Serotonergic modulation of the limbic system

Early postnatal stress affects the serotonergic function in the median raphe nuclei of adult rats

Recent studies have focused on the serotonergic mechanism mediated via serotonin (5-HT) receptors underlying regulation of emotional stress during the developmental period. The present study was undertaken to elucidate whether early postnatal stress affects rat brain development and influences the serotonergic function in the midbrain median raphe nuclei (MRN) and dorsal raphe nuclei (DRN) in the adult, focusing on the response to unconditioned fear stress. Rats received aversive foot shock (FS) stimuli at the third week of the postnatal period (3wFS), but not those at the second week (2wFS), had increased percentage of time spent on open arms, estimated by the elevated plus maze test, at the postadolescent period (10-12 weeks old). The anxiolytic behavior observed in 3wFS was similar to that in rats having electrolytic lesion of the MRN, but not the DRN. In addition, the number of MRN 5-HT-immunoreactive cells in 3wFS remarkably was reduced compared to the non-FS control and 2wFS groups. These data suggest that aversive stress at the third week is attributable to the serotonergic function in the MRN underlying regulation of unconditioned fear stress. In other words, the "critical period" appears to be the time of neural circuit development of the MRN serotonergic system, which may be implicated in lifelong susceptibility to emotional stress.

Anxiolytic-like and antidepressant-like effects produced by the selective 5-HT6 receptor antagonist SB-258585 after intrahippocampal administration to rats

The purpose of this study was to investigate the effect of SB-258585, a selective 5-HT6 receptor antagonist, administered intrahippocampally to rats, in the conflict drinking and forced swim tests, that is models used for evaluating anxiolytic-like and antidepressant-like activity, respectively. Diazepam and imipramine were used as reference drugs. SB-258585 at a dose of 1 microg (but not 0.3 and 3 microg) showed an anticonflict effect that was weaker than that of diazepam (40 microg). SB-258585 at a dose of 3 microg (but not 1 and 10 microg) produced a marked anti-immobility effect comparable with that of imipramine (0.1 microg). The anxiolytic-like and antidepressant-like activity of SB-258585 seemed to be specific, as that compound--when given by the same route in doses effective in each model--did not affect the shock threshold, nonpunished water consumption, or exploratory activity of rats. The results obtained indicate that the hippocampus is one of the neuroanatomical sites involved in the anxiolytic-like and antidepressant-like activity of the selective 5-HT6 receptor antagonist SB-258585.

Understanding genetic risk for aggression: clues from the brain's response to social exclusion

BACKGROUND

Although research indicates a relationship between the monoamine oxidase-A (MAOA) gene and aggression, the intervening neural and psychological mechanisms are unknown. Individuals with the low expression allele (MAOA-L) of a functional polymorphism in the MAOA gene might be prone to aggression because they are socially or emotionally hyposensitive and thus care less about harming others or because they are socially or emotionally hypersensitive and thus respond to negative social experiences with defensively aggressive behavior.

METHODS

We investigated the relationships between the MAOA polymorphism, trait aggression, trait interpersonal hypersensitivity, and neural responses to social exclusion in 32 healthy men and women.

RESULTS

The MAOA-L individuals (men and women) reported higher trait aggression than individuals with the high expression allele (MAOA-H). The MAOA-L individuals reported higher trait interpersonal hypersensitivity and showed greater dorsal anterior cingulate cortex (dACC) activity (associated with rejection-related distress) to social exclusion compared with MAOA-H individuals, consistent with a social hypersensitivity hypothesis. Moreover, the MAOA-aggression relationship was mediated by greater dACC reactivity to social exclusion, suggesting that MAOA might relate to aggression through socioemotional hypersensitivity.

CONCLUSIONS

These data suggest that the relationship between MAOA and aggression might be due to a heightened rather than a reduced sensitivity to negative socioemotional experiences like social rejection.

Neurobiology of memory and anxiety: from genes to behavior

Interaction of anxiety and memory represents an essential feature of CNS functioning. This paper reviews experimental data coming from neurogenetics, neurochemistry, and behavioral pharmacology (as well as parallel clinical findings) reflecting different mechanisms of memory-anxiety interplay, including brain neurochemistry, circuitry, pharmacology, neuroplasticity, genes, and gene-environment interactions. It emphasizes the complexity and nonlinearity of such interplay, illustrated by a survey of anxiety and learning/memory phenotypes in various genetically modified mouse models that exhibit either synergistic or reciprocal effects of the mutation on anxiety levels and memory performance. The paper also assesses the putative role of different neurotransmitter systems and neuropeptides in the regulation of memory processes and anxiety, and discusses the role of neural plasticity in these mechanisms.

5-HT1A receptors and memory

The study of 5-hydroxytryptamine (5-HT) systems has benefited from the identification, classification and cloning of multiple 5-HT receptors (5-HT(1)-5-HT(7)). Increasing evidence suggests that 5-HT pathways, reuptake site/transporter complex and 5-HT receptors represent a strategic distribution for learning and memory. A key question still remaining is whether 5-HT markers (e.g., receptors) are directly or indirectly contributing to the physiological and pharmacological basis of memory and its pathogenesis or, rather, if they represent protective or adaptable mechanisms (at least in initial stages). In the current paper, the major aim is to revise recent advances regarding mammalian 5-HT(1A) receptors in light of their physiological, pathophysiological and therapeutic implications in memory. An attempt is made to identify and discuss sources of discrepancies by employing an analytic approach to examine the nature and degree of difficulty of behavioral tasks used, as well as implicating other factors (for example, brain areas, training time or duration, and drug administration) which might offer new insights into the understanding and interpretation of these data. In this context, 8-OH-DPAT deserves special attention since for many years it has been the more selective 5-HT drug and, hence, more frequently used. As 5-HT(1A) receptors are key components of serotonergic signaling, investigation of their memory mechanisms and action sites and the conditions under which they might operate, could yield valuable insights. Moreover, selective drugs with agonists, neutral antagonists or inverse agonist properties for 5-HT(1A) (and 5-HT(7)) receptors may constitute a new therapeutic opportunity for learning and memory disorders.

Serotonin and Dopamine Interactions in Rodents and Primates: Implications for Psychosis and Antipsychotic Drug Development

Since the late 1950s, appreciation of dopamine receptor blockade has played a primary role in understanding the mechanism underlying the therapeutic effects of antipsychotic drugs in schizophrenic patients in treating the positive symptoms of schizophrenia (e.g., delusions and hallucinations). Development of the second generation of antipsychotic drugs, otherwise known as atypical antipsychotic drugs, has resulted in treatments with improved subjective tolerability but relatively modest improvements in the negative symptoms of schizophrenia such as avolition, flat affect, and anhedonia. The major current challenge is to develop medications which can further improve negative symptoms treatment and also tackle the intractable clinical problems of cognitive impairment associated with schizophrenia. Further advances along these lines with respect to the dopaminergic and serotonergic neurostransmitter systems will be aided by an appreciation of the interaction between dopamine and serotonin receptor subtypes in a range of key brain structures, such as the prefrontal cortex, thalamus, striatum, amygdala, hippocampus, and the brain stem nuclei, from which the cell bodies of monoaminergic-containing neurons originate. Increasing emphasis on the use of animal models which are homologous to critical aspects of the pathophysiology in the brains of schizophrenic patients will also be required, especially as negative symptoms and cognitive impairment become an important focus for generating novel therapeutics.

Potential anxiolytic and antidepressant effects of the selective 5-HT7 receptor antagonist SB 269970 after intrahippocampal administration to rats

Using conflict drinking and forced swimming tests in rats, we examined the anxiolytic- and the antidepressant-like activity, respectively, of (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]-pyrrolidine (SB 269970), a selective 5-HT(7) receptor antagonist, after its intrahippocampal administration. SB 269970 at doses of 0.3, 1 and 3 mug showed an anticonflict effect which was weaker than that of diazepam (40 mug), whereas SB 269970 at doses of 3 and 10 mug had marked anti-immobility action comparable to that of imipramine (0.1 mug). Importantly, the anxiolytic- and antidepressant-like activity of SB 269970 seemed to be specific, since that agent - when given by the same route in doses effective in either model - affected neither the shock threshold, nor the non-punished water consumption, nor the exploratory activity of rats. The obtained results indicate that the hippocampus is one of the neuroanatomical structures involved in the potential anxiolytic and, in particular, antidepressant activity of SB 269970.

Fatty acids differentially affect serotonin receptor and transporter binding in the rat brain

The aim of this study was to examine the influence of different fat diets on serotonin receptor and transporter binding. Male Sprague-Dawley rats were fed a diet of either high saturated fat, omega-6 polyunsaturated fatty acid, omega-3 polyunsaturated fatty acid or low fat (control) for eight weeks. Using Beta-Imager quantification techniques, [(3)H]ketanserin, [(3)H]mesulergine and [(3)H]paroxetine binding to serotonin (5-HT)(2A), 5-HT(2C) receptors and 5-HT transporters (5-HTT) was measured throughout the brain in all four groups. All three high fatty acid diets influenced serotonin receptor binding, however the most pronounced effects were that compared with the low fat control group, i) 5-HT(2A) receptor binding was increased in the caudate putamen, but reduced in the mammillary nucleus in high saturated fat and high omega-6 polyunsaturated fatty acid diet groups; ii) 5-HT(2C) receptor binding was reduced in the mamillary nucleus of saturated fat group and reduced in prefrontal cortex of the omega-6 polyunsaturated fatty acid and omega-3 polyunsaturated fatty acid groups; and iii) 5-HTT binding was reduced in the hippocampus in the omega-6 polyunsaturated fatty acid group. Overall, the omega-6 polyunsaturated fatty acid diet exerted the most influence on serotonin receptor and transporter binding. These results may be of importance in relation to neuropsychiatric diseases such as schizophrenia, where associations between altered fatty acid levels and the serotonergic system have been made.