Development of tolerance after repeated administration of a selective muscarinic M1 antagonist biperiden in healthy human volunteers

Calcium and Neural Stem Cell Proliferation

Intracellular calcium plays a pivotal role in central nervous system (CNS) development by regulating various processes such as cell proliferation, migration, differentiation, and maturation. However, understanding the involvement of calcium (Ca2+) in these processes during CNS development is challenging due to the dynamic nature of this cation and the evolving cell populations during development. While Ca2+ transient patterns have been observed in specific cell processes and molecules responsible for Ca2+ homeostasis have been identified in excitable and non-excitable cells, further research into Ca2+ dynamics and the underlying mechanisms in neural stem cells (NSCs) is required. This review focuses on molecules involved in Ca2+ entrance expressed in NSCs in vivo and in vitro, which are crucial for Ca2+ dynamics and signaling. It also discusses how these molecules might play a key role in balancing cell proliferation for self-renewal or promoting differentiation. These processes are finely regulated in a time-dependent manner throughout brain development, influenced by extrinsic and intrinsic factors that directly or indirectly modulate Ca2+ dynamics. Furthermore, this review addresses the potential implications of understanding Ca2+ dynamics in NSCs for treating neurological disorders. Despite significant progress in this field, unraveling the elements contributing to Ca2+ intracellular dynamics in cell proliferation remains a challenging puzzle that requires further investigation.

Development of tolerance to chemokine receptor antagonists: current paradigms and the need for further investigation

Chemokine G-protein coupled receptors are validated drug targets for many diseases, including cancer, neurological, and inflammatory disorders. Despite much time and effort spent on therapeutic development, very few chemokine receptor antagonists are approved for clinical use. Among potential reasons for the slow progress in developing chemokine receptor inhibitors, antagonist tolerance, a progressive reduction in drug efficacy after repeated administration, is likely to play a key role. The mechanisms leading to antagonist tolerance remain poorly understood. In many cases, antagonist tolerance is accompanied by increased receptor concentration on the cell surface after prolonged exposure to chemokine receptor antagonists. This points to a possible role of altered receptor internalization and presentation on the cell surface, as has been shown for agonist (primarily opioid) tolerance. In addition, examples of antagonist tolerance in the context of other G-protein coupled receptors suggest the involvement of noncanonical signal transduction in opposing the effects of the antagonists. In this review, we summarize the available progress and challenges in therapeutic development of chemokine receptor antagonists, describe the available knowledge about antagonist tolerance, and propose new avenues for future investigation of this important phenomenon. Furthermore, we highlight the modern methodologies that have the potential to reveal novel mechanisms leading to antagonist tolerance and to propel the field forward by advancing the development of potent "tolerance-free" antagonists of chemokine receptors.

Biperiden Challenge Model in Healthy Elderly as Proof-of-Pharmacology Tool: A Randomized, Placebo-Controlled Trial

Selective M1 muscarinic acetylcholine receptor (mAChR) agonists are being developed as symptomatic treatment for neurodegenerative and neuropsychiatric disorders that lead to cognitive dysfunction. Demonstrating cognition-enhancing effects in early-phase clinical development in healthy subjects is difficult. A challenge with the M1 mAChR antagonist biperiden could be used to demonstrate procognitive and pharmacological effects of selective M1 mAChR agonists. The aim of this study was to develop such a model. To this end, 12 healthy elderly subjects participated in a randomized, placebo-controlled, 3-way crossover study investigating tolerability, pharmacokinetic (PK) and pharmacodynamic (PD) effects of 2 and 4 mg biperiden. Repeated PD assessments were performed using neurocognitive tasks and electrophysiological measurements. A population PK-PD model was developed. Four milligrams of biperiden showed significant impairment of sustained attention (-2.1 percentage point in adaptive tracking [95%CI, -3.043 to -1.148], verbal memory (2-3 fewer words recalled [95%CI, -5.9 to -0.2]) and working memory (up to a 50-millisecond increase in the n-back task reaction time [95%CI, 21.854-77.882]) compared with placebo. The PK data were best fitted by a 2-compartment model and showed high interoccasion and intersubject variability. Population PK-PD analysis quantified significant concentration-effect relationships for the n-back reaction time, n-back accuracy, and adaptive tracking. In conclusion, biperiden caused M1 mAChR-related dose- and concentration-dependent temporary declines in cognitive functioning. Therefore a biperiden pharmacological challenge model can be used for proof-of-pharmacology studies and to demonstrate cognition-enhancing effects of new cholinergic compounds that are being developed.

Biperiden Selectively Impairs Verbal Episodic Memory in a Dose- and Time-Dependent Manner in Healthy Subjects

PURPOSE/BACKGROUND

Biperiden is a muscarinic antagonist that produces memory impairments without impairing attention or motor functions in healthy subjects. It has been suggested that a biperiden-induced memory deficit could model age- and dementia-related memory impairments. The goal of the current study was to determine the dose- and time-dependent effects of biperiden on cognition in healthy volunteers.

METHODS/PROCEDURES

Twenty-one healthy volunteers participated in a placebo-controlled, 3-way, crossover study. After a baseline test, cognitive performance was tested at 3 time points after a single dose of biperiden 2 or 4 mg, or placebo. Episodic memory was measured using a 15-word verbal learning task (VLT). Furthermore, n-back tasks, a sustained attention to response task and a reaction time task were used, as well as subjective alertness and a side effects questionnaire. In addition, blood serum values and physiological measures were taken.

FINDINGS/RESULTS

Biperiden decreased the number of words recalled in immediate and delayed recall of the VLT 90 minutes after drug intake. A dose-dependent impairment was found for the delayed recall, whereas the immediate recall was equally impaired by the 2 doses. Biperiden did not affect the performance on the VLT 4 hours after administration. Performance in the n-back task and the sustained attention to response task were not affected by biperiden at any time point. Both doses were well tolerated as reported side effects were mild at Tmax and were minimal at the other time points.

IMPLICATIONS/CONCLUSIONS

Biperiden exerts effects on episodic memory without negatively affecting other cognitive performance and behavioral measures that were assessed in this study. The data provide further evidence that biperiden has selective effects on cognition, even after a high dose.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Differential response of rapid eye movement sleep to cholinergic blockade by scopolamine in currently depressed, remitted, and normal control subjects

The degree of cholinergic dysregulation of sleep in adult depression was evaluated using scopolamine. On separate sessions, placebo and scopolamine (4.5 micrograms/kg, IM) were administered to 14 patients with unipolar major depression, 16 recovered/remitted patients, and 18 normal controls. Scopolamine increased rapid eye movement (REM) latency (RL), reduced REM activity (RA), REM density (RD), and REM duration, and increased the percentage of stage 4 sleep in all groups. There was a differential effect of scopolamine on RL, RA, and REM duration for the first REM period, and on percentage of stage 4 sleep. Whereas a primary cholinergic hyperactivity could account for the RA and RD responses, the response profile for RL was more compatible with reduced aminergic tone as the proximal cause of the cholinergic hyperactivity. Whether the sleep abnormalities observed in remitted patients reflect an underlying vulnerability for development or recurrence of depression, and/or a scar, remains to be determined.

No antidepressant effect of biperiden compared with placebo in depression: a double-blind 6-week clinical trial

Depression may result from an increased ratio of cholinergic to aminergic central neurotransmission. The effect of centrally active anticholinergic agents on depressive ratings in depressed patients, however, has not been well studied. In a previous open label 4-week clinical trial, Kasper et al. (1981) reported that biperiden (Akineton) had antidepressant effects in depressed patients. We compared the effects of placebo and biperiden in a randomized, double-blind, parallel-design 6-week study. All patients received placebo for the first week and then received either biperiden (< or = 12 mg/day) or a peripherally acting anticholinergic agent, glycopyrrolate (Robinul, 1 mg/day) for 4 weeks. All patients received placebo during week 6. Both groups significantly improved, but biperiden did not demonstrate any significant benefit compared with glycopyrrolate. In a subgroup of patients receiving biperiden, the percentage of rapid eye movement sleep was significantly decreased only in the first week of biperiden compared with the initial placebo week, suggesting that tolerance to the central effects of biperiden developed. Further studies would be needed to determine whether anticholinergic agents possess antidepressant properties.

Cholinergic receptor subtypes and REM sleep in animals and normal controls

As reviewed here and elsewhere in this symposium, acetylcholine, in conjunction with other neurotransmitter systems, plays a very important role in the regulation of circadian and sleep-wake states. To briefly recapitulate, several current basic concepts about the regulation of sleep-wake states include: (a) REM sleep, or at least its phasic events (eye movements and PGO spikes), are promoted by cholinergic neurons originating within the peribrachial regions [LDT/PPT] (Mitani et al., 1988; Shiromani et al., 1988; Datta et al., 1991; Shouse and Siegel, 1992); (b) REM sleep may be inhibited by noradrenergic and serotonergic neurons in the locus coeruleus and dorsal raphe, respectively (Siegel, 1989; Steriade and McCarley, 1990; Jones, 1991); (c) stages 3 and 4 (Delta) sleep are inhibited by cholinergic terminals from basal forebrain to cortex (Buzsaki et al., 1988) and from LDT/PPT to thalamus (Steriade and McCarley, 1990; Steriade et al., 1991); (d) Delta sleep is modulated by complex serotonergic mechanisms; for example, it is increased by pharmacological antagonists of 5HT2 receptors (Declerck et al., 1987; Dugovic et al., 1989; Benson et al., 1991), although the mechanism and neuroanatomical site at which this effect occurs is unknown. Given the importance of mACHR mediation of components of REM sleep, it is unfortunate that so little is known about the distribution of the various subtypes of mACHRs in brainstem areas which regulate REM sleep. mACHR subtypes have been identified by molecular, biological and pharmacological methods.(ABSTRACT TRUNCATED AT 250 WORDS)