Neuroscience
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Comparative Study
Autoradiographic localization of 5-hydroxytryptamine1A, 5-hydroxytryptamine1B and 5-hydroxytryptamine1C/2 binding sites in the rat spinal cord.
Autoradiographic techniques revealed that 5-hydroxytryptamine1A, 5-hydroxytryptamine1B and 5-hydroxytryptamine1C/2 binding sites are differentially distributed in the spinal cords of adult male rats. In the dorsal horn, 5-hydroxytryptamine1A sites were dense in all laminae; 5-hydroxytryptamine1B sites were more dense in laminae I, III and IV than in lamina II; while 5-hydroxytryptamine1C/2 sites were very sparse. The dorsal commissure gray matter also exhibited very dense 5-hydroxytryptamine1A and 5-hydroxytryptamine1B binding. ⋯ In the ventral horn, 5-hydroxytryptamine1A and 5-hydroxytryptamine1B sites were very sparse (except for very dense 5-hydroxytryptamine1A sites located in the dorsolateral nucleus of the pudendal nerve), while 5-hydroxytryptamine1C/2 sites were relatively dense in motor nuclei. Surprisingly, 5-hydroxytryptamine1B sites were moderately dense in the dorsal column corticospinal tract. These studies will provide an anatomical perspective for interpretation of the complex role of 5-hydroxytryptamine in regulating spinal cord function.
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Dopamine-mediated behaviors and certain biochemical and molecular events associated with these behaviors were examined following continuous infusion of the D1 dopamine agonist SKF38393 or the D2 dopamine agonist quinpirole into mice for six days. SKF38393 produced a transient grooming behavior while quinpirole initially induced stereotypy, which was followed by an increased locomotor behavior. Continuous infusion of quinpirole caused a significant down-regulation of striatal D2 dopamine receptors without significantly changing the density of D1 receptors. ⋯ This treatment also induced a significant decrease in proenkephalin messenger RNA in striatum. Taken together, these results suggest that the down-regulation of D2 dopamine receptor and D2 receptor messenger RNA is the result of the persistent stimulation of D2 receptors and that the up-regulation of mu opioid receptors may be a compensatory response to a decreased biosynthesis of enkephalin. They suggest further that the biochemical and molecular changes that take place in dopaminergic and enkephalinergic systems following continuous treatment with dopamine agonists may underlie the mechanisms by which certain dopamine-mediated behaviors occur.
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The segmental and laminar origin of propriospinal antinociceptive systems in the cat spinal cord and the modes to activate them are characterized. The experiments were performed on pentobarbital-anesthetized cats with a high cervical spinalization. Recordings were made from single lumbar spinal dorsal horn neurons responding to noxious radiant skin heating and to innocuous mechanical skin stimuli. ⋯ Glutamate microinjections into the superficial layers of the thoracic, upper lumbar or sacral dorsal horn ipsi- or contralateral to the recording sites or into lamina VIII of the ipsilateral thoracic or upper lumbar cord reduced noxious heat-evoked responses with or without changes in the level of background activity. It is concluded that propriospinal neurons originating from circumscribed areas of the cervical, thoracic, lumbar or sacral spinal cord independently modulate background activity and noxious heat-evoked responses of multireceptive lumbar spinal dorsal horn neurons. The incidence and efficacy of propriospinal antinociceptive stimulation sites was found to be as high as for the classical region of endogenous antinociception, the midbrain periaqueductal gray.
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The rostral ventrolateral medulla oblongata plays an important role in the control of arterial blood pressure and it has strong descending projections into the intermediolateral nucleus of the thoracic spinal cord, where the majority of sympathetic preganglionic neurons are located. The purpose of this study was to see whether these projections form synaptic contacts with sympathetic preganglionic neurons in the rat. Projections from both the lateral part of the rostral ventrolateral medulla (rostroventrolateral reticular nucleus) and from the more medial region (lateral paragigantocellular nucleus) were investigated separately in view of their different functional roles in sympatho-regulation and their different chemical composition. ⋯ Synaptic specializations were of both the symmetrical and asymmetrical type. The targets of boutons forming asymmetrical synaptic contacts differed according to their origin: boutons originating from neurons in the rostroventrolateral reticular nucleus were mainly in contact with dendrites of sympathetic preganglionic neurons, while those originating from the lateral paragigantocellular nucleus mainly innervated the cell bodies. Our observations provide anatomical support for the view that there are two distinct classes of sympatho-regulatory cells in the rostral ventrolateral medulla, each of which can directly influence the activity of sympathetic preganglionic neurons; they also emphasize the importance of detailed investigation of the subregions of the ventrolateral medulla with respect to their sympatho-regulatory functions.
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The effect of continuous intrathecal infusion with morphine (5 mu/h) or naloxone (2 micrograms/h) was investigated with regard to analgesia and the apparent density of mu- and delta-opioid and neurokinin-I/substance P receptors in the rat spinal cord. Morphine infusion increased tail-flick and paw-pressure responses until day 4 after the mini-osmotic pump implant. A decline in antinociception, reflecting tolerance to morphine, was then apparent in both tests. ⋯ In contrast, the density of mu-opioid receptors was only affected by naloxone (50% increase). Neurokinin-I/substance P binding parameters were not altered by these treatments. Thus, it appears that delta-opioid binding sites may be of special relevance with regard to the development of tolerance to opiates in the spinal cord.