Morphine administration to the region of the solitary tract nucleus produces analgesia in rats

Twenty-three Sprague-Dawley rats received 0, 60, and 45 or 75 μg of morphine in 0.5 μl of Ringer's solution in the region of the dorsal medullary nucleus tractus solitarius (NTS). Morphine produced naloxone-reversible analgesia (as measured by the tail-flick test) and catatonia, and non-naloxone-reversible barrel rolling, depending on cannula placement. The NTS region thus appears to participate in an opiate-sensitive pain-suppressive system.     

Comparison of the effects of ventral medullary lesions on systemic and microinjection morphine analgesia

The effects of electrolytic lesions of the nucleus raphe magnus (NRM), nucleus reticularis paragigantocellularis (PGC) and nucleus raphe alatus (NRA) on analgesia elicited in the rat from systemic morphine and morphine microinjection into the periaqueductal gray (PAG) were evaluated using the tail flick test. No consistent change in baseline pain sensitivity was observed following lesions of the NRM, PGC or NRA. To determine the effect of ventral medullary lesions on systemic porphine analgesia, pain sensitivity was assessed prior to and 40 min after 6 mg/kg morphine administration (i.p.) at 2 days preceding lesioning and 5, 12 and 19 days post-lesion. NRM and PGC lesions produced only slight reductions in analgesia at 5 days after surgery. It was observed that large NRM, large PGC, and NRA lesions significantly attenuated analgesia evaluated at 12 days post-lesion. Smaller lesions confined within the NRM or PGC were reliably less effective than the larger lesions in reducing analgesia. In a subsequent study, 5 μg morphine in 0.5 μl saline was microinjected into the ventral PAG at the level of the dorsal raphe. Identical testing procedures were used and the analgesia was assessed at 2 days before lesioning and 5 and 12 days post-lesion. In contrast to the previous study, large NRM lesions abolished analgesia as early as 5 days following lesioning. Small NRM lesions were less effective and PGC lesions were generally ineffective in attenuating analgesia induced by morphine microinjection. We conclude that the NRA may act as a functional unit in the mediation of systemic morphine analgesia. In contrast, analgesia elicited from intracerebral (PAG) morphine microinjection is mediated via the NRM.     

Footshock-induced analgesia: Its opioid nature depends on the strain of rat

Previous studies have indicated that stressful footshock can induce both opioid, naloxone-sensitive, and non-opioid, naloxone-insensitive forms of analgesia, depending on stimulation parameters used with 30 min of intermittent footshock (3 mA, 1 s on, 5 s off) producing opioid analgesia and 3 min of continuous shock (3 mA) producing non-opioid analgesia. Using a local strain of Charles River (CR)-derived rats we conducted a parametric investigation of footshock-induced analgesia applying both AC and DC scrambled shock ranging from 1 to 4 mA, continuous shock of 1, 3 and 5 min in duration and intermittent shock lasting 1, 3, 5, 10, 20, 30 and 80 min. All shock parameters produced potent analgesia. In no case did 10 mg/kg of naloxone block this analgesia. Varying the dose of the antagonist (0.1-10 mg/kg) and testing the animals at different points in the diurnal cycle did not result in the emergence of naloxone-sensitive anangesia. Based on the assumption that non-opioid systems may mask the activity of opioid analgesia systems, we attempted to either enhance opioid analgesia by: preventing enkephalin degradation by the use of D-phenylalanine; increasing the entry of blood-borne opioids into the brain by the use of DMSO; and the attenuation of non-opioid analgesia by the use of reserpine. In no case did a naloxone-sensitive component of analgesia emerge. To test whether the animals possess an intact opioid analgesia system, both electrical stimulation of, and injection of opiates into the periaqueductal gray (PAG) were examined. Both procedures produced analgesia which was reversed by naloxone.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a noxious visceral stimulus (colorectal balloon distension; CRD) and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity. Sixty-four (59%) responded to CRD, of which 52 (81 %) had excitatory and 12 (19%) had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralateral or bilateral. Ninety-four percent of these neurons responded to both noxious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/ kg, i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are consistent with a role of VPL in sensory-discriminative aspects of nociception.     

Morphine withdrawal-induced morphological changes in the nucleus accumbens

Morphine withdrawal produces a hypofunction of mesencephalic dopamine neurons that impinge upon medium spiny neurons (MSN) of the forebrain. After chronic treatment (from 20 to 140 mg/kg of morphine twice a day over 14 days at escalating doses) rats were withdrawn from chronic morphine spontaneously and pharmacologically. In these two distinct conditions we studied the effects of withdrawal on the morphology of MSN of the core and shell of the nucleus accumbens (Nacc). MSN were stained with the Golgi-Cox procedure and analysed by a confocal laser-scanning microscope (CLSM). Our analysis shows that, shell and core MSN differed significantly for perikarya size and spine density, and the various morphine treatments did not affect the perikarya morphometry. Both spontaneous and naloxone-induced withdrawal produced a similar reduction in spine density in MS shell neurons, as compared with MS core neurons. This effect is selectively localized at the level of second order dendritic trunks where afferents converge. By contrast, spine density counts of accumbens MSN from rats chronically treated with morphine, did not reveal any change. Collectively, the results of the present study are twofold: (i) spontaneous and pharmacologically precipitated withdrawal, but not chronic morphine per se, affects spine density of target structures of a reduced mesolimbic dopamine transmission, and (ii) the reduction of spine density in second order dendritic trunks is selectively segregated in the MSN of the shell of the Nacc. In conclusion, morphine withdrawal dramatically alters spine density, selectively in second order dendritic trunks of Nacc shell MSN, thereby further impoverishing the already abated dopamine (DA) transmission. This is in line with recent views suggesting the hypodopaminergic state as a cardinal feature of opioid dependence.     

Microinjection of morphine into nucleus reticularis paragigantocellularis of the rat suppresses spontaneous activity of nucleus raphe magnus neurons

Female rats were neonatally treated with estradiol-17 beta-benzoate or the long-acting dibenzoate esters of the isomeric catecholestrogens, 2-hydroxyestradiol-17 beta and 4-hydroxyestradiol-17 beta. Estrogen benzoates were administered subcutaneously from day 1 to 5 of life at doses of 0.05, 0.10, 0.50 and 1.00 micrograms/day. All rats were ovariectomized as adults and, 4 weeks later, the luteinizing hormone (LH) response to progesterone (2.5 mg) was tested after priming with estradiol-17 beta-benzoate (20 micrograms). At a dose of 0.5 micrograms/day, estradiol-17 beta-benzoate and 4-hydroxyestradiol-17 beta-dibenzoate were equally effective in neonatally defeminizing the LH surge mechanism. In contrast, up to a dose of 1.00 micrograms/day, 2-hydroxyestradiol-17 beta-dibenzoate did not interfere with the LH response in adult life. In the pituitary gland and uterus of the neonatally defeminized rats estrogen responsiveness of cytosolic progestin receptor induction was unimpaired. Moreover, in the uterus of these rats nuclear translocation of cytosolic progestin receptors was intact.     

高频电刺激伏隔核对吗啡诱导大鼠条件性位置偏爱行为的影响

目的研究高频电刺激伏隔核(nucleus accumbens,NAc)对吗啡诱导戒断大鼠觅药行为的影响,分析NAc在大鼠成瘾行为中的作用。方法26只雄性SD大鼠通过吗啡强化形成条件性位置偏爱后,13只予以120Hz高频电刺激伏隔核(吗啡刺激组),另13只予以假刺激(吗啡假刺激组)。15d后给予注射吗啡,诱导大鼠恢复位置偏爱行为,测量并比较两组的位置偏爱得分。结果吗啡电刺激组大鼠在注射吗啡诱导下不易恢复对吗啡的条件性位置偏爱,位置偏爱得分为(237.59±79.89)s,吗啡假刺激组为(441.29±212.68)s,两组比较,差异有统计学意义(P<0.01)。结论电刺激NAc能阻断注射吗啡诱导戒断大鼠恢复觅药行为。在成瘾药物诱导戒断动物觅药行为的过程中,NAc起重要作用。     

Microinjection of morphine into nucleus reticularis paragigantocellularis of the rat: Suppression of noxious-evoked activity of nucleus raphe magnus neurons

Microinjection of 1 microgram of morphine into nucleus reticularis paragigantocellularis (Pgc) of anesthetized rats depressed both noxious-evoked and spontaneous activity of nociresponsive neurons in the nucleus raphe magnus (NRM). This effect was naloxone-reversible, and was not observed after control injections dorsal to Pgc. The percent change in spontaneous firing was significantly greater than the percent change in pinch-evoked firing. This reduction in NRM neuronal discharge may contribute to the antinociceptive effects produced by microinjection of morphine into Pgc.     

Potentiation of morphine analgesia by the cholecystokinin antagonist proglumide

Recent evidence has suggested that cholecystokinin (CCK) may act as a physiological opiate antagonist. Both the overlap of CCK and opiate systems within the central nervous system and the fact that exogenous CCK can antagonize opiate analgesia suggest that endogenous CCK systems interact with opiate-mediated pain inhibitory systems. In the present series of experiments, we examined the effect of the CCK receptor antagonist proglumide on various forms of morphine analgesia. We have observed that proglumide can potentiate morphine analgesia following systemic, intrathecal or intracerebral administration of these drugs. Endogenous CCK systems do not appear to be tonically active since neither systemic, intrathecal nor intracerebral proglumide typically produced measurable analgesia in the absence of morphine. These data suggest that CCK may be released in response to opiate administration and acts to return the organism toward its basal level of pain sensitivity. If such a hypothesis is in fact true, then CCK blockade may be of clinical value in the treatment of pain.     

Differential responses of serotonergic and non-serotonergic neurons in nucleus raphe magnus to systemic morphine in rats

Forty-eight raphe-spinal units in nucleus raphe magnus were identified by antidromic stimulation and further classified into serotonergic and non-serotonergic populations according to their conduction velocity and spontaneous discharge rate. Following morphine administration (5 mg/kg, i.p.) they showed different responses: excitatory, depressive or non-responsive. It was found that 6 of 7 nonresponsive, 3 of 20 depression-responsive and only 1 of 21 excitation-responsive units proved to be serotonergic neurons, indicating that the involvement of serotonergic neurons in morphine analgesia is probably insignificant. It is suggested that the raphe-spinal fiber systems, both excitatory and inhibitory are originated mainly from the non-serotonergic neurons.     

Topical application of morphine to the rat somatosensory cortex produces analgesia to tonic pain

Topical application of 0.01 or 0.1% morphine solution to the somatosensory SI area of the rat cerebral cortex significantly decreased the pain intensity rating in the formalin test without producing motor side effects or sensory deficits. Naloxone partially antagonizes this effect. Topical application of morphine to the striate cortex did not induce analgesia. It is suggested that the primary somatosensory SI area of the cerebral cortex plays a role in opiate pain control.     

The effect of different lesions of the median raphe on morphine analgesia

The effect of different manipulations of the nucleus medianus raphe (MR) on morphine analgesia was investigated in rats using the tail-immersion test. Electrolytic lesions of this structure antagonized morphine analgesia, while injections of 5,7-dihydroxytryptamine (to destroy serotonergic neurons) or ibotenic acid (to destroy cell bodies) in the medianus raphe did not alter the effect of morphine. Injection of naloxone (0.5 and 0.1 micrograms) in the MR antagonized morphine analgesia. These results suggest the importance of this structure for morphine analgesia in this test, although the substrates within the nucleus that mediate this action are still unknown.     

Distribution of amygdala input to the nucleus accumbens septi: An electrophysiological investigation

Summary The nucleus accumbens septi (NAS) receives afferent input from the amygdala via the stria terminalis and from the hippocampus via the fimbria. Extracellular recordings from 196 NAS neurons in halothane-anesthetized rats revealed heterogeneous response patterns following stimulation of the amygdala. The observation that 30% of anterior NAS units but only 16% of posterior NAS units were responsive to amygdala stimulation suggested a topographical arrangement of amygdala efferents. Comparing the effects of amygdala and fimbria stimulation revealed that the two afferent pathways converge onto individual NAS neurons, but that the two sites of stimulation can differentially influence other neurons. The present results clarify the topographical distribution of amygdala input to the NAS, confirm that inputs from two limbic structures are integrated within the NAS, and further illustrate the electrophysiological heterogeneity of NAS neurons.     

苍白球在电针镇痛及兴奋尾壳核镇痛中的作用

用行为学和电生理学的方法 ,探讨苍白球在电针镇痛及兴奋尾壳核镇痛中的作用。结果表明 :电针可以延长辐射热引起的缩腿潜伏期 ,电针或兴奋尾壳核可抑制丘脑束旁核神经元的伤害性反应 ;苍白球微量注射红藻氨酸 7d后 ,电针对辐射热引起的大鼠缩腿潜伏期无明显影响 ,电针或兴奋尾壳核对丘脑束旁核神经元的伤害性反应亦无明显影响 ,与毁损前相比有显著性差异 (P <0 .0 5 ) ,与苍白球微量注射生理盐水 7d后 ,电针可延长大鼠缩腿潜伏期 ,及电针或兴奋尾壳核对束旁核神经元伤害性反应的抑制作用相比有显著性差异 (P <0 .0 5 )。结果提示 :苍白球在电针及兴奋尾壳核镇痛中发挥重要作用     

An investigation of the origin of extracellular GABA in rat nucleus accumbens measuredin vivo by microdialysis

Summary GABA transmission in the nucleus accumbens is believed to play a central role in motivational processes and the expression of psychostimulant drug action. Here we report measurements of extracellular GABA in nucleus accumbens of the rat and investigate its origin. Extracellular GABA was detected using microdialysis in combination with a novel HPLC-based assay. In the awake rat, GABA in the microdialysates (1) increased 10-fold following perfusion with 0.5 mM nipecotic acid, a GABA releasing agent and uptake blocker, (2) increased 7-fold following local perfusion with 50 mM KCl, (3) decreased 50% following perfusion with tetrodotoxin, (4) decreased 50% following perfusion with a Ca²⁺-free medium and (5) decreased 40% following perfusion with high (12.5mM) MgCl. Finally, in the anaesthetized rat, GABA in the microdialysates decreased 50% following i.p. injection of 100mg/kg 3-mercap-toproprionic acid, a GABA synthesis inhibitor. We conclude that GABA in microdialysates from nucleus accumbens of the rat (awake) responds appropriately to selected pharmacological agents and derives at least in part (50%) from neurones.     

Response of nucleus accumbens neurons to amygdala stimulation and its modification by dopamine

Extracellular single unit recordings were obtained from the nucleus accumbens of urethane anesthetized rats. It was found that electrical stimulation of the basal lateral and basal medial nuclei of the amygdala produced strong excitatory responses in neurons of the nucleus accumbens, in particular the medial region. Latencies of activation were relatively short with a mean of 10.7 ms.Dopamine applied iontophoretically had a marked attenuating effect on the excitatory response of nucleus accumbens neurons to amygdala stimulation. The spontaneous activity of all neurons recorded from the nucleus accumbens was also suppressed by dopamine, but the excitatory response was more sensitive to dopamine inhibition than the spontaneous activity.Neurons in the nucleus accumbens showed a variety of responses to single-pulse electrical stimulation of the ventral tegmental area (VTA). Some units in the nucleus accumbens received convergent inputs from both the amygdala and the VTA. Stimulation of the VTA also attenuated the response of nucleus accumbens neurons to excitatory inputs from the amygdala. A train of 10 pulses (0.15 ms, 200–600 αA) at 10 Hz delivered to the VTA at 100 ms before stimulation of the amygdala caused attenuation of the original excitatory response. The attenuating effect could be observed irrespective of whether individual single-pulse stimulation of the VTA elicited a response in that particular accumbens neuron or not. 6-Hydroxydopamine injected into the VTA 2 days prior to the recording experiment, or haloperidol injected intraperitoneally 1 h before the recording session, abolished this attenuating effect. However, responses to single-pulse stimulations of the VTA were not abolished. The results suggest that the attenuation of the excitatory response to amygdala stimulation was due to the release of dopamine from mesolimbic dopaminergic neurons. Responses to single-pulse stimulations of the VTA were probably due to activation of non-dopaminergic neurons projecting from the same area.It is suggested as a working hypothesis that this inhibitory effect of dopamine may be an important function of the mesolimbic dopamine pathway in modulating the extent to which limbic structures can exert an influence on the motor system through the accumbens.     

Rewarding and aversive effects of nicotine are segregated within the nucleus accumbens

Forebrain dopamine plays a critical role in motivated behavior. According to the classic view, mesolimbic dopamine selectively guides behavior motivated by positive reinforcers. However, this has been challenged in favor of a wider role encompassing aversively motivated behavior. This controversy is particularly striking in the case of nicotine, with opposing claims that either the rewarding or the aversive effect of nicotine is critically dependent on mesolimbic dopamine transmission. In the present study, the effects of 6-hydroxydopamine lesions of nucleus accumbens core vs. medial shell on intravenous nicotine conditioned place preference and conditioned taste aversion were examined in male adult rats. Dopaminergic denervation in accumbens medial shell was associated with decreased nicotine conditioned place preference. Conversely, denervation in accumbens core was associated with an increase in conditioned place preference. In addition, dopaminergic denervation of accumbens core but not medial shell abolished conditioned taste aversion for nicotine. We conclude that nucleus accumbens core and medial shell dopaminergic innervation exert segregated effects on rewarding and aversive effects of nicotine. More generally, our findings indicate that dopaminergic transmission may mediate or enable opposing motivational processes within functionally distinct domains of the accumbens.     

A projection from the entorhinal cortex to the nucleus accumbens in the rat

A study was performed in which both anterograde ([³H]leucine radioautography) and retrograde (horseradish peroxidase (HRP) histochemistry) tracing methods were employed to identify the origin of the fimbrial projection to the nucleus accumbens in the rat. The data reveal that this pathway arises predominantly from layers II–III of the anterior two-thirds of entorhinal cortex rather than from any part of hippocampal formation.     

Brainstem lesions dissociate neural mechanisms of morphine analgesia in different kinds of pain

The effects of brainstem lesions on morphine analgesia were examined using the formalin test which produces moderate pain that lasts about 2 h, and the tail-flick test which measures brief threshold-level pain. Lesions of the nucleus raphe magnus attenuated and small lesions of the central tegmental nucleus potentiated the effects of morphine in the tail-flick test. Lesions of the median raphe nucleus potentiated the effects of morphine in the formalin test. Large lesins of the pontine reticular formation had no effect in either pain test. These results indicate that the neural mechanisms underlying morphine analgesia are different in different kinds of pain.     

Naloxone-reversible analgesia produced by microstimulation in the rat medulla

Using microstimulation of the rostral medulla in the barbiturate-anesthetized rat, a map was constructed of loci for inhibition of the tail-flick response to noxious heat. Low threshold sites (</ 10 μA) were found in both the nucleus raphe magnus and the nucleus reticularis paragigantocellularis. Chronaxie determinations indicate that analgesia was not produced by activation of large myelinated axons of passage. Systemic naloxone only antagonized the inhibition generated from stimulation at low threshold sites. Inhibition from higher threshold sites, for example from the nucleus reticularis gigantocellularis, was not naloxone reversible. Depending on the area stimulated, either an opioid- or a non-opioid-mediated inhibition results from microstimulation within the rat medulla.