Features peculiar to the trigeminal innervation

Development of resurgent and persistent sodium currents in mesencephalic trigeminal neurons

Sodium channels play multiple roles in the formation of neural membrane properties in mesencephalic trigeminal (Mes V) neurons and in other neural systems. Mes V neurons exhibit conditional robust high-frequency spike discharges. As previously reported, resurgent and persistent sodium currents (I_NaR and I_NaP , respectively) may carry small currents at subthreshold voltages that contribute to generation of spike firing. These currents play an important role in maintaining and allowing high-frequency spike discharge during a burst. In the present study, we investigated the developmental changes in tetrodotoxin-sensitive I_NaR and I_NaP underlying high-frequency spike discharges in Mes V neurons. Whole-cell patch-clamp recordings showed that both current densities increased one and a half times from postnatal day (P) 0-6 neurons to P7-14 neurons. Although these neurons do not exhibit subthreshold oscillations or burst discharges with high-frequency firing, I_NaR and I_NaP do exist in Mes V neurons at P0-6. When the spike frequency at rheobase was examined in firing Mes V neurons, the developmental change in firing frequency among P7-14 neurons was significant. I_NaR and I_NaP density at -40 mV also increased significantly among P7-14 neurons. The change to an increase in excitability in the P7-14 group could result from this quantitative change in I_NaP. In neurons older than P7 that exhibit repetitive firing, quantitative increases in I_NaR and I_NaP density may be major factors that facilitate and promote high-frequency firing as a function of age in Mes V neurons.

Participation of sodium currents in burst generation and control of membrane excitability in mesencephalic trigeminal neurons

Subthreshold sodium currents are important in sculpting neuronal discharge and have been implicated in production and/or maintenance of subthreshold membrane oscillations and burst generation in mesencephalic trigeminal neurons (Mes V). Moreover, recent data suggest that, in some CNS neurons, resurgent sodium currents contribute to production of high-frequency burst discharge. In the present study, we sought to determine more directly the participation of these currents during Mes V electrogenesis using the action potential-clamp method. In postnatal day 8-14 rats, the whole-cell patch-clamp method was used to record sodium currents by subtraction in response to application of TTX in voltage-clamp mode using the action potential waveform as the command protocol. We found that TTX-sensitive sodium current is the main inward current flowing during the interspike interval, compared with the h-current (Ih) and calcium currents. Furthermore, in addition to the transient sodium current that flows during the upstroke of action potential, we show that resurgent sodium current flows at the peak of afterhyperpolarization and persistent sodium current flows in the middle of the interspike interval to drive high-frequency firing. Additionally, transient, resurgent, and persistent sodium current components showed voltage- and time-dependent slow inactivation, suggesting that slow inactivation of these currents can contribute to burst termination. The data suggest an important role for these components of the sodium current in Mes V neuron electrogenesis.

Lack of global vagal propensity in patients with oculocardiac reflex

BACKGROUND

Profound bradycardia during ophthalmic surgery is a rare but potentially serious event. Little is known about the predictability of the oculocardiac reflex.

METHODS

Four vagotonic maneuvers were performed on six patients who had profound oculocardiac reflex (3- to 10-second asystole) during eye surgery, the results of which were compared with 30 previously studied control subjects. Electrocardiographs were monitored during the following vagotonic maneuvers: diving response (apneic facial immersion), Valsalva maneuver, ocular compression, and carotid sinus massage.

RESULTS

The degree of heart rate slowing as a result of diving response, Valsalva maneuver, and, notably, ocular compression did not differ when these patients were compared with the 30 previously studied control subjects. Carotid sinus massage produced significantly (P = 0.01) more bradycardia in the six patients (mean +/- standard deviation, -24% +/- 6%) than in the 30 control subjects (-12% +/- 7%). The heart rate response to pressure on the eyes did not correlate with prior intraoperative oculocardiac reflex.

CONCLUSION

The discrepancy in heart rate sensitivity between surgical extraocular muscle tension and ocular compression may be due to different sensory receptors and brain stem processing for the trigeminally mediated oculocardiac reflex. Carotid sinus massage may help predict low heart rates during eye surgery.

Bradycardia during cold ocular irrigation under general anaesthesia: an example of the diving reflex

A case of bradycardia is reported which was precipitated by cold normal saline applied to the eye during general anaesthesia. The history and physiology of the diving reflex is discussed and we believe that these data suggest that this patient's bradycardia was induced by the diving reflex, and not by the oculocardiac reflex.

Trigeminal primary afferent projections to "non-trigeminal" areas of the rat central nervous system

The central projections of rat trigeminal primary afferent neurons to various "non-trigeminal" areas of the central nervous system were examined by labeling the fibers with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) transported anterogradely from the trigeminal ganglion. This technique produced a clear and comprehensive picture of trigeminal primary afferent connectivity that was in many ways superior to that which may be obtained by using degeneration, autoradiography, cobalt labeling, or HRP transganglionic transport techniques. Strong terminal labeling was observed in all four rostrocaudal subdivisions of the trigeminal brainstem nuclear complex, as well as in the dorsal horn of the cervical spinal cord bilaterally, numerous brainstem nuclei, and in the cerebellum. Labeling in the ipsilateral dorsal horn of the cervical spinal cord was very dense at C1, moderately dense at C2 and C3, and sparse at C4-C7. Numerous fibers crossed the midline in the medulla and upper cervical spinal cord and terminated in the contralateral pars caudalis and dorsal horn of the spinal cord from C1-C5. The latter axons terminated most heavily in the mandibular and ophthalmic regions of the contralateral side. Extremely dense terminal labeling was observed in the ipsilateral paratrigeminal nucleus and the nucleus of the solitary tract, moderate labeling was seen in the supratrigeminal nucleus and in the dorsal reticular formation, and small numbers of fibers were observed in the cuneate, trigeminal motor, lateral and superior vestibular nuclei, and in the cerebellum. The latter fibers entered the cerebellum in the superior cerebellar peduncle and projected to the posterior and anterior lobes as well as to the interposed and lateral deep cerebellar nuclei. Most projections in this study originated from fibers in the dorsal part of the spinal tract of V, suggesting a predominantly mandibular origin for these fibers. Projections from the ophthalmic and maxillary divisions, in contrast, were directed mainly to the cervical spinal cord bilaterally, to contralateral pars caudalis, and to certain areas of the reticular formation. In conclusion, this study has demonstrated that somatosensory information from the head and face may be transmitted directly to widespread and functionally heterogeneous areas of the rat central nervous system, including the spinal cord dorsal horn, numerous brainstem nuclei, and the cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)

The neurobiology of vascular head pain

Nervous connections between the trigeminal ganglia and cerebral blood vessels have recently been identified in experimental animals and have been termed the trigeminovascular system. Existence of this system in humans is inferential. Trigeminovascular neurons and their peripheral unmyelinated nerve fibers contain the neurotransmitter peptide substance P. Most newly synthesized substance P is transported from ganglion cell bodies to afferent nerve fibers, where depolarization-induced release of neurotransmitter into the wall of the cerebral blood vessel occurs. Substance P dilates pial arteries, increases vascular permeability, and activates cells that participate in the inflammatory response. The relationship of trigeminovascular fibers to the pathogenesis of vascular head pain sheds light on possible mechanisms of migraine and other central nervous system conditions associated with headache and inflammation.