Autoreceptors in the central nervous system

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part I. General background and discovery of serotonin as a basis for 5-HT receptor identification

This review contains background information on the serotonin system, furthermore the suggestion to introduce the term Contemporary Witness Report (CWR) for a novel type of review and, as the main part, an overview over the history of serotonin discovery as a basis for the identification of its receptor heterogeneity and the increase in complexity by genetic and allosteric variation. The present article conforms to CWRs in historical and autobiographical elements, in more emphasis on the author's work than in conventional reviews and in aspects neglected in previous reviews, but not in the main feature namely the work of a scientist with comprehensive expertise in a field in which, over long time, he/she continuously performed research and published. A scientist complying with these requirements is a contemporary witness in that field. His report on the scientific achievements in that period, a CWR, comprises confirmation and putative re-interpretation of data from a superior viewpoint. Identification of serotonin's vascular properties (publication year: 1912) as an "adrenaline mimicking substance" (without attempt to isolate it) by O'Connor preceded the discovery of serotonin in the gastrointestinal tract by Erspamer [1937] and in blood by Rapport [1948, 1949], who identified its structure as 5-hydroxytryptamine [1949]. Detection as a neurotransmitter in invertebrates suggested its occurrence in vertebrate CNS as well. This was verified by finding it in dog, rat and rabbit brain [1953]. The Falck-Hillarp technique [1962] visualized serotonin neurones as fluorescent structures. The neurotoxin 5,7-dihydroxytryptamine [1972] indirectly proved the involvement of 5-HT in multiple CNS functions.

Dopamine and opioid regulation of the memory retrieval recovery in mice

The reactivation effects of the delta-opioid receptor blockade and D2 dopamine receptor activation on the detention-induced memory deficit in mice were investigated, in order to study possible interactions between opioid and dopamine systems in memory retrieval. Animals were trained in a one-trial passive-avoidance task. Pretesting treatment with ICI 174,864 (1, 3 or 5 mg/kg, i.p.) or quinpirole (0.5, 1 or 2 mg/kg, i.p.) facilitated retrieval of memory trace in saline-pretreated mice. Pretraining injection of the dopamine autoreceptor agonist, (+)-3PPP (2 mg/kg), having no effect alone in learning, prevented the ability of ICI 174,864 to produce the memory-enhancing effect. It is suggested that the normal functioning of the dopamine system was critical for the facilitation of retrieval by delta-antagonist. Quinpirole-induced reactivátion of memory retrieval was enhanced by pretreatment with Leu-enkephalin (0.2 mg/kg), inducing increased retention. We discuss these results in the context of an important interactions between D2 dopamine and delta-opioid receptors.

Galanin stimulates striatal acetylcholine release via a mechanism unrelated to cholinergic receptor stimulation

The effect of galanin (GAL) on the basal and the muscarinic agonist/antagonist mediated release of acetylcholine (ACh) in the striatum was investigated in male rats using in vivo microdialysis and HPLC techniques. GAL (300 microM or 3 nmol/10 microliters), perfused through the microdialysis membrane into the striatum, was found to enhance basal ACh release. The GAL evoked ACh release was completely prevented by bupivacaine, a sodium channel blocker (1.5 mM) when coperfused with GAL. This suggests that the effect of GAL depends on intact neuronal activity and thus derives from impulse-dependent release. The muscarinic agonists oxotremorine (0.3 mg/kg, i.p.) or carbachol (100 microM, infusion) reduced ACh release and reduced the stimulation of ACh release by GAL with a magnitude corresponding to that of oxotremorine or carbachol alone. Thus, the resultant effect of GAL on ACh release remained unchanged. When GAL was given at a threshold dose (30 microM), which by itself did not stimulate ACh release, it was unable to attenuate the muscarinic agonist induced inhibition of ACh releases. Furthermore, GAL given in combination with scopolamine (0.25 mg/kg, i.p.) or pirenzepine (1 microM, infusion) added to the stimulating effect by the two muscarinic antagonists. In contrast to the GAL (300 microM) evoked ACh release, the scopolamine (0.25 mg/kg, i.p.) stimulated ACh release was not blocked by M15, a putative GAL antagonist, indicating that the mechanisms behind GAL and scopolamine evoked striatal ACh release differ. These results suggest that the mechanisms behind GAL evoked release do not involve direct interactions with pre- or postsynaptic muscarinic receptor mediated events. It is concluded that the stimulation of the basal ACh release by GAL in the striatum occurs via occupation of GAL receptors located on cholinergic interneurons and that the release process is dependent on intact neuronal activity.

Comparison of two radioiodinated ligands of dopamine D2 receptors in animal models: iodobenzamide and iodoethylspiperone

Several iodinated compounds have been developed for in vivo exploration of dopamine D2 receptors by SPECT. It is of great value to understand if the same information could be obtained with different radioligands. For this purpose, we compared in vivo properties of two iodinated ligands, iodoethylspiperone (IES) and iodobenzamide (IBZM), using different pharmacological and lesioning treatments in rats. Cerebral biodistribution performed by ex vivo autoradiograms and dissection of brain areas showed that IES and IBZM bound specifically to D2 receptors since a pre-injection of haloperidol prevented accumulation of both ligands. In contrast, when haloperidol was injected after IES or IBZM, only IBZM was displaced from its binding sites. This could be explained partly by a process of dopamine-dependent internalization with IES. The response to striatal quinolinic acid infusion for lesioning post-synaptic neurons was very different for IES and IBZM. In this model a decrease in IBZM accumulation occurred, corresponding to the loss of D2 receptors located on post-synaptic neurons. In contrast, a unexpected increase in IES accumulation was observed on the lesioned side. From these results we concluded that IES and IBZM, two iodinated ligands belonging to different pharmacological families, bound specifically to dopamine D2 receptors. However they have different properties in animal models. Therefore, it appears that IBZM is a more suitable ligand than IES to detect modifications of D2 receptors by in vivo exploration.

[Release of acetylcholine and its regulation]

The mechanism of acetylcholine (ACh) release and its regulation is a widely studied subject still underdebated. Although the vesicular hypothesis for ACh release is at present largely accepted, alternative theories have been proposed. ACh release is triggered by calcium influx through specific presynaptic Ca2+ channels. The modulation of this calcium influx appears as the main mechanism through which ACh release is regulated. This can be achieved by direct modification of the presynaptic Ca2+ channel opening or indirectly by a change in the polarization level of the presynaptic membrane due to the opening or closing of other presynaptic channels (usually K+ channels). The increase in the intracellular Ca2+ concentration that triggers ACh release is also under the control of Ca2+ membrane exchanges and intracellular Ca2+ buffers. ACh synthesis that takes place in the cytoplasm of the terminal, can itself be modulated leading to changes in the quantity of ACh available for release. All these regulatory mechanisms can be initiated by the activation of presynaptic receptors to either ACh itself (autoreceptors) or to other transmitters (heteroreceptors). Most often, these presynaptic receptors seem to require the transducing role of G proteins and the involvement of various second messengers. Some illnesses concerning the cholinergic system can be related to a disfunction of one of these presynaptic regulatory mechanisms.

Anpirtoline, a novel, highly potent 5-HT1B receptor agonist with antinociceptive/antidepressant-like actions in rodents

1. The purpose of the present study was to relate the effects of the novel drug, anpirtoline, on 5-hydroxytryptamine (5-HT) receptor subtypes to its antinociceptive and antidepressant-like actions in rodents. 2. Binding assays with rat brain membranes have shown that anpirtoline bound with a much higher affinity to 5-HT1B receptor (Ki = 28 nM) than to 5-HT1A (Ki = 150 nM) and 5-HT2 (Ki = 1.49 microM) receptors. 3. Like 5-HT, anpirtoline concentration-dependently inhibited forskolin-stimulated adenylate cyclase activity in homogenates from the rat substantia nigra. Both effects were not additive, and could be prevented by 5-HT1B receptor antagonists such as propranolol and penbutolol. 4. In superfused rat and pig brain cortex slices preincubated with [3H]-5-HT, the electrically evoked tritium overflow was inhibited by anpirtoline and 5-HT. Whereas 5-HT was equipotent in both tissues (EC50 = 69 nM), anpirtoline was markedly less potent in pig brain cortex slices (EC50 = 1190 nM) than in rat brain cortex slices (EC50 = 55 nM). The concentration-response curve for anpirtoline was shifted to the right by metitepine in both preparations. 5. In the social behaviour deficit test, anpirtoline and trifluoromethylphenyl-piperazine were effective in reversing the isolation-induced impairments in mice, an effect shown only by compounds with agonist properties at the 5-HT1B receptor. 6. In the electrostimulated pain test using mice, anpirtoline dose-dependently increased the pain threshold with an ED50 of 0.52 mg kg-1, i.p. The antinociceptive activity of anpirtoline was abolished by pretreatment with cyproheptadine or propranolol.7. In the forced swimming test in rats, anpirtoline induced a dose-related increase in swimming activity. With an ED50 value of 4.6mgkg-1, i.p., anpirtoline was 4 times more potent than the two standard compounds imipramine and desipramine. The decrease of immobility time or the increase of active periods in this model of behavioural despair is suggested to be characteristic of antidepressant drugs.8. Anpirtoline exhibits both antinociceptive and antidepressant-like activities in animals. It is probable that anpirtoline elicits these pharmacological effects via its agonist effect on 5-HT1B and 5-HT1A receptors.

Presynaptic serotonin receptors in the central nervous system

Presynaptic 5-HT autoreceptors have been identified in any region of the mammalian CNS containing 5-HT nerve terminals that has been investigated for this purpose. They belong to the 5-HT1B receptor subclass in the rat and to the 5-HT1D subclass in the pig, guinea pig, and probably man. The presence and operation of presynaptic 5-HT autoreceptors have been proven by the ability of 5-HT receptor agonists to inhibit 5-HT release and of 5-HT receptor antagonists not only to competitively antagonize this effect but also to disclose the autoinhibitory effect of endogenous 5-HT by blocking the autoreceptor, thus interrupting the negative feedback loop. There is evidence that presynaptic 5-HT autoreceptors are operative in vivo. Presynaptic inhibitory 5-HT heteroreceptors have also been identified in various brain regions of the rat. DA nerve terminals in the striatum and nucleus accumbens as well as GLU nerve terminals in the cerebellum are endowed with such receptors, which were either not yet classified (DA neurone) or represent a not yet specified 5-HT1 subtype (GLU neurone). Release-inhibiting 5-HT receptors on the acetylcholine nerve terminals in the hippocampus are of the 5-HT1B subtype, and those in the striatum were not yet classified in detail. A 5-HT heteroreceptor mediating stimulation of release occurs on rat striatal DA nerve terminals; it belongs to the 5-HT3 class. Thus, presynaptic inhibitory 5-HT auto- and heteroreceptors as well as presynaptic excitatory 5-HT heteroreceptors are involved in the regulation of transmitter release in the brain.

Catecholamines and the initiation of sexual behavior in male rats without sexual experience

The purpose of the present experiments was to investigate the effects of modified catecholaminergic neurotransmission upon sexual behavior in inexperienced males. Such males are critically dependent on stimuli from the female in order to initiate sexual behavior, and catecholamines are known to modulate interactions with environmental stimuli. It was found that D-amphetamine, 0.5 and 1 mg/kg, and amfonelic acid, 0.25 and 0.5 mg/kg, reduced mount and intromission latencies. Pimozide, in doses between 0.25 and 1 mg/kg, and cis(Z)-flupentixol, 0.5 mg/kg, reduced the proportion of animals displaying sexual behavior. The noradrenergic neurotoxin DSP4 (50 mg/kg one week before behavioral observation) shortened intromission latency while the noradrenaline precursor threo-dihydroxyphenylserine (10 mg/kg + carbidopa 50 mg/kg) increased mount and intromission latencies. In a test for social and exploratory behaviors it was found that amfonelic acid, in a dose of 0.5 mg/kg, augmented sniffing and rearing without affecting nonsexual interaction with a receptive female. Flupentixol (0.5 mg/kg) had a slight inhibitory effect on exploratory behavior and no effect on nonsexual interaction with a female. It is suggested that enhanced dopaminergic activity facilitates the initiation of sexual behavior due to an increased general arousal and not because of a specific effect on that behavior. The role of noradrenaline is less clear at present.

Presynaptic cholinergic mechanisms in the rat cerebellum: evidence for nicotinic, but not muscarinic autoreceptors

The present study shows that N-[3H]methylcarbamylcholine ([3H]MCC) binds to a single population of high-affinity/low-density (KD = 5.0 nM; Bmax = 8.2 fmol/mg of protein) nicotinic binding sites in the rat cerebellum. Also, there exists a single class of high-affinity binding sites (KD = 4.8 nM; Bmax = 24.2 fmol/mg of protein) in the cerebellum for the M1 specific muscarinic ligand [3H]pirenzepine. In contrast, the M2 ligand, [3H]AF-DX 116, appears to bind to two classes of binding sites, i.e., a high-affinity (KD = 3 nM)/low-capacity (Bmax = 11.7 fmol/mg of protein) class, and a second class of lower affinity (KD = 28.4 nM) and higher capacity (Bmax = 36.3 fmol/mg of protein) sites. The putative M3 selective ligand [3H]4-diphenylacetoxy-N-methylpiperidine also binds to two distinct classes of binding sites in cerebellar homogenates, one of high affinity (KD = 0.5 nM)/low capacity (Bmax = 19.5 fmol/mg of protein) and one of low affinity (KD = 57.5 nM)/high capacity (Bmax = 140.6 fmol/mg of protein). In experiments which tested the effects of cholinergic drugs on acetylcholine release from cerebellar brain slices, the nicotinic agonist MCC enhanced spontaneous acetylcholine release in a concentration-dependent manner, and the maximal increase in acetylcholine release (59.0-68.0%) occurred at 10(-7) M. The effect of MCC to increase acetylcholine release was Ca2+-dependent and tetrodotoxin-insensitive, suggesting an action on cholinergic terminals. Also, the MCC-induced increase in acetylcholine release was effectively antagonized by dihydro-beta-erythroidine, d-tubocurarine, and kappa-bungarotoxin, but was insensitive to either atropine or alpha-bungarotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding sites for [3H]AF-DX 116 and effect of AF-DX 116 on endogenous acetylcholine release from rat brain slices

The present study shows that the putative M2 ligand, [3H]AF-DX 116, binds to two classes of muscarinic sites in homogenates of rat hippocampus, striatum and cerebral cortex: one with a high affinity (Kd less than 5 nM)/low capacity (Bmax = 30-63 fmol/mg protein), and a second of lower affinity (Kd greater than 65 nM) and higher capacity (Bmax greater than 190 fmol/mg protein). In experiments which tested the effects of the muscarinic antagonists on acetylcholine (ACh) release from brain slices, the non-selective antagonist (-)-quinuclidinyl benzylate and atropine significantly enhanced the potassium (25 mM)-evoked release of ACh. This effect was mimicked by the M2 ligand AF-DX 116, but neither the M1-selective antagonist pirenzepine, nor the putative M3-muscarinic antagonist, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), altered ACh release. Also, the muscarinic agonist, oxotremorine, significantly depressed evoked ACh release from brain slices, an effect that was completely antagonized by atropine or by AF-DX 116, but not by pirenzepine or 4-DAMP. Thus, it appears that presynaptic muscarinic autoreceptors in the rat hippocampus, striatum and cerebral cortex belong to the M2 subtype of muscarinic receptors.

The pharmacological properties of the presynaptic serotonin autoreceptor in the pig brain cortex conform to the 5-HT1D receptor subtype

The effects of serotonin receptor agonists and antagonists on the electrically (3 Hz) evoked 3H overflow were determined on pig brain cortex slices preincubated with 3H-serotonin and superfused with physiological salt solution containing indalpine (an inhibitor of serotonin uptake) plus phentolamine. The potencies of the serotonin receptor agonists and antagonists were compared with their affinities for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT1D binding sites in pig or rat tissue membranes; in addition, the potencies of the agonists were compared to their potencies in inhibiting adenylate cyclase activity in membranes of calf substantia nigra. In the superfusion experiments on pig brain cortex slices the following rank orders of potencies were obtained: agonists, serotonin greater than 5-methoxytryptamine = 5-carboxamidotryptamine greater than RU 24969 (5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole) greater than SDZ 21009 (4(3-terbutylamino-2-hydroxypropoxy)indol-2-carbonic-acid-isopr opylester) greater than or equal to yohimbine greater than or equal to cyanopindolol greater than 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) greater than or equal to CGS 12066 B (7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline); ipsapirone and urapidil were ineffective; antagonists (antagonism determined against 5-methoxytryptamine as an agonist), metitepine greater than metergoline greater than mianserin. Propranolol, spiperone or mesulergine did not produce a shift of the concentration-response curve for 5-methoxytryptamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of acetylcholine from tissue slices of the rat nucleus basalis magnocellularis

We investigated the release of acetylcholine (ACh) from tissue slices obtained from the nucleus basalis magnocellularis (nbM) of the rat brain. Potassium (35 mM) depolarization produced a 10- to 12-fold increase in the release of endogenous ACh above spontaneous release. Potassium-evoked ACh release was Ca2+ dependent. Injection of the excitotoxin quinolinic acid into the nbM produced a 72.8 +/- 13.0% decrease in spontaneous ACh release and a 60.4 +/- 8.2% decrease in potassium-evoked release. A fourfold increase in ACh release was observed following perfusion of the tissue with 1 mM 3,4-diaminopyridine (3,4-DAP) whereas 10 mM 3,4-DAP caused a sevenfold increase. The increase in ACh release caused by 3,4-DAP was inhibited by tetrodotoxin. Tissue slices accumulated [3H]choline by high-affinity choline uptake and this could be inhibited by hemicholinium-3. These results indicate that ACh can be released from tissue slices of the nbM by a calcium-dependent process and that a part of this release appears to be from the cholinergic neurons of the nbM.

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.

Characterization of N-[3H]methylcarbamylcholine binding sites and effect of N-methylcarbamylcholine on acetylcholine release in rat brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.