Noradrenaline Neurons Research Articles

Sustained administration of bupropion alters the neuronal activity of serotonin, norepinephrine but not dopamine neurons in the rat brain

Bupropion is widely used in the treatment of depression. There are, however, limited data on its long-term effects on monoaminergic neurons and therefore the mechanism of its delayed onset of action is at present not well understood. The present study was conducted to examine the effects of prolonged bupropion administration on the firing activity of dorsal raphe nucleus (DRN), locus coeruleus (LC), and ventral tegmental area (VTA) neurons. Spontaneously firing neurons were recorded extracellularly in rats anesthetized with chloral hydrate. Bupropion (30 mg/kg/day) was administered using subcutaneously implanted minipumps. In the DRN, the firing rate of serotonin (5-HT) neurons was significantly increased after 2, 7 and 14 days of administration. The suppressant effect of LSD was significantly diminished after the two-day regimen, indicating a desensitization of 5-HT 1A autoreceptors. In the LC, the firing rate of norepinephrine (NE) neurons was significantly attenuated after a 2-day regimen, but recovered progressively over 14 days of administration. The suppressant effect of clonidine on NE neuronal firing was significantly attenuated in rats treated with bupropion for 14 days, indicating a desensitization of α 2-adrenoceptors. In the VTA, neither 2 nor 14 days of bupropion administration altered the firing and burst activity of dopamine neurons. These results indicate that bupropion, unlike 5-HT reuptake inhibitors, promptly increased 5-HT neuronal activity, due to early desensitization of the 5-HT 1A autoreceptor. The gradual recovery of neuronal firing of NE neurons, due to the desensitization of α 2-adrenoceptors, in the presence of the sustained increase in 5-HT neuronal firing, may explain in part the delayed onset of action of bupropion in major depression.

Regulation of the neuronal norepinephrine transporter by endothelin-1 and -3 in the rat anterior and posterior hypothalamus

We previously reported that endothelin-1 and endothelin-3 modulate norepinephrine neuronal release and tyrosine hydroxylase activity and expression in the hypothalamus. In the present study we sought to establish the role of endothelin-1 and -3 in the regulation of norepinephrine uptake in the anterior and posterior hypothalamus. Results showed that in the anterior hypothalamus endothelin-3 increased neuronal norepinephrine uptake whereas endothelin-1 decreased it. Conversely, in the posterior hypothalamic region both endothelins diminished the neuronal uptake of the amine. Endothelins response was concentration dependent and maintained at all studied times. Endothelins also modified the kinetic and internalization of the NE neuronal transporter. In the anterior hypothalamic region endothelin-3 increased the V max and the B max whereas endothelin-1 decreased them. However, in the posterior hypothalamic region both endothelins diminished the V max as well as B max. Neither endothelin-1 nor endothelin-3 modified neuronal norepinephrine transporter K d in the studied hypothalamic regions. These findings support that in the posterior hypothalamic region both endothelins diminished neuronal norepinephrine transporter activity by reducing the amine transporter expression on the plasmatic membrane. Conversely, in the anterior hypothalamic region endothelin-3 enhanced neuronal norepinephrine transporter activity by increasing the expression of the transporter on the presynaptic membrane, whereas endothelin-1 induced the opposite effect. Present results permit us to conclude that both endothelins play an important role in the regulation of norepinephrine neurotransmission at the presynaptic nerve endings in the hypothalamus.

Cardiac norepinephrine transporter protein expression is inversely correlated to chamber norepinephrine content

The cardiac neuronal norepinephrine (NE) transporter (NET) in sympathetic neurons is responsible for uptake of released NE from the neuroeffector junction. The purpose of this study was to assess the chamber distribution of cardiac NET protein measured using [(3)H]nisoxetine binding in rat heart membranes and to correlate NE content to NET amount. In whole mounts of atria, NET was colocalized in nerve fibers with tyrosine hydroxylase (TH) immunoreactivity. NE content expressed as micrograms NE per gram tissue was lowest in the ventricles; however, NET binding was significantly higher in the left ventricle than the right ventricle and atria (P < 0.05), resulting in a significant negative correlation (r(2) = 0.922; P < 0.05) of NET to NE content. The neurotoxin 6-hydroxydopamine, an NET substrate, reduced NE content more in the ventricles than the atria, demonstrating functional significance of high ventricular NET binding. In summary, there is a ventricular predominance of NET binding that corresponds to a high NE reuptake capacity in the ventricles, yet negatively correlates to tissue NE content.

Resiliency of monoaminergic systems: the 80% rule and its relevance to drug development

Monoaminergic systems have long been implicated in the pathophysiology and/or the therapeutics of mood disorders. These are old systems on the phylogenic scale, which has allowed their in-depth studies in the rodent brain with the results being extrapolated to humans with striking consistency. Mother Nature has built-in mechanisms in these systems to maintain their homeostasis. Indeed, they exert such important roles in the brain within the animal kingdom, including nonhuman primates and humans, and the maintenance of their normal function plays a crucial role in survival. The level of redundancy appears to be of the order of 80%, meaning that alterations of various parameters controlling the function of these system has to reach an approximate threshold of 80% change before a net change in transmission can occur. This is highly relevant to human therapeutics because it implies that there should not be a linear relationship between interference with a monoaminergic neuronal element, such as reuptake transporter, and a clinical phenomenon. Some examples are given here. In Parkinson’s disease, patients begin to show motor symptoms when there is an approximate loss of 80% of dopamine neurons in the substantia nigra (Agid, et al., 1987). These neurons give rise to the dopaminergic innervation of the striatum, which plays a crucial role in motor function. Consistent with the above-mentioned figure, 80% of striatal dopamine type 2 receptors need to be blocked by antipsychotic medications to lead to extrapyramidal symptoms. In Alzheimer’s disease, approximately 75% of the cholinergic neurons in the basal forebrain are lost when symptoms (Farde et al., 1992) appear (McGeer, et al., 1984). These neurons provide the cholinergic innervation to the cerebral cortex. Acetylcholine is generally not thought of as a monoamine, such as serotonin (5-HT), noradrenaline (NE) or dopamine, but it has a single quaternary amine (a positively charged nitrogen atom with three methyl groups). Approximately, 70–80% of monoamine oxidase (MAO) need to be inhibited before an antidepressant action can occur with inhibitors of this enzyme (Zimmer, 1990). This was determined from MAO-B inhibition and the decrease of 3,4-dihydroxyphenyl-ethylene glycol in the blood to assess MAO-A inhibition. Finally, the minimal effective doses of the serotonin reuptake inhibitors (SRIs) produce an 80% occupancy of 5-HT transporters (5-HTTs) in the human brain, as measured by positron emission tomography (PET; Meyer, et al., 2004). Experimental results in the rodent brain are consistent with the above observations. In the NE system, approximately 90% of NE neurons need to be lesioned before attenuating the effect of the stimulation of the NE pathway on postsynaptic neuron firing in the hippocampus (Curet and de Montigny, 1988). In the 5-HT system, an approximate 80% decrease in tissue 5-HT content leaves extracellular 5-HT levels unaltered in the rat striatum (Kirby, et al., 1995). In the dopamine system, extracellular levels are maintained until 80% of control tissue content is eliminated (Castaneda, et al., 1990). Why is this relevant to current drug development? Because selective 5-HT, NE or dopamine agents can produce an antidepressant action on their own, it is generally accepted that the combination of at least two effects can produce a greater therapeutic effect. This is supported by the effectiveness of augmentation or combination strategies in patients not responding to a selective agent (Blier, 2006). In the case of the dual 5-HT/NE reuptake inhibitors, venlafaxine and duloxetine, biochemical data are consistent in showing their potent action on the 5-HT transporter (Turcotte, et al., 2001; Vincent, et al., 2004; Blier, et al., 2007). With concentration or dose increments, the NE transporters (NETs) are progressively inhibited. Regarding NET inhibition, the main issue is at what dose(s) or concentration(s) are NETs inhibited to an extent that significantly impacts overall NE transmission? In the case of venlafaxine, 75 mg/day produces the above mentioned 80% occupancy of the 5-HTT, which corresponds to its minimal effective dose in depression (Meyer, et al., 2004). Similarly, the minimal Editorial

Norepinephrine and serotonin imbalance in the locus coeruleus in bipolar disorder

Abnormalities in norepinephrine (NE) and serotonin (5-HT) are implicated in bipolar disorder (BD). We examined 5-HT input and NE neurons in the locus coeruleus (LC, the NE nucleus that innervates the forebrain) in BD by quantifying immunoreactivity (IR) for tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), the biosynthetic enzymes for NE and 5-HT, respectively. Six suicides with BD were compared to matched normal controls and unipolar major depression suicides, using immunocytochemistry with computer-assisted quantification of immunoreactivity. Depressed bipolar suicides had 26.7 +/- 1.3% of LC area occupied by the TH immunoreactive (TH-IR) process, while controls had 50.7 +/- 8% (p = 0.002) and unipolar depressed suicides had 50.3 +/- 2.5% (p = 0.003). In bipolars, these processes did not stain as darkly (1.9 +/- 0.5 x background) as controls (2.9 +/- 0.9 x background; p = 0.01) or unipolars (2.9 +/- 0.6 x background; p = 0.002). Bipolar suicides also had less TPH-IR processes in the LC (11.7 +/- 10%) compared with controls (32.8 +/- 8.8%; p = 0.01) or unipolar suicides (30.3 +/- 8%; p = 0.02). The TPH-IR intensity did not differ between groups. We found less TH-IR and TPH-IR in the LC in depressed bipolar suicides, but not unipolar suicides, suggesting that both NE and 5-HT activity is lower in BD. Studies during manic or euthymic states will determine whether these changes are mood state dependent.

Transgenic Mice Overexpressing Neuropeptide Y in Noradrenergic Neurons

A functional polymorphism leucine 7 proline in the human neuropeptide Y (NPY) gene leading to increased NPY release from sympathetic nerves is associated with traits of metabolic syndrome. Although hypothalamic NPY neurons play an established role in promoting positive energy balance, the role of NPY colocalized with norepinephrine in sympathetic nervous system and brain noradrenergic neurons remains obscure. To clarify the role of NPY in noradrenergic neurons, we generated a transgenic mouse overexpressing NPY under dopamine-beta-hydroxylase promoter and characterized the metabolic phenotype of the OE-NPY(DbetaH) mouse. NPY levels are increased by 1.3-fold in adrenal glands and 1.8-fold in the brainstem but not in the hypothalamus in OE-NPY(DbetaH) mice. They display increased white adipose tissue mass and cellularity and liver triglyceride accumulation without hyperphagia or increased body weight. Hyperinsulinemia and impaired glucose tolerance develop by the age of 6 months in the OE-NPY(DbetaH) mice. Furthermore, circulating ghrelin is significantly increased in comparison with wild-type mice. The present study shows that even a moderate increase in NPY levels in noradrenergic neurons leads to disturbances in glucose and lipid metabolism. The OE-NPY(DbetaH) mouse is an interesting new model to investigate the pathophysiology of some key components of the cluster of abnormalities characterizing the metabolic syndrome.

The role of plasma noradrenaline in health and disease

This paper proposes that plasma noradrenaline plays a central role in the physiology and pathophysiology of the macrocirculation, the heart, veins and arteries. The proposal stems, in the final analysis, from the finding that noradrenaline dilates the canine lateral saphenous vein when it is released from its microcirculation, the vasa venarum. The concentration threshold of the effect is estimated to be at least eight times lower than the threshold of the constrictor effect of intralumenal noradrenaline. Combined with other evidence, the finding indicates that, contrary to opinion, plasma noradrenaline has an effective β 1-agonist hormonal effect on the macrocirculation, at normal concentrations. It also indicates that noradrenaline has a bi-polar effect. The reason it has that effect is that noradrenaline is a primary biological stimulus and like all primary stimuli investigated to date, it can be expected to have two cross-inhibitory components, commonly referred to as excitor and lateral inhibitory. When examined, neuronal noradrenaline shows the features characteristic of excitor components in general and plasma noradrenaline shows those characteristic of inhibitory components. The most significant of the latter is that inhibitory components are the most potent physiological modulators of excitor components. A striking example of that modulation effect occurs in smooth muscle contraction where muscles contracted by neuronal noradrenaline stimulation appear to be incapable of relaxing without stimulation by plasma noradrenaline-hence the proposition that vascular stenosis and cardiospasm are caused by a pathological loss of plasma noradrenaline stimulation. By triggering turbulence, cholesterol plaques increase the arterial microcirculatory flow and so increase the β 1-agonist effect of plasma noradrenaline. This paper proposes that this cholesterol driven increase in the microcirculatory effect of plasma noradrenaline is the cause of the arteriosclerotic syndrome, a proposition that is consistent with the success of beta-blockade in treating manifestations of the syndrome. The paper concludes by examining a variety of conditions, including dissecting aneurysms, dilator cardiac failure, angina, myocardial infarction, hypertension, eclampsia and cirrhosis and pointing out that all of them, like varicose veins, show evidence consistent with having been caused by a turbulence induced increase in a β 1-agonist stimulatory effect of microcirculatory plasma noradrenaline.

Neurobehavioral mechanisms of impulsivity: Fronto-striatal systems and functional neurochemistry

Impulsive acts and decisions are a part of everyday normal behavior. However, in its pathological forms, impulsivity can be a debilitating disorder often associated with a number of neuropsychiatric disorders, including attention-deficit hyperactivity disorder (ADHD). This article reviews recent progress in our understanding of the neurobiology of impulsivity using examples from recent investigations in experimental animals. Evidence is reviewed from several well-established paradigms with putative utility in assessing distinct forms of impulsive behavior in rodents, including the 5-choice serial reaction time (5CSRT) task and the delay discounting paradigm. We discuss, in particular, recent psychopharmacological and in-vivo neurochemical data in task-performing rats showing functional heterogeneity of the forebrain dopamine (DA), noradrenaline (NA), serotonin (5-HT) and acetylcholine (ACh) systems and identify how these systems normally function to facilitate flexible goal-directed behavior in situations that tax basic attentional functions and inhibitory response control mechanisms. We also discuss future research needs in terms of understanding the functional diversity of different sub-regions of prefrontal cortex (PFC) and how these systems normally interact with the striatum and main nuclei of origin of DA and NA neurons. Finally, we argue in line with others that animal paradigms are unlikely to model all aspects of complex psychiatric conditions such as ADHD but components of such syndromes may be amenable to investigation using sophisticated animal models based on highly-defined psychiatric endophenotypes.

In vivo electrophysiological assessment of the putative antidepressant Wf-516 in the rat raphe dorsalis, locus coeruleus and hippocampus

Wf-516 is a potential novel antidepressant. It has high affinity for serotonin (5-hydroxytryptamine; 5-HT) transporters, 5-HT(1A) and 5-HT(2A) receptors. In the present study, the pharmacologic properties of Wf-516 were thus assessed using in vivo electrophysiology in the rat dorsal raphe nucleus (DRN), locus coeruleus (LC) and hippocampus. Glass microelectrodes were lowered into the DRN, LC or hippocampus, and neurons were recorded and tested using systemic or microiontophoretic injections of drugs. In the DRN, cumulative doses of 0.5 mg/kg of Wf-516 were injected intravenously and total inhibition of 5-HT neurons firing was obtained with 2.8 +/- 0.3 mg/kg. The administration of 1 mg/kg of Wf-516, which by itself did not induce a change in the firing of 5-HT neurons, markedly attenuated the inhibitory effect of the 5-HT(1A) autoreceptor agonist LSD, indicating that Wf-516 is a 5-HT(1A) autoreceptor antagonist. In the LC, 1 mg/kg of Wf-516 dampened the inhibitory effect of the preferential 5-HT(2A) agonist DOI on norepinephrine (NE) neurons, indicating that Wf-516 is also a 5-HT(2A) receptor antagonist. In the hippocampus, cumulative intravenous doses of Wf-516 significantly increased the recovery time of firing activity of CA(3) pyramidal neurons after 5-HT applications, indicating an inhibitory effect on 5-HT reuptake. Unlike the 5-HT(1A) antagonist WAY100635, Wf-516 did not block the inhibitory effect of microiontophoretic application of 5-HT, indicating that this drug is devoid of 5-HT(1A) receptor antagonistic activity in this postsynaptic structure. These properties of WF-516 define the transporter/receptorial profile of an antidepressant with superior effectiveness.

Stress-Induced Intracellular Trafficking of Corticotropin-Releasing Factor Receptors in Rat Locus Coeruleus Neurons

Corticotropin-releasing factor (CRF) activates locus coeruleus (LC)-norepinephrine neurons during stress. Previous stress or CRF administration attenuates the magnitude of this response by decreasing postsynaptic sensitivity to CRF. Here we describe the fate of CRF receptors (CRFr) in LC neurons after stress. Rats were exposed to swim stress or handling and perfused 1 or 24 h later. Sections through the LC were processed for immunogold-silver labeling of CRFr. CRFr in LC dendrites was present on the plasma membrane and within the cytoplasm. In control rats, the ratio of cytoplasmic to total dendritic labeling was 0.55 +/- 0.01. Swim stress increased this ratio to 0.77 +/- 0.01 and 0.80 +/- 0.02 at 1 and 24 h after stress, respectively. Internalized CRFr was associated with different organelles at different times after stress. At 1 h after stress, CRFr was often associated with early endosomes in dendrites and perikarya. By 24 h, more CRFr was associated with multivesicular bodies, suggesting that some of the internalized receptor is targeted for degradation. In perikarya, more internalized CRFr was associated with Golgi apparatus 24 vs. 1 h after stress. This is suggestive of changes in CRFr synthesis. Alternatively, this may indicate communication between multivesicular bodies and Golgi apparatus in the process of recycling. Administration of the selective CRF(1) antagonist, antalarmin, before swim stress attenuated CRFr internalization. The present demonstration of stress-induced internalization of CRFr in LC neurons provides evidence that CRF is released in the LC during swim stress to activate this system and initiate cellular trafficking of the receptor that determines subsequent sensitivity of LC neurons to CRF.

Chronic repeated restraint stress increases prolactin‐releasing peptide/tyrosine‐hydroxylase ratio with gender‐related differences in the rat brain

In this study, we investigated the effect of chronic repeated restraint (RR) on prolactin-releasing peptide (PrRP) expression. In the brainstem, where PrRP colocalize with norepinephrine in neurons of the A1 and A2 catecholaminergic cell groups, the expression of tyrosine hydroxylase (TH) has also been examined. In the brainstem, but not in the hypothalamus, the basal PrRP expression in female rats was higher than that in the males that was abolished by ovariectomy. RR evoked an elevation of PrRP expression in all areas investigated, with smaller reaction in the brainstems of females. There was no gender-related difference in the RR-evoked TH expression. Elevation of PrRP was relatively higher than elevation of TH, causing a shift in PrRP/TH ratio in the brainstem after RR. Estrogen alpha receptors were found in the PrRP neurons of the A1 and A2 cell groups, but not in the hypothalamus. Bilateral lesions of the hypothalamic paraventricular nucleus did not prevent RR-evoked changes. Elevated PrRP production parallel with increased PrRP/TH ratio in A1/A2 neurons indicate that: (i) there is a clear difference in the regulation of TH and PrRP expression after RR, and (ii) among other factors this may also contribute to the changed sensitivity of the hypothalamo-pituitary-adrenal axis during chronic stress.

P.1.c.046 Asenapine displays high affinity and potent antagonism at an ensemble of human serotonin receptor subtypes

Substance P (neurokinin-1; NK1) receptor antagonists represent a putative new class of antidepressant/anxiolytic drugs. Using in vivo electrophysiological paradigms in rats, this study examined the effects of acute, sub-acute and long-term administration of these drugs on the firing of rat noradrenaline and serotonin (5-HT) neurons. In the locus coeruleus, neither a 2-day treatment with the tachykinin NK1 receptor antagonists [(2S,3S)-cis-2-(diphenylmethyl)-N-[(2-methoxyphenyl) methyl]-1-azabicyclo[2.2.2]octan-3-amine (CP-96,345, 10 mg/kg/day, i.p.), CP-99,994 (10 mg/kg/day, i.p.), nor a 14-day of treatment with (+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine (CP-99,994, 10 mg/kg/day, s.c.) significantly modified the firing rate of noradrenaline neurons. However, all these treatments attenuated the inhibitory action of the α2-adrenoceptor agonist clonidine on noradrenaline neuronal firing. While acute administration of the tachykinin NK1 receptor antagonist CP-96,345 (10 mg/kg, i.p.) attenuated the responsiveness of dorsal raphe 5-HT1A autoreceptors, lesioning noradrenaline neurons with the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) prevented the enhancing action of a 2-day treatment with CP-96,345 on 5-HT neuronal firing, suggesting that tachykinin NK1 receptor antagonists influence 5-HT system via noradrenaline neurons independently of their firing rate.

P.2.d.016 Effects of long-term treatment with bupropion on the firing activity of serotonin and norepinephrine neurons

Background: Major depressive disorder (MDD) is a common, chronic illness associated with substantial disability and economic burden. Although a number of effective antidepressants are available, the need for new medications that are effective and well tolerated remains.Objective: The aim of this study was to compare the efficacy and tolerability of fixed-dose desvenlafaxine 50 and 100 mg/d with placebo for MDD. A post hoc pooled analysis was conducted to evaluate this study in the context of all similarly designed, completed studies with the 2 doses.Methods: This was an 8-week, Phase III, randomized, double-blind, duloxetine-referenced, placebo-controlled, parallel-group trial conducted in 21 centers across the United States. Duloxetine was included for assay sensitivity as a positive control; the study was not designed or powered to compare desvenlafaxine with duloxetine. Participants were outpatients aged ≥18 years with Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition—defined MDD and a 17-item Hamilton Rating Scale for Depression (HAM-D17) score ≥20. Patients were randomly assigned at baseline to fixed-dose desvenlafaxine (50 or 100 mg/d), fixed-dose duloxetine (60 mg/d), or placebo. The primary outcome measure was HAM-D17 total score at the final evaluation. Additional measures included the Clinical Global Impressions-Improvement (CGI-I) score, Montgomery Åsberg Depression Rating Scale (MADRS) score, Clinical Global Impressions-Severity (CGI-S) score, and 6-item Hamilton Rating Scale for Depression, Bech version (HAM-D6). Tolerability assessments included discontinuation rates, adverse events (AEs), vital signs, and laboratory tests. The post hoc pooled analysis was performed using data from the current study and 2 previously published, positive studies that compared the efficacy and tolerability of desvenlafaxine 50 and 100 mg/d with placebo for MDD. The design and methodologies of the 2 studies were similar to the methodology of the current trial, other than not including a reference compound.Results: Of the 925 patients who were screened, 287 did not meet entry criteria, and 638 patients enrolled in the study; the intent-to-treat (ITT) population included 615 patients who were evaluated for efficacy (mean [SD] age range, 38.8–40.7 [12.1–13.2] years; mean weight range, 83.3–87.0 [22.8–23.9] kg; female sex, 398 [64.7%]; white race, 458 [74.5%]). The primary end point did not reach significance based on the global F test for controlling multiplicity of the desvenlafaxine doses. Based on pairwise comparison, significantly greater improvements on the HAM-D17 were observed in the desven-lafaxine 100 mg/d (−10.5; P = 0.028, unadjusted for multiple comparisons) and duloxetine 60 mg/d groups (−10.3; P = 0.047) compared with placebo (−8.7). Desvenlafaxine 100 mg/d and duloxetine 60 mg/d were associated with significantly better scores compared with placebo on the CGI-I, MADRS, CGI-S, and HAM-D6. No significant differences were observed in any scale between the desvenlafaxine 50 mg/d and placebo groups. Discontinuation rates due to AEs were 5%, 7%, 13%, and 6% for the desvenlafaxine 50-mg/d, desvenlafaxine 100-mg/d, duloxetine 60-mg/d, and placebo groups, respectively. The ITT population from all 3 studies in the pooled analysis consisted of 1388 patients (mean [SD] age range, 38.8–45.7 [12.1–12.6] years; mean weight range, 73.1–87.0 [17.6–23.9] kg; female sex, 896 [64.6%]; white race, 1136 [81.8%]). Significantly greater improvements on the HAM-D17 were observed for desvenlafaxine 50 mg/d (−11.5; P < 0.001) and 100 mg/d (−11.8; P < 0.001) versus placebo (−9.6). Both doses were significantly better than placebo on the CGI-I, MADRS, and HAM-D6.Conclusions: The current study failed to meet its primary efficacy end point based on the a priori analysis plan. Desvenlafaxine was generally well tolerated. A post hoc pooled analysis of this trial and 2 previously published trials with both desvenlafaxine 50 and 100 mg/d found both doses to be effective for MDD compared with placebo. ClinicalTrials.gov Identifier: 00384033.

P.2.d.026 Functional interactions between serotonin, norepinephrine and dopamine neurons: an electrophysiological study

P.2.d.026 Functional interactions between serotonin, norepinephrine and dopamine neurons: an electrophysiological study

Sympathetic activity in major depressive disorder: identifying those at increased cardiac risk?

Evidence exists linking major depressive disorder (MDD) with clinical cardiovascular events. The importance of the sympathetic nervous system in the generation of cardiac risk in other contexts is established. To examine the importance of the sympathetic nervous system in the generation of cardiac risk in patients with major depressive disorder (MDD). Studies were performed in 39 patients meeting the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV) criteria for MDD and in 76 healthy subjects. Treatment for patients consisted of selective serotonin reuptake inhibition (SSRI) for 12 weeks. Whole body and cardiac sympathetic activity were examined using noradrenaline isotope dilution methodology and sympathetic nerve recording techniques. Measurement of the extraction of infused tritiated noradrenaline by the heart, and estimation of cardiac dihydroxyphenylglycol production provided direct quantification of neuronal noradrenaline reuptake. Sympathetic activity, particularly in the heart and for the whole body, in patients with MDD followed a bimodal distribution. Elevated values were observed in patients with co-morbid panic disorder (P = 0.006). Consistent with a defect in noradrenaline reuptake, the cardiac extraction of tritiated noradrenaline (0.80 +/- 0.01 versus 0.56 +/- 0.04%, P < 0.001) and cardiac dihydroxyphenylglycol overflow (109 +/- 8 versus 73 +/- 11, P = 0.01) were reduced in patients with MDD. SSRI therapy abolished the excessive sympathetic activation, with whole body noradrenaline spillover falling from 518 +/- 83 to 290 +/- 41 ng/min (P = 0.008). We have identified a subset of patients with MDD in whom sympathetic nervous activity is extraordinarily high, including in the sympathetic outflow to the heart. Treatment with an SSRI may reduce sympathetic activity in a manner likely to reduce cardiac risk.

Norepinephrine loss produces more profound motor deficits than MPTP treatment in mice

Although Parkinson's disease (PD) is characterized primarily by loss of nigrostriatal dopaminergic neurons, there is a concomitant loss of norepinephrine (NE) neurons in the locus coeruleus. Dopaminergic lesions induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are commonly used to model PD, and although MPTP effectively mimics the dopaminergic neuropathology of PD in mice, it fails to produce PD-like motor deficits. We hypothesized that MPTP is unable to recapitulate the motor abnormalities of PD either because the behavioral paradigms used to measure coordinated behavior in mice are not sensitive enough or because MPTP in the absence of NE loss is insufficient to impair motor control. We tested both possibilities by developing a battery of coordinated movement tests and examining motor deficits in dopamine beta-hydroxylase knockout (Dbh-/-) mice that lack NE altogether. We detected no motor abnormalities in MPTP-treated control mice, despite an 80% loss of striatal dopamine (DA) terminals. Dbh-/- mice, on the other hand, were impaired in most tests and also displayed spontaneous dyskinesias, despite their normal striatal DA content. A subset of these impairments was recapitulated in control mice with 80% NE lesions and reversed in Dbh-/- mice, either by restoration of NE or treatment with a DA agonist. MPTP did not exacerbate baseline motor deficits in Dbh-/- mice. Finally, striatal levels of phospho-ERK-1/2 and DeltaFosB/FosB, proteins which are associated with PD and dyskinesias, were elevated in Dbh-/- mice. These results suggest that loss of locus coeruleus neurons contributes to motor dysfunction in PD.

Outcomes after Isolated Mirtazapine (Remeron™) Supratherapeutic Ingestions

Mirtazapine (Remeron™) is a newly approved medication for the treatment of depression. It is an alpha 2-adrenergic antagonist that causes increased levels of neuronal norepinephrine and serotonin. It is also believed to be an antagonist at the serotonin receptors 5-HT 2 and 5-HT 3. Little is known about isolated mirtazapine ingestions. We conducted a retrospective chart review of mirtazapine ingestions reported to our Poison Center during 2004. A standardized data sheet was completed collecting information regarding standard demographic data along with co-ingestants, neurologic and cardiovascular symptoms, and disposition. Data collection was reviewed by a second investigator, and a kappa score was calculated. Of 71 patients identified with mirtazapine ingestions, there were 33 isolated exposures that were further reviewed. A kappa score for inter-reviewer reliability was calculated and at 0.61, 95% confidence interval 56–70. The average age of these patients was 27 years (range 6–82 years), with the mean ingestion of 343 mg (range 15–1500 mg). The most common neurologic symptom was drowsiness seen in 8/23 patients, 1 patient became agitated, and 14 patients had no abnormal neurologic findings. Cardiovascular effects were recorded in 4/23 patients, with 3 patients exhibiting tachycardia and 1 patient with bradycardia and hypotension. Seven of 23 patients required admission; there were no deaths. Mirtazapine overdoses are generally very well tolerated, with the most common symptoms being drowsiness and lethargy. This study is limited by being a retrospective chart review.

Prenatal cocaine exposure enhances responsivity of locus coeruleus norepinephrine neurons: Role of autoreceptors

Children exposed to cocaine during gestation have a higher incidence of neurobehavioral deficits. The neurochemical bases of these deficits have not been determined, but the pharmacology of cocaine and the nature of the abnormalities suggest that disruptions in catecholaminergic systems may be involved. In the current study, we used a rat model of prenatal cocaine exposure to examine the impact that this exposure has on the locus coeruleus (LC) noradrenergic system in offspring. Pregnant rats received twice-daily i.v. injections of cocaine (3 mg/kg) or saline between gestational days 10 and 20, and progeny were tested as juveniles. Exposure to a mild stressor elevated an index of norepinephrine turnover in the prefrontal cortex and also increased Fos expression in tyrosine hydroxylase–positive LC neurons in rats exposed to prenatal cocaine but not in rats exposed to prenatal saline. No change in the number of tyrosine hydroxylase-positive neurons in the LC was observed between the two prenatal treatment groups. Specific binding of [ 125I]-para-iodoclonidine, a radioligand with specificity for high affinity α2A-adrenergic receptors, was decreased in the LC of rats exposed to prenatal cocaine compared with prenatal saline controls. As α 2-adrenergic receptors on LC norepinephrine neurons function as autoreceptors, their down-regulation by prenatal cocaine exposure provides a plausible mechanism for the observed heightened reactivity of norepinephrine neurons in these animals. These data indicate that prenatal cocaine exposure results in lasting changes to the regulation and responsivity of rat LC norepinephrine neurons. A similar dysregulation of LC norepinephrine neurons may occur in children exposed to cocaine during gestation, and this may explain, at least partly, the increased incidence of cognitive deficits that have been observed in these subjects.

Regulation and Role of the Presynaptic and Myocardial Na+/H+ Exchanger NHE1: Effects on the Sympathetic Nervous System in Heart Failure

In acute myocardial ischemia and in chronic heart failure, sympathetic activation with excessive norepinephrine (NE) release from and reduced NE reuptake into sympathetic nerve endings is a prominent cause of arrhythmias and cardiac dysfunction. The Na(+)/H(+) exchanger NHE1 is the predominant isoform in the heart. It contributes to cellular acid-base balance, and electrolyte, and volume homeostasis, and is activated in response to intracellular acidosis and/or activation of guanine nucleotide binding (G) protein-coupled receptors. NHE1 mediates its signaling via protein kinases A (PKA) or C (PKC). In cardiomyocytes, NHE1 is restricted to specialized membrane domains, where it regulates the activity of pH-sensitive proteins and modulates the driving force of the Na(+)/Ca(2+) exchanger. During acute ischemia/reperfusion and in heart failure the activity/amount of NHE1 is increased, leading to intracellular Ca(2+) overload and promoting structural (apoptosis, hypertrophy) and functional (arrhythmias, hypercontraction) myocardial damage. In sympathetic nerve endings, increased NHE1 activity results in the accumulation of axoplasmic Na(+) that diminishes the inward and/or favors the outward transport of NE via the neuronal norepinephrine transporter (NET). The increased NE levels within the nerve-muscle junction facilitate the sustained stimulation of myocardial alpha- and beta-adrenoceptors (ARs), which in turn aggravate the increases in myocardial NHE1 activity and the associated deleterious effects. Furthermore, the responsiveness of the beta-AR declines overtime, which results in further release of NE, initiating a vicious cycle. Accordingly, NHE1 is a potential candidate for targeted intervention to suppress this feedback loop.

Distinct electrophysiological effects of paliperidone and risperidone on the firing activity of rat serotonin and norepinephrine neurons

Paliperidone (9-OH-risperidone) is the main metabolite of the atypical antipsychotic risperidone. While both drugs are potent dopamine (D)2 antagonists, they have quantitative differential affinities for serotonin (5-HT) and norepinephrine (NE) receptor binding sites. The present study aimed to determine if paliperidone exerts distinct effects on 5-HT and NE neuronal activity from those of risperidone. Risperidone and paliperidone were administered to Sprague-Dawley rats. Neuronal activity of 5-HT and NE neurons was assessed using in vivo electrophysiology. Acute administration of risperidone but not paliperidone inhibited the firing of 5-HT neurons, as previously reported. This inhibition was partially antagonized by the NE reuptake inhibitor desipramine, by the 5-HT(1A) receptor antagonist WAY 100635, and completely reversed when both drugs were given consecutively. Risperidone inhibited the firing of 5-HT neurons after 2 and 14 days of administration, with or without escitalopram. Paliperidone did not alter the firing rate of NE neurons by itself, but it reversed the suppression of NE neurons induced by escitalopram, as it was previously reported for risperidone. These results indicate that although risperidone and paliperidone share a qualitatively similar receptor binding profile in vitro, they differentially alter the firing of 5-HT and NE neurons in vivo. The capacity of paliperidone to reverse the selective serotonin reuptake inhibitor (SSRI)-induced inhibition of NE neuronal firing, without interfering with the effect of SSRIs of 5-HT neuronal activity, suggests that paliperidone may be a very effective adjunct in SSRI-resistant depression.