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Modulation of hippocampal theta rhythm by the opioid system of the pedunculopontine tegmental nucleus

100%
Leszkowicz E., Kusmierczak M., Matulewicz P., Trojniar W.
Acta Neurobiologiae Experimentalis
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2007
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vol. 67
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issue 4
447-460
EN
The pedunculopontine tegmental nucleus (PPN) belongs to the brainstem system which synchronizes hippocampal activity. Theta relevant intra-PPN circuitry involves its cholinergic, GABA-ergic and glutamatergic neurons and Substance P as neuromodulator. Evidence that PPN opioid elements also modulate the hippocampal theta is provided here. In urethane-anesthetized rats a unilateral microinjection of morphine (MF) (1.5 and 5 micrograms) increased the maximal peak power of tail pinch-induced theta. The higher dose also increased the corresponding frequency. When the theta was evoked by intra-PPN injection of carbachol (10 micrograms), the addition of MF (5 micrograms) prolonged theta latency and shortened the duration of the theta. These effects of MF were blocked by naloxone (5 micrograms). The results obtained suggest that the PPN opioid system can enhance or suppress the hippocampal theta depending on the actual level of PPN activation.
2
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The effect of naloxone on object exploration, object recognition and other types of spontaneous behavior

88%
Lukaszewska I., Klepaczewska A.
Acta Neurobiologiae Experimentalis
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1997
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vol. 57
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issue 2
123-133
EN
Object exploration was examined in naloxone injected (1 mg/kg or 4 mg/kg) and saline control rats. Naloxone rats explored an object for a shorter time than did controls, thus indicating a lower investigatory motivation. This effect was dose dependent. Higher drug dose (4 mg/kg) decreased the number of contacts with an object. Both doses increased the mean duration of contacts with an object. The naloxone groups showed intact recognition of a familiar object paired with a new one in two sessions 4 h and 24 h after the injections. The higher drug dose depressed the locomotor activity and wall leaning. Grooming was not influenced by naloxone. The normal daily fluctuations in the level of grooming and locomotion were distorted following the injection of the higher dose of naloxone. The lower dose (1 mg/kg) did not affect the rats' performance in some tests. The results could be viewed as a naloxone related depression of the behavior containing motor elements like locomotion, wall leaning and object approaching. The prolonged contact time with an object could be the result of a lowered flexibility of movement. However, the decrease of rewarding value of exploration could not be ruled out. Possibly, naloxone exerts several different interacting behavioral effects.
3
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Naloxone impairs spatial performance in rats

88%
Lukaszewska I.
Acta Neurobiologiae Experimentalis
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1997
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vol. 57
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issue 1
71-74
EN
Naloxone injected (1.0 mg/kg) and saline injected control rats were subjected to a two trial test of object localization memory. In trial I rats were allowed to explore for 5 min an enclosed T maze with an object (plastic bottle) placed in one maze arm. Then, the object was removed and after a 20 min retention interval rats were faced with two empty arms of the same maze (trial II). Control rats showed good retention of the place occupied by the object, displaying a significant preference (74%, ) for the arm which previously contained the object. Naloxone treated rats responded at chance levels (53%). An accurate performance in this task is normally based on information provided by spatial cues outside the maze (cognitive map), so that a random performance of Naloxone treated rats could supposedly be related to some disorders in the internal representation of the environment.
4
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Effective stimulation of daily LH secretion by the combined treatment with melatonin and naloxone in luteal-phase ewes

75%
Misztal T., Romanowicz K.
Acta Neurobiologiae Experimentalis
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2005
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vol. 65
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issue 1
1-9
EN
This study was designed to test the hypothesis that melatonin would intensify daily LH release after central blockade of the opiate receptors in sexually active ewes. The intracerebroventricular infusions of vehicle (control), melatonin, naloxone and melatonin in combination with naloxone were made in ewes in the luteal phase of the estrous cycle, from 2:00 P.M. to 5:00 P.M. Blood samples were collected from 11:00 A.M. to 8:00 P.M. at 10-min intervals. The mean plasma LH concentrations were measured before, during and after the infusions. The frequency and amplitude of LH pulses were determined during the whole experimental period. The LH concentrations recorded during melatonin or naloxone infusions were significantly higher than the concomitant concentration in vehicle-infused animals. The mean LH pulse amplitude in melatonin- and naloxone-treated ewes was also significantly higher than in controls. The LH concentration measured during the combined infusion of melatonin and naloxone was significantly higher than that during vehicle infusion. The LH concentration recorded in turn after the treatment was significantly higher than the concomitant concentrations in vehicle-, melatonin- and naloxone-infused animals. The mean LH pulse amplitude in this group was significantly higher than in the vehicle-infused group. These results indicate that blockade of the opiate receptors within the CNS facilitated effective stimulation of daily LH secretion by exogenous melatonin. In conclusion, a relationship between melatonin and endogenous opioid peptides may be crucial in enabling melatonin to exhibit stimulatory action on LH secretion during the luteal phase of the estrous cycle in ewes.
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