Membrane resistance increase induced in thalamic neurons by stimulation of brainstem cholinergic afferents

Laser-Induced Apoptosis of Corticothalamic Neurons in Layer VI of Auditory Cortex Impact on Cortical Frequency Processing

Corticofugal projections outnumber subcortical input projections by far. However, the specific role for signal processing of corticofugal feedback is still less well understood in comparisonto the feedforward projection. Here, we lesioned corticothalamic (CT) neurons in layers V and/or VI of the auditory cortex of Mongolian gerbils by laser-induced photolysis to investigate their contribution to cortical activation patterns. We have used laminar current-source density (CSD) recordings of tone-evoked responses and could show that, particularly, lesion of CT neurons in layer VI affected cortical frequency processing. Specifically, we found a decreased gain of best-frequency input in thalamocortical (TC)-recipient input layers that correlated with the relative lesion of layer VI neurons, but not layer V neurons. Using cortical silencing with the GABA _a -agonist muscimol and layer-specific intracortical microstimulation (ICMS), we found that direct activation of infragranular layers recruited a local recurrent cortico-thalamo-cortical loop of synaptic input. This recurrent feedback was also only interrupted when lesioning layer VI neurons, but not cells in layer V. Our study thereby shows distinct roles of these two types of CT neurons suggesting a particular impact of CT feedback from layer VI to affect the local feedforward frequency processing in auditory cortex.

Neuropeptides and Neurotransmitters That Modulate Thalamo-Cortical Pathways Relevant to Migraine Headache

Dynamic thalamic regulation of sensory signals allows the cortex to adjust better to rapidly changing behavioral, physiological, and environmental demands. To fulfill this role, thalamic neurons must themselves be subjected to constantly changing modulatory inputs that originate in multiple neurochemical pathways involved in autonomic, affective, and cognitive functions. This review defines a chemical framework for thinking about the complexity of factors that modulate the response properties of relay trigeminovascular thalamic neurons. Following the presentation of scientific evidence for monosynaptic connections between thalamic trigeminovascular neurons and axons containing glutamate, GABA, dopamine, noradrenaline, serotonin, histamine, orexin, and melanin-concentrating hormone, this review synthesizes a large body of data to propose that the transmission of headache-related nociceptive signals from the thalamus to the cortex is modulated by potentially opposing forces and that the so-called 'decision' of which system (neuropeptide/neurotransmitter) will dominate the firing of a trigeminovascular thalamic neuron at any given time is determined by the constantly changing physiological (sleep, wakefulness, food intake, body temperature, heart rate, blood pressure), behavioral (addiction, isolation), cognitive (attention, learning, memory use), and affective (stress, anxiety, depression, anger) adjustment needed to keep homeostasis.

Neurochemical pathways that converge on thalamic trigeminovascular neurons: potential substrate for modulation of migraine by sleep, food intake, stress and anxiety

Dynamic thalamic regulation of sensory signals allows the cortex to adjust better to rapidly changing behavioral, physiological and environmental demands. To fulfill this role, thalamic neurons must themselves be subjected to constantly changing modulatory inputs that originate in multiple neurochemical pathways involved in autonomic, affective and cognitive functions. Our overall goal is to define an anatomical framework for conceptualizing how a ‘decision’ is made on whether a trigeminovascular thalamic neuron fires, for how long, and at what frequency. To begin answering this question, we determine which neuropeptides/neurotransmitters are in a position to modulate thalamic trigeminovascular neurons. Using a combination of in-vivo single-unit recording, juxtacellular labeling with tetramethylrhodamine dextran (TMR) and in-vitro immunohistochemistry, we found that thalamic trigeminovascular neurons were surrounded by high density of axons containing biomarkers of glutamate, GABA, dopamine and serotonin; moderate density of axons containing noradrenaline and histamine; low density of axons containing orexin and melanin concentrating hormone (MCH); but not axons containing CGRP, serotonin 1D receptor, oxytocin or vasopressin. In the context of migraine, the findings suggest that the transmission of headache-related nociceptive signals from the thalamus to the cortex may be modulated by opposing forces (i.e., facilitatory, inhibitory) that are governed by continuous adjustments needed to keep physiological, behavioral, cognitive and emotional homeostasis.

Microtopography of the dual corticothalamic projections originating from domains along the frequency axis of the cat primary auditory cortex

Spatial relationships between clusters of corticothalamic (CT) large terminals originating from cortical domains tuned to different frequencies were examined by pair-injecting two different anterograde tracers. Large-terminal CT projection originating from layer 5 was highly divergent with each injection site producing, on average, 15 local clusters distributing throughout non-lemniscal thalamic nuclei following a single anterograde tracer injection in the cat primary auditory cortex. Paired injections in higher- and lower-frequency cortical domains, resulting in labeling of two independent sets of terminal clusters, showed five recognizable patterns of spatial interaction between them. (1) In the ventral division of the medial geniculate complex (vMGC), sheet-like plexuses of small terminals of different origins were situated in parallel, with minimal overlap. (2) Extensive overlap of two low-density plexuses of differently labeled small terminals was observed in the medial division of the medial geniculate complex (MGC). (3) At the transition zones between the vMGC and the superficial dorsal nucleus of the MGC dorsal division, and between the vMGC and the ventrolateral nucleus, there were relatively broad clusters of a high density of large-terminal structures from the two cortical domains, which overlapped extensively. (4) At multiple loci in the nonlemniscal nuclei, pairing of two small clusters of differently labeled large terminals was observed. (5) Small unpaired clusters of large terminals were also found in the nonlemniscal nuclei. For large terminals, approximately 14%, 59%, and 27% clusters per injection demonstrated patterns 3, 4, and 5, respectively. The results provide evidence for the precise topographical organization for the large-terminal CT system at the microscopic level despite its highly divergent projection. This microtopographical projection from the tonotopic cortical field to non-tonotopic thalamic nuclei may raise the possibility of presence of a map that has not been defined in auditory non-lemniscal thalamic nuclei yet.

Different composition of glutamate receptors in corticothalamic and lemniscal synaptic responses and their roles in the firing responses of ventrobasal thalamic neurons in juvenile mice

Thalamic ventrobasal (VB) relay neurons receive information via two major types of glutamatergic synapses, that is, from the medial lemniscus (lemniscal synapses) and primary somatosensory cortex (corticothalamic synapses). These two synapses influence and coordinate firing responses of VB neurons, but their precise operational mechanisms are not yet well understood. In this study, we compared the composition of glutamate receptors and synaptic properties of corticothalamic and lemniscal synapses. We found that the relative contribution of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) to non-NMDA receptor-mediated EPSCs was significantly greater in corticothalamic synapses than in lemniscal synapses. Furthermore, NMDA receptor 2B-containing NMDA receptor- and kainate receptor-mediated currents were observed only in corticothalamic synapses, but not in lemniscal synapses. EPSCs in corticothalamic synapses displayed the postsynaptic summation in a frequency-dependent manner, in which the summation of the NMDA receptor-mediated component was largely involved. The summation of kainate receptor-mediated currents also partially contributed to the postsynaptic summation in corticothalamic synapses. In contrast, the contribution of NMDA receptor-mediated currents to the postsynaptic summation of lemniscal EPSCs was relatively minor. Furthermore, our results indicated that the prominent NMDA receptor-mediated component in corticothalamic synapses was the key determinant for the late-persistent firing of VB neurons in response to corticothalamic stimuli. In lemniscal synapses, in contrast, the onset-transient firing in response to lemniscal stimuli was regulated mainly by AMPA receptors.

Cholinergic modulation of vibrissal receptive fields in trigeminal nuclei

In sensory systems, it is usually considered that mesopontine cholinergic neurons exert their modulatory action in the thalamus by enhancing the relay of sensory messages during states of neural network desynchronization. Here, we report a projection heretofore unknown of these cholinergic cells to the interpolar division of the brainstem trigeminal complex in rats. After FluoroGold injection in the interpolar nucleus, a number of retrogradely labeled cells were found bilaterally in the pedunculopontine tegmental nucleus, and immunostaining revealed that the vast majority of these cells were also positive for choline acetyltransferase. Immunostaining for the acetylcholine vesicular transporter confirmed the presence of cholinergic terminals in the interpolar nucleus, where electron microscopy showed that they make symmetric and asymmetric synaptic contacts with dendrites and axon terminals. In agreement with these anatomical data, recordings in slices showed that the cholinergic agonist carbachol depolarizes large-sized interpolaris cells and increases their excitability. Local application of carbachol in vivo enhances responses to adjacent whiskers, whereas systemic administration of the cholinergic antagonist scopolamine produces an opposite effect. Together, these results show that mesopontine cholinergic neurons exert a direct, effective control over receptive field size at the very first relay stations of the vibrissal system in rodents. As far as receptive field synthesis in the lemniscal pathway relies on intersubnuclear projections from the spinal complex, it follows that cholinergic modulation of sensory transmission in the interpolar nucleus will have a direct bearing on the type of messages that is forwarded to the thalamus and cerebral cortex.

Active zones for presynaptic plasticity in the brain

Some of the most abundant synapses in the brain such as the synapses formed by the hippocampal mossy fibers, cerebellar parallel fibers and several types of cortical afferents express presynaptic forms of long-term potentiation (LTP), a putative cellular model for spatial, motor and fear learning. Those synapses often display presynaptic mechanisms of LTP induction, which are either NMDA receptor independent of dependent of presynaptic NMDA receptors. Recent investigations on the molecular mechanisms of neurotransmitter release modulation in short- and long-term synaptic plasticity in central synapses give a preponderant role to active zone proteins as Munc-13 and RIM1-alpha, and point toward the maturation process of synaptic vesicles prior to Ca(2+)-dependent fusion as a key regulatory step of presynaptic plasticity.

Neuronal response to local electrical stimulation in rat thalamus: physiological implications for mechanisms of deep brain stimulation

High-frequency deep brain stimulation (DBS) of sensorimotor thalamus containing "tremor cells" leads to tremor arrest in humans with parkinsonian and essential tremor. To examine the possible underlying mechanism(s), we recorded in vitro intracellular responses of rat thalamic neurons to local intrathalamic stimulation. Such simulated DBS (sDBS) induced a sustained membrane depolarization accompanied by an increase in apparent membrane conductance in both motor and sensory neurons. With stimulation frequency above approximately 100 Hz, the sDBS-induced depolarization most typically led to repetitive neuronal firing or less frequently resulted in a complete blockade of action potential genesis. When regular intracellular current pulses were injected into cells to mimic "tremor" activity, such rhythmic discharges were invariably disrupted or abolished by the random spike firing induced during high-frequency sDBS. Low-frequency sDBS left rhythmicity unaffected.We conclude that clinical thalamic DBS may lead to a neuronal de-rhythmicity and tremor stoppage through masking and/or blocking rhythmic firing of tremor cells.

Different temporal processing of sensory inputs in the rat thalamus during quiescent and information processing states in vivo

Sensory inputs from the whiskers reach the primary somatosensory thalamus through the medial lemniscus tract. The main role of the thalamus is to relay these sensory inputs to the neocortex according to the regulations dictated by behavioural state. Intracellular recordings in urethane-anaesthetized rats show that whisker stimulation evokes EPSP-IPSP sequences in thalamic neurons. Both EPSPs and IPSPs depress with repetitive whisker stimulation at frequencies above 2 Hz. Single-unit recordings reveal that during quiescent states thalamic responses to repetitive whisker stimulation are suppressed at frequencies above 2 Hz, so that only low-frequency sensory stimulation is relayed to the neocortex. In contrast, during activated states, induced by stimulation of the brainstem reticular formation or application of acetylcholine in the thalamus, high-frequency whisker stimulation at up to 40 Hz is relayed to the neocortex. Sensory suppression is caused by the depression of lemniscal EPSPs in relatively hyperpolarized thalamocortical neurons. Sensory suppression is abolished during activated states because thalamocortical neurons depolarize and the depressed lemniscal EPSPs are able to reach firing threshold. Strong IPSPs may also contribute to sensory suppression by hyperpolarizing thalamocortical neurons, but during activated states IPSPs are strongly reduced altogether. The results indicate that the synaptic depression of lemniscal EPSPs and the level of depolarization of thalamocortical neurons work together in thalamic primary sensory pathways to suppress high-frequency sensory inputs during non-activated (quiescent) states while permitting the faithful relay of high-frequency sensory information during activated (processing) states.

Pedunculopontine stimulation induces prolonged activation of pontine reticular neurons

Extracellular and intracellular recordings were carried out from neurons in the region of the pontine reticular formation at the transition between the nucleus reticularis pontis oralis and caudalis, and in the pontis caudalis. Responses were studied after stimulation of the mesopontine cholinergic pedunculopontine nucleus in precollicular-postmammillary transected, paralyzed preparations. Recordings of neurographic activity in hindlimb flexor and extensor nerves served to detect changes in fictive locomotion and muscle tone induced by pedunculopontine nucleus stimulation or occurring spontaneously. Short duration trains of pedunculopontine nucleus stimulation induced long lasting responses, on average over 12s in duration, in one-third of pontine reticular neurons. These prolonged responses were stimulation frequency-dependent such that the longest durations were induced by stimulation at 20-60Hz. In some cells, stimulation at lower (10Hz) or higher (100Hz) frequencies induced responses of shorter duration or were absent, while in others, higher frequencies prolonged the excitatory effects of pedunculopontine nucleus stimulation. We conclude that these stimulation frequency-dependent effects may be related to the modulation of postural muscle tone and locomotion by the pedunculopontine nucleus.

Cholinergic excitation of dendrites in neocortical neurons

Discharge patterns were studied in response to iontophoretic application of acetylcholine to the soma and dendrites of 128 neocortical pyramidal neurons of layer V. Extracellular recordings were obtained from slices of the guinea-pig parietal cortex. All responses found were excitatory and were better expressed in spontaneously firing cells than in silent ones. Sensitivity to acetylcholine was approximately the same at somatic and dendritic sites in all the cells. Activation of muscarinic receptors gave rise to firing patterns with equal latencies and intensities when applied to both soma and dendrites. The latter suggests that membrane excitation elicited in dendrites by binding of acetylcholine to muscarinic cholinoreceptors is likely to propagate towards the soma through intracellular biochemical processes. Modulating effect of acetylcholine on output firing patterns, elicited by dendritic application of excitatory amino acids, included shortening of the somatic response latency and increase of response intensity and duration. We propose that, in contrast to glutamatergic excitation, the spread of cholinergic excitation along dendrites involves intra-cellular chemical signalling and results in changing the electrical properties of dendrites all over their length.

Responses of cortical neurons to microiontophoretic application of acetylcholine to their dendrites

Spike responses of neurons to the microiontophoretic application of acetylcholine to the soma and the dendrites were studied. The somatic and dendritic membranes had virtually equal sensitivity to acetylcholine. Only activatory responses were seen, which were most typical of spontaneously active neurons. Muscarinic activation induced spike responses with equal latent periods and equal intensities on application of acetylcholine to dendrites and the soma. It is suggested that intracellular chemical signaling is involved in the propagation of cholinergic excitation via dendrites.

Sleep and arousal: thalamocortical mechanisms

Thalamocortical activity exhibits two distinct states: (a) synchronized rhythmic activity in the form of delta, spindle, and other slow waves during EEG-synchronized sleep and (b) tonic activity during waking and rapid-eye-movement sleep. Spindle waves are generated largely through a cyclical interaction between thalamocortical and thalamic reticular neurons involving both the intrinsic membrane properties of these cells and their anatomical interconnections. Specific alterations in the interactions between these cells can result in the generation of paroxysmal events resembling absence seizures in children. The release of several different neurotransmitters from the brain stem, hypothalamus, basal forebrain, and cerebral cortex results in a depolarization of thalamocortical and thalamic reticular neurons and an enhanced excitability in many cortical pyramidal cells, thereby suppressing the generation of sleep rhythms and promoting a state that is conducive to sensory processing and cognition.

The brain-stem parabrachial region controls mode of response to visual stimulation of neurons in the cat's lateral geniculate nucleus

We recorded the responses of neurons from the cat's lateral geniculate nucleus to drifting sine-wave grating stimuli both before and during electrical stimulation of the parabrachial region of the midbrain. The parabrachial region provides a mostly cholinergic input to the lateral geniculate nucleus, and our goal was to study its effect on responses of geniculate cells to visual stimulation. Geniculate neurons respond to visual stimuli in one of two modes. At relatively hyperpolarized membrane potentials, low threshold (LT) Ca2+ spikes are activated, leading to high-frequency burst discharges (burst mode). At more depolarized levels, the low threshold Ca2+ spike is inactivated, permitting a more tonic response (relay or tonic mode). During our intracellular recordings of geniculate cells, we found that, at initially hyperpolarized membrane potentials, LT spiking in response to visual stimulation was pronounced, but that parabrachial activation abolished this LT spiking and associated burst discharges. Coupled with the elimination of LT spiking, parabrachial activation also led to a progressive increase in tonic responsiveness. Parabrachial activation thus effectively switched the responses to visual stimulation of geniculate neurons from the burst to relay mode. Accompanying this switch was a gradual depolarization of resting membrane potential by about 5-10 mV and a reduction in the hyperpolarization that normally occurs in response to the inhibitory phase of the visual stimulus. Presumably, the membrane depolarization was sufficient to inactivate the LT spikes. We were able to extend and confirm our intracellular observations on the effects of parabrachial activation to a sample of cells recorded extracellularly. This was made possible by adopting empirically determined criteria to distinguish LT bursts from tonic responses solely on the basis of the temporal pattern of action potentials. During parabrachial activation, every cell responded only in the relay mode, an effect that corresponds to our intracellular observations. We quantified the effects of parabrachial activation on various response measures. The fundamental Fourier response amplitude (F1) was calculated separately for the total response, the tonic response component, and the LT burst component. Parabrachial activation resulted in an increased F1 amplitude for the total response. This increase was due to an increase in the tonic response component. For a subset of cells showing epochs of LT bursting, parabrachial activation concurrently reduced LT bursting and increased the amplitude of the tonic response. Parabrachial activation, by eliminating LT bursting, also caused cells to respond with more linearity.(ABSTRACT TRUNCATED AT 400 WORDS)