5-HT1A, 5-HT1B and 5-HT2 receptor agonists induce differential behavioral responses in neonatal rat pups

Sprague-Dawley rat pups at 3–4 days prenatally were tested in both the absence and presence of milk following administration of various doses of either the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), the 5-HT_1B agonist 1-(3-chlorophenyl)piperazine (mCPP), or the 5-HT₂ agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Administration of 8-OHDPAT decreased mouthing, increased probing and increased behavioral activation. Conversely, the 5-HT₂ agonist DOI and the 5-HT_1B agonist mCPP increased mouthing and decreased probing. mCPP and DOI differed in their effects on behavioral activation, with mCPP decreasing and DOI increasing this composite behavioral score. mCPP increased grooming, whereas DOI elicited a characteristic unusual positioning of the limbs. Thus it appears that 5-HT_1A, 5-HT_1B and 5-HT₂ receptor subtypes are present in the neonate and elicit differential behavioral responses upon stimulation with selective agonists. Ontogenetic variations in the balance among these receptor subtypes during development may be related to the ontogenetic reversal that has been previously reported in the impact of serotonin manipulations on mouthing and suckling behavior during the neonatal to weanling age period.     

5-HT1A, 5-HT1B and 5-HT2 receptor agonists induce differential behavioral responses in preweanling rat pups

Preweanling (postnatal day 17-18) Sprague-Dawley rat pups were tested in both the absence and presence of milk following administration of various doses of the 5-HT1A agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or ipsapirone, the 5-HT1B agonist 1-(3-chlorophenyl)piperazine (mCPP) or the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI). 8-OH-DPAT decreased mouthing while ipsapirone, mCPP and DOI had no effect upon this behavior. However, all four agonists significantly decreased grooming. Both 8-OH-DPAT and mCPP produced alterations in limb positioning, with 8-OH-DPAT administration resulting in a poor control of the hindlimbs and mCPP inducing a hindlimb straddle position. These functional responses to 5-HT1A, 5-HT1B and 5-HT2 agonists in preweanling pups vary from those observed previously in neonates. For instance, whereas inhibitory effects of 5-HT1A stimulation on mouthing are observed in both neonatal and preweanling pups, facilitory effects of 5-HT1B and 5-HT2 stimulation are only seen in neonates. These ontogenetic alterations may be related to the previously reported ontogenetic reversal in the effect of serotonergic activation upon mouthing and suckling that occurs during the neonatal to weanling age period.     

5-HT1A receptor and 5-HT1B receptor knockout mice in stress and anxiety paradigms

Generation of receptor knockout mice has offered a new approach to study processes underlying anxiety. In this paper, studies focusing on anxiety using 5-HT1A receptor knockout (1AKO) and 5-HT1B receptor knockout (1BKO) mice are reviewed. 1AKO mice on different genetic background strains have initially been described as more anxious. In 1AKO mice on the 129/Sv background strain, the initial findings could not always be replicated, although under certain conditions, mild anxiety-like responses were observed in these 1AKO mice. In 1BKO mice, some indications of reduced anxiety have been found, but these observations may be confounded partly with increased motor impulsivity of these mutants. To study whether the putative effects of the null mutations on anxiety were reflected in the autonomic nervous system, basal heart rate and body temperature of 1AKO and 1BKO mice were measured, as well as their autonomic responses to novel cage exposure and to reversal of the light-dark rhythm. 1AKO mice did not differ from wild-type mice in any parameter, neither under non-stress conditions, nor following novel cage exposure. In 1BKO mice, basal heart rate was reduced and body temperature was increased. 1BKO mice showed exaggerated autonomic responses to novel cage stress. Adaptation to the reversal of the light-dark cycle was comparable in the three genotypes. The stress-induced hyperthermia procedure showed no differential responses of the three genotypes to the stressor. Pharmacological responses to various psychotropic drugs in the stress-induced hyperthermia test were also comparable in 1AKO, 1BKO and wild-type mice. The present data illustrate the complexity of studying the behavioural and physiological consequences of deletion of genes coding for important receptors in the CNS.     

Conflict behavior and the effects of 8-OHDPAT treatment in rats selectively bred for differential 5-HT(1A)-induced hypothermia

The high DPAT sensitivity (HDS) and low DPAT sensitivity (LDS) rat lines are the result of selective breeding for differences in the hypothermic response to acute treatment with the 5-HT(1A) receptor agonist 8-hydroxydipropylaminotetralin (8-OHDPAT). The HDS rats exhibit a much greater hypothermic response than do the LDS rats. The present study examined conflict anxiety-like behavior and the effects of acute challenges with 8-OHDPAT and phenobarbital (PhB) on conflict behavior in HDS and LDS rats. Water-restricted (24-h deprivation) HDS and LDS rats were trained to drink from a tube that was occasionally electrified. The 5-s bouts of drinking tube electrification occurred on a fixed interval (FI) 30-s schedule and were signaled by the presence of a tone. Under this schedule, responding is suppressed approximately 10-fold during the tone-on periods compared to the no-tone periods. After two weeks of training in this repeated measures drink suppression conflict paradigm, the effects of acute challenges with 8-OHDPAT (30-500 microg/kg, SC, +10 min) or PhB (20 mg/kg, IP, +10 min) were determined. In control (i.e. , non-drug) conflict test sessions, rats of the HDS line accepted significantly fewer shocks than did rats of the LDS line. Acute treatment with 8-OHDPAT resulted in a modest increase in punished responding (maximum increase: +30-40 shocks/session) in both lines at doses of 60 and 125 microg/kg. Higher doses produced significant general behavioral disruption and substantial reductions in water intake (unpunished responding) in both HDS and LDS rats. Neither the increase in shocks received nor the decrease in water intake produced by these 8-OHDPAT challenges differed between HDS and LDS rats. In both lines, acute PhB treatment resulted in a more dramatic increase in punished responding than did 8-OHDPAT (+55-65 shocks/session) and an increase in water intake. The effects of PhB also did not differ between HDS and LDS rats. These data suggest that the HDS and LDS rats exhibit differences in baseline anxiety-like behavior in the conflict task, but do not differ in their response to acute challenges with PhB or 8-OHDPAT.     

Long-term fluoxetine treatment decreases 5-HT1A receptor responsivity in obsessive-compulsive disorder

Fluoxetine (FLX) is a selective serotonin (5-HT) reuptake inhibitor with therapeutic benefit in patients with obsessive-compulsive disorder (OCD). To evaluate the effect of chronic FLX treatment on 5-HT_1A receptor responsivity, hypothermic, neuroendocrine, and behavioral responses to the selective 5-HT_1A receptor ligand ipsapirone (IPS) were examined in patients with primary OCD. A single dose of 0.3 mg/kg of IPS or placebo were given under double-blind, random-assignment conditions to ten patients before and during FLX treatment. The ability of IPS to induce hypothermia and ACTH/cortisol release was significantly attenuated during chronic FLX as compared to the pretreatment IPS challenge. The behavioral effects of IPS, though minimal, were less pronounced during FLX treatment. While FLX was effective in reducing the severity of OC symptoms, no significant correlation between attenuation of 5-HT_1A receptor-mediated functional measures and FLX-induced improvement in OC symptoms was detected. These findings are consistent with the development of adaptive hyporesponsivity of the 5-HT_1A receptor-effector system complex possibly involving subsensitivity of the 5-HT_1A receptor itself and/or decreased functional activity of the postreceptor signal transduction. Modulation of 5-HT_1A receptor-effector system function may be critical to the antidepressant/anti-OC efficacy of 5-HT reuptake inhibitors.Part of this work was presented at the 17th Congress of the CINP, Kyoto, Japan, September 10–14, 1990     

Urocortin 1 expression in five pairs of rat lines selectively bred for differences in alcohol drinking

Rationale There is accumulating evidence that the neuropeptide urocortin 1 (Ucn1) is involved in alcohol consumption. Thus far, however, most studies have been performed in mice. Objectives The purpose of the present study was to characterize Ucn1 expression in rats selectively bred for either high or low alcohol intake. Methods Brains from naive male rats of five pairs of independently selected lines (iP/iNP, AA/ANA, HARF/LARF, HAD1/LAD1, and HAD2/LAD2) were analyzed by immunohistochemistry. Results Significant differences were found between iP/iNP, HARF/LARF, and HAD2/LAD2 in number of Ucn1-containing cells in the Edinger–Westphal (EW) nucleus (the main source of Ucn1 in the brain), whereas no significant differences were found between HAD1/LAD1 and AA/ANA. Similarly, significant differences in the optical density of Ucn1 immunoreactivity in EW were found between iP/iNP, HARF/LARF, and HAD2/LAD2, whereas no differences on this measure were found between HAD1/LAD1 and AA/ANA. In the lateral septum (LS, the main projection area of Ucn1-containing neurons in the rat), significant differences were found only between AA/ANA and HAD2/LAD2; however, a meta-analysis indicated that across all five lines, preferring animals had a significantly greater number of Ucn1-positive fibers than nonpreferring animals. Conclusions These results provide evidence that, in rats, Ucn1 may be involved in regulation of alcohol intake, and that this regulation may occur through the Ucn1 projections to LS.     

Chronic effects of corticosterone on GIRK1-3 subunits and 5-HT1A receptor expression in rat brain and their reversal by concurrent fluoxetine treatment

Dysregulation of the serotonergic system and abnormalities of the hypothalamic–pituitary–adrenal axis have been demonstrated in major depression. Animal studies indicate that 5-HT_1A receptor expression may be reduced by long-term administration of corticosterone. However, similar studies on the regulation of GIRK channels, one of the most important effectors of the neuronal 5-HT_1A receptor, are limited. In order to address these issues, slow-release corticosterone pellets were implanted subcutaneously to adrenal intact male rats (200 mg pellets, 35 days release). Starting on day 15, animals were treated for 21 days with fluoxetine (5 mg/kg/day, i.p.), or vehicle. Using in situ hybridization histochemistry and receptor autoradiography, we found that chronic corticosterone treatment was accompanied by a significant decrease on the mRNAs coding for mineralocorticoid receptors in hippocampal areas. Under these conditions, 5-HT_1A receptor mRNA expression decreased in dorsal raphe nucleus and dentate gyrus. However, 5-HT_1A receptor levels, as measured by [³H]-8-OH-DPAT binding, diminished significantly only in dentate gyrus. It is noteworthy that chronic treatment with fluoxetine reversed the alterations on 5-HT_1A receptor mRNA levels only in dorsal raphe. Finally, chronic corticosterone treatment produced an increase on the mRNA coding for the GIRK2 subunit in several hypothalamic and thalamic areas, which was reversed by fluoxetine. Measurements of cell density and volume of the granular layer of the dentate gyrus did not reveal significant changes after corticosterone or corticosterone plus fluoxetine treatments. These data are relevant for a better understanding of the differential regulation of pre- and postsynaptic 5-HT_1A receptors by corticosterone flattened rhythm.     

Alcohol self-administration in two rat lines selectively bred for extremes in anxiety-related behavior.

According to the tension reduction hypothesis, individuals with an elevated anxiety level may be more sensitive to the anxiolytic effects of alcohol and may, therefore, have a higher predisposition to consume alcohol. To examine this hypothesis, we studied the drinking behavior as well as the sensitivity to the anxiolytic effect of alcohol in two rat lines that were bred and selected for differences in anxiety-related behavior on the elevated plus-maze: the extremely anxious HAB (high anxiety-related behavior) and the non-anxious LAB (low anxiety-related behavior) lines. Alcohol self-administration and the occurrence of an alcohol deprivation effect were studied in female and male HAB and LAB rats in a free-choice, 4-bottle home cage paradigm. The sensitivity of HAB and LAB rats to the anxiolytic effect of alcohol was assessed by testing their behavior on the elevated plus-maze after an acute application of ethanol. During the first days of voluntary ethanol drinking, the ethanol intake and preference of female LABs was significantly higher than that of female HABs. Although not statistically significant, the same trend could be seen in male LABs. Moreover, male as well as female LAB but not HAB rats showed a significant alcohol deprivation effect after abstinence. There were no differences when saccharin was presented to naive animals, indicating that the different ethanol drinking behavior of HAB and LAB rats does not represent a general difference in the consumption of new liquids. Application of ethanol resulted in an anxiolytic effect in HAB but not in LAB rats on the elevated plus-maze. In summary, increased inborn anxiety and voluntary ethanol consumption of HAB and LAB rats were correlated to some extent; however, this relationship was a negative one. It is concluded that, although such a relationship might exist in some individuals, increased levels of inborn anxiety and alcohol consumption are not necessarily related.     

Modulation of 5-HT4 receptor function in the rat isolated ileum by fluoxetine: the involvement of endogenous 5-hydroxytryptamine

The effect of the selective serotonin reuptake inhibitor fluoxetine was examined on the 5-HT4 receptor-mediated relaxation in the rat isolated ileum. Fluoxetine unsurmountably antagonized the relaxation to exogenous 5-HT with abolition of the response at 10 microM. Fluoxetine (10 microM) also caused a gradual loss of the resting tension. These effects of fluoxetine were prevented by a prior addition of the 5-HT4 receptor selective antagonist GR113808 (100 nM), which itself caused a contraction of the tissues when administered alone. Fluoxetine (10 microM) also failed to prevent the relaxation due to exogenous 5-HT and the 5-HT4 receptor agonist 5-methoxytryptamine in tissues taken from the rats treated with para-chlorophenylalanine (300 mg kg-1) for 3 and 6 days, which reduced the 5-HT level in the mucosa by 88 and 97.5% respectively. The contraction of the tissues with GR113808 indicates the presence of an endogenous 5-HT tone at the 5-HT4 receptor in the rat ileum. It is hypothesized that in the presence of fluoxetine, the concentration of endogenous 5-HT at the receptor was increased sufficiently to reduce or abolish the relaxation to 5-HT added exogenously. The inability of fluoxetine to prevent the relaxation to 5-HT in the presence of GR113808 or after the p-CPA treatment supports this hypothesis.     

Chronic fluoxetine induces opposite changes in G protein coupling at pre and postsynaptic 5-HT1A receptors in rat brain

Chronic treatment with the antidepressant fluoxetine may lead to changes in the properties of pre- and postsynaptic 5-HT(1A) receptors due to modifications in the receptor-G protein coupling process. We have evaluated, in rats, the effect of chronic fluoxetine (10 mg/kg/day) at brain 5-HT(1A) receptors using different techniques. The density of 5-HT(1A) receptors was unchanged in fluoxetine-treated rats vs. vehicle group. Stimulation of [(35)S]GTPgammaS binding induced by (+/-)8-OH-DPAT was significantly attenuated in dorsal raphe nucleus after fluoxetine (+3.7 vs. +31.2% in vehicle). The inhibition of dorsal raphe firing by (+/-)8-OH-DPAT (ED(50) in vehicle = 2.1 microg/kg, i.v.) was also attenuated in rats treated with fluoxetine (ED(50)=4.7 microg/kg). In contrast, a significant increase on (+/-)8-OH-DPAT-induced stimulation of [(35)S]GTPgammaS binding was observed in CA(1) (+53.4 vs.+20.2% in vehicle) and dentate gyrus (+105.7 vs. +52.6% in vehicle) but not in entorhinal cortex. Our data demonstrate that fluoxetine-induced desensitization of 5-HT(1A) autoreceptors occurs at G protein level. Moreover, a relevant finding is the region-specific hypersensitivity of postsynaptic 5-HT(1A) receptors, in the hippocampus but not in entorhinal cortex, following chronic fluoxetine. These differential adaptive changes in brain 5-HT(1A) receptors could underlie the mechanism of action of antidepressants and also contribute to their clinical effects.     

Evidence for the differential co-localization of neurokinin-1 receptors with 5-HT receptor subtypes in rat forebrain

Studies suggest that like selective 5-hydroxytryptamine (5-HT; serotonin) reuptake inhibitors, antagonists at neurokinin-1 receptors (NK(1)Rs) may have antidepressant and anxiolytic properties. NK(1)Rs are present in 5-HT innervated forebrain regions which may provide a common point of interaction between these two transmitter systems. This study aimed to investigate for cellular co-localization between NK(1)Rs and 5-HT receptor subtypes in mood-related brain regions in the rat forebrain. With experiments using fluorescence immunocytochemistry, double-labelling methods demonstrated a high degree of co-localization between NK(1)Rs and 5-HT(1A) receptors in most regions examined. Co-localization was highest in the medial septum (88% NK(1)R expressing cells were 5-HT(1A) receptor-positive) and hippocampal regions (e.g. dentate gyrus, 65%), followed by the lateral/basolateral amygdala (35%) and medial prefrontal cortex (31%). In contrast, co-localization between NK(1)Rs and 5-HT(2A) receptors was infrequent (< 8%) in most areas examined except for the hippocampus (e.g. CA3, 43%). Overall co-localization between NK(1)Rs and 5-HT(1A) receptors was much greater than that between NK(1)Rs and 5-HT(2A) receptors. Thus, these experiments demonstrate a high degree of co-localization between NK(1)Rs and 5-HT(1A) receptors in cortical and limbic regions of the rat forebrain. These findings suggest a novel site of interaction between NK(1)R antagonists and the 5-HT system.     

5-HT1A receptor knockout mouse as a genetic model of anxiety

The major targets of current drugs used in mental health, such as neurotransmitter receptors and transporters, are based on serendipitous findings from several decades ago, and there is currently a severe drought of new drug targets. There is a pressing need for novel drugs, and much hope has been placed on the use of molecular genetics to help define them. However, despite evidence for a genetic basis to schizophrenia stretching back for over a century, and a heritability of about 80%, the identification of susceptibility genes has been an uphill struggle. Candidate gene studies, which have generally focussed on obvious candidates from the dopamine and serotonin systems, as well as genes involved in brain development, have not generally been successful, although meta-analysis indicates that the dopamine D3 receptor gene (DRD3) and the serotonin receptor gene type 2A (HTR2A) may have a very small influence on risk. Linkage analysis has provided robust evidence of genetic loci, for example, on chromosomes 8p, 13q and 22q, and also implies shared genetic aetiology with bipolar disorder. The identification of these loci together with advances in genetic technology, especially the characterisation of polymorphisms, the understanding of haplotypes and the development of statistical methods, has lead to the identification of several plausible susceptibility genes, including neuregulin 1, proline dehydrogenase and dysbindin. Interestingly, these genes point more towards a role for the glutamate pathway rather than the dopamine pathway in schizophrenia. We have attempted to replicate some of these findings in schizophrenic patients from SW China, and we find significant association with a novel neuregulin 1 haplotype, with proline dehydrogenase polymorphisms, but not with catechol-O-methyltransferase (COMT). The replication of neuregulin 1 association on chromosome 8p by several investigators is the most convincing to date, and the presence of a syndrome similar to dementia praecox of 8p linked families, and the lack of linkage of bipolar disorder to this region is a testament to the ideas of Kraepelin more than 100 years ago.     

Corticosterone modulation of somatodendritic 5-HT1A receptor function in mice

Corticosteroid modulation of serotonergic function may play a central role in mood disorders. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) produces a hypothermia in mice that serves as an in-vivo model of somatodendritic 5-HT1A autoreceptor function. Daily injections (s.c.) of 50 mg/kg of corticosterone (CORT) for 3 days attenuates 8-OH-DPAT hypothermia tested 24 h later. This study sought to further clarify the nature of the CORT-mediated attenuation of somatodendritic 5-HT1A receptor function. Mice underwent various CORT manipulations prior to an 8-OH-DPAT challenge. Neither 14-day bilateral adrenalectomy (ADX), nor CORT 50 mg/kg/day, administered continuously by osmotic minipump over 72 h had any effect on the 8-OH-DPAT hypothermic response. In contrast, daily injections of CORT over three consecutive days caused a significant attenuation in 8-OH-DPAT hypothermia when tested 24 h later. However, administration of an additional dose of CORT 2 h prior to the 8-OH-DPAT challenge occluded this CORT-mediated attenuation in a dose-dependent fashion. The findings demonstrate that CORT modulates somatodendritic 5-HT1A receptor function in a complex manner. Attenuation is seen only after intermittent administration of CORT. In addition, the degree of attenuation depends on CORT concentrations at the time of testing. These findings may have implications regarding mechanisms of adaptation to stress.     

Avoidance behavior in rats selectively bred for differential alcohol sensitivity

Most affected (MA) and least affected (LA) rats, bred for extremes in motor impairment following an alcohol challenge, differed in their performance on two active avoidance tasks. In two-way shuttle avoidance, the MA line performed significantly better than the LA group, both in terms of response latencies and percent avoidances. The inferior performance of the LA line persisted across the 15 days of testing, and appeared to reflect an difference in asymptotic performance levels. In one-way avoidance, the MA line showed significantly better acquisition than the LA group; however, this difference dissipated across the 3 days of training. When tested following alcohol administration in either the one-or two-way avoidance paradigm, the MA rats showed a greater performance deficit than LA animals. These data were interpreted as indicating the generality of alcoholrelated line differences to a situation motivated by aversive consequences. Moreover, the line difference in avoidance acquisition represents one of the few non-drug-related phenotypic differences that have been found in these lines. In previous generations, disparate base rates of wheel running have been reported, and the data presented here confirm and extend this finding.     

The 5-HT(1A) receptor knockout mouse and anxiety.

The 5-HT(1A) receptor has been implicated in the modulation of anxiety processes, mainly via pharmacological experiments. The recent production, in three independent research groups, of 5-HT(1A) receptor knockout (R KO) mice in three different genetic backgrounds (C57BL/6J, 129/Sv, Swiss-Webster) led to the intriguing finding that all mice, independent from the genetic background strain from which the null mutants were made, showed an "anxious" phenotype compared to corresponding wild-type mice. The present paper reviews the behavioral findings in these three KO lines and focuses on new findings in the 129/Sv-KO mice. These mice were more anxious or stress-prone only under specific conditions (high stress) and not as broadly as suggested from the initial studies. The 5-HT(1A) R KO made in the Swiss-Webster background displays disturbances in the GABA(A)-benzodiazepine (BZ) receptor system in the brain, including downregulation of GABA(A) alpha1 and alpha2 subunits in the amygdala. In contrast, the GABA(A)-BZ receptor system seems to function normally in the 5-HT(1A) R KO in the 129/Sv background suggesting that changes in the GABA(A)-BZ receptor system may not be a prerequisite for anxiety but rather could have a modifying effect on this phenotype. It can be concluded that the constitutive absence of the 5-HT(1A) receptor gene and receptor leads to a more "anxious" mouse, dependent on the stress level but independent from the strain. Depending on the genetic background, this null mutation may be associated with changes in GABA(A)-ergic neurotransmission. It is as yet unclear which mechanisms are involved in this intriguing differentiation.     

Voluntary alcohol intake in two rat lines selectively bred for learned helpless and non-helpless behavior

Rationale A high comorbidity between depression and alcoholism has been reported in several studies, but the mechanisms underlying this relationship remain unknown.Objectives We tested whether learned helplessness in rats as a model for depression is associated with enhanced alcohol intake and relapse behavior.Methods Congenital learned helplessness (cLH) and congenital non-learned helplessness (cNLH) rats were selectively bred for differences in an escape paradigm. Sucrose preference was tested at the first hour of the dark phase. In order to study an association with alcohol drinking behavior, rats underwent a free-choice procedure with access to water, and 5% and 20% alcohol solutions for 6 weeks. After acquisition of alcohol drinking behavior, the alcohol deprivation effect (ADE) was assessed. Sensitivity to the sedative-hypnotic effect of alcohol was measured by loss of the righting reflex.Results cLH rats showed significantly lower preference for sucrose solutions during the second half hour of the dark phase than cNLH rats. Alcohol intake of male cLH rats was not significantly different from that of male cNLH rats. In contrast, cLH female rats consumed higher amounts of alcohol than female cNLH rats. The ADE was more pronounced in female animals, although the magnitude of the ADE was similar in both cNLH and cLH female rats. The time to regain the righting reflex was significantly higher in both male and female cLH rats than in cNLH rats.Conclusions In summary, these data suggest that an inborn depressive-like behavior in female rats is associated with enhanced alcohol intake.     

Chronic fluoxetine treatment selectively uncouples raphe 5-HT(1A) receptors as measured by [(35)S]-GTP gamma S autoradiography

1. Selective Serotonin Reuptake Inhibitors (SSRIs) are thought to have a delay in therapeutic efficacy because of the need to overcome the inhibitory influence of raphe 5-HT(1A) autoreceptors. Prolonged SSRI administration has been reported to desensitize these autoreceptors. We have used [(35)S]-GTP gamma S autoradiography to determine whether this desensitization occurs at the level of receptor/G protein coupling. 2. Male mice were injected intraperitoneally once a day with saline or 20 mg kg(-1) fluoxetine for either 2 days or 14 days. 5-HT(1A) receptor binding and coupling to G proteins were assessed using [(3)H]-8-OH-DPAT and [(35)S]-GTP gamma S autoradiography, respectively. 3. The 5-HT receptor agonist 5-carboxamidotryptamine (5-CT) stimulated [(35)S]-GTP gamma S binding in the substantia nigra, as well as in hippocampus and dorsal raphe nucleus. The 5-HT(1A) receptor antagonist p-MPPF (4-fluoro-N-(2-[4-(2-methoxyphenyl)1-piperazinyl]ethyl)-N-(2-pyridinyl)benzamide) blocked this effect in the latter regions, whereas the 5-HT(1B/D) antagonist GR-127,935 (2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-l-yl)-phenyl]-amide) only decreased labelling in substantia nigra. 4. Fourteen-day fluoxetine treatment decreased 5-CT-stimulated [(35)S]-GTP gamma S binding in dorsal raphe (saline: 112 +/- 12% stimulation; fluoxetine: 66 +/- 13%), but not in substantia nigra (99 +/- 14% vs 103 +/- 7%) or hippocampus (157 +/- 3% vs 148 +/- 18%). Two-day fluoxetine treatment did not alter 5-CT-stimulated [(35)S]-GTP gamma S binding in any of the brain areas investigated. 5. Decreased [(35)S]-GTP gamma S binding was not due to receptor down-regulation, since the density of raphe [(3)H]-8-OH-DPAT binding sites was unaffected by fluoxetine treatment. 6. These results suggest that the desensitization of presynaptic 5-HT(1A) receptor function occurs at the level of receptor-G protein interaction on dorsal raphe neurons, and may underlie the therapeutic efficacy of long-term SSRI treatment.     

Domestication alters 5-HT1A receptor binding in rat brain.

Serotonin-1A receptor binding density was compared in the brains of wild and domesticated adult male Rattus norvegicus using in vitro receptor autoradiography of [3H]8-hydroxy-2-[n-dipropylamino]tetraline (DPAT). While both groups exhibited similar patterns of labeling, [3H]DPAT binding density was significantly (p less than or equal to 0.05) lower in the median raphe nucleus and greater in superficial entorhinal cortex and rostral dentate gyrus of domesticated compared to wild rats. The results suggest that specific serotonergic circuits from the median raphe nucleus to the entorhinal and hippocampal regions might be involved in regulation of the defensive behaviors that differ profoundly between wild and domesticated rats. The relationship of these putative differences to behavioral disorders such as anxiety and depression in humans is discussed.