Genetic Insights Into Serotonin Function

CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector SEROTONIN RECEPTORS MARK J.S. HEATH AND RENtRENE HEN SEROTONIN RECEPTORS Genetic insights into serotonin function Targeted disruption of the genes for the 5-HT 1Band 5-HT 2c serotonin receptors and monoamine oxidase A have confirmed pharmacological experiments and revealed unexpected behavioral roles for serotonin. Serotonin (5-hydroxytryptamine, 5-HT) is a biogenic but pharmacological experiments suggest that 5-HT1B monoamine found in a wide variety of sites in the central receptor activation may increase anxiety and locomotion and peripheral nervous systems. Serotonergic signaling and decrease food intake, sexual activity and aggression. appears to play a key role in the generation and modula- Targeted disruption was achieved by deleting a fragment tion of a broad array of behaviors, including sleep, loco- of the coding sequence, and functional inactivation of the motion, feeding, vomiting, addiction, aggression, sexual 5-HTB receptor was confirmed by autoradiography activity and affect [1]. It has been the focus of intense with 'lI-cyanopindolol, which binds 5-HTIB receptors, investigation because of these roles and the extensive along with appropriate blockers of non-5-HTIB binding opportunities it provides for therapeutic manipulation. sites. Heterozygote animals expressed the same levels of receptor as wild-types, suggesting that a feedback mecha- In recent years, our view of serotonin has become nism may regulate the transcriptional activity of the gene. tremendously complex as we have learned that multiple specific proteins mediate each of the steps of receptor Two of the behaviors postulated to be modulated by binding, re-uptake and degradation. The advent of mol- 5-HT1B receptors were analyzed: locomotion and aggres- ecular techniques has provided a structural basis for sub- sion. Untreated wild-type and homozygous mutant (5- classifying and expanding the large family of serotonin HTB/-) mice were found to display similar levels of receptors; at present, at least 15 mammalian serotonin locomotor activity in an open field. However, when receptors have been identified [2]. A wide functional treated with the 5-HT 1 agonist RU24969, the 5-HTiB/- diversity exists within this family: serotonin receptors mice displayed no change in activity, whereas wild-type may be ionotropic or metabotropic, and may couple via mice doubled their activity. This result confirmed pre- G proteins to numerous intracellular signaling pathways. vious pharmacological studies indicating that RU24969 Serotonin receptors also display specific patterns of operates through 5-HT1B receptors. expression both during development and in the mature animal. The pathways involved in serotonin re-uptake The aggressiveness of 5-HTIB/- male mice was assessed and degradation add further layers of complexity to by isolating them for four weeks and then exposing them serotonin biology. to a non-isolated male wild-type intruder mouse. The latency and number of attacks displayed by the knockout The classical pharmacological techniques of using selec- mice during a three-minute period were used as indices tive agonists, antagonists and inhibitors provided the ini- of aggression. The 5-HTiB/- mice, when compared with tial basis for categorizing the receptors, transporters and wild-type mice, exhibited faster, more intense and more degrading enzymes involved in serotonergic pathways. frequent attacks. For example, 46 % of the 5-HTIB/- But the complexity of the system has overwhelmed the mice attacked the intruder within 10 seconds of intro- armamentarium of selective drugs, leaving many proteins duction, whereas no heterozygous or wild-type mice without specific probes of their function. Recent advan- attacked during this interval. Tail rattling, an aggressive ces in the field have exploited deliberate or naturally display, was also more frequent in 5-HTIB/- mice. occurring disruptions of the genes that code for proteins important to serotonergic transmission. Four recent con- These experiments suggest an involvement of 5-HTIB tributions [3-6] describe the effects of disrupting the receptors in the modulation of aggressive behavior. Elto- genes encoding the 5-HT 1B and 5-HT 2c receptors in prazine and fluprazine, drugs classified as serenics because mice, and the monoamine oxidase A (MAOA) genes in of their anti-aggressive properties, are believed to operate mouse and human (Table 1). as agonists of 5-HT 1 receptors. The lack of subtype- specific antagonists has impeded specific identification of 5-HTls knockout - locomotion and aggression the molecular target of such serenics; the availability of The 5-HTIB receptor was the first protein involved in 5-HTB/- mice should allow more detailed evaluation of serotonergic signaling to have its gene disrupted [3]. The the pathways leading to aggression. 5-HT1B receptor is expressed in the basal ganglia, central gray, hippocampus, amygdala and raphe nuclei; it is tar- 5-HT2c knockout - seizures and overeating geted primarily to presynaptic terminals, where it can The 5-HT2 c gene knockout was generated by insertion inhibit neurotransmitter release. Highly specific agonists of a stop codon that eliminated the carboxyl terminus of and antagonists are not available for 5-HT1B receptors, the protein [4]. The 5-HT2 c gene is X-linked; brains of C Current Biology 1995, Vol 5 No 9 997 998 Current Biology 1995, Vol 5 No 9 mutant males were devoid of 5-HT 2c immunoreactivity, 3 of the MAOA gene with a -interferon transgene [5]. and mRNA from such males failed to produce functional Brain and liver MAOA activity was abolished in the receptor when injected into Xenopus oocytes. mutant mice. The patterns of expression of 5-HT 2C receptors in the MAOA-deficient mice displayed a wide array of behav- hippocampus and spinal cord suggest that they might ioral abnormalities, from birth through to maturity. modulate memory and nociception. However, hippo- Importantly, all of these abnormalities could be suppres- campal LTP and nociceptive thresholds were the same sed by administration of a serotonin synthesis inhibitor, in wild-type and 5-HT2c/ - mice. Video monitoring whereas catecholamine synthesis suppression had no / revealed that 5-HT2 c- mice displayed spontaneous epi- effect. Thus, even though both serotonin and norepi- leptic seizures that were sometimes fatal; survival plots nephrine levels were elevated in MAOA-deficient mice, indicated that almost half of the mice had died by 25 the observed behavioral abnormalities are likely to be weeks of age. Metrazol, a y-amino butyric acid (GABA) specifically the result of decreased serotonin degradation. receptor antagonist, was used to quantify seizure suscepti- Behavioral abnormalities included tremulousness, hyper- bility. Relative to wild-type mice, the 5-HT2c/ - mice active startle responses, violent motions during sleep and displayed a reduced seizure threshold and a more rapid abnormal posture. Increased male aggressiveness was doc- progression through the tonic-clonic phase of the seizure. umented by resident-intruder tests and increased wound- The unexpected susceptibility of 5-HT2 c'- mice to ing between male cage-mates. Interestingly, MAOA- epileptic seizures raises the possibility that this receptor deficient mice fail to develop cortical whisker barrels, participates in the modulation of neuronal network indicating that MAOA plays a developmental role in the excitability. The ability to affect the seizure characteristics nervous system. The fact that MAOA-deficient mice have of normal mice with 5-HT2 c ligands suggests that the abnormal brain development leaves open the question of - - phenotype of the 5-HT2 c mice is not a result of whether the behavioral abnormalities seen in the adult chronic compensation by other receptors for the loss of animals are due to abnormal monoamine metabolism or 5-HT 2c receptors. to structural abnormalities. /- - 5-HT2 c mice were also noted to be significantly MAOA disruption and human aggression heavier than matched wild-type controls. Close analysis The behavior of the MAOA-deficient mice described of their feeding behavior indicated that the weight above provides a timely complement to the description increase was due entirely to increased intake, and not to of a human mutation of MAOA [6]. The X-linked muta- a metabolic derangement that increases the efficiency tion was detected in several kindred males with border- of caloric storage. The serotonergic agonist 1-(3-chloro- line mental retardation and histories of abnormal behav- phenyl)piperazine (mCPP) drastically reduced food intake ior including impulsive aggression, arson, attempted rape by wild-type mice, but had no effect on food intake by and exhibitionism. Affected males exhibit markedly dis- /- 5-HT2 c mice, demonstrating that appetite suppressants turbed monoamine metabolism and an absence of func- may operate via 5-HT 2c receptors. tional MAOA in cultured fibroblasts. A nonconservative point mutation was found in all affected males and all MAOA knockout - aggression and elevated brain serotonin carrier females; the mutation introduces a stop codon at MAOA is a mitochondrial enzyme that degrades and position 296 of the deduced amino-acid sequence. The inactivates biogenic amines, including serotonin, dopa- inhibition of MAOA in adult humans has not been mine and norepinephrine. MAOA-deficient mice were reported to induce aggressive behavior; thus, the abnor- generated accidentally

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