Aaron J. Camp

Calretinin: Modulator of neuronal excitability

Calretinin is a member of the calcium-binding protein EF-hand family first identified in the retina. As with the other 200-plus calcium-binding proteins, calretinin serves a range of cellular functions including intracellular calcium buffering, messenger targeting, and is involved in processes such as cell cycle arrest, and apoptosis. Calcium-binding proteins including calretinin are expressed differentially in neuronal subpopulations throughout the vertebrate and invertebrate nervous system and their expression has been used to selectively target specific cell types and isolate neuronal networks. More recent experiments have revealed that calretinin plays a crucial role in the modulation of intrinsic neuronal excitability and the induction of long-term potentiation (LTP). Furthermore, selective knockout of calretinin in mice produces disturbances of motor coordination and suggests a putative role for calretinin in the maintenance of calcium dynamics underlying motor adaptation.

Adaptable Mechanisms That Regulate the Contrast Response of Neurons in the Primate Lateral Geniculate Nucleus

The response of the classical receptive field of visual neurons can be suppressed by stimuli that, when presented alone, cause no change in the discharge rate. This suppression reveals the presence of an extraclassical receptive field (ECRF). In recordings from the lateral geniculate nucleus (LGN) of a New World primate, the marmoset, we characterize the mechanisms that contribute to the ECRF by measuring their spatiotemporal tuning during prolonged exposure to a high-contrast grating (contrast adaptation). The ECRF was strongest in magnocellular cells, where contrast adaptation reduced suppression from the ECRF: adaptation of the ECRF transferred across spatial frequency, temporal frequency, and orientation, but not across space. This implies that the ECRF of LGN cells comprises multiple adaptable mechanisms, each broadly tuned but spatially localized, and consistent with a retinal origin. Signals from the ECRF saturated at high contrasts, and so adaptation of one part of the ECRF brought into its operating range signals from other parts of the visual field. Although the ECRF is adaptable, its major impact during contrast adaptation to a spatially extended pattern was to reduce visual response and hence reduce a neurons susceptibility to contrast adaptation; in normal viewing, a major role of the ECRF might be to protect visual sensitivity in scenes dominated by high contrast.

Visual motion integration by neurons in the middle temporal area of a New World monkey, the marmoset

Non‐technical summary  The machinery of motion vision is highly conserved across New World and Old World monkeys, according to our study of the marmoset visual cortex. The marmoset is a New World primate, part of a lineage that diverged from Old World monkeys some 30–40 million years ago. A small part of the cerebral cortex, area MT, can be identified anatomically in both New and Old World primates. In the macaque, an Old World primate, this area is thought to be important in analysing the motion of complex patterns. Here we quantified the capacity of neurons in area MT of marmosets to extract motion from complex patterns. We find the responses of neurons in area MT of marmosets to be indistinguishable from those in macaques, suggesting that the functional role of this small area of the visual cortex is highly conserved over evolution.

Linear and nonlinear contributions to the visual sensitivity of neurons in primate lateral geniculate nucleus.

Several parallel pathways convey retinal signals to the visual cortex of primates. The signals of the parvocellular (P) and magnocellular (M) pathways are well characterized; the properties of other rarely encountered cell types are distinctive in many ways, but it is not clear that they can provide signals with the same fidelity. Here we study this by characterizing the temporal receptive field of neurons in the lateral geniculate nucleus of anesthetized marmosets. For each neuron, we measured the response to a flickering uniform field, and, from this, estimated the linear and nonlinear receptive fields using spike-triggered average (STA) and spike-triggered covariance (STC) analyses. As expected the response of most P-cells was dominated by the STA, but the response of most M-cells required additional nonlinear components, and these usually acted to suppress cell responses. The STC analysis showed stronger suppressive axes in suppressed-by-contrast cells, and both suppressive and excitatory axes in on-off cells. Together, the STA and the STC analyses form a model of the temporal response to a large uniform field: under this model, the information that was provided by suppressed-by-contrast cells or on-off cells approached that provided by the P- and M-cells. However, whereas P- and M-cells provided more information about luminance, the nonlinear cells provided more information about the contrast energy. This suggests that the nonlinear cells provide complimentary signals to those of P- and M-cells, with reasonably high fidelity, and may play an important role in normal visual processing.

Attenuated glycine receptor function reduces excitability of mouse medial vestibular nucleus neurons

Spontaneous activity in medial vestibular nucleus (MVN) neurons is modulated by synaptic inputs. These inputs are crucial for maintaining gaze and posture and contribute to vestibular compensation after lesions of peripheral vestibular organs. We investigated how chronically attenuated glycinergic input affects excitability of MVN neurons. To this end we used three mouse strains (spastic, spasmodic, and oscillator), with well-characterized naturally occurring mutations in the inhibitory glycine receptor (GlyR). First, using whole-cell patch-clamp recordings, we demonstrated that the amplitude of the response to rapidly applied glycine was dramatically reduced by 25 to 90% in MVN neurons from mutant mice. We next determined how reduced GlyR function affected MVN neuron output. Neurons were classified using two schemas: (1) the shape of their action potential afterhyperpolarization (AHP); and (2) responses to hyperpolarizing current injection. In the first schema, neurons were classified as types A, B and C. The prevalence of type C neurons in the mutant strains was significantly increased. In the second schema, the proportion of neurons lacking post inhibitory rebound firing (PRF-deficient) was increased. In both schemas an increase in AHP amplitude was a common feature of the augmented neuron group (type C, PRF-deficient) in the mutant strains. We suggest increased AHP amplitude reduces overall excitability in the MVN and thus maintains network function in an environment of reduced glycinergic input.

Vestibular primary afferent activity in an in vitro preparation of the mouse inner ear

Most information on the properties of mammalian vestibular primary afferents has been obtained in deeply anesthetized animals, in vivo. Generally, non-human primates and larger rodents have been the species of choice. Investigations using smaller rodents, such as the laboratory mouse, have been limited despite the increasing availability of naturally occurring or engineered mutants that result in balance disorders. Furthermore, in vitro preparations of the intact peripheral vestibular apparatus are only available for non-mammalian vertebrates. To take advantage of the genetic/molecular advances available in mice and the utility of in vitro preparations that permit manipulations of the extracellular milieu, we developed an isolated mouse inner ear preparation with the attached eighth cranial nerve for electrophysiological recording. Intra-axonal recordings of background activity in vestibular primary afferents were obtained in a modified Ringers solution (0.25 mM Ca2+; 3.25 mM Mg2+) at 22 degrees C. We also recorded afferent activity in the presence of neuroactive drugs known to affect various stages of the transduction cascade. These results, together with responses to sinusoidal mechanical deformation of the membranous ducts, showed that transduction mechanisms remain viable. Where possible, we also obtained results in vivo for comparison. In future, the in vitro mouse preparation will allow investigation of the effects of genetic manipulations and pharmacological agents on the intact peripheral vestibular apparatus.

Vestibular Interactions in the Thalamus

It has long been known that the vast majority of all information en route to the cerebral cortex must first pass through the thalamus. The long held view that the thalamus serves as a simple hi fidelity relay station for sensory information to the cortex, however, has over recent years been dispelled. Indeed, multiple projections from the vestibular nuclei to thalamic nuclei (including the ventrobasal nuclei, and the geniculate bodies)- regions typically associated with other modalities- have been described. Further, some thalamic neurons have been shown to respond to stimuli presented from across sensory modalities. For example, neurons in the rat anterodorsal and laterodorsal nuclei of the thalamus respond to visual, vestibular, proprioceptive and somatosensory stimuli and integrate this information to compute heading within the environment. Together, these findings imply that the thalamus serves crucial integrative functions, at least in regard to vestibular processing, beyond that imparted by a “simple” relay. In this mini review we outline the vestibular inputs to the thalamus and provide some clinical context for vestibular interactions in the thalamus. We then focus on how vestibular inputs interact with other sensory systems and discuss the multisensory integration properties of the thalamus.

Behavioral assessment of the aging mouse vestibular system.

Age related decline in balance performance is associated with deteriorating muscle strength, motor coordination and vestibular function. While a number of studies show changes in balance phenotype with age in rodents, very few isolate the vestibular contribution to balance under either normal conditions or during senescence. We use two standard behavioral tests to characterize the balance performance of mice at defined age points over the lifespan: the rotarod test and the inclined balance beam test. Importantly though, a custom built rotator is also used to stimulate the vestibular system of mice (without inducing overt signs of motion sickness). These two tests have been used to show that changes in vestibular mediated-balance performance are present over the murine lifespan. Preliminary results show that both the rotarod test and the modified balance beam test can be used to identify changes in balance performance during aging as an alternative to more difficult and invasive techniques such as vestibulo-ocular (VOR) measurements.

Noise normalizes firing output of mouse lateral geniculate nucleus neurons.

The output of individual neurons is dependent on both synaptic and intrinsic membrane properties. While it is clear that the response of an individual neuron can be facilitated or inhibited based on the summation of its constituent synaptic inputs, it is not clear whether subthreshold activity, (e.g. synaptic “noise”- fluctuations that do not change the mean membrane potential) also serve a function in the control of neuronal output. Here we studied this by making whole-cell patch-clamp recordings from 29 mouse thalamocortical relay (TC) neurons. For each neuron we measured neuronal gain in response to either injection of current noise, or activation of the metabotropic glutamate receptor-mediated cortical feedback network (synaptic noise). As expected, injection of current noise via the recording pipette induces shifts in neuronal gain that are dependent on the amplitude of current noise, such that larger shifts in gain are observed in response to larger amplitude noise injections. Importantly we show that shifts in neuronal gain are also dependent on the intrinsic sensitivity of the neuron tested, such that the gain of intrinsically sensitive neurons is attenuated divisively in response to current noise, while the gain of insensitive neurons is facilitated multiplicatively by injection of current noise- effectively normalizing the output of the dLGN as a whole. In contrast, when the cortical feedback network was activated, only multiplicative gain changes were observed. These network activation-dependent changes were associated with reductions in the slow afterhyperpolarization (sAHP), and were mediated at least in part, by T-type calcium channels. Together, this suggests that TC neurons have the machinery necessary to compute multiple output solutions to a given set of stimuli depending on the current level of network stimulation.

Intrinsic neuronal excitability: a role in homeostasis and disease.

The neuronal components of brain circuitry are generally considered “stable” throughout an animals’ life; with the exception of times of growth and degeneration that occur during development, aging, or pathology. To maintain this stability, neurons must achieve a balance between changing output to meet new requirements, and keeping output within a satisfactory operating range. This balancing act is done through the combination of synaptic plasticity and changes in intrinsic neuronal excitability. While synaptic plasticity has been the focus of most research into neuronal excitability, growing evidence including that described in the recent review by Beraneck and Idoux (2012), highlights the fact that changes in intrinsic membrane properties also shape the output of single neurons. In their article Beraneck and Idoux (2012) provide evidence for changes in intrinsic excitability of second order vestibular neurons that occur during vestibular compensation – a remarkable example of homeostatic plasticity whereby spontaneous discharge of neurons in the ipsi and contralesional vestibular nuclei is recalibrated following unilateral vestibular damage or labyrinthectomy. While the authors describe clearly how the output of second order vestibular neurons changes with respect to action potential shape and discharge dynamics within the context of vestibular compensation, it is worth noting that the underlying mechanisms of these changes are not isolated to the vestibular system, but appear to be a common feature of homeostatic plasticity in other central nervous system (CNS) structures and during disease. In the following commentary some of the findings highlighted by Beraneck and Idoux are discussed with respect to work on neuronal excitability in other CNS regions. Beraneck and Idoux (2012) detail evidence to support the hypothesis that changes in background discharge during vestibular compensation of vestibular nucleus neurons appears to be predominately expressed as changes in the excitability of the type B medial vestibular nucleus (MVN) neuron subpopulation. Broadly speaking the changes described in their review can be characterized as those that affect discharge rate (e.g., spike threshold, action potential shape), or those that affect discharge pattern. Importantly, similar modifications of intrinsic membrane properties have also been shown in various CNS pathologies, highlighting their importance as potential therapeutic targets during disease. In the commonly used Pilocarpine model of epilepsy where injection of the muscarinic agonist causes seizures similar to those observed in human temporal lobe epilepsy (Sanabria et al., 2001), down-regulation of the slow, non-inactivation voltage-dependent potassium current (Ih) has been observed in pyramidal neurons of the hippocampus (Jung et al., 2007), as well as entorhinal cortex (Shah et al., 2004). In these structures attenuation of Ih results in hyperpolarization of the resting membrane potential, increased cell resistivity (making the cell more sensitive to small voltage fluctuations), and increased EPSP summation (Shah et al., 2004; Jung et al., 2007). Similarly, changes in the number of voltage-gated sodium (Na+) channels have been demonstrated in animal models of chronic pain. In this example, the threshold required to elicit an action potential from dorsal root ganglion (DRG) neurons was reduced following up-regulated expression of voltage-gated Na+ channels (Hong et al., 2004; Kirita et al., 2007; Wang et al., 2007). In addition, down-regulation of somatic IA has been shown in DRG neurons following crush injury of the nerve root (Hu and Gereau, 2003; Hu et al., 2006; Karim et al., 2006; Tan et al., 2006). These changes in DRG neuron intrinsic excitability, presumably underlie the hyperalgesia and allodynia commonly observed in chronic pain models. While the changes in intrinsic excitability described above can be broadly characterized as those that affect the properties of all-or-none action potentials, experience-dependent changes can also impact on the pattern of neuronal discharge, that is, the number or regularity of action potentials. Beraneck and Idoux (2012) discuss a homeostatic shift toward the phasic type B-like discharge phenotype (including the subset of neurons with low-threshold “burst” spikes) following chronic attenuation of inhibitory drive, and on the contralesional side following vestibular compensation. Importantly, this change in discharge phenotype is also observed in a number of pathophysiological states (Beck and Yaari, 2008). For example, changes in firing mode toward a phasic phenotype have been reported in models of chronic epilepsy (Sanabria et al., 2001), and in other epilepsy-associated disorders (Baraban and Schwartzkroin, 1995). Further, increases in the number of neurons displaying the burst-firing phenotype have been implicated in other CNS disorders including chronic stress (Okuhara and Beck, 1998), pain (for review see Cummins et al., 2007; Hains and Waxman, 2007), and neuroinflammation (Saab et al., 2004). What underlies experience-dependent changes in firing mode? In burst-firing neurons such as the phasic type B plus low-threshold spikes MVN neuron population, a subthreshold depolarization (termed an afterdepolarization; ADP) can exceed action potential threshold and elicit a burst of high frequency APs. Changes in the ionic conductances underlying the ADP presumably contribute to the changes in firing mode observed under both homeostatic and pathological conditions. Indeed, blockade of dendritic IA (generated by the fast transient A-type K channels, and as pointed out by Beraneck and Idoux, 2012, a key signature of the long-term shift toward the type A AP profile in ipsilesional MVN neurons following compensation) increases the amplitude of the ADP in CA1 neuron dendrites beyond that required for action potential generation, and results in a burst-firing phenotype (Magee and Carruth, 1999; Metz et al., 2007). Similarly, there is substantial evidence for the involvement of other voltage-gated channels in regulating the magnitude of the ADP (and firing mode) including ICa,T channels, which, when up-regulated as in the pilocarpine model of epilepsy or in response to ischemia, also cause an increase in ADP magnitude and burst-firing (Yue et al., 2005; Yaari et al., 2007). Taken together, it is clear that while modifications in intrinsic neuronal excitability outlined by Beraneck and Idoux (2012) are crucial during short and long-term homeostatic phenomenon like vestibular compensation, similar modifications also underlie the excitability of neurons in other CNS structures during pathology. It is worth noting that work on the role of intrinsic neuronal excitability during health and disease is still young when compared with work focused on synaptic mechanisms, and as such represents an exciting avenue for unraveling the mechanisms that contribute to disorders of the CNS, and potentially, novel targets for their treatment.