dCTP misincorporation in a Chinese hamster mutator phenotype: the role of GGA genetic context

Clone CSA7 is a CHEF18 hamster cell line that shows an increasedintracellular accumulation of dCTP. To localize the mutationsthat accumulate spontaneously in a functional gene of such amutator phenotype, independent CSA7 mutants of the hypoxanthine–guaninephosphoribosyl transferase (hprt) gene were isolated and screenedby a polymerase chain reaction–single strand conformationpolymorphism technique. Sixty-two percent of mutants produceddetectable changes of the strand migration profile and the mutationswere preferentially localized in the exons 3 (31%) and 6 (62%).The sequencing of such exons revealed that the rate of C baseincorporation was the major mutation pathway and that the Abase of a GGA sequence was the preferential site of misincorporation. ³To whom correspondence should be addressed     

Desensitization of AMPA Receptors Limits the Amplitude of EPSPs and the Excitability of Motoneurons of the Rat Isolated Spinal Cord

Intracellular recording was used to study the effect of cyclothiazide, a selective blocker of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor desensitization, on lumbar motoneurons of the rat isolated spinal cord. Cyclothiazide (25 μM) enhanced the responses to AMPA in a tetrodotoxin-insensitive fashion, without affecting those produced by N -methyl-D-aspartate or γ-aminobutyric acid. Excitatory postsynaptic potentials (EPSPs) evoked by dorsal root stimulation were strongly potentiated in amplitude while paired-pulse depression (produced by applying pairs of pulses at 2 s interval) of the EPSP was decreased. In the presence of cyclothiazide the frequency of spontaneous synaptic events was greatly increased and network-driven bursting activity developed with eventual loss of electrical excitability. The present results suggest that pharmacological block of AMPA receptor desensitization led to strong excitation of motoneurons and indicate a physiological role of desensitization in protecting these nerve cells from overactivity.     

Excitatory effects of serotonin on rat striatal cholinergic interneurones

We investigated the effects of 5-hydroxytryptamine (5-HT, serotonin) in striatal cholinergic interneurones with gramicidin-perforated whole-cell patch recordings. Bath-application of serotonin (30 μ m ) significantly and reversibly increased the spontaneous firing rate of 37/45 cholinergic interneurones tested. On average, in the presence of serotonin, firing rate was 273 ± 193% of control. Selective agonists of 5-HT_1A, 5-HT₃, 5-HT₄ and 5-HT₇ receptors did not affect cholinergic interneurone firing, while the 5-HT₂ receptor agonist α-methyl-5-HT (30 μ m ) mimicked the excitatory effects of serotonin. Consistently, the 5-HT₂ receptor antagonist ketanserin (10 μ m ) fully blocked the excitatory effects of serotonin. Two prominent after-hyperpolarizations (AHPs), one of medium duration that was apamin-sensitive and followed individual spikes, and one that was slower and followed trains of spikes, were both strongly and reversibly reduced by serotonin; these effects were fully blocked by ketanserin. Conversely, the depolarizing sags observed during negative current injections and mediated by hyperpolarization-activated cationic currents were not affected. In the presence of apamin and tetrodotoxin, the slow AHP was strongly reduced by 5-HT, and fully abolished by the calcium channel blocker nickel. These results show that 5-HT exerts a powerful excitatory control on cholinergic interneurones via 5-HT₂ receptors, by suppressing the AHPs associated with two distinct calcium-activated potassium currents.     

Metastatic Psammomatous Somatostatinoma of the Pancreas Causing Severe Ketoacidotic Diabetes Cured by Surgery

A case of somatostatin-producing pancreatic tumor associated with severe insulindependent diabetes mellitus and ketoacidotic coma is reported. The tumor, a 10-cm expansile mass arising from the pancreatic tail of a 70-yr-old woman, was first detected by ultrasonography, performed because of abdominal pain, and subsequently confirmed by computed tomography and fine-needle tumor aspiration. Pathologic investigation showed a predominatly solid-trabecular structure with scattered microacini and psammomatous bodies. A large proportion of tumor cells expressed somatostatin and/or calcitonin. Following resection of the primary tumor and three peripancreatic lymph nodes with metastases, the patient recovered rapidly from her diabetic syndrome and remained in substantially good health during a subsequent 8-yr follow-up period, without evidence of tumor recurrence.     

Age-dependent cerebral metabolic effects of unilateral nucleus basalis magnocellularis ablation in rats

To investigate the age-dependent functional importance of cholinergic neocortical inputs, and to explore whether cortical cholinergic denervation in aged animals might better model the cerebral metabolic changes of Alzheimer's disease, the effects of unilateral ablation of the nucleus basalis magnocellularis (NBM) on cerebral glucose metabolism were studied in young and aged rats. Regional cerebral metabolic rates for glucose (rCMRglc) were determined, using the [14C]deoxyglucose method, in 48 brain regions of 3- and 24-month old Fischer-344 rats at 3, 7, 14 and 28 days after stereotaxic injection of ibotenate into the right NBM, and in sham-operated animals at 3 and 14 days later. For both ages the peak effect of unilateral NBM ablation occurred 3 days later: in young rats, rCMRglc was significantly reduced (compared to the contralateral side) in all 24 anterior cortical areas examined (mean decline 20%), whereas in aged animals, only 9 of 24 areas showed a significant decline in glucose utilization, and the magnitude of rCMRglc reduction (9%) was smaller. Near complete recovery of rCMRglc occurred by 7 days in young and old rats. We conclude that the basalocortical cholinergic projection plays a smaller role in neocortical function of aged rats, possibly because its tonic activity is reduced. Both young and aged rats undergo cortical metabolic normalization after unilateral NBM ablation; hence the NBM-lesioned aged rat is not a better model of the progressive decline in rCMRglc that occurs in Alzheimer's disease.     

Behavioral and hormonal responses to stress in the newborn mouse: Effects of maternal deprivation and chlordiazepoxide

These studies investigated behavioral and hormonal responses to stress in developing mice. Experiment 1 examined the effects of 24-hr maternal deprivation on corticosterone (CORT) secretion and ultrasonic vocalization (UVZ) rate in 4-, 8-, and 12-day-old mice. At these ages, exposure to a novel environment resulted in minimal changes in CORT secretion. Maternal deprivation increased pups′ CORT secretion in an age-dependent fashion but did not affect their UVZ rate. The aim of experiment 2 was to test the effects of cholordiazepoxide (CDP), an anxyolytic compound, on CORT secretion and UVZ in both normally reared and in maternally deprived 8-day-old mice. CDP administration elevated CORT increases in deprived (DEP) animals. CDP affected UVZ only in nondcprived (NDEP) animals: UVZ ratewas decreased by high CDP doses Overall, these findings demonstrate that the infant mouse shows a period of stress hyp9oresponsiveness similar to the rat and that maternal presence contributes to inhibit adrenocorticalactivity. CDP administration, butnot novelty exposure, increased CORT secretion in 8-day- old normally reared mice suggesting that during the stress hyporesponsive period, the HPA axis is capable of responding only to specific stimuli. Changes in HPA axis activity and UVZ rateresulting from maternal deprivation and/or CDP challenge do not seem to be directly related. ©1994 John Wiley & Sons, Inc.     

Isolation of mutants of CHO cells resistant to 6(p-hydroxyphenylazo)-uracil I. A novel BrdU cross-resistant phenotype

Three classes of mutants resistant to the drug 6(p-hydroxyphenylazo)-uracil have been isolated from mutagenized cultures of CHO cells. One class of these mutants designated HPU ^R A exhibits a unique form of cross-resistance to bromodeoxyuridine in that it is resistant to this drug only in the presence of thymidine. The molecular basis of the BrdU resistance is unknown but does not appear to involve the known targets of the drug. An interesting feature of these mutants is that they give rise, at a high frequency, to a subpopulation of cells which are much more resistant to BrdU.