Martin A. Wikström

Mechanisms for selection of basic motor programs - roles for the striatum and pallidum

The nervous system contains a toolbox of motor programs in the brainstem and spinal cord--that is, neuronal networks designed to handle the basic motor repertoire required for survival, including locomotion, posture, eye movements, breathing, chewing, swallowing and expression of emotions. The neural mechanisms responsible for selecting which motor program should be recruited at a given instant are the focus of this review. Motor programs are kept under tonic inhibition by GABAergic pallidal neurons (the output nuclei of the basal ganglia). The motor programs can be relieved from pallidal inhibition through activation of striatal neurons at the input stage of the basal ganglia. It is argued that the striatum has a prominent role in selecting which motor program should be called into action.

Transient Calcium and Dopamine Increase PKA Activity and DARPP-32 Phosphorylation

Reinforcement learning theorizes that strengthening of synaptic connections in medium spiny neurons of the striatum occurs when glutamatergic input (from cortex) and dopaminergic input (from substantia nigra) are received simultaneously. Subsequent to learning, medium spiny neurons with strengthened synapses are more likely to fire in response to cortical input alone. This synaptic plasticity is produced by phosphorylation of AMPA receptors, caused by phosphorylation of various signalling molecules. A key signalling molecule is the phosphoprotein DARPP-32, highly expressed in striatal medium spiny neurons. DARPP-32 is regulated by several neurotransmitters through a complex network of intracellular signalling pathways involving cAMP (increased through dopamine stimulation) and calcium (increased through glutamate stimulation). Since DARPP-32 controls several kinases and phosphatases involved in striatal synaptic plasticity, understanding the interactions between cAMP and calcium, in particular the effect of transient stimuli on DARPP-32 phosphorylation, has major implications for understanding reinforcement learning. We developed a computer model of the biochemical reaction pathways involved in the phosphorylation of DARPP-32 on Thr34 and Thr75. Ordinary differential equations describing the biochemical reactions were implemented in a single compartment model using the software XPPAUT. Reaction rate constants were obtained from the biochemical literature. The first set of simulations using sustained elevations of dopamine and calcium produced phosphorylation levels of DARPP-32 similar to that measured experimentally, thereby validating the model. The second set of simulations, using the validated model, showed that transient dopamine elevations increased the phosphorylation of Thr34 as expected, but transient calcium elevations also increased the phosphorylation of Thr34, contrary to what is believed. When transient calcium and dopamine stimuli were paired, PKA activation and Thr34 phosphorylation increased compared with dopamine alone. This result, which is robust to variation in model parameters, supports reinforcement learning theories in which activity-dependent long-term synaptic plasticity requires paired glutamate and dopamine inputs.

The action of 5-HT on calcium-dependent potassium channels and on the spinal locomotor network in lamprey is mediated by 5-HT1A-like receptors.

5-HT has a powerful modulatory action on the firing properties of single neurons as well as on locomotor activity. In lamprey, 5-HT increases the neuronal firing frequency in spinal neurons by reducing the conductance in Ca(2+)-dependent K+ channels (KCa) underlying the slow afterhyperpolarization (sAHP), and it also lowers the burst frequency of the spinal locomotor network. To elucidate which type of 5-HT receptor mediates these effects, different specific receptor agonists and antagonists were applied during intracellular current clamp recordings and during NMDA-induced fictive locomotion in the lamprey spinal cord in vitro preparation. The 5-HT1A receptor agonist 8-OH-DPAT ((+/-)-8-hydroxy-dipropylaminotetralin hydrobromide), the 5-HT1 receptor agonist 5-CT (5-carboxyamidotryptamine maleate) and the 5-HT2 receptor agonist alpha-CH3-5-HT (alpha-methylserotonin maleate) all reproduced the actions of 5-HT at both the cellular and the network levels. The effects of all agonists were completely or partially blocked by the 5-HT1A and 5-HT2 receptor antagonist spiperone (spiroperidol hydrochloride) while selective 5-HT2 receptor antagonists were ineffective. The selective 5-HT1A receptor antagonist S(-)-UH301 (S(-)-5-fluoro-8-hydroxy-dipropylaminotetralin hydrochloride) also counteracted the effect of 5-HT on the sAHP. 5-HT3 and 5-HT4 receptor agonists and antagonists were without effects. The intracellular coupling mechanism was not sensitive to pertussis toxin nor to the cAMP dependent protein kinase blocker (Rp)-cAMPS.(ABSTRACT TRUNCATED AT 250 WORDS)

Bi-directional modulation of AMPA receptor unitary conductance by synaptic activity

BackgroundKnowledge of how synapses alter their efficiency of communication is central to the understanding of learning and memory. The most extensively studied forms of synaptic plasticity are long-term potentiation (LTP) and its counterpart long-term depression (LTD) of AMPA receptor-mediated synaptic transmission. In the CA1 region of the hippocampus, it has been shown that LTP often involves a rapid increase in the unitary conductance of AMPA receptor channels. However, LTP can also occur in the absence of any alteration in AMPA receptor unitary conductance. In the present study we have used whole-cell dendritic recording, failures analysis and non-stationary fluctuation analysis to investigate the mechanism of depotentiation of LTP.ResultsWe find that when LTP involves an increase in unitary conductance, subsequent depotentiation invariably involves the return of unitary conductance to pre-LTP values. In contrast, when LTP does not involve a change in unitary conductance then depotentiation also occurs in the absence of any change in unitary conductance, indicating a reduction in the number of activated receptors as the most likely mechanism.ConclusionsThese data show that unitary conductance can be bi-directionally modified by synaptic activity. Furthermore, there are at least two distinct mechanisms to restore synaptic strength from a potentiated state, which depend upon the mechanism of the previous potentiation.

Age-related changes in electrophysiological properties of the mouse suprachiasmatic nucleus in vitro

Endogenous biological rhythms are altered at several functional levels during aging. The major pacemaker driving biological rhythms in mammals is the suprachiasmatic nucleus of the hypothalamus. In the present study we used tissue slices from young and old mice to analyze the electrophysiological properties of the retinorecipient ventrolateral part of the suprachiasmatic nucleus. Loose patch and whole-cell recordings were performed during day and night. Both young and old mice displayed a significant variation between day and night in the mean firing rate of suprachiasmatic nucleus neurons. The proportion of cells not firing spontaneous action potentials showed a clear day/night rhythm in young but not in old animals, that had an elevated number of such silent cells during the day compared to young animals. Analysis of firing patterns revealed a more regular spontaneous firing during the day than during the night in the old mice, while there was no difference between day and night in young animals. The frequency of spontaneous inhibitory postsynaptic currents was reduced in ventrolateral suprachiasmatic nucleus neurons in the old animals. Since the inhibitory input to these neurons is mainly derived from within the suprachiasmatic nucleus, this reduction most likely reflects the greater proportion of silent cells found in old animals. The results show that the suprachiasmatic nucleus of old mice is subject to marked electrophysiological changes, which may contribute to physiological and behavioral changes associated with aging.

Parallel kinase cascades are involved in the induction of LTP at hippocampal CA1 synapses.

To identify the enzymes involved in the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) at CA1 synapses of two-week-old rats we have tested various kinase inhibitors. Surprisingly, given the large body of evidence supporting a role for calcium/calmodulin-dependent protein kinase II (CaMKII) in LTP, inhibition of this enzyme did not affect the induction of LTP at this age. Similarly inhibition of protein kinase A (PKA) or protein kinase C (PKC) was also without effect. However, inhibition of CaMKII together with inhibition of either PKA or PKC fully blocked the induction of LTP. These experiments reveal, unexpectedly, the existence of two parallel kinase pathways, one involving CaMKII and the other PKA and PKC, either of which can fully support the induction of LTP, at this stage of development.

An Ancient Duplication of Exon 5 in the Snap25 Gene Is Required for Complex Neuronal Development/Function

Alternative splicing is an evolutionary innovation to create functionally diverse proteins from a limited number of genes. SNAP-25 plays a central role in neuroexocytosis by bridging synaptic vesicles to the plasma membrane during regulated exocytosis. The SNAP-25 polypeptide is encoded by a single copy gene, but in higher vertebrates a duplication of exon 5 has resulted in two mutually exclusive splice variants, SNAP-25a and SNAP-25b. To address a potential physiological difference between the two SNAP-25 proteins, we generated gene targeted SNAP-25b deficient mouse mutants by replacing the SNAP-25b specific exon with a second SNAP-25a equivalent. Elimination of SNAP-25b expression resulted in developmental defects, spontaneous seizures, and impaired short-term synaptic plasticity. In adult mutants, morphological changes in hippocampus and drastically altered neuropeptide expression were accompanied by severe impairment of spatial learning. We conclude that the ancient exon duplication in the Snap25 gene provides additional SNAP-25-function required for complex neuronal processes in higher eukaryotes.

Endogenous dopaminergic modulation of the lamprey spinal locomotor network.

The lamprey spinal cord contains three dopaminergic systems. The most extensive is the ventromedial plexus in which dopamine is co-localized with 5-HT and tachykinins. In this study we have investigated the effects of endogenously released dopamine on NMDA-induced spinal activity, and for comparison applied dopamine exogenously. The dopamine reuptake blocker bupropion increases the levels of extracellular dopamine in the spinal cord. Bath application of bupropion during ongoing NMDA-induced network activity (around 2 Hz) resulted in an initial increase of the burst rate followed by a transitional phase with the fast rhythm superimposed on a much slower ventral root burst activity (below 0.25 Hz). Finally only the slow rhythm was observed. The same response pattern with regard to the fast and slow rhythms was observed when dopamine was slowly perfused over the spinal cord, resulting in a gradual build-up of dopamine concentration. At low constant dopamine concentrations, however, an increased burst frequency was maintained, but at somewhat higher concentrations the fast burst rate instead was decreased. The degree of modulation of fictive locomotion by dopamine was also tested at low and high NMDA concentrations. Dopamine was found to exert stronger effects at low NMDA concentrations. With high NMDA concentrations dopamine did not induce the transition phase or the slow ventral root bursting. The slow alternating ventral root bursts, induced by bupropion, shifted to synchronized activity when glycinergic synaptic transmission was blocked with strychnine, testifying that the alternation depended on a crossed glycinergic action as previously shown for the fast rhythm.

Endogenous and exogenous dopamine presynaptically inhibits glutamatergic reticulospinal transmission via an action of D2‐receptors on N‐type Ca2+ channels

In this study, the effects of exogenously applied and endogenously released dopamine (DA), a powerful modulator of the lamprey locomotor network, are examined on excitatory glutamatergic synaptic transmission between reticulospinal axons and spinal neurons. Bath application of DA (1–50 µm) reduced the amplitude of monosynaptic reticulospinal‐evoked glutamatergic excitatory postsynaptic potentials (EPSPs). The effect of DA was blocked by the D2‐receptor antagonist eticlopride, and mimicked by the selective D2‐receptor agonist 2,10,11 trihydroxy‐N‐propyl‐noraporphine hydrobromide (TNPA). Bath application of the DA reuptake blocker bupropion, which increases the extracellular level of dopamine, also reduced the monosynaptic EPSP amplitude. This effect was also blocked by the D2‐receptor antagonist eticlopride. To investigate if the action of DA was exerted at the presynaptic level, the reticulospinal axon action potentials were prolonged by administering K+ channel antagonists while blocking l‐type Ca2+ channels. A remaining Ca2+ component, mainly dependent on N and P/Q channels, was depressed by DA. When DA (25–50 µm) was applied in the presence of ω‐conotoxin GVIA, a toxin specific for N‐type Ca2+ channels, it failed to affect the monosynaptic EPSP amplitude. DA did not affect the response to extracellularly ejected d‐glutamate, the postsynaptic membrane potential, or the electrical component of the EPSPs. DA thus acts at the presynaptic level to modulate reticulospinal transmission.

Mathematical modelling of non‐stationary fluctuation analysis for studying channel properties of synaptic AMPA receptors

1 The molecular properties of synaptic α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) receptors are an important factor determining excitatory synaptic transmission in the brain. Changes in the number (N) or single‐channel conductance (γ) of functional AMPA receptors may underlie synaptic plasticity, such as long‐term potentiation (LTP) and long‐term depression (LTD). These parameters have been estimated using non‐stationary fluctuation analysis (NSFA). 2 The validity of NSFA for studying the channel properties of synaptic AMPA receptors was assessed using a cable model with dendritic spines and a microscopic kinetic description of AMPA receptors. Electrotonic, geometric and kinetic parameters were altered in order to determine their effects on estimates of the underlying γ. 3 Estimates of γ were very sensitive to the access resistance of the recording (RA) and the mean open time of AMPA channels. Estimates of γ were less sensitive to the distance between the electrode and the synaptic site, the electrotonic properties of dendritic structures, recording electrode capacitance and background noise. Estimates of γ were insensitive to changes in spine morphology, synaptic glutamate concentration and the peak open probability (Po) of AMPA receptors. 4 The results obtained using the model agree with biological data, obtained from 91 dendritic recordings from rat CA1 pyramidal cells. A correlation analysis showed that RA resulted in a slowing of the decay time constant of excitatory postsynaptic currents (EPSCs) by approximately 150 %, from an estimated value of 3.1 ms. RA also greatly attenuated the absolute estimate of γ by approximately 50‐70 %. 5 When other parameters remain constant, the model demonstrates that NSFA of dendritic recordings can readily discriminate between changes in γvs. changes in N or Po. Neither background noise nor asynchronous activation of multiple synapses prevented reliable discrimination between changes in γ and changes in either N or Po. 6 The model (available online) can be used to predict how changes in the different properties of AMPA receptors may influence synaptic transmission and plasticity.