Structure, function, and cortical representation of the rat submandibular whisker trident.

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.

In vivo dual intra- and extracellular recordings suggest bidirectional coupling between CA1 pyramidal neurons.

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.

Rhythmic episodes of subthreshold membrane potential oscillations in the rat inferior olive nuclei in vivo.

In vitro studies 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 observed in vitro and the sporadic reports on rhythmicity of complex spikes can be reconciled by recording intracellularly from inferior olive neurons in 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.

State-dependent modification of complex spike waveforms in the cerebellar cortex.

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.

Representation of egomotion in rat's trident and E-row whisker cortices.

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.

Mitogen-activated protein kinase mediates luteinizing hormone-induced breakdown of communication and oocyte maturation in rat ovarian follicles.

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.

Impact of spikelets on hippocampal CA1 pyramidal cell activity during spatial exploration.

In vivo intracellular recordings of hippocampal neurons reveal the occurrence of fast events of small amplitude called spikelets or fast prepotentials. Because intracellular recordings have been restricted to anesthetized or head-fixed animals, it is not known how spikelet activity contributes to hippocampal spatial representations. We addressed this question in CA1 pyramidal cells by using in vivo whole-cell recording in freely moving rats. We observed a high incidence of spikelets that occurred either in isolation or in bursts and could drive spiking as fast prepotentials of action potentials. Spikelets strongly contributed to spiking activity, driving approximately 30% of all action potentials. CA1 pyramidal cell firing and spikelet activity were comodulated as a function of the animal's location in the environment. We conclude that spikelets have a major impact on hippocampal activity during spatial exploration.

Electrophysiological recordings from behaving animals--going beyond spikes.

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.