Qing-Ping Ma

Serotonergic projections from the nucleus raphe dorsalis to the amygdala in the rat

Previous studies have shown a possible connection between the nucleus raphe dorsalis (NRD) and the amygdala in mediating opioid analgesia. In the present study, horseradish peroxidase (HRP) retrograde tracing was used in combination with serotonin (5-HT) immunocytochemical staining in an attempt to search for serotonergic projections from the NRD to the amygdala. In rats which received an injection of HRP into the amygdala, HRP retrogradely labelled 5-HT-immunoreactive cells were observed in the NRD. About 10% of the 5-HT-immunoreactive neurons in the NRD give rise to axons to the amygdala. These cells are predominantly situated in the ipsilateral wing and ventromedial part of the NRD. These data indicate the existence of serotonergic projections from the NRD to the amygdala, providing a morphological substrate for the putative antinociceptive pathway from the NRD to the amygdala.

Further studies on interactions between periaqueductal gray, nucleus accumbens and habenula in antinociception

Previous findings from this laboratory with the intracerebral microinjection technique suggested that the periaqueductal gray (PAG), nucleus accumbens, and habenula might constitute a unidirectional loop to play their roles in pain modulation. In the present study we demonstrate that intra-habenular injection of naloxone antagonizes the analgesia elicited by morphine injected into the periaqueductal gray (PAG) and that intra-accumbens injection of naloxone is capable of attenuating the analgesic effects of morphine injected into the habenula. These results indicate that the relationships between these nuclei may be more complex than the putative unidirectional loop.

Neurochemical and Morphological Evidence of an Antinociceptive Neural Pathway From Nucleus Raphe Dorsalis to Nucleus Accumbens in the Rabbit

Previous studies using a pharmacological approach suggested a neural pathway emanating from the periaqueductal gray (PAG) to the nucleus accumbens relevant to antinociception. This was investigated with neurochemical and histochemical methods in the present study. Push-pull perfusion and radioimmunoassay were used to measure the release of immunoreactive-(ir) enkephalin (ir-ENK) and ir-beta-endorphin (ir-beta-EP) in the nucleus accumbens after microinjection of morphine into the PAG and the nucleus raphe dorsalis (NRD) of the rabbit. Morphine administration elicited an increase in ir-ENK and ir-beta-EP in the nucleus accumbens. Horseradish peroxidase (HRP) retrograde tracing in combination with 5-hydroxytryptamine (5-HT) immunocytochemistry revealed a serotonergic projection from the NRD and ventral PAG to the nucleus accumbens in the rabbit. About 7% of the serotonin-positive cells in the NRD and ventral PAG send fibers directly to the nucleus accumbens, with an ipsilateral dominance. These results indicate the existence of a serotonergic pathway from the NRD to the N. accumbens involved in opioid analgesia.

Noxious stimulation accelerated the expression of c-fos protooncogene in cholecystokininergic and dopaminergic neurons in the ventral tegmental area

Cholecystokininergic (CCK) and dopaminergic systems in the midbrain have been implicated in pain modulation. In the present study, Fos immunoreactivity was used as a marker of neuronal activity to investigate if subcutaneous injection of formalin (a model of noxious stimulation) could activate the CCKergic and dopaminergic neurons in the midbrain. Noxious stimulation increased significantly the numbers of Fos-positive CCKergic and dopaminergic neurons in the ventral tegmental area (VTA), whereas no significant changes in the numbers of Fos-positive CCKergic or dopaminergic neurons were observed in other midbrain nuclei. These results indicate that the VTA may play a role in pain modulation.

Naloxone blocks the release of opioid peptides in periaqueductal gray and N. accumbens induced by intra-amygdaloid injection of morphine

The working hypothesis that the periaqueductal gray (PAG), N. accumbens and amygdala were connected serially in a unidirectional loop for antinociception, in which Metenkephalin and beta-endorphin were considered to be two important analgesic neurotransmitters, was examined by simultaneously perfusing the PAG and N. accumbens after microinjection of morphine into the amygdala. Intra-amygdaloid injection of morphine increased the release of enkephalins and beta-endorphin in the PAG and N. accumbens. When the perfusion fluid contained 3 microM of naloxone, the release of enkephalins and beta-endorphin was reduced in both the PAG and the N. accumbens. These results do not support the hypothesis of a unidirectional loop and its putative sequence.

Electroacupuncture Accelerated the Expression of C-Fos Protooncogene in Dopaminergic Neurons in the Ventral Tegmental Area of the Rat

The mesolimbic dopaminergic system has been implicated in mediating morphine and electroacupuncture analgesia. In the present study, Fos immunoreactivity was used as a marker of neuronal activity to investigate if electroacupuncture could activate the dopaminergic neurons in the midbrain. Electroacupuncture stimulation significantly increased the number of Fos-positive dopaminergic neurons in the ventral tegmental area (VTA), whereas no significant c-fos expression in the dopaminergic neurons was observed in the substantia nigra (SN). These results indicate that the VTA rather than the SN may play a role in mediating electroacupuncture analgesia.

Naloxone blocks opioid peptide release in periaqueductal gray and amygdala elicited by morphine injected into N. accumbens

Previous studies from this laboratory suggested that the periaqueductal gray (PAG), nucleus accumbens, and amygdala might take part in a serial, unidirectional mesolimbic loop to play their roles in pain modulation. It has been proposed that morphine injected into one of these nuclei would cause the release of opioid peptides in one nucleus after another. This working hypothesis was examined in the present study by perfusing simultaneously the PAG and the amygdala after microinjection of morphine into the N. accumbens. It was found that microinjection of morphine increased the content of immunoreactive enkephalins (ir-ENK) and immunoreactive beta-endorphin (ir-beta-EP) in the perfusate of the PAG and the amygdala. When the perfusion fluid contained 3 microM of naloxone, the increase of ir-ENK and ir-beta-EP was reduced significantly. These results indicate that the three nuclei were not serially connected in a unidirectional loop.

Brief Communication: Intrathecally Injected Antibody Can Diffuse Into Spinal Cord

Antibody microinjection has been widely used to investigate the function of neuropeptides, but the capability of antibody to diffuse in the brain tissue has not been well characterized. The present study was conducted with an immunohistochemical method to determine if the anti-enkephalin serum injected intrathecally could diffuse into the spinal cord. Enkephalin immunoreactivity was observed in laminae I and II in slight amounts after 10 min, in moderate amounts after 30 minutes and very clearly after 60 min of the intrathecal injection of enkephalin antiserum. In addition, marked nonspecific staining was observed in the dorsal part but not in the ventral part of the white matter. These results indicate that antibodies injected intrathecally are capable of diffusing into the spinal cord within a time period of 10-60 min.