Reply to: "Reflecting the causes of variability of EEG responses elicited by cerebellar TMS"

In their commentary on our recently published paper about electroencephalographic responses induced by cerebellar transcranial magnetic stimulation (Fong et al., 2023), Gassmann and colleagues (Gassmann et al., 2023b) try to explain the differences between our results and their own previous work on the same topic. We agree with them that many of the differences arise from our use of a different magnetic stimulation coil. However, two unresolved questions remain. (1) Which method is most likely to achieve optimal activation of cerebellar output? (2) To what extent are the evoked cerebellar responses contaminated by concomitant sensory input? We highlight the role of careful experimental design and of combining electrophysiological and behavioural data to obtain reliable TMS-EEG data.

Standard intensities of transcranial alternating current stimulation over the motor cortex do not entrain corticospinal inputs to motor neurons

Transcranial alternating current stimulation (TACS) is commonly used to synchronize a cortical area and its outputs to the stimulus waveform, but gathering evidence for this based on brain recordings in humans is challenging. The corticospinal tract transmits beta oscillations (∼21 Hz) from the motor cortex to tonically contracted limb muscles linearly. Therefore, muscle activity may be used to measure the level of beta entrainment in the corticospinal tract due to TACS over the motor cortex. Here, we assessed whether TACS is able to modulate the neural inputs to muscles, which would provide indirect evidence for TACS-driven neural entrainment. In the first part of the study, we ran simulations of motor neuron (MN) pools receiving inputs from corticospinal neurons with different levels of beta entrainment. Results suggest that MNs are highly sensitive to changes in corticospinal beta activity. Then, we ran experiments on healthy human subjects (N = 10) in which TACS (at 1 mA) was delivered over the motor cortex at 21 Hz (beta stimulation), or at 7 Hz or 40 Hz (control conditions) while the abductor digiti minimi or the tibialis anterior muscle were tonically contracted. Muscle activity was measured using high-density electromyography, which allowed us to decompose the activity of pools of motor units innervating the muscles. By analysing motor unit pool activity, we observed that none of the TACS conditions could consistently alter the spectral contents of the common neural inputs received by the muscles. These results suggest that 1 mA TACS over the motor cortex given at beta frequencies does not entrain corticospinal activity. KEY POINTS: Transcranial alternating current stimulation (TACS) is commonly used to entrain the communication between brain regions. It is challenging to find direct evidence supporting TACS-driven neural entrainment due to the technical difficulties in recording brain activity during stimulation. Computational simulations of motor neuron pools receiving common inputs in the beta (∼21 Hz) band indicate that motor neurons are highly sensitive to corticospinal beta entrainment. Motor unit activity from human muscles does not support TACS-driven corticospinal entrainment.

EEG responses induced by cerebellar TMS at rest and during visuomotor adaptation

BACKGROUND

Connections between the cerebellum and the cortex play a critical role in learning and executing complex behaviours. Dual-coil transcranial magnetic stimulation (TMS) can be used non-invasively to probe connectivity changes between the lateral cerebellum and motor cortex (M1) using the motor evoked potential as an outcome measure (cerebellar-brain inhibition, CBI). However, it gives no information about cerebellar connections to other parts of cortex.

OBJECTIVES

We used electroencephalography (EEG) to investigate whether it was possible to detect activity evoked in any areas of cortex by single-pulse TMS of the cerebellum (cerebellar TMS evoked potentials, cbTEPs). A second experiment tested if these responses were influenced by the performance of a cerebellar-dependent motor learning paradigm.

METHODS

In the first series of experiments, TMS was applied over either the right or left cerebellar cortex, and scalp EEG was recorded simultaneously. Control conditions that mimicked auditory and somatosensory inputs associated with cerebellar TMS were included to identify responses due to non-cerebellar sensory stimulation. We conducted a follow-up experiment that evaluated whether cbTEPs are behaviourally sensitive by assessing individuals before and after learning a visuomotor reach adaptation task.

RESULTS

A TMS pulse over the lateral cerebellum evoked EEG responses that could be distinguished from those caused by auditory and sensory artefacts. Significant positive (P80) and negative peaks (N110) over the contralateral frontal cerebral area were identified with a mirrored scalp distribution after left vs. right cerebellar stimulation. The P80 and N110 peaks were replicated in the cerebellar motor learning experiment and changed amplitude at different stages of learning. The change in amplitude of the P80 peak was associated with the degree of learning that individuals retained following adaptation. Due to overlap with sensory responses, the N110 should be interpreted with caution.

CONCLUSIONS

Cerebral potentials evoked by TMS of the lateral cerebellum provide a neurophysiological probe of cerebellar function that complements the existing CBI method. They may provide novel insight into mechanisms of visuomotor adaptation and other cognitive processes.

Altered motor cortex physiology and dysexecutive syndrome in patients with fatigue and cognitive difficulties after mild COVID-19

Background and purpose

Fatigue and cognitive difficulties are reported as the most frequently persistent symptoms in patients after mild SARS‐CoV‐2 infection. An extensive neurophysiological and neuropsychological assessment of such patients was performed focusing on motor cortex physiology and executive cognitive functions.

Methods

Sixty‐seven patients complaining of fatigue and/or cognitive difficulties after resolution of mild SARS‐CoV‐2 infection were enrolled together with 22 healthy controls (HCs). Persistent clinical symptoms were investigated by means of a 16‐item questionnaire. Fatigue, exertion, cognitive difficulties, mood and ‘well‐being’ were evaluated through self‐administered tools. Utilizing transcranial magnetic stimulation of the primary motor cortex (M1) resting motor threshold, motor evoked potential amplitude, cortical silent period duration, short‐interval intracortical inhibition, intracortical facilitation, long‐interval intracortical inhibition and short‐latency afferent inhibition were evaluated. Global cognition and executive functions were assessed with screening tests. Attention was measured with computerized tasks.

Results

Post COVID‐19 patients reported a mean of 4.9 persistent symptoms, high levels of fatigue, exertion, cognitive difficulties, low levels of well‐being and reduced mental well‐being. Compared to HCs, patients presented higher resting motor thresholds, lower motor evoked potential amplitudes and longer cortical silent periods, concurring with reduced M1 excitability. Long‐interval intracortical inhibition and short‐latency afferent inhibition were also impaired, indicating altered GABA_B‐ergic and cholinergic neurotransmission. Short‐interval intracortical inhibition and intracortical facilitation were not affected. Patients also showed poorer global cognition and executive functions compared to HCs and a clear impairment in sustained and executive attention.

Conclusions

Patients with fatigue and cognitive difficulties following mild COVID‐19 present altered excitability and neurotransmission within M1 and deficits in executive functions and attention.

Alzheimer disease and neuroplasticity

Alzheimer's disease (AD) is considered the most harmful form of dementia in the elderly population. At present, there are no effective treatments and this is likely due to the incomplete understanding of the pathophysiology. Recent data indicate that synaptic dysfunction could be a central element of AD pathophysiology. It was found that a synaptic breakdown is an early event that heralds neuronal degeneration. Transcranial magnetic stimulation (TMS) has been recently introduced as a novel approach to identify the early signatures of synaptic dysfunction characterizing AD pathophysiology. In this chapter, we review the new neurophysiologic signatures of AD that have been emphasized by TMS studies. We show how TMS measurement of neuroplasticity identified long-term potentiation (LTP)-like cortical plasticity as a key element of AD synaptic dysfunction. These measurements are useful to increase the accuracy of differential diagnosis, predict disease progression, and anticipate response to therapy. Moreover, enhancing neuroplasticity holds as a promising therapeutic approach to improve cognition in AD. In recent years, studies showed treatments with multiple sessions of rTMS can influence cognition in people with neurodegenerative diseases. In the second part of this chapter, we also consider novel therapeutic approaches based on the clinical use of rTMS.

Consensus Paper: Novel Directions and Next Steps of Non-invasive Brain Stimulation of the Cerebellum in Health and Disease

The cerebellum is involved in multiple closed-loops circuitry which connect the cerebellar modules with the motor cortex, prefrontal, temporal, and parietal cortical areas, and contribute to motor control, cognitive processes, emotional processing, and behavior. Among them, the cerebello-thalamo-cortical pathway represents the anatomical substratum of cerebellum-motor cortex inhibition (CBI). However, the cerebellum is also connected with basal ganglia by disynaptic pathways, and cerebellar involvement in disorders commonly associated with basal ganglia dysfunction (e.g., Parkinson's disease and dystonia) has been suggested. Lately, cerebellar activity has been targeted by non-invasive brain stimulation (NIBS) techniques including transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) to indirectly affect and tune dysfunctional circuitry in the brain. Although the results are promising, several questions remain still unsolved. Here, a panel of experts from different specialties (neurophysiology, neurology, neurosurgery, neuropsychology) reviews the current results on cerebellar NIBS with the aim to derive the future steps and directions needed. We discuss the effects of TMS in the field of cerebellar neurophysiology, the potentials of cerebellar tDCS, the role of animal models in cerebellar NIBS applications, and the possible application of cerebellar NIBS in motor learning, stroke recovery, speech and language functions, neuropsychiatric and movement disorders.

Experimental Protocol to Test Explicit Motor Learning–Cerebellar Theta Burst Stimulation

Implicit and explicit motor learning processes work interactively in everyday life to promote the creation of highly automatized motor behaviors. The cerebellum is crucial for motor sequence learning and adaptation, as it contributes to the error correction and to sensorimotor integration of on-going actions. A non-invasive cerebellar stimulation has been demonstrated to modulate implicit motor learning and adaptation. The present study aimed to explore the potential role of cerebellar theta burst stimulation (TBS) in modulating explicit motor learning and adaptation, in healthy subjects. Cerebellar TBS will be applied immediately before the learning phase of a computerized task based on a modified Serial Reaction Time Task (SRTT) paradigm. Here, we present a study protocol aimed at evaluating the behavioral effects of continuous (cTBS), intermittent TBS (iTBS), or sham Theta Burst Stimulation (TBS) on four different conditions: learning, adaptation, delayed recall and re-adaptation of SRTT. We are confident to find modulation of SRTT performance induced by cerebellar TBS, in particular, processing acceleration and reduction of error in all the conditions induced by cerebellar iTBS, as already known for implicit processes. On the other hand, we expect that cerebellar cTBS could induce opposite effects. Results from this protocol are supposed to advance the knowledge about the role of non-invasive cerebellar modulation in neurorehabilitation, providing clinicians with useful data for further exploiting this technique in different clinical conditions.

Two forms of short-interval intracortical inhibition in human motor cortex

Background

Pulses of transcranial magnetic stimulation (TMS) with a predominantly anterior-posterior (AP) or posterior-anterior (PA) current direction over the primary motor cortex appear to activate distinct excitatory inputs to corticospinal neurons. In contrast, very few reports have examined whether the inhibitory neurons responsible for short-interval intracortical inhibition (SICI) are sensitive to TMS current direction.

Objectives

To investigate whether SICI evaluated with AP and PA conditioning stimuli (CS_PA and CS_AP) activate different inhibitory pathways. SICI was always assessed using a PA-oriented test stimulus (TS_PA).

Methods

Using two superimposed TMS coils, CS_PA and CS_AP were applied at interstimulus intervals (ISI) of 1–5 ms before a TS_PA, and at a range of different intensities. Using a triple stimulation design, we then tested whether SICI at ISI of 3 ms using opposite directions of CS (SICI_CSPA3 and SICI_CSAP3) interacted differently with three other forms of inhibition, including SICI at ISI of 2 ms (SICI_CSPA2), cerebellum-motor cortex inhibition (CBI 5 ms) and short-latency afferent inhibition (SAI 22 ms). Finally, we compared the effect of tonic and phasic voluntary contraction on SICI_CSPA3 and SICI_CSAP3.

Results

CS_AP produced little SICI at ISIs = 1 and 2 ms. However, at ISI = 3 ms, both CS_AP and CS_PA were equally effective at the same percent of maximum stimulator output. Despite this apparent similarity, combining SICI_CSPA3 or SICI_CSAP3 with other forms of inhibition led to quite different results: SICI_CSPA3 interacted in complex ways with CBI, SAI and SICI_CSPA2, whereas the effect of SICI_CSAP3 appeared to be quite independent of them. Although SICI_CSPA and SICI_CSAP were both reduced by the same amount during voluntary tonic contraction compared with rest, in a simple reaction time task SICI_CSAP was disinhibited much earlier following the imperative signal than SICI_CSPA.

Conclusions

SICI_CSPA appears to activate a different inhibitory pathway to that activated by SICI_CSAP. The difference is behaviourally relevant since the pathways are controlled differently during volitional contraction. The results may explain some previous pathological data and open the possibility of testing whether these pathways are differentially recruited in a range of tasks.

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Opposite directions of conditioning stimulus (CS) used to suppress MEPs evoked by a conventional test stimulus.

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Different directions of CS have different time courses of short-interval intracortical inhibition (SICI).

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They also interact differently with short-latency afferent inhibition and with cerebellar inhibition.

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They are differently affected in a reaction time task.

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We suggest there are two forms of SICI in motor cortex.

Frequency-dependent modulation of cerebellar excitability during the application of non-invasive alternating current stimulation

Background

it is well-known that the cerebellum is critical for the integrity of motor and cognitive actions. Applying non-invasive brain stimulation techniques over this region results in neurophysiological and behavioural changes, which have been associated with the modulation of cerebellar-cerebral cortex connectivity. Here, we investigated whether online application of cerebellar transcranial alternating current stimulation (tACS) results in changes to this pathway.

Methods

thirteen healthy individuals participated in two sessions of cerebellar tACS delivered at different frequencies (5Hz and 50Hz). We used transcranial magnetic stimulation to measure cerebellar-motor cortex (M1) inhibition (CBI), short-intracortical inhibition (SICI) and short-afferent inhibition (SAI) before, during and after the application of tACS.

Results

we found that CBI was specifically strengthened during the application of 5Hz cerebellar tACS. No changes were detected immediately following the application of 5Hz stimulation, nor at any time point with 50Hz stimulation. We also found no changes to M1 intracortical circuits (i.e. SICI) or sensorimotor interaction (i.e. SAI), indicating that the effects of 5Hz tACS over the cerebellum are site-specific.

Conclusions

cerebellar tACS can modulate cerebellar excitability in a time- and frequency-dependent manner. Additionally, cerebellar tACS does not appear to induce any long-lasting effects (i.e. plasticity), suggesting that stimulation enhances oscillations within the cerebellum only throughout the stimulation period. As such, cerebellar tACS may have significant implications for diseases manifesting with abnormal cerebellar oscillatory activity and also for future behavioural studies.

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Cerebellar tACS increases the inhibitory tone that the cerebellum exerts over M1 (CBI).

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CBI changes were found only during the online application of 5Hz tACS and not immediately following stimulation.

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The effects are specific to the cerebellum, as no changes were found in intracortical measures (e.g. SICI and SAI).

Multiple Motor Learning Processes in Humans: Defining Their Neurophysiological Bases

Learning new motor behaviors or adjusting previously learned actions to account for dynamic changes in our environment requires the operation of multiple distinct motor learning processes, which rely on different neuronal substrates. For instance, humans are capable of acquiring new motor patterns via the formation of internal model representations of the movement dynamics and through positive reinforcement. In this review, we will discuss how changes in human physiological markers, assessed with noninvasive brain stimulation techniques from distinct brain regions, can be utilized to provide insights toward the distinct learning processes underlying motor learning. We will summarize the findings from several behavioral and neurophysiological studies that have made efforts to understand how distinct processes contribute to and interact when learning new motor behaviors. In particular, we will extensively review two types of behavioral processes described in human sensorimotor learning: (1) a recalibration process of a previously learned movement and (2) acquiring an entirely new motor control policy, such as learning to play an instrument. The selected studies will demonstrate in-detail how distinct physiological mechanisms contributions change depending on the time course of learning and the type of behaviors being learned.

Dissecting two distinct interneuronal networks in M1 with transcranial magnetic stimulation

Interactions from both inhibitory and excitatory interneurons are necessary components of cortical processing that contribute to the vast amount of motor actions executed by humans daily. As transcranial magnetic stimulation (TMS) over primary motor cortex is capable of activating corticospinal neurons trans-synaptically, studies over the past 30 years have provided how subtle changes in stimulation parameters (i.e., current direction, pulse width, and paired-pulse) can elucidate evidence for two distinct neuronal networks that can be probed with this technique. This article provides a brief review of some fundamental studies demonstrating how these networks have separable excitatory inputs to corticospinal neurons. Furthermore, the findings of recent investigations will be discussed in detail, illustrating how each network's sensitivity to different brain states (i.e., rest, movement preparation, and motor learning) is dissociable. Understanding the physiological characteristics of each network can help to explain why interindividual responses to TMS exist, while also providing insights into the role of these networks in various human motor behaviors.

Temporal dynamics of cerebellar and motor cortex physiological processes during motor skill learning

Learning motor tasks involves distinct physiological processes in the cerebellum (CB) and primary motor cortex (M1). Previous studies have shown that motor learning results in at least two important neurophysiological changes: modulation of cerebellar output mediated in-part by long-term depression of parallel fiber-Purkinje cell synapse and induction of long-term plasticity (LTP) in M1, leading to transient occlusion of additional LTP-like plasticity. However, little is known about the temporal dynamics of these two physiological mechanisms during motor skill learning. Here we use non-invasive brain stimulation to explore CB and M1 mechanisms during early and late motor skill learning in humans. We predicted that early skill acquisition would be proportional to cerebellar excitability (CBI) changes, whereas later stages of learning will result in M1 LTP-like plasticity modifications. We found that early, and not late into skill training, CBI changed. Whereas, occlusion of LTP-like plasticity over M1 occurred only during late, but not early training. These findings indicate a distinct temporal dissociation in the physiological role of the CB and M1 when learning a novel skill. Understanding the role and temporal dynamics of different brain regions during motor learning is critical to device optimal interventions to augment learning.

Modulating Motor Learning through Transcranial Direct-Current Stimulation: An Integrative View

Motor learning consists of the ability to improve motor actions through practice playing a major role in the acquisition of skills required for high-performance sports or motor function recovery after brain lesions. During the last decades, it has been reported that transcranial direct-current stimulation (tDCS), consisting in applying weak direct current through the scalp, is able of inducing polarity-specific changes in the excitability of cortical neurons. This low-cost, painless and well-tolerated portable technique has found a wide-spread use in the motor learning domain where it has been successfully applied to enhance motor learning in healthy individuals and for motor recovery after brain lesion as well as in pathological states associated to motor deficits. The main objective of this mini-review is to offer an integrative view about the potential use of tDCS for human motor learning modulation. Furthermore, we introduce the basic mechanisms underlying immediate and long-term effects associated to tDCS along with important considerations about its limitations and progression in recent years.

Laterality Differences in Cerebellar-Motor Cortex Connectivity

Lateralization of function is an important organizational feature of the motor system. Each effector is predominantly controlled by the contralateral cerebral cortex and the ipsilateral cerebellum. Transcranial magnetic stimulation studies have revealed hemispheric differences in the stimulation strength required to evoke a muscle response from the primary motor cortex (M1), with the dominant hemisphere typically requiring less stimulation than the nondominant. The current study assessed whether the strength of the connection between the cerebellum and M1 (CB-M1), known to change in association with motor learning, have hemispheric differences and whether these differences have any behavioral correlate. We observed, in right-handed individuals, that the connection between the right cerebellum and left M1 is typically stronger than the contralateral network. Behaviorally, we detected no lateralized learning processes, though we did find a significant effect on the amplitude of reaching movements across hands. Furthermore, we observed that the strength of the CB-M1 connection is correlated with the amplitude variability of reaching movements, a measure of movement precision, where stronger connectivity was associated with better precision. These findings indicate that lateralization in the motor system is present beyond the primary motor cortex, and points to an association between cerebellar M1 connectivity and movement execution.

Molecular determinants of insulin resistance, cell apoptosis and lipid accumulation in non-alcoholic steatohepatitis

BACKGROUNDS AND AIMS

Non-alcoholic-steatohepatitis (NASH) is closely related to insulin resistance, but it is unknown whether insulin resistance may be localized in hepatocytes. This study investigates insulin signalling in liver tissue from NASH, and the molecular mechanisms by which insulin-resistance could lead to liver damage (apoptosis). Moreover, to investigate the mechanisms of lipid overload we studied key enzymes in hepatocytes lipid metabolism.

METHODS AND RESULTS

In liver specimens from 11 patients with NASH and 7 histological normal livers, we measured total and phosphorylated Akt (active form), Bax and Bcl-2 by Western-blot analysis. In addition, we studied AMP-activated protein Kinase and Carnitine-Palmitoyl-Transferase-1 gene expression, key regulators of non-esterified fatty acid synthesis and oxidation, by reverse transcription polymerase chain reaction. In NASH, phosphorylated Akt was impaired (104.3+/-10.6 vs 152.6+/-22.4 AU, p<0.002) and correlated with necroinflammatory score (r=-0.62; p<0.05). Bax/Bcl-2 ratio was increased in NASH. Moreover, we observed a decrease of AMP-activated protein Kinase (10.74+/-6 vs 144.7+/-41.6 AU, p<0.0001) and Carnitine-Palmitoyl-Transferase-1 gene expression (38.7+/-14.6 vs 192.1+/-26.2 AU, p<0.0001), and both were correlated with steatosis score (r=-0.56, p<0.05, r=-0.87, p<0.05 respectively).

CONCLUSIONS

Akt, a key molecule of insulin signalling and cell apoptosis is impaired in NASH, suggesting an important role of hepatic insulin resistance in liver failure. Moreover, decreased non-esterified fatty acid oxidation may cause hepatic lipid overload.

Functional and morphological alterations of mitochondria in pancreatic beta cells from type 2 diabetic patients

AIMS/HYPOTHESIS

Little information is available on the insulin release properties of pancreatic islets isolated from type 2 diabetic subjects. Since mitochondria represent the site where important metabolites that regulate insulin secretion are generated, we studied insulin release as well as mitochondrial function and morphology directly in pancreatic islets isolated from type 2 diabetic patients.

METHODS

Islets were prepared by collagenase digestion and density gradient purification, and insulin secretion in response to glucose and arginine was assessed by the batch incubation method. Adenine nucleotides, mitochondrial membrane potential, the expression of UCP-2, complex I and complex V of the respiratory chain, and nitrotyrosine levels were evaluated and correlated with insulin secretion.

RESULTS

Compared to control islets, diabetic islets showed reduced insulin secretion in response to glucose, and this defect was associated with lower ATP levels, a lower ATP/ADP ratio and impaired hyperpolarization of the mitochondrial membrane. Increased protein expression of UCP-2, complex I and complex V of the respiratory chain, and a higher level of nitrotyrosine were also found in type 2 diabetic islets. Morphology studies showed that control and diabetic beta cells had a similar number of mitochondria; however, mitochondrial density volume was significantly higher in type 2 diabetic beta cells.

CONCLUSIONS/INTERPRETATION

In pancreatic beta cells from type 2 diabetic subjects, the impaired secretory response to glucose is associated with a marked alteration of mitochondrial function and morphology. In particular, UCP-2 expression is increased (probably due to a condition of fuel overload), which leads to lower ATP, decreased ATP/ADP ratio, with consequent reduction of insulin release.

Evidence for genetic epistasis in human insulin resistance: the combined effect of PC-1 (K121Q) and PPAR?2 (P12A) polymorphisms

Insulin resistance is believed to be under the control of several genes often interacting each other. However, whether genetic epistasis does in fact modulate human insulin sensitivity is unknown. In 338 healthy unrelated subjects from Sicily, all nondiabetic and not morbidly obese, we investigated whether two gene polymorphisms previously associated with insulin resistance (namely PC-1 K121Q and PPARgamma2 P12A) affect insulin sensitivity by interacting. PC-1 X121Q subjects showed higher level of fasting glucose, lower insulin sensitivity (by both the Matsuda insulin sensitivity index and M values at clamp, the latter performed in a subgroup of 113 subjects representative of the overall cohort) and higher insulin levels during the oral glucose tolerance test (OGTT) than PC-1 K121K subjects. In contrast, no difference in any of the measured variables was observed between PPARgamma2 P12P and X12A individuals. The deleterious effect of the PC-1 X121Q genotype on each of these three variables was significant and entirely dependent upon the coexistence of the PPARgamma2 P12P genotype. Among PPARgamma2 P12P carriers also fasting insulin and glucose levels during OGTT were higher in PC-1 X121Q than in K121K individuals. In contrast, no deleterious effect of the PC-1 X121Q genotype was observed among PPARgamma2 X12A carriers; rather, in these subjects a lower body mass index and consequently lower fasting insulin level was observed in PC-1 X121Q than in K121K carriers. Overall, a significant interaction between the two genes was observed on body mass index, insulin levels (both fasting and after OGTT) and both insulin sensitivity (i.e., insulin sensitivity index and M value) and insulin secretion (i.e., HOMA-B%) indexes.

Rats that are made insulin resistant by glucosamine treatment have impaired skeletal muscle insulin receptor phosphorylation

The current study sought to verify whether glucosamine (GlcN)-induced insulin resistance is associated with impaired insulin receptor (IR) autophosphorylation. Rats were given either saline or primed continuous GlcN infusion (5 micromol x kg(-1) x min(-1)) 10 minutes prior to and during euglycemic hyperinsulinemic clamp (primed continuous infusion of 20 mU x kg(-1) x min(-1) insulin for 2 hours). IR autophosphorylation was measured in skeletal muscle after in vivo insulin stimulation (ie, during clamp) by Western blot and then retested after subsequent in vitro 0.1 to 100 nmol/L insulin stimulation (by enzyme-linked immunosorbent assay [ELISA]). Tissue PC-1 enzymatic activity was also measured. In vivo, insulin/GlcN rats had decreased (P <.01) whole body glucose uptake (37.7 +/- 2.1 v 49.7 +/- 2.7 mg x kg(-1) x min(-1) in respect to insulin/saline), receptor autophosphorylation (37 +/- 5 v 82 +/-.0 arbitrary units/mg protein), and insulin receptor substrate-1 (IRS-1) phosphorylation (112% +/- 15% v 198% +/- 23% of saline infusion rats). Receptor autophosphorylation was correlated with whole body glucose uptake (r = 0.62, P <.05). Skeletal muscle PC-1 activity (58.8 +/- 10.7 v 55.7 +/- 5.8 nmol x mg(-1) x min(-1)) was not different in the 2 groups. Our data show that GlcN-induced insulin resistance is mediated, at least in part, by impaired skeletal muscle IR autophosphorylation.

The Q121 PC-1 variant and obesity have additive and independent effects in causing insulin resistance

PC-1 is a membrane glycoprotein that impairs insulin receptor function. Its K121Q polymorphism is a genetic determinant of insulin resistance. We investigated whether the PC-1 gene modulates insulin sensitivity independently of weight status (i.e. both in nonobese and obese individuals). Nondiabetic subjects [164 males, 267 females; age, 37 +/- 0.6 yr, mean +/- SEM; body mass index (BMI), 32.7 +/- 0.5 kg/m(2)], who were subdivided into 220 nonobese (BMI < or = 29.9) and 211 obese, were studied. Although subjects were nondiabetic by selection criteria, plasma insulin concentrations during oral glucose tolerance test were higher (P < 0.05) in Q allele-carrying subjects (K121Q or Q121Q genotypes), compared with K121K individuals, in both the nonobese and obese groups. Insulin sensitivity, measured by euglycemic clamp in a representative subgroup of 131 of 431 randomly selected subjects, progressively decreased (P < 0.001) from nonobese K121K [n = 61; glucose disposal (M) = 34.9 +/- 1.1 micromol/kg/min] to nonobese Q (n = 21; M = 29.9 +/- 2.0), obese K121K (n = 31, M = 18.5 +/- 1.2), and obese Q (n = 18, M = 15.5 +/- 1.2) carriers. The K121Q polymorphism was correlated with insulin sensitivity independently (P < 0.05) of BMI, gender, age, and waist circumference. In conclusion, the Q121 PC-1 variant and obesity have independent and additive effects in causing insulin resistance.

A cluster of three single nucleotide polymorphisms in the 3'-untranslated region of human glycoprotein PC-1 gene stabilizes PC-1 mRNA and is associated with increased PC-1 protein content and insulin resistance-related abnormalities

Glycoprotein PC-1 inhibits insulin signaling and, when overexpressed, plays a role in human insulin resistance. Mechanisms of PC-1 overexpression are unknown. We have identified a haplotype in the 3'-untranslated region of the PC-1 gene that may modulate PC-1 expression and confer an increased risk for insulin resistance. Individuals from Sicily, Italy, carrying the "P" haplotype (i.e., a cluster of three single nucleotide polymorphisms: G2897A, G2906C, and C2948T) were at higher risk (P < 0.01) for insulin resistance and had higher (P < 0.05) levels of plasma glucose and insulin during an oral glucose tolerance test and higher levels of cholesterol, HDL cholesterol, and systolic blood pressure. They also had higher (P < 0.05-0.01) PC-1 protein content in both skeletal muscle and cultured skin fibroblasts. In CHO cells transfected with either P or wild-type cDNA, specific PC-1 mRNA half-life was increased for those transfected with P (t/2 = 3.73 +/- 1.0 vs. 1.57 +/- 0.2 h; P < 0.01). In a population of different ethnicity (Gargano, East Coast Italy), patients with type 2 diabetes (the most likely clinical outcome of insulin resistance) had a higher P haplotype frequency than healthy control subjects (7.8 vs. 1.5%, P < 0.01), thus replicating the association between the P allele and the insulin resistance-related abnormalities observed among Sicilians. In conclusion, we have identified a possible molecular mechanism for PC-1 overexpression that confers an increased risk for insulin resistance-related abnormalities.

The Q allele variant (GLN121) of membrane glycoprotein PC-1 interacts with the insulin receptor and inhibits insulin signaling more effectively than the common K allele variant (LYS121)

When overexpressed, the membrane glycoprotein PC-1 may play a role in human insulin resistance through the inhibition of insulin receptor (IR) autophosphorylation. A PC-1 variant (K121Q, with lysine 121 replaced by glutamine) is also associated with whole-body insulin resistance when not overexpressed. To better understand the effects of the Q allele on IR function and downstream signaling, we transfected cultured cells with cDNAs for either the Q or the K alleles. In human MCF-7 cells, the Q allele was severalfold more effective (P < 0.05-0.01) than the K allele in reducing insulin stimulation of IR autophosphorylation, insulin receptor substrate-1 phosphorylation, phosphatidylinositol 3-kinase activity, glycogen synthesis, and cell proliferation. Similar data on IR autophosphorylation inhibition were also obtained in mouse R-/hIR and human HEK 293 cell lines. In transfected MCF-7 cells, 125I-labeled insulin binding and IR content were unchanged, and PC-1 overexpression did not influence IGF-1 stimulation of IGF-1 receptor autophosphorylation. Both the Q and K alleles directly interacted with the IR, as documented by coimmunoprecipitation assays. This interaction was greater for the Q allele than for the K allele (P < 0.01), suggesting that direct PC-1-IR interactions are important for the PC-1 inhibitory effect on insulin signaling. In conclusion, the Q allele has stronger inhibitory activity on IR function and insulin action than the more common K allele, and this is likely a consequence of the intrinsic characteristics of the molecule, which more strongly interacts with the IR.

Insulin/insulin-like growth factor I hybrid receptors overexpression is not an early defect in insulin-resistant subjects

Hybrid receptors (HRs), insulin receptor (IR)/insulin-like growth factor I receptor (IGF-I-R) heterodimers have been reported increased in skeletal muscle of obese and type 2 diabetic patients and to contribute to the patient insulin resistance. To investigate whether or not the increased expression of hybrid receptors is an early defect (probably genetic) of insulin resistance, we measured by specific enzyme-linked immunosorbent assays both IR, IGF-I-R, and HR content in skeletal muscle of healthy nonobese, nondiabetic subjects either insulin sensitive or insulin resistant, and also in patients with moderate obesity. IR content was significantly reduced in insulin-resistant subjects both nonobese and obese, compared with insulin-sensitive subjects (2.32+/-0.26, 2.36+/-0.18, and 3.45+/-0.42 ng/mg protein, respectively, P = 0.002). In contrast, IGF-I-R content was similar in the three groups. Muscle HR content was not different in insulin-sensitive vs. insulin-resistant subjects (both nonobese and obese) (4.90+/-0.46, 4.69+/-0.29, and 4.91+/-0.25 ng/mg protein, respectively, P = not significant). These studies indicate that, in insulin-resistant subjects without diabetes or severe obesity, muscle IR content but not IGF-I-R or HR content is reduced. They do not suggest, therefore, a primary (genetic) role of increased HR as a cause of IR decrease and insulin resistance.

High insulin levels do not influence PC-1 gene expression and protein content in human muscle tissue and hepatoma cells

BACKGROUND

To verify whether insulin levels influence PC-1 tissue content, we studied PC-1 gene expression and protein content in skeletal muscle of patients with insulinoma, a model of primary hyperinsulinemia. Data were compared with those obtained in matched insulin sensitive or resistant healthy subjects. In addition, the effect of high insulin concentration on PC-1 protein content was studied in HepG2 cells.

METHODS

The following measurements were performed: insulin sensitivity by euglycemic clamp; PC-1 protein content and insulin receptor autophosphorylation by specific ELISAs; PC-1 gene expression by competitive polymerase chain reaction (PCR); phosphatidyl-inositol-3 kinase by immunoprecipitation and thin layer chromatography; glycogen synthesis by (14)C-glucose incorporation.

RESULTS

Muscle PC-1 content was similar in the insulinoma patients and in insulin sensitive controls but higher (p<0.01) in insulin resistant controls (21.9+/-4.6 ng/mg protein, 23.8+/-3.9, 48.0+/-8.7, respectively). PC-1 protein content was inversely correlated with insulin sensitivity (r=-0.5, p<0.015) but with neither plasma insulin nor glucose levels. PC-1 protein content was correlated with PC-1 gene expression (r=0.53, p<0.05, n=14). Exposure to high insulin (100 nmol/l for 16 h) caused a significant (p<0.05-0.01) impairment of insulin receptor autophosphorylation, phosphatidyl-inositol-3 kinase activity and glycogen synthesis, but not of PC-1 protein content (114+/-3 vs 102+/-14 ng/mg protein) in HepG2 cells.

CONCLUSION

These findings suggest that chronic high insulin levels do not influence PC-1 expression.

A soluble PC-1 circulates in human plasma: relationship with insulin resistance and associated abnormalities

An increased tissue content of PC-1, an inhibitor of insulin receptor signaling, may play a role in insulin resistance. Large scale prospective studies to test this hypothesis are difficult to carry out because of the need for tissue biopsies. The aim of this study was to investigate whether PC-1 is measurable in human plasma and whether its concentration is related to insulin sensitivity. A soluble PC-1, with mol wt and enzymatic activity similar to those of tissue PC-1, was measurable in human plasma by a specific enzyme-linked immunosorbent assay and was inversely correlated to skeletal muscle PC-1 content (r = -0.5; P < 0.01). The plasma PC-1 concentration was decreased (P < 0.05) in insulin-resistant (22.7 +/- 3.0 ng/mL; n = 25) compared to insulin-sensitive (36.7 +/- 4.5; n = 25) nondiabetic subjects and was correlated negatively with the waist/hip ratio (r = -0.48; P < 0.001) and mean blood pressure (r = -0.3; P < 0.05) and positively with high density lipoprotein/total cholesterol (r = 0.38; P < 0.01) and both the M value and the plasma free fatty acid level decrement at clamp studies (r = 0.28; n = 50; P = 0.05 and r = 0.43; n = 22; P < 0.05, respectively). A plasma PC-1 concentration of 19 ng/mL or less identified a cluster of insulin resistance-related alterations with 75% accuracy. In conclusion, PC-1 circulates in human plasma, and its concentration is related to insulin sensitivity. This may help to plan studies aimed at understanding the role of PC-1 in insulin resistance.

Elevated PC-1 content in cultured skin fibroblasts correlates with decreased in vivo and in vitro insulin action in nondiabetic subjects: evidence that PC-1 may be an intrinsic factor in impaired insulin receptor signaling

Membrane glycoprotein PC-1 inhibits insulin receptor (IR) tyrosine kinase activity and subsequent cellular signaling. PC-1 content is elevated in muscle and adipose tissue from insulin-resistant subjects, and its elevation correlates with in vivo insulin resistance. To determine whether elevated PC-1 content is a primary cause of insulin resistance, we have now measured PC-1 content in cultured skin fibroblasts from nonobese nondiabetic insulin-resistant subjects and found that 1) PC-1 content was significantly higher in these cells when compared with cells from insulin-sensitive subjects (6.7 +/- 0.9 vs. 3.1 +/- 0.6 ng/0.1 mg protein, mean +/- SE, P < 0.01); 2) PC-1 content in fibroblasts was highly correlated with PC-1 content in muscle tissue (r = 0.95, P = 0.01); 3) PC-1 content in fibroblasts negatively correlated with both decreased in vivo insulin sensitivity and decreased in vitro IR autophosphorylation; and 4) in cells from insulin-resistant subjects, insulin stimulation of glycogen synthetase was decreased. These studies indicate, therefore, that the elevation of PC-1 content may be a primary factor in the cause of insulin resistance.

Effects of hyaluronic acid fractions in the rabbit eye

Two fractions of hyaluronic acid with different molecular weight (Ial, molecular weight 500,000-730,000 and Healon, molecular weight 750,000) were injected intracamerally or intravitreously in the rabbit eye. Although the fraction with higher molecular weight caused an increase in intraocular pressure, no change of this parameter was found after administration of the fraction with the lower molecular weight. Furthermore, various inflammatory reactions in ocular tissues were observed during slit-lamp biomicroscopy after administration of Healon but not of Ial. No inflammatory reaction was found after subchronic instillation of the compounds.

Neurotensin-induced miosis: the role of dopaminergic pathways

In this study we investigated the role that dopaminergic pathways play in the miotic effect exerted by neurotensin after intracameral administration. Neurotensin was injected into the anterior chamber (AC) at a dose of 30 micrograms to 4 groups of albino rabbits which had previously undergone the following treatment: a) desmethylimipramine IM and, after 30 min, 6-hydroxydopamine IV 7 days prior to the neurotensin administration; b) haloperidol IM for 15 days; c) haloperidol AC 10 minutes before the neurotensin administration. Our data confirm previous observations regarding the miotic activity of neurotensin and suggest that the dopaminergic system plays an important role in the miotic effect of neurotensin.

Antagonism of neurotensin induced miosis by thyrotropin-releasing hormone (TRH) in rabbits

In previous studies we have shown that thyrotropin-releasing hormone (TRH) antagonizes many of the neural effects of neurotensin (NT). This study, evaluated the ability of TRH and two TRH analogs: 3 methyl-His-TRH and Phe2-TRH to affect NT-induced miosis in rabbits. In confirmation of previous findings, NT (30 micrograms) produced a significant miosis. The high (60 micrograms), but not the low (30 micrograms) dose of TRH significantly antagonized NT (30 micrograms)-induced miosis. Of interest was the observation that 3 methyl-His-TRH and Phe2-TRH were more effective than native TRH in blocking NT-induced miosis. The inhibitory effect of 3 methyl-His-TRH on the miotic response to NT exhibited long duration (approximately 60 min) when compared to native TRH and Phe2-TRH. TRH or the TRH congeners had no appreciable effects on pupillary diameter when administered alone. These findings indicate that TRH antagonizes the miotic response to NT, and suggest a hitherto undescribed peptide-peptide interaction involved in regulation of iris motility.

Intracameral administration of alpha-MSH increases intraocular pressure in rabbits

A growing body of evidence suggests that neural peptides may induce important modulations on vegative and motor functions of the eye. The present study was designed to evaluate the effect of intracameral (I.C.) administration of alpha-melanocyte-stimulating hormone (alpha-MSH) and several other ocular peptides on intraocular pressure (IOP) in rabbits. alpha-MSH (5 micrograms) produced a significant and prolonged unilateral increase of IOP. This effect of I.C. alpha-MSH was dose-dependent (ED50 = 2.5 micrograms). Structure-activity studies revealed that equimolar doses of beta-MSH and gamma-MSH, unlike alpha-MSH, were totally ineffective. In addition, the structurally unrelated peptides beta-endorphin, thyrotropin-releasing hormone (TRH) and gonadotropin-releasing hormone (Gn-RH) did not affect IOP, when tested in a dose equimolar to 5 micrograms of alpha-MSH. These results confirm and extend previous observations, suggesting that alpha-MSH may be an important factor involved in regulation of IOP.

Pupillary effects of neurotensin: structure-activity relationships

We have previously reported that intracameral (I.C.) administration of neurotensin (NT) potently induces a time- and dose-dependent miosis in rabbits. This study was designed to determine structure-function relationships for NT-induced miosis. NT and twelve different fragments and analogs of NT, and the structurally-unrelated peptides beta-endorphin (beta-end), somatostatin (SRIF) and thyrotropin-releasing hormone (TRH) were tested in a dose equimolar to 30 micrograms of NT for their effects on pupillary diameter (PD) in rabbits. In confirmation of previous findings, NT produced significant miosis. Followed in order of duration of effect were D-Trp11-NT, D-Tyr11-NT, the N-terminal fragment NT1-12, [Gln4] - NT and NMe-NT. The N-terminal fragment NT1-8, D-Arg8-NT, and D-Phe11-NT were weakly active. In addition, the initial N-terminal fragment NT1-6 and the C-terminal fragments NT8-13 and NT9-13 did not affect PD. D-Pro10-NT, beta-end, SRIF, and TRH were totally ineffective. The results of this investigation contribute to support a role for NT on regulation of pupillary function, and suggest that the midportion of NT appears to be critical for the expression of NT-induced miosis.

Ocular instillation of naloxone increases intraocular pressure in morphine-addicted patients: a possible test for detecting misuse of morphine

The effect of conjunctival instillation of naloxone on intraocular pressure has been examined in morphine-addicted patients as compared to non-addicted healthy volunteers. Morphine-addicted subjects showed a lower basal value of intraocular pressure as compared to the control volunteers. The instillation of naloxone caused a normalization of intraocular pressure to a level similar to that of control volunteers. This test seems to be a useful screening method for detecting morphine addiction.