Teresa F. Burke

Density and function of central serotonin (5-HT) transporters, 5-HT1A and 5-HT2A receptors, and effects of their targeting on BTBR T+tf/J mouse social behavior

J. Neurochem. (2011) 116, 291–303.

Interleukin-1beta Causes Fluoxetine Resistance in an Animal Model of Epilepsy-Associated Depression

Depression represents a common comorbidity of epilepsy and is frequently resistant to selective serotonin reuptake inhibitors (SSRI). We tested the hypothesis that the SSRI resistance in epilepsy associated depression may be a result of a pathologically enhanced interleukin-1β (IL1-β) signaling, and consequently that the blockade of IL1-β may restore the effectiveness of SSRI. Epilepsy and concurrent depression-like impairments were induced in Wistar rats by pilocarpine status epilepticus (SE). The effects of the 2-week long treatment with fluoxetine, interleukin-1 receptor antagonist (IL-1ra), and their combination were examined using behavioral, biochemical, neuroendocrine, and autoradiographic assays. In post-SE rats, depression-like impairments included behavioral deficits indicative of hopelessness and anhedonia; the hyperactivity of the hypothalamo-pituitary-adrenocortical axis; the diminished serotonin output from raphe nucleus; and the upregulation of presynaptic serotonin 1-A (5-HT1A) receptors. Fluoxetine monotherapy exerted no antidepressant effects, whereas the treatment with IL-1ra led to the complete reversal of anhedonia and to a partial improvement of all other depressive impairments. Combined administration of fluoxetine and IL-1ra completely abolished all hallmarks of epilepsy-associated depressive abnormalities, with the exception of the hyperactivity of the hypothalamo-pituitary-adrenocortical axis, the latter remaining only partially improved. We propose that in certain forms of depression, including but not limited to depression associated with epilepsy, the resistance to SSRI may be driven by the pathologically enhanced interleukin-1β signaling and by the subsequent upregulation of presynaptic 5-HT1A receptors. In such forms of depression, the use of interleukin-1β blockers in conjunction with SSRI may represent an effective therapeutic approach.

Plasticity of Presynaptic and Postsynaptic Serotonin 1A Receptors in an Animal Model of Epilepsy-Associated Depression

Depression is a common comorbidity of temporal lobe epilepsy and has highly negative impact on patients’ quality of life. We previously established that pilocarpine-induced status epilepticus (SE) in rats, concurrently with chronic epilepsy leads to depressive impairments, and that the latter may stem from the dysregulation of hypothalamo–pituitary–adrenocortical (HPA) axis and/or diminished raphe–hippocampal serotonergic transmission. We examined possible involvement of presynaptic and postsynaptic serotonin 1A (5-HT1A) receptors in epilepsy-associated depression. Based on their performance in the forced swim test (FST), post-SE animals were classified as those with moderate and severe depressive impairments. In moderately impaired rats, the activity of the HPA axis (examined using plasma corticosterone radioimmunoassay) was higher than in naive subjects, but the functional capacity of presynaptic 5-HT1A receptors (measured in raphe using autoradiography) remained unaltered. In severely depressed animals, both the activity of the HPA axis and the function of presynaptic 5-HT1A receptors were increased as compared with naive and moderately depressed rats. Pharmacological uncoupling of the HPA axis from raphe nucleus exerted antidepressant effects in severely impaired rats, but did not modify behavior in both naive and moderately depressed animals. Further, the function of postsynaptic 5-HT1A receptors was diminished in the hippocampus of post-SE rats. Pharmacological activation of postsynaptic 5-HT1A receptors improved depressive deficits in epileptic animals. We suggest that under the conditions of chronic epilepsy, excessively hyperactive HPA axis activates presynaptic 5-HT1A receptors, thus shifting the regulation of serotonin release in favor of autoinhibition. Downregulation of postsynaptic 5-HT1A receptors may further exacerbate the severity of epilepsy-associated depression.

Acetaminophen Differentially Enhances Social Behavior and Cortical Cannabinoid Levels in Inbred Mice

Supratherapeutic doses of the analgesic acetaminophen (paracetomol) are reported to promote social behavior in Swiss mice. However, we hypothesized that it might not promote sociability in other strains due to cannabinoid CB(1) receptor-mediated inhibition of serotonin (5-HT) transmission in the frontal cortex. We examined the effects of acetaminophen on social and repetitive behaviors in comparison to a cannabinoid agonist, WIN 55,212-2, in two strains of socially-deficient mice, BTBR and 129S1/SvImJ (129S). Acetaminophen (100mg/kg) enhanced social interactions in BTBR, and social novelty preference and marble burying in 129S at serum levels of ≥70 ng/ml. Following acetaminophen injection or sociability testing, anandamide (AEA) increased in BTBR frontal cortex, while behavior testing increased 2-arachidonyl glycerol (2-AG) levels in 129S frontal cortex. In contrast, WIN 55,212-2 (0.1mg/kg) did not enhance sociability. Further, we expected CB(1)-deficient (+/-) mice to be less social than wild-type, but instead found similar sociability. Given strain differences in endocannabinoid response to acetaminophen, we compared cortical CB(1) and 5-HT(1A) receptor density and function relative to sociable C57BL/6 mice. CB(1) receptor saturation binding (Bmax=958±117 fmol/mg protein), and affinity for [(3)H] CP55,940 (K(D)=3±0.8 nM) was similar in frontal cortex among strains. CP55,940-stimulated [(35)S] GTPγS binding in cingulate cortex was 136±12, 156±22, and 75±9% above basal in BTBR, 129S and C57BL/6 mice. The acetaminophen metabolite para-aminophenol (1 μM) failed to stimulate [(35)S] GTPγS binding. Hence, it appears that other indirect actions of acetaminophen, including 5-HT receptor agonism, may underlie its sociability promoting properties outweighing any CB(1) mediated suppression by locally-elevated endocannabinoids in these mice.

Autoreceptor‐mediated inhibition of norepinephrine release in rat medial prefrontal cortex is maintained after chronic desipramine treatment

Alterations in noradrenergic neurotransmission are important in the mechanism of action of many antidepressant drugs, including selective norepinephrine (NA) reuptake inhibitors such as desipramine (DMI). It has been suggested that chronic NA reuptake blockade induces a desensitization of inhibitory α2‐adrenergic autoreceptors. This hypothesis was tested in experiment 1 using in vivo microdialysis to examine the degree of α2‐autoreceptor‐mediated inhibition of NA release in rat medial prefrontal cortex exerted by endogenous NA following chronic treatment with vehicle or DMI. This was accomplished by measuring the elevation of extracellular NA levels induced by acute administration of the α2‐receptor antagonist yohimbine. An 8‐fold increase in basal NA levels was observed after 21 days of DMI treatment. Further, acute yohimbine administration induced a robust elevation in NA levels which was not attenuated, and in fact at lower doses was greater in DMI‐treated rats compared with vehicle‐treated controls. In experiment 2, we addressed directly the functional status of terminal α2‐autoreceptors in frontal cortex in vitro, in the absence of potentially confounding competition from elevated levels of endogenous NA, after chronic reuptake blockade. We observed no difference in the degree to which the α2‐receptor agonist clonidine inhibited potassium‐evoked [3H]‐NA release from cortical slices taken from DMI‐ or vehicle‐treated rats. Together, these data suggest that endogenous activation of α2‐autoreceptors persists in restraining NA neurotransmission in the face of tonically elevated basal NA levels following chronic reuptake blockade.

Serotonin‐1A receptor function in the dorsal raphe nucleus following chronic administration of the selective serotonin reuptake inhibitor sertraline

Serotonin‐1A (5‐HT1A) receptors in the dorsal raphe nucleus (DRN) function as somatodendritic autoreceptors, and therefore play a critical role in controlling serotonergic cell firing and serotonergic neurotransmission. We hypothesized that a decrease in the capacity of 5‐HT1A receptors to activate G proteins was a general mechanism by which 5‐HT1A receptors in the DRN are desensitized following chronic administration of selective serotonin reuptake inhibitors (SSRIs). Using in vivo microdialysis, we found that the ability of the 5‐HT1A receptor agonist 8‐hydroxydipropylaminotetralin hydrobromide (8‐OH‐DPAT) (0.025 mg/kg, s.c.) to decrease extracellular 5‐HT levels in striatum was attenuated following chronic treatment of rats with the SSRIs sertraline or fluoxetine. This apparent desensitization of somatodendritic 5‐HT1A autoreceptor function was not accompanied by a decrease in 5‐HT1A receptor sites in the coupled, high‐affinity agonist state as measured by the binding of [3H]8‐OH‐DPAT. In marked contrast to what was observed following chronic administration of fluoxetine, 5‐HT1A receptor‐stimulated [35S]GTPγS binding in the DRN was not altered following chronic sertraline treatment. Thus, desensitization of 5‐HT1A somatodendritic autoreceptor function following chronic sertraline administration appears not to be due to a decrease in the capacity 5‐HT1A receptors to activate G proteins in the DRN. Our findings suggest that the SSRIs may not be a homogeneous class of antidepressant drug with regard to the mechanism by which the function of somatodendritic 5‐HT1A autoreceptors is regulated.

GABAB Receptor-Positive Modulators: Brain Region-Dependent Effects

This study examined the positive modulatory properties of 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF) at γ-aminobutyric acid B (GABAB) receptors in different brain regions. Using quantitative autoradiography, we measured GABAB receptor-stimulated binding of guanosine 5′-O-(3-[35S]thiotriphosphate) ([35S]GTPγS) to G proteins in medial prefrontal cortex (mPFC), hippocampus, and cerebellum. CGP7930 and rac-BHFF enhanced baclofen-stimulated [35S]GTPγS binding similarly in mPFC and hippocampus, but were more effective in cerebellum. CGP7930 (100 μM) increased [35S]GTPγS binding stimulated by baclofen (30 μM) from 29 to 241% above basal in mPFC and from 13 to 1530% above basal in cerebellum. Likewise, rac-BHFF (10 μM) increased baclofen-stimulated [35S]GTPγS binding more in cerebellum (from 13 to 1778% above basal) than in mPFC (from 29 to 514% above basal). rac-BHFF (10 μM) in combination with γ-hydroxybutyrate (20 mM) increased [35S]GTPγS binding in cerebellum but not in mPFC. rac-BHFF also enhanced the effects of 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348). Consistent with its partial agonist properties, CGP35348 stimulated [35S]GTPγS binding in mPFC when given alone (to 18% above basal), but less extensively than baclofen (140% above basal), and antagonized baclofen when given together. CGP35348 (1 mM) in combination with rac-BHFF (100 μM) produced an increase in [35S]GTPγS binding that was larger in cerebellum (from 61 to 1260% above basal) than in mPFC (from 18 to 118% above basal). Taken together, the results show that GABAB receptor-positive modulators enhance [35S]GTPγS binding stimulated by GABAB receptor agonists in a brain region-dependent manner. This regionally selective enhancement is further evidence of pharmacologically distinct GABAB receptor populations, possibly allowing for more selective therapeutic targeting of the GABAB system.

Sensitivity of hippocampal 5-HT1A receptors to mild stress in BDNF-deficient mice

Serotonin 1A (5-HT(1A)) receptors in brain play an important role in cognitive and integrative functions, as well as emotional states. Decreased brain-derived neurotrophic factor (BDNF) expression and/or function, particularly in hippocampus, are implicated in the pathophysiology of stress-related disorders such as major depression. BDNF(+/-) mice are more vulnerable to stress than wild-type mice, exhibiting behavioural despair after mild handling stress. We examined the effect of mild handling stress on 5-HT(1A) receptor function, as measured by 8-OH-DPAT stimulated [(35)S]GTPγS binding, in BDNF(+/-) mice and mice with a forebrain-specific reduction in BDNF (embryonic BDNF inducible knockout mice). Our data show a remarkable sensitivity of hippocampal 5-HT1A receptors to mild stress and a deficiency in BDNF. Other 5-HT(1A) receptor populations, specifically in frontal cortex and dorsal raphe, were resistant to the combined detrimental effects of mild stress and reductions in BDNF expression. Decreases in hippocampal 5-HT(1A) receptor function induced by mild stress in BDNF-deficient mice were prevented by administration of the selective serotonin reuptake inhibitor fluoxetine, which increased activation of TrkB, the high affinity receptor for BDNF, in wild-type and BDNF(+/-) mice. In hippocampal cultures, BDNF increased the capacity of 5-HT(1A) receptors to activate G proteins, an effect eliminated by the knockout of TrkB, confirming TrkB activation increases 5-HT(1A) receptor function. The mechanisms underlying the sensitivity of hippocampal 5-HT(1A) receptors to mild stress and decreased BDNF expression remain to be elucidated and may have important implications for the emotional and cognitive impairments associated with stress-related mental illness.

Effect of valproic acid on serotonin‐2A receptor signaling in C6 glioma cells

Valproic acid (VPA), which has demonstrated efficacy in the treatment of bipolar disorder, has been shown to alter components of the phosphoinositide (PI) signaling cascade and to increase gene expression mediated by the transcription factor activator protein 1 (AP‐1). Central serotonin‐2A (5‐HT2A) receptors, which have been implicated in the pathophysiology of manic‐depressive illness, are coupled to PI hydrolysis. The promoter region of the 5‐HT2A receptor gene contains AP‐1 binding sites. We examined in C6 glioma cells the effect of VPA on 5‐HT2A receptor signaling. Treatment of cells with VPA (100 µg/mL) for 20 h, but not 1.5 h, resulted in an enhancement of 5‐HT2A receptor‐stimulated PI hydrolysis. This effect of 20‐h VPA exposure appeared not to be at the level of G protein or effector (i.e. phospholipase C: PLC) as inositol phosphate accumulation stimulated by aluminum fluoride or the PLC activator 2,4,6‐trimethyl‐N‐(m‐3‐trifluromethylphenyl) benzenesulfonamide was not increased. The number of 5‐HT2A receptors, as determined in saturation binding experiments using [3H]ketanserin, was increased by 20‐h VPA treatment, with no change in affinity (KD). Taken together, our data suggest that the increase in 5‐HT2A receptor‐mediated PI hydrolysis following 20‐h VPA exposure is not due to a general effect of VPA on this signaling cascade, but due to the up‐regulation of 5‐HT2A receptor number.

Hippocampal serotonin-1A receptor function in a mouse model of anxiety induced by long-term voluntary wheel running

We have recently demonstrated that, in C57/Bl6 mice, long‐term voluntary wheel running is anxiogenic, and focal hippocampal irradiation prevents the increase in anxiety‐like behaviors and neurobiological changes in the hippocampus induced by wheel running. Evidence supports a role of hippocampal 5‐HT1A receptors in anxiety. Therefore, we investigated hippocampal binding and function of 5‐HT1A receptors in this mouse model of anxiety. Four weeks of voluntary wheel running resulted in hippocampal subregion‐specific changes in 5‐HT1A receptor binding sites and function, as measured by autoradiography of [3H] 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin binding and agonist‐stimulated binding of [35S]GTPγS to G proteins, respectively. In the dorsal CA1 region, 5‐HT1A receptor binding and function were not altered by wheel running or irradiation. In the dorsal dentate gyrus and CA2/3 region, 5‐HT1A receptor function was decreased by not only running but also irradiation. In the ventral pyramidal layer, wheel running resulted in a decrease of 5‐HT1A receptor function, which was prevented by irradiation. Neither irradiation nor wheel running affected 5‐HT1A receptors in medial prefrontal cortex or in the dorsal or median raphe nuclei. Our data indicate that downregulation of 5‐HT1A receptor function in ventral pyramidal layer may play a role in anxiety‐like behavior induced by wheel running. Synapse 67:648–655, 2013.