Distribution of omega-Conotoxin GVIA binding sites in teleost cerebellar and electrosensory neurons

Topological Regulation of Synaptic AMPA Receptor Expression by the RNA-Binding Protein CPEB3

Synaptic receptors gate the neuronal response to incoming signals, but they are not homogeneously distributed on dendrites. A spatially defined receptor distribution can preferentially amplify certain synaptic inputs, resize receptive fields of neurons, and optimize information processing within a neuronal circuit. Thus, a longstanding question is how the spatial organization of synaptic receptors is achieved. Here, we find that action potentials provide local signals that influence the distribution of synaptic AMPA receptors along dendrites in mouse cerebellar stellate cells. Graded dendritic depolarizations elevate CPEB3 protein at proximal dendrites, where we suggest that CPEB3 binds to GluA2 mRNA, suppressing GluA2 protein synthesis leading to a distance-dependent increase in synaptic GluA2 AMPARs. The activity-induced expression of CPEB3 requires increased Ca(2+) and PKC activation. Our results suggest a cell-autonomous mechanism where sustained postsynaptic firing drives graded local protein synthesis, thus directing the spatial organization of synaptic AMPARs.

Voltage-gated currents of tilapia prolactin cells

The first recordings of neuron-like electrical activity from endocrine cells were made from fish pituitary cells. However, patch-clamping studies have predominantly utilized mammalian preparations. This study used whole-cell patch-clamping to characterize voltage-gated ionic currents of anterior pituitary cells of Oreochromis mossambicus in primary culture. Due to their importance for control of hormone secretion we emphasize analysis of calcium currents (I(Ca)), including using peptide toxins diagnostic for mammalian neuronal Ca(2+) channel types. These appear not to have been previously tested on fish endocrine cells. In balanced salines, inward currents consisted of a rapid TTX-sensitive sodium current and a smaller, slower I(Ca); there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K(+) current. About half of cells tested from a holding potential (V(h)) of -90 mV showed early transient K(+) current; most cells showed a small Ca(2+)-mediated outward current. I-V plots of isolated I(Ca) with 15 mM [Ca(2+)](o) showed peak currents (up to 20 pA/pF from V(h) -90 mV) at approximately +10 mV, with approximately 60% I(Ca) for V(h) -50 mV and approximately 30% remaining at V(h) -30 mV. Plots of normalized conductance vs. voltage at several V(h)s were nearly superimposable. Well-sustained I(Ca) with predominantly Ca(2+)-dependent inactivation and inhibition of approximately 30% of total I(Ca) by nifedipine or nimodipine suggests participation of L-type channels. Each of the peptide toxins (omega-conotoxin GVIA, omega-agatoxin IVA, SNX482) alone blocked 36-54% of I(Ca). Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. I(Ca) of up to 30% of total remained with any combination of inhibitors, but 0.1mM cadmium blocked all I(Ca) rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, I(Ca) is carried by high voltage-activated Ca(2+) channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.

Distribution and function of potassium channels in the electrosensory lateral line lobe of weakly electric apteronotid fish

Potassium channels are one of the fundamental requirements for the generation of action potentials in the nervous system, and their characteristics shape the output of neurons in response to synaptic input. We review here the distribution and function of a high-threshold potassium channel (Kv3.3) in the electrosensory lateral line lobe of the weakly electric fish Apteronotus leptorhynchus, with particular focus on the pyramidal cells in this brain structure. These cells contain both high-threshold Kv3.3 channels, as well as low-threshold potassium channels of unknown molecular identity. Kv3.3 potassium channels regulate burst discharge in pyramidal cells and enable sustained high frequency firing through their ability to reduce an accumulation of low-threshold potassium current.

A prominent soma-dendritic distribution of Kv3.3 K+ channels in electrosensory and cerebellar neurons

The expression pattern and subcellular distribution of a teleost homologue of the mammalian Kv3.3 potassium channel, AptKv3.3, was examined in the electrosensory lateral line lobe (ELL) and two cerebellar lobes in the hindbrain of the weakly electric gymnotiform Apteronotus leptorhynchus. AptKv3.3 expression was brain specific, with the highest level of expression in the cerebellum and 56% relative expression in the ELL. In situ hybridization revealed that AptKv3.3 mRNA was present in virtually all cell classes in the ELL as well as in the cerebellar lobes eminentia granularis pars posterior (EGp) and corpus cerebellum (CCb). Immunocytochemistry indicated a distribution of AptKv3.3 channels over the entire soma-dendritic axis of ELL pyramidal, granule, and polymorphic cells and over the soma and at least proximal dendrites (100 microm) of multipolar cells and neurons of the ventral molecular layer. AptKv3.3 immunolabel was present at the soma of cerebellar granule, golgi, eurydendroid, and CCb Purkinje cells, with an equally intense label throughout the dendrites of CCb Purkinje cells and EGp eurydendroid cells. Immunolabel was virtually absent in afferent or efferent axon tracts of the ELL but was detected on climbing fiber axons and on the axons and putative terminal boutons of CCb Purkinje cells. These data reveal a prominent soma-dendritic distribution of AptKv3.3 K+ channels in both principal output and local circuit neurons, a pattern that is distinct from the soma-axonal distribution that characterizes all other Kv3 K+ channels examined to date. The widespread distribution of AptKv3.3 immunolabel in electrosensory cells implies an important role in several aspects of signal processing.

Neurogenesis, cell death and regeneration in the adult gymnotiform brain

Gymnotiform fish, like all teleosts examined thus far, are distinguished by their enormous potential for the production of new neurons in the adult brain. In Apteronotus leptorhynchus, on average 10(5) cells, corresponding to approximately 0.2 % of the total population of cells in the adult brain, are in S-phase within any period of 2 h. At least a portion of these newly generated cells survive for the rest of the fish's life. This long-term survival, together with the persistent generation of new cells, leads to a continuous growth of the brain during adulthood. Zones of high proliferative activity are typically located at or near the surface of the ventricular, paraventricular and cisternal systems. In the central posterior/ prepacemaker nucleus, for example, new cells are generated, at very high rates, in areas near the wall of the third ventricle. At least some of these cells differentiate into neurons, express immunoreactivity against the neuropeptide somatostatin and migrate into more lateral areas of this complex. Approximately 75 % of all new brain cells are generated in the cerebellum. In the corpus cerebelli and the valvula cerebelli, they are produced in the molecular layers, whereas in the eminentia granularis the newborn cells stem from proliferation zones in the pars medialis. Within the first few days of their life, these cells migrate towards specific target areas, namely the associated granule cell layers. At least some of them develop into granule neurons. The high proliferative activity is counterbalanced by apoptosis, a mechanism that resembles the processes known from embryonic development of the vertebrate brain. Apoptosis also appears to be used as an efficient mechanism for the removal of cells damaged through injury in the brain of adult Apteronotus leptorhynchus. Since apoptosis is not accompanied by the side effects known from necrosis, this ‘clean’ type of cell death may, together with the enormous proliferative activity in the brain, explain, at least partially, the tremendous capability of teleost fish to replace damaged neurons with newly generated ones. One factor that appears to play a major role in the generation of new cells and in their further development is the neuropeptide somatostatin. In the caudal cerebellum of the gymnotiform brain, somatostatin-binding sites are expressed, at extremely high densities, at sites corresponding to the areas of origin, migration and differentiation of the newborn cells. This pattern of expression resembles the expression pattern in the rat cerebellum, where somatostatin immunoreactivity and somatostatin-binding sites are transiently expressed at the time when the granule cells of the cerebellum are generated. Moreover, after mechanical lesions of the corpus cerebelli, the expression of somatostatin-like immunoreactivity is tremendously increased in several cell types (presumably astrocytes, microglia and granule cell neurons) near the path of the lesion; the time course of this expression coincides with the temporal pattern underlying the recruitment of new cells incorporated at the site of the lesion.

Oscillatory and burst discharge in the apteronotid electrosensory lateral line lobe

Oscillatory and burst discharge is recognized as a key element of signal processing from the level of receptor to cortical output cells in most sensory systems. The relevance of this activity for electrosensory processing has become increasingly apparent for cells in the electrosensory lateral line lobe (ELL) of gymnotiform weakly electric fish. Burst discharge by ELL pyramidal cells can be recorded in vivo and has been directly associated with feature extraction of electrosensory input. In vivo recordings have also shown that pyramidal cells are differentially tuned to the frequency of amplitude modulations across three ELL topographic maps of electroreceptor distribution. Pyramidal cell recordings in vitro reveal two forms of oscillatory discharge with properties consistent with pyramidal cell frequency tuning in vivo. One is a slow oscillation of spike discharge arising from local circuit interactions that exhibits marked changes in several properties across the sensory maps. The second is a fast, intrinsic form of burst discharge that incorporates a newly recognized interaction between somatic and dendritic membranes. These findings suggest that a differential regulation of oscillatory discharge properties across sensory maps may underlie frequency tuning in the ELL and influence feature extraction in vivo.

Inhibition evoked from primary afferents in the electrosensory lateral line lobe of the weakly electric fish (Apteronotus leptorhynchus)

Inhibition evoked from primary afferents in the electrosensory lateral line lobe of the weakly electric fish (Apteronotus leptorhynchus). J. Neurophysiol. 80: 3173-3196, 1998. The responses of two types of projection neurons of the electrosensory lateral line lobe, basilar (BP) and nonbasilar (NBP) pyramidal cells, to stimulation of primary electrosensory afferents were determined in the weakly electric fish, Apteronotus leptorhynchus. Using dyes to identify cell type, the response of NBP cells to stimulation of primary afferents was inhibitory, whereas the response of BP cells was excitation followed by inhibition. gamma-Aminobutyric acid (GABA) applications produced biphasic (depolarization then hyperpolarization) responses in most cells. GABAA antagonists blocked the depolarizing effect of GABA and reduced the hyperpolarizing effect. The GABAB antagonists weakly antagonized the hyperpolarizing effect. The early depolarization had a larger increase in cell conductance than the late hyperpolarization. The conductance changes were voltage dependent, increasing with depolarization. In both cell types, baclofen produced a slow small hyperpolarization and reduced the inhibitory postsynaptic potentials (IPSPs) evoked by primary afferent stimulation. Tetanic stimulation of primary afferents at physiological rates (100-200 Hz) produced strongly summating compound IPSPs (approximately 500-ms duration) in NBP cells, which were usually sensitive to GABAA but not GABAB antagonists; in some cells there remained a slow IPSP that was unaffected by GABAB antagonists. BP cells responded with excitatory or mixed excitatory + inhibitory responses. The inhibitory response had both a fast (approximately 30 ms, GABAA) and long-lasting slow phase (approximately 800 ms, mostly blocked by GABAA antagonists). In some cells there was a GABAA antagonist-insensitive slow IPSP (approximately 500 ms) that was sensitive to GABAB antagonists. Application of glutamate ionotropic receptor antagonists blocked the inhibitory response of NBP cells to primary afferent stimulation and the excitatory response of BP cells but enhanced the BP cell slow IPSP; this remaining slow IPSP was reduced by GABAB antagonists. Unit recordings in the granule cell layer and computer simulations of pyramidal cell inhibition suggested that the duration of the slow GABAA inhibition reflects the prolonged firing of GABAergic granule cell interneurons to primary afferent input. Correlation of the results with known GABAergic circuitry in the electrosensory lobe suggests that the GABAergic type 2 granule cell input to both pyramidal cell types is via GABAA receptors. The properties of the GC2 GABAA input are well suited to their putative role in gain control, regulation of phasicness, and coincidence detection. The slow GABAB IPSP evoked in BP cells is likely due to ovoid cell input to their basal dendrites.