A longitudinal study of central and peripheral nerve conduction in hypothyroid rats

Pattern Reversal Visual Evoked Potential and Cognitive Functions in Subclinical Hypothyroid Subjects

Background:

Central nervous system (CNS) involvement is insidious and may occur early in subclinical hypothyroid (SCH) state which can be picked up by electrophysiological study. This study aims to record visual evoked potential (VEP), event-related latency and cognitive functions, and find their association with the levels of serum thyroid-stimulating hormone (TSH) in patients with SCH.

Materials and Methods:

In this cross-sectional study, 36 adult SCH patients and an equal number of age- and sex-matched euthyroid controls were included. Pattern reversal VEP, visual reaction time (RT), digit spanning test, and AB clock test (ABCT) were done in both SCH cases and euthyroid controls. The observed values were analyzed for comparison of mean values between the groups and correlation of recorded variables with the levels of serum TSH.

Results:

SCH cases showed a higher P100 (VEP) latency in both the right (103.2 ± 12.3 vs. 102.7 ± 6.8 ms) and left eye (101.1 ± 9.1 vs. 96.2 ± 10.7 ms) as compared to controls, but the difference was statistically insignificant. A significant delay in RT was observed on visible spectra of light in SCH cases (P < 0.001). Digit spanning score (forward and backward) in SCH cases was significantly lower than controls (P < 0.001), and a lower standardized score (<124 or <95^th percentile) was significantly associated with SCH state (P = 0.027). No significant difference was observed in visuospatial domain by ABCT between both the groups although the median score was lower in SCH cases. Only digit spanning score showed a significant negative correlation with TSH levels (r = −0.4; P = 0.001).

Conclusion:

Decline in working memory and RT to visual stimuli is an evidence of the involvement of CNS in SCH. Prolonged latency in VEP may depend on the duration of SCH.

Evaluation of the Female Patients with Subclinical Hypothyroidism by Brainstem Auditory Evoked Potentials: Case-Control Study

INTRODUCTION

Hypothyroidism, a common endocrinological disorder is quite prevalent in its subclinical state in the adult population. Nervous system involvement is frequent in hypothyroidism with documentation of peripheral and central conduction delays as abnormal latency prolongations in Brainstem Auditory Evoked Potential (BAEP) records. Subclinical hypothyroidism however, has been less extensively studied for investigating the involvement of the auditory functions.

AIM

To assess the auditory functions and Central Nervous System (CNS) involvement in the patients with subclinical hypothyroidism by recording BAEP.

MATERIALS AND METHODS

The study comprised of 50 females (25 females with subclinical hypothyroidism and 25 age-matched healthy females) in the age-group of 30-50 years. BAEP absolute and Interpeak Latencies (IPLs) (I, III and V, I-III, III-V and I-V) were compared between the two groups by unpaired t-test. The p<0.05 was considered as statistically significant.

RESULTS

Mean BAEP absolute latencies (III and V) increased in the subjects with subclinical hypothyroidism as compared to controls (p<0.001) (both the ears) and wave I absolute latency also increased with p<0.001 (both the ears) by unpaired t-test. Among IPLs (interpeak latencies), III-V and I-V IPLs exhibited increase (p<0.01), while I-III IPL did not vary with statistical significance (both the ears) in the two groups.

CONCLUSION

Patients with subclinical hypothyroidism were found to demonstrate central as well as peripheral auditory pathway affections. BAEPs can prove valuable and sensitive tests to detect involvement of the CNS and auditory dysfunctions earlier in hypothyroidism.

Peripheral gene transfer of glial cell-derived neurotrophic factor ameliorates neuropathic deficits in diabetic rats

Deprivation of neurotrophic factors contributes to the pathogenesis of diabetic neuropathy. However, the role of glial cell-derived neurotrophic factor (GDNF) in the pathogenesis of diabetic neuropathy remains unclear. The present study evaluated the pathogenic role of GDNF deficiency and the therapeutic potential of GDNF gene transfer for diabetic neuropathy. After injection of streptozotocin (STZ) for 2 weeks, diabetic rats displayed significant alteration in electrophysiological parameters, which was associated with structural changes and defective myelination in the sciatic nerves. The early diabetic neuropathy was accompanied by attenuation of the GDNF/GFRalpha1/Akt signaling cascade and depletion of sensory neuropeptides in the peripheral nerves. After detection of neuropathy, intramuscular GDNF gene transfer reversed the deficiency of GDNF/Akt signaling in the sciatic nerve and improved the neurological functions of diabetic rats. Moreover, GDNF gene delivery alleviated the axonal demyelination and restored the sensory neuropeptide levels in the sciatic nerve of diabetic rats. In summary, peripheral GDNF gene delivery ameliorates the diabetes-induced downregulation of the GDNF signaling complex in the peripheral nervous system and holds promises for treatment of diabetic neuropathy.

Thyroid hormone regulation of N-methyl-D-aspartic acid receptor subunit mRNA expression in adult brain

Thyroid hormone is an essential modulator of brain development, but little is known about its actions in the adult brain. Hypothyroidism is associated with gene expression changes in both central and peripheral nervous tissue. Functional consequences of adult-onset hypothyroidism include an inability to produce long-term potentiation in rat hippocampus and impaired learning and memory in both rats and man. Long-term potentiation is a form of learning that is dependent on functional N-methyl-d-aspartic acid (NMDA)-preferring ionotropic glutamate receptors. This work examines the expression of ionotropic glutamate receptor subunit mRNA following surgical thyroidectomy with or without thyroid hormone replacement. In situ hybridization histochemistry was used to determine the mRNA levels of the NMDA receptor subunits NR1, NR2A, NR2B, the AMPA receptor subunit GluR1, and the kainate receptor subunit KA2. Reducing circulating concentrations of thyroid hormone by surgical removal of the thyroid gland 2 weeks before sacrifice decreased the expression of NR1 mRNA exclusively in the hippocampus. Conversely, hyperthyroidism selectively reduced NR2B mRNA expression in the dorsal hippocampus. Altering thyroid hormone status had no effect on the expression of KA2 or GluR1 subunit mRNA. The regulation of expression of NR1 and NR2B mRNA by thyroid hormone is a novel mechanism for explaining the relationship between thyroid hormone and cognitive function.

EMG myokymia as a cause of ptosis in hypothyroidism

Ptosis is known to be associated with thyroid disorders. We describe two biochemically corrected hypothyroid patients presenting with isolated bilateral ptosis. EMG of the orbicularis oculi showed continuous grouped motor unit potentials. In the absence of obvious aetiology, it is hypothesised that focal demyelination of terminal branches to the orbicularis oculi may play a role in the generation of the discharges.

Hearing loss in congenital hypothalamic hypothyroidism: a wide therapeutic window

In this study, which aims to investigate for the first time auditoty sensitivity in congenital central hypothyroidism (CH), we concluded that: (1) permanent sensorineural hearing loss may be observed not only in congenital primary disorders of thyroid function, as reported hitherto; (2) it may also be found in patients with congenital CH, at least when substitutive treatment is started many years after thyroid failure presentation; (3) the concept of a 'critical therapeutic window' in preventing hearing impairment also holds true for children with congenital CH; (4) the therapeutic window in CH is wider than in primary hypothyroidism; (5) this is probably due to the less severe thyroid impairment in the cases with a central origin of hypothyroidism. These conclusions were suggested by the following data obtained in a cohort of 10 patients with congenital CH who were euthyroid at the time of audiological evaluation thanks to long-standing substitutive therapy: (a) four out of 10 patients exhibited a bilateral sensorineural hearing loss; (b) hearing impaired patients were the oldest of the entire series at diagnosis of hypothyroidism and none of them was aged less than 7 years at the time of the start of replacement therapy; (c) a strong positive relationship was found between age at therapy start and hearing loss.

Click-evoked otoacoustic emissions recorded from untreated congenital hypothyroid newborns

Thyroid hormone plays an important role in hearing development. Both a genetic or non-genetic hypothyroidism is often associated with congenital hearing loss. The exact incidence of hearing impairment in untreated congenital hypothyroid (CH) patients is unknown. This paper will present the results of measuring of the transient-evoked otoacoustic emissions (TEOAE) in a population of 29 newborns, who tested positive on a screening test for hypothyroidism (CH group) and in 68 well babies (control group) randomly chosen from all the newborns, classified as PASS, included in the Hearing Screening Program of the San Raffaele Hospital in Milan. TEOAE were recorded in all newborns within 1 month after birth and before beginning L-thyroxine treatment with conventional commercial instrumentation. Both temporal and time-frequency analyses of the emitted responses were conducted by means of a wavelet transform. The comparison of the characteristics of the temporal and frequency content of the responses of the two groups (CH and control) showed no statistically significant difference. No correlation was found between outer hair cell dysfunction and hypothyroidism.

Differential effect of thyroid hormone deficiency on the growth of calretinin-expressing neurons in rat spinal cord and dorsal root ganglia

The development of spinal cord or dorsal root ganglia neurons expressing calretinin (CR) was studied in thyroid hormone-deficient rats. Immunocytochemical and morphometric analyses showed that the hypothyroidism induced a significant decrease in the number and size of immunoreactive neurons in the spinal cord, as well as stunted growth and arborization of the axons and dendrites. These alterations were observed at different embryonic ages and persisted during the whole postnatal life. In adult hypothyroid rats, the mean number of CR-positive neurons per spinal cord section (31.2 +/- 2.3 in laminae I and II and 30.5 +/- 5.5 in laminae III-X) was significantly decreased (P < 0.001 and P = 0.024, respectively) compared with adult normal rats (68.7 +/- 8.9 and 50.0 +/- 11.0, respectively). In the peripheral nervous system, hypothyroidism altered the growth of sensory neurons expressing CR protein mainly during embryonic life. In comparison with normal rats, hypothyroid embryonic animals showed not only reduced cell size but also a significantly decreased percentage of CR-positive neurons (6.6 +/- 0. 9% in normal, 2.1 +/- 0.3% in hypothyroid rats, P < 0.001). In contrast, although the size of neurons was reduced in hypothyroid young and adult rats, there was no reduction in the percentage of CR-positive neurons. These results showed that thyroid hormone deficiency altered differentially the development of neurons expressing CR protein in the central and peripheral nervous systems. This suggests that central and peripheral neurons are heterogeneous in their sensitivity to thyroid hormone.

The recovery potential of central conduction disorder in hypothyroid rats

In an aim to detect the dysfunction of central nervous system among rats with varied durations of hypothyroidism and to elucidate the recovery potential after thyroxine replacement, a series of BAEP were conducted and compared with age-matched controls. BAEP was performed in five groups of the hypothyroid animals 1, 3, 5, 7, and 9 months after thyroidectomy respectively. Following initial electrophysiological assessment, thyroxine replacement was administered to each group of hypothyroid rats, and BAEP was performed at two month intervals, up to two successive normal studies or six months after the initiation of therapy, whichever came first. Before thyroxine treatment, prolonged I-V interpeak latency was the most consistent abnormal finding in all groups of hypothyroid rats, and longer hypothyroid state correlated well with more severe central conduction disorder. Hearing impairment was also noted among those with long duration of hypothyroidism. After thyroxine replacement, the central conduction dysfunction usually returned to normal if the hypothyroid state was not more than 5 months in duration. However, when hypothyroid state persisted over 7 months or more, there would be an incomplete recovery for central conduction disorder. The present study brings out the concept of 'therapeutic window' in reversing the central nervous dysfunction caused by hypothyroidism in adult rats.

Effect of primary congenital hypothyroidism upon expression of genes mediating murine brain glucose uptake

Using hyt/hyt mice that exhibit naturally occurring primary hypothyroidism (n = 72) and Balb/c controls (n = 66), we examined the mRNA, protein, and activity of brain glucose transporters (Glut 1 and Glut 3) and hexokinase I enzyme at various postnatal ages (d 1, 7, 14, 21, 35, and 60). The hyt/hyt mice showed an age-dependent decline in body weight (p < 0.04) and an increase in serum TSH levels (p < 0.001) at all ages. An age-dependent translational/posttranslational 40% decline in Glut 1 (p = 0.02) with no change in Glut 3 levels was observed. These changes were predominant during the immediate neonatal period (d 1). A posttranslational 70% increase in hexokinase enzyme activity was noted at d 1 alone (p < 0.05) with no concomitant change in brain 2-deoxy-glucose uptake. This was despite a decline in the hyt/hyt glucose production rate. We conclude that primary hypothyroidism causes a decline in brain Glut 1 associated with no change in Glut 3 levels and a compensatory increase in hexokinase enzyme activity. These changes are pronounced only during the immediate neonatal period and disappear in the postweaned stages of development. These hypothyroid-induced compensatory changes in gene products mediating glucose transport and phosphorylation ensure an adequate supply of glucose to the developing brain during transition from fetal to neonatal life.

Metabolic neuropathies

A large epidemiological study has documented that one-third of diabetic patients have peripheral neuropathy. Diabetes duration, poor glycaemic control, smoking and hypertension are all independent predictors of the incidence of diabetic polyneuropathy. High prevalence of autonomic dysfunctions, both sympathetic and parasympathetic, has been found in patients with nonalcoholic chronic liver disease. The pathogenesis of metabolic neuropathy is unclear; even immunologic factors might play a role in the development of diabetic autonomic neuropathy. No specific treatments are available for these neuropathies. Correction of metabolic derangement is fundamental, as shown by the amelioration of peripheral nerve function obtained after successful simultaneous pancreas-kidney transplantation. The therapeuthic potentials of neurotrophins for the prevention and treatment of diabetic neuropathy have to be confirmed in future studies.