Block of Delayed-Rectifier Potassium Channels by Reduced Haloperidol and Related Compounds in Mouse Cortical Neurons

Genotype-phenotype correlation in Taiwanese children with diazoxide-unresponsive congenital hyperinsulinism

Objective

Congenital hyperinsulinism (CHI) is a group of clinically and genetically heterogeneous disorders characterized by dysregulated insulin secretion. The aim of the study was to elucidate genetic etiologies of Taiwanese children with the most severe diazoxide-unresponsive CHI and analyze their genotype-phenotype correlations.

Methods

We combined Sanger with whole exome sequencing (WES) to analyze CHI-related genes. The allele frequency of the most common variant was estimated by single-nucleotide polymorphism haplotype analysis. The functional effects of the ATP-sensitive potassium (KATP) channel variants were assessed using patch clamp recording and Western blot.

Results

Nine of 13 (69%) patients with ten different pathogenic variants (7 in ABCC8, 2 in KCNJ11 and 1 in GCK) were identified by the combined sequencing. The variant ABCC8 p.T1042QfsX75 identified in three probands was located in a specific haplotype. Functional study revealed the human SUR1 (hSUR1)-L366F KATP channels failed to respond to intracellular MgADP and diazoxide while hSUR1-R797Q and hSUR1-R1393C KATP channels were defective in trafficking. One patient had a de novo dominant mutation in the GCK gene (p.I211F), and WES revealed mosaicism of this variant from another patient.

Conclusion

Pathogenic variants in KATP channels are the most common underlying cause of diazoxide-unresponsive CHI in the Taiwanese cohort. The p.T1042QfsX75 variant in the ABCC8 gene is highly suggestive of a founder effect. The I211F mutation in the GCK gene and three rare SUR1 variants associated with defective gating (p.L366F) or traffic (p.R797Q and p.R1393C) KATP channels are also associated with the diazoxide-unresponsive phenotype.

Hypofunctional Dopamine Uptake and Antipsychotic Treatment-Resistant Schizophrenia

Antipsychotic treatment resistance in schizophrenia remains a major issue in psychiatry. Nearly 30% of patients with schizophrenia do not respond to antipsychotic treatment, yet the underlying neurobiological causes are unknown. All effective antipsychotic medications are thought to achieve their efficacy by targeting the dopaminergic system. Here we review early literature describing the fundamental mechanisms of antipsychotic drug efficacy, highlighting mechanistic concepts that have persisted over time. We then reconsider the original framework for understanding antipsychotic efficacy in light of recent advances in our scientific understanding of the dopaminergic effects of antipsychotics. Based on these new insights, we describe a role for the dopamine transporter in the genesis of both antipsychotic therapeutic response and primary resistance. We believe that this discussion will help delineate the dopaminergic nature of antipsychotic treatment-resistant schizophrenia.

Ventricular Tachycardia Triggered by Loperamide and Famotidine Abuse

A 32-year-old male developed recurrent ventricular tachycardia after taking mega doses of loperamide and famotidine in order to experience an opiate-like euphoric effect. He was taking up to 200 mg of loperamide and multiple doses of famotidine each day. He developed palpitations and syncope. Electrocardiography demonstrated ventricular tachycardia and QT interval prolongation (corrected QT interval was 597 ms). He was diagnosed with loperamide-induced QT prolongation resulting in incessant ventricular tachycardia. Loperamide was discontinued, and he was treated with electrolyte replacement, supportive care, and monitoring. After 5 days, his electrocardiogram (ECG) normalized and he had no more ventricular tachycardia. A Naranjo assessment score of 8 was obtained, indicating a probable relationship between QT prolongation and his use of loperamide. Large doses of loperamide can cause QT interval prolongation and life-threatening arrhythmias. These effects may be accentuated when histamine-2 receptor blockers are also abused.

Loperamide inhibits sodium channels to alleviate inflammatory hyperalgesia

Previous studies demonstrated that Loperamide, originally known as an anti-diarrheal drug, is a promising analgesic agent primarily targeting mu-opioid receptors. However some evidences suggested that non-opioid mechanisms may be contributing to its analgesic effect. In the present study, Loperamide was identified as a Nav1.7 blocker in a pilot screen. In HEK293 cells expressing Nav1.7 sodium channels, Loperamide blocked the resting state of Nav1.7 channels (IC₅₀ = 1.86 ± 0.11 μM) dose-dependently and reversibly. Loperamide produced a 10.4 mV of hyperpolarizing shift for the steady-state inactivation of Nav1.7 channels without apparent effect on the voltage-dependent activation. The drug displayed a mild use- and state-dependent inhibition on Nav1.7 channels, which was removed by the local anesthetic-insensitive construct Nav1.7-F1737A. Inhibition of Nav1.7 at resting state was not altered significantly by the F1737A mutation. Compared to its effects on Nav1.7, Loperamide exhibited higher potency on recombinant Nav1.8 channels in ND7/23 cells (IC₅₀ = 0.60 ± 0.10 μM) and weaker potency on Nav1.9 channels (3.48 ± 0.33 μM). Notably more pronounced inhibition was observed in the native Nav1.8 channels (0.11 ± 0.08 μM) in DRG neurons. Once mu-opioid receptor was antagonized by Naloxone in DRG neurons, potency of Loperamide on Nav1.8 was identical to that of recombinant Nav1.8 channels. The inhibition on Nav channels may be the main mechanism of Loperamide for pain relief beyond mu-opioid receptor. In the meanwhile, the opioid receptor pathway may also influence the blocking effect of Loperamide on sodium channels, implying a cross-talk between sodium channels and opioid receptors in pain processing.

Effects of haloperidol on Kv4.3 potassium channels

Haloperidol is commonly used in clinical practice to treat acute and chronic psychosis, but it also has been associated with adverse cardiovascular events. We investigated the effects of haloperidol on Kv4.3 currents stably expressed in CHO cells using a whole-cell patch-clamp technique. Haloperidol did not significantly inhibit the peak amplitude of Kv4.3, but accelerated the decay rate of inactivation of Kv4.3 in a concentration-dependent manner. Thus, the effects of haloperidol on Kv4.3 were estimated from the integral of the Kv4.3 currents during the depolarization pulse. The Kv4.3 was decreased by haloperidol in a concentration-dependent manner with an IC50 value of 3.6 μM. Haloperidol accelerated the decay rate of Kv4.3 inactivation and activation kinetics in a concentration-dependent manner, thereby decreasing the time-to-peak. Haloperidol shifted the voltage dependence of the steady-state activation and inactivation of Kv4.3 in a hyperpolarizing direction. Haloperidol also caused an acceleration of the closed-state inactivation of Kv4.3. Haloperidol produced a use-dependent block of Kv4.3, which was accompanied by a slowing of recovery from the inactivation of Kv4.3. These results suggest that haloperidol blocks Kv4.3 by both interacting with the open state of Kv4.3 channels during depolarization and accelerating the closed-state inactivation at subthreshold membrane potentials.

Peripherally acting mu-opioid receptor agonist attenuates neuropathic pain in rats after L5 spinal nerve injury

Studies in experimental models and controlled patient trials indicate that opioids are effective in managing neuropathic pain. However, side effects secondary to their central nervous system actions present barriers to their clinical use. Therefore, we examined whether activation of the peripheral mu-opioid receptors (MORs) could effectively alleviate neuropathic pain in rats after L5 spinal nerve ligation (SNL). Systemic loperamide hydrochloride (0.3-10 mg/kg, s.c.), a peripherally acting MOR-preferring agonist, dose-dependently reversed the mechanical allodynia at day 7 post-SNL. This anti-allodynic effect produced by systemic loperamide (1.5mg/kg, s.c.) was blocked by systemic pretreatment with either naloxone hydrochloride (10 mg/kg, i.p.) or methyl-naltrexone (5 mg/kg, i.p.), a peripherally acting MOR-preferring antagonist. It was also blocked by ipsilateral intraplantar pretreatment with methyl-naltrexone (43.5 microg/50 microl) and the highly selective MOR antagonist CTAP (5.5 microg/50 microl). However, this anti-allodynic effect of systemic loperamide was not blocked by intraplantar pretreatment with the delta-opioid receptor antagonist naltrindole hydrochloride (45.1 microg/50 microl). The anti-allodynic potency of systemic loperamide varied with time after nerve injury, with similar potency at days 7, 28, and 42 post-SNL, but reduced potency at day 14 post-SNL. Ipsilateral intraplantar injection of loperamide also dose-dependently (10-100 microg/50 microl) reversed mechanical allodynia on day 7 post-SNL. We suggest that loperamide can effectively attenuate neuropathic pain, primarily through activation of peripheral MORs in local tissue. Therefore, peripherally acting MOR agonists may represent a promising therapeutic approach for alleviating neuropathic pain.

Direct Inhibition of Ih by Analgesic Loperamide in Rat DRG Neurons

Hyperpolarization-activated cyclic nucleotide–gated (HCN) channels are responsible for the functional hyperpolarization-activated current ( Ih) in dorsal root ganglion (DRG) neurons, playing an important role in pain processing. We found that the known analgesic loperamide inhibited Ih channels in rat DRG neurons. Loperamide blocked Ih in a concentration-dependent manner, with an IC50 = 4.9 ± 0.6 and 11.0 ± 0.5 μM for large- and small-diameter neurons, respectively. Loperamide-induced Ih inhibition was unrelated to the activation of opioid receptors and was reversible, voltage-dependent, use-independent, and was associated with a negative shift of V1/2 for Ih steady-state activation. Loperamide block of Ih was voltage-dependent, gradually decreasing at more hyperpolarized membrane voltages from 89% at –60 mV to 4% at –120 mV in the presence of 3.7 μM loperamide. The voltage sensitivity of block can be explained by a loperamide-induced shift in the steady-state activation of Ih. Inclusion of 10 μM loperamide into the recording pipette did not affect Ih voltage for half-maximal activation, activation kinetics, and the peak current amplitude, whereas concurrent application of equimolar external loperamide produced a rapid, reversible Ih inhibition. The observed loperamide-induced Ih inhibition was not caused by the activation of peripheral opioid receptors because the broad-spectrum opioid receptor antagonist naloxone did not reverse Ih inhibition. Therefore we suggest that loperamide inhibits Ih by direct binding to the extracellular region of the channel. Because Ih channels are involved in pain processing, loperamide-induced inhibition of Ih channels could provide an additional molecular mechanism for its analgesic action.

Amphetamine-induced 50kHz calls from rat nucleus accumbens: A quantitative mapping study and acoustic analysis

Emission of 50 kHz ultrasonic calls in rats is known to be associated with appetitive behavioural situations and positive social interactions. The purpose of the study was to pharmacologically characterize amphetamine-induced 50 kHz calls and to perform quantitative mapping of this response in the nucleus accumbens. Injections of amphetamine into the nucleus accumbens induced species-typical 50 kHz calls in adult rats. The acoustic parameters of the calls were not affected by different amphetamine doses or combination of agents. The increase in the number of calls occurred predominantly from the accumbens shell and to a lesser degree from the core region. This effect was dose-dependent within the range of 1-20 microg of amphetamine and was reversed by pretreatment with D1 or D2 dopamine antagonists (SKF-83566 or raclopride) administered to the same brain site. However, another D2 dopamine receptor antagonist, haloperidol, which is known to increase the accumbens dopamine level, was ineffective in reversing the increase in call number at the dose studied. On the contrary, intraacumbens haloperidol, when injected alone, caused an increase in 50 kHz calls. It is concluded that the release of dopamine, predominantly in the accumbens shell region, is responsible for production of 50 kHz calls and the calls may indicate an appetitive state compatible with anticipation of reward and positive affect. Both D1 and D2 subtypes of dopamine receptors may be necessary to induce 50 kHz calls and signal the appetitive state.