3-Acetylpyridine lesions and four serotonergic behavioral syndromes in the rat

The Neuroanatomical Basis of the 5-HT Syndrome and Harmalineinduced Tremor

The 5-HT syndrome in rats is composed of head weaving, body shaking, forepaw treading, flat body posture, hindlimb abduction, and Straub tail. The importance of the brainstem and spinal cord for the syndrome is underlined by findings of 5,7-dihydroxytryptamine (5,7-DHT)-induced denervation supersensitivity in response to 5-HT-stimulant drugs. For head weaving and Straub tail, supersensitivity occurred when the neurotoxin was injected into the cisterna magna or spinal cord, for forepaw treading in cisterna magna, and for hindlimb abduction in the spinal cord. Although 5,7- DHT-related body shaking increased in the spinal cord, the sign decreased when injected into the striatum, indicating the modulatory influence of the basal ganglia. Further details on body shaking are provided by its reduced response to harmaline after 5-HT depletion caused by intraventricular 5,7-DHT, electrolytic lesions of the medial or dorsal raphe, and lesions of the inferior olive caused by systemic injection of 3-acetylpyridine along with those found in Agtpbp1pcd or nr cerebellar mouse mutants. Yet the influence of the climbing fiber pathway on other signs of the 5-HT syndrome remains to be determined.

Altered behavioral responses mediated by serotonin receptors in the genetically dystonic (dt) rat

The genetically dystonic (dt) rat is an animal model of dystonia that displays sustained abnormal movements that include: torticollis, clasping of the hindlimbs, rigidity of the limbs, and contortions of the trunk. Since serotonin (5-HT) has been shown to be involved in some animal models of movement disorders, the functional responsiveness of the 5-HT system in dt rats and phenotypical normal littermates was examined by administering 5-HT agonists selective for different receptor subtypes and observing behavioral responses associated with the activation of specific 5-HT receptor subtypes. The dt rats were 6-fold more sensitive to the ability of the 5-HT1A agonist 8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT) to produce the 5-HT behavioral syndrome. The dt rats demonstrated a diminished head-shaking response following administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB). However, the dt rats also displayed significantly fewer head shakes following mechanical stimulation of the aural pinnae. The inability of the dt rats to demonstrate head-shaking behavior following stimulation of 5-HT2 receptors is probably due to the dt rat's difficulty in producing the motor responses involved in this behavioral response and do not reflect alterations in 5-HT2 receptor sensitivity. These results suggest that the 5-HT system, particularly 5-HT1A receptors, may have an integral role in the abnormal movements displayed by the genetically dystonic rat and movement disorders in general.

Enhanced sensitivity to quipazine in the genetically dystonic rat (dt)

Both normal and genetically dystonic (dt) rats show a high-frequency forepaw tremor in response to systemic administration of the serotonin (5-HT) agonist quipazine at 8 days of age. The response declines with age in normal, but not dystonic, rats. By 16 days of age and after the development of a generalized movement disorder, the dystonic rat exhibits enhanced sensitivity to the tremorogenic effects of the drug in comparison with normal rats. Tremor was blocked by pretreatment with ketanserin, suggesting that it is mediated by 5-HT2 receptors. The dystonic rat has previously been shown to be insensitive to the tremorogenic effects of harmaline, a drug presumed to act indirectly through serotonergic neurons. This finding, coupled with the increased sensitivity to quipazine, suggests the presence of an abnormality in serotonergic systems in the mutants. Since there is evidence of abnormality in the olivo-cerebellar system in the dystonic rat, the alternative hypothesis that a nonserotonergic defect in the olivo-cerebellar system accounts for both the failure of behavioral response to harmaline and the persistent expression of a response to quipazine is also discussed.

Destruction of the nucleus raphe obscurus and potentiation of serotonin-mediated behaviors following administration of the neurotoxin 3-acetylpyridine

Systemic administration of the neurotoxin 3-acetylpyridine (3-AP) to rats produced spontaneous episodes of spasmodic movement involving the trunk and limbs including torticollis, contortions of the trunk and rigid extension of the limbs. Because the neurotransmitter serotonin (5-HT) has been implicated in various human involuntary movement disorders, the functional and anatomical integrity of the 5-HT system in rats treated with 3-AP were examined. 5-HT-containing neurons in the brain stem were studied using immunohistochemical labeling with antiserum to 5-HT. Cells in the nucleus raphe obscurus were found to be altered following 3-AP treatment as shown by a decrease in 5-HT immunoreactivity as compared to control rats. No changes in 5-HT immunoreactivity were observed in any other region containing 5-HT cell bodies. Behaviorally, rats treated with 3-AP were 2.5-fold more sensitive to the ability of the 5-HT1A agonist 8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.33-3.3 mg/kg) to produce the 5-HT syndrome. Similarly, 3-AP-treated rats were 2-fold more sensitive to the selective 5-HT2 agonist 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB; 0-1.0 mg/kg) at producing the head shake response. Although these behaviors associated with brain stem 5-HT receptors were potentiated by 3-AP, the hypothermic effect of 8-OH-DPAT which involves ascending mesencephalic 5-HT neurons was unchanged following 3-AP treatment. Treatment with 3-AP did not produce significant alterations of 5-HT or 5-hydroxyindoleacetic acid (5-HIAA) content in any brain region studied. Quantitative autoradiographic analysis of the density of 5-HT1A receptors labeled with [3H]8-OH-DPAT revealed that these sites were unchanged in regions of the brain (frontal cortex, hippocampus and brain stem) and in the spinal cord. Similarly, few changes in the density of 5-HT2 receptors measured with [3H]ketanserin were observed in various brain regions. These results suggest that neurons from the nucleus raphe obscurus are involved in the elicitation of 5-HT-mediated behavioral responses by 5-HT1A and 5-HT2 receptor agonists that are though to be mediated through brain stem and spinal cord mechanisms. In addition, because of the close neuroanatomical relationship of the nucleus raphe obscurus with various brain regions known to be involved in motor control, the destruction of this region by 3-AP may contribute to the spasmodic motor behaviors observed following 3-AP treatment.