Superior Salivatory Nucleus Neurons Research Articles

Orexin A and B in the rat superior salivatory nucleus.

Orexin (OX), which regulates sleep and wakefulness and feeding behaviors has 2 isoforms, orexin-A and -B (OXA and OXB). In this study, the distribution of OXA and OXB was examined in the rat superior salivatory nucleus (SSN) using retrograde tracing and immunohistochemical and methods. OXA- and OXB-immunoreactive (-ir) nerve fibers were seen throughout the SSN. These nerve fibers surrounded SSN neurons retrogradely labeled with Fast blue (FB) from the corda-lingual nerve. FB-positive neurons had pericellular OXA- (47.5%) and OXB-ir (49.0%) nerve fibers. Immunohistochemistry for OX receptors also demonstrated the presence of OX1R and OX2R in FB-positive SSN neurons. The majority of FB-positive SSN neurons contained OX1R- (69.7%) or OX2R-immunoreactivity (57.8%). These neurons had small and medium-sized cell bodies. In addition, half of FB-positive SSN neurons which were immunoreactive for OX1R (47.0%) and OX2R (52.2%) had pericellular OXA- and OXB-ir nerve fibers, respectively. Co-expression of OX1R- and OX2R was common in FB-positive SSN neurons. The present study suggests a possibility that OXs regulate the activity of SSN neurons through OX receptors.

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.

Stimulation of Baroresponsive Parts of the Nucleus of the Solitary Tract Produces Nitric Oxide-mediated Choroidal Vasodilation in Rat Eye.

Preganglionic parasympathetic neurons of the ventromedial part of the superior salivatory nucleus (SSN) mediate vasodilation of orbital and choroidal blood vessels, via their projection to the nitrergic pterygopalatine ganglion (PPG) neurons that innervate these vessels. We recently showed that the baroresponsive part of the nucleus of the solitary tract (NTS) innervates choroidal control parasympathetic preganglionic neurons of SSN in rats. As this projection provides a means by which blood pressure (BP) signals may modulate choroidal blood flow (ChBF), we investigated if activation of baroresponsive NTS evokes ChBF increases in rat eye, using Laser Doppler Flowmetry (LDF) to measure ChBF transclerally. We found that electrical activation of ipsilateral baroresponsive NTS and its efferent fiber pathway to choroidal SSN increased mean ChBF by about 40–80% above baseline, depending on current level. The ChBF responses obtained with stimulation of baroresponsive NTS were driven by increases in both choroidal blood volume (ChBVol; i.e., vasodilation) and choroidal blood velocity (ChBVel; possibly due to orbital vessel dilation). Stimulation of baroresponsive NTS, by contrast, yielded no significant mean increases in systemic arterial blood pressure (ABP). We further found that the increases in ChBF with NTS stimulation were significantly reduced by administration of the neuronal nitric oxide (NO) synthase inhibitor Nω-propyl-l-arginine (NPA), thus implicating nitrergic PPG terminals in the NTS-elicited ChBF increases. Our results show that the NTS neurons projecting to choroidal SSN do mediate increase in ChBF, and thus suggest a role of baroresponsive NTS in the BP-dependent regulation of ChBF.

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.

Muscarinic receptor immunoreactivity in the superior salivatory nucleus neurons innervating the salivary glands of the rat

The superior salivatory nucleus (SSN) contains preganglionic parasympathetic neurons to the submandibular and sublingual salivary glands. Cevimeline, a muscarinic acetylcholine receptor agonist, stimulates the salivary glands and is presently used as sialogogue in the treatment of dry mouth. Since cevimeline passes through the blood–brain barrier, it is also able to act on muscarinic acetylcholine receptors in the central nervous system. Our preliminary experiment using the whole-cell patch-clamp technique has shown that cevimeline excites SSN neurons in rat brain slices, suggesting that SSN neurons have muscarinic acetylcholine receptors; however, it is unclear which subtypes of muscarinic acetylcholine receptors exist in SSN neurons. In the present study, we investigated immunohistochemically muscarinic acetylcholine receptor subtypes, M1 receptor (M1R), M2R, M3R, M4R, and M5R in SSN neurons. SSN neurons innervating the salivary glands, retrogradely labeled with a fluorescent tracer from the chorda-lingual nerve, mostly expressed M3R immunoreactivity (-ir) (92.3%) but not M1R-ir. About half of such SSN neurons also showed M2R- (40.1%), M4R- (54.0%) and M5R-ir (46.0%); therefore, it is probable that SSN neurons co-express M3R-ir with at least two of the other muscarinic receptor subtypes. This is the first report to show that SSN neurons contain muscarinic acetylcholine receptors.

Projections from the hypothalamic paraventricular nucleus and the nucleus of the solitary tract to prechoroidal neurons in the superior salivatory nucleus: Pathways controlling rodent choroidal blood flow

Using intrachoroidal injection of the transneuronal retrograde tracer pseudorabies virus (PRV) in rats, we previously localized preganglionic neurons in the superior salivatory nucleus (SSN) that regulate choroidal blood flow (ChBF) via projections to the pterygopalatine ganglion (PPG). In the present study, we used higher-order transneuronal retrograde labeling following intrachoroidal PRV injection to identify central neuronal cell groups involved in parasympathetic regulation of ChBF via input to the SSN. These prominently included the hypothalamic paraventricular nucleus (PVN) and the nucleus of the solitary tract (NTS), both of which are responsive to systemic BP and are involved in systemic sympathetic vasoconstriction. Conventional pathway tracing methods were then used to determine if the PVN and/or NTS project directly to the choroidal subdivision of the SSN. Following retrograde tracer injection into SSN (biotinylated dextran amine 3K or Fluorogold), labeled perikarya were found in PVN and NTS. Injection of the anterograde tracer, biotinylated dextran amine 10K (BDA10K), into PVN or NTS resulted in densely packed BDA10K+terminals in prechoroidal SSN (as defined by its enrichment in nitric oxide synthase-containing perikarya). Double-label studies showed these inputs ended directly on prechoroidal nitric oxide synthase-containing neurons of SSN. Our study thus establishes that PVN and NTS project directly to the part of SSN involved in parasympathetic vasodilatory control of the choroid via the PPG. These results suggest that control of ChBF may be linked to systemic blood pressure and central control of the systemic vasculature.

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.

Immunohistochemical study on the distribution and origin of GABAergic nerve terminals in the superior salivatory nucleus

The superior salivatory nucleus (SSN) is the primary parasympathetic center controlling submandibular salivatory secretion. Our previous electrophysiological study revealed that many SSN neurons receive GABAergic and glycinergic synaptic inputs. In the present study, we examined the distribution of GABAergic and glycinergic nerve terminals, GABA(A) receptors in the SSN, and the origin of GABAergic nerve terminals innervating the SSN. Glutamic acid decarboxylase (GAD) and glycine transporter 2 (GLYT2) were used as markers of GABAergic and glycinergic nerve terminals, respectively. GAD- and GLYT2-positive nerve terminals and GABA(A) receptors were examined immunohistochemically in SSN neurons labeled by the retrograde axonal transport of FastBlue (FB) injected into the chorda-lingual nerve. The SSN neurons abundantly contained GAD-positive nerve terminals and GABA(A) receptors, suggesting that SSN neurons undergo strong GABAergic inhibition. The origin of GABAergic terminals was examined in neurons labeled by the retrograde transport of FluoroGold (FG) injected into the SSN. GAD was used as a marker of GABAergic neurons. Numerous FG-labeled neurons were found in the forebrain and brainstem. However, in FG-labeled neurons, GAD-positive neurons were occasionally observed in the reticular formation of the brainstem. These findings suggest that SSN neurons mainly receive GABAergic projections from the reticular formation.

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

We investigated the effects of the ionophoretic application of ionotropic non-NMDA receptor agonist (AMPA) and NMDA receptor agonist (NMDA) on extracellularly recorded and antidromically identified superior salivatory nucleus (SSN) neurons. A great majority (93%) of SSN neurons was induced to fire by ionophoretic application of AMPA, and they were classified into high firing rate (more than 6 spikes/s), and low firing rate (less than 3 spikes/s) neurons. No clear differences were found between high firing rate and low firing rate neurons according their fibre type and histological locations. Of the SSN neurons that excited by AMPA, 22% (4/18) and 50% (5/9) of the neurons also were induced to fire following ionophoretic application of the NMDA receptor agonist NMDA in different concentrations, 20 mM and 100 mM, respectively. In neurons that induced firing by AMPA and by NMDA, AMPA-evoked firings were induced by the lower intensities of applied current and had higher mean firing rates than NMDA-evoked firing. Neurons that were induced firing by AMPA and by NMDA had B fibre and C fibre axons as well as those that induced firing only by AMPA. Neurons that were fired only by AMPA were found in whole SSN area, whereas neurons that were induced firing by AMPA and by NMDA were mainly found in intermediate SSN area. In conclusion, activation of ionotoropic non-NMDA receptor has a greater excitatory effect on the SSN neurons than that of ionotropic of NMDA receptor. Our data support the view that non-NMDA receptor plays a major role, whereas NMDA receptor plays a minor role, in the activation of SSN neurons.

Ionotropic NMDA receptor evokes an excitatory response in superior salivatory nucleus neurons in anaesthetized rats

Extracellular recordings were taken from preganglionic superior salivatory nucleus (SSN) neurons projecting to submandibular and intra-lingual ganglia, in order to study the action of SSN neurons resulting from ionophoretic application of ionotropic NMDA receptor agonist in urethane-chloralose anaesthetized rats. Single SSN neurons were identified by their antidromic spike responses following stimulation of the chorda–lingual nerve (CLN), chorda tympani branches (CTBs) and the lingual nerve (LN). About one-third (33%, 10/30) of the identified SSN neurons were induced to fire by ionophoretic application of the NMDA receptor agonists used, dl-homocysteic acid (DLH) and N-methyl- d-aspartic acid (NMDA). More than half exhibited firing at high frequencies, often exceeding 40 Hz. About one-fifth (20%; 6/30) of the identified SSN neurons exhibited orthodromic spike responses to the combination of NMDA receptor agonist application and sensory nerve (CLN or LN) stimulus. These excitatory responses evoked by application of NMDA receptor agonist were attenuated ( n=4) by ionophoretic application of dl-2-amino-5-phosphonovaleric acid (AP5; NMDA receptor antagonist). About half (47%) of the neurons did not respond to any combination of NMDA receptor agonist and sensory nerve stimuli. No differences were observed between SSN neurons with B fibre axons and those with C fibre axons in response to ionophoresis of the NMDA receptor agonists. The NMDA-sensitive neurons, which exhibited high frequency firing, were predominantly found in the rostral part of the SSN. In summary, activation of ionotropic NMDA receptors exerts an excitatory effect on about half of the SSN neurons. These data support the view that NMDA receptors are involved in information processing and transmission on SSN neurons.

Localization of preganglionic neurons that innervate choroidal neurons of pterygopalatine ganglion.

The pterygopalatine ganglion (PPG) receives preganglionic input from the superior salivatory nucleus (SSN) of the facial motor complex and is the main source of parasympathetic input to the choroid in mammals. The present study was undertaken to determine in rats the location and neurotransmitters of SSN neurons innervating those PPG neurons that target the choroid and to determine the location and neurotransmitters of the PPG choroidal neurons themselves. Retrograde labeling from rat choroid using a fluorescent tracer, in combination with immunofluorescence labeling for nitric oxide synthase (NOS), vasoactive intestinal polypeptide (VIP), and choline acetyltransferase (ChAT), was used to characterize the location and neurotransmitters of choroidal PPG neurons. To identify SSN neurons that innervate the choroidal PPG neurons, the Bartha strain of the retrograde transneuronal tracer pseudorabies virus (PRV-Ba) was injected into rat choroid, and immunolabeling for NOS or ChAT was used to characterize their neurochemistry. Fluorescent retrograde labeling showed that PPG neurons projecting to the choroid contained NOS, VIP, and ChAT and were widely distributed in PPG and its preganglionic root, the greater petrosal nerve. SSN neurons were ChAT(+), and a subset of them was found to contain NOS. PRV-Ba transneuronal retrograde labeling revealed that choroidal preganglionic neurons were localized to the rostral medioventral part of the ipsilateral SSN. The choroidal SSN neurons were ChAT(+) and appeared largely to correspond to the NOS(+) neurons of the SSN. These results show that preganglionic neurons in rats that are presumed to regulate choroidal blood flow through the PPG reside within the rostral medioventral SSN, and that NOS is a marker for these SSN neurons.

Evidence for a glutamatergic input to pontine preganglionic neurons of the superior salivatory nucleus in rat

Parasympathetic preganglionic neurons of the superior salivatory nucleus (SSN), which projects to the pterygopalatine ganglion (PPG), modulate salivation, lacrimation, and cerebrovascular tone. Our previous studies suggest that excitatory projections from the nucleus tractus solitarii modulate cerebrovascular tone by actions on SSN neurons. In this study we sought to test the hypothesis that N-methyl- d-aspartate (NMDA) type glutamate receptors and vesicular glutamate transporters (VGLUT) are present in the SSN and that SSN neurons receive glutamatergic input. In six rats we injected tetramethylrhodamine dextran (TRD), a fluorescent tracer, unilaterally into the PPG to label SSN neurons. Four days later, rats were perfused and brain stem sections containing the SSN were processed for fluorescent immunohistochemistry for N-methyl- d-aspartate receptor subunit 1 (NMDAR1) and vesicular glutamate transporters (VGLUT1 and VGLUT2). Confocal laser scanning microscopy showed that 88±3% of TRD-labeled SSN neurons contained NMDAR1-immunoreactivity (IR). The surrounding neuropil contained numerous fibers labeled for VGLUT2-IR, but not VGLUT1-IR. Double fluorescent immunohistochemistry for NMDAR1 and VGLUT2 revealed that fibers containing VGLUT2-IR were often in close proximity to cell bodies or proximal dendrites of TRD-labeled SSN neurons that were positive for NMDAR1-IR. These studies support our hypothesis that NMDA receptors and VGLUT are present in the SSN. They further provide support for the suggestion that there are glutamatergic inputs to SSN neurons and would be consistent with an excitatory input that could regulate cerebrovascular tone.

Analysis of the Central Pathway of the Trigeminal-Salivary Reflex by the Spinal Trigeminal Tractotomy.

The aim of this study was to clarify the role of spinal trigeminal subnucleus interporalis (Sp 5 I) and subnucleus caudalis (Sp 5 C) on the trigeminal-salivary reflex. Extracellular recordings were made from a total of 119 superior salivatory nucleus (SSN) neurons innervating the submandibular and sublingual glands in urethane-chloralose anaesthetized cats. Responses to stimulation of the inferior alveolar (IA) nerve were investigated in trigeminal tractotomized animals at the level of the obex (Sp 5 C animals) and at the level of 2.5-3mm rostral from the obex (Sp 5 I animals) as well as in non-tractotomized (control) animals. In control, Sp 5 C and Sp 5 I animals, 50% (23/46), 46% (15/33) and 10% (4/40) of the identified SSN neurons orthodromically responded to stimulation of the IA nerve, respectively.In Sp 5 C animals, no significant reduction of the response rate was found in comparison with control animals, and there was no significant difference in the mean onset latency between control and Sp 5 C animals. However, only the responses activated by the medium-threshold afferent fibres or by high-threshold afferent fibres could be observed.In Sp 5 I animals, a marked reduction of the response rate was noted in comparison with control animals, and there was no significant difference in the mean onset latency between control and Sp 5 I animals. However, only the responses excited by the medium-threshold afferent fibres or by high-threshold afferent fibres could be observed.As the relay nucleus in the trigeminal-salivary reflex, Sp 5 I has a more important role and Sp 5 C has a lesser role.

Immunohistochemical studies on glutamatergic, GABAergic and glycinergic axon varicosities presynaptic to parasympathetic preganglionic neurons in the superior salivatory nucleus of the rat

After the superior salivatory nucleus (SSN) neurons were labeled by administration of cholera toxin B subunit (CTB) or wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) to the pterygopalatine ganglion, morphological interactions between SSN neurons and fibers afferent to SSN neurons were examined by light and electron microscopy with double-immunostaining techniques. Antibodies to either the neurotransmitters or its receptor were used to label glutamatergic, GABAergic and glycinergic synapses on these neurons. By light microscopy, SSN neurons were identified in the ipsilateral ventrolateral part of the rostral medulla oblongata, and rich distributions of glutamate- and GABA-immunoreactive (ir) axon varicosities were observed around SSN neurons. Electron microscopy revealed that dendrites of SSN neurons received asymmetric synapses from glutamate-ir axon varicosities. Somata as well as dendrites received symmetric synapses from GABA-ir varicosities, or showed immunoreactivity for glycine receptors. Quantitative analysis by electron microscopy showed that glutamate-ir axon varicosities comprised 45.3% of total axon profiles in the SSN region, while GABA-ir varicosities were 20.8% and varicosities presynaptic to glycine receptors were 19.9%. These findings suggest that glutamatergic, GABAergic and glycinergic inputs, originated from a variety of nuclei, directly affect the activity of SSN neurons, and play a role in the regulation of the pterygopalatine ganglion of the rat.

Responses of the superior salivatory nucleus neurons to stimulation of the lateral hypothalamic area in the cat.

Responses of the superior salivatory nucleus neurons to stimulation of the lateral hypothalamic area in the cat.

Direct amygdaloid projections to the superior salivatory nucleus: a light and electron microscopic study in the cat

Amygdaloid projections to the superior salivatory nucleus (SSN) were investigated in the cat by using the anterograde and retrograde tracing techniques of horseradish peroxidase (HRP). After HRP injections were made into the lingual nerve, retrogradely labeled SSN neurons were located in the lateral tegmental field medial to the spinal trigeminal nucleus from the middle level of the superior olivary nucleus to the caudal level of the facial nucleus. These labeled neurons, triangular, oval or polygonal in shape, were small to medium-sized (12–29 μm) and formed loosely packed clusters. In further HRP studies, HRP injections were made into the amygdala and in the reticular formation containing the SSN neurons. The results suggested that the SSN receives direct afférents from the central nucleus of the amygdala with ipsilateral predominance. Final proof of such direct connections from amygdala to the SSN can be obtained only by electron microscopic study. Therefore, HRP injections were made into the lingual nerve and in the amygdala in the same animal and electron microscopic observations were carried out on the SSN. It appeared that anterogradely labeled amygdalotegmental fibers formed axosomatic and axodendritic synaptic contacts with retrogradely labeled SSN neurons.

Convergence of excitatory inputs from the chorda tympani, glossopharyngeal and vagus nerves onto superior salivatory nucleus neurons in the cat

Superior salivatory nucleus (SSN) neurons were identified by antidromic spike responses to stimulation of the chorda tympani nerve, and were tested to stimulation of the ipsilateral chorda tympani, glossopharyngeal, vagus and lingual nerves in urethane-chloralose anesthetized cats. In 48 SSN neurons identified, 33 (69%) responded with spikes to stimulation of at least one of these nerves, and 24 (50%) were excited with inputs from more than one stimulated nerve. The mean latencies of the reflex responses to stimulation of the chorda tympani, glossopharyngeal, vagus or lingual nerve were 13.2, 18.9, 24.6 or 11.4 ms, respectively.

Response of rat superior salivatory units to chorda tympani stimulation

Unit responses to chorda tympani stimulation in urethane anesthetized rats were characterized by response latency and frequency following ability. One hundred and fifty one units responded with constant latencies, mean: 16.9 msec, S.D.: 7.8 msec. Responses recorded from within the superior salivatory nucleus (SSN) had significantly longer latencies (18.8 ± 6.8, n = 94) than those from surrounding areas (13.5 ± 8.3, n = 50) ( p < 0.001). This difference was due to the higher incidence of responses with latencies between 2.7 and 10 msec outside of the SSN (44%) compared to SSN responses (8%). Ability to follow high frequency stimulation ranged from 2–550 Hz, and was the same for SSN neurons and those outside the nucleus. The population of responses localized to the SSN, with latencies greater than 10 msec and with high (> 100 Hz) frequency following, had a mean latency of 20.5 msec. The estimated conduction velocity for these presumed preganglionic, parasympathetic neurons is 0.85 M/sec ± 0.25, significantly slower than that reported for similar responses in cats. The antidromic nature of these responses requires confirmation.

A direct hypothalamic projection to the superior salivatory nucleus neurons in the rat. A study using anterograde autoradiographic and retrograde HRP methods

The location of the superior salivatory nucleus and terminal labelings of the hypothalamic descending fibers were demonstrated in the nucleus reticularis parvocellularis using HRP and the autoradiographic techniques, respectively. When both techniques were used in the same animals, some HRP-labeled neurons were seen among the accumulations of silver grains, suggesting pericellular terminations. The present study demonstrates that the hypothalamic efferents project directly to the superior salivatory nucleus innervating salivary and lacrimal glands.