The role of the dopamine D1 receptor in social cognition: studies using a novel genetic rat model

Social cognition is an endophenotype that is impaired in schizophrenia and several other (comorbid) psychiatric disorders. One of the modulators of social cognition is dopamine, but its role is not clear. The effects of dopamine are mediated through dopamine receptors, including the dopamine D1 receptor (Drd1). Because current Drd1 receptor agonists are not Drd1 selective, pharmacological tools are not sufficient to delineate the role of the Drd1. Here, we describe a novel rat model with a genetic mutation in Drd1 in which we measured basic behavioural phenotypes and social cognition. The I116S mutation was predicted to render the receptor less stable. In line with this computational prediction, this Drd1 mutation led to a decreased transmembrane insertion of Drd1, whereas Drd1 expression, as measured by Drd1 mRNA levels, remained unaffected. Owing to decreased transmembrane Drd1 insertion, the mutant rats displayed normal basic motoric and neurological parameters, as well as locomotor activity and anxiety-like behaviour. However, measures of social cognition like social interaction, scent marking, pup ultrasonic vocalizations and sociability, were strongly reduced in the mutant rats. This profile of the Drd1 mutant rat offers the field of neuroscience a novel genetic rat model to study a series of psychiatric disorders including schizophrenia, autism, depression, bipolar disorder and drug addiction.

Reduced cocaine-induced serotonin, but not dopamine and noradrenaline, release in rats with a genetic deletion of serotonin transporters

It has recently been proposed that the increased reinforcing properties of cocaine and ecstasy observed in rats with a genetic deletion of serotonin transporters are the result of a reduction in the psychostimulant-induced release of serotonin. Here we provide the neurochemical evidence in favor of this hypothesis and show that changes in synaptic levels of dopamine or noradrenaline are not very likely to play an important role in the previously reported enhanced psychostimulant intake of these serotonin transporter knockout rats. The results may very well explain why human subjects displaying a reduced expression of serotonin transporters have an increased risk to develop addiction.

Noradrenaline-induced release of newly-synthesized accumbal dopamine: differential role of alpha- and beta-adrenoceptors

Previous studies have shown that intra-accumbens infusion of isoproterenol (ISO), a beta-adrenoceptor-agonist, and phenylephrine (PE), an alpha-adrenoceptor-agonist, increase the release of accumbal dopamine (DA). In the present study we analyzed whether the ISO-induced release of DA is sensitive to pretreatment with the DA synthesis inhibitor alpha-methyl-para-tyrosine (AMPT). Earlier studies have shown that the PE-induced release of DA is derived from DA pools that are resistant to AMPT. In addition to PE, the alpha-adrenoceptor-antagonist phentolamine (PA) was also found to increase accumbal DA release. Therefore, we investigated whether similar to the DA-increasing effect of PE, the DA increase induced by PA is resistant to AMPT. Pretreatment with AMPT prevented the ISO-induced increase of accumbal DA. The accumbal DA increase after PA was not reduced by the DA synthesis inhibitor, independently of the amount of DA released. These results show that mesolimbic beta-, but not alpha-adrenoceptors, control the release of accumbal newly-synthesized DA pools. The DA-increasing effects of PE have previously been ascribed to stimulation of presynaptic receptors located on noradrenergic terminals, whereas the DA-increasing effects of PA and ISO have been ascribed to an action of these drugs at postsynaptic receptors on dopaminergic terminals. The fact that AMPT did not affect the accumbal DA response to PE and PA, whereas it did prevent the accumbal DA increase to ISO, supports our previously reported hypothesis that the noradrenergic neurons of the nucleus accumbens containing presynaptic alpha-adrenoceptors impinge upon the dopaminergic terminals in the nucleus accumbens containing postsynaptic adrenoceptors of the alpha but not of the beta type. The putative therapeutic effects of noradrenergic agents in the treatment of DA-related disorders are shortly discussed.

Maternal care affects the phenotype of a rat model for schizophrenia

Schizophrenia is a complex mental disorder caused by an interplay between genetic and environmental factors, including early postnatal stressors. To explore this issue, we use two rat lines, apomorphine-susceptible (APO-SUS) rats that display schizophrenia-relevant features and their phenotypic counterpart, apomorphine-unsusceptible (APO-UNSUS) rats. These rat lines differ not only in their gnawing response to apomorphine, but also in their behavioral response to novelty (APO-SUS: high, APO-UNSUS: low). In this study, we examined the effects of early postnatal cross-fostering on maternal care and on the phenotypes of the cross-fostered APO-SUS and APO-UNSUS animals later in life. Cross-fostered APO-UNSUS animals showed decreased body weights as pups and decreased novelty-induced locomotor activity as adults (i.e., more extreme behavior), in accordance with the less appropriate maternal care provided by APO-SUS vs. their own APO-UNSUS mothers (i.e., the APO-SUS mother displayed less non-arched-back nursing and more self-grooming, and was more away from its nest). In contrast, cross-fostered APO-SUS animals showed increased body weights as pups and reduced apomorphine-induced gnawing later in life (i.e., normalization of their extreme behavior), in line with the more appropriate maternal care provided by APO-UNSUS relative to their own APO-SUS mothers (i.e., the APO-UNSUS mother displayed more non-arched-back nursing and similar self-grooming, and was not more away). Furthermore, we found that, in addition to arched-back nursing, non-arched-back nursing was an important feature of maternal care, and that cross-fostering APO-SUS mothers, but not cross-fostering APO-UNSUS mothers, displayed increased apomorphine-induced gnawing. Thus, cross-fostering not only causes early postnatal stress shaping the phenotypes of the cross-fostered animals later in life, but also affects the phenotypes of the cross-fostering mothers.

Accumbal core: essential link in feed-forward spiraling striato-nigro-striatal in series connected loop

The goal of the present study was to establish the behavioral role of the nucleus accumbens (Nacc) core in the feed-forward spiraling striato-nigro-striatal circuitry that transmits information from the Nacc shell toward the dorsal subregion of the neostriatum (DS) in freely moving rats. Unilateral injection of μ-opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO; 1 and 2 μg), but not the δ 1-opioid receptor agonist [D-Pen(2,5)]-enkephalin (4 μg) or the δ2-opioid receptor agonist [D-Ala(2),Glu(4)]-deltorphin (2 μg), into the ventral tegmental area (VTA) produced contraversive circling in a dose-dependent manner. The effect of DAMGO was μ-opioid receptor-specific, because the μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2 (0.1 and 1 μg), which alone did not elicit any turning behavior, dose-dependently inhibited the effect of DAMGO. Injection of the dopamine D1/D2 receptor antagonist cis-(Z)-flupentixol (1 and 10 μg) into the Nacc shell ipsilaterally to the VTA significantly inhibited DAMGO (2 μg)-induced circling. Similar injections of cis-(Z)-flupentixol into the Nacc core inhibited DAMGO-induced circling, but, in addition, replaced circling by pivoting, namely turning behavior during which the rat rotates around its disfunctioning hindlimb. The present findings show that unilateral stimulation of μ-, but not δ-, opioid receptors in the VTA elicits contraversive circling that requires a relatively hyperdopaminergic activity in both the shell and the core of the Nacc at the opioid-stimulated side of the brain. The Nacc core plays an essential role in the transmission of information directing the display of pivoting that is elicited by an increased dopaminergic activity in the Nacc shell. It is concluded that the Nacc core is an essential link in the feed-forward spiraling striato-nigro-striatal circuitry that transmits information from the Nacc shell toward the DS in freely moving rats.

Spiraling dopaminergic circuitry from the ventral striatum to dorsal striatum is an effective feed-forward loop

Central dopamine systems are key players in the cerebral organization of behavior and in various neurological and psychiatric diseases. We demonstrate the presence of a neurochemical feed-forward loop characterized by region-specific changes in dopamine efflux in serially connected striatal regions, providing evidence in favor of the existence of so-called spiraling striato-nigro-striatal connections. Using in vivo microdialysis of rats, we show that simultaneous stimulation of dopamine D1 and D2 receptors in the accumbal shell decreased dorsal striatal dopamine efflux via a direct or indirect feed-forward loop involving shell, core, ventrolateral and dorsal part of the striatum: simultaneous stimulation of dopamine D1 and D2 receptors in the shell decreased dopamine efflux in the core; flupenthixol-induced inhibition of dopamine D1 and D2 receptors in the core increased dopamine efflux in the ventrolateral part of the striatum, and simultaneous stimulation of dopamine D1 and D2 receptors in the ventrolateral part of the striatum decreased dopamine efflux in the dorsal part of the striatum. Finally, simultaneous stimulation of dopamine D1 and D2 receptors in the shell decreased dopamine efflux in the dorsal part of the striatum. Thus, distinct striatal regions act also in series, providing a better understanding of the neural mechanisms underlying dopamine-dependent behaviors and the progression of dopamine-dependent disorders such as depression, schizophrenia, attention deficit hyperactivity disorder (ADHD), and addiction.

In vivo neurochemical evidence that newly synthesised GABA activates GABA(B), but not GABA(A), receptors on dopaminergic nerve endings in the nucleus accumbens of freely moving rats

GABA released from accumbal GABAergic interneurons plays an inhibitory role in the regulation of dopamine efflux through GABA(B) and GABA(A) receptors located on accumbal dopaminergic nerve endings. The cytosolic newly synthesised GABA alters vesicular GABA levels and, accordingly, the amount of GABA released from the neuron. Therefore, we hypothesised that glutamic acid decarboxylase (GAD) which generates GABA in accumbal GABAergic neurons, at least partly determines the GABA receptor subtype-mediated GABAergic tonus. To (in)validate this hypothesis, in vivo microdialysis was used to study the effects of an intra-accumbal infusion of the GAD inhibitor l-allylglycine (allylglycine) on the accumbal dopamine efflux of freely moving rats. The intra-accumbal infusion of allylglycine (50.0, 250.0 and 500.0 nmol) dose-dependently increased the accumbal dopamine levels. The co-administration of tetrodotoxin (720 pmol) suppressed the allylglycine (500.0 nmol)-induced dopamine efflux. The intra-accumbal infusion of GABA(B) receptor agonist baclofen (2.5 and 5.0 nmol) inhibited the allylglycine (500.0 nmol)-induced dopamine efflux. The baclofen's effects were counteracted by GABA(B) receptor antagonist saclofen (10.0 nmol). Neither GABA(A) receptor agonist (muscimol: 25.0 and 250.0 pmol) nor antagonist (bicuculline: 50.0 pmol) altered the allylglycine (250.0 and 500.0 nmol)-induced dopamine efflux. The present study provides in vivo neurochemical evidence that newly synthesised GABA that exerts an inhibitory tonus on the accumbal dopaminergic activity, acts at the level of GABA(B) receptors, but not GABA(A) receptors. The present study also shows that there is an allylglycine-insensitive GABA pool that release GABA exerting an inhibitory control of the accumbal dopaminergic activity, at the level of GABA(A) receptors. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

Blockade of dopamine, but not noradrenaline, transporters produces hyperthermia in rats that lack serotonin transporters

To investigate whether life-long disturbed serotonin neurotransmission may result in adaptive changes of dopaminergic and noradrenergic systems, effects of drugs on stress-induced hyperthermia were studied in serotonin transporter knockout rats. The noradrenalin transporter blocker atomoxetine was more effective in reducing stress-induced hyperthermia, induced by an injection, in serotonin transporter (SERT) knockout (SERT(-/-)) rats compared to SERT(+/+) rats. The dopamine transporter blocker GBR12909 increased the core body temperature in SERT(-/-) rats, and had no effect on the SERT(+/+) rats. Finally, the noradrenalin transporter together with dopamine transporter blocker bupropion was more effective in decreasing the stress of an injection in SERT(-/-) rats than in SERT(+/+) rats. These data suggest that the sensitivity of dopamine and noradrenalin receptors is changed in serotonin transporter knockout rats. The lack of the serotonin transporter in SERT(-/-) rats might reflect humans with a life-long disturbed serotonin system, making this rat a good model to study possible changes in dopaminergic and noradrenergic systems in psychiatric disorders.

Individual differences in the sensitivity to serotonergic drugs: a pharmacobehavioural approach using rats selected on the basis of their response to novelty

RATIONALE

The mechanisms underlying individual differences in the response to serotonergic drugs are poorly understood. Rat studies may contribute to our knowledge of the neuronal substrates that underlie these individual differences.

OBJECTIVES

A pharmacobehavioural study was performed to assess individual differences in the sensitivity to serotonergic drugs in rats that were selected based on their response to a novel environment.

METHODS

Low responders (LR) and high responders (HR) to novelty rats were tested on the elevated T-maze following systemic injections of increasing doses of various serotonergic agents. The duration of avoidance of the open arms was scored for five trials.

RESULTS

The duration of avoidance behaviour was larger in saline-treated LR rats compared to saline-treated HR rats. The 5-HT1A agonist 8-OH-DPAT and the 5-HT2 agonists mCPP and DOI decreased the duration of avoidance behaviour in LR rats, but increased it in HR rats. The 5-HT3 agonist SR57227A and the 5-HT releaser/reuptake inhibitor d-fenfluramine increased the duration of avoidance behaviour in both types of rat. However, higher doses of SR57227A were required to alter avoidance behaviour in HR than in LR rats. The onset of the effects of SR57227A, d-fenfluramine and WAY100635 was faster in LR than in HR rats. The described effects were receptor specific. A model explaining the data is presented.

CONCLUSIONS

These data demonstrate that LR and HR rats differ in their sensitivity to serotonergic drugs that act at 5-HT3, 5-HT2 and 5-HT1A receptors. The implications of these individual differences for individual-specific treatment of substance abuse are briefly discussed.

Mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles

Mesolimbic beta-, but not alpha-adrenoceptors control the accumbal release of dopamine that is derived from alpha-methyl-para-tyrosine-sensitive pools of newly synthesized neurotransmitter. The aim of this study was to investigate which of these adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive pools of previously stored neurotransmitter. Rats, that were divided in low-responders and high-responders to novelty, were pretreated with 1 mg/kg of reserpine before the alpha-adrenergic-agent phentolamine or the beta-adrenergic-agent isoproterenol was locally applied into the nucleus accumbens. The original finding that phentolamine and isoproterenol increased accumbal dopamine levels in low-responders and high-responders was replicated. Reserpine reduced the phentolamine-induced increase of accumbal dopamine in both types of rat. However, phentolamine could still increase accumbal dopamine levels in reserpine-treated high-responders, but not anymore in reserpine-treated low-responders. Reserpine did not reduce the isoproterenol-induced increase of accumbal dopamine in any type of rat. This study demonstrates that mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles. In addition, these data confirm our previous finding that dopamine can still be released from storage vesicles of reserpinized high-responders, but not of reserpinized low-responders. The collected data underline our notion that alpha- and beta-adrenergic drugs may have therapeutic effects in patients suffering from diseases in which accumbal dopamine is involved.

Accumbal noradrenaline that contributes to the alpha-adrenoceptor-mediated release of dopamine from reserpine-sensitive storage vesicles in the nucleus accumbens is derived from alpha-methyl-para-tyrosine-sensitive pools

Alpha-adrenoceptors in the nucleus accumbens are known to inhibit accumbal dopamine release from reserpine-sensitive pools. The aim of this study was to test our previously reported hypothesis that accumbal noradrenaline that controls the dopamine release from these storage vesicles is derived from alpha-methyl-para-tyrosine-sensitive pools. The sensitivity of accumbal alpha-adrenoceptors to noradrenergic agents depends on the amount of noradrenaline that is available in the synapse. In case the synaptic noradrenaline levels decrease, the conformation of alpha-adrenoceptors changes into a state that makes these receptors more sensitive to its agonists. The effects of alpha-methyl-para-tyrosine, respectively reserpine, on the alpha-adrenoceptor-agonist-induced changes of accumbal dopamine release were investigated. Alpha-methyl-para-tyrosine, but not reserpine, made accumbal postsynaptic alpha-adrenoceptors more sensitive to phenylephrine. These results indicate that noradrenaline that inhibits the release of dopamine from reserpine-sensitive storage vesicles, via stimulation of accumbal postsynaptic alpha-adrenoceptors, is derived from alpha-methyl-para-tyrosine-sensitive pools. The clinical impact of these data is discussed.

Neural correlates of strategic memory retrieval: differentiating between spatial-associative and temporal-associative strategies

Remembering complex, multidimensional information typically requires strategic memory retrieval, during which information is structured, for instance by spatial- or temporal associations. Although brain regions involved in strategic memory retrieval in general have been identified, differences in retrieval operations related to distinct retrieval strategies are not well-understood. Thus, our aim was to identify brain regions whose activity is differentially involved in spatial-associative and temporal-associative retrieval. First, we showed that our behavioral paradigm probing memory for a set of object-location associations promoted the use of a spatial-associative structure following an encoding condition that provided multiple associations to neighboring objects (spatial-associative condition) and the use of a temporal-associative structure following another study condition that provided predominantly temporal associations between sequentially presented items (temporal-associative condition). Next, we used an adapted version of this paradigm for functional MRI, where we contrasted brain activity related to the recall of object-location associations that were either encoded in the spatial- or the temporal-associative condition. In addition to brain regions generally involved in recall, we found that activity in higher-order visual regions, including the fusiform gyrus, the lingual gyrus, and the cuneus, was relatively enhanced when subjects used a spatial-associative structure for retrieval. In contrast, activity in the globus pallidus and the thalamus was relatively enhanced when subjects used a temporal-associative structure for retrieval. In conclusion, we provide evidence for differential involvement of these brain regions related to different types of strategic memory retrieval and the neural structures described play a role in either spatial-associative or temporal-associative memory retrieval.

The reboxetine-induced increase of accumbal dopamine efflux is inhibited by l-propranolol: a microdialysis study with freely moving rats

In vivo microdialysis was used to study the effects of the locally applied selective noradrenaline uptake inhibitor reboxetine on the baseline noradrenaline and dopamine efflux in the nucleus accumbens of freely moving rats. The effects of intra-accumbal infusion of the beta-adrenoceptor antagonist l-propranolol on the reboxetine-elicited noradrenaline and dopamine efflux in the nucleus accumbens were also analysed. The intra-accumbal infusion of reboxetine (1.2 and 12 pmol) significantly increased both the accumbal noradrenaline efflux and the accumbal dopamine efflux. The intra-accumbal infusion of the chosen doses of l-propranolol (300 and 1200 pmol) did not alter the accumbal noradrenaline and dopamine efflux. The l-propranolol treatment did not affect the reboxetine-elicited accumbal noradrenaline efflux, but it significantly inhibited the reboxetine-elicited increase of accumbal dopamine efflux. The doses mentioned are the total amount of drug over the infusion period that varied across the drugs (60 or 120 min). The present study shows that the intra-accumbal infusion of selective noradrenaline uptake inhibitor reboxetine increases noradrenaline as well as dopamine efflux in the nucleus accumbens of freely moving rats. This study also indicates that inhibition of accumbal beta-adrenoceptors prevented the increase of the reboxetine-induced accumbal dopamine efflux. It is suggested that the reboxetine-induced increase of the endogenous accumbal noradrenaline activates among others accumbal beta-adrenoceptors that, in turn, stimulate the accumbal release of dopamine.

Acute tryptophan depletion dose dependently impairs object memory in serotonin transporter knockout rats

RATIONALE

Acute tryptophan depletion (ATD) transiently lowers central serotonin levels and can induce depressive mood states and cognitive defects. Previous studies have shown that ATD impairs object recognition in rats.

OBJECTIVES

As individual differences exist in central serotonin neurotransmission, the impact of ATD may vary accordingly. In this experiment, we investigated the hypothesis that male serotonin transporter knockout (SERT(-/-)), rats marked by a lower SERT function, are more vulnerable to the effects of ATD in an object recognition task than male wildtype (SERT(+/+)) and heterozygous (SERT(+/-)) rats.

MATERIALS AND METHODS

Twelve male SERT(+/+), SERT(+/-), and SERT(-/-) rats were treated with standard dose and low-dose ATD using a gelatine-based protein-carbohydrate mixture lacking tryptophan. In the control treatment, L: -tryptophan was added to the mixture. Four hours after treatment, the rats were subjected to the object recognition task. In addition, the effects of ATD on plasma amino acid concentrations were measured, and concentrations of 5-HT and 5-HIAA were measured in the frontal cortex and hippocampus of these rats.

RESULTS

Plasma TRP levels and central 5-HT and 5-HIAA levels were decreased in all genotypes after ATD, but effects were stronger in SERT(-/-) rats. The standard dose of ATD impaired object recognition in all genotypes. SERT(-/-) and SERT(+/-) rats were more vulnerable to low dose of ATD in the object recognition task compared to SERT(+/+) rats.

CONCLUSIONS

These results indicate a greater sensitivity to ATD in SERT(-/-) and SERT(+/-) rats, which may be related to stronger central depletion effects in these rats.

Twenty years of dopamine research: individual differences in the response of accumbal dopamine to environmental and pharmacological challenges

Individual differences in the dopaminergic system of the nucleus accumbens of rats have extensively been reported. These individual differences have frequently been used to explain individual differences in response to environmental and pharmacological challenges. Remarkably, only little attention is paid to the factors that underlie these individual differences. This review gives an overview of the studies that have been performed in our institute during the last 20 years to investigate individual differences in accumbal dopamine release. Data are summarised demonstrating that individual differences in accumbal dopamine release are due to individual differences in: the functional reactivity of the noradrenergic system, the accumbal concentration of vesicular monoamine transporters and tyrosine hydroxylase as well as in the quantal size of the presynaptic pools of dopamine. Our data are embedded in the available literature to create a model that illustrates the putative hardware giving rise to the individual-specific release of accumbal dopamine. An important role is contributed to individual differences in the reactivity of the: hypothalamic-pituitary-adrenal axes, the reactivity of second messenger systems as well in the aminergic reactivity of the accumbens shell and core. The consequences of the individual-specific make-up and reactivity of the nucleus accumbens on the regulation of behaviour and the response to drugs of abuse will also be discussed. Apart from agents that interact with dopaminergic receptors, re-uptake or breakdown, noradrenergic agents as well as agents that interact with vesicular monoamine transporters or tyrosine hydroxylase are suggested to have therapeutic effects in subjects that are suffering from diseases in which the dopaminergic system is disturbed.

A study in male and female 5-HT transporter knockout rats: an animal model for anxiety and depression disorders

Human studies have shown that a reduction of 5-HT transporter (SERT) increases the vulnerability for anxiety and depression. Moreover, women are more vulnerable to develop depression and anxiety disorders than men. For that reason we hypothesized that homozygous 5-HT transporter knockout rat (SERT(-/-)) models, especially female, are valuable and reliable animal models for humans with an increased vulnerability for anxiety- and depression-related disorders. As rats are extensively used in neuroscience research, we used the unique 5-HT transporter knockout rat, that was recently generated using N-ethyl-N-nitrosurea (ENU) -driven mutagenesis, to test this hypothesis. Behavioral testing revealed that male and female SERT(-/-) rats spent less time in the center of the open field and spent less time on the open arm of the elevated plus maze compared with wild-type 5-HT transporter knockout rats (SERT(+/+)). In the novelty suppressed feeding test, only male SERT(-/-) rats showed a higher latency before starting to eat in a bright novel arena compared with SERT(+/+) controls. Both male and female SERT(-/-) rats showed a higher escape latency from their home cage than SERT(+/+) littermates. Moreover, SERT(-/-) rats were less mobile in the forced swim test, and sucrose consumption was reduced in SERT(-/-) rats relative to SERT(+/+) rats. Both effects were sex-independent. Neurochemically, basal extracellular 5-HT levels were elevated to a similar extent in male and female SERT(-/-) rats, which was not influenced by the selective 5-HT reuptake inhibitor citalopram. 5-HT immunostaining revealed no difference between SERT(+/+) and SERT(-/-) rats in the dorsal raphe nuclei, in both males and females. These findings demonstrate that SERT(-/-) rats show anxiety and depression-related behavior, independent of sex. Genetic inactivation of the SERT has apparently such a great impact on behavior, that hardly any differences are found between male and female rats. This knockout rat model may provide a valuable model to study anxiety- and depression-related disorders in male and female rats.

Role of GABA B receptors in the endomorphin-1-, but not endomorphin-2-, induced dopamine efflux in the nucleus accumbens of freely moving rats

In vivo microdialysis was used to study the effects of the locally applied GABA B receptor antagonist 2-hydroxysaclofen and GABA B receptor agonist baclofen on the basal dopamine efflux as well as on the endomorphin-1- and endomorphin-2-induced dopamine efflux in the nucleus accumbens of freely moving rats. 2-Hydroxysaclofen (100 and 500 nmol) increased basal dopamine efflux. Baclofen (2.5 and 5 nmol) failed to affect basal dopamine efflux. 2-Hydroxysaclofen (1 and 10 nmol) which did not alter the basal dopamine efflux, enhanced the endomorphin-1 (25 nmol)-induced dopamine efflux. Baclofen (2.5 and 5 nmol) failed to affect endomorphin-1 (25 nmol)-induced dopamine efflux, but it counteracted the 2-hydroxysaclofen-induced increase of the endomorphin-1-elicited dopamine efflux. Neither 2-hydroxysaclofen (10 nmol) nor baclofen (5 nmol) affected the endomorphin-2 (25 nmol)-induced dopamine efflux. The doses mentioned are the total amount of drug over the infusion period that varied across the drugs (25 or 50 min). These results suggest that accumbal GABA B receptor plays an inhibitory role on the basal as well as the endomorphin-1-elicited accumbal dopamine efflux. The present results support our earlier reported notion that endomorphin-1 and endomorphin-2 increase accumbal dopamine efflux by different mechanisms. Finally, it is suggested that a decrease of endogenous accumbal GABA reduces the accumbal GABA B receptor-mediated GABA-ergic inhibition, enhancing thereby the accumbal dopamine efflux.

Effects of acute and chronic apomorphine on sex behavior and copulation-induced neural activation in the male rat

Apomorphine is a non-selective dopaminergic receptor agonist. Because of its pro-erectile effects, apomorphine is clinically used for treatment of erectile dysfunction. We investigated the effects of subcutaneous apomorphine administration (0.4 mg/kg rat) on sexual behavior and mating-induced Fos-expression following acute (day 1) or chronic apomorphine treatment (days 8 and 15) in sexually experienced male rats. Consistent facilitatory effects of apomorphine were observed in the reduced numbers of mounts and intromissions over time and an increased ejaculation frequency on day 1. The first post-ejaculatory interval, however, was lengthened, while other behavioral parameters were unaffected. Fos-immunoreactivity induced by acute apomorphine administration (barrel cortex, paraventricular hypothalamic nucleus, central amygdala and locus coeruleus) was strongly reduced after chronic administration. After mating, induction of Fos-immunoreactivity was observed in well-known areas like medial preoptic nucleus and the posterodorsal medial amygdaloid area. Apomorphine, however, reduced mating-induced Fos-immunoreactivity in the nucleus accumbens shell and prevented its occurrence in its core area. This remarkable apomorphine effect was not observed in any other brain area. We conclude that the behavioral (pro-erectile) effects of apomorphine are consistent over time, and that the diminished accumbens-Fos-immunoreactivity and the elongated post-ejaculatory interval may reflect a decreased response to remote cues from the estrus female.

Role of orexin receptors in the nucleus accumbens in dopamine-dependent turning behaviour of rats

The role of orexin receptors in the nucleus accumbens shell in rat turning behaviour of rats was studied. Unilateral injection of neither the orexin 1 and 2 receptor agonist orexin A (2 microg) nor the orexin 1 receptor antagonist SB 334867 (20 ng) into the nucleus accumbens shell elicited turning behaviour. Unilateral injection of a mixture of dopamine D(1) (SKF 38393) and D2 (quinpirole) receptor agonists into the nucleus accumbens shell has been found to elicit contraversive pivoting. Orexin A (1 and 2 microg) dose-dependently potentiated the contraversive pivoting induced by a mixture of SKF 38393 (1 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell whereas SB 334867 (10 and 20 ng) did not significantly affect the pivoting. The potentiating effect of orexin A (2 microg) on the dopaminergic pivoting was not significantly inhibited by SB 334867 (10 and 20 ng) injected into the nucleus accumbens shell. The contraversive pivoting induced by a mixture of SKF 38393 (1 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell was also potentiated by the orexin 2 receptor agonist orexin B (0.5, 1 and 2 microg), which alone did not elicit turning behaviour. These results suggest that orexin 2 receptors in the nucleus accumbens shell play a modulatory role in rat turning behaviour.

Role of GABAA receptors in the endomorphin-1-, but not endomorphin-2-, induced dopamine efflux in the nucleus accumbens of freely moving rats

In vivo microdialysis was used to study the effects of the locally applied GABA(A) receptor agonist muscimol and GABA(A) receptor antagonist bicuculline on the basal dopamine efflux as well as on the endomorphin-1- and endomorphin-2-induced dopamine efflux in the nucleus accumbens of freely moving rats. Muscimol (2500 pmol) and bicuculline (5 and 10 nmol) increased basal dopamine efflux. Bicuculline (50 pmol) inhibited the muscimol (2500 pmol)-induced dopamine efflux. Muscimol (250 pmol), but not bicuculline (50 and 500 pmol), enhanced the endomorphin-1 (25 nmol)-induced dopamine efflux. Bicuculline (50 pmol) counteracted the muscimol (250 pmol)-induced increase of the endomorphin-1-elicited dopamine efflux. Neither muscimol (25 and 250 pmol) nor bicuculline (50 and 500 pmol) affected the endomorphin-2 (25 nmol)-induced dopamine efflux. The doses mentioned are the total amount of drug over the infusion period (25 or 50 min) that varied across the drugs. The finding that muscimol and bicuculline increased basal dopamine efflux may imply that these drugs acted at different sites. It is suggested that (1) muscimol acts at GABA(A) receptors on GABA-ergic neurons that exert an inhibitory control of dopaminergic neurons and, accordingly, disinhibits these dopaminergic neurons, and that (2) bicuculline acts directly at GABA(A) receptors on dopaminergic neurons and, accordingly, removes the inhibitory control of these dopaminergic neurons. The finding that an agonist, but not antagonist, of GABA(A) receptors enhanced the endomorphin-1's effects might indicate that endomorphin-1 produced a floor effect at the level of GABA(A) receptors located on presynaptic, dopaminergic terminals. Finally, the present results support our earlier reported notion that endomorphin-1 and endomorphin-2 increase accumbal dopamine efflux by different mechanisms.

A vehicle injection into the right core of the nucleus accumbens both reverses the region-specificity and alters the type of contralateral turning elicited by unilateral stimulation of dopamine D2/D3 and D1 receptors in the left core of the nucleus accumbens

The goal of the present study was to analyse to what extent variables such as (1) injected volume, (2) nature of the solvent of drugs (saline versus distilled water) and (3) placement of an additional cannula to inject the solvent of the drugs at the opposite side of the brain, influenced the behavioural effects of the combined administration of the dopamine D(1) receptor agonist (+/-)-1-phenyl-2,3,4,5-tetrahydro-[1H]-3-benzazepine-7,8-diol (SKF 38393, 5.0 microg) and the dopamine D(2)/D(3) receptor agonist quinpirole (10.0 microg) into the shell or core of the nucleus accumbens of freely moving rats. First, we found that increasing the injected volume from 0.2 microl to 0.5 microl significantly increased the amount of contralateral turning after injection of the drugs into the shell and, especially, the core of rats equipped with one cannula. More importantly, the type of turning behaviour changed: instead of a predominance of pivoting, both pivoting and circling appeared. Second, replacing the solvent saline by distilled water resulted in a minor, but significant, decrease of the amount of contralateral turning elicited from either the shell or the core of the nucleus accumbens of rats equipped with one cannula. The type of turning was not changed by this new solvent. Third, and most importantly, this study showed that the vehicle injection into the right core exerted a potentiating effect on the number of contralateral rotations elicited by injections of SKF 38393+quinpirole into the left core, whereas such a vehicle injection into the right shell did not affect the number of contralateral rotations elicited by injections of SKF 38393+quinpirole into the left shell. The type of turning in these rats was not changed when compared to rats equipped with one cannula. It is hypothesized that the fluid injected into the core, directly or indirectly, enhanced the dopaminergic asymmetry between the left and the right brain, implying that this manipulation anyhow reduced the dopaminergic activity in the region under discussion. In conclusion, subtle changes in the methodology used to study both the behaviour-specificity and the region-specificity of drug injections into the brain significantly directs the outcome of such studies.

Role of alpha adrenoceptors in the nucleus accumbens in the control of accumbal noradrenaline efflux: a microdialysis study with freely moving rats

Microdialysis technique was used to study the effects of the locally applied alpha adrenoceptor agonist phenylephrine and antagonist phentolamine on the basal noradrenaline efflux as well as on the noradrenaline uptake inhibitor desipramine-elicited noradrenaline efflux in the nucleus accumbens (NAc) of freely moving rats. Tetrodotoxin reduced basal noradrenaline efflux by 72%, whereas desipramine increased it by 204%. Phenylephrine reduced the basal noradrenaline efflux by 32% and phentolamine blocked this effect. Phentolamine elevated the basal noradrenaline efflux by 150% and phenylephrine counteracted this effect. The desipramine-elicited noradrenaline efflux was not affected by phenylephrine, but enhanced by phentolamine. Desipramine counteracted the effects of phenylephrine and potentiated those of phentolamine. These results indicate that the accumbal noradrenaline efflux is under inhibitory control of alpha adrenoceptors that are suggested to be presynaptically located on adrenergic nerve terminals in the NAc. Furthermore, this study suggests that the conformational state of alpha adrenoceptors varies across the available amount of noradrenaline. The clinical impact of these data is discussed.

Characterization of the serotonin transporter knockout rat: a selective change in the functioning of the serotonergic system

Serotonergic signaling is involved in many neurobiological processes and disturbed 5-HT homeostasis is implicated in a variety of psychiatric and addictive disorders. Here, we describe the functional characterization of the serotonin transporter (SERT) knockout rat model, that is generated by N-ethyl-N-nitrosurea (ENU)-driven target-selected mutagenesis. Biochemical characterization revealed that SERT mRNA and functional protein are completely absent in homozygous knockout (SERT-/-) rats, and that there is a gene dose-dependent reduction in the expression and function of the SERT in heterozygous knockout rats. As a result, 5-HT homeostasis was found to be severely affected in SERT-/- rats: 5-HT tissue levels and depolarization-induced 5-HT release were significantly reduced, and basal extracellular 5-HT levels in the hippocampus were ninefold increased. Interestingly, we found no compensatory changes in in vitro activity of tryptophan hydroxylase and monoamine oxidase, the primary enzymes involved in 5-HT synthesis and degradation, respectively. Similarly, no major adaptations in non-serotonergic systems were found, as determined by dopamine and noradrenaline transporter binding, monoamine tissue levels, and depolarization-induced release of dopamine, noradrenaline, glutamate and GABA. In conclusion, neurochemical changes in the SERT knockout rat are primarily limited to the serotonergic system, making this novel rat model potentially very useful for studying the behavioral and neurobiological consequences of disturbed 5-HT homeostasis.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

Differences in the cellular mechanism underlying the effects of amphetamine on prepulse inhibition in apomorphine-susceptible and apomorphine-unsusceptible rats

BACKGROUND

Amphetamine is often used to mimic certain aspects of schizophrenia in laboratory animals, such as a decreased prepulse inhibition.

MATERIALS AND METHODS

Apomorphine-susceptible and apomorphine-unsusceptible rats represent a well-characterized animal model for individual differences in the sensitivity to dopaminergic drugs. Moreover, apomorphine-susceptible rats show a wide variety of schizophrenia-like abnormalities. The differential response to administration of amphetamine (1-4 mg/kg, i.p.) was investigated in these two rat lines using the prepulse inhibition paradigm. Because amphetamine promotes dopamine release, the cellular mechanism underlying the line-specific effects of amphetamine was investigated by administration of alpha-methyl-para-tyrosine (aMpT) and reserpine, substances that are known to deplete the cytosolic dopamine pool and the vesicular dopamine pool, respectively, the former being primarily implicated in mediating the effects of amphetamine.

RESULTS

All doses of amphetamine decreased prepulse inhibition in apomorphine-susceptible rats, whereas only the highest doses (2 and 4 mg/kg, i.p.) of amphetamine decreased prepulse inhibition in apomorphine-unsusceptible rats. Alpha-methyl-para-tyrosine, but not reserpine, blocked the amphetamine-induced disruption in prepulse inhibition in apomorphine-unsusceptible rats, whereas both substances alone had no effect in apomorphine-susceptible rats. However, the combination of alpha-methyl-para-tyrosine and reserpine did block the amphetamine-induced effects in the latter rat line.

DISCUSSION

The present study suggests that apomorphine-susceptible rats are more sensitive to systemic administration of amphetamine than apomorphine-unsusceptible rats. In addition, the data show that the cellular mechanism underlying the effects of amphetamine differs between apomorphine-susceptible and apomorphine-unsusceptible rats. Whereas the effects of amphetamine on prepulse inhibition in apomorphine-unsusceptible rats just require the alpha-methyl-para-tyrosine sensitive dopamine pool, the effects in apomorphine-susceptible rats require both the alpha-methyl-para-tyrosine sensitive and the reserpine sensitive dopamine pool. Because apomorphine-susceptible rats share many features with schizophrenic patients, these data open the perspective that in these patients amphetamine may induce dopamine release from both types of dopamine pool. This might provide an explanation for the increased dopamine release after this psychostimulant drug in patients vs controls.

Adrenergic receptors in the nucleus accumbens shell differentially modulate dopamine and acetylcholine receptor-mediated turning behaviour

The role of alpha- and beta-adrenoceptors in the nucleus accumbens shell in turning behaviour of rats was investigated. Unilateral injections of the alpha-adrenoceptor agonist (phenylephrine; 10 microg) and antagonist (phentolamine; 10 microg) as well as the beta-adrenoceptor agonist (isoprenaline; 1 microg) and antagonist (propranolol; 5 microg) into the nucleus accumbens shell did not produce turning behaviour more than that of control vehicle injection. Unilateral injection of a mixture of dopamine D(1) ((+/-)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol, SKF 38393; 5 microg) and D(2) (quinpirole; 10 microg) receptor agonists into the nucleus accumbens shell has been found to elicit contraversive pivoting. Such pivoting was dose-dependently inhibited by phenylephrine (5, 10 microg), injected into the nucleus accumbens shell, and the inhibitory effect of phenylephrine (10 microg) was antagonised by phentolamine (10 microg) that per se had no effect on this pivoting. Isoprenaline (0.5, 1 microg) dose-dependently increased the contraversive pivoting induced by the mixture of SKF 38393 (1 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell. The effect of isoprenaline (1 microg) was antagonised by propranolol (5 microg) that per se had no effect on this pivoting. It is concluded that stimulation of accumbal alpha-adrenoceptors inhibits the dopamine-dependent pivoting in contrast to stimulation of accumbal beta-adrenoceptors that facilitates this dopamine-dependent pivoting. Unilateral injection of the acetylcholine receptor agonist carbachol (5 microg) into the nucleus accumbens shell has been found to elicit contraversive circling. Such circling was significantly reduced by accumbal administration of either phenylephrine (10, 20 microg) or phentolamine (5, 10 microg) in a dose-independent manner; moreover, both drugs potentiated, but did not counteract, each other's effects. Carbachol-induced circling was also reduced by propranolol (2.5, 5 microg), but again in an aspecific manner. It is concluded that alpha- and beta-adrenergic agents have an effect on accumbal acetylcholine receptor-mediated circling through a non-adrenergic mechanism. The impact of the present study for putative new treatments of various neuropsychiatric and neurological disorders is discussed.

Cocaine strongly reduces prepulse inhibition in apomorphine-susceptible rats, but not in apomorphine-unsusceptible rats: Regulation by dopamine D2 receptors

Dopaminergic agonists, such as apomorphine and amphetamine, have been shown to drastically reduce prepulse inhibition of the acoustic startle reflex. The effects of the indirect dopamine agonist cocaine on prepulse inhibition have only been described in a few reports and have yielded conflicting results, possibly due to individual differences within and between rat strains. In this study we therefore used apomorphine-susceptible and apomorphine-unsusceptible rats, as an animal model for individual differences, to study the effects of cocaine (20, 30 mg/kg i.p.) on prepulse inhibition. In addition we tested whether the cocaine-induced deficit in prepulse inhibition could be reversed by the D2-antagonist remoxipride (5 mg/kg i.p.), the alpha-1 adrenoceptor antagonist prazosin (2.5 mg/kg i.p.) and the 5-HT2-antagonist ketanserin (2.0 mg/kg i.p.). Cocaine strongly reduced prepulse inhibition in apomorphine-susceptible rats, but had no effect at all on apomorphine-unsusceptible rats. Remoxipride had no effect on prepulse inhibition, but prazosin and ketanserin increased prepulse inhibition. Both remoxipride and prazosin reversed the cocaine-induced deficit in prepulse inhibition, whereas ketanserin did not. We conclude that apomorphine-susceptible rats are extremely sensitive to the effects of cocaine on prepulse inhibition, while apomorphine-unsusceptible rats are not. The effects of cocaine on prepulse inhibition in apomorphine-susceptible rats were mediated by D2-receptors, but not by 5-HT2-receptors or alpha-1 adrenoceptors.

Writer's cramp: restoration of striatal D2-binding after successful biofeedback-based sensorimotor training

INTRODUCTION

Previous studies of writer's cramp have detected cerebral sensorimotor abnormalities in this disorder and, more specifically, a reduced striatal D2-binding as assessed by [(123)I]IBZM SPECT. However, empirical data were lacking about the influence of effective biofeedback-based sensorimotor training on D2 receptor binding.

METHODS

To determine whether there is a restoration of D2-binding after successful sensorimotor treatment, pre- and posttreatment SPECTs were compared in five patients with writer's cramp and correlated with improvement in handwriting.

RESULTS

After treatment, the clinical and electromyographic picture appeared substantially improved connected with a significant increase in D2-binding to nearly normal levels similar to normative data in age/sex-matched healthy subjects.

CONCLUSION

The current study supported the view that writer's cramp results from a plastic adaptation of a rectifiable nigrostriatal dopaminergic system and that effective sensorimotor training leads to increased efficacy of striatal dopaminergic transmission.

Ontogenic reduction of Aph-1b mRNA and gamma-secretase activity in rats with a complex neurodevelopmental phenotype

Selectively bred apomorphine susceptible (APO-SUS) rats display a complex behavioral phenotype remarkably similar to that of human neurodevelopmental disorders, such as schizophrenia. We recently found that the APO-SUS rats have only one or two Aph-1b gene copies (I/I and II/II rats, respectively), whereas their phenotypic counterpart has three copies (III/III). Aph-1b is a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. Nevertheless, surprisingly little is known about gamma-secretase expression during development. Here, we performed a longitudinal quantitative PCR study in embryos and the hippocampus of I/I, II/II and III/III rats, and found gene-dosage dependent differences in Aph-1b, but not Aph-1a, mRNA expression throughout pre- and post-natal development. On the basis of the developmental mRNA profiles, we assigned relative activities to the various Aph-1a and -1b gene promoters. Furthermore, in the three rat lines, we observed both tissue-specific and temporal alterations in gamma-secretase cleavage activity towards one of its best-known substrates, the amyloid-beta precursor protein APP. We conclude that the low levels of Aph-1b mRNA and gamma-secretase activity observed in the I/I and II/II rats during the entire developmental period may well underlie their complex phenotype.

Generation of gene knockouts and mutant models in the laboratory rat by ENU-driven target-selected mutagenesis

OBJECTIVE

The rat is one of the most important model organisms for biomedical and pharmacological research. However, the generation of novel models for studying specific aspects of human diseases largely depends on selection for specific traits using existing rat strains, thereby solely depending on naturally occurring variation. This study aims to provide the tools to manipulate the rat genome in a more directed way.

METHODS

We developed robust, automated, and scaleable reverse genetic methodology based on ENU (N-ethyl-N-nitrosourea)-driven target-selected mutagenesis. Optimal mutagenesis conditions have been determined in three different rat strains and a universal, rapid, and cost-effective dideoxy resequencing-based screening setup was established for mutation discovery. The effectiveness of the approach is illustrated by the identification of 120 induced mutations in a set of genes of interest, including six that result in unique rat knockout models due to the introduction of premature stop codons. In addition, 56 mutations were found that change amino acids, including critical residues in transmembrane domains of receptors and channels.

CONCLUSIONS

The approach described here allows for the systematic generation of knockout and protein function altering alleles in the rat. The resulting induced rat models will be powerful tools for studying many aspects of a wide variety of human diseases.

Expression of cocaine-induced conditioned place preference in apomorphine susceptible and unsusceptible rats

Differences in cocaine self-administration can be attributed to differences in the rewarding value that cocaine has for the individual. An ongoing debate, however, exists whether a high rewarding or a low rewarding value leads to an increase in self-administration. To investigate which of these two alternatives is correct, we investigated the occurrence of cocaine-induced conditioned place preference in apomorphine susceptible and apomorphine unsusceptible rats. We have recently shown that under specific environmental conditions (challenged-not habituated to the environment-as measured by distance travelled) apomorphine susceptible rats consistently self-administer more cocaine than apomorphine unsusceptible rats do. As conditioned place preference allows the assessment of the rewarding value of cocaine, we investigated the expression of cocaine-induced conditioned place preference in apomorphine susceptible and apomorphine unsusceptible rats under the same conditions as the self-administration experiments in order to establish whether the rewarding value of cocaine is greater or smaller in challenged apomorphine susceptible rats than in challenged apomorphine unsusceptible rats. The data clearly showed that challenged apomorphine susceptible rats had a preference for the cocaine-paired compartment with lower doses of cocaine (10 mg/kg) than challenged apomorphine unsusceptible rats. Apomorphine unsusceptible rats expressed conditioned place preference only with the highest dose tested (20 mg/kg). On the basis of these data, we concluded that the rewarding value that cocaine has in challenged apomorphine susceptible rats is greater than that in challenged apomorphine unsusceptible rats. It is suggested that challenged apomorphine susceptible rats self-administer more of a lower dose of cocaine than challenged apomorphine unsusceptible rats do, because the rewarding value of cocaine is greater in challenged apomorphine susceptible rats than in challenged apomorphine unsusceptible rats.

Topographical resolution of jaw movements mediated by cyclase- vs. non-cyclase-coupled dopamine D1-like receptors: Studies with SK&F 83822

This study examined the effects on orofacial movement topography of SK&F 83822 ([R/S]-6-chloro-7,8-dihydroxy-3-allyl-1-[3-methylphenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), which stimulates dopamine D(1)-like receptors coupled to stimulation of adenylyl cyclase (AC) but not phosphoinositide (PI) hydrolysis, in comparison with SK&F 83959 ([R/S]-3-methyl-6-chloro-7,8-dihydroxy-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), which stimulates PI hydrolysis but not AC. SK&F 83822 alone induced chattering, while SK&F 83959 alone exerted little effect. SK&F 83822 and SK&F 83959 each in combination with the dopamine D(2)-like agonist quinpirole resulted in synergistic induction of non-chattering movements with tongue protrusions. These effects were blocked by the dopamine D(1)-like receptor antagonist SCH 23390 ([R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine). However, the dopamine D(2)-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-methylaminobenzamide) exerted a biphasic effect on synergism with SK&F 83822: chattering was initially released but antagonised thereafter. Only antagonism was seen for synergism with SK&F 83959. While both AC- and PI-coupled dopamine D(1)-like receptors participate in synergistic dopamine D(1)-like:D(2)-like receptor interactions, topographically specific synergistic and oppositional dopamine D(1)-like:D(2)-like interactions evident with SK&F 83822 reflect the involvement primarily of D(1)-like receptors coupled to AC rather than PI.

Endomorphin-2 and endomorphin-1 promote the extracellular amount of accumbal dopamine via nonopioid and mu-opioid receptors, respectively

Activation of mu-opioid receptors in the nucleus accumbens (NAc) is known to increase accumbal dopamine efflux in rats. Endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2); EM-2) and endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2); EM-1) are suggested to be the endogenous ligands for the mu-opioid receptor. As the ability of EM-2 and EM-1 to alter the accumbal extracellular dopamine level has not yet been studied in freely moving rats, the present study was performed, using a microdialysis technique that allows on-line monitoring of the extracellular dopamine with a temporal resolution of 5 min. A 25 min infusion of either EM-2 or EM-1 into the NAc (5, 25, and 50 nmol) produced a dose-dependent increase of the accumbal dopamine level. The EM-2 (50 nmol)- and EM-1 (25 and 50 nmol)-induced dopamine efflux were abolished by intra-accumbal perfusion of tetrodotoxin (2 muM). Intra-accumbal perfusion of the mu-opioid receptor antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2); 3 nmol) failed to affect the EM-2 (50 nmol)-induced dopamine release, whereas it significantly inhibited the EM-1 (25 and 50 nmol)-induced dopamine release. The EM-1 (50 nmol)-induced accumbal dopamine efflux was significantly reduced by the systemic administration of the putative mu1-opioid receptor antagonist naloxonazine (15 mg/kg, intraperitoneally (i.p.), given 24 h before starting the perfusion). Systemic administration of the aspecific opioid receptor antagonist naloxone (1 mg/kg, i.p., given 10 or 20 min before starting the perfusion) also failed to affect the EM-2 (50 nmol)-induced dopamine efflux, whereas it significantly inhibited the EM-1 (25 and 50 nmol)-induced dopamine efflux. The present study shows that the intra-accumbal infusion of EM-2 and EM-1 increases accumbal dopamine efflux by mechanisms that fully differ. It is concluded that the effects of EM-2 are not mediated via opioid receptors in contrast to the effects of EM-1 that are mediated via mu1-opioid receptors in the NAc.

Effects of chronic treatment with fluvoxamine and paroxetine during adolescence on serotonin-related behavior in adult male rats

Selective Serotonin Reuptake Inhibitors (SSRIs) are designed to treat adults, but are increasingly prescribed for adolescents. SSRIs might cause permanent changes in serotonin-related behavior in adolescents, since their serotonergic system is still developing. Male Wistar rats were treated with paroxetine (15 mg/kg p.o.) or fluvoxamine (30 mg/kg p.o.) throughout adolescence. After a washout period their behavior in the elevated plus-maze, prepulse inhibition test, Forced swimming test and elevated T-maze were studied. In addition, the effects of the 5-HT(1A) receptor agonist 8-OH-DPAT on sexual behavior and lower lip retraction were measured. Paroxetine mildly inhibited weight gain during treatment. Both SSRIs caused a reduction in ejaculation frequency and in time spent on the open arm of the elevated plus-maze in adult rats. Fluvoxamine slightly increased avoidance latency in the elevated T-maze compared to paroxetine. No differences between the groups were found in the other tests. Apparently, chronic treatment with SSRIs during adolescence may cause mild changes in adult behavior.

Effects of chronic paroxetine pretreatment on (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin induced c-fos expression following sexual behavior

Chronic treatment with the selective serotonin reuptake inhibitor paroxetine impairs the functioning of 5-HT(1A) receptors involved in ejaculation. This could underlie the development of delayed ejaculation often reported by men treated with paroxetine. The neurobiological substrate linking the effects of selective serotonin reuptake inhibitor-treatment and 5-HT(1A) receptor activation with ejaculation was investigated. Male Wistar rats that were pretreated with paroxetine (20 mg/kg/day p.o.) or vehicle for 22 days and had received an additional injection with the 5-HT(1A) receptor agonist 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.4 mg/kg s.c.) or saline on day 22, 30 min prior to a sexual behavior test, were perfused 1 h after the sexual behavior test. Brains were processed for Fos-, and oxytocin immunohistochemistry. The drug treatments markedly changed both sexual behavior and the pattern and number of Fos-immunoreactive cells in the brain. Chronic pretreatment with paroxetine caused delayed ejaculation. Acute injection with 8-OH-DPAT facilitated ejaculation in vehicle-pretreated rats, notably evident in a strongly reduced intromission frequency, whereas 8-OH-DPAT had no effects in paroxetine-pretreated rats. Chronic treatment with paroxetine reduced Fos-immunoreactivity in the locus coeruleus, and prevented the increase in Fos-immunoreactive neurons induced by 8-OH-DPAT in the oxytocinergic magnocellular part of the paraventricular nucleus as well as in the locus coeruleus. Since oxytocin and noradrenalin facilitate ejaculation, the alterations in Fos-IR in these areas could connect selective serotonin reuptake inhibitor treatment and 5-HT(1A) receptor activation to ejaculation. Chronic paroxetine treatment and 8-OH-DPAT changed c-fos expression in a number of other brain areas, indicating that Fos-immunohistochemistry is a useful tool to find locations where selective serotonin reuptake inhibitors and 8-OH-DPAT exert their effects.

Individual differences in drug dependence in rats: the role of genetic factors and life events

Drug dependence and addiction is a chronic mental illness that has far reaching consequences for society in terms of economic loss, health costs and judicial problems. A crucial question in drug addiction, is what factors are involved in its aetiology, and especially what mediates the shit from use to abuse. As in most other mental illnesses, addiction can best be described using the so-called three hit model, which states that a disease results from an interaction between genetic factors, early lie events and late environmental factors. However, the precise nature of these factors still remains to be elucidated. This present review discusses the results from an animal model in which these three different hit are currently being investigated. The apomorphine susceptible (APO-SUS) and apomorphine unsusceptible (APO-UNSUS) rats, originally selected on the basis of their behavioural response to the dopaminergic agonist apomorphine, were recently found to be genetically different in the number of gene copies of a component of the gamma-secretase complex called Aph-1b. Whereas APO-UNSUS rats have three copies of the gene, APO-SUS rats have either 1 or 2 copies. In addition we have shown that these rats show differences in cocaine and alcohol self-administration, and that both early life events and late environmental factors can alter this self-administration behaviour. Thus, the data so far support the hypothesis that the APO-SUS and APO-UNSUS rats offer an interesting animal model for drug dependence in which genes and environment interact. We finally propose a theoretical model which can explain this gene-environment interaction.

Reduced Aph‐1b expression causes tissue‐ and substrate‐specific changes in γ‐secretase activity in rats with a complex phenotype

The gamma-secretase enzyme complex displays intramembrane catalytic activity toward many type I transmembrane proteins, including the Alzheimer-linked amyloid-beta-protein precursor (APP) and the neuregulin receptor ErbB4. Active gamma-secretase is a tetrameric protein complex consisting of presenilin-1 (or -2), nicastrin, PEN-2, and Aph-1a (or -1b). We have recently discovered that pharmacogenetically bred apomorphine-susceptible Wistar rats (APO-SUS) have only one or two copies of the Aph-1b gene (termed I/I and II/II rats, respectively), whereas their phenotypic counterparts (APO-UNSUS) have three copies (III/III). As a result, APO-SUS rats display reduced Aph-1b expression and a complex phenotype reminiscent of neurodevelopmental disorders. Here we determined in the I/I and III/III rats the gamma-secretase cleavage activity toward the three APP superfamily members, p75 neurotrophin receptor, ErbB4, and neuregulin-2, and found that the cleavage of only a subset of the substrates was changed. Furthermore, the observed differences were restricted to tissues that normally express relatively high Aph-1b compared with Aph-1a levels. Thus, we provide in vivo evidence that subtle alterations in gamma-secretase subunit composition may lead to a variety of affected (neuro)developmental signaling pathways and, consequently, a complex phenotype.

Acetylcholine receptor effects on accumbal shell dopamine-mediated turning behaviour in rats

The nature of acetylcholine receptor effects on dopaminergic functions within the nucleus accumbens shell was studied in rats, using turning behaviour as read-out parameter. Unilateral injections of the acetylcholine receptor agonist, carbachol (1.0-5.0 microg), into the nucleus accumbens shell dose-dependently elicited contraversive circling. Unilateral injections of the combination of a fixed dose of the dopamine D(2) receptor agonist, quinpirole (10.0 microg), with increasing doses of the dopamine D(1) receptor agonist, SKF 38393 (1.0-5.0 microg), into the nucleus accumbens shell dose-dependently elicited contraversive pivoting. The same held for the combination of a fixed dose of SKF 38393 (5.0 microg) with increasing doses of quinpirole (5.0 and 10.0 microg), which was injected into the nucleus accumbens shell. The nicotinic acetylcholine receptor antagonist, mecamylamine (5.0 and 10.0 microg), injected into the nucleus accumbens shell, which alone did not elicit any turning behaviour, significantly suppressed both the contraversive circling induced by carbachol (5.0 microg) and the contraversive pivoting induced by the mixture of SKF 38393 (5.0 microg) and quinpirole (10.0 microg). The muscarinic acetylcholine receptor antagonist, methylscopolamine (1.0 and 2.5 microg), injected into the nucleus accumbens shell, which alone did not elicit any turning behaviour, significantly suppressed the contraversive circling induced by carbachol (5.0 microg), whereas it significantly increased the contraversive pivoting induced by both the mixture of SKF 38393 (1.0 microg) and quinpirole (10.0 microg) and the mixture of SKF 38393 (5.0 microg) and quinpirole (5.0 microg). Neither SKF 38393 (5.0 microg) nor quinpirole (10.0 microg) injected into the nucleus accumbens shell affected the contraversive circling induced by carbachol (5.0 microg). Carbachol (1.0 microg) injected into the nucleus accumbens shell caused a slight initial potentiation followed by an inhibition of the contraversive pivoting induced by the mixture of SKF 38393 (5.0 microg) and quinpirole (10.0 microg). These results confirm that stimulation of both nicotinic and muscarinic acetylcholine receptors in the nucleus accumbens shell is required for the accumbens-dependent, acetylcholine-mediated circling. The study provides the original evidence that stimulation of nicotinic acetylcholine receptors in the nucleus accumbens shell is required for the accumbens-dependent, dopamine-mediated pivoting. Finally, the present study shows that muscarinic acetylcholine receptors in the nucleus accumbens shell play an inhibitory role in the production of the accumbens-dependent, dopamine-mediated pivoting.

The development of various somatic markers is retarded in an animal model for schizophrenia, namely apomorphine-susceptible rats

Although schizophrenia usually sets on after puberty, deviations of normal development exist in pre-schizophrenic children. To investigate the presence of early developmental abnormalities in a valid animal model for schizophrenia, we delineated line-specific developmental differences between apomorphine-susceptible rats (APO-SUS), which share many features with schizophrenic patients, and their counterpart, apomorphine-unsusceptible rats (APO-UNSUS). A battery of somatic developmental markers was assessed in naive animals on postnatal day (PND) 4 and in animals from PND 0 to PND 60. Three comparisons were made: naive APO-SUS and naive APO-UNSUS rats on PND 4; naive and handled APO-SUS and APO-UNSUS rats on PND 4; handled APO-SUS rats and handled APO-UNSUS rats across the initial 60 PND's. Naive APO-SUS rats developed much slower than naive APO-UNSUS rats as far as it concerns digit-separation, anogenital-distance, rooting-reflex, and body-displacement on PND 4, thereby underlining the validity of the APO-SUS rats as model for aspects of schizophrenia. Handling on PND 0-3 retarded the development of both types of rat, implying that early life events have long-lasting effects on pure-somatic markers. Finally, handling from PND 0 to PND 60 had a more pronounced retardation effect in APO-UNSUS rats than in APO-SUS rats. It is suggested that the APO-SUS rats are not affected as much as the APO-UNSUS rats, because they are already overwhelmed by other subliminal stimuli that have no effect on APO-UNSUS rats.

IN CONCLUSION

(1) the APO-SUS rat, which is a valid model for schizophrenia, has a retarded development just as pre-schizophrenic children have; (2) early postnatal manipulations have immediate and long-lasting effects on the rodents' morphology; and (3) subchronic, early postnatal handling has a greater effect in APO-UNSUS rats than in APO-SUS rats. The impact of these data for APO-SUS rats as a model for schizophrenia is discussed.

Individual differences in male rat ejaculatory behaviour: searching for models to study ejaculation disorders

In addition to investigating sexual function in rats that display normal ejaculatory behaviour, studying rats that are either 'hyposexual' or 'hypersexual' may provide important insights into the aetiology of ejaculatory dysfunctions in men, such as premature and retarded ejaculation. To this end, rats were matched into groups of 'sluggish', 'normal' and 'rapid' ejaculators based on their ejaculation frequencies displayed in a series of weekly sexual behaviour tests. Selecting rats on this parameter revealed large and stable differences in other parameters of sexual behaviour as well, including ejaculation latency and mount frequency but not intromission frequency and mount latency, putative indices of sexual motivation. Neuroanatomically, Fos immunoreactivity as a measure of neuronal activation was increased in rapid ejaculators compared with sluggish ejaculators in ejaculation-related brain areas, presumably associated with the differences in ejaculatory behaviour. Although the total number of oxytocin neurones within subregions of the hypothalamus did not differ between groups, in the supraoptic nucleus of the hypothalamus more oxytocin neurones were activated in rapid ejaculators compared with the other groups. Apart from the differences observed in ejaculatory behaviour, groups did not differ with respect to their locomotor activity and approach-avoidance behaviour as measured in the elevated plus-maze. Finally, apomorphine-induced stereotypy was similar in sluggish and rapid ejaculators, suggesting no large differences in dopamine susceptibility. Altogether, the present results suggest stable differences in male rat ejaculatory behaviour. Further exploring the neurobiological mechanisms underlying these differences may be a promising approach to gain insights into the aetiology of sexual dysfunctions such as premature, retarded or an-ejaculation.

Role of GABA(A) receptors in the retrorubral field and ventral pallidum in rat jaw movements elicited by dopaminergic stimulation of the nucleus accumbens shell

The role of gamma-aminobutyric acid (GABA)(A) receptors in the retrorubral field in the production of rat repetitive jaw movements was examined, as this nucleus receives a GABAergic, inhibitory input from the nucleus accumbens and is connected with the parvicellular reticular formation, a region that is directly connected with the orofacial motor nuclei. The GABA(A) receptor antagonist bicuculline (150 ng/0.2 microl per side) significantly produced repetitive jaw movements when injected bilaterally into the retrorubral field, but not the ventral pallidum. The effects of bicuculline were GABA(A) receptor specific, because the effects were abolished by muscimol, a GABA(A) receptor agonist, given into the same site. The bicuculline-induced jaw movements differed qualitatively from those elicited by injection of a mixture of (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 82958; 5 microg) and quinpirole (10 microg), agonist at dopamine D1 and D2 receptors respectively, into the nucleus accumbens shell. Nevertheless, bilateral injections of muscimol (10 ng, 25 ng and 50 ng/0.2 microl per side) into the retrorubral field significantly inhibited jaw movements evoked by the dopamine D1/D2 receptor stimulation in the nucleus accumbens shell. Bilateral injections of bicuculline (50 ng and 150 ng/0.2 microl per side) also reduced the dopamine D1/D2 receptor-mediated jaw movements. Essentially similar effects were obtained when muscimol and bicuculline were given into the ventral pallidum, a region that is also known to receive GABAergic inhibitory inputs from the nucleus accumbens. In conclusion, GABA(A) receptor blockade in the retrorubral field elicits characteristic repetitive jaw movements, and the GABA(A) receptors in that region as well as in the ventral pallidum modulate the accumbens-specific, dopamine D1/D2 receptor-mediated jaw movements.

Gene - environment interactions determine the individual variability in cocaine self-administration

Research into factors that determine the propensity to self-administer cocaine has shown that stressors can determine the amount of cocaine self-administered as well as the rate of acquisition. However, the interaction between the genetic make-up of the animal and stress is unknown. This study investigated this interaction by using the genetic animal model consisting of apomorphine susceptible (APO-SUS) and unsusceptible (APO-UNSUS) rats. Animals were allowed to self-administer 0.25 mg/kg cocaine under stressful and habituated conditions. This study revealed that the amount of cocaine consumed was highly dependent on the genetic make-up of the animal as well as the amount of stress during self-administration. Under habituated circumstances the APO-UNSUS rats took far more cocaine than the APO-SUS rats. Under stressful circumstances, however, the APO-SUS rats took far more cocaine than the APO-UNSUS rats. This difference in the amount consumed by APO-SUS and APO-UNSUS rats is likely to be due to the specific neurobiological features of their dopaminergic and, possibly, noradrenergic system as well as the reactivity of their HPA-axis. It is suggested that the amount of a drug consumed and, accordingly, its addictive potential and 'drug-vulnerability' are determined by the interaction between the genetic make-up of the animals and stress, and not by either component alone.

Effects of chronic selective serotonin reuptake inhibitors on 8-OH-DPAT-induced facilitation of ejaculation in rats: comparison of fluvoxamine and paroxetine

RATIONALE

Chronic treatment with selective serotonin reuptake inhibitors (SSRIs) can delay ejaculation in humans, but the extent of this effect differs between SSRIs. The involvement of 5-HT1A receptors is likely, since 5-HT1A receptor agonists accelerate ejaculation and chronic SSRI treatment is thought to desensitize 5-HT1A receptors.

OBJECTIVES

This study was conducted to examine the effects of chronic pretreatment with the SSRIs fluvoxamine and paroxetine on the facilitation of ejaculation induced by the 5-HT1A receptor agonist 8-OH-DPAT.

METHODS

Sexually experienced Wistar rats with normal ejaculatory behavior were treated for 22 days with vehicle, fluvoxamine (30 mg/kg/day), or paroxetine (10 or 20 mg/kg/day, p.o.). On day 22, rats received a challenge with saline or 8-OH-DPAT (0.4 mg/kg, s.c.). Sexual behavior was tested on days 1, 8, 15, and 22 of the SSRI-treatment.

RESULTS

Treatment with both doses of paroxetine, but not fluvoxamine, delayed ejaculation. 8-OH-DPAT strongly accelerated ejaculation under vehicle conditions. Pretreatment with paroxetine reduced the effects of 8-OH-DPAT on ejaculation in a dose-dependent manner and more strongly than fluvoxamine.

CONCLUSIONS

SSRIs affect 5-HT1A receptors involved in ejaculation. The degree to which this occurs, with paroxetine exerting a stronger effect than fluvoxamine, might determine the extent of SSRI-induced delayed ejaculation.

Gene dosage effect on gamma-secretase component Aph-1b in a rat model for neurodevelopmental disorders

A combination of genetic factors and early life events is thought to determine the vulnerability of an individual to develop a complex neurodevelopmental disorder like schizophrenia. Pharmacogenetically selected, apomorphine-susceptible Wistar rats (APO-SUS) display a number of behavioral and pathophysiological features reminiscent of such disorders. Here, we report microarray analyses revealing in APO-SUS rats, relative to their counterpart APO-UNSUS rats, a reduced expression of Aph-1b, a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. The reduced expression is due to a duplicon-based genomic rearrangement event resulting in an Aph-1b dosage imbalance. The expression levels of the other gamma-secretase components were not affected. However, gamma-secretase cleavage activity was significantly changed, and the APO-SUS/-UNSUS Aph-1b genotypes segregated with a number of behavioral phenotypes. Thus, a subtle imbalance in the expression of a single, developmentally important protein may be sufficient to cause a complex phenotype.