Non-NMDA and NMDA receptor agonists induced excitation and their differential effect in activation of superior salivatory nucleus neurons in anaesthetized rats

Microinjection of NMDA-neurotoxin into the superior salivatory nucleus of the rat: Short-term secretory and long-term drinking behavior effects

The anatomical location of the superior salivatory nucleus (SSN), the site of origin of the parasympathetic preganglionic cell bodies that innervate the submandibular-sublingual salivary glands, is well established in rats. However, as of yet there is no functional data that convincingly shows the secretory nature of this region. Previous studies have not been able to differentiate between interventions on efferent or afferent fibers connected to the SSN versus interventions on the salivatory nucleus itself. Taking advantage of the fact that salivatory neurons express NMDA-receptors on their somas, in the present study SSN cell bodies were activated and lesioned sequentially by means of intracerebral application of NMDA-neurotoxin. In exp. 1 two effects, a short- and a long-term effect, were observed following NMDA administration. The first effect was high submandibular-sublingual saliva secretion during the hour following administration of the neurotoxin and the second was a profound change in drinking behavior once the animals recovered from the lesion. Thus, on post-surgery days 16, 17 and 18, the rats exhibited hyperdipsia in the presence of dry food but not in the presence of wet food. In expt. 2 results showed that saliva hypersecretion observed after NMDA-microinjection was completely blocked by the administration of atropine (a cholinergic blocker) but not after the administration of dihydroergotamine plus propranolol (α and β-adrenergic blockers, respectively). From a functional perspective, these data suggest that the somata of the parvocellular reticular formation control the secretory activity of the submandibular-sublingual salivary glands and thus constitute the SSN.

Effects of cevimeline on excitability of parasympathetic preganglionic neurons in the superior salivatory nucleus of rats

The superior salivatory nucleus (SSN) contains parasympathetic preganglionic neurons innervating the submandibular and sublingual salivary glands. Cevimeline, a muscarinic acetylcholine receptor (mAChR) agonist, is a sialogogue that possibly stimulates SSN neurons in addition to the salivary glands themselves because it can cross the blood-brain barrier (BBB). In the present study, we examined immunoreactivities for mAChR subtypes in SSN neurons retrogradely labeled with a fluorescent tracer in neonatal rats. Additionally, we examined the effects of cevimeline in labeled SSN neurons of brainstem slices using a whole-cell patch-clamp technique. Mainly M1 and M3 receptors were detected by immunohistochemical staining, with low-level detection of M4 and M5 receptors and absence of M2 receptors. Most (110 of 129) SSN neurons exhibited excitatory responses to application of cevimeline. In responding neurons, voltage-clamp recordings showed that 84% (101/120) of the neurons exhibited inward currents. In the neurons displaying inward currents, the effects of the mAChR antagonists were examined. A mixture of M1 and M3 receptor antagonists most effectively reduced the peak amplitude of inward currents, suggesting that the excitatory effects of cevimeline on SSN neurons were mainly mediated by M1 and M3 receptors. Current-clamp recordings showed that application of cevimeline induced membrane depolarization (9/9 neurons). These results suggest that most SSN neurons are excited by cevimeline via M1 and M3 muscarinic receptors.

Proposal for research and education: joint lectures and practicals on central nervous system anatomy and physiology

We coordinated anatomy and physiology lectures and practicals to facilitate an integrated understanding of morphology and function in a basic medical science program for dental students and to reduce the time spent on basic science education. This method is a means to provide the essential information and skills in less time. The overall impression was that the practice of joint central nervous system lectures and practicals was an efficient method for students, which suggests that joint lectures might also be useful for clinical subjects. About two-thirds of students felt that the joint anatomy and physiology lecture on the central nervous system was useful and necessary in understanding the relationship between morphology and function, at least for this subject. One-third of students were neutral on the effectiveness of this method. However, the survey results suggest that improvements are needed in the method and timing of joint lectures and practicals. The present teaching approach can be further improved by conducting combined lectures in which the form and function of anatomic structures are presented by the relevant departments during the same lecture. Finally, joint lecturers and practicals offer an opportunity to increase student understanding of the importance of new research findings by the present authors and other researchers.

The identification and neurochemical characterization of central neurons that target parasympathetic preganglionic neurons involved in the regulation of choroidal blood flow in the rat eye using pseudorabies virus, immunolabeling and conventional pathway tracing methods

The choroidal blood vessels of the eye provide the main vascular support to the outer retina. These blood vessels are under parasympathetic vasodilatory control via input from the pterygopalatine ganglion (PPG), which in turn receives its preganglionic input from the superior salivatory nucleus (SSN) of the hindbrain. The present study characterized the central neurons projecting to the SSN neurons innervating choroidal PPG neurons, using pathway tracing and immunolabeling. In the initial set of studies, minute injections of the Bartha strain of the retrograde transneuronal tracer pseudorabies virus (PRV) were made into choroid in rats in which the superior cervical ganglia had been excised (to prevent labeling of sympathetic circuitry). Diverse neuronal populations beyond the choroidal part of ipsilateral SSN showed transneuronal labeling, which notably included the parvocellular part of the paraventricular nucleus of the hypothalamus (PVN), the periaqueductal gray, the raphe magnus (RaM), the B3 region of the pons, A5, the nucleus of the solitary tract (NTS), the rostral ventrolateral medulla (RVLM), and the intermediate reticular nucleus of the medulla. The PRV+ neurons were located in the parts of these cell groups that are responsive to systemic blood pressure signals and involved in systemic blood pressure regulation by the sympathetic nervous system. In a second set of studies using PRV labeling, conventional pathway tracing, and immunolabeling, we found that PVN neurons projecting to SSN tended to be oxytocinergic and glutamatergic, RaM neurons projecting to SSN were serotonergic, and NTS neurons projecting to SSN were glutamatergic. Our results suggest that blood pressure and volume signals that drive sympathetic constriction of the systemic vasculature may also drive parasympathetic vasodilation of the choroidal vasculature, and may thereby contribute to choroidal baroregulation during low blood pressure.

Multi-source inputs converge on the superior salivatory nucleus neurons in anaesthetized rats

Activation of parasympathetic nerves innervating salivary glands evokes not only salivation but also vascular responses. These parasympathetic nerves may have cardiac and/or respiratory-related activity as well as the cardiovascular sympathetic nerves that control vascular bed of salivary glands. Therefore, we investigated whether preganglionic superior salivatory nucleus (SSN) neurons projecting to the submandibular and intra-lingual ganglia exhibit pulse-related and/or respiratory-related activity, and whether they can be excited by electrical stimulation of the lingual nerve. 25% of SSN neurons were found to have pulse-related and tracheal pressure-related activities, implying that they receive cardiac and respiratory inputs. 44% of neurons exhibited only pulse-related activity, whereas 31% of the neurons had neither pulse-related nor tracheal pressure-related activity. Neurons with pulse and tracheal pressure-related activities, and those only with pulse-related activity, had B and C fibre range axons. 53% of SSN neurons received both cardiac and lingual nerve inputs. 16% of neurons recorded were found to receive only cardiac inputs, and 26% only lingual nerve inputs; whereas 5% received neither cardiac nor lingual nerve inputs. We conclude that the inputs from diverse sources converge on the SSN neurons, and they can cooperate to modulate SSN neuronal activity.

Peripheral glutamate signaling in head and neck areas

The major excitatory neurotransmitter glutamate is also found in the periphery in an increasing number of nonexcitable cells. In line with this it became apparent that glutamate can regulate a broad array of peripheral biological responses, as well. Of particular interest is the discovery that glutamate receptor reactive reagents can influence tumor biology. However, the knowledge of glutamate signaling in peripheral tissues is still incomplete and, in the case of head and neck areas, is almost lacking. The roles of glutamate signaling pathways in these regions are manifold and include orofacial pain, periodontal bone production, skin and airway inflammation, as well as salivation. Furthermore, the interrelations between glutamate and cancers in the oral cavity, thyroid gland, and other regions are discussed. In summary, this review shall strengthen the view that glutamate receptor reagents may also be promising targets for novel therapeutic concepts suitable for a number of diseases in peripheral tissues. The contents of this review cover the following sections: Introduction; The "Glutamate System"; The Taste of Glutamate; Glutamate Signaling in Dental Regions; Glutamate Signaling in Head and Neck Areas; Glutamate Signaling in Head and Neck Cancer; A Brief Overview of Glutamate Signaling in Other Cancers; and Conclusion.