Hypomyelination in the cerebrum of the congenitally hypothyroid mouse (hyt)

Type 2 Deiodinase Disruption in Astrocytes Results in Anxiety-Depressive-Like Behavior in Male Mice

Millions of levothyroxine-treated hypothyroid patients complain of impaired cognition despite normal TSH serum levels. This could reflect abnormalities in the type 2 deiodinase (D2)-mediated T4-to-T3 conversion, given their much greater dependence on the D2 pathway for T3 production. T3 normally reaches the brain directly from the circulation or is produced locally by D2 in astrocytes. Here we report that mice with astrocyte-specific Dio2 inactivation (Astro-D2KO) have normal serum T3 but exhibit anxiety-depression-like behavior as found in open field and elevated plus maze studies and when tested for depression using the tail-suspension and the forced-swimming tests. Remarkably, 4 weeks of daily treadmill exercise sessions eliminated this phenotype. Microarray gene expression profiling of the Astro-D2KO hippocampi identified an enrichment of three gene sets related to inflammation and impoverishment of three gene sets related to mitochondrial function and response to oxidative stress. Despite normal neurogenesis, the Astro-D2KO hippocampi exhibited decreased expression of four of six known to be positively regulated genes by T3, ie, Mbp (∼43%), Mag (∼34%), Hr (∼49%), and Aldh1a1 (∼61%) and increased expression of 3 of 12 genes negatively regulated by T3, ie, Dgkg (∼17%), Syce2 (∼26%), and Col6a1 (∼3-fold) by quantitative real-time PCR. Notably, in Astro-D2KO animals, there was also a reduction in mRNA levels of genes known to be affected in classical animal models of depression, ie, Bdnf (∼18%), Ntf3 (∼43%), Nmdar (∼26%), and GR (∼20%), which were also normalized by daily exercise sessions. These findings suggest that defects in Dio2 expression in the brain could result in mood and behavioral disorders.

The T3-induced gene KLF9 regulates oligodendrocyte differentiation and myelin regeneration

Hypothyroidism is a well-described cause of hypomyelination. In addition, thyroid hormone (T3) has recently been shown to enhance remyelination in various animal models of CNS demyelination. What are the ways in which T3 promotes the development and regeneration of healthy myelin? To begin to understand the mechanisms by which T3 drives myelination, we have identified genes regulated specifically by T3 in purified oligodendrocyte precursor cells (OPCs). Among the genes identified by genomic expression analyses were four transcription factors, Kruppel-like factor 9 (KLF9), basic helix-loop-helix family member e22 (BHLHe22), Hairless (Hr), and Albumin D box-binding protein (DBP), all of which were induced in OPCs by both brief and long term exposure to T3. To begin to investigate the role of these genes in myelination, we focused on the most rapidly and robustly induced of these, KLF9, and found it is both necessary and sufficient to promote oligodendrocyte differentiation in vitro. Surprisingly, we found that loss of KLF9 in vivo negligibly affects the formation of CNS myelin during development, but does significantly delay remyelination in cuprizone-induced demyelinated lesions. These experiments indicate that KLF9 is likely a novel integral component of the T3-driven signaling cascade that promotes the regeneration of lost myelin. Future analyses of the roles of KLF9 and other identified T3-induced genes in myelination may lead to novel insights into how to enhance the regeneration of myelin in demyelinating diseases such as multiple sclerosis.

Aroclor 1254 impairs the hearing ability of Xenopus laevis

In this study we assessed the effects of chronic, dietary exposure of Aroclor 1254 (A1254) on the hearing of Xenopus frogs. We used the auditory brainstem response (ABR) to assay changes in hearing physiology; ABR thresholds, as well as latency-intensity and amplitude-intensity profiles of the initial positive (P1) and negative (N1) peaks were measured. Two groups of animals that received 50 ppm and 100 ppm of A1254 in their diet from 5 days post-fertilization through metamorphosis were compared to a control group that received untreated chow. The results showed significant threshold elevations in the 3-4 kHz range and significantly delayed peak latencies and reduced amplitudes at these frequencies in A1254 treated animals as compared to control animals. These findings indicate that A1254 selectively damages the high-frequency sensorineural hearing system associated with the basilar papilla of frogs. This preferential damage may be related to inherent differences in the vulnerability of the basilar versus amphibian papilla in the frog. The overall results of this study are also consistent with the reported A1254-induced auditory deficits in mammals indicating that the basilar papilla of the Xenopus frog may serve as an effective model for studying the effects of A1254 on the auditory system.

Pediatric Endocrinology

Endocrine and metabolic disorders affecting puppies and kittens from birth until 6 months of age may manifest as clinical problems related to growth or to water metabolism (polydipsia and polyuria). Most commonly,endocrine and metabolic disorders affect growth of the animal,and puppies are often presented to the veterinarian for assessment of delayed or aberrant growth. Other endocrine disorders of small animals,such as juvenile-onset diabetes insipidus or diabetes mellitus, affect water metabolism, resulting in excessive thirst and urination and resultant difficulty in house-breaking.

Diagnosis of Congenital and Adult-Onset Hypothyroidism in Cats

Whereas hyperthyroidism is the most common endocrine disorder in the cat, hypothyroidism is the least common feline endocrine disorder. This is a the result of several factors including low index of suspicion, rarity of the naturally occurring hypothyroidism in cats, and a lack of species specific tests for endogenous TSH and antithyroglobulin antibodies. Nonetheless, hypothyroidism does occur in cats, especially in kittens and after radioactive treatment for hyperthyroidism. The clinician should become familiar with the common presentations of congenital and adult-onset hypothyroidism in cats. In addition, some of the tests specific to dogs (such as endogenous canine TSH) may be utilized to diagnose subclinical hypothyroidism in cats. Fortunately, the treatment of feline hypothyroidism with synthetic levothyroxine is both straightforward and effective.

Spontaneous circling behavior and dopamine neuron loss in a genetically hypothyroid mouse

The genetically hypothyroid mouse, Tshr(hyt), has a single point mutation resulting in a defective thyroid-stimulating hormone receptor, and therefore a non-functional thyroid gland. This is an autosomal recessive disorder and affected mice have been reported to have a number of somatic and behavioral deficits. This study reports a pronounced, spontaneous, asymmetrical circling behavior in the Tshr(hyt) mouse. The spontaneous circling behavior appeared in about 25% of the homozygous animals, in both males and females. The circling usually appeared by postnatal day 35 and continued throughout the lifespan of the animal. The circling was in one direction only, either clockwise or counterclockwise, with the directional preference being almost absolute. A stereological analysis of tyrosine hydroxylase immunoreactive neurons in the substantia nigra and adjacent ventral tegmental area of circling homozygous mice, non-circling homozygous mice and heterozygous mice revealed that the circlers had significantly fewer (40% reduction) midbrain dopamine neurons than those animals that did not circle. There was not an association between the direction of the circling and an asymmetry in the number of dopamine neurons in the midbrains of these mice. There was no difference in the number of dopamine neurons in the midbrain of the homozygous non-circlers and the heterozygous mice. These studies indicate that about 25% of genetically hypothyroid mice demonstrated a spontaneous, perseverative, unilateral circling behavior that was associated with a significant reduction in the number of their midbrain dopamine neurons. Thus congenitally hypothyroid mice are at risk for a reduction in the number of nigral dopamine neurons and an associated repetitive movement disorder.

Diagnosis and treatment of juvenile endocrine disorders in puppies and kittens

Endocrine and metabolic disorders affecting puppies and kittens from birth until 6 months of age may manifest as clinical problems related to growth, water metabolism (polydipsia or polyuria), or as episodic weakness. Endocrine and metabolic disorders that affect stature, such as pituitary or hypothyroid dwarfism, present to the veterinarian for assessment of delayed or aberrant growth. Conversely, juvenile-onset diabetes mellitus and diabetes insipidus cause excessive thirst, urination, and difficulty in house-breaking.

Molecular cloning and brain localization of HZF-2 alpha, a new member of the Rev-erb subfamily of orphan nuclear receptors

We have isolated a novel cDNA homologous to the family of nuclear hormone receptors which we have designated hippocampal zinc finger-2 alpha (HZF-2 alpha). HZF-2 alpha encodes a protein 494 amino acids in length and predicted molecular size of 56 kD. HZF-2 alpha shares sequence similarities with orphan nuclear receptors related to thyroid hormone and retinoic acid receptors. An additional HZF-2 cDNA clone, HZF-2 beta, was isolated. HZF-2 beta differs from HZF-2 alpha in a nine-amino-acid region carboxyl to the DNA-binding domain thought to participate in enhancing the interaction of nuclear receptors with DNA. Northern blot analysis with a probe homologous to both HZF-2 alpha and beta identified two transcripts, sizes 4.4 and 5.5 kb in several rat tissues. HZF-2 alpha/beta mRNA levels are low in embryonic rat brain, increase during neonatal brain development and remain elevated in the adult rat brain. In situ hybridization analyses localized HZF-2 alpha/beta mRNA expression to all hippocampal subfields. Within the hippocampus heaviest expression was observed within the dentate gyrus. High levels of HZF-2 alpha/beta mRNA were also detected in Purkinje cells and the granular cell layer of the cerebellum. In summary, HZF-2 alpha/beta are novel orphan nuclear receptors which may play an important role in hormone-dependent aspects of developmental processes modulating hippocampal and cerebellar plasticity.

Drug-induced taste and smell disorders. Incidence, mechanisms and management related primarily to treatment of sensory receptor dysfunction

Drugs in every major pharmacological category can impair both taste and smell function and do so more commonly than presently appreciated. Impairment usually affects sensory function at a molecular level, causing 2 major behavioural changes--loss of acuity (i.e. hypogeusia and hyposmia) and/or distortion of function (i.e. dysgeusia and dysosmia). These changes can impair appetite, food intake, cause significant lifestyle changes and may require discontinuation of drug administration. Loss of acuity occurs primarily by drug inactivation of receptor function through inhibition of tastant/odorant receptor: (i) binding; (ii) Gs protein function; (iii) inositol trisphosphate function; (iv) channel (Ca++,Na++) activity; (v) other receptor inhibiting effects; or (vi) some combination of these effects. Distortions occur primarily by a drug inducing abnormal persistence of receptor activity (i.e. normal receptor inactivation does not occur) or through failure to activate: (i) various receptor kinases; (ii) Gi protein function; (iii) cytochrome P450 enzymes; or other effects which usually (iv) turn off receptor function; (v) inactivate tastant/odorant receptor binding; or (vi) some combination of these effects. Termination of drug therapy is commonly associated with termination of taste/smell dysfunction, but occasionally effects persist and require specific therapy to alleviate symptoms. Treatment primarily requires restoration of normal sensory receptor growth, development and/or function. Treatment which restores sensory acuity requires correction of steps initiating receptor and other pathology and includes zinc, theophylline, magnesium and fluoride. Treatment which inhibits sensory distortions requires reactivation of biochemical inhibition at the receptor or inactivation of inappropriate stimulus receptor binding and/or correction of other steps initiating pathology including dopaminergic antagonists, gamma-aminobutyric acid (GABA)-ergic agonists, calcium channel blockers and some orally active local anaesthetic, antiarrhythmic drugs.

Subcellular distribution of carbonic anhydrase and Na+,K(+)-ATPase in the brain of the hyt/hyt hypothyroid mice

Activities of carbonic anhydrase and Na+,K(+)-ATPase in tissue homogenates and in subcellular fractions from different brain regions were studied in inherited primary hypothyroid (hyt/hyt) mice. The body weight, the weight of different brain regions, and the plasma thyroxine and triiodothyronine levels of hyt/hyt mice were significantly lower than those of the age-matched hyt/+ controls. In tissue homogenates of cerebral cortex, brain stem and cerebellum of hypothyroid mice, the activity of carbonic anhydrase (units/mg protein) was 59.2, 57.6, and 43.2%, and the activity of Na+,K(+)-ATPase (nmol Pi/mg protein/min) was 73.7, 74.4 and 68.7%, respectively, of that in corresponding regions of euthyroid littermates. The decrease in enzyme activity in tissue homogenates was also reflected in different subcellular fractions. In cerebral cortex and brain stem, carbonic anhydrase activity in cytosol, myelin and mitochondrial fractions of hypothyroid mice was about 45-50% of that in euthyroid mice, while in cerebellum the carbonic anhydrase activity in these subcellular fractions of hyt/hyt mice was only 33-38% of that in hyt/+ controls. Na+,K(+)-ATPase activity in myelin fraction of different brain regions of hyt/hyt mice was about 34-42% of that in hyt/+ mice, while in mitochondria, synaptosome and microsome fractions were about 44-52, 46-53, and 66-68%, respectively of controls. These data indicate that the activity of both carbonic anhydrase and Na+,K(+)-ATPase was affected more in the myelin than other subcellular fractions and more in the cerebellum than cerebral cortex and brain stem by deficiency of thyroid hormones.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal hypothyroidism affects the timely expression of myelin-associated glycoprotein in the rat brain

Congenital hypothyroidism strongly affects myelination. To assess the role of thyroid hormone on myelin gene expression, we have studied the effect of hypothyroidism on the steady state levels of myelin-associated glycoprotein (MAG) and its mRNA in rat brain during the first postnatal month. As studied by immunoblot analysis of several brain regions, MAG increased from days 10-15 onwards, reaching constant levels by days 20-25. Hypothyroid samples showed a delay in the accumulation of MAG that was more severe in rostral regions, such as cortex and hippocampus. The effect of hypothyroidism on the accumulation of the protein correlated with mRNA levels. MAG mRNA started to accumulate in the cerebrum of normal animals by postnatal day 7, reaching maximal levels by day 20. Hypothyroid rats showed a delay of several days in the onset of mRNA expression, increasing thereafter at the same rate as in normal animals, and eventually reaching similar values. When individual brain regions were analyzed, we found strong regional differences in the effect of hypothyroidism. The cerebral cortex was most affected, with messenger levels lower than in normal animals at all ages. In more caudal regions differences between control and hypothyroid rats were evident only at the earlier stages of myelination, with spontaneous recovery at later ages. By run on analysis, we found no differences in transcriptional activities of the MAG gene in normal, hypothyroid, or T4-treated rats. Therefore, the effects of hypothyroidism on MAG mRNA and protein levels were most likely caused by decreased mRNA stability. We propose that thyroid hormone contributes to enhanced myelin gene expression by affecting the stability of newly transcribed mRNA in the early phases of myelination.

Microcephalic cerebrum with hypomyelination in the congenital goiter mouse (cog)

2',3'-Cyclic nucleotide 3'-phosphohydrolase activity in the cerebrum of the congenital goiter mouse (cog/cog) is reduced in comparison with the normal heterozygote (cog/+). The weight of the cog/cog cerebrum and cerebellum were significantly less than those of the normal controls, 89.0% less for the cerebrum, and 81.1% less for the cerebellum. However, no differences were observed with regard to DNA and RNA content and the RNA/DNA ratio. The results of this study indicate that hypomyelination in the congenital goiter mouse is restricted to the cerebrum, and is not related to arrested glial proliferation.

Hypothyroidism selectively reduces the rate and amount of transport for specific SCb proteins in the hyt/hyt mouse optic nerve

Thyroid hormone significantly affects molecular and neuroanatomical properties of the developing nervous system. Altered connectivity in hypothyroidism may reflect reductions in process growth, alterations in process maintenance, or changes in synaptogenesis or synaptic maintenance. These events are dependent on microtubules, neurofilaments, microfilaments, and associated molecular components. Reductions in delivery of microtubules and neurofilaments to the distal axon by slow component a (SCa) of axonal transport may contribute to the neuroanatomical abnormalities of hypothyroidism (Stein et al., J Neurosci Res 28:121-133, 1991). However, hypothyroidism might also affect the axon and synaptic connections by altering slow component b (SCb), which includes actin microfilaments and proteins that contribute to synaptic function, i.e., clathrin, HSC70 (clathrin uncoating ATPase), spectrin, and calmodulin. To determine the effect of hypothyroidism on SCb proteins, slow axonal transport was analyzed in optic nerves of hyt/hyt hypothyroid mice, which have severe primary hypothyroidism, and euthyroid control mice. Clathrin, spectrin, HSC70, and actin showed significant reductions in transport velocity in hyt/hyt optic nerves relative to euthyroid nerves, but the transport rate for calmodulin was less affected. However, the amount of calmodulin was significantly elevated in hyt/hyt nerve over euthyroid nerves. Hypothyroidism selectively reduces transport of SCb proteins, which are thought to play significant roles in synaptic function and in the growth cone. The effects of hypothyroidism on microtubules and neurofilaments combined with actions on SCb suggest that changes in neuronal function associated with reduced thyroid hormone during development and maturity (i.e., alterations in neuronal connectivity, nerve conduction, and synaptic function) may be mediated in part by effects on slow axonal transport.

Congenital hypothyroid dwarfism in a family of giant schnauzers

Congenital hypothyroid dwarfism was diagnosed in a family of Giant Schnauzers. Three female and two male puppies from different litters were evaluated for dwarfism, lethargy, somnolence, gait abnormalities, and constipation. On physical examination, disproportionate dwarfism (n = 5), macroglossia (n = 3), hypothermia (n = 3), delayed dental eruption (n = 3), ataxia (n = 2), and abdominal distension (n = 1) were identified. Results of initial laboratory tests showed anemia (n = 4), hypercholesterolemia (n = 4), hypercalcemia (n = 2), and transudative abdominal effusion (n = 1). Radiographic skeletal surveys disclosed epiphyseal dysgenesis and delayed skeletal maturation (n = 5). A diagnosis of hypothyroidism was established on the basis of low basal serum thyroxine concentrations that failed to increase following the administration of TSH (n = 5) and markedly reduced to absent thyroid image when evaluated with gamma camera imaging of the thyroid gland (n = 4). In the two dogs that were most thoroughly evaluated, the results of thyroid histology, prolonged TSH testing, and repeat thyroid imaging, after three daily injections of TSH, were all consistent with secondary or tertiary, rather than primary, hypothyroidism. When TSH was administered over a period of 3 consecutive days (5 IU/day, subcutaneously), serum thyroid hormone response became normal and resulted in a normal thyroid image in the two dogs re-evaluated with gamma camera imaging. Daily treatment with oral levothyroxine (20 micrograms/kg) resulted in complete remission in puppies (n = 4) treated prior to 4 months of age. The other puppy failed to attain normal breed standards for height.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothyroidism reduces the rate of slow component A (SCa) axonal transport and the amount of transported tubulin in the hyt/hyt mouse optic nerve

Thyroid hormone deficiency in the developing brain leads to disorders of neuronal process growth. This is evidenced by reduced axonal and dendritic size and complexity (Garza et al.: Developmental Brain Research 43:287-297, 1988; Ruiz-Marcos: Iodine and the Brain. New York: Plenum Press, pp 91-102, 1989). These findings may be related to alterations in the neuronal cytoskeleton in hypothyroidism, such as reduced or abnormal microtubular number and density (Faivre et al.: Developmental Brain Research 8: 21-30, 1983), and altered assembly, stabilization, and composition of microtubule protein in the hypothyroid brain. Neurofilaments also contribute to axonal caliber and process stability. Similar to microtubules, certain properties of neurofilaments are altered in developing hypothyroid axons (Marc and Rabie: International Journal of Developmental Neuroscience 3: 353-358, 1985; Faivre et al.: Developmental Brain Research 8:21-30, 1983) that may affect axonal caliber and process stability. Normal process growth is predicted on formation of appropriate numbers of microtubules and on the normal synthesis and axonal transport of cytoskeletal components [tubulin, microtubule associated proteins (MAPs), and neurofilament proteins]. Hypothyroidism might alter the neuronal cytoskeleton and neuronal growth either by affecting the developmental programs for expression of specific isoforms of cytoskeletal proteins or by changing the delivery of cytoskeletal proteins via slow axonal transport, particularly slow component a (SCa). Previous studies had demonstrated changes in the amount of specific microtubule protein isoforms and mRNAs (Stein et al.: Iodine and the Brain. New York: Plenum Press, pp 59-78, 1989a). To further elucidate the molecular basis for process growth abnormalities in the hypothyroid brain, we investigated slow axonal transport in the mouse to determine the effects of thyroid hormone deficiency on the rate and composition of SCa. Comparisons of SCa in the optic nerve of hyt/hyt hypothyroid mouse and euthyroid hyt/+ littermates and euthyroid progenitor strain, BALB/cBY +/+ mice, indicated that the velocity of SCa was significantly reduced in hyt/hyt optic nerve relative to hyt/+ and +/+. The axonal transport rate for tubulin, which is carried in SCa, was 0.118 mm/day in the hyt/hyt optic nerves. This rate was significantly different for the tubulin rates for the hyt/+ optic nerves (0.127 mm/day) and for the +/+ optic nerves (0.138 mm/day). Neurofilament proteins, as measured by the 140,000 daltons component, NFM, also appeared to be reduced in velocity in the hyt/hyt versus the hyt/+ and +/+ optic nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Retarded growth of the suprachiasmatic nucleus and pineal body in dw and lit dwarf mice

The suprachiasmatic nucleus (SCN) and the pineal body in 3 types of inherited hormone-deficient mice, the dw, lit and hyt mice were examined by morphological, morphometric and biochemical techniques. In the dw and lit mice the SCN was underdeveloped. In the ventral part of the SCN, where most of the retinal fibers appeared to terminate, both cell number and cell size were decreased, although the size of the SCN was unaltered. In addition, the pineal bodies of both mice were morphologically underdeveloped and showed low levels of N-acetyltransferase activity. In contrast, the hyt SCN was comparable to the normal controls in every respect. The hyt pineal was well developed and showed levels of enzyme activity comparable to the controls. However, in all the deficient mice, the optic nerve appeared to be normal in morphological and biochemical studies. These results suggest that the underdevelopment of the pineal body, the reduced levels of spontaneous locomotion and the indistinct diurnal periodicity of the dw and lit mice might be related to the retarded neuronal growth of the SCN, and that growth hormone likely is indispensable for the development of the SCN.

Spermatogenesis in Snell dwarf, little and congenitally hypothyroid mice

The status of spermatogenesis in Snell dwarf, little and congenitally hypothyroid mice was studied. In all of these mice with a hormone deficiency the seminiferous tubules were smaller in size and contained fewer spermatogonia, spermatocytes, spermatids and spermatozoa than did those of normal control mice. There was no substantial difference in the Johnsen score between the hormone-deficient mice and normal control mice, but the former had underdeveloped seminiferous tubules with a corresponding paucity of germ cells, which may be partly responsible for the infertility of these mice. In the present study, growth hormone and thyroxine were administered separately to growth hormone-deficient and thyroxine-deficient mice, respectively. Such replacement therapy brought about an increase in cell counts of the seminiferous tubules and in sperm counts in both groups.

Suppression of 2', 3'-cyclic nucleotide 3'-phosphohydrolase activity in suckling rat brain by ACTH

Administration of ACTH at two different doses (0.05 and 0.5 microgram/g/day) to suckling rats resulted in the suppression of both the body and brain weight gains and the developmental increase in brain CNPase activity, and the suppression of the brain CNPase activity persisted for 3 weeks (up to the end of the experiment) after the cessation of ACTH administration in the suckling period, while the suppression of the body and brain weight gains was noticed only during the administration period. The authors emphasized the possibility that long-term therapy with massive doses of ACTH for infantile spasms may be hazardous to the developing brain in many ways.

A new myelin-deficient mutant hamster: biochemical and morphological studies

Biochemical and morphological studies were done on a new trembling mutant hamster CBB. The yield of myelin from the mutant was 30 and 40% of the control at 46 and 140 days of age, respectively, but myelin composition and 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNPase) activity were normal. Morphologically, about 18% of the axons were myelinated in the mutant optic nerve at 46 days of age, in which the myelinated fibers were those with larger diameters (more than 0.6 micron), while the control had a peak at 0.4 micron in diameter. The ultrastructure and thickness of compact myelin lamellae in the mutant were normal. Myelination and the structure of peripheral nerve myelin appeared normal. The results indicate that the essential defect is the delay and arrest of myelination in the CNS, which is probably caused by either a decreased rate of synthesis of myelin components in oligodendrocytes or a defect in the oligodendrocyte-axon recognition in smaller axons.

Partial restoration of cerebral myelination of the congenitally hypothyroid mouse by parenteral or breast milk administration of thyroxine

We attempted to define whether thyroid hormone can ameliorate the cerebral hypomyelination present in the congenitally hypothyroid (hyt) neonatal mouse, and to define the critical time period during early postnatal life when thyroxine (T4) is essential for myelin formation. We administered T4 to the hyt mouse by breast milk during the first 20 days of postnatal life, and through the diet during the second 20 days of postnatal life. Positive results were obtained only when hormone was given during the first 20 days of postnatal life. A distinct increase in cerebral 2',3'-cyclic nucleotide 3'-phosphohydrolase activity was noted, and brain sections stained for myelin basic protein correlated with the biochemical findings. The later administration of hormone through diet was ineffective.

Microcephalic cerebrum with hypomyelination in the growth hormone-deficient mouse (lit)

To determine whether GH has an independent action on cerebral development, we examined the central nervous system of the little mouse (lit), a promising model of isolated growth hormone deficiency. Our findings are (A); the weights of two parts of the lit brain were significantly less than those of the normal controls, 81.5% less for the cerebrum, and 81.6% for the cerebellum, (B): the total DNA content was reduced to approximately 80% in the cerebrum and 84% in the cerebellum compared to those of the normal controls, (C); the total RNA content was also reduced in the cerebrum and cerebellum, proportional to the reduction in DNA, (D); CNPase activity was reduced selectively in the cerebrum of the lit mouse (74.4% of the normal control), and (E); the lit mice exhibited a strikingly reduced level of activity with an indistinct diurnal periodicity. These results indicate that GH has independent actions on cerebral development, especially on glial cell proliferation as a precondition of myelin formation.

Abnormal neuronal growth in the little (lit) cerebrum

Mice genetically deficient in growth hormone were compared with normal littermates for evidence of altered neuronal development. In the lit/lit motor cortex, area 6 of Caviness, cell number, stratification of neurons, and the proportionate layer widths were identical to those of the controls (lit/+). Examined by the Golgi-Cox method modified by Ramon-Moliner, however, the pyramidal neuron was found to have short primary dendrites with sparse branchings. This result was confirmed in sections stained for neuron-specific enolase. Therefore, we conclude that growth hormone has an independent action on neuronal development.