Telemetric animal model to evaluate visceral pain in the freely moving rat.

Several research groups have measured the visceromotor response to visceral distension by electromyography (EMG) in the conscious restraint, wrapped or lightly anaesthetized rat. Our aim was to develop a more physiological and stress-free technique that enables the simultaneous measurement of duodenal distension-induced visceromotor and cardiovascular responses in the conscious, freely moving rat. A telemetry transmitter, consisting of a bipolar electrode pair and arterial catheter, was chronically implanted into the rat to measure abdominal EMG, mean arterial pressure (MAP) and heart rate (HR). Furthermore, a balloon catheter was chronically implanted in the duodenum to deliver volume-fixed staircase (0.1-0.6 ml) or phasic (0.1, 0.3, 0.5 ml) distensions. The area under the curve (AUC; mVs) and maximal amplitude (EMG(max); mV) during distension were analyzed. The model was validated by pre-treatment with morphine (0.3, 1.5 and 3 mg/kg, intraperitoneally). Staircase and phasic distension produced a volume-dependent increase in AUC and EMG(max), HR and MAP. Pre-treatment with morphine inhibited the distension-induced visceromotor response, i.e. abdominal contractions, increase in AUC and EMG(max). These findings indicate that telemetry is an adequate tool to measure visceromotor and cardiovascular responses to averse, noxious duodenal distension continuously and simultaneously in the rats home cage, without additional handling-related or restraint-induced stress. The presented animal visceral model is intended for studying acute and chronic analgesic properties of new pharmaceutical compounds.

CRH signalling in the bed nucleus of the stria terminalis is involved in stress-induced cardiac vagal activation in conscious rats.

The bed nucleus of the stria terminalis (BNST) is involved in autonomic and behavioral reactions to fearful stimuli and contains corticotropin-releasing hormone (CRH) fibers and terminals. The role of CRH in the medial part of the BNST in the regulation of heart rate (HR) and PQ interval of the electrocardiogram was studied under resting conditions and conditioned fear stress in freely moving rats. Microinfusion of CRH (0.2 microg/0.6 microl) in the medial BNST under resting conditions significantly enhanced HR as compared to saline treatment, but did not reduce the PQ interval, indicating that exogenous CRH in the medial BNST can activate both the sympathetic and parasympathetic cardiac outflow. In addition, CRH induced a slight increase in gross locomotor activity, an effect that succeeded the tachycardiac response, indicating that the HR response was not a consequence of increased locomotor activity, but likely a direct effect of CRH. CF was induced by 10-min forced exposure to a cage in which the rat had experienced footshocks (5 x 0.5 mA x 3s) the day before. alpha-helical CRH(9-41) (alphahCRH; 5 microg/0.6 microl), a non-selective CRH receptor antagonist, or saline was infused into the medial BNST of rats prior to CF. CF induced freezing behavior, associated with an increase in HR and PQ interval, indicating activation of sympathetic and vagal outflow to the heart. alphahCRH significantly reduced the PQ response, but enhanced the tachycardia, suggesting inhibition of vagal activity. In addition, alpha-helical CRH(9-41) reduced the freezing response. Taken together, the data provide first evidence that CRH, released in the medial BNST during stress, contributes to cardiac stress responses, particularly by activating vagal outflow.

Effect of gamma-melanocyte-stimulating hormones on baroreflex sensitivity and cerebral blood flow autoregulation in rats.

OBJECTIVE: In the present paper, we are interested in the effects of gamma-melanocyte-stimulating hormones (gamma-MSHs) on cardiovascular regulatory systems. METHODS: Mean arterial pressure (MAP), cerebral blood flow (CBF) and heart rate (HR) were measured in urethane-anaesthetised rats after intravenous administration of lysgamma(2)-MSH, gamma(2)-MSH, gamma(2)-MSH(6-12) or phenylephrine. RESULTS: The gamma-MSHs caused an increase in MAP, CBF and HR, whereas phenylephrine caused an increase in MAP and CBF and baroreceptor reflex-mediated bradycardia. All tested gamma-MSHs showed a significant impairment of the baroreceptor reflex sensitivity and CBF autoregulation as compared to the phenylephrine group. gamma(2)-MSH shows identical effects on the baroreceptor reflex and CBF as the endogenous occurring lysgamma(2)-MSH. In addition, the C-terminal fragment of gamma(2)-MSH, gamma(2)-MSH(6-12), induced similar effects as gamma(2)-MSH. The level of increase in MAP was comparable between the gamma-MSHs and the phenylephrine group. CONCLUSIONS: The present study suggests that gamma(2)-MSH and the shorter fragment gamma(2)-MSH(6-12) impair baroreceptor reflex sensitivity, due to a strong increase in sympathetic tone and/or change in baroreceptor reflex setpoint, and induce cerebrovasodilatation, which can counteract an autoregulation-mediated cerebrovasoconstriction due to systemic pressor effects. Furthermore, the results indicate that the C-terminal site of gamma(2)-MSH is relevant for its central-mediated inhibitory effects on the baroreceptor reflex and CBF.

Relevance of the C-terminal Arg-Phe sequence in gamma(2)-melanocyte-stimulating hormone (gamma(2)-MSH) for inducing cardiovascular effects in conscious rats.

1. The cardiovascular effects by gamma(2)-melanocyte-stimulating hormone (gamma(2)-MSH) are probably not due to any of the well-known melanocortin subtype receptors. We hypothesize that the receptor for Phe-Met-Arg-Phe-amide (FMRFa) or Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-amide (neuropeptide FF; NPFFa), other Arg-Phe containing peptides, is the candidate receptor. Therefore, we studied various Arg-Phe containing peptides to compare their haemodynamic profile with that of gamma(2)-MSH(6 - 12), the most potent fragment of gamma(2)-MSH. 2. Mean arterial pressure (MAP) and heart rate (HR) changes were measured in conscious rats after intravenous administration of gamma(2)-MSH related peptides. 3. Phe-Arg-Trp-Asp-Arg-Phe-Gly (gamma(2)-MSH(6 - 12)), FMRFa, NPFFa, Met-enkephalin-Arg-Phe-amide (MERFa), Arg-Phe-amide (RFa), acetyl-Phe-norLeu-Arg-Phe-amide (acFnLRFa) and desamino-Tyr-Phe-norLeu-Arg-Phe-amide (daYFnLRFa) caused a dose-dependent increase in MAP and HR. gamma(2)-MSH(6 - 12) showed the most potent cardiovascular effects (ED(50)=12 nmol kg(-1) for delta MAP; 7 nmol kg(-1) for delta HR), as compared to the other Arg-Phe containing peptides (ED(50)=177 - 292 nmol kg(-1) for delta MAP; 130 - 260 nmol kg(-1) for delta HR). 4. Peptides, which lack the C-terminal Arg-Phe sequence (Lys-Tyr-Val-Met-Gly-His-Phe-Arg-Trp-Asp-Arg-Pro-Gly (gamma(2)-pro(11)-MSH), desamino-Tyr-Phe-norLeu-Arg-[L-1,2,3,4 tetrahydroisoquinoline-3-carboxylic acid]-amide (daYFnLR[TIC]a) and Met-enkephalin (ME)), were devoid of cardiovascular actions. 5. The results indicate that the baroreceptor reflex-mediated reduction of tonic sympathetic activity due to pressor effects is inhibited by gamma(2)-MSH(6 - 12) and that its cardiovascular effects are dependent on the presence of a C-terminal Arg-Phe sequence. 6. It is suggested that the FMRFa/NPFFa receptor is the likely candidate receptor, involved in these cardiovascular effects.

Role of corticotropin-releasing factor, vasopressin and the autonomic nervous system in learning and memory.

Learning and memory are essential requirements for every living organism in order to cope with environmental demands, which enables it to adapt to changes in the conditions of life. Research on the effects of hormones on memory has focused on hormones such as adrenocorticotropic hormone (ACTH), glucocorticoids, vasopressin, oxytocin, epinephrine, corticotropin-releasing factor (CRF) that are released into the blood and brain following arousing or stressful experiences. Most of the information have been derived from studies on conditioned behavior, in particular, avoidance behavior in rats. In these tasks, an aversive situation was used as a stimulus for learning. Aversive stimuli are associated with the release of stress hormones and neuropeptides. Many factors play a role in different aspects of learning and memory processes. Neuropeptides not only affect attention, motivation, concentration and arousal or vigilance, but also anxiety and fear. In this way, they participate in learning and memory processes. Furthermore, neuropeptides such as CRF and vasopressin modulate the release of stress hormones such as epinephrine. In turn, systemic catecholamines enhance memory consolidation. CRF and vasopressin are colocalized in neurons from the nucleus paraventricularis, which project to nuclei in the brainstem involved in autonomic regulation. The objective of this paper is to discuss the role of CRF, vasopressin, and the autonomic nervous system (ANS) in learning and memory processes. Both CRF and vasopressin have effects in the same direction on behavior, learning and memory processes and stress responses (release of catecholamines and ACTH). These neuropeptides may act synergistically or in a concerted action aimed to learn to adapt to environmental demands.

The role of the CRH type 1 receptor in autonomic responses to corticotropin- releasing hormone in the rat.

The involvement of the corticotropin-releasing hormone (CRH) type 1 receptor in CRH-induced cardiac responses was studied in freely moving rats. Intracerebroventricular (icv) infusion of 2 microg CRH under resting conditions resulted in a significant increase in heart rate (HR), but did not significantly affect the PQ interval of the electrocardiogram. This effect involves sympathetic nervous system (SNS) activation, since CRH-treatment resulted in a marked increase in plasma norepinephrine (NE) and epinephrine (E), and sympathetic blockade by subcutaneously injected atenolol (1 mg/kg), a beta1-selective adrenergic antagonist, completely prevented the CRH-induced tachycardia. CRH infusion after sympathetic blockade resulted in an elongation of the PQ interval, indicating CRH-induced vagal activation. Gross locomotor activity (GA) was determined to study its possible indirect effects on cardiac activity. Although CRH induced a marked increase in GA, this effect followed the tachycardiac response, indicating that the HR response was not a consequence of increased locomotor activity, but was a direct effect of icv CRH. Treatment with CP-154,526 (icv, 10 or 25 microg), a selective CRH type 1 receptor antagonist, did not affect baseline HR, plasma NE and E, whereas it partially blocked the CRH-induced increase in HR, plasma NE and E levels. CP-154,526 treatment had no significant effects on baseline or CRH-induced changes in GA. These results indicate that CRH activates the sympathetic nervous system at least in part via the CRH type 1 receptor.

Endogenous corticotropin-releasing hormone inhibits conditioned-fear-induced vagal activation in the rat.

The role of the endogenous corticotropin-releasing hormone (CRH) system in the regulation of heart rate, PQ interval (a measure of vagal activity), gross activity and release of adrenocorticotropic hormone (ACTH), noradrenaline and adrenaline into the blood during conditioned fear was studied in freely moving rats. Intracerebroventricular (i.c.v.) infusion of alpha-helical CRH-(9-41) (10 microgram/3 microliter), a non-selective CRH receptor antagonist, under resting conditions had no significant effect on gross activity, heart rate and PQ interval, indicating that alpha-helical CRH at this dose was devoid of agonist effects. Conditioned fear was induced by 10 min forced exposure to a cage in which the rat had experienced footshocks (5x0.5 mAx3 s) 1 day before. Conditioned-fear rats showed freezing behaviour, associated with an increase in heart rate, PQ interval, noradrenaline and adrenaline, indicating that the conditioned-fear-induced cardiac effects were the result of coactivation of the sympathetic and parasympathetic nervous system. The i.c.v. pre-treatment of rats with alpha-helical CRH significantly reduced the conditioned-fear-induced tachycardiac and ACTH response, and enhanced the increase in PQ interval, without affecting the noradrenaline and adrenaline response. These results suggest that endogenous CRH reduces the vagal response to conditioned-fear stress in rats. To test this, rats were pre-treated with atropine methyl nitrate (0.3 mg/kg, subcutaneously; s.c.), a peripherally acting cholinergic receptor antagonist. This resulted in a complete blockade of the alpha-helical CRH-induced decrease in heart rate response and increase in PQ interval. From these findings, it is concluded that endogenous CRH in the brain inhibits vagal outflow induced by emotional stress.

Conditioned fear-induced tachycardia in the rat: vagal involvement.

The effects of conditioned fear on gross activity, heart rate, PQ interval, noradrenaline and adrenaline were studied in freely moving rats. Subcutaneous (s.c.) injections of atropine methyl nitrate (0.5 mg/kg) during rest resulted in a significant shortening of the PQ interval, indicating that the PQ interval can be used as a measure of vagal activity. Conditioned fear was induced by 10-min forced exposure to a cage in which the rat had previously experienced footshocks (5 x 0.5 mA x 3 s). In non-shocked controls, an increase in gross activity was found and a pronounced tachycardia, without changes in PQ interval. Conditioned fear rats showed immobility behaviour, associated with a less pronounced tachycardia and an increase in PQ interval. Noradrenaline was similarly increased in both groups, whereas adrenaline was increased in conditioned fear rats only. To further evaluate the role of the vagus, rats were exposed to conditioned fear after pre-treatment with atropine methyl nitrate (0.5 mg/kg, s.c.). Again, immobility was observed with a concomitant tachycardia, but without an increase in PQ interval. These results indicate that the autonomic nervous system is differentially involved in heart rate regulation in conditioned fear rats and in non-shocked controls: in non-shocked controls a predominant sympathetic nervous system activation results in an increase in heart rate, whereas in conditioned fear rats the tachycardiac response is attenuated by a simultaneous activation of sympathetic nervous system and parasympathetic nervous system.

Vagal activation in novelty-induced tachycardia during the light phase in the rat.

The effects of repeated exposure to a novel test box on cardiac and behavioral activities (locomotion, rearing, grooming, scanning, and immobility) were studied in rats tested during the dark phase ("dark" rats) or the light phase ("light" rats) of the lighting cycle, using a telemetry system for registration of ECGs during the first and fifth tests. Heart rate (HR) was used to monitor sympathetic and parasympathetic activity; the PQ interval was used to monitor parasympathetic activity. Behavior was videotaped simultaneously. In light rats, the first and fifth exposures to the test box resulted in higher increases of active behavior and HR than in dark rats, whereas the duration of the PQ interval of the ECG was increased in light rats only. This indicates that in the light phase novelty induces active behavior associated with an increase in both sympathetic and vagal outflow, whereas in the dark phase behavioral activation is predominantly associated with increased sympathetic activity, without appreciable changes in vagal outflow. In addition, light rats showed less active behavior during the fifth than during the first exposure, indicating behavioral habituation. This behavioral habituation to the test box in the light phase coincided with vagal habituation (a diminution of the PQ interval). The increase of the tachycardiac response during the fifth exposure as compared to the first exposure suggests that it is not likely that sympathetic outflow was part of the habituation process. In dark rats no behavioral or cardiac habituation was found.

Cisplatin-induced autonomic neuropathy: does it really exist?

The neurotoxic side-effects of cisplatin affect predominantly the large, myelinated fibres of peripheral nerves, leading to a sensory neuropathy. Several reports of cisplatin-associated autonomic neuropathy have been published. Autonomic dysfunction however, is caused by a neuropathy of small unmyelinated nerve fibres. By using the absolute pupil diameter as a parameter of autonomic nervous system function, we studied autonomic neuropathy in the eye of cisplatin-intoxicated rats. In addition, we examined autonomic cardiovascular function by measuring the change in heart rate (HR) and mean arterial blood pressure (MAP) in response to intravenous phenylephrine (PHE) and tyramine (TYR). No significant differences in mean pupil diameter developed in cisplatin-intoxicated rats (n = 12) in the course of 9 weeks (total cumulative dose cisplatin 18 mg/kg) compared with normal controls (n = 9) MANOVA, F1,19 = 0.88, P < 0.36). The PHE- and TYR-induced changes in MAP and HR were virtually the same in cisplatin-intoxicated rats when compared with normal controls. We conclude that cisplatin probably does not cause autonomic dysfunction, at least not in rats, in doses commonly used and which are known to cause a peripheral sensory neuropathy.

PVH lesions do not inhibit stressor-induced grooming in the rat.

Electrical and chemical stimulation of specific parts of the paraventricular hypothalamus (PVH) and the adjacent hypothalamus induce self-grooming responses in the rat. The function of this hypothalamic grooming area (HGA) is not understood. The localization of the HGA in the hypothalamus suggests that grooming, a behavioural response to stressors, is somehow linked to the neuroendocrine response to stressors. In this study it is shown that grooming induced by the stressors, mild restraint and moistening of the fur of the rat, is not inhibited by complete, bilateral radiofrequency lesions of the HGA. The changes in grooming patterns observed following lesions suggest that the HGA may have a function in the timing of different grooming elements.