Frequency dependent activation of a slow N-methyl-D-aspartate-dependent excitatory postsynaptic potential in turtle cerebellum by mossy fibre afferents

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Direct neural current imaging in an intact cerebellum with magnetic resonance imaging

The ability to detect neuronal currents with high spatiotemporal resolution using magnetic resonance imaging (MRI) is important for studying human brain function in both health and disease. While significant progress has been made, we still lack evidence showing that it is possible to measure an MR signal time-locked to neuronal currents with a temporal waveform matching concurrently recorded local field potentials (LFPs). Also lacking is evidence that such MR data can be used to image current distribution in active tissue. Since these two results are lacking even in vitro, we obtained these data in an intact isolated whole cerebellum of turtle during slow neuronal activity mediated by metabotropic glutamate receptors using a gradient-echo EPI sequence (TR=100ms) at 4.7T. Our results show that it is possible (1) to reliably detect an MR phase shift time course matching that of the concurrently measured LFP evoked by stimulation of a cerebellar peduncle, (2) to detect the signal in single voxels of 0.1mm(3), (3) to determine the spatial phase map matching the magnetic field distribution predicted by the LFP map, (4) to estimate the distribution of neuronal current in the active tissue from a group-average phase map, and (5) to provide a quantitatively accurate theoretical account of the measured phase shifts. The peak values of the detected MR phase shifts were 0.27-0.37°, corresponding to local magnetic field changes of 0.67-0.93nT (for TE=26ms). Our work provides an empirical basis for future extensions to in vivo imaging of neuronal currents.

A computational mechanism for unified gain and timing control in the cerebellum

Precise gain and timing control is the goal of cerebellar motor learning. Because the basic neural circuitry of the cerebellum is homogeneous throughout the cerebellar cortex, a single computational mechanism may be used for simultaneous gain and timing control. Although many computational models of the cerebellum have been proposed for either gain or timing control, few models have aimed to unify them. In this paper, we hypothesize that gain and timing control can be unified by learning of the complete waveform of the desired movement profile instructed by climbing fiber signals. To justify our hypothesis, we adopted a large-scale spiking network model of the cerebellum, which was originally developed for cerebellar timing mechanisms to explain the experimental data of Pavlovian delay eyeblink conditioning, to the gain adaptation of optokinetic response (OKR) eye movements. By conducting large-scale computer simulations, we could reproduce some features of OKR adaptation, such as the learning-related change of simple spike firing of model Purkinje cells and vestibular nuclear neurons, simulated gain increase, and frequency-dependent gain increase. These results suggest that the cerebellum may use a single computational mechanism to control gain and timing simultaneously.

Computational models of timing mechanisms in the cerebellar granular layer

A long-standing question in neuroscience is how the brain controls movement that requires precisely timed muscle activations. Studies using Pavlovian delay eyeblink conditioning provide good insight into this question. In delay eyeblink conditioning, which is believed to involve the cerebellum, a subject learns an interstimulus interval (ISI) between the onsets of a conditioned stimulus (CS) such as a tone and an unconditioned stimulus such as an airpuff to the eye. After a conditioning phase, the subject’s eyes automatically close or blink when the ISI time has passed after CS onset. This timing information is thought to be represented in some way in the cerebellum. Several computational models of the cerebellum have been proposed to explain the mechanisms of time representation, and they commonly point to the granular layer network. This article will review these computational models and discuss the possible computational power of the cerebellum.

Trace eyeblink conditioning in decerebrate guinea pigs

We investigated the trace eyeblink conditioning in decerebrate guinea pigs to elucidate the possible role of the cerebellum and brainstem in this hippocampus-dependent task. A 350-ms tone conditioned stimulus was paired with a 100-ms periorbital shock unconditioned stimulus with a trace interval of either 0, 100, 250 or 500 ms. Decerebrate animals readily acquired the conditioned response with a trace interval of 0 or 100 ms. Even in the paradigm with a 500-ms trace interval, which is known to depend critically on the hippocampus in all animal species examined, the decerebrate guinea pigs acquired the conditioned response, which had adaptive timing as well as in the other paradigms with a shorter trace interval. However, it took many more trials to learn in the 500-ms trace paradigm than in the shorter trace interval paradigms, and the conditioned response expression was unstable from trial to trial. When decerebrate animals were conditioned step by step with a trace interval of 100, 250 and 500 ms, sequentially, they easily acquired the adaptive conditioned response to a 500-ms trace interval. However, the frequency of conditioned responses decreased after the trace interval was shifted from 250 ms to 500 ms, which was not observed after the shift from 100 ms to 250 ms. These results suggest that the cerebellum and brainstem could maintain the 'trace' of the conditioned stimulus and associate it with the unconditioned stimulus even in the 500-ms trace paradigm, but that the forebrain might be required for facilitating and stabilizing the association.

Short-lasting conditioned stimulus applied to the middle cerebellar peduncle elicits delayed conditioned eye blink responses in the decerebrate ferret

In delay eye blink conditioning, the conditioned stimulus (CS) ends at the time of the unconditioned stimulus (US). If the CS duration is decreased, there will be a 'trace' period with no ongoing CS before the onset of the US. During this period some neural activity has to continue after the CS offset to: (i) permit association between the CS and the US; and (ii) elicit a conditioned response appearing after the CS offset. In this study we test the role of the cerebellum in maintaining CS activity required for eliciting a conditioned response after the CS offset. Decerebrate ferrets were trained in a delay conditioning paradigm with an electrical stimulation of the forelimb as CS and of the periorbital area as US. The conditioned responses in the upper eyelid were monitored with electromyographical techniques. In well-trained animals, test CSs of short duration down to 0.2 ms were applied to the forelimb or the middle cerebellar peduncle, while the interstimulus interval between CS onset and US onset was kept constant at 300 ms. Test CSs of short duration applied to the forelimb elicited conditioned responses. More importantly, also a short-lasting CS to the middle cerebellar peduncle could elicit conditioned responses. The results indicate that precerebellar CS pathways are not required for maintaining the neural activity that elicits conditioned responses after the CS offset. It is suggested that neurons maintaining such activity are located in the cerebellum, either the cortex alone or the cortex and the deep nuclei.

Motor discoordination results from combined gene disruption of the NMDA receptor NR2A and NR2C subunits, but not from single disruption of the NR2A or NR2C subunit

NMDA receptors consist of two distinct classes of subunits. The NR1 subunit possesses all properties of the NMDA receptor-channel complex, whereas four NR2 subunits (NR2A-2D) potentiate and differentiate NMDA receptor responses by heteromeric assemblies with NR1. The mRNAs for the five NMDA receptor subunits are expressed in the cerebellum in a distinct temporospatial manner. To study functions of the NMDA receptors in the cerebellum, we generated knockout mice deficient in either NR2A or NR2C or both of these subunits. All three mutant mice developed normally and showed normal overall morphology of the cerebellum. The NMDA receptor-mediated components of EPSCs in granule cells, as assessed by whole-cell recordings of cerebellar slices, were reduced in NR2A- and NR2C-deficient mice and nearly abolished in mice lacking both NR2A and NR2C. The NR2A- and NR2C-deficient granule cells were different in the current-voltage relationship and time course of NMDA receptor responses. The NR2A and NR2C subunits thus contribute to distinct NMDA receptor-mediated excitatory transmission in mossy fiber-granule cell synapses in the mature cerebellum. Both NR2A- and NR2C-deficient mice showed no impaired movements in the motor coordination tasks tested. The mutant mice deficient in both NR2A and NR2C could also manage simple coordinated tasks, such as staying on a stationary or a slowly rotating rod, but failed more challenging tasks such as staying on a quickly rotating rod. These data demonstrate that the NMDA receptors play an active role in motor coordination.

Impaired cerebellar synaptic plasticity and motor performance in mice lacking the mGluR4 subtype of metabotropic glutamate receptor

The application of the glutamate analog L-2-amino-4-phosphonobutyric acid (L-AP4) to neurons produces a suppression of synaptic transmission. Although L-AP4 is a selective ligand at a subset of metabotropic glutamate receptors (mGluRs), the precise physiological role of the L-AP4-activated mGluRs remains primarily unknown. To provide a better understanding of the function of L-AP4 receptors, we have generated and studied knockout (KO) mice lacking the mGluR4 subtype of mGluR that displays high affinity for L-AP4. The mGluR4 mutant mice displayed normal spontaneous motor activity and were unimpaired on the bar cross test, indicating that disruption of the mGluR4 gene did not cause gross motor abnormalities, impairments of novelty-induced exploratory behaviors, or alterations in fine motor coordination. However, the mutant mice were deficient on the rotating rod motor-learning test, suggesting that mGluR4 KO mice may have an impaired ability to learn complex motor tasks. Patch-clamp and extracellular field recordings from Purkinje cells in cerebellar slices demonstrated that L-AP4 had no effect on synaptic responses in the mutant mice, whereas in the wild-type mice 100 microM L-AP4 produced a 23% depression of synaptic responses with an EC50 of 2.5 microM. An analysis of presynaptic short-term synaptic plasticity at the parallel fiber-->Purkinje cell synapse demonstrated that paired-pulse facilitation and post-tetanic potentiation were impaired in the mutant mice. In contrast, long-term depression (LTD) was not impaired. These results indicate that an important function of mGluR4 is to provide a presynaptic mechanism for maintaining synaptic efficacy during repetitive activation. The data also suggest that the presence of mGluR4 at the parallel fiber-->Purkinje cell synapse is required for maintaining normal motor function.

N-methyl-D-aspartate receptors mediate a slow excitatory postsynaptic potential in the rat midbrain dopaminergic neurons

Repetitive local application of a short train of stimuli to the rat substantia nigra and ventral tegmental area elicited a predominant depolarizing, slow, long-lasting synaptic response in the dopaminergic cells intracellularly recorded in vitro. This slow excitatory postsynaptic potential ranged between 13 and 27 mV at holding potentials of about-75 mV and lasted for 0.2-6 s. It was not greatly affected by the perfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (10-20 microM), while it was potentiated in the presence of bicuculline methiodide (30 microM) or picrotoxin (50-100 microM) and 2-hydroxysaclofen (100-300 microM). In contrast, a substantial component of the slow excitatory postsynaptic potential was reversibly depressed, in a concentration-dependent manner, by the application of the N-methyl-D-aspartate receptor antagonists D,1-2-amino-5-phosphonovalerate (10-100 microM). Furthermore, the slow excitatory postsynaptic potential was reversibly increased by the superfusion of nominally magnesium-free solution. It was graded, increasing in amplitude with increased stimulus intensity, and was blocked by tetrodotoxin (0.5 microM). We suggest that a sustained activation of synaptic terminals containing excitatory amino acids mediates a slow excitatory postsynaptic potential in the dopaminergic cells of the midbrain. N-Methyl-D-aspartate receptors participate in the generation of this slow potential, while the alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate/kainate receptors do not seem to contribute substantially to this potential. This N-methyl-D-aspartate-mediated synaptic event could be implicated in the release of dopamine as well as in the excitotoxic injury of the dopaminergic neurons.

Identification of central 5-HT and 5-HT1A receptors in the turtle brain (Chrysemys picta)

Radiologand binding studies were undertaken in the turtle whole brain, cerebellum and raphe using the selective radioligands [3H]5-hydroxytryptamine trifluoroacetate ([3H]5-HT) amd [3H]+/-8-hydroxy-2-(di-N-propylamino) tetralin hydrobromide ([3H]DPAT) to identify serotonin (5-HT) receptors and the specific 5-HT1A receptor subtype. Scatchard analysis identified a nanomolar affinity binding site for [3H]5-HT (12 nM) in turtle whole brain assays. A low affinity 5-HT1A site (102 nM) was also identified in turtle whole brain assays, with a higher affinity site noted in binding studies performed with tissue from the inferior raphe (20 nM). The difference in affinity for 5-HT receptors in reptilian versus mammalian brain may prove characteristic of lower vertebrate brains with implications for the physiologic effects of this neurotransmitter in the central nervous system.