Elisabeth Y. Bijlsma

Sexual side effects of serotonergic antidepressants: Mediated by inhibition of serotonin on central dopamine release?

Antidepressant-induced sexual dysfunction adversely affects the quality of life of antidepressant users and reduces compliance with treatment. Animal models provide an instructive approach for examining potential sexual side effects of novel drugs. This review discusses the stability and reproducibility of our standardized test procedure that assesses the acute, subchronic and chronic effects of psychoactive compounds in a 30 minute mating test. In addition, we present an overview of the effects of several different (putative) antidepressants on male rat sexual behavior, as tested in our standardized test procedure. By comparing the effects of these mechanistically distinct antidepressants (paroxetine, venlafaxine, bupropion, buspirone, DOV 216,303 and S32006), this review discusses the putative mechanism underlying sexual side effects of antidepressants and their normalization. This review shows that sexual behavior is mainly inhibited by antidepressants that increase serotonin neurotransmission via blockade of serotonin transporters, while those that mainly increase the levels of dopamine and noradrenaline are devoid of sexual side effects. Those sexual disturbances cannot be normalized by simultaneously increasing noradrenaline neurotransmission, but are normalized by increasing both noradrenaline and dopamine neurotransmission. Therefore, it is hypothesized that the sexual side effects of selective serotonin reuptake inhibitors may be mediated by their inhibitory effects on dopamine signaling in sex brain circuits. Clinical development of novel antidepressants should therefore focus on compounds that simultaneously increase both serotonin and dopamine signaling.

Human Fear Acquisition Deficits in Relation to Genetic Variants of the Corticotropin Releasing Hormone Receptor 1 and the Serotonin Transporter

The ability to identify predictors of aversive events allows organisms to appropriately respond to these events, and failure to acquire these fear contingencies can lead to maladaptive contextual anxiety. Recently, preclinical studies demonstrated that the corticotropin-releasing factor and serotonin systems are interactively involved in adaptive fear acquisition. Here, 150 healthy medication-free human subjects completed a cue and context fear conditioning procedure in a virtual reality environment. Fear potentiation of the eyeblink startle reflex (FPS) was measured to assess both uninstructed fear acquisition and instructed fear expression. All participants were genotyped for polymorphisms located within regulatory regions of the corticotropin releasing hormone receptor 1 (CRHR1 - rs878886) and the serotonin transporter (5HTTLPR). These polymorphisms have previously been linked to panic disorder and anxious symptomology and personality, respectively. G-allele carriers of CRHR1 (rs878886) showed no acquisition of fear conditioned responses (FPS) to the threat cue in the uninstructed phase, whereas fear acquisition was present in C/C homozygotes. Moreover, carrying the risk alleles of both rs878886 (G-allele) and 5HTTLPR (short allele) was associated with increased FPS to the threat context during this phase. After explicit instructions regarding the threat contingency were given, the cue FPS and context FPS normalized in all genotype groups. The present results indicate that genetic variability in the corticotropin-releasing hormone receptor 1, especially in interaction with the 5HTTLPR, is involved in the acquisition of fear in humans. This translates prior animal findings to the human realm.

Local repeated corticotropin-releasing factor infusion exacerbates anxiety- and fear-related behavior: differential involvement of the basolateral amygdala and medial prefrontal cortex.

Increased central corticotropin-releasing factor (CRF) signaling has been associated with various psychiatric symptoms, including anxiety, depression and psychosis. CRF signaling in both the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) has been implicated in anxiety-like behavior. In addition, repeated activation of CRF receptors within the BLA induces a chronic anxious state. Here we studied the effects of local repeated CRF infusion in the BLA and mPFC on different forms of anxiety, as assessed during light-enhanced startle (LES, general anxiety) and acquisition of fear-potentiated startle (FPS, cue-conditioned fear). In addition, as CRF has been implicated in sensorimotor gating, prepulse inhibition (PPI) was assessed to determine if local CRF infusion within the BLA or mPFC would interfere with the processing of sensory information. To this end, canulas were placed bilaterally in either the BLA or mPFC of Wistar rats. After recovery, animals were infused with h/rCRF (200 ng/side) or vehicle for five consecutive days. Long term effects of local CRF infusion on LES and acquisition of FPS were measured 4 and 10 days post-treatment, respectively. In addition, the acute (day 1), sub-chronic (day 5) and long-term (7 days post treatment) effects on PPI were measured in the same animals. A clear regional differentiation was found on the long lasting effect of CRF on anxiety-like behavior: infusion into the BLA only enhanced acquisition of FPS, whereas infusion into the mPFC only enhanced LES. Sub-chronic CRF infusion into the BLA, but not the mPFC, disrupted PPI. This disturbed PPI was normalized 7 days post-treatment. Together, the current study shows that local repeated CRF receptor activation in the BLA and mPFC is differentially involved in anxiety- and fear-related behavior. In addition, the BLA may be involved in CRF-induced sensorimotor gating deficits. The absence of a long-term effect on these PPI deficits suggests that lasting activation of CRF receptors is a prerequisite for CRF-mediated effects on sensorimotor gating. The long-term effects of repeated CRF infusion on LES and acquisition of FPS on the other hand, show that in case of anxiety-related processes repeated CRF infusion may have lasting effects.

Medial amygdala lesions differentially influence stress responsivity and sensorimotor gating in rats

BACKGROUND The amygdala is involved in the coordination of stress but is also an important gatekeeper involved in the regulation of vigilance. The amygdala is structurally complex, consisting of several nuclei with specific functions in the affective response to environmental stimuli. There are indications that the medial amygdaloid nucleus may be a pivotal player in acute responses to emotional environmental stimuli. METHODS The present study therefore aimed to study the effects of bilateral electrolytic lesions of the medial amygdala on unconditioned anxiety-related behavior as well as a sensorimotor gating parameter (prepulse inhibition, PPI) in rats. Anxiety-related behavior was assessed with the use of stress-induced hyperthermia (SIH), light-enhanced startle (LES) and open field behavior. RESULTS Bilateral electrolytic lesions of the medial amygdala decreased the SIH response and anxiety-related open field behavior. In contrast, lesioned animals displayed augmented LES and disrupted PPI. No changes in basal locomotor activity, body temperature and acoustic startle were found between lesioned and sham animals. CONCLUSIONS The present study suggests that the medial amygdala is an important player in response to acute environmental stimuli. Decreased unconditioned psychological stress responses were found, whereas LES was enhanced and sensorimotor processing was disrupted. However, considering the existing data on basolateral amygdala involvement in PPI and bed nucleus of the stria terminalis involvement in LES, local infusion studies into the MeA should be performed to further substantiate these findings.

Disrupted startle modulation in animal models for affective disorders

Affective startle modulation is used to study emotional reactivity in humans, and blunted affective startle modulation has been reported in depressed patients. To determine whether blunted affective startle modulation is also a common feature in animal models for affective disorders, light-enhanced startle was studied in three models: inescapable foot shock (IFS), repeated restraint stress (RRS) and olfactory bulbectomy (OBX). In addition, prepulse inhibition was studied in these models. Light-enhanced startle was blunted following IFS and OBX and RRS decreased overall startle responding. Prepulse inhibition, however, was unaffected. These findings indicate that induction models for affective disorders may be associated with long term effects on affective startle modulation. The lack of changes in sensory motor gating suggests that these changes can be ascribed to alterations in emotional reactivity. In conclusion, our results indicate that the blunted affective startle modulation seen in animal models for affective disorders may be used to examine the mechanisms underlying altered emotional reactivity.

Fear-potentiated startle, but not light-enhanced startle, is enhanced by anxiogenic drugs

RATIONALE AND OBJECTIVES The light-enhanced startle paradigm (LES) is suggested to model anxiety, because of the non-specific cue and the long-term effect. In contrast, the fear-potentiated startle (FPS) is suggested to model conditioned fear. However, the pharmacological profiles of these two paradigms are very similar. The present study investigated the effects of putative anxiogenic drugs on LES and FPS and aimed at determining the sensitivity of LES for anxiogenic drugs and to potentially showing a pharmacological differentiation between these two paradigms. METHODS Male Wistar rats received each dose of the alpha(2)-adrenoceptor antagonist yohimbine (0.25-1.0mg/kg), the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP, 0.5-2.0mg/kg) or the GABA(A) inverse receptor agonist pentylenetetrazole (PTZ, 3-30mg/kg) and were subsequently tested in either LES or FPS. RESULTS None of the drugs enhanced LES, whereas mCPP increased percentage FPS and yohimbine increased absolute FPS values. Furthermore, yohimbine increased baseline startle amplitude in the LES, while mCPP suppressed baseline startle in both the LES and FPS and PTZ suppressed baseline startle in the FPS. CONCLUSIONS In contrast to findings in the FPS paradigm, none of the drugs were able to exacerbate the LES response. Thus, a clear pharmacological differentiation was found between LES and FPS.

Influence of transgenic corticotropin-releasing factor (CRF) over-expression on social recognition memory in mice

We examined juvenile social recognition and discrimination in mice with early post-natal onset, transgenic CRF over-expression (CRF-OE) and in their wild-type littermates (WT). CRF-OE mice showed enhanced social investigation during the first encounter, normal short-term and facilitated long-term social recognition memory, compared to WT. These results suggest that chronically elevated brain CRF tone may contribute in better remembering ethologically relevant and emotionally salient stimuli, such as social interaction.

Lifelong disturbance of serotonin transporter functioning results in fear learning deficits : Reversal by blockade of CRF1 receptors

The inability to associate aversive events with relevant cues (i.e. fear learning) may lead to maladaptive anxiety. To further study the role of the serotonin transporter (SERT) in fear learning, classical fear conditioning was studied in SERT knockout rats (SERT(-/-)) using fear potentiation of the startle reflex. Next, fear acquisition and concomitant development of contextual conditioned fear were monitored during training. To differentiate between developmental and direct effects of reduced SERT functioning, effects of acute and chronic SSRI treatment were studied in adult rats. Considering the known interactions between serotonin and corticotropin-releasing factor (CRF), we studied the effect of the CRFR1 antagonist CP154,526 on behavioral changes observed and determined CRF1 receptor levels in SERT(-/-) rats. SERT(-/-) showed blunted fear potentiation and enhanced contextual fear, which resulted from a deficit in fear acquisition. Paroxetine treatment did not affect acquisition or expression of fear-potentiated startle, suggesting that disturbed fear learning in SERT(-/-) results from developmental changes and not from reduced SERT functioning. Although CRF1 receptor levels did not differ significantly between genotypes, CP154,526 treatment normalized both cue- and contextual fear in SERT(-/-) during acquisition, but not expression of fear-potentiated startle. The disrupted fear acquisition and concomitant increase in contextual conditioned fear-potentiated startle fear in SERT(-/-) resembles the associative learning deficit seen in patients with panic disorder and suggests that normal SERT functioning is crucial for the development of an adequate fear neuro-circuitry. Moreover, the normalization of fear acquisition by CP154,526 suggests a role for central CRF signaling in the generalization of fear.

Cocaine-induced changes in affective state modulate the light-enhanced startle response

In order to evaluate the influence of changes in affective state on light-enhanced startle, the effects of positive affect, induced by acute cocaine administration, and the effect of negative affect, induced by spontaneous cocaine withdrawal-induced anxiety, were studied. Acute cocaine administration decreased LES, whereas withdrawal from chronic cocaine administration exacerbated LES 24h after withdrawal, an effect indicative of increased anxiety. This exacerbated LES was reduced, but not back to normal, 4 days after withdrawal. The finding that both cocaine-induced positive and negative affect can be detected in LES, suggests that this may be a valuable tool in studying affect regulation in rodents.

Fear‐Potentiated Startle and Light‐Enhanced Startle Models in Drug Discovery

Described in this unit are the fear‐potentiated startle (FPS) and light‐enhanced startle (LES) tests. These protocols have proven reliable in detecting the anxiolytic properties of test compounds. The principle of these tests is that the magnitude of the acoustic startle reflex is an index of anxiety. The FPS test includes two training sessions in which an intrinsically aversive foot shock is paired with a neutral cue light. In the test session presentation of this cue light is subsequently used to elicit startle potentiation. In the LES test startle reactivity is increased by presentation of bright light. Because LES is based on the innate aversion of rodents for bright light it does not require training sessions. Although LES has been used less frequently than FPS for screening compounds, it has an advantage in that drug effects on startle potentiation are independent of memory retrieval. Further, the contextual anxiety measured in the LES test could be more relevant for pathological anxiety than the conditioned fear associated with the FPS test. Curr. Protoc. Pharmacol. 41:5.48.1‐5.48.15.