Response properties of the periodontal mechanosensitive neurons in the trigeminal main sensory nucleus of the cat

Summary Periodontal mechanosensitive units (PM units) were recorded from the trigeminal main sensory nucleus (Vms) of the cat. The receptive fields of PM units were arranged from mandibular to maxillary divisions dorso-ventrally. The majority of PM units were single tooth units responsive to the canine tooth. They were directionally selective and had sustained responses to pressure applied to the tooth. The optimal stimulus direction of maxillary and mandibular PM units when the canine tooth was stimulated was single and it was oriented predominantly in the caudio-medial or rostrolateral direction. The threshold intensity of canine tooth stimulation was less than 0.05 N in most of the units. These findings indicate that the response properties of PM units in the Vms resemble fairly closely those of the primary afferent nerves arising from the periodontal mechanoreceptors.     

Functional relationship between cat brainstem neurons during sleep and wakefulness.

The mode of interaction between so-called sleep-wakefulness centers in the cat lower brainstem was studied on 100 neurons. The magnitude of the spike response of a neuron in one center to electrical stimulation of another center was measured to calculate the index of responsiveness. During REM sleep the index was, in a great majority of cases, significantly smaller as compared with that during wakefulness and slow wave sleep. This reduction in the effectiveness of information transmission between different centers might be the basis of characteristic events occurring during REM sleep. From the behavior of the indexes during sleep-wakefulness cycle, it is suggested that different phases of sleep and wakefulness are realized by a complicated interplay of many sleep-wakefulness centers which would be communicating with one another not only through channels which are activated in a phase-specific manner, but also through a larger number of channels of which activity is modulated differentially during different phases.     

Response of neurons in primary somatosensory cortex of cat following prolonged exposure to strong sinusoidal mechanical stimuli

Psychophysical studies in humans have demonstrated adaptation of the mechanoreceptive submodalities of flutter and vibration after prolonged presentation of a strong adapting, or conditioning, stimulus. In our studies we recorded from single neurons in cat primary somatosensory cortex, and followed a paradigm in which the response of a single unit was measured before and after the presentation of a strong conditioning stimulus. Our findings show no adaptive response in the 30 sec period immediately after cessation of the conditioning stimulus, and suggest that other parallel ascending pathways or somatosensory cortical areas other than SI account for these psychophysical observations.     

Gaze-related activity of brainstem omnipause neurons during combined eye-head gaze shifts in the alert cat

Summary Studies undertaken in head-restrained animals have long implicated the omnipause neurons (OPNs) in the initiation of saccadic eye movements. These inhibitory neurons discharge tonically but cease firing just before and during saccades in all directions. By recording from OPNs in alert behaving head-unrestrained cats, we have demonstrated that the activity of these cells is related to the displacement of the visual axis in space (gaze), which is the sum of the eye movement relative to the head and head movement relative to space. OPNs were found to exhibit a complete cessation of discharge for a period equivalent to the duration of the gaze shift, and not to the duration of either the rapid eye movement or the head movement components. In large gaze shifts, OPNs were silent even when the eye was immobile in the orbit, as long as the gaze shift was not completed. The results of this study show that OPNs are controlled by neural elements that take into account the actual position of the visual axis relative to its final desired position, irrespective of the trajectory of the eye in the orbit or of whether the head is moving or not.     

Neuronal activity of identified posterior hypothalamic neurons projecting to the brainstem peribrachial area of the cat

Following horseradish peroxidase injections in the brainstem peribrachial (PB) area, massive retrograde labeling was found in the posterior hypothalamic region. Single-unit recordings posterior hypothalamic neurons with antidromically identified projections to the PB area revealed that these neurons have higher firing rates in waking than in slow-wave sleep and dissimilar discharge patterns as compared with intralaminar thalamic neurons. The results are discussed in the context of reciprocal hypothalamo-brainstem circuits.     

Response properties of the periodontal mechanosensitive neurons in the thalamus of the cat: a comparison between the slowly adapting and rapidly adapting neurons

Slowly adapting (SA) and rapidly adapting (RA) types of the periodontal mechanosensitive units (PM units) were recorded in the thalamus and their response properties were examined in the cat. Both types of the PM units were located in the medial area (PM area) of the nucleus ventralis posteromedialis (VPM) of the thalamus. The SA units were located in a rostro-medial part of the PM area, while the RA units were distributed in the caudo-lateral part. An incidence of the SA and RA units was 45.5 and 54.5%, respectively. The single-tooth units were found in 23.5% of the SA units and in 14.9% of the RA units, and they responded chiefly to mechanical stimulation of the contralateral canine tooth. The multi-tooth units of the SA type had smaller receptive fields than those of the RA type, because the majority of the RA units had their receptive fields at the bilateral and/or bimaxillary area. In total, the units having the contralateral receptive fields were also dominant in both adaptation types. The latency of the neuronal discharges to electrical stimulation of the receptive field was fairly shorter in the SA units than in the RA units. These findings suggest that the SA units of the thalamus receive periodontal inputs directly from the trigeminal nuclear complex (Vcomp) of the brain stem, while the RA units receive them polysynaptically from the Vcomp via other pathways.     

c-FOS-like immunoreactivity in rat brainstem neurons following noxious chemical stimulation of the nasal mucosa

It has previously been shown that noxious and non-noxious peripheral stimuli induce c-fos expression in spinal dorsal horn neurons. In the present study we have examined the expression of c-fos in brainstem neurons following noxious chemical stimulation of the respiratory region of the nasal mucosa. In urethane-anaesthetized rats we injected mustard oil or applied CO2 pulses to the right nasal cavity. In control animals we applied paraffin oil or a continuous flow of air. A further group of control animals was anaesthetized and not subjected to any experimental treatment. Two hours after the first stimulus the rats were perfused with 4% phosphate-buffered paraformaldehyde. Brainstem sections were incubated with primary antiserum against the FOS protein and processed according to the ABC method. Only the mustard oil-treated rats had obvious signs of rhinitis and displayed FOS-positive cells in laminae I and II of the subnucleus caudalis and in the subnucleus interpolaris of the trigeminal brainstem nuclear complex as well as in the medullary lateral reticular nucleus. These areas are known to be involved in the processing of nociceptive information. Although CO2 pulses applied to the nasal mucosa are known to evoke pain sensations in man we did not observe any FOS-positive neurons in trigeminal and reticular brainstem areas of CO2-treated rats. This lack of c-fos expression probably results from the fact that unlike mustard oil, CO2 did not induce any apparent inflammatory reactions. In all animals c-fos expression was found in the nucleus of the solitary tract and in the area postrema. Staining in these areas might partly result from factors related to anaesthesia, changed respiration parameters and stress. Since the mustard oil-treated rats displayed the highest levels of immunoreactivity in the nucleus of the solitary tract and in the area postrema, additional effects specifically related to nociceptive input are very likely.     

Membrane ionic currents and properties of freshly dissociated rat brainstem neurons

It is well known that neuronal firing properties are determined by synaptic inputs and inherent membrane functions such as specific ionic currents. To characterize the ionic currents of brainstem cardio-respiratory neurons, cells from the hypoglossal (XII) nucleus and the dorsal motor nucleus of the vagus (DMX) were freshly dissociated and membrane ionic currents were studied under whole-cell voltage and current clamp. Both of these neurons showed a TTX-sensitive Na⁺ current with a much larger current density in XII than DMX neurons. This Na⁺ current had two (fast and slow) distinct inactivation decay components. The ratio of the magnitudes of the fast to slow component was roughly two-fold greater in DMX than in XII cells. Both DMX and XII neurons also showed a high voltage-activated Ca²⁺ current, but this current density was significantly greater (three-fold) in DMX than XII neurons. A relatively small amount of low-voltage activated Ca²⁺ current was also observed in DMX neurons, but not in the majority of XII cells. A transient and a sustained outward current components were observed in DMX cells, but only sustained currents were present in XII neurons. These outward currents had a reversal potential of about -70 mV with 3 mM external K⁺ and -30mV with 25 mM K⁺, and substitution of K⁺ with cesium and tetraethylammonium suppressed more than 90% the outward currents, indicating that most outward currents were carried by K⁺. The transient outward current consisted of two components with one sensitive to 4-aminopyridine and the other to intracellular Ca²⁺. In XII neurons, BRL 38227 (lemakalim), an ATP-sensitive K⁺ (K_ATP) channel activator, increased the sustained K⁺ currents by 10% of control, and glibenclamide, a K_ATP channel blocker, decreased the sustained K⁺ currents by 20%. Evidence for the presence of an inward rectifier K⁺ current was also obtained from both XII and DMX neurons. These results on XII and DMX neurons indicate that (1) the methods used to dissociate neurons provide a useful means to overcome voltage clamp technical difficulties; (2) ion channel characteristics such as density and biophysical properties of DMX neurons are very different from those of XII neurons; and (3) several newly discovered membrane ionic currents are present in these cells.     

Spatial properties of second-order vestibulo-ocular relay neurons in the alert cat

Second-order vestibular nucleus neurons which were antidromically activated from the region of the oculomotor nucleus (second-order vestibuloocular relay neurons) were studied in alert cats during whole-body rotations in many horizontal and vertical planes. Sinusoidal rotation elicited sinusoidal modulation of firing rates except during rotation in a clearly defined null plane. Response gain (spike/s/deg/s) varied as a cosine function of the orientation of the cat with respect to a horizontal rotation axis, and phases were near that of head velocity, suggesting linear summation of canal inputs. A maximum activation direction (MAD) was calculated for each cell to represent the axis of rotation in three-dimensional space for which the cell responded maximally. Second-order vestibuloocular neurons divided into 3 non-overlapping populations of MADs, indicating primary canal input from either anterior, posterior or horizontal semicircular canal (AC, PC, HC cells). 80/84 neurons received primary canal input from ipsilateral vertical canals. Of these, at least 6 received input from more than one vertical canal, suggested by MAD azimuths which were sufficiently misaligned with their primary canal. In addition, 21/80 received convergent input from a horizontal canal, with about equal number of type I and type II yaw responses. 4/84 neurons were HC cells; all of them received convergent input from at least one vertical canal. Activity of many vertical second-order vestibuloocular neurons was also related to vertical and/or horizontal eye position. All AC and PC cells that had vertical eye position sensitivity had upward and downward on-directions, respectively. A number of PC cells had MADs centered around the MAD of the superior oblique muscle, and 2/3 AC cells recorded in the superior vestibular nucleus had MADs near that of the inferior oblique. Thus, signals with spatial properties appropriate to activate oblique eye muscles are present at the second-order vestibular neuron level. In contrast, none of the second-order vestibuloocular neurons had MADs near those of the superior or inferior rectus muscles. Signals appropriate to activate these eye muscles might be produced by combining signals from ipsilateral and contralateral AC neurons (for superior rectus) or PC neurons (for inferior rectus). Alternatively, less direct pathways such as those involving third or higher order vestibular or interstitial nucleus of Cajal neurons might play a crucial role in the spatial transformations between semicircular canals and vertical rectus eye muscles.     

Membrane ionic currents and properties of freshly dissociated rat brainstem neurons

It is well known that neuronal firing properties are determined by synaptic inputs and inherent membrane functions such as specific ionic currents. To characterize the ionic currents of brainstem cardio-respiratory neurons, cells from the hypoglossal (XII) nucleus and the dorsal motor nucleus of the vagus (DMX) were freshly dissociated and membrane ionic currents were studied under whole-cell voltage and current clamp. Both of these neurons showed a TTX-sensitive Na⁺ current with a much larger current density in XII than DMX neurons. This Na⁺ current had two (fast and slow) distinct inactivation decay components. The ratio of the magnitudes of the fast to slow component was roughly two-fold greater in DMX than in XII cells. Both DMX and XII neurons also showed a high voltage-activated Ca²⁺ current, but this current density was significantly greater (three-fold) in DMX than XII neurons. A relatively small amount of low-voltage activated Ca²⁺ current was also observed in DMX neurons, but not in the majority of XII cells. A transient and a sustained outward current components were observed in DMX cells, but only sustained currents were present in XII neurons. These outward currents had a reversal potential of about − 70 mV with 3 mM external K⁺ and −30 mV with 25 mM K⁺, and substitution of K⁺ with cesium and tetraethylammonium suppressed more than 90% the outward currents, indicating that most outward currents were carried by K⁺. The transient outward current consisted of two components with onesensitive to 4-aminopyridine and the other to intracellular Ca²⁺. In XII neurons, BRL 38227 (lemakalim), an ATP-sensitive K⁺ (K_ATP) channel activator, increased the sustained K⁺ currents by 10% of control, and glibenclamide, a K_ATP channel blocker, decreased the sustained K⁺ currents by 20%. Evidence for the presence of an inward rectifier K⁺ current was also obtained from both XII and DMX neurons. These results on XII and DMX neurons indicate that (1) the methods used to dissociate neurons provide a useful means to overcome voltage clamp technical difficulties; (2) ion channel characteristics such as density and biophysical properties of DMX neurons are very different from those of XII neurons; and (3) several newly discovered membrane ionic currents are present in these cells.     

Auditory brainstem responses in the aged cat

Auditory brainstem responses (ABRs) were compared in young adult and aged cats. Mean thresholds for click-evoked ABRs were greater in the aged cats. Clicks normalized to 15 and 30 dB above individual thresholds at rates of 10, 20, 50 and 100/sec evoked ABRs with similar latencies and central conduction times in both groups. Background noise at equal intensity for all cats completely suppressed ABRs evoked by clicks 30 dB above threshold in 2/3 of the young but none of the old cats. As rise time of a 25 msec noise burst at equal intensity for all cats increased 1, 2, 5, and 10 msec, latency of wave 4 increased more for the old cats than for the young. Summed monaural ABRs from both ears were greater than binaural ABRs for waves 4 and 5 in both groups. These data indicate peripheral auditory dysfunction in aged cats but little abnormality in auditory brainstem transmission with click intensity normalized for ABR threshold.     

The location of brainstem neurons with bilateral projections to the motor nuclei of jaw openers in the cat

Symmetrical motor output is the rule in the masticatory system. We examined morphologically how this functional symmetry might be reflected in the organization of premotor neurons that could mediate excitation of jaw-opener motoneurons. Premotor neurons projecting bilaterally to jaw-opener motoneurons by way of axon collaterals were identified by retrograde dual-labeling with cholera toxin B-conjugated fluorescein isothiocyanate (CTb-FITC) and tetramethylrhodamine (TMR). In each cat, CTb-FITC and TMR were injected into the digastric motoneuron pools, respectively, on the left and right sides. In three animals, 69-147 neurons were labeled with both tracers, comprising approximately 44% of all retrogradely labeled cells. Double-labeled cells were located bilaterally in the trigeminal oral nucleus (Vo) and the adjacent reticular formation (RF), the former containing a larger number of cells. Neurons labeled with only one tracer were also distributed bilaterally in the Vo and RF. The results indicated that the bilaterally projecting premoter neurons identified mainly in the Vo and RF represent neuronal substrates for the symmetry that characterizes most jaw movements.     

Response of cat semicircular canal afferents to sinusoidal polarizing currents: implications for input-output properties of second-order neurons

1. We studied the response of cat vestibular afferents, most likely innervating the semicircular canals, to sinusoidal polarizing currents applied to an electrode implanted near the horizontal ampulla. 2. Electrode implantation abolished responses to natural stimulation and reduced the level of resting activity compared to a population of afferents from unimplanted animals. The distribution of coefficients of variation of resting activity was, however, similar to that seen when the labyrinth is intact. 3. Many fibers were modulated sinusoidally by polarizing currents in the frequency range 0.175-4 Hz. Phase was mainly constant and typically led stimulus negativity by approximately 14 degrees, although about half the regular fibers had a phase lead that increased with frequency. Mean sensitivity (spikes X s-1 X microA-1) of regular and irregular fibers increased by a factor of about 1.5 over the frequency studied. Absolute sensitivity was about 7 times higher for irregular than for regular fibers. The overall behavior of the afferents could be well described by a transfer function in the form, sk, with 0 less than k less than 1. 4. We compared the response of afferent fibers to sinusoidal current with the response of second-order neurons studied under similar conditions in earlier experiments (15, 23). While the slopes of the sensitivities were similar, second-order neurons developed a phase advance over afferents at frequencies around 1 Hz. This difference in dynamics can be described by a transfer function in the form tau S + 1, with tau = 12 ms. This predicts that second-order neurons can develop a phase lead of about 25 degrees with respect to afferents at 6 Hz, a frequency still in the physiological range. It remains to be determined whether this applies to a particular subset of second-order neurons contributing to vestibulocollic reflexes.     

Mapping of neurokinin b in the cat brainstem

We studied the distribution of neurokinin B-immunoreactive cell bodies and fibers in the cat brainstem using an indirect immunoperoxidase technique. The highest density of immunoreactive fibers was found in the motor trigeminal nucleus, the laminar and alaminar spinal trigeminal nuclei, the facial nucleus, the marginal nucleus of the brachium conjunctivum, the locus coeruleus, the cuneiform nucleus, the dorsal motor nucleus of the vagus, the postpyramidal nucleus of the raphe, the lateral tegmental field, the K?lliker-Fuse nucleus, the inferior central nucleus, the periaqueductal gray, the nucleus of the solitary tract, and in the inferior vestibular nucleus. Immunoreactive cell bodies containing neurokinin B were observed, for example, in the locus coeruleus, the dorsal motor nucleus of the vagus, the median division of the dorsal nucleus of the raphe, the lateral tegmental field, the pericentral nucleus of the inferior colliculus, the internal division of the lateral reticular nucleus, the inferior central nucleus, the periaqueductal gray, the postpyramidal nucleus of the raphe, and in the medial nucleus of the solitary tract. This widespread distribution of neurokinin B in the cat brainstem suggests that the neuropeptide could be involved in many different physiological functions. In comparison with previous studies carried out in the rat brainstem on the distribution of neurokinin B, our results point to a more widespread distribution of this neuropeptide in the cat brainstem.     

Projections of brainstem core cholinergic and non-cholinergic neurons of cat to intralaminar and reticular thalamic nuclei

We combined the retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase with choline acetyltransferase immunohistochemistry to study the projections of cholinergic and non-cholinergic neurons of the upper brainstem core to rostral and caudal intralaminar thalamic nuclei, reticular thalamic complex and zona incerta in the cat. After wheat germ agglutinin-horseradish peroxidase injections in the rostral pole of the reticular thalamic nucleus, the distribution and amount of retrogradely labeled brainstem neurons were similar to those found after tracer injection in thalamic relay nuclei (see preceding paper). After wheat germ agglutinin-horseradish peroxidase injections in the caudal intralaminar centrum medianum-parafascicular complex, rostral intralaminar central lateral-paracentral wing, and zona incerta, the numbers of retrogradely labeled brainstem neurons were more than three times higher than those found after injections in thalamic relay nuclei. The larger numbers of horseradish peroxidase-positive brainstem reticular neurons after tracer injections in intralaminar or zona incerta injections results from a more substantial proportion of labeled neurons in the central tegmental field at rostral midbrain (perirubral) levels and in the ventromedial part of the pontine reticular formation, ipsi- and contralaterally to the injection site. Of all retrogradely labeled neurons in the caudal midbrain core at the level of the cholinergic peribrachial area and laterodorsal tegmental nucleus, 45-50% were also choline acetyltransferase-positive after the injections into central lateral-paracentral and reticular nuclei, while only 25% were also choline acetyltransferase-positive after the injection into the centrum medianum-parafascicular complex. These findings are discussed in the light of physiological evidence of brainstem cholinergic mechanisms involved in the blockade of synchronized oscillations and in activation processes of thalamocortical systems.