Multiunit recording from medial basal hypothalamus following acute and chronic morphine treatment

Acute and chronic dose-response effect of methylphenidate on ventral tegmental area neurons correlated with animal behavior

Methylphenidate (MPD) is used to treat ADHD and as a cognitive enhancement and recreationally. MPD's effects are not fully understood. One of the sites of psychostimulant action is the ventral tegmental area (VTA). The VTA neuronal activity was recorded from freely behaving rats using a wireless system. 51 animals were divided into groups: saline, 0.6, 2.5, and 10.0 mg/kg MPD. The same repetitive MPD dose can elicit either behavioral sensitization or tolerance; thus the evaluation of the VTA neuronal activity was based on the animals' behavioral response to chronic MPD exposure: animals exhibiting behavioral tolerance or sensitization. Acute MPD elicits dose-related increases in behavioral activity. About half of the animals exhibited behavioral sensitization or tolerance to each of the MPD doses. 361 units were recorded from the VTA and exhibited similar spike shape on experimental day 1 (ED1) and on ED10. 71, 84, and 79 % of VTA units responded to acute 0.6, 2.5, and 10.0 mg/kg MPD, respectively. The neuronal baseline activity at ED10 was significantly modified in 94, 95, and 100 % of VTA units following 0.6, 2.5 and 10.0 mg/kg MPD, respectively. Following chronic MPD exposure, 91, 98, and 100 % exhibit either electrophysiological tolerance or sensitization of 0.6, 2.6, or 10.0 mg/kg MPD, respectively. In conclusion, the chronic administration of the same dose of MPD caused some animals to exhibit behavioral sensitization and other animals to exhibit tolerance. The VTA units recorded from animals exhibiting behavioral sensitization responded significantly differently to MPD from animals that exhibited behavioral tolerance.

Behavioral and dorsal raphe neuronal activity following acute and chronic methylphenidate in freely behaving rats

Concomitant behavioral and dorsal raphe (DR) neuronal activity were recorded following acute and chronic dose response of methylphenidate (MPD) in freely moving rats previously implanted with permanent semi-microelectrodes using telemetric (wireless) technology. On experimental day (ED) 1, the neuronal and locomotor activity were recorded after saline (baseline) and MPD (0.6, 2.5 or 10.0mg/kg) injection (i.p.). Animals were injected daily with a single dose of MPD for five consecutive days (ED 2-6) to elicit behavioral sensitization or tolerance. After three washout days, the neuronal and locomotor activity recording was resumed on ED 10 followed by saline and MPD rechallenge injection. The main findings were: (1) the same dose of chronic MPD administration elicited behavioral sensitization in some animals and behavioral tolerance in others. (2) 46%, 56% and 73% of DR units responded to acute 0.6, 2.5 and 10.0mg/kg MPD respectively. (3) 89%, 70% and 86% of DR units changed their baseline activity on ED 10 compared to that on ED 1 in the 0.6, 2.5 and 10.0mg/kg MPD groups respectively. (4) A significant difference in ED 10 baseline activity was observed in the DR neuronal population recording from animals expressing behavioral sensitization compared to that of animals expressing behavioral tolerance. (5) 89%, 78% and 88% of DR units responded to chronic 0.6, 2.5 and 10.0mg/kg MPD respectively. (6) The DR neuronal population recording following acute MPD on ED 1 and rechallenge MPD on ED 10 from animals expressing behavioral sensitization was significantly different from the neuronal population recorded from animals exhibited behavioral tolerance. The correlation between the DR neuronal activity and animal's behavior following chronic MPD exposure suggested that the DR neuronal activity may play an important role in the expression of behavioral sensitization and tolerance induced by chronic MPD administration.

Dorsal raphe neuronal activities are modulated by methylphenidate

This study investigated the electrophysiological properties of the dorsal raphe nucleus (DR) neurons in response to the acute and repetitive administration of methylphenidate (MPH). Activities of DR neurons were recorded from non-anesthetized, freely behaving rats previously implanted bilaterally with permanent semi microelectrodes. The main findings were: (1) after initial (acute) administration of MPH (2.5 mg/kg i.p.) on experimental day one (ED1), 56 % of DR units significantly changed their firing rates. The majority of the responsive units (88 %) exhibited increased firing rate; (2) daily MPH injections were given on ED2 through ED6 followed by 3 washout days. On ED10, 83 % of the DR units significantly changed their baseline activity compared to the baseline activity on ED1; (3) after rechallenge MPH administration on ED10, 63 % of DR units exhibited significant change in their firing rate; the majority of the responsive units (76 %) exhibited a significant increase in their firing rate; (4) The effect of rechallenge MPH administration on ED10 was compared to the effect of initial MPH on ED1, 47 % DR units exhibited a further significant increase in their firing rate while 53 % DR units exhibited decrease or non-change in their firing rate which can be interpreted as electrophysiological sensitization or tolerance. In conclusion, this study demonstrated that acute MPH administration modulated the DR neuronal activities. Repetitive MPH administration modulated the baseline activities of DR units and elicited neurophysiological sensitization or tolerance. The results indicated that MPH affects DR neuronal activity.

Methylphenidate modulates the locus ceruleus neuronal activity in freely behaving rat

The electrophysiological properties of the locus coeruleus (LC) neurons in response to acute and chronic administration of methylphenidate (MPD) were investigated. The extracellular LC neuronal activities were recorded from non-anesthetized, freely behaving rats previously implanted bilaterally with permanent semi microelectrodes. The main findings were: (1) On experimental day 1 (ED1), 87% (94/108) of LC units significantly changed their firing rate after initial (acute) MPD (2.5mg/kg, i.p.) administration. The majority of the responsive units (80%, 75/94) increased their firing rate; (2) Daily MPD (2.5mg/kg) injection was given on ED2 through ED6 followed by 3 washout days (ED7 to 9). On ED10, all LC units exhibited a significant change of their baseline activity compared to their baseline activity on ED1; (3) MPD rechallenge on ED10 elicits 94% (101/108) of LC units significantly changed their firing rate; the majority of them (78%, 79/101) increased their firing rate; (4) The effect of rechallenge MPD administration on ED10 were compared to the effect of initial MPD on ED1, 98% of the LC units exhibited a significant change in their firing rate. 41% (43/106) of them exhibited a significant increase in their firing rate while 59% (63/106) units significantly decreased their firing rate which can be interpreted as electrophysiological sensitization or tolerance respectively. In conclusion, the majority of LC neurons significantly increased their firing rate after acute and chronic MPD administration. This data demonstrated that enhanced LC neuronal activities play important role in the effect of MPD.

Acute and chronic methylphenidate modulates the neuronal activity of the caudate nucleus recorded from freely behaving rats

Methylphenidate (MPD) is currently one of the most prescribed drug therapies for attention deficit/hyperactivity disorder (ADHD) and moreover is abused for cognitive enhancement and used for recreation by the young and adults. Methylphenidate is used for prolonged periods of time and its mechanism of action on the brain is still unknown. The main action of MPD is known to act on the motive circuit of the brain, and one of these structures is the caudate nucleus (CN). The objective of this study was to investigate the neurophysiological properties of the CN neurons in response to acute and chronic administration of MPD in freely behaving animals, previously implanted with permanent semi microelectrodes. Twenty-six rats were permanently implanted with semi microelectrodes into the CN using general anesthesia. On experimental day one (ED1) the rat was placed into the testing chamber, and neuronal activity was recorded using a wireless (telemetric) headstage device following both a saline and a 2.5 mg/kg MPD injection. From ED2 to ED6 daily injections of 2.5 mg/kg MPD were administered without recordings to induce a chronic effect of the drug, preceded by three days of washout (ED7-ED9) where no injections were given. On ED10 rats were placed back into the testing chamber, the wireless headstage device was attached to skull cap and recordings were resumed for 1 h each following both a saline and re-challenge administration of 2.5 mg/kg MPD. Sixty-seven CN neuronal recorded units from twenty-six animals with identical shape and amplitude at ED1 and ED10 were evaluated. All the 67 CN units responded to MPD administration, 70% (47/67) CN units exhibited an increase in activity following initial 2.5 mg/kg MPD administration and 30% (20/67) exhibited a decrease in neuronal activity. On ED10 all the CN units showed a significant change in their firing rate baseline compared to ED1 baseline, 52% (35/67) exhibiting an increase in their ED10 baseline activity compared to ED1 baseline activity and 48% (32/67) of the CN units at ED10 exhibited decreasing activity. All the CN units responded significantly to MPD rechallenge at ED10, 57% (38/67) of the units exhibited increased neuronal activity while 43% (29/67) exhibited decreasing neuronal activity. The results indicate that the majority of the CN units exhibited neurophysiological sensitization.

Effects of Morphine on: Spontaneous, Dorsal Raphe, Spinal Tract of Trigeminal Nucleus, Medial Lemniscus and Reticular Lateral Magnocellular Evoked Responses of Hypothalamic Units, in Naive and Morphine Physically Dependent Rats

The spontaneous activity and the inputs to the medial basal hypothalamus (MBH) following dorsal raphe (DR), spinal tract of the trigeminal nerve (SpV), medial lemniscus (ML), reticular lateral magnocellular nucleus ( RLM ) and acoustic (Ac) stimulation and the effects of morphine and the opioid antagonist, naloxone, on these inputs, were investigated in morphine-naive and morphine-dependent animals. The observations were obtained in freely behaving animals previously implanted with permanent electrodes. The spontaneous activity of MBH neurons exhibits heterogenic spontaneous firing rates. This spontaneous activity is affected by acute and chronic morphine treatment. The MBH neuronal population exhibits neurophysiological patterns of tolerance of morphine dependence and withdrawal. The central input exerts a marked influence on MBH neurons in both naive and morphine-dependent animals. These inputs are modified by morphine challenge dose in both preparations, i.e., in morphine-naive and morphine-dependent animals, and are reversed by naloxone. The DR and Ac inputs affect the MBH neuronal activity differently from that observed following SpV, ML and RLM stimuli. The effects of morphine and naloxone on the DR and Ac input in morphine-naive and morphine-dependent animals differ from those observed following SpV, ML and RLM inputs. The MBH neurons exhibited a high percentage of convergence to Ac, DR, SpV, ML and RLM stimulation.

Collateralization of periaqueductal gray neurons to forebrain or diencephalon and to the medullary nucleus raphe magnus in the rat

Antinociceptive effects elicited from the midbrain may involve both ascending and descending projections from the periaqueductal gray and dorsal raphe nucleus. To investigate the relationship between these different efferent pathways in the rat, we performed a double-labeling study using two retrograde tracers, colloidal gold-coupled wheatgerm agglutinin-apo horseradish peroxidase and a fluorescent dye. One tracer was microinjected in the medullary nucleus raphe magnus; the second was injected into one of several regions rostral to the periaqueductal gray that have been implicated in nociceptive and antinociceptive processes. The results can be grouped into two categories. First, injections into the ventrobasal thalamus, lateral hypothalamus, amygdala, and cerebral cortex labeled neurons in the dorsal raphe nucleus but not in the periaqueductal gray. Up to 90% of these projection neurons were serotonin immunoreactive, and up to 17% were also retrogradely labeled from the nucleus raphe magnus. Second, only injections into the ventrobasal hypothalamus (which included the beta-endorphin-containing arcuate neurons) or into the medial thalamus labeled neurons in the periaqueductal gray itself. Injections into the medial thalamus, but not into the ventrobasal hypothalamus, also labeled neurons in the dorsal raphe nucleus. Up to 20% of the neurons retrogradely labeled from these regions were also retrogradely labeled from nucleus raphe magnus. The presence of large populations of rostrally projecting periaqueductal gray neurons that collateralize to the nucleus raphe magnus implies that activity in ascending projections necessarily accompanies any activation of the periaqueductal gray-nucleus raphe magnus pathway. Possibly, projections from the medial thalamus and medial hypothalamus mediate antinociceptive effects that complement descending inhibition. Finally, possible antidromic activation of these pathways must be considered when interpreting the results of electrical brain stimulation studies.

Dose-related differential accumulation of morphine in specific regions of rat brain determined by mass fragmentography

Regional morphine accumulation was examined in 8 brain areas (cerebral cortex, hippocampus, striatum, midbrain, hypothalamus, thalamus, medulla oblongata and cerebellum) following either a single dose or incremental doses administered by intraperitoneal injection. Morphine levels were determined by gas chromatography-mass spectrometry with chemical ionization detection. Prior to morphine assay, the vasculature was cleared of blood by saline perfusion to eliminate distortion of tissue-morphine by blood-morphine. Results indicate a dose-dependent, differential accumulation of morphine in different brain regions following incremental morphine administration. Three distinct uptake profiles were obtained, with the cerebellum, hippocampus, medulla and cortex showing roughly linear accumulation the midbrain, striatum and thalamus showing nonlinear accumulation, and the hypothalamus showing significantly higher absolute morphine levels than any other brain region.

Suppression of the induction of delayed hypersensitivity in rats by repetitive morphine treatments

Previous in vitro experiments suggest that lymphocyte function may be altered in the presence of opiates. The effect of morphine treatment upon specific T lymphocyte function in vivo was investigated in rats. Following morphine treatment, rats were incapable of responding to immunization with the T-dependent antigens of Mycobacterium bovis, strain BCG. The absence of a delayed hypersensitive skin response to tuberculin in chronic morphine-treated rats supports the hypothesis that opiates may interact with the cells of the immune system. A potential communication between the nervous system and cells of the immune system is proposed.

Microiontophoretic application of morphine and naloxone to neurons in hypothalamus of rat

The present experiments used urethane-anesthetized rats and single cell recording to study the electrophysiological properties of ventromedial hypothalamic (VMH) cells following different doses of morphine and naloxone, applied microiontophoretically. More than 45% of ventromedial hypothalamic units reacted in a dose-response fashion to local application of morphine. In the majority of the ventromedial hypothalamic neurons, naloxone failed to reverse the effects of morphine. Naloxone alone had effects on 37% of the ventromedial hypothalamic units. The ventromedial hypothalamic units exhibited different response patterns from those observed from other CNS sites in response to the microiontophoretic application of morphine and naloxone; this difference is discussed. The present neurophysiological findings support the existence of opiate target sites with multiple opiate receptors within the ventromedial hypothalamus.

Depression of neuronal activity in the hypothalamic ventromedial nucleus of the rat following microinjection of morphine in the amygdala or the periaqueductal gray

The effects of morphine administered intravenously and intracerebrally in the cortical amygdala and periaqueductal gray were examined on spontaneous neuronal activity in the hypothalamic ventromedial nucleus in both intact and castrated adult male rats. Spontaneous extracellular single unit activity in the ventromedial nucleus was significantly faster in chronically castrated (19-23 days) rats compared with intact control rats (6.0 +/- 1.1 and 2.5 +/- 0.3 spikes/s, respectively). Neurons in the ventromedial nucleus exhibited a significant attenuation of neuronal activity following the intravenous administration of morphine; the maximal per cent decrease (to 25% of the control unit activity) was the same in both castrated and intact rats, but cell firing in castrated rats was affected more at lower doses of morphine. When microinjected bilaterally in the cortical amygdala or periaqueductal gray (5 micrograms/site), morphine produced the same maximal depression of unit activity in the ventromedial nucleus as when administered intravenously. The time required to significantly attenuate unit activity in the ventromedial nucleus was the same (3 min) whether morphine was administered intravenously or bilaterally in the periaqueductal gray. When microinjected bilaterally in the cortical amygdala, the depressant effect of morphine on cell firing in the ventromedial nucleus was significant first at 10 min. These data correlate well with the time courses for depression of serum levels of luteinizing hormone produced by morphine microinjected in the same doses in the same extrahypothalamic sites in the rat. In view of the relatively high levels of specific opiate receptor binding in both the periaqueductal gray and amygdala, this report further supports a role for modulation by extrahypothalamic areas of opioid-induced changes in the functional activity of the hypothalamic-pituitary axis, perhaps via the ventromedial nucleus.