Low Analgesia Research Articles

The Influence of Cross-Fostering on Alcohol Consumption and Depressive-Like Behaviors in HA and LA Mice: The Role of the Endogenous Opioid System.

The development of alcohol dependence and depression is determined by various genetic and environmental factors. In the presented study, we used high analgesia (HA) and low analgesia (LA) mouse lines, characterized by different endogenous opioid system activity and divergent blood–brain barrier permeability, to determine the influence of cross-fostering of these lines raised by surrogate mothers on ethanol consumption and development of depressive-like behaviors. We also investigated ethanol drinking by biological parents or surrogate mothers. Furthermore, we investigated whether these parental changes would alter the effect of naloxone on ethanol intake and depressive-like behaviors in offspring. Our results reveal that cross-fostering of HA and LA raised by surrogate mothers has a greater impact on depressive-like behaviors than ethanol consumption. Ethanol intake by biological parents substantially affected depressive-like behaviors and ethanol consumption in offspring. Moreover, ethanol intake by biological parents or an adoptive mother modified the effect of naloxone on ethanol consumption and preference and depressive-like behaviors in the HA offspring only. Together, these results indicate that cross-fostering differentially affects the effect of naloxone on alcohol consumption and the development of depression.

Association between theA107Vsubstitution in the δ‐opioid receptors and ethanol drinking in mice selected for high and low analgesia

Experimental evidence suggests that endogenous opioids play an important role in the development of ethanol addiction. In this study, we employed two mouse lines divergently bred for opioid-mediated stress-induced analgesia. In comparison with HA (high analgesia line) mice, LA (low analgesia line) mice, having lower opioid receptor system activity, manifest enhanced basal as well as stress-induced ethanol drinking. Here, we found that recently discovered C320T transition in exon 2 of the δ-opioid receptor gene (EU446125.1), which results in an A107V substitution (ACA23171.1), leads to higher ethanol preference in CT mice compared with CC homozygotes. This genetic association is particularly evident under chronic mild stress (CMS) conditions. The interaction between stress and ethanol intake was significantly stronger in HA than in LA mice. Ethanol almost completely attenuated the pro-depressive effect of CMS (assessed with the tail suspension test) in both the CC and CT genotypes in the HA line. In the LA mice, a lack of response to ethanol was observed in the CC genotype, whereas ethanol consumption strengthened depressive-like behaviours in CT individuals. Our results suggest that constitutively active A107V substitution in δ-opioid receptors may be involved in stress-enhanced vulnerability to ethanol abuse and in the risk of ethanol dependence.

Acoustic startle and disruption of prepulse inhibition by dizocilpine in selectively bred mice.

In this study we examined the relationship between genetically produced differences in the magnitude of prepulse inhibition (PPI) of the acoustic startle response (ASR) and stress induced swim analgesia in genetically different strains of mice. Prepulse inhibition of the ASR and its changes due to dizocilpine (MK-801) injection were studied in 180 mice. The animals used in this study were obtained from our colony of 54 generation, Swiss-Webster mice selectively bred for high and low magnitude of analgesia. Three month old male mice of the high analgesia (HA) and the low analgesia (LA) lines, in addition to randomly bred controls (C) were used in the experiment. Thirty minutes before the ASR session the mice were injected intraperitoneally with saline or with 0.15, 0.25, 0.5 mg÷kg of dizocilpine maleate. Prepulses suppressed the acoustic startle response in all lines in a prepulse intensity-dependent manner, but only the differences between the weakest and the strongest prepulses appeared significant. Two-way ANCOVA performed separately for each line revealed a significant effect of dizocilpine and prepulse intensity. Only in the HA line, however, the disruption of PPI from the injection of dizocilpine was evidenced by significant treatment by prepulse interaction. That means the prepulses decreased ASR significantly less in dizocilpine- treated animals than in saline-treated animals. The results confirmed that the mouse lines manifesting differential ASR magnitudes along with different degrees of PPI sensitivity to dizocilpine, might be suitable for pharmacogenetic studies on the glutaminergic mechanism of the startle response.

Cytogenetic comparison of the sensitivity to mutagens in mice selected for high (HA) and low (LA) swim stress-induced analgesia

Sensitivity to mutagens was studied in mouse lines selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20 °C water. Apart from pain-related traits HA mice also manifest, as compared to the LA line, higher emotionality in various behavioural tests, and cope worse with the hypothermic challenge of swimming in cold water. In the present study HA mice appeared more susceptible to the mutagenic effect of whole-body γ-radiation and mitomycin-C injection. Both treatments caused higher frequencies of chromosomal aberrations and micronucleus in bone marrow cells in the HA than in the LA line. The results are discussed in terms of a genetic correlation between animals’ susceptibility to environmental stressors and the mechanism of mutagenesis. As shown by our recent molecular study, the selection for magnitude of swim analgesia has differentiated the parental outbred population into two distinct genotypes characterised by specific minisatellite and microsatellite sequences for each line, which may be genetic markers of particular traits. We conceive that the breeding strategy, along with the differentiation of stress-related phenomena, has altered the frequencies of genes controlling DNA repair in each line.

Chromosomal NOR activity in mice selected for high and low swim stress-induced analgesia.

Metaphase nucleolar organizer region (NOR) activity was studied in mouse lines selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20 degrees C water. Apart from pain-related traits, HA mice also manifest, as compared to the LA line, higher emotionality in certain behavioral tests and are less capable of coping with the hypothermic challenge of swimming in cold water, in addition to being more susceptible to the mutagenic effect of whole-body gamma-radiation and mitomycin C injection. We here compared NOR activity in HA and LA mice. A statistically significant difference (p < or =.01) was detected in mean silver-stained nucleolar number/cell between the HA and LA lines, being lower for HA mice. We propose that the breeding strategy, along with the differentiation of stress-related phenomena, has altered the activity of genes coding rRNA and that this activity is important in controlling DNA repair in each line. It is concluded that quantitative studies of nucleolar changes may be useful in evaluating the biological reactivity to stress stimuli.

Antidepressant-type effect of the NK3 tachykinin receptor agonist aminosenktide in mouse lines differing in endogenous opioid system activity

The influence of the tachykinin NK3 receptor agonist, aminosenktide on the immobility in the forced swimming test was studied in mouse lines selectively bred for divergent magnitudes of stress-induced analgesia. The high analgesia (HA) line is known to display enhanced, and the low analgesia (LA) line displays reduced activity of the opioid system. Aminosenktide at doses of 125 μg/kg or 250 μg/kg intraperitoneally (IP) reduced, in naltrexone-reversible manner, the immobility more of opioid receptor-dense HA than of unselected mice, but was ineffective in the opioid receptor-deficient LA line. The effect of aminosenktide was quite similar to the antiimmobility action of desipramine (10 mg/kg IP), a prototypic antidepressant agent. None of the compounds increased animals’ locomotion as found with an open field test; therefore their antiimmobility effect cannot be attributed to a change in general motility. The results claim that aminosenktide causes an antidepressant effect, and endogenous opioids are involved in this process.

Acoustic startle and open-field behavior in mice bred for magnitude of swim analgesia

Acoustic startle response (ASR) and open-field activity was examined in the 46th generation of mice that have been selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20°C water. These lines were earlier found to differ in brain opioid receptor density and in the expression of opioid-mediated phenomena, as analgesic sensitivity to opiates and reversibility of swim stress-induced analgesia (SSIA) by naloxone. For comparison, a randomly bred control (C) line was used. To measure the amplitude of ASR, the mice were exposed to 110-dB acoustic stimuli in a Coulbourn apparatus. In saline-injected mice, the ASR force was found significantly lower in the LA than in the HA, as well in the C line, but did not differ between the two last lines. Naltrexone hydrochloride (10 mg/kg IP 30 min before ASR testing) augmented the startle in the opioid receptor-dense HA line, but had no effect in the opioid receptor-deficient LA line, as well in the C line; therefore, the ASR magnitude in naltrexone-injected HA mice was significantly higher compared to the C line. HA mice displayed less activity in an open-field test; that is, they remained immobile longer in the center of the field, and thereafter performed less ambulation and less rearing against the wall compared to the LA line. Naltrexone failed to modify the open-field activity in any line. The results confirm that the pattern of ASR depends on the genetic makeup of the animals. The higher amplitude of ASR, taken together with the lower open-field activity of HA mice, can be interpreted in terms of higher anxiety level, compared to the LA line. It is suggested that the higher ASR in HA mice relies on a nonopioid mechanism, which is tonically inhibited by the opioid system.

Analgesia in Selectively Bred Mice Exposed to Cold in Helium/Oxygen Atmosphere

In order to evaluate the stressing role of swim hypothermia in producing swim stress-induced analgesia (SSIA), we examined whether a mere decrease in the animals’ core temperature without swimming would be sufficient to elicit analgesia. The subjects were Swiss–Webster mice selectively bred for 37 and 40 generations for divergent magnitudes of SSIA. High (HA) and low analgesia (LA) mice were exposed for 15 min to temperatures in the range between −5 and +20°C in 79% He/21% O 2 (Heliox) atmosphere. The Heliox exposure produced ambient temperature-dependent hypothermia and analgesia, as assessed with a hot-plate test (56°C). The post-Heliox analgesia was of much higher magnitude in HA than in LA mice. The steeper slope of regression of the magnitude of analgesia upon hypothermia in HA mice indicates that these mice are far more sensitive to the analgesic effect of hypothermia than LA mice. Naltrexone HCl (10 mg/kg i.p.) attenuated analgesia in ambient temperature-dependent manner in HA, but not in LA mice. In view of the apparent similarity of Heliox-induced analgesia and SSIA we suggest that hypothermia is a powerful component of swim stress to induce SSIA in the mouse.

Cross-tolerance between morphine and swim analgesia in mice selectivity bred for high and low stress-induced analgesia

Mice selectively bred for high (HA) and for low analgesia (LA) induced by 3-min swimming at 20°C and unselected controls (C) were injected three times daily for 3 days with 20 mg/kg morphine HCl. The analgesic effect of 10 mg/kg morphine in nontolerant mice differed between the lines in the rank order of HA > C > LA and significantly decreased after repeated treatment with morphine, as revealed by the hotplate test (56°C). The tolerance to morphine analgesia was more pronounced in HA than in C mice but did not develop at all in LA mice. Similarly, the magnitude of swim-induced analgesia in morphine tolerant mice decreased to a greater degree in the HA than the C line but did not change in LA mice. Naloxone HCl (1 and 10 mg/kg) attenuated swim analgesia more in nontolerant HA than C mice but had no effect in morphine-tolerant HA and C and in all LA mice. The differential degree of morphine tolerance and cross-tolerance with swim analgesia suggests that the strategy of selective breeding toward divergent magnitudes of stress-induced analgesia has differentiated opioid involvement in endogenous pain inhibition in the selected lines.

Differentiation of neurochemical basis of stress-induced analgesia in mice by selective breeding

Two selectively bred lines of mice, one responding with high (HA) and the other with low analgesia (LA) to 3 min swims at 20 °C were compared with respect to naloxone ability to reverse such swim induced pain threshold elevation. Also, the analgesic effect of morphine was tested in both lines of mice. Naloxone at a dose of 1 mg/kg significantly reduced postswim analgesia determined with hot plate and tail flick tests in HA mice, but was not effective in LA mice. HA mice responded with significant elevation of hot plate threshold to 0.1 mg/kg of morphine hydrochloride, whereas in LA mice comparable level of analgesia developed after 12.8 mg/kg. The data argue for greater involvement of opioid mechanisms in producing analgesia in HA mice than in LA mice, and so for inheritance of endorphin system(s) activity.