Electrophysiological evidence for the tonic activation of 5-HT(1A) autoreceptors in the rat dorsal raphe nucleus.

Serotonin (5-hydroxytryptamine, 5-HT) and norepinephine (NE) neurons have reciprocal connections. These may thus interfere with anticipated effects of selective pharmacological agents targeting these neurons. The main goal of the present study was to assess whether the somatodendritic 5-HT(1A) autoreceptor is tonically activated by endogenous 5-HT in anesthetised rats, using in vivo extracellular unitary recordings. In rats with their NE neurons lesioned using 6-hydroxydopamine (6-OHDA) and in controls administered the NE reuptake inhibitor desipramine to suppress NE neuronal firing, the alpha2-adrenoceptor agonist clonidine no longer inhibited 5-HT neuron firing, therefore indicating the important modulation of the firing activity of 5-HT neurons by NE neurons. In control rats, the administration of the potent and selective 5-HT(1A) receptor antagonist WAY 100,635 ((N-[2-[4(2-methoxyphenyl)-1-piperazinyl]ethy]-N-(2-pyridinyl)cyclohexanecarboxamide trihydroxychloride) (100 microg/kg, i.v.) did not modify the spontaneous firing activity of 5-HT neurons, but in NE-lesioned rats using either 6-OHDA or DSP-4, WAY 100,635 produced a mean firing increase of 80 and 69%, respectively. When desipramine and D-amphetamine were used in control rats to prevent alterations in the availability of NE in the dorsal raphe, again WAY 100,635 produced a significant disinhibition of the firing of 5-HT neurons (83 and 53%, respectively). These data support the notion that the NE system tonically activates the firing activity of 5-HT neurons. When the fluctuations of the function of NE neurons normally produced by WAY 100,635 were prevented, a tonic activation of 5-HT(1A) autoreceptors by endogenous 5-HT was unmasked.     

Electrophysiological evidence that spiperone is an antagonist of 5-HT1A receptors in the dorsal raphe nucleus

The neuroleptic spiperone, which binds to 5-HT1A, 5-HT2 and dopamine (DA) receptors, was studied for its effects on serotonin (5-HT) and DA neurons in dorsal raphe nucleus and substantia nigra pars compacta, respectively. We found that 1 mg/kg i.v. spiperone, but not LY53837 (a 5-HT2 antagonist), antagonized the inhibition induced by 5-HT1A agonists 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and buspirone in the dorsal raphe nucleus. Lower spiperone doses blocked DA receptors in substantia nigra pars compacta, but did not affect 5-HT neurons. Doses of 8-OH-DPAT completely silencing dorsal raphe neurons were ineffective in substantia nigra pars compacta. However, buspirone antagonized DA receptors in substantia nigra pars compacta with doses similar to those depressing dorsal raphe neurons. It is concluded that spiperone is an antagonist of 5-HT1A receptors in the dorsal raphe nucleus.     

Nicotine inhibits firing activity of dorsal raphe 5-HT neurones in vivo

It is established that the brain monoaminergic systems are among the main targets of several dependence-inducing drugs, including nicotine. In the present study extracellular electrophysiological recordings were performed to investigate the effects of nicotine on dorsal raphe 5-HT neurones. Nicotine, administered systemically (50-400 microg/kg, i.v.) in chloral hydrate-anaesthetised rats, induced a transient inhibition of the majority of 5-HT neurones recorded (38 of 45). The inhibition was rapid in onset (about 30 s) and the firing rate returned to baseline within 1-3 min. No apparent tachyphylaxis was observed to this inhibitory effect. The centrally acting nicotine antagonist mecamylamine (4 mg/kg, i.v.), but not the peripherally acting nicotine antagonist chlorisondamine (0.3 mg/kg, i.v.) antagonised the nicotine-induced inhibition of 5-HT neurones. The inhibition of 5-HT neurones was also blocked with a selective 5-HT1A receptor antagonist (WAY 100635; 0.1 mg/kg, i.v.), indicating a possible involvement of somato-dendritic 5-HT1A receptors in the effect of nicotine. Interestingly, microiontophoretic application of nicotine into the dorsal raphe failed to inhibit 5-HT neurones, suggesting an indirect effect of nicotine on 5-HT neurones, possibly involving afferent pathways.     

Inhibition of the 5-HT(1A) receptor-mediated inwardly rectifying K(+) current by dextromethorphan in rat dorsal raphe neurones

The effect of dextromethorphan (DM) on the inwardly rectifying K(+) currents mediated by 5-HT(1A) receptors in acutely dissociated dorsal raphe (DR) neurones of rats was studied using nystatin-perforated patch and conventional whole-cell patch recording configurations under voltage-clamp conditions. DM rapidly and reversibly inhibited the K(+) currents induced by 10(-7) M 5-HT in a concentration-dependent manner with a half-maximum inhibitory concentration of 1.43 x 10(-5) M. The inhibitory effect of DM was neither voltage- nor use-dependent. DM caused a suppression of the maximum response of the 5-HT concentration-response curve, thus suggesting a non-competitive type of inhibition. In neurones perfused intracellularly with a pipette-solution containing the nonhydrolyzable GTP analog GTPgammaS, 5-HT activated K(+) currents in an irreversible manner. DM suppressed the current irreversibly activated by intracellular GTPgammaS even in the absence of the agonist. DM also inhibited the inwardly rectifying K(+) currents regulated by alpha(2)-adrenoceptors in freshly isolated rat locus coeruleus neurones. These results suggest that DM may inhibit the G-protein coupled inwardly rectifying K(+) channels, but not the neurotransmitter receptors, in the central nervous system.     

Electrophysiological evidence for a functional differentiation between subtypes of the 5-HT1 receptor

Serotonin (5-HT) neurons in the dorsal (DRN) and median (MRN) raphe nuclei, and dopamine (DA) neurons in the substantia nigra (SN) were recorded extracellularly in the anesthetized rat. Compounds which have a relatively high affinity for the 5-HT1A or 5-HT1B subtypes of the 5-HT1 receptor were administered and their effect on the firing rate of the monoamine cells was determined. 5-HT1A ligands were more potent in inhibiting impulse activity in the DRN than in the MRN, but had little effect in the SN. In contrast, 5-HT1B ligands increased the firing rate of MRN 5-HT units at low doses, and were also effective inhibitors of DA cell firing in the SN. These results could be correlated with recently described differences in the distribution of the 5-HT1A and 5-HT1B receptor subtypes, and were interpreted as indicating possible functional differentiation between these subtypes. In particular, agonist activity at the 5-HT1B autoreceptor site may decrease 5-HT release, suggesting a presynaptic locus for this receptor in the somatodendritic region. The site also appears to be implicated in 5-HT modulation of nigral DA impulse flow.     

Effect of 5-HT1A receptor agonists in two models of anxiety after dorsal raphe injection

The purpose of the present study was two-fold. Firstly, to present a more comprehensive analysis of the disinhibitory effects of 5-HT_1A receptor agonists after discrete dorsal raphe (DRN) injections (Higgins et al. 1988). Secondly, the effects of the 5-HT_1B receptor agonist CGS12066B and the 5-HT_1B/1C agonist mCPP were examined following injection into this nucleus. The increases in social interaction (SI) induced by intra-raphe injections of 8-OH DPAT (0.02–1 μg), buspirone (0.04–0.2 μg), ipsapirone (0.2 μg) and gepirone (0.2–1 μg) under a high light unfamiliar paradigm (HLU) were typically due to increased bout frequency, duration and a higher incidence of sniff, follow, allogroom behaviour. These increases were qualitatively similar to those seen in control animals tested under low light/familiar (LLF) conditions, thus supporting the belief that the drug-induced increases in SI reflected decreases in anxiety. Furthermore, at doses effective under the HLU condition, 8-OH DPAT, buspirone and gepirone failed to modify SI under conditions of minimal suppression (LLF paradigm). At doses which significantly increased punished responding in a water-lick conflict test 8-OH DPAT, ipsapirone and gepirone tended to also increase unpunished rates of drinking. However, in drug untreated rats, prior habituation to the test apparatus also increased unpunished drinking, suggesting some neophobia-induced suppression. At a comparatively high dose, the 5-HT_1B agonist CGS12066B (2.5 μg), but not the putative 5-HT_1B/1c agonist mCPP (0.5–12.5 μg), increased SI under the HLU condition. Considered along-side the other compounds described in this report, the relative potency of CGS12066B may be reflective of a 5-HT_1A receptor interaction. Together, these data support the proposal that the DRN is an important site through wich 5-HT_1A receptor agonists express their anxiolytic actions.     

Cardiovascular effects of 5-HT1A receptor agonists injected into the dorsal raphe nucleus of conscious rats

The aim of this study was to investigate the cardiovascular effects of the 5-HT1A receptor agonists, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), flesinoxan and 5-carboxamidotryptamine (5-CT) following injection into the dorsal raphe nucleus of conscious rats. 8-OH-DPAT (0.5-2.5 micrograms), hypotension, bradycardia and flat body posture. In contrast, injection of 8-OH-DPAT (0.5 microgram) into the median raphe nucleus caused no cardiovascular changes or flat body posture. (-)Pindolol (0.5 microgram dorsal raphe nucleus) had little effect on cardiovascular parameters, but significantly attenuated the cardiovascular effects of 8-OH-DPAT (0.5 microgram dorsal raphe nucleus). N-Methylatropine (1 mg/kg i.v.) antagonised the cardiovascular effects of 8-OH-DPAT (0.5 microgram dorsal raphe nucleus), suggesting these were vagally mediated. Both pretreatments also appeared to reduce 8-OH-DPAT-induced flat body posture. The results suggest that 8-OH-DPAT activates 5-HT1A receptors in the dorsal raphe nucleus to cause hypotension and bradycardia.     

Inhibition of dorsal raphe cell firing by MDL 73005EF, a novel 5-HT1A receptor ligand.

The effect of a novel ligand for the 5-HT1A receptor subtype, MDL 73005EF, on the firing rate of serotonergic dorsal raphe neurons was assessed in rat midbrain slices maintained in vitro. Superfusion with MDL 73005EF inhibited neuronal firing in a concentration-dependent manner. Based upon IC50 values, MDL 73005EF was equipotent with buspirone (129 +/- 34 vs. 97 +/- 8 nM, respectively) but significantly less potent than 8-OH-DPAT (8-hydroxy-2(di-n-propylamino)tetralin; 7 +/- 2 nM). Pretreatment with (-)-propranolol (1 microM), a mixed 5-HT1A/B receptor antagonist, blocked by 50% the inhibition of unit activity elicited by MDL 73005EF. Taken together, these data suggest that MDL 73005EF is an agonist at the somatodendritic autoreceptor on dorsal raphe neurons, a 5-HT1A receptor which regulates in part the pacemaker activity of these cells. The results are discussed in the context of receptor reserve, recently proposed to explain apparent discrepancies in the actions of agonists at pre- and postsynaptic 5-HT1A sites.     

Perospirone, a novel antipsychotic agent, hyperpolarizes rat dorsal raphe neurons via 5-HT1A receptor

To investigate the effect of cis-N-[4-[4-(1,2-benz-isozole-3-yl)-1-piperazinyl]butyl] cyclohexane-1,2-dicarboximide hydrochloride (perospirone), a novel antipsychotic agent with high affinities for D(2)/5-HT(2) receptors, on the rat dorsal raphe (DR) neurons, an electrophysiological study was performed using the tight-seal whole-cell patch-clamp technique. Applications of perospirone at the concentration between 10(-)(9) and 10(-)(5) M hyperpolarized the membrane potential and inhibited spontaneous action potentials of the DR neurons in a concentration-dependent manner. This effect of perospirone on DR neurons is similar to that of typical 5HT(1A)-receptor agonists, including 8-OH-DPAT or tandospirone. In addition, WAY100635, a 5-HT(1A)-receptor antagonist, inhibited this perospirone-induced hyperpolarization of DR neurons, suggesting that perospirone physiologically acts on DR neurons as a 5HT(1A)-receptor agonist. These results provide new profiles of perospirone as an antipsychotic drug.     

Localisation of mRNA for h5-HT1B and h5-HT1D receptors in human dorsal raphe

In the mammalian mesencephalon, virtually all serotoninergic neurons are located in the raphe nuclei and the adjacent reticular formation. Pharmacological evidence obtained in rodents suggests that terminal and somatodendritic autoreceptors controlling serotonin (5-hydroxytryptamine, 5-HT) release belong to the 5-HT1B/D subtype of receptors, whereas somatodendritic autoreceptors controlling neuronal cell firing are predominantly of the 5-HT1A subtype. This study investigated the presence of h5-HT1D and h5-HT1B receptor mRNA within the subdivisions of the dorsal raphe of post-mortem human brains by means of in situ hybridisation. Although differences in the labelling intensity, which may be caused by different pre- and/or post-mortem conditions, were obvious among the specimens, all brains expressed both the h5-HT1D and the h5-HT1B mRNA in dorsal raphe neurons. In comparison to h5-HT1D mRNA, expression of h5-HT1B mRNA was slightly more abundant. Information on the existence and localisation of h5-HT1D and h5-HT1B receptors in human dorsal raphe neurons confirms that both subtypes may serve an autoreceptor function in humans. This finding is of pharmacological relevance since these receptors are potential new targets for therapeutic interventions in psychiatric disorders such as depression and anxiety.     

Effects of 5-HT and 5-HT1A receptor agonists and antagonists on dorsal vagal preganglionic neurones in anaesthetized rats: an ionophoretic study.

1. Effects of ionophoretic administration of 5-hydroxytryptamine (5-HT) and selective 5-HT1A receptor agonists and antagonists on identified dorsal vagal preganglionic and dorsal raphe neurones were studied in pentobarbitone sodium or chloral hydrate-anaesthetized rats, respectively. 2. Extracellular recordings were made from 176 preganglionic neurones in the dorsal vagal nucleus (DVN). Application of 5-HT at low currents (< or = 10 nA) increased the activity of these neurones. However, at increased currents (10-60 nA), it had a predominantly depressant effect. Application of selective 5-HT1A receptor antagonists, (+/-)-pindolol or WAY-100635, attenuated the excitatory responses evoked by 5-HT. 3. Ionophoresis of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (5-30 nA) increased the firing rate of 19 and decreased that of 67 of the 104 vagal neurones tested. Other 5-HT1A receptor agonists, flesinoxan and N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) also had predominantly depressant effects. 4. (+/-)-Pindolol attenuated excitations but not inhibitions evoked by 8-OH-DPAT. Surprisingly, WAY-100635 and 8-OH-DPAT produced the same effect on these neurones and when applied together, WAY-100635 failed to attenuate the 8-OH-DPAT responses. 5. Dorsal raphe neurones were identified by their low, regular firing rate and their subsequent histological localization. 8-OH-DPAT reversibly reduced the activity in all 7 neurones tested and this was antagonized by WAY-100635 in all 3 neurones tested. 6. In conclusion, 5-HT applied to vagal preganglionic neurones evokes excitatory and inhibitory responses. The excitatory, but not the inhibitory responses may be mediated, at least in part, by activation of 5-HT1A receptors.     

Role of 5-HT(1A) and 5-HT(2) receptors of dorsal and median raphe nucleus in tolerance to morphine analgesia in rats

Several studies indicate that central serotonergic neurons have important role in morphine analgesia and tolerance. The aim of this study was to investigate possible role of 5-HT(1A) and 5-HT(2) receptors in dorsal and median raphe nucleus on development of tolerance to analgesic effect of morphine using hot plate test. Chronic injection of 5-HT(1A) receptor agonist 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino]tetralin) (2, 4 and 8 mug/rat/day) to dorsal raphe nucleus (DRN) delayed tolerance to morphine analgesia, whereas injection of the same doses of 8-OH-DPAT to the median raphe nucleus (MRN) did not alter tolerance to morphine. In addition, chronic administration of ketanserin (1.5, 3 and 6 mug/rat/day), as a 5-HT(2) receptors antagonist, in DRN and MRN did not produce any significant effect. We conclude that 5-HT(1A) receptors of DRN are involved in tolerance to antinociceptive effect of morphine. However, the exact mechanism of interaction between serotonergic and opioidergic systems is not clear and remains to be elucidated.     

Evidence for postsynaptic mediation of the hypothermic effect of 5-HT1A receptor activation.

1. The 5-HT1A ligand BMY 7378 (8-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]8-azaspirol [4,5]-decane-7,9-dione dihydrochloride, 0.032-2 mg kg-1, s.c.) caused hyperphagia, a response to the activation of presynaptic 5-HT1A receptors. 2. BMY 7378 (8 mg kg-1, s.c.) and the 5-HT1A agonist (8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), 0.10 and 0.25 mg kg-1 s.c.) also caused hypothermia. This was inhibited by (-)-pindolol (1-mg kg-1, i.p.) and not prevented by pretreatments with p-chlorophenylalanine which grossly depleted 5-hydroxytryptamine (5-HT) from terminal regions. The hypothermic effects are explicable by activation of postsynaptic 5-HT1A receptors. Infusion of BMY 7378 (8-64 micrograms) into the dorsal raphe was without convincing hypothermic effect. 3. BMY 7378 (8 mg kg-1, s.c.) inhibited another effect of activation of postsynaptic 5-HT1A receptors, i.e., the induction of components of the 5-HT syndrome by 8-OH-DPAT (0.5, 1.0 mg kg-1, s.c.) which suggests that BMY 7378 has antagonistic as well as agonistic effects at these sites. 4. Partial agonist properties of BMY 7378 at postsynaptic sites were also indicated by doses for hypothermia being much greater than those for hyperphagia i.e., ED50 (hypothermia) greater than 2 mg kg-1, ED50 (hyperphagia) = 0.010 mg kg-1. This contrasts with the similar ED50 values for both the hypothermic (ED50 = 0.08-0.10 mg kg-1) and hyperphagic (ED50 = 0.06-0.10 mg kg-1) effects of 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic clomipramine alters presynaptic 5-HT(1B) and postsynaptic 5-HT(1A) receptor sensitivity in rat hypothalamus and hippocampus, respectively

Clomipramine is a tricyclic antidepressant drug with a high affinity for the serotonin (5-HT) uptake site or transporter. Electrophysiological experiments have provided evidence that repeated administration of clomipramine induces an increase in the sensitivity of postsynaptic 5-HT(1A) receptors in the hippocampus. We have studied the effects of clomipramine, administered to rats at a dose of 10mg/kg/day for 28 days by osmotic minipumps, on presynaptic 5-HT(1A) and 5-HT(1B) autoreceptors in the hypothalamus, and on postsynaptic 5-HT(1A) receptors in the hippocampus, by using in vivo microdialysis to measure 5-HT and cyclic adenosine monophosphate (cAMP) levels. Postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus was determined by means of a neuroendocrine challenge procedure. Although the sensitivity of presynaptic 5-HT(1A) autoreceptors, as measured by the effect of a subcutaneous (s.c.) injection of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.2mg/kg or 50 microg/kg) to reduce 5-HT levels, did not change, there was a reduction in sensitivity of presynaptic 5-HT(1B) receptors, as measured by the effect of an injection of the 5-HT(1B/1D) antagonist GR 127935 (5mg/kg, s.c.) to increase 5-HT levels. This effect probably accounted for the increase in basal 5-HT levels observed in the hypothalamus after chronic clomipramine administration. Postsynaptic 5-HT(1A) receptor sensitivity in the hippocampus, measured by the effect of 8-OH-DPAT to increase cAMP levels in the dialysate, was increased after chronic clomipramine. Animals that had received daily intraperitoneal injections of 10mg/kg clomipramine for 28 days did not show a change in postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus as measured by the ability of 8-OH-DPAT (50 microg/kg, s.c.) to stimulate secretion of corticosterone. Taken together with the results of previous experiments involving the cerebral cortex, these in vivo results show that chronic clomipramine exerts effects on both pre- and postsynaptic serotonin receptors, but that these effects are highly region-specific.     

Effect of the postsynaptic 5-HT1A receptor antagonist MM-77 on stressed mice treated with 5-HT1A receptor agents

The pharmacological effect of the 5-HT1A receptor ligands, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), indorenate, and buspirone, alone or in combination with the antagonist MM-77, was studied in mice subjected to forced swimming. It was confirmed that this stressful factor produces an anxiolytic-like effect, which is reversed by the mentioned 5-HT1A receptor agonists. Only the 8-OH-DPAT-induced decrease of such an effect could be blocked by the postsynaptic antagonist of the 5-HT1A receptor 1-(2-methoxyphenyl)-4-[(4-succinimido)butyl]-piperazine (MM-77). Stressing by forced swimming seems to induce plastic changes in 5-HT1A receptors, which in turn modify the behavioural actions of 5-HT1A receptor agents.     

(-)-Propranolol blocks the inhibition of serotonergic dorsal raphe cell firing by 5-HT1A selective agonists

The ability of the beta-adrenoceptor antagonist propranolol to block the effects of serotonin (5-HT) and 5-HT1A-selective agonists on the spontaneous firing of serotonergic dorsal raphe neurons was assessed. During microiontophoretic application, (-)- but not (+)-propranolol rapidly and reversibly blocked the suppressant effects of the 5-HT1A-selective agonists ipsapirone (TVX Q 7821) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). However, (-)-propranolol was a relatively weak antagonist of 5-HT itself, suggesting that the endogenous neurotransmitter may have actions on dorsal raphe neurons in addition to those mediated by 5-HT1A receptors.     

Diltiazem potentiates pentobarbital-induced hypnosis via 5-HT1A and 5-HT2A/2C receptors: role for dorsal raphe nucleus

It has been reported that the sedative component of pentobarbital is mediated by GABA receptors in an endogenous sleep pathway and the ventrolateral preoptic area (VLPO)-tuberomammillary nucleus (TMN) or VLPO-dorsal raphe nucleus (DRN) neural circuit is important in the sedative response to pentobarbital. Our previous findings indicated that the VLPO-TMN neuronal circuit may play crucial part in the augmentative effect of diltiazem on pentobarbital sleep and the serotonergic system may be involved. This study was designed to investigate the role of DRN and the serotonergic receptors 5-HT_1A and 5-HT_2A/2C in the augmentative effect of diltiazem on pentobarbital-induced hypnosis in rats. The results showed that diltiazem (5 mg/kg, i.g.) significantly reversed pentobarbital-induced (35 mg/kg, i.p.) reduction of c-Fos expression in 5-HT neurons of DRNV (at − 7.5 mm Bregma), DRND, DRNVL and MRN (at − 8.0 mm Bregma). However it did not influence this reducing effect of pentobarbital on non-5-HT neurons either in DRN or in MRN. Moreover, the effect of diltiazem (1 or 2 mg/kg, i.g.) on pentobarbital-induced (35 mg/kg, i.p.) hypnosis was significantly inhibited by 5-HT_1A agonist 8-OH-DPAT (0.5 mg/kg, i.p.) and 5-HT_2A/2C agonist DOI (0.5 mg/kg, i.p.), and potentiated by 5-HT_1A antagonist p-MPPI (2 mg/kg, i.p.) and 5-HT_2A/2C antagonist ritanserin (2 mg/kg, i.p.), respectively. From these results, it should be presumed that the augmentative effect of diltiazem on pentobarbital-induced sleep may be related to 5-HT_1A and 5-HT_2A/2C receptors, and DRN may be involved. In addition, it also suggested that the DRN may play a multi-modulating role in sleep-wake regulation rather than being recognized simply as arousal nuclei.     

Pharmacology of a cholecystokinin receptor on 5-hydroxytryptamine neurones in the dorsal raphe of the rat brain.

1. The effect of bath application of sulphated cholecystokinin octapeptide (CCK-8) was studied on neurones in slices containing rat raphe nucleus. 2. Intracellular recordings were made from neurones in the dorsal raphe nucleus. Some of the neurones with the characteristics of 5-hydroxytryptamine (5-HT)-containing cells which were inhibited by 5-HT and excited by noradrenaline were excited by cholecystokinin. The response to cholecystokinin was dose-dependent over the range 10 to 1000 nM. 3. The response to CCK-8 persisted in the presence of tetrodotoxin. Either reduction of extracellular calcium or addition of 25 mM magnesium did not block the CCK response, suggesting it was mediated by receptors located on the membrane of the raphe neurones. 4. The agonist and antagonist specificity of the CCK response was determined. The CCKB selective agonist, pentagastrin, was inactive when applied at concentrations up to 10 microM. the CCKA receptor antagonist L-364,718 (1 to 100 nM) blocked the response to cholecystokinin. Much higher (1-10 microM) concentrations of the CCKB receptor antagonist L-365,260 were required for inhibition of the CCK response. 5. These data support the existence of a CCK receptor, located on raphe neurones in the rat, with a pharmacological profile very similar to that described for the CCKA type.     

The effect of 5-HT and selective 5-HT receptor agonists and antagonists on rat dorsal vagal preganglionic neurones in vitro.

1. Whole-cell patch-clamp recordings were made from 142 visually identified rat dorsal vagal preganglionic neurones (DVMs). Applications of 5-hydroxytryptamine (5-HT, 20 microM, 2 min) elicited a slow depolarization (8.2 +/- 0.5 mV, n = 59) in 95% of the cells tested, accompanied by an increase in excitability. In (68%) of DVMs the depolarization was associated with an increase in apparent membrane resistance (Rmt 22.7 +/- 2.2%). These depolarizations and increases in Rm (14.3 +/- 2.6%, n = 8) were maintained in a medium which blocked synaptic transmission. 2. The response to 5-HT was associated with a reversal potential (Erev) of -91 +/- 1 mV at an extracellular K+ concentration (LK+]o) of 4.2 mM. This correlated well with the K+ equilibrium potential (Ek = -89 mV). 3. The depolarizing effect of 5-HT was attenuated by the 5-HT2A/2C receptor antagonists, ketanserin (1 microM), LY 53,857 (1 microM) and the 5-HT1A/2A receptor antagonist, spiperone (1 microM). The 5-HT1A receptor antagonist, pindobind 5-HT1A (5 microM), had no effect on the depolarizing response to 5-HT. 4. The effect of 5-HT was mimicked by the 5-HT2A/2C receptor agonist, alpha-methyl-5-HT (50 microM), the 5-HT1 receptor agonist, 5-carboxamidotryptamine (20 microM) and the putative 5-HT4 agonist, 5-methyoxytryptamine (5 microM). The selective 5-HT4 receptor antagonist, GR113808, had no effect on the depolarizing effect of 5-HT or 5-MEOT on DVMs. 5. The 5-HT3 antagonists, MDL 72222 (10 microM) and ICS-205-930 (1 and 10 microM), partially reduced the effect of 5-HT. The 5-HT3 receptor agonist, 2-methyl-5-HT (100-300 microM), excited a proportion of neurones tested (56%) by evoking a depolarizing and/or an increase in postsynaptic potentials (p.s.ps). 6. These results are consistent with direct, postsynaptic actions of 5-HT on DVMs via 5-HT2A receptors, being mediated, in part, by the reduction of K+ conductance.     

Effects of repeated treatment with 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on the autoregulatory control of dorsal raphe 5-HT neuronal firing and cortical 5-HT release.

These experiments were designed to examine the effects of repeated 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) treatment on the autoregulatory control of cortical 5-HT release and dorsal raphe nucleus (DRN) 5-HT neuronal cell firing. Repeated DOI treatment decreased the behavioural responsiveness (wet-dog shakes) of 5-HT2 receptors and attenuated the inhibitory effects of the 5-HT1A receptor agonist, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), on both cortical 5-HT release and DRN 5-HT neuronal firing. In contrast, the inhibitory effect of acute DOI on cortical 5-HT release and DRN 5-HT neuronal firing was unaffected by repeated DOI treatment. The results demonstrate that changes in the responsiveness of 5-HT2 receptor function may influence the responsiveness of presynaptic 5-HT1A receptors regulating 5-HT neuronal function. The results also provide further evidence that the inhibition of cortical 5-HT release and DRN 5-HT neuronal firing produced by DOI is not mediated by 5-HT2 receptor activation.