Understanding mental retardation in Down's syndrome using trisomy 16 mouse models

Mental retardation in Down's syndrome, human trisomy 21, is characterized by developmental delays, language and memory deficits and other cognitive abnormalities. Neurophysiological and functional information is needed to understand the mechanisms of mental retardation in Down's syndrome. The trisomy mouse models provide windows into the molecular and developmental effects associated with abnormal chromosome numbers. The distal segment of mouse chromosome 16 is homologous to nearly the entire long arm of human chromosome 21. Therefore, mice with full or segmental trisomy 16 (Ts65Dn) are considered reliable animal models of Down's syndrome. Ts65Dn mice demonstrate impaired learning in spatial tests and abnormalities in hippocampal synaptic plasticity. We hypothesize that the physiological impairments in the Ts65Dn mouse hippocampus can model the suboptimal brain function occuring at various levels of Down's syndrome brain hierarchy, starting at a single neuron, and then affecting simple and complex neuronal networks. Once these elements create the gross brain structure, their dysfunctional activity cannot be overcome by extensive plasticity and redundancy, and therefore, at the end of the maturation period the mind inside this brain remains deficient and delayed in its capabilities. The complicated interactions that govern this aberrant developmental process cannot be rescued through existing compensatory mechanisms. In summary, overexpression of genes from chromosome 21 shifts biological homeostasis in the Down's syndrome brain to a new less functional state.

Increased expression of NR2A subunit does not alter NMDA-evoked responses in cultured fetal trisomy 16 mouse hippocampal neurons

The trisomy 16 (Ts16) mouse is an animal model for human trisomy 21 (Down's syndrome). The gene encoding the NR2A subunit of the NMDA receptor has been localized to mouse chromosome 16. In the present study, western blot analysis revealed a 2.5-fold increase of NR2A expression in cultured Ts16 embryonic hippocampal neurons. However, this increase did not affect the properties of NMDA-evoked currents in response to various modulators. The sensitivity of NMDA receptors to transient applications of NMDA, spermine, and Zn(2+) was investigated in murine Ts16 and control diploid cultured embryonic hippocampal neurons. Peak and steady-state currents evoked by NMDA were potentiated by spermine at concentrations < 1 mM, and inhibited by Zn(2+) in a dose-dependent and voltage-independent manner. No marked difference was observed between Ts16 and control diploid neurons for any of these modulators with regard to IC(50) and EC(50) values or voltage dependency. Additionally, inhibition by the NR2B selective inhibitor, ifenprodil, was similar. These results demonstrate that NMDA-evoked currents are not altered in cultured embryonic Ts16 neurons and suggest that Ts16 neurons contain similar functional properties of NMDA receptors as diploid control neurons despite an increased level of NR2A expression.

Ts65Dn mouse, a Down syndrome model, exhibits elevated myo-inositol in selected brain regions and peripheral tissues

myo-Inositol is elevated in the Down syndrome (DS; trisomy 21) brain and may play a role in mental retardation. In the present study, we examined brain regions and peripheral tissues of Ts65Dn mouse, a recently characterized genetic model of DS, for abnormal myo-inositol accumulation. A GC/MS technique was used to quantitate myo-inositol and other polyol species (ribitol, arabitol, xylitol, and 1,5-anhydrosorbitol) in tissues from the Ts65Dn mice and control diploid mice. myo-Inositol was found to be elevated in frontal cortex, hippocampus, and brain stem but not in cerebellum of the Ts65Dn mouse. Among peripheral organs examined, liver and skeletal muscle were found to excessively accumulate myo-inositol. In all tissues, concentrations of polyol internal controls were normal. The Ts65Dn mouse is useful to study the possible effect of elevated myo-inositol on cellular processes.

Increased synaptic depression in the Ts65Dn mouse, a model for mental retardation in Down syndrome

Long-term potentiation (LTP) and depression (LTD) were investigated in hippocampus of a genetic model of Down syndrome, the segmental trisomy (Ts65Dn) mouse. Field excitatory postsynaptic potentials were recorded from hippocampal slices and LTP and LTD evoked sequentially. LTP decreased whereas LTD increased significantly in Ts65Dn compared with control hippocampus.

Altered long-term potentiation in the young and old Ts65Dn mouse, a model for Down Syndrome

We investigated the phenomenon of long-term potentiation (LTP) in a genetic model of Down Syndrome, the segmental trisomy mouse (Ts65Dn). Ts65Dn mice survive to adulthood and have an extra chromosome that contains a segment of chromosome 16 homologous to human chromosome 21. In this study, field excitatory postsynaptic potentials (fEPSP) were recorded from the CA1 area of in vitro hippocampal slices from diploid and Ts65Dn mice, and LTP was induced by a single tetanizing pulse train (1 sec in duration) at 100 Hz. The hippocampus from both young (2 months) and older (9 months) Ts65Dn mice had a reduced LTP over a period of 60 min compared with LTP in age-matched controls. This finding may explain the reported behavioral and learning impairments in Ts65Dn mice; it suggests that this mouse model can be used to study the role of altered synaptic plasticity in mental retardation of Down Syndrome.

Responses to NMDA in cultured hippocampal neurons from trisomy 16 embryonic mice

The trisomy 16 (Ts16) mouse is regarded as a model of human trisomy 21 (Ts21), or Down syndrome. The ionic current evoked by the glutamate receptor agonist N-methyl-D-aspartate (NMDA) was investigated in cultured hippocampal neurons from embryonic Ts16 and control diploid mice. In both Ts16 and control neurons, NMDA- (6-150 microM) evoked a similar inward current. The reversal potential, the minimum current, the dose response plot of the conductance, the effect of Mg2+ on the current-voltage plot and the inhibition by D-2-amino-5-phosphonovaleric acid (AP5; 50 microM) showed no significant difference between Ts16 and control neurons. These data suggest that, although voltage-dependent ion channels are reported to have altered active properties in Ts16 neurons, NMDA-evoked currents are not altered.

Serotoninergic modulation of excitability in area CA1 of the in vitro rat hippocampus

Intra- and extracellular recordings from the in vitro rat hippocampal slice preparation have been used to investigate the influence of serotoninergic, adrenergic and cholinergic receptor antagonists on the excitability of CA1 pyramidal neurones. The serotonin receptor antagonist 4-amino-N-(1-azabicyclo[2.2.2]oct-3yl)-5-chloro-2- methoxybenzamide(E)-2-butenedioate (zacopride, 100 microM) produced multiple population spikes on the orthodromically evoked field potential, in contrast to the lack of effect of another serotonin antagonist 1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate (MDL 72222, 30 microM), as well as the cholinergic antagonists atropine (10 microM) and hexamethonium (100 microM) and the noradrenergic antagonist atenolol (10 microM). Monosynaptic inhibitory postsynaptic potentials (IPSPs) recorded in the presence of the glutamatergic antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and ketamine (50 microM) were recorded from CA1 pyramidal neurones. Zacopride (100 microM) and MDL 72222 (30 microM) both reduced the isolated IPSP to 54 +/- 9% (n = 8) and 78 +/- 4% (n = 3), respectively. Neither of the cholinergic antagonists had any effect, while atenolol reduced the IPSP to 87 +/- 3% (n = 7) of the control IPSP. We propose that the difference in action of zacopride and MDL 72222 on the field potentials is due to zacopride activating postsynaptic 5HT4 receptors on the pyramidal neurone, thereby reducing a Ca(2+)-activated K(+)-conductance. This, in combination with a 5HT3 receptor-mediated reduction in gamma-aminobutyric acid (GABA)-ergic inhibition, leads to an increase in pyramidal cell excitability evident as epileptic field potentials.

The effects of central myorelaxants on synaptically-evoked primary afferent depolarization in the immature rat spinal cord in vitro

1. In the immature rat in vitro hemisected spinal cord preparation the dorsal root-evoked depolarizing potential recorded from an adjacent dorsal root DR-DRP had a mean peak amplitude (+/- s.e.mean, n = 27) of 2.9 +/- 0.2 mV and a mean latency to peak amplitude of 106 +/- 3 ms. The DR-DRP amplitude was maximal with a stimulus intensity of four times the threshold intensity required to activate the lowest threshold fibres. The peak amplitude and/or integral over a time-source of 0.5 s were used to assess the effects of applied drugs. 2. The DR-DRP was abolished by baclofen (mean IC50 190 +/- 46 nM, n = 7). The depressant effect of baclofen was reversed by CGP35348 (1 mM). The mean apparent Kd value calculated from dose-ratios was 16.7 +/- 6.4 microM (n = 3). 3. At a maximally effective concentration, tizanidine (1 microM) produced at the most only a 14% depression of the DR-DRP (n = 4). Clonidine (0.3 microM) had an effect similar to that of tizanidine. These depressant effects were reversed by idazoxan (1 microM). 4. The DR-DRP was potentiated by diazepam in a flumazenil (1 microM)-reversible manner. A maximal potentiation of 23.2 +/- 2.7% (n = 5) was produced by 1 microM diazepam. 5. Diazepam (1 microM) induced a mean bicuculline- (10 microM, n = 2) and flumazenil- (1 microM, n = 8) sensitive depolarization in the dorsal root of 0.25 +/- 0.03 mV (n = 8). However, diazepam failed to depolarize dorsal roots (n = 3) which had been excised from the spinal cord. 6. Comparison of the above effects with previously reported depressant effects of these drugs on the synaptic output from ventral roots suggests that actions on presynaptic inhibition, as reflected in the DR-DRP, are of subsidiary importance in explaining the muscle relaxant actions of tizanidine or diazepam.

The effect of centrally acting myorelaxants on NMDA receptor-mediated synaptic transmission in the immature rat spinal cord in vitro

1. The effect of the myorelaxant drugs baclofen, diazepam and tizanidine have been compared on in vitro preparations of baby rat spinal cord and adult rat superior cervical ganglion. 2. Dorsal root-elicited long duration (time to half decay 9.71 +/- 0.29 s.e. mean, n = 31) ipsilateral ventral root reflexes (DR-VRP), measured as integrated area, of immature rat spinal cord preparations were abolished by RS-2-amino-5-phosphonopentanoate (AP5) (EC50 8.13 +/- 0.92 microM, n = 3). The initial short latency component of DR-VRP was resistant to AP5. 3. Baclofen abolished both components of the DR-VRP. Respective EC50 values for the AP5-insensitive and AP5-sensitive components were 237 +/- 68 nM (n +/- 7) and 57 +/- 10 nM (n = 7). These effects of baclofen were reversed by the GABAB antagonist, CGP35348. The apparent Kd values (16.7 +/- 6.4 microM, n = 3 and 14.3 +/- 3.9 microM, n = 6 respectively) for this reversal were not significantly different. 4. Tizanidine, clonidine and diazepam had no effect on the AP5-insensitive component of the DR-VRP. 5. The AP5-sensitive long duration component of the DR-VRP was depressed to respective maximal levels of 23.2 +/- 1.4% (n = 7), 18.8 +/- 3.8% (n = 4) and 47.6 +/- 1.6% (n = 5) of control (100%) levels by tizanidine (EC50 135 +/- 33 nM), clonidine (EC50 26.0 +/- 2.2 nM) and diazepam (EC25 114 +/- 12 nM, n = 4). The depressant effects of tizanidine and clonidine were reversed by idazoxan (1 microM). Flumazenil (I microM) failed to reverse the depressant effect of tizanidine. The depressant effect of diazepam was reversed by flumazenil (1 microM) but not by idazoxan (1 microM). Naloxone 1 M did not reverse the effects of either tizanidine or diazepam.6. In the presence of tetrodotoxin (0.1 SAM) which abolished synaptic activity, clonidine, tizanidine or diazepam (10, 100 and 101JM respectively) produced no significant antagonism of NMDA-induced depolarizations recorded from ventral roots.7. Control (100%) synaptic responses of rat superior cervical ganglion preparations were depressed respectively to near maximal levels of 60.0 +/- 5.2% (n = 4) and 60.7 +/- 5.6% (n = 5) by clonidine (0.5 JAM,EC25 15.3 +/- 3.0 nM) and tizanidine (1 JAM, EC25 227 +/- 83 nM). These depressant effects were reversed by idazoxan (1 AM). Baclofen (EC25 28.7 +/- 10.0, n = 3) depressed the postganglionic response to a maximum level of 71.8 + 2.4% (n = 4) control at a concentration of 100 microM. The latter depressant action was reversed by the GABAB receptor antagonist, CGP35348 (1 mM). Diazepam (1 microM) had no significant effect on ganglionic transmission.8. It is concluded that the activation of benzodiazepine or M2-noradrenaline receptors can modulate NMDA receptor-mediated excitatory synaptic pathways whereas synaptic excitation from primary afferent terminals, mediated by non-NMDA receptors, appears to lack the propensity for this type of modulation. The results show also that the isolated spinal preparation can be used to identify central myorelaxant actions that are mediated through functional benzodiazepine or X2-noradrenaline receptors.

Chronic dihydropyridine treatment can reverse the behavioural consequences of and prevent adaptations to, chronic ethanol treatment

1. Chronic treatment with the dihydropyridine calcium channel antagonist, nitrendipine, given concurrently with ethanol, prevented the ethanol withdrawal syndrome in mice, even though the chronic nitrendipine treatment was stopped 24 h or 48 h before the withdrawal testing. 2. This effect was seen in two strains of mice with different methods of ethanol administration. Nitrendipine was effective when given for two weeks but not after only two days' treatment. 3. Two other dihydropyridine calcium antagonists, nimodipine and PN 200-110, given chronically with ethanol, also prevented the withdrawal syndrome. The tests were again made 24 h after the last administration of dihydropyridine. 4. The chronic nitrendipine treatment also prevented the rise in the number of central dihydropyridine binding sites that occurs on chronic ethanol administration. 5. Chronic administration of nitrendipine alone did not cause any withdrawal behaviour. 6. Chronic nitrendipine treatment did not affect the seizure threshold to bicuculline in mice that were not given ethanol. 7. Whole brain concentration measurements showed that the effects were not due to residual nitrendipine in the CNS at the time of withdrawal testing or to differences in central ethanol concentrations during the treatment. 8. It is suggested that the results provide evidence for a functional role for dihydropyridine-sensitive calcium channels in ethanol dependence.

Potentiation of synaptic reflexes by D-serine in the rat spinal cord in vitro

6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) depressed dorsal root-evoked ventral and dorsal root potentials of the in vitro immature rat spinal cord to 26.3 +/- 5.2 S.E.M. and 40.8 +/- 2.7% of control values respectively. These depressant effects of CNQX were partially reversed by D-serine (EC50 values 39.7 microM +/- 8.7 S.E.M. N = 6 and 34.9 +/- 12.5 microM, N = 5 for ventral root potential and dorsal root potential respectively). Under our experimental conditions, which included the presence of Mg2+ (0.75 mM) in the bathing medium, no measurable potentiation of these synaptic reflexes by D-serine was recorded in the absence of CNQX. These data indicate that CNQX, in addition to its depressant effect at non-NMDA receptors, depresses an NMDA receptor-mediated component of segmental transmission through its action at the glycine site of the NMDA receptor complex.

Nitrendipine decreases benzodiazepine withdrawal seizures but not the development of benzodiazepine tolerance or withdrawal signs

1. The effects of the calcium channel blocking agent, nitrendipine, were studied on seizures in mice produced during withdrawal from chronic benzodiazepine treatment and on the development of tolerance to benzodiazepines. 2. Nitrendipine produced a dose-dependent decrease in seizure incidence, when seizures were produced by the partial inverse agonist FG7142 during withdrawal from seven days treatment with flurazepam. 3. Nitrendipine did not raise the seizure thresholds in naïve mice to the full inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), or to the gamma-aminobutyric acid (GABA) antagonist, bicuculline. 4. When given concurrently with flurazepam for seven days, nitrendipine did not affect the incidence of seizures during flurazepam withdrawal. 5. When given concurrently with the benzodiazepines, nitrendipine did not prevent the development of tolerance to midazolam general anaesthesia or tolerance to the ataxic actions of flurazepam or midazolam. 6. Chronic treatment with flurazepam for seven days did not affect the Kd or Bmax of [3H]-nimodipine binding in mouse whole brain or cerebral cortex. 7. These results with benzodiazepines are partially in contrast with those for ethanol, where nitrendipine not only decreased ethanol withdrawal seizures when given acutely, but also prevented the development of tolerance and withdrawal signs when given concurrently with ethanol. However, they do confirm the selectivity of nitrendipine for withdrawal-induced seizures.

Effect of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) on dorsal root-, NMDA-, kainate- and quisqualate-mediated depolarization of rat motoneurones in vitro

1. Mature in vitro rat spinal cord preparations have been used to compare the depressant effects of 6-cyano-2,3-dihydroxy-7-nitroquinoxalinedione (CNQX) and kynurenate on transmission from low threshold myelinated primary afferents in dorsal roots. EC50 values +/- s.e.mean (number of preparations in parentheses) for depression of the monosynaptic ventral root reflex were respectively 1.0 +/- 0.3 microM (5) and 135 +/- 15 microM (3) for CNQX and kynurenate. Transmission through superior cervical ganglia was not significantly affected by concentrations of CNQX up to 100 microM or kynurenate up to 5 mM. 2. Immature in vitro rat spinal cord preparations were used to measure dose-ratios for antagonism of depolarizations induced by N-methyl-D-aspartate (NMDA), kainate or quisqualate by 4, 10 and 25 microM CNQX. In the presence of 0.75 mM Mg2+ pA2 values +/- s.e.mean were respectively 4.62 +/- 0.05 (16), 5.79 +/- 0.01 (4) and 5.59 +/- 0.05 (16) for each agonist. These values were not significantly altered in the absence of added Mg2+. The mean pA2 values for kainate were significantly higher than those for quisqualate (P less than 0.01). 3. Antagonism of NMDA-induced depolarizations was evident at 10 and 25 but not 4 microM CNQX. The antagonism of NMDA was reversed by D-serine (100 and 200 microM). 4. A similarity between the relative potencies of both CNQX and kynurenate for depression of synaptic transmission and antagonism of amino acid-induced depolarizations indicates that monosynaptic transmission from myelinated primary afferents to motoneurones is mediated by kainate and/or quisqualate sub-types of non-NMDA receptors.