased efficacy at h 5-HT_1A receptors. The maximal response to the 8-OH-DPAT dimer was systematically greater than the response to 8-OH-DPAT and identical to that to 5-HT; moreover in contrast to the 8-OH-DPAT monomer, the maximal response to the dimer was unaffected by increasing the GDP concentration. An enhanced [³⁵S]GTPγS binding response (44 to 63% vs 8-OH-DPAT) was also observed in the hippocampus, lateral septum, dorsal raphe and cingulate cortex of guinea-pig brain sections using autoradiography of 5-HT_1A receptor-activated G-proteins. Hence, the 8-OH-DPAT dimer shows increased efficacy at 5-HT_1A receptors compared to 8-OH-DPAT. The differential regulation of the maximal agonist responses by GDP suggests that the [³⁵S]GTPγS binding responses to these two ligands could be mediated by different G-protein subtypes upon activation of the 5-HT_1A receptor. Received: 23 June 1998 / Accepted: 29 July 1998     

Estrogen modulates 5-HT(1A) agonist inhibition of lordosis behavior but not binding of [(3)H]-8-OH-DPAT

Previous studies showed that repeated estrogen treatment reduces the ability of the 5-HT(1A) receptor agonist, 8-hydroxy-2(di-n-propylamino) tetralin (8-OH-DPAT), to inhibit lordosis behavior of female rats. The present study evaluated the effects of repeated estrogen treatment on lordosis behavior and 5-HT(1A) receptor binding and coupling to G protein in the hypothalamus-preoptic area using the agonist ligand [3H]-8-OH-DPAT, which binds selectively to G-protein-coupled 5-HT(1A) receptors. Rats were injected twice with 25 or 50 microg of estradiol benzoate (EB) 7 days apart followed by 500 microg of progesterone (P) 48 h after the second EB injection. Controls received a single injection of 25 or 50 microg EB followed 48 h later by 500 microg of P. Four hours after P, 0.15 mg/kg 8-OH-DPAT was injected, and lordosis behavior examined for 30 min. Rats treated twice with EB showed significantly less 8-OH-DPAT inhibition of lordosis behavior than rats receiving a single EB injection. For receptor binding, rats received EB without P treatment. None of the estrogen treatments reduced [3H]-8-OH-DPAT binding density or affinity in the hypothalamus-preoptic area or hippocampus. These studies suggest that estrogen modulates 5-HT(1A) agonist potency without a measurable change in 5-HT(1A) receptor density or coupling to G protein.     

Suppression of lordosis behavior by the putative 5-HT receptor agonist 8-OH-DPAT in the rat

The administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), inhibited the lordosis induced by estradiol benzoate or estradiol benzoate plus progesterone in ovariectomized rats. There was no facilitation of lordosis by 8-OH-DPAT in animals pretreated with a threshold dose of estradiol benzoate. The results are consistent with the view that 8-OH-DPAT is an agonist at 5-HT receptors and provide further support for an inhibition role of central 5-HT in the mediation of lordosis behavior.     

The effect of the 5-HT1A receptor agonist, 8-OH-DPAT, on motion-induced emesis in Suncus murinus

In the present study we evaluated the role of 5-HT(1A) receptors in mediating the inhibitory action of 8-OH-DPAT, a 5-HT(1A) receptor agonist, in motion sickness in Suncus murinus. 8-OH-DPAT (0.1 mg/kg, i. p) attenuated motion-induced emesis which was associated with an increase in the latency of the onset to the first emetic episode. Pre-treatment with methysergide (a 5-HT(1/2/7) receptor antagonist, 1.0 mg/kg, i. p.), WAY-100635 (a 5-HT(1A) receptor antagonist, 1.0 mg/kg, i. p.), SB269970A (a 5-HT(7) receptor antagonist, 1.0 and 5.0 mg/kg, i. p.), ondansetron (a 5-HT(3) receptor antagonist, 1.0 mg/kg, i. p) or GR13808 (a 5-HT(4) receptor antagonist, 0.5 mg/kg, i. p) failed to modify the inhibitory action of 8-OH-DPAT on motion sickness. Furthermore, the application of either methysergide, WAY-100635, SB269970A, ondansetron or GR13808 alone had no effect on motion sickness in its own right. These data indicate that neither 5-HT(1A) nor any 5-HT(2) receptor subtypes, 5-HT(3), 5-HT(4) and 5-HT(7) receptors are likely to be involved in the inhibition of motion-induced emesis mediated by 8-OH-DPAT.     

Comparison of the cardiovascular effects of the 5-HT1A receptor agonist flesinoxan with that of 8-OH-DPAT in the rat

The cardiovascular response to flesinoxan and 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino)tetralin), 5-HT_1A receptor agonists, has been investigated in anaesthetized Wistar rats and spontaneously hypertensive rats (SHR) and in conscious SHR. Flesinoxan and 8-OH-DPAT potently lowered blood pressure and heart rate in these models. In conscious SHR, atropine reversed the bradycardia induced by flesinoxan partially and that induced by 8-OH-DPAT completely. In pithed rats with vasopressin-raised blood pressure, neither flesinoxan nor 8-OH-DPAT lowered blood pressure or heart rate. Intracisternal administration of either flesinoxanor 8-OH-DPAT was less efficacious than intravenous administration. The cardiovascular responses to flesinoxan and 8-OH-DPAT in the anaesthetized Wistar were inhibited by the putative 5-HT_1A antagonists methiothepin, buspirone, spiroxatrine and 8-MeO-C1EPAT (8-methoxy-2-(N-2-chloroethyl-N-n-propylamino)tetralin). 8-MeO-C1EPAT appeared to be the most suitable antagonist in this model. The 5-HT_1C, 5-HT₂ antagonist ritanserin or the 5-HT₃ antagonist GR 38032F had no effect on the responses to flesinoxan or 8-OH-DPAT. In conscious SHR however, 8-MeO-C1EPAT did not antagonize these cardiovascular responses. This study confirms the involvement of central 5-HT_1A receptors in the cardiovascular effects of flesinoxan and 8-OH-DPAT.     

A single dose of 8-OH-DPAT reduces raphe binding of [3H]8-OH-DPAT and increases the effect of raphe stimulation on 5-HT metabolism.

8-Hydroxy-(di-n-propylamino)tetralin (8-OH-DPAT) has antidepressant-like effects in rats and selectively reduces presynaptic 5-HT1A function a day after administration. In the present study, the effect of 8-OH-DPAT (1 mg/kg s.c.) pretreatment on presynaptic (raphe nuclei) and postsynaptic (frontal cortex and hippocampus) [3H]8-OH-DPAT binding was studied. Bmax values were markedly reduced in the raphe, but not in the hippocampus and frontal cortex. Kd values were unchanged. Electrical stimulation of the dorsal raphe (300 microA, 1 ms, 20 Hz, 30 min) significantly increased 5-hydroxyindoleacetic acid in the frontal cortex, but not in the amygdala or the nucleus accumbens and caused smaller increases in the rest of the brain. The increase in the frontal cortex was significantly potentiated one day after giving 8-OH-DPAT. These results confirm the ability of 8-OH-DPAT to desensitise presynaptic 5-HT1A receptors and suggest that this may lead to a loss of feedback control so that, on neuronal stimulation, the increase of 5-HT function is enhanced. This effect may underlie the antidepressant-like action of 8-OH-DPAT pretreatment, i.e. its ability to oppose restraint-induced defects in locomotion on placement in an open field one day later. A requirement of presynaptic 5-HT for this behavioural effect is consistent with its prevention by the 5-HT synthesis inhibitor parachlorophenylalanine.     

Gender and estrous cycle differences in the response to the 5-HT1A agonist 8-OH-DPAT.

The effects of the 5-HT_1A agonist, 8-hydroxy-2-9(di-n-propylamino)tetralin (8-OH-DPAT) on eating behavior and on rectal temperature were examined in adult male rats and in diestrous, proestrous, and estrous female rats. The 5-HT_1A agonist produced evidence of hyperphagia at some dose (0.125, 0.25, 0.5 and 1.0 mg/kg) in all groups examined. However, hyperphagia was most evident in diestrous females and least evident in proestrous and estrous rats. These findings are interpreted as an estrous cycle modulation of somatodendritic 5-HT_1A autoreceptors. The hypothermic response to 8-OH-DPAT was present in all females and at all doses of 8-OH-DPAT (0.1, 0.25 and 0.5 mg/kg). These findings suggest that postsynaptic 5-HT_1A sites involved in 8-OH-DPAT-induced hypothermia do not vary during the estrous cycle. However, males showed less hypothermia following 8-OH-DPAT than did females. The gender difference was evidenced primarily as a slower onset of hypothermia in males treated with the lower doses of the drug.     

Nicotine withdrawal leads to increased sensitivity of serotonergic neurons to the 5-HT1A agonist 8-OH-DPAT

In order to explore the neurophysiology of nicotine withdrawal, we examined the activity of serotonergic neurons in the dorsal raphe nucleus in rats undergoing withdrawal from chronic exposure to nicotine. Animals were exposed to nicotine (6 mg/kg per day base) via SC implanted osmotic minipumps. After 12 days the pumps were removed and the animals allowed to go through spontaneous withdrawal. Rats were anesthetized on various days of the procedure and the effect of the 5-hydroxytryptamine (5-HT)-1A agonist 8-OH-DPAT on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus was examined. The sensitivity of serotonergic neurons to 8-OH-DPAT was not changed by the chronic administration of nicotine or saline; slightly increased on day 2 of withdrawal; significantly increased on days 3 and 4 of withdrawal; and no longer significantly increased by day 7 of withdrawal. These results indicate that serotonergic neurons in the dorsal raphe nucleus have an increased sensitivity to systemically administered 8-OH-DPAT in rats undergoing nicotine withdrawal and that the serotonergic system may play a role in the symptoms of nicotine withdrawal. Received: 13 September 1996/Final version: 29 April 1997