Psychopharmacological evidences for the involvement of muscarinic and nicotinic cholinergic receptors on sweet substance-induced analgesia in Rattus norvegicus

Long-term effects of neonatal pain and sucrose treatment

Purpose

In neonatal intensive care units, applying sucrose solution for analgesia is now a routine treatment for mild procedural pain. Studies of animal and human infants provide clear evidence of benefits in the short term, but few studies have investigated the long term benefits. Thus, we determined whether sucrose could ameliorate painful stimulation during infancy in Sprague-Dawley rats and also explored the long-term effects of repeated sucrose administration during infancy. Female and male rats were included to investigate sex-related differences.

Methods

Rat pups were stimulated either with painful or tactile stimuli for the first 14 days of their lives. Pups were pretreated either with sucrose or not treated before stimulation. Behavioral tests were conducted during adolescence and adulthood. Hotplate, rotarod, open field, elevated plus maze, and radial arm water maze tests were employed to assess the behavioral consequences of early life manipulations and treatments.

Results

Painful stimulation during infancy increased the sensitivity to pain later in life, and sucrose did not remedy this effect. Motility, coordination, anxiety, and cognition tests in adulthood obtained mixed results. Pain during infancy appeared to increase anxiety during adulthood. Learning and memory in adulthood were affected by pain during infancy, and sucrose had a negative effect even in the absence of pain. No sex-related differences were observed in any of the behavioral tests by employing this model of neonatal pain.

Conclusion

Painful stimulation during infancy resulted in deficiencies in some behavioral tests later in life. Sucrose pretreatment did not mitigate these shortcomings and it actually resulted in negative outcomes.

Acute and Chronic Pain from Facial Skin and Oral Mucosa: Unique Neurobiology and Challenging Treatment

The oral cavity is a portal into the digestive system, which exhibits unique sensory properties. Like facial skin, the oral mucosa needs to be exquisitely sensitive and selective, in order to detect harmful toxins versus edible food. Chemosensation and somatosensation by multiple receptors, including transient receptor potential channels, are well-developed to meet these needs. In contrast to facial skin, however, the oral mucosa rarely exhibits itch responses. Like the gut, the oral cavity performs mechanical and chemical digestion. Therefore, the oral mucosa needs to be insensitive, to some degree, in order to endure noxious irritation. Persistent pain from the oral mucosa is often due to ulcers, involving both tissue injury and infection. Trigeminal nerve injury and trigeminal neuralgia produce intractable pain in the orofacial skin and the oral mucosa, through mechanisms distinct from those seen in the spinal area, which is particularly difficult to predict or treat. The diagnosis and treatment of idiopathic chronic pain, such as atypical odontalgia (idiopathic painful trigeminal neuropathy or post-traumatic trigeminal neuropathy) and burning mouth syndrome, remain especially challenging. The central integration of gustatory inputs might modulate chronic oral and facial pain. A lack of pain in chronic inflammation inside the oral cavity, such as chronic periodontitis, involves the specialized functioning of oral bacteria. A more detailed understanding of the unique neurobiology of pain from the orofacial skin and the oral mucosa should help us develop novel methods for better treating persistent orofacial pain.

Effect of chronic intake of sweet substance on nociceptive thresholds and feeding behavior of Rattus norvegicus (Rodentia, Muridae)

The investigation of the influence of sweetened food on feeding behavior targeted to non-sucrose nutrients as well as the sensitivity to painful stimuli in isolated and grouped animals is the aim of the present work. The tail withdrawal latencies in the tail-flick test (a spinal reflex) were measured before and immediately after the treatment with tap water or sucrose (62, 125 or 250 g/l). Our findings suggest that: (a) The analgesic effect of sucrose intake depends on the concentration of sucrose solution and on the time during which the solution is consumed; (b) the most effective concentration of sucrose followed by antinociceptive effect was the one of 250 g/l in both isolated and grouped animals; (c) considering the individually caged rats, the intake of sucrose in the highest concentration (250 g/l) was the smallest as compared with the consumption of sucrose in more diluted solutions (62.5 and 125 g/l), but this higher sweetened solution was followed by antinociception; (d) animals treated with concentrated sucrose solution ate smaller quantities of pellets than animals treated with tap water; (e) tonic intake of highly concentrated sweet substance seems to be crucial for the increase of the nociceptive threshold in our model of sweet substance-induced antinociception.

Pattern of biphasic response to various noxious stimuli in rats ingesting sucrose ad libitum

Sucrose ingestion is reported to produce an initial (20-30min) analgesia and late (<5h) hyperalgesia. However, the influence of the characteristics of noxious stimuli and sweet substances on the pattern of transition from analgesia to hyperalgesia is not known. Therefore, we investigated the effect of sucrose (20%, sucrose fed group), saccharin (0.1%, saccharin fed group) and water ingestion (control group) on pain responses to various noxious stimuli for 5h. Latency of motor response of tail (TFL), paws to noxious thermal stimuli, threshold for elicitation of motor responses to electrical stimulation of tail nociceptive afferents in 5 sessions (0, 0.25, 1, 3 and 5h) and pain-related behavior to tonic noxious stimulus in 3 sessions at 1, 3 and 5h were recorded. In sucrose fed rats as compared to controls, the TFL sequentially increased (9.29+/-0.47s from 8.41+/-0.25; p<0.01), recovered to base-line and decreased (6.61+/-0.61sec; p<0.0001) in sessions II, III and V indicating analgesia, eualgesia and hyperalgesia, respectively. In saccharin fed rats the initial analgesia extended until session III followed by eualgesia and hyperalgesia in sessions IV and V. Pain related behaviour to tonic noxious stimulus also indicated an initial analgesia (0-5min), intermediate eualgesia and late hyperalgesia (3-5h) in sucrose fed rats, whereas only analgesia in saccharin fed rats. The results of our study suggest that sucrose ingestion for 5h leads to a bi-phasic response to both phasic and tonic noxious stimuli, albeit there are variations in their durations. Therefore, the temporal relationship of the nociceptive responses to palatable food is a function of the stimulus quality of both.

Dorsal raphe nucleus and locus coeruleus neural networks and the elaboration of the sweet-substance-induced antinociception

In order to investigate the effects of monoaminergic neurons of the dorsal raphe nucleus (DRN) and locus coeruleus (LC) on the elaboration and control of sweet-substance-induced antinociception, male albino Wistar rats weighing 180-200 g received sucrose solution (250 g/L) for 7-14 days as their only source of liquid. After the chronic consumption of sucrose solution, each animal was pretreated with unilateral microinjection of ibotenic acid (1.0 microg/0.2 microL) in the DRN or in the LC. The tail withdrawal latencies of the rats in the tail-flick test were measured immediately before and 7 days after this treatment. The neurochemical lesion of locus coeruleus, but not of DRN neural networks with ibotenic acid, after the chronic intake of sweetened solution, decreased the sweet-substance-induced antinociception. These results indicate the involvement of noradrenaline-containing neurons of the LC in the sucrose-induced antinociception. We also consider the possibility of DRN serotonergic neurons exerting some inhibitory effect on the LC neural networks involved with the elaboration of the sweet-substance-induced antinociception.

Effect of the blockade of mu1-opioid and 5HT2A-serotonergic/alpha1-noradrenergic receptors on sweet-substance-induced analgesia

RATIONALE

Sweet-substance-induced analgesia has been widely studied, and the investigation of the neurotransmitters involved in this antinociceptive process is an important way for understanding the involvement of the neural system controlling this kind of antinociception.

OBJECTIVE

The aim of this study was to investigate the involvement of opioid and monoaminergic systems in sweet-substance-induced analgesia.

METHODS

The present work was carried out in an animal model with the aim of investigating whether acute (24 h) or chronic (14 days) intake of a sweet substance, such as sucrose (250 g/l), is followed by antinociception. Tail withdrawal latencies in the tail-flick test were measured before and immediately after this treatment. Immediately after the recording of baseline values, independent groups of rats were submitted to sucrose or tap-water intake and, after chronic treatment, they were pretreated with intraperitoneal administration of (1) naltrexone at 0.5, 1, 2 or 3 mg/kg; (2) naloxonazine at 5, 10, 20 or 30 mg/kg; (3) methysergide at 0.5, 1, 2 or 3 mg/kg; (4) ketanserin at 0.5, 1, 2 or 3 mg/kg; or (5) physiological saline.

RESULTS

Naltrexone and methysergide at two major doses decreased sweet-substance-induced analgesia after chronic intake of a sweet substance. These effects were corroborated by peripheral administration of naloxonazine and ketanserin.

CONCLUSIONS

These data give further evidence for: (a) the involvement of endogenous opioids and a mu1-opioid receptor in the sweet-substance-induced antinociception; (b) the involvement of monoamines and 5HT2A serotonergic/alpha1-noradrenergic receptors in the central regulation of the sweet-substance-produced analgesia.

Involvement of pre- and post-synaptic serotonergic receptors of dorsal raphe nucleus neural network in the control of the sweet-substance-induced analgesia in adult Rattus norvegicus (Rodentia, Muridae)

In order to investigate the effects of monoaminergic mechanisms of the dorsal raphe nucleus on the elaboration and control of sweet-substance-induced antinociception, male albino Wistar rats weighing 180-200 g received sucrose solution (250 g/L) for 14 days as their only source of liquid. After the chronic consumption of sucrose solution, each animal was pretreated with unilateral microinjection of methiothepin mesylate (5.0 microg/0.2 microL), or methysergide maleate (5.0 microg/0.2 microL) in the dorsal raphe nucleus. Each rat consumed an average of 15.6g sucrose/day. Their tail withdrawal latencies in the tail-flick test were measured immediately before and after this treatment. An analgesia index was calculated from the withdrawal latencies before and after the pharmacological treatment. The blockade of serotonergic receptor in the dorsal raphe nucleus with methysergide after the chronic intake of sucrose decreased the sweet-induced antinociception. However, microinjections of methiothepin in the dorsal raphe nucleus did not cause a similar effect on the tail-flick latencies after the chronic intake of sucrose solution, increasing the sweet-substance-induced analgesia. These results indicate the involvement of serotonin as a neurotransmitter in the sucrose-produced antinociception. Considering that the blockade of pre-synaptic serotonergic receptors of the neural networks of the dorsal raphe nucleus with methiothepin did not decrease the sweet-substance-induced antinociception, and the central blockade of post-synaptic serotonergic receptors decreased the sucrose-induced analgesia, the modulation of the release of serotonin in the neural substrate of the dorsal raphe nucleus seems to be crucial for the organization of this interesting antinociceptive process.

Role of muscarinic and nicotinic cholinergic receptors in an experimental model of epilepsy-induced analgesia

The blockade of GABA-mediated Cl(-) influx with pentylenetetrazol (PTZ) was used in the present work to induce seizures in animals. The neurotransmission in the postictal period has been the focus of many studies, and there is evidence suggesting antinociceptive mechanisms following tonic-clonic seizures in both animals and men. The aim of this work was to study the involvement of acetylcholine in the antinociception induced by convulsions elicited by peripheral administration of PTZ (64 mg/kg). Analgesia was measured by the tail-flick test in eight albino Wistar rats per group. Convulsions were followed by significant increases in tail-flick latencies (TFLs) at least for 120 min of the postictal period. Peripheral administration of atropine (0.25, 1 and 4 mg/kg) caused a significant dose-dependent decrease in the TFL in seizing animals, as compared to controls. These data were corroborated by peripheral administration of mecamylamine, a nicotinic cholinergic receptor blocker, at the same doses (0.25, 1 and 4 mg/kg) used for the muscarinic cholinergic receptor antagonist. The recruitment of the muscarinic receptor was made 10 min postconvulsions and in subsequent periods of postictal analgesia, whereas the involvement of the nicotinic cholinergic receptor was implicated only after 30 min postseizures. The cholinergic antagonists caused a minimal reduction in body temperature, but did not impair baseline TFL, spontaneous exploration or motor coordination in the rotarod test at the maximal dose of 4 mg/kg. These results indicate that acetylcholine may be involved as a neurotransmitter in postictal analgesia.

[Treatment of postoperative pain in elderly oncology patients with intravenous administration of a 50% glucose solution]

Postoperative pain is the most important factor od so called "tumor-promotive effect of surgery" i.e. of endocrine-metabolic changes having the consequence drop in immune, antiinfective and antitumor defense. Due to presence of organic involutive changes, old people (= 65 years), often have serious side effects during application of usual analgetics. Since hypertonic glucose (33%) given i.v. or per os, works analgesically in small children there is assumption that it can be used in treatment of postoperative pain in old oncology patients. We tested the hypothesis that postoperative pain in old oncology patients can be treated with i.v. application of 50% of glucose solution. 37 oncology patients over 65 years, 26 females and 11 males, operated for breast cancer and soft tissue cancer, were investigated. Average age of the patients was 72 +/- 4 years. 50% Glucose solution was given in two boluses of 20 ml each: the first bolus was given to all patients at the end of anesthesia, and the other bolus was given individually after appearance of post-operative pain. Pain intensity (in coefficients of the visual analogue scale VAK = 1-100) and its characteristics were tested by oral testing of operated patients; after weakening from anesthesia, after the first appearance of the pain and 15 minutes after giving of the second glucose bolus. None patient had pain weakening from anesthesia. All tested patients experienced pain during the first 70 minutes and it could be categorized as very strong pain (= 82 VAK). The pain was decreased with another glucose bolus by approximately (= 56% VAK) so it was classifies in category of bearable pains (= 36 VAK). In 9 patients (24.3%) the pain had neuropathic component (filing of "burning") which could not be eliminated by hypertonic glucose, but only with application of tramadol. Activation of the central cholinergic transmission is the most significant mechanism of analgesic glucose effect, but, probably there is another one: facilitation of entrance of formerly given analgesics in the brain cells. As energetic substrate, entering all organism cells, glucose could make easier intracell breakthrough of any other analgesic drug, of the peripheral or central action, and final antipain effect could be potential or additional one. It was concluded that 40 ml of 50% glucose solution given in two identical boluses, has good analgesic effect in treatment of postoperative pain in old oncology patient: the pain was not completely eliminated, but it was significantly decreased and became tolerable. Hypertonic glucose neither eliminates, nor decreases neuropathic component of the pain, so, when the pain appears the therapy should be supplemented with other drugs, which may completely eliminate all pain components.