CRF microinjected into the dorsal vagal complex inhibits TRH analog- and kainic acid-stimulated gastric contractility in rats

The effect of CRF microinjected into the dorsal vagal complex (DVC) on centrally-stimulated gastric contractility was investigated in fasted, urethane-anesthetized rats. Miniature strain gauge force transducers were acutely implanted on the corpus of the stomach and contractility was analyzed by computer. Microinjection of the stable thyrotropin-releasing hormone (TRH) analog, RX 77368, (26 pmol) into the DVC induced a 12.2-fold stimulation of gastric contractility within 30 min. Corticotropin-releasing factor (CRF) (63-210 pmol) microinjected into the DVC concomitantly with RX 77368 (26 pmol) induced a dose-related inhibition of stimulated gastric contractility. Neither CRF alone (210 pmol) nor vehicle modified basal gastric contractility. Microinjection of kainic acid (141 pmol) into the raphe pallidus nucleus induced a 3.6-fold stimulation of gastric contractility after 45 min. This stimulation was suppressed by bilateral microinjection of CRF (105 pmol/site) into the DVC. These results demonstrate that CRF acts in the DVC to inhibit centrally-stimulated gastric contractility and suggest that TRH and CRF may interact in the DVC to regulate gastric motor function.     

Microinjection of thyrotropin-releasing hormone analogue into the central nucleus of the amygdala stimulates gastric contractility in rats

The effect on gastric contractility following bilateral microinjection of thyrotropin-releasing hormone (TRH) analog. RX 77368, into the central nucleus of the amygdala was examined in fasted. urethane-anesthetized rats. Extraluminal force transducers were used to measure gastric corpus contractility. Bilateral microinjection of RX 77368 (0.5 μg. 1.0 μg,n = 6 each) stimulated gastric contractility for up to 120 min post-injection,P < 0.05. Gastric contractility was not significantly stimulated by microinjection of 0.1 μg RX 77368. 0.1% bovine serum albumin (BSA) into the central nucleus or RX 77368 (0.5 μg. 1.0 μg) into sites adjacent to the central nucleus. Peak responses (1.0 μg) occurred 40 min post-injection and represented a 16-26-fold increase over basal values. The frequency of gastric contraction waves was attenuated for 0–90 min in rats receiving central amygdaloid microinjection of RX 77368 (0.1. 0.5 or 1.0 μg) versus rats microinjected with the vehicle or RX 77368 into sites adjacent to the central nuclei. The stimulatory effect of RX 77368 (1.0 μg) on gastric contractility was abolished by subdiaphragmatic vagotomy. These results indicate that the TRH analog. RX 77368, acts within the central amygdala to vagally stimulate gastric contractility.     

Raphe pallidus stimulation increases gastric contractility via TRH projections to the dorsal vagal complex in rats

The role of thyrotropin releasing hormone (TRH) in the dorsal vagal complex (DVC) in mediating the enhanced gastric contractility induced by glutamate (100 pmol) microinjected into the raphe pallidus (Rpa) was investigated in urethane-anesthetized rats acutely implanted with miniature strain gauge force transducers on the corpus of the stomach. Glutamate-induced stimulation of gastric contractility was dose-dependently inhibited by bilateral microinjection into the DVC of TRH antibody (0.17, 0.85 or 1.7 μg/100 nl/site) but not by vehicle. TRH antibody microinjected into the dorsal medullary reticular field had no effect. These data indicate that activation of Rpa neurons by glutamate increases gastric motor function through TRH release in the DVC.     

Microinjection of thyrotropin-releasing hormone in the paraventricular nucleus of the hypothalamus stimulates gastric contractility

Changes in gastric contractility following microinjection of thyrotropin-releasing hormone (TRH) into the paraventricular nucleus of the hypothalamus (PVN) were examined in fasted, urethane-anesthetized rats. Gastric contractility was measured with extraluminal force transducers and analysed by computer. Unilateral and bilateral PVN microinjections of TRH (0.5 and 1.0 μg) significantly increased the force index of gastric contractions from 0 to 60 min postinjection, when compared with animals microinjected with 0.1 μg TRH, 0.1% BSA or TRH (0.5 and 1.0 μg TRH) in sites adjacent to the PVN. The gastric force index was also significantly elevated from 61 to 120 min postinjection in rats receiving bilateral PVN microinjections of TRH (0.5 and 1.0 μg). Peak gastric responses occurred within 10–20 min postinjection and represented an approximately eight-fold increase over basal values. In the remaining groups, the force index was not significantly altered from preinjection values. The excitatory action of TRH (1.0 μg) on gastric contractility was completely abolished by subdiaphragmatic vagotomy. These results suggest that TRH acts within the PVN to stimulate gastric contractility via vagal-dependent pathways.     

Structure-activity relationships of TRH analogs in rat spinal cord injury

Effects of thyrotropin-releasing hormone (TRH) analogs were compared in rats to evaluate the structure-activity relationships of such compounds in the treatment of traumatic spinal cord injury. CG3703, a TRH analog having a modified amino-terminus, significantly improved motor recovery and somatosensory-evoked responses after trauma; in contrast, RX77368, which has a modified carboxy-terminus, was without effect, even at doses up to 10 mg/kg. These findings confirm and extend findings in cats, using other TRH analogs in a different model of spinal trauma. Together, data from rat and cat studies are consistent with the hypothesis that the integrity of the C-terminal amino acid may be critical for the beneficial effects of treatment with TRH and TRH analogs in experimental spinal injury, and suggest that a variety of other TRH analogs having substitutions of the pyroglutamyl or histidyl moieties of the tripeptide may also prove to be effective in the treatment of such injury.     

TRH stimulation andl-glutamic acid inhibition of proximal gastric motor activity in the rat dorsal vagal complex

The importance of the dorsal vagal complex (DVC) in the control of gastric motor activity has been previously established by electrical and chemical stimulation of this region. We have further evaluated excitatory and inhibitory influences on motor activity of the gastric corpus by microinjection ofl-glutamic acid (GLU) and thyrotropin-releasing hormone (TRH) into the DVC. GLU and TRH were ejected by pressure (20–30 psi) in 1–10 nl vol. from multibarreled micropipettes and intraluminal pressure and the gastric corpus was measured using a manometric catheter placed into the stomach through the pylorus of urethane-chloralose anesthetized rats. Gastric motor activity was monitored while micropipettes were advanced from the surface of the dorsomedial medulla to a depth of 1 mm in 100 μm increments. Microinjections of GLU (1–10 pmol) at depths of 200–600 μm below the surface of the brainstem caused a decrease in tonic intraluminal pressure and amplitude of phasic contactions of the gastric corpus. Injection of TRH (1–10 pmol) at depths of 200–800 μm increased both tonic intraluminal pressure and amplitude of phasic contractions. The responses to GLU (10 pmol) and TRH (10 pmol) were abolished by hexamethonium and vagotomy; atropine abolished the effect of TRH and attenuated that of GLU. It is concluded that GLU evokes only vagally mediated inhibitory effects on tonic and phasic gastric motor activity when microinjected into the DVC. In contrast, injection of TRH at the same loci causes only vagal cholinergic increases in motor activity. Subpopulations of neurons in the DVC may, therefore, be activated by specific neurotransmitters having opposite effects on gastric motor activity.     

Bombesin microinfusion into the paraventricular nucleus suppresses gastric acid secretion in the rat

Bombesin is a particularly potent inhibitor of gastric acid secretion when injected intracisternally in the rat. Because bombesin-like immunoreactivity is found in several forebrain regions implicated in gut regulation, the ability of bombesin to affect gastric secretion was tested in these areas by direct microinfusion. Bombesin significantly and dose-relatedly suppressed gastric acid secretion when it was infused into the hypothalamic paraventricular nucleus. Bombesin microinfusion into the ventromedial or lateral hypothalamic areas, or the caudate-putamen, had no significant effect. A further experiment using glass micropipets showed that back-diffusion of bombesin along the cannula track to a distant site of action was unlikely to account for the results obtained, and provided further evidence that the active site is limited to the paraventricular nucleus and possibly the ventralmost nucleus reuniens. The results suggest that the bombesin receptors and immunoreactive terminals previously identified in this region may be involved in the central regulation of gastric secretion.     

Hypothalamic control of gastric acid secretion

Study of hypothalamic control of gastric acid scretion (GAS) has revealed GAS-related neurons, their location in the lateral hypothalamic area (LHA), their characteristics, and implications of their relations to feeding and other functions. Some LHA glucose-sensitive neurons are referred to as gastric type because of their effects on gastric oxyntic cells via specific gastric related neurons of the medulla oblongata and the vagus. The 2-deoxy-D-glucose (2-DG), or insulin induced GAS was completely abolished by bilateral subdiaphragmatic vagotomy, or micro-lesions in specific sites of the LHA. These gastric type glucose-sensitive neurons were thus believed to contribute to control of GAS. The paraventricular nucleus (PVN) was also found to affect GAS. GAS-related PVN neurons were observed in the rostral PVN. Electrophoretic application of various chemicals, especially glucose, also affected neurons in the rostral PVN. Electrophoretically applied norepinephrine (NE) increased PVN single neuron activity and suppressed GAS. Results suggest that the rostral PVN may be another site to modulate LHA control of GAS, and NE may be a transmitter or modulator.     

Gastric vagal functional distribution in the secretion of gastric acid produced by sweet taste

The sweet signal pathway relevant to lingual control of gastric acid secretion was examined in bilaterally adrenalectomized rats. The increase in the acid output after lingual glucose application was completely blocked by prior section of both sides of the vagus nerve at the cervical level. However, the reduction in the acid output showed no laterality following vagotomy on either side. At the subdiaphragmatic level, the acid response was mainly suppressed by section of the dorsal vagus trunk. Both sides of trunk vagotomy abolished the acid response. These results suggest that the sweet signal evoking gastric acid secretion has a specific pathway from the tongue to the stomach, and that there is functional laterality in this pathway in the visceral cavity.     

Bombesin microinjected into the dorsal vagal complex inhibits TRH-stimulated gastric contractility in rats

The effects of centrally injected bombesin on central and peripheral stimulated gastric contractility were investigated in fasted urethane-anesthetized rats. Miniature strain gauge force transducers were acutely implanted on the corpus of the stomach and gastric contractility was analyzed by computer. Intracisternal injection of the stable thyrotropin-releasing hormone (TRH)-analog RX 77368 (77 pmol) induced a stimulation of gastric contractility for 40 min. Intracisternal injection of bombesin (62-620 pmol) followed 30 min later by that of RX 77368 resulted in a dose-related inhibition of the TRH-analog-induced gastric contractility. Intracisternal injection of bombesin (620 pmol) did not modify gastric contractility stimulated by intravenous carbachol. Stimulation of gastric contractility induced by TRH-analog microinjected into the dorsal vagal complex (DVC) was dose-related suppressed by concomitant injections of bombesin (6.2-620 pmol). Neither bombesin alone (6.2 pmol) nor vehicle modified basal gastric contractility. These results demonstrate that bombesin acts within the brain to inhibit vagally stimulated gastric contractility and that the DVC is a sensitive site for bombesin inhibitory action. These findings suggest a possible interaction between TRH and bombesin in the central vagal regulation of gastric contractility.     

Microelectrophoretic application of kainic acid into the globus pallidus: Disturbances in feeding behavior

Body weight changes, food and water intake, and sensorimotor disturbances of male rats were studied after bilateral kainic acid-(KA) induced lesions of the globus pallidus (GP). To minimize the extent of damages, KA was applied electrophoretically by means of glass micropipettes (tip diameter of the pipettes was 10-15 microns). The neuron-specific damages of the GP resulted in aphagia and adipsia and rapid body weight decrease. Lesioned animals showed permanent motor disturbances but only temporary difficulties in the orientation toward sensory stimuli. Our data show that the selective destruction of the GP neurons results in a complex disorder that has motivational, (sensori)motor, and metabolic components.     

Electrical stimulation of the medial amygdala facilitates gastric acid secretion in conscious rats

One hundred and twenty-seven conscious rats prepared with chronic gastric fistula were studied to investigate the effect of stimulation of the medial amygdala on gastric acid secretion. Gastric acid output was significantly increased by electrical stimulation of the medial amygdala in normal rats and the increase in acid secretion was completely abolished by vagotomy. Vagotomized rats, with or without amygdaloid stimulation, showed comparable levels of gastric acid output which were significantly lower than in controls. These results indicate that the amygdala effect on gastric acid secretion is carried via the vagus nerve. Subcutaneous injections of high doses of histamine increased gastric acid secretion which was further increased by amygdaloid stimulation. Plasma levels of gastrin were not significantly changed by stimulation of the medial amygdala with or without vagotomy. From the above results, we concluded that in conscious rats the medial amygdala plays a significant role in stimulating gastric acid secretion, the vagus nerve is involved in this process, but it is not mediated by release of either histamine or gastrin.     

Raphe pallidus and raphe obscurus projections to the intermediolateral cell column in the rat

The descending spinal projections from the raphe pallidus and raphe obscurus nuclei were studied by the anterograde autoradiographic technique in the rat. Both regions project to the intermediolateral cell columns. These pathways were destroyed by intraventricular injections of 5,7-dihydroxytryptamine, a neurotoxin relatively specific for serotonergic cells.     

Activation of muscarinic receptors stimulates GABA release in the rat globus pallidus

The effect of carbachol on the spontaneous release of ³H-GABA was investigated on rat globus pallidus (GP) slices. Carbachol dose-dependently enhanced the release of ³H-GABA. The carbachol (5 × 10⁻⁴ M) induced ³H-GABA release is mediated by muscarinic receptors since atropine (10⁻⁶ M), pirenzepine (10⁻⁶ M) and AF-DX384MS (10⁻⁶ M) abolished the effect. An indirect carbachol effect mediated by dopaminergic and glutamatergic afferents was ruled out since the effect was not blocked by either D1 (SCH23390 10⁻⁶) and D2 (sulpiride 10⁻⁵ M) receptor antagonists or by ionotropic glutamate receptor antagonists (CNQX 10⁻⁶ M and 10⁻⁵ M, MK801 10⁻⁶ M). A direct effect is further evidenced by the persistence of the carbachol effect in the presence of tetrodotoxin (5 × 10⁻⁷ M). Surprisingly the carbachol effect was not abolished by lowering the Ca²⁺ concentration of the superfusion medium or by increasing concomitantly the Mg²⁺ concentration. The involvement of a GABA transporter can partially explain this latter result, as nipecotic acid (10⁻³ M) blocked the effect by only 50%. Carbachol stimulated the accumulation of ³H-phosphoinositides in pallidal slices, an effect that was antagonized by atropine (10⁻⁶ M), pirenzepine (10⁻⁶ M), and AF-DX384MS (10⁻⁶ M). These results suggest that the activation of muscarinic receptors localized on striatopallidal terminals stimulates the release of GABA in the globus pallidus through inositol phosphate hydrolysis. Synapse 26:131–139, 1997. © 1997 Wiley-Liss, Inc.     

TRH and TRH-like peptide expression in rat following episodic or continuous corticosterone

Sustained abnormalities of glucocorticoid levels have been associated with neuropsychiatric illnesses such as major depression, posttraumatic stress disorder (PTSD), panic disorder, and obsessive compulsive disorder. The pathophysiological effects of glucocorticoids may depend not only on the amount of glucocorticoid exposure but also on its temporal pattern, since it is well established that hormone receptors are down-regulated by continuously elevated cognate hormones. We have previously reported that TRH (pGlu-His-Pro-NH(2)) and TRH-like peptides (pGlu-X-Pro-NH(2)) have endogenous antidepressant-like properties and mediate or modulate the acute effects of a single i.p. injection of high dose corticosterone (CORT) in rats. For these reasons, two accepted methods for inducing chronic hyperglucocorticoidemia have been compared for their effects on brain and peripheral tissue levels of TRH and TRH-like peptides in male, 250g, Sprague-Dawley rats: (1) the dosing effect of CORT hemisuccinate in drinking water, and (2) s.c. slow-release pellets. Overall, there were 93% more significant changes in TRH and TRH-like peptide levels in brain and 111% more in peripheral tissues of those rats ingesting various doses of CORT in drinking water compared to those with 1-3 s.c. pellets. We conclude that providing rats with CORT in drinking water is a convenient model for the pathophysiological effects of hyperglucocorticoidemia in rodents.     

Central control of gastric acid secretion by extralateralhypothalamic nuclei

Whether secretion of gastric acid (GAS) is in response to peripheral and/or central administration of chemical or electrical stimuli can be differentiated by vagotomy. GAS has been shown to be controlled by specific lateral hypothalamic (LHA) neurons. Application of 2-deoxy-D-glucose (2-DG) or insulin to the LHA by microinjection or iontophoresis has experimentally induced GAS. The paraventricular nucleus (PVN) has now been found to also affect GAS. GAS was produced more copiously and more quickly by rostral PVN lesion than by lesion of the ventromedial (VMH) or dorsomedial (DMH) nucleus, and nearly as much by caudal PVN lesion. Microinjection of 2-DG into the LHA induced GAS more potently in animals with rostral PVN lesions than in those with caudal PVN, VMH or DMH lesions, or in intact animals. Results indicate that the PVN may be an additional central site from which GAS is affected.     

Serotonin-mediated excitation of recurrent laryngeal and phrenic motoneurons evoked by stimulation of the raphe obscurus

Short-latency averaged responses in the recurrent laryngeal nerve (RLN) and C₅ phrenic nerve to electrical stimulation (2.5–80 μA; 2.5–160 Hz; 150 μs pulse duration) of the medullary nucleus raphe obscurus (RO) were investigated in anaesthetized, paralyzed and artificially ventilated cats. The response evoked in RLN by stimulation within RO was excitatory and consisted of a single peak. Characteristics of this response in RLN were compared with those of the delayed excitatory response in C₅ phrenic nerve, which we previously showed to be elicited by stimulation within RO. Mean latency to onset for the excitatory response in RLN was5.7 ± 0.3ms, while the delayed excitatory response in C₅ phrenic nerve occurred at7.0 ± 0.3ms. The excitatory response in both could be evoked when stimulation was applied during inspiration as well as during expiration. The stimulus threshold varied between 2.5 and 5 μA for evoked the inspiratory-phase response in each nerve. The magnitude of this response in RLN and in C₅ phrenic nerve was directly related to current intensity and was dependent upon stimulus frequency. Intravenous administration of the serotonin receptor antagonist, methysergide (0.1–2.4 mg/kg) caused significant dose-related reductions in the response in each nerve. In summary, characteristics of the evoked response in RLN and phrenic nerve are similar in several ways. Both responses are: (1) excitatory in nature, (2) elicited at small stimulus currents, (3) affected similarly by increasing stimulus current and frequency, (4) elicited by stimulation during inspiration and expiration, and (5) mediated at least in part by activation of pathways using serotonin as a neurotransmitter.     

Endogenous sugar acid control of hypothalamic neuron activity and gastric acid secretion in rats

The feeding related endogenous sugar acids, 2-deoxytetronic acid, 2-DTA and 3-deoxypentonic acid, 3-DPA, were investigated for their peripheral and central (hypothalamic) control of gastric acid secretion, and effects on activity of lateral hypothalamic (LHA) neurons in rats. Peripheral gastric acid secretion was not affected by either 2-DTA or 3-DPA. Slight gastric acid secretion was elicited by 3-DPA only when it was applied directly into the gastric related site of the LHA. Gastric acid secretion induced by 2-deoxy-D-glucose (2DG) was suppressed by application of 2-DTA in the LHA. 3-DPA had no effect on 2DG induced secretion. Electrophoretic application of 2-DTA significantly inhibited the activity of both gastric and non-gastric type glucose-sensitive neurons in the LHA, and 3-DPA significantly excited both types of glucose-sensitive neurons. The results agree with previous reports that 2-DTA and 3-DPA are endogenous satiety and hunger factors, respectively, and act by modulating hypothalamic control of gastric acid secretion which is mediated through gastric type and non-gastric type glucose-sensitive neurons.     

Neuronal responses to 5-hydroxytryptamine and dorsal raphe stimulation within the globus pallidus of the rat

The projection from the dorsal raphe nucleus (DRN) to the globus pallidus (GP) was investigated electrophysiologically, in the urethane-anesthetized rat together with the responsiveness of cells in the GP to 5-hydroxytryptamine (5-HT) and noradrenaline (NA). The majority of spontaneously active cells in the GP had high regular firing rates. They were unaffected by both DRN stimulation (69/83 cells) and iontophoretically applied 5-HT (38/63 units) or NA (30/42 units) but were inhibited by GABA. A few cells (N = 10) were recorded from, that were spontaneously active but with a much lower and less regular firing rate, which, however, seemed to be much more responsive to 5-HT. In addition, DL-homocysteic acid (DLH) was used to activate silent cells and all seven cells activated in this manner were inhibited by 5-HT. In addition 5/6 cells that had their firings maintained by DLH were inhibited by stimulation of the dorsal raphe. The results show a lack of responsiveness to both 5-HT and DRN stimulation of the typically regular spontaneously active pallidal neurons. There seems to be a small population of normally quiescent cells, however, that is sensitive to 5-HT and receives an input from the DRN.     

Central muscarinic cholinergic antagonists block wet-dog shakes produced by the TRH analog MK-771 in the rat

Thyrotropin-releasing hormone (TRH) is known to elicit wet-dog shakes in rats through a central mechanism of action. In the present study, the ability of muscarinic cholinergic antagonists to inhibit TRH-mediated wet-dog shakes was examined. The longer-acting TRH analog, MK-771, at doses of 1.0, 1.5 and 3.0 mg/kg elicited wet-dog shakes in a dose-dependent manner. The centrally-acting muscarinic cholinergic antagonists, atropine and scopolamine, at doses of 5 and 0.8 mg/kg, respectively, significantly reduced the ability of only the highest dose of MK-771 to elicit wet-dog shakes. When the peripherally-acting antagonists, methylscopolamine and methylatropine, were examined, they were not found to significantly reduce wet-dog shakes produced by MK-771 at doses of 5 and 0.8 mg/kg, respectively. The results of this study suggest that cholinergic antagonists inhibit Mk-771 induced wet-dog shakes in a non-competitive manner and support the view that TRH-mediated wet-dog shakes are modulated by central muscarinic cholinergic systems.