Neuronal responses to turtle head rotationin vitro

Sensory-evoked turning locomotion in red-eared turtles: kinematic analysis and electromyography

We examined the limb kinematics and motor patterns that underlie sensory-evoked turning locomotion in red-eared turtles. Intact animals were held by a band-clamp in a water-filled tank. Turn-swimming was evoked by slowly rotating turtles to the right or left via a motor connected to the shaft of the band-clamp. Animals executed sustained forward turn-swimming against the direction of the imposed rotation. We recorded video of turn-swimming and computer-analyzed the limb and head movements. In a subset of turtles, we also recorded electromyograms from identified limb muscles. Turning exhibited a stereotyped pattern of (1) coordinated forward swimming in the hindlimb and forelimb on the outer side of the turn, (2) back-paddling in the hindlimb on the inner side, (3) a nearly stationary, "braking" forelimb on the inner side, and (4) neck bending toward the direction of the turn. Reversing the rotation caused animals to switch the direction of their turns and the asymmetric pattern of right and left limb activities. Preliminary evidence suggested that vestibular inputs were sufficient to drive the behavior. Sensory-evoked turning may provide a useful experimental platform to examine the brainstem commands and spinal neural networks that underlie the activation and switching of different locomotor forms.

Evaluation of rebound tonometry in red-eared slider turtles (Trachemys scripta elegans)

OBJECTIVE

To evaluate feasibility and accuracy of intraocular pressure (IOP) measurement by rebound tonometry in adult red-eared slider turtles and determine the effects of manual and chemical restraint on IOP.

ANIMAL STUDIED

Seventeen adult red-eared slider turtles.

PROCEDURES

Intraocular pressure was measured with TonoLab® and TonoVet® tonometers in conscious, unrestrained turtles. To evaluate the effects of manual restraint, turtles were restrained by digital pressure on the rostral head or proximal neck. The effect of two chemical restraint protocols (dexmedetomidine, ketamine, midazolam [DKM] and dexmedetomidine, ketamine [DK] subcutaneously) on IOP was evaluated. Triplicate TonoLab® and TonoVet® readings were compared with direct manometry in three ex vivo turtle eyes.

RESULTS

TonoLab® correlated better with manometry at IOPs < 45 mmHg than TonoVet® (linear regression slopes of 0.89 and 0.30, respectively). Mean (±SD) IOP in unrestrained conscious turtles was significantly lower (P < 0.01) with TonoLab® (10.02 ± 0.66 mmHg) than with TonoVet® (11.32 ± 1.57 mmHg). Manual neck restraint caused a significant increase in IOP (+6.31 ± 5.59 mmHg), while manual rostral head restraint did not. Both chemical restraint protocols significantly reduced IOP (DKM: −1.0 ± 0.76 mmHg; DK: −1.79 ± 1.17) compared with measurements in conscious unrestrained turtles.

CONCLUSIONS

Chemical and manual neck restraint affected IOP. Rostral head restraint had no significant effect on IOP and is, therefore, recommended as the appropriate restraint technique in red-eared slider turtles. TonoLab® measurements estimated actual IOP more accurately, within physiologic range, than measurements obtained using the TonoVet®.

A novel path for rapid transverse communication of vestibular signals in turtle cerebellum

Voltage-sensitive dye activity within the thin, unfoliated turtle cerebellar cortex (Cb) was recorded in vitro during eighth cranial nerve (nVIII) stimulation. Short latency responses were localized to the middle of the lateral edges of both ipsilateral and contralateral Cb [vestibulocerebellum (vCb)]. Even with a severed contralateral Cb peduncle, stimulation of the nVIII ipsilateral to the intact peduncle evoked contralateral vCb responses with a mean latency of only 0.25 ms after the ipsilateral responses, even though the distance between them was ∼ 5 mm. We investigated whether a rapidly conducting commissure exists between each vCb by stimulating one of them directly. Responses in both vCb spread sagittally, but, surprisingly, there was no sequential activation along a transverse Cb beam between them. In contrast, stimulation medial to either vCb evoked transverse beams that required ∼ 20 ms to cross the Cb. Therefore, the rapid commissural connection between each vCb is not mediated by slowly conducting parallel fibers. Also, the vCb was not strongly activated by climbing fiber stimulation, suggesting that inputs to vCb involve distinct cerebellar circuits. Responses between the two vCb remained following knife cuts through the rostral and caudal Cb along the midline, through both peduncles, and even shallow midline cuts to the middle Cb through its white matter and granule cell layer. Commissural responses were still observed only with a narrow transverse bridge between each vCb or in thick transverse Cb slices. Horseradish peroxidase transport from one vCb labeled transverse axons traveling within the Purkinje cell layer that were larger than parallel fibers and lacked varicosities. In sagittal sections, cross-section profiles of myelinated axons were observed around Purkinje cells midway between the rostral and caudal Cb. This novel pathway for transverse communication between lateral edges of turtle Cb suggests that afferents may directly conduct vestibular information rapidly across the Cb to coordinate vestibulomotor reflex behaviors.

Pupil constriction evoked in vitro by stimulation of the oculomotor nerve in the turtle (Trachemys scripta elegans)

The pond turtle (Trachemys scripta elegans) exhibits a notably sluggish pupillary light reflex (PLR), with pupil constriction developing over several minutes following light onset. In the present study, we examined the dynamics of the efferent branch of the reflex in vitro using preparations consisting of either the isolated head or the enucleated eye. Stimulation of the oculomotor nerve (nIII) using 100-Hz current trains resulted in a maximal pupil constriction of 17.4% compared to 27.1% observed in the intact animal in response to light. When current amplitude was systematically increased from 1 to 400 microA, mean response latency decreased from 64 to 45 ms, but this change was not statistically significant. Hill equations fitted to these responses indicated a current threshold of 3.8 microA. Stimulation using single pulses evoked a smaller constriction (3.8%) with response latencies and threshold similar to that obtained using train stimulation. The response evoked by postganglionic stimulation of the ciliary nerve using 100-Hz trains was largely indistinguishable from that of train stimulation of nIII. However, application of single-pulse stimulation postganglionically resulted in smaller pupil constriction at all current levels relative to that of nIII stimulation, suggesting that there is amplification of efferent drive at the ganglion. Time constants for constrictions ranged from 88 to 154 ms with relaxations occurring more slowly at 174-361 ms. These values for timing from in vitro are much faster than the time constant 1.66 min obtained for the light response in the intact animal. The rapid dynamics of pupil constriction observed here suggest that the slow PLR of the turtle observed in vivo is not due to limitations of the efferent pathway. Rather, the sluggish response probably results from photoreceptive mechanisms or central processing.

Topography and response timing of intact cerebellum stained with absorbance voltage-sensitive dye

Physiological activity of the turtle cerebellar cortex (Cb), maintained in vitro, was recorded during microstimulation of inferior olive (IO). Previous single-electrode responses to such stimulation showed similar latencies across a limited region of Cb, yet those recordings lacked spatial and temporal resolution and the recording depth was variable. The topography and timing of those responses were reexamined using photodiode optical recordings. Because turtle Cb is thin and unfoliated, its entire surface can be stained by a voltage-sensitive dye and transilluminated to measure changes in its local absorbance. Microstimulation of the IO evoked widespread depolarization from the rostral to the caudal edge of the contralateral Cb. The time course of responses measured at a single photodiode matched that of single-microelectrode responses in the corresponding Cb locus. The largest and most readily evoked response was a sagittal band centered about 0.7 mm from the midline. Focal white-matter (WM) microstimulation on the ventricular surface also activated sagittal bands, whereas stimulation of adjacent granule cells evoked a radial patch of activation. In contrast, molecular-layer (ML) microstimulation evoked transverse beams of activation, centered on the rostrocaudal stimulus position, which traveled bidirectionally across the midline to the lateral edges of the Cb. A timing analysis demonstrated that both IO and WM microstimulation evoked responses with a nearly simultaneous onset along a sagittal band, whereas ML microstimulation evoked a slowly propagating wave traveling about 25 cm/s. The response similarity to IO and WM microstimulation suggests that the responses to WM microstimulation are dominated by activation of its climbing fibers. The Cb's role in the generation of precise motor control may result from these temporal and topographic differences in orthogonally oriented pathways. Optical recordings of the turtle's thin flat Cb can provide insights into that role.

Morphology, Intrinsic Membrane Properties, and Rotation-Evoked Responses of Trochlear Motoneurons in the Turtle

Intrinsic properties and rotation-evoked responses of trochlear motoneurons were investigated in the turtle using an in vitro preparation consisting of the brain stem with attached temporal bones that retain functional semicircular canals. Motoneurons were divided into two classes based on intrinsic properties. The first class exhibited higher impedance (123.0 ± 11.0 MΩ), wider spikes (0.99 ± 0.05 ms), a single spike afterhyperpolarization (AHP), little or no spike frequency adaptation (SFA), and anomalous rectification, characterized by an initial “sag” in membrane potential in response to hyperpolarizing current injection. The second class exhibited lower impedance (21.8 ± 2.5 MΩ), narrower spikes (0.74 ± 0.03 ms), a double AHP, substantial SFA, and little or no rectification. Vestibular responses were evoked by horizontal sinusoidal rotation (1/12-1/3 Hz; peak velocity: 30–100°/s). Spiking in higher-impedance cells was recruited earlier in the response and exhibited a more limited dynamic range relative to that of lower impedance cells. Spiking evoked by injecting depolarizing current during rotation was blocked during contraversive motion and was consistent with a shunting inhibition. No morphological features were identified in neurobiotin-filled cells that correlated with the two physiological classes. Recovered motoneurons were multipolar but exhibited a less-complex dendritic morphology than ocular motoneurons of similarly sized mammals. The two physiologically defined cell classes have homologues in other vertebrates, suggesting that intrinsic membrane properties play an important role in oculomotor processing.

The effects of unilateral eighth nerve block on fictive VOR in the turtle

Multiunit activity during horizontal sinusoidal motion was recorded from pairs of oculomotor, trochlear, or abducens nerves of an in vitro turtle brainstem preparation that received inputs from intact semicircular canals. Responses of left oculomotor, right trochlear and right abducens nerves were approximately aligned with leftward head velocity, and that of the respective contralateral nerves were in-phase with rightward velocity. We examined the effect of sectioning or injecting lidocaine (1-2 microL of 0.5%) into the right vestibular nerve. Nerve block caused a striking phase shift in the evoked response of right oculomotor and left trochlear nerves, in which (rightward) control responses were replaced by a smaller-amplitude response to leftward table motion. Such "phase-reversed" responses were poorly defined in abducens nerve recordings. Frequency analysis demonstrated that this activity was advanced in phase relative to post-block responses of the respective contralateral nerves, which were in turn phase-advanced relative to pre-block controls. Phase differences were largest (approximately 10 degrees) at low frequencies (approximately 0.1 Hz) and statistically absent at 1 Hz. The phase-reversed responses were further investigated by eliminating individual canal input from the left labyrinth following right nVIII block, which indicated that the activation of the vertical canal afferents is the source of this activity.

Modulation of visual inputs to accessory optic system by theophylline during hypoxia

Neural tissues from fresh water turtles have been electrophysiologically studied in vitro due to their significant resistance to hypoxia. Such neurons have resting membrane potentials that are similar to intact animals and receive similar synaptic inputs evoked by sensory stimuli. One mechanism to reduce the brain's metabolic requirement in the absence of oxygenated blood flow was investigated by blocking adenosine receptors before and during hypoxia. Extracellular and whole-cell patch recordings were made from the basal optic nucleus, whose neurons respond to visual stimuli in vitro. While the addition of the adenosine antagonist theophylline to oxygenated superfusate had minimal effect on the neural activity, theophylline in superfusate bubbled with nitrogen strongly increased activity compared to either oxygenated theophylline or control superfusate bubbled with nitrogen. The increase in spontaneous activity was due to increases to both amplitude and frequency of excitatory synaptic events. Even during these increases, the neurons continued to exhibit their direction-sensitive responses. These results indicate that adenosine may play a role in protecting the viability of the brainstem during hypoxia without reducing visually mediated brainstem reflex control.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Bilateral processing of vestibular responses revealed by injecting lidocaine into the eighth cranial nerve in vitro

Extracellular unit responses were recorded from the vestibular nucleus (VN) and medial longitudinal fasciculus during horizontal head rotation of an in vitro turtle brainstem in which the temporal bones remained attached. Units were characterized as type I or type II based on the responses to ipsiversive or contraversive rotation, respectively. Lidocaine injections (0.5-2 microl of 0.5%) into the root of the eighth cranial nerve within the cranium caused rapid effects on unit responses to head rotation. Responses of type I units were reduced by ipsilateral injection but enhanced following contralateral injection. On the other hand, type II units had their responses increased by ipsilateral injections yet decreased by contralateral injections. In approximately half of the type II cells, decrease of the contraversive response was accompanied by the appearance of latent ipsiversive activity. Our findings not only confirm that each eighth nerve has afferents that drive ipsiversive excitation of both vestibular nuclei but also suggest that both nerves compete to dominate a central neuron's vestibular response. These results may be inconsistent with the push-pull vestibular model in which each nerve drives the central neuron with a complementary response that enhances the vestibular output. An alternate model is described in which vestibular neurons receive bilateral excitation, and that excitatory input is antagonized by crossed inhibition during contraversive motion.

An in vitro technique for electrophysiological mapping of reptilian retinotectal projections

An in vitro procedure is described for electrophysiological mapping of the retinotectal projections using an eye-cup and brain stem preparation which remains viable for up to 30 h. The technique has been found to be successful in turtles and lizards and may be useful for other species in which metabolism is greatly depressed by low temperatures. There are several advantages over in vivo recording, including the longevity and stability of the preparation, an absence of confounding anaesthetic effects and the ability to record from the retina as well as from the brain. The technique offers opportunities to introduce pharmacological agents via the perfusate or to conduct anatomical tracing studies coincident with electrophysiological recording.