The whisker trident, a three-whisker array on the rat's chin, has been implicated in egomotion sensing and might function as a tactile speedometer. Here we study the cortical representation of trident whiskers and E-row whiskers in barrel cortex. Neurons identified in trident cortex of anesthetized animals showed sustained velocity-sensitive responses to ground motion. In freely moving animals, about two-thirds of the units in the trident and E-row whisker cortices were tuned to locomotion speed, a larger fraction of speed-tuned cells than in the somatosensory dysgranular zone. Similarly, more units were tuned to acceleration and showed sensitivity to turning in trident and E-row whisker cortices than in the dysgranular zone. Microstimulation in locomoting animals evoked small but significant speed changes, and such changes were larger in the trident and E-row whisker representations than in the dysgranular zone. Thus, activity in trident and E-row cortices represents egomotion information and influences locomotion behavior.

Although the neurobiology of rodent facial whiskers has been studied intensively, little is known about sensing in other vibrissae. Here we describe the under-investigated submandibular "whisker trident" on the rat's chin. In this three-whisker array, a unique unpaired midline whisker is laterally flanked by two slightly shorter whiskers. All three whiskers point to the ground and are curved backwards. Unlike other whiskers, the trident is not located on an exposed body part. Trident vibrissae are not whisked and do not touch anything over long stretches of time. However, trident whiskers engage in sustained ground contact during head-down running while the animal is exploring or foraging. In biomechanical experiments, trident whiskers follow caudal ground movement more smoothly than facial whiskers. Remarkably, deflection angles decrease with increasing ground velocity. We identified one putative trident barrel in the left somatosensory cortex and two barrels in the right somatosensory cortex. The elongated putative trident-midline barrel is the longest and largest whisker barrel, suggesting that the midline trident whisker is of great functional significance. Cortical postsynaptic air-puff responses in the trident representation show much less temporal precision than facial whisker responses. Trident whiskers do not provide as much high-resolution information about object contacts as facial whiskers. Instead, our observations suggest an idiothetic function: their biomechanics allow trident whiskers to derive continuous measurements about ego motion from ground contacts. The midline position offers unique advantages in sensing heading direction in a laterally symmetric manner. The changes in trident deflection angle with velocity suggest that trident whiskers might function as a tactile speedometer.

Spikelets, small spikelike membrane potential deflections, are prominent in the activity of hippocampal pyramidal neurons in vivo. The origin of spikelets is still a source of much controversy. Somatically recorded spikelets have been postulated to originate from dendritic spikes, ectopic spikes, or spikes in an electrically coupled neuron. To differentiate between the different proposed mechanisms we used a dual recording approach in which we simultaneously recorded the intracellular activity of one CA1 pyramidal neuron and the extracellular activity in its vicinity, thus monitoring extracellularly the activity of both the intracellularly recorded cell as well as other units in its surroundings. Spikelets were observed in a quarter of our recordings ( n = 36). In eight of these nine recordings a second extracellular unit fired in correlation with spikelet occurrences. This observation is consistent with the idea that the spikelets reflect action potentials of electrically coupled nearby neurons. The extracellular spikes of these secondary units preceded the onset of spikelets. While the intracellular spikelet amplitude was voltage dependent, the simultaneously recorded extracellular unit remained unchanged. Spikelets often triggered action potentials in neurons, resulting in a characteristic 1- to 2-ms delay between spikelet onset and firing. Here we show that this relationship is bidirectional, with spikes being triggered by and also triggering spikelets. Secondary units, coupled to pyramidal neurons, showed discharge patterns similar to the recorded pyramidal neuron. These findings suggest that spikelets reflect spikes in an electrically coupled neighboring neuron, most likely of pyramidal cell type. Such coupling might contribute to the synchronization of pyramidal neurons with millisecond precision.

Space and Spikelets In neurons, spikelets are voltage fluctuations of small amplitude with a spike-like waveform. Spikelets are difficult to detect with extracellular techniques traditionally used to record neuronal activity in freely moving animals. Epsztein et al. (p. 474 ) used head-anchored whole-cell recordings to analyze spikelet activity during spatial exploration in freely moving rats. A high incidence of spikelets was often followed by action potentials. Like action potentials, spikelets were all-or-none, but had different kinetics and amplitude, and were clearly distinct from excitatory postsynaptic potentials and occurred to a different extent in different cells. In cells with clear place fields, spikelets had similar spatial firing preferences, as did regular action potentials. Thus, spatially modulated spikelets may be involved in information processing in cortical neuronal networks.

Most of our current knowledge about the neural control of behavior is based on electrophysiology. Here we review advances and limitations of current electrophysiological recording techniques applied in behaving animals. Extracellular recording methods have improved with respect to sampling density and miniaturization, and our understanding of the nature of the recorded signals has advanced. Juxtacellular recordings have become increasingly popular as they allow identification of the recorded neurons. Juxtacellular recordings are relatively easy to apply in behaving animals and can be used to stimulate individual neurons. Methods for intracellular recordings in awake behaving animals also advanced, and it has become clear that long-duration intracellular recordings are possible even in freely moving animals. We conclude that the electrophysiological methods repertoire has greatly diversified in recent years and that the field has moved beyond what used to be a mere spike counting business.

The cerebellum has been the focus of extensive research for more than a century. However, its functional role is still under debate. The comprehensive description of its anatomy and physiology seem to deepen rather than resolve the controversy about its function. Recently, it was shown that Purkinje cells' (PC) membrane potential is bistable and can be found in one of two states: periods of simple spike bursting ("up state"), followed by periods of electrical quiescence and hyperpolarized membrane potential ("down state"). This bistability, which challenges the current dogma regarding the functional organization of the cerebellum, has immediate implications on the mode by which the cerebellar cortex reads incoming input. The well-documented, all-or-none response of PCs to climbing fiber input is generated by complex interactions between the synaptic currents and intrinsic properties of PCs. Hence, it is bound to change as a function of PC membrane potential. Therefore, we compared complex spike waveforms occurring during down and up states, as recorded in both slice preparations and the intact brain of anesthetized rats. We then used the voltage derivative of the intracellular recording to compare the in-vitro intracellular recording to the in-vivo extracellular unit recordings. We found highly significant differences between CSs that occur during the up state and those occurring during the down state. CSs at the up state have a longer duration, and their wavelets have a slower rate of rise than those occurring in the down state. Corresponding changes in the extracellular unit recordings suggests that these changes are manifested in the intact brain. Hence, these state-dependent modifications have immediate, as well as long-term, effects on the output and dynamics of the cerebellar cortex.

In vitrostudies of inferior olive neurons demonstrate that they are intrinsically active, generating periodic spatiotemporal patterns. These self-generated patterns of activity extend the role of olivary neurons beyond that of a deliverer of teaching or error signals. However, autorhythmicity or patterned activity of complex spikes in the cerebellar cortex was observed in only a few studies. This discrepancy between the self-generated rhythmicity in the inferior olive observedin vitroand the sporadic reports on rhythmicity of complex spikes can be reconciled by recording intracellularly from inferior olive neuronsin situ. To this end, we recorded intracellularly from olivary neurons of anesthetized rats. We demonstrate that,in vivo, olivary neurons show both slow and fast rhythmic processes. The slow process (0.2–2 Hz) is expressed as rhythmic transitions from quiescent periods to periods of fast rhythm, manifested as subthreshold oscillations of 6–12 Hz. Spikes, if they occur, are locked to the depolarized phase of these subthreshold oscillations and, therefore, hold and transfer rhythmic information. The transient nature of these oscillatory epochs accounts for the difficulties to uncover them by prolonged recordings of complex spikes activity in the cerebellar cortex.

The electroresponsiveness fingerprint of a neuron reflects the types and distributions of the ionic channels that are embedded in the neuronal membrane as well as its morphology. Theoretical analysis shows that subtle changes in the density of channels can contribute substantially to the electroresponsive fingerprints of neurons. We have confirmed these predictions, using the dynamic clamp approach to emulate changes in channels densities in neurons from the inferior olive. We demonstrate how the density of T-type channels determines the behavioral destiny of neurons. We argue that regulation of channel densities could be an efficient mechanism for controlling the electrical activity of single cells, as well as the output of neuronal networks.

Resumption of meiosis, induced by LH, is preceded by the breakdown of gap junctional communication, which terminates the supply of cAMP from the somatic cells of the ovarian follicle to the oocyte. It has recently been shown that LH-induced reinitiation of meiosis is mediated by MAPK; however, the underlying molecular mechanism involved in the action of this enzyme remains unknown. We hypothesized that activation of MAPK interrupts junctional communication within the ovarian follicle, leading, in turn, to oocyte maturation. To test this hypothesis, we blocked the activation of MAPK by UO126, which specifically inhibits the MAPK signaling pathway. We analyzed junctional communication using three complementary methods: 1) patch-clamp analysis, which determined changes in the electrical coupling between two adjacent granulosa cells; 2) the scrape-loading technique, which monitored the spread of dyes through a granulosa cell layer; and 3) a metabolic coupling assay, which evaluated the transfer of radiolabeled uridine from the cumulus cells to the oocyte. We show, herein, that the somatic follicle cells, rather than the oocyte, activate MAPK immediately after their exposure to LH. Moreover, inhibition of LH-induced MAPK activation not only prevents oocyte maturation but also blocks the reduction in junctional communication. In addition, the appearance of the two phosphorylated forms of the gap junction protein, connexin 43, in response to LH, was avoided by UO126. We concluded that MAPK mediates LH-induced oocyte maturation by interrupting cell-to-cell communication within the ovarian follicle, possibly through phosphorylation of connexin 43.

It has been recently shown that LH-induced reinitiation of meiosis in mouse oocytes is mediated by mitogen activated protein kinase (MAPK), however, the mechanisms involved in this action remained unknown. We hypothesized that activation of MAPK terminates gap junctional communication in the ovarian follicle leading in turn to oocyte maturation. Accordingly, we demonstrated that LH down regulates the translation of the ovarian gap junction protein, connexin43 (Cx43) and that this response is mediated by MAPK. However, LH-induced arrest of Cx43 synthesis was observed after, but not prior to reinitiation of meiosis. On the other hand, uncoupling of the ovarian cells as well as Cx43 phosphorylation, that is also MAPK-dependent, took place immediately after exposure to LH. We conclude that MAPK mediates LH-induced oocyte maturation by reducing the permeability of gap junctions within the ovarian follicle possibly through phosphorylation of Cx43.

Compensatory mechanisms after genetic manipulations have been documented extensively for the nervous system. In many cases, these mechanisms involve genetic regulation at the transcription or expression level of existing isoforms. We report a novel mechanism by which single neurons compensate for changes in network connectivity by retuning their intrinsic electrical properties. We demonstrate this mechanism in the inferior olive, in which widespread electrical coupling is mediated by abundant gap junctions formed by connexin 36 (Cx36). It has been shown in various mammals that this electrical coupling supports the generation of subthreshold oscillations, but recent work revealed that rhythmic activity is sustained in knock-outs of Cx36. Thus, these results raise the question of whether the olivary oscillations in Cx36 knock-outs simply reflect the status of wild-type neurons without gap junctions or the outcome of compensatory mechanisms. Here, we demonstrate that the absence of Cx36 results in thicker dendrites with gap-junction-like structures with an abnormally wide interneuronal gap that prevents electrotonic coupling. The mutant olivary neurons show unusual voltage-dependent oscillations and an increased excitability that is attributable to a combined decrease in leak conductance and an increase in voltage-dependent calcium conductance. Using dynamic-clamp techniques, we demonstrated that these changes are sufficient to transform a wild-type neuron into a knock-out-like neuron. We conclude that the absence of Cx36 in the inferior olive is not compensated by the formation of other gap-junction channels but instead by changes in the cytological and electroresponsive properties of its neurons, such that the capability to produce rhythmic activity is maintained.

The cells of the inferior olivary nucleus, the sole source of the cerebellar climbing fibers, form a network of electrically coupled neurons. Experimental observations show that these neurons produce a large repertoire of electrical signals, among which sub-threshold oscillations of the membrane potential. Simultaneous recordings from pairs of neurons and optical imaging of voltage sensitive dyes show that sub-threshold activity occurs in synchrony throughout the network. The mechanism underlying the generation of the sub-threshold oscillations is not fully understood. Experimental observations suggest that the electrical coupling is essential but insufficient for their generation. Several theoretical mechanisms have been suggested to explain these observations. Up-to-date, the most realistic model is the heterogeneity model, that assumes a certain degree of heterogeneity among olivary neurons. The heterogeneity model proposes that sub-threshold oscillations are produced by electrical coupling of neurons with the same types of ionic conductances, but with different densities. The variability in channel densities yield neurons of different functional types. The main prediction of the model is that different functional types of neurons should be found in the inferior olive. Dynamic clamp experiments support this prediction.