Disproportionate increases in locomotor activity in response to hormonal and photic stimuli following regional neurochemical depletions of serotonin.
The role of forebrain serotonin in behavior-related energy output was assessed in two locomotor activity tests conducted 3 and 6 months after bilateral, intrahypothalamic microinfusion of the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). The serotonin-depleted animals exhibited a long-lasting and stable increase in energy expenditure as locomotor activity. This increased activity was investigated at the behavioral level by relating the hyperactivity to estrous cycle, photoperiod and body weight. Although the serotonin depletion-induced hyperactivity occurred in all photoperiod and estrous cycle stages, its magnitude was disproportionately increased during light and estrus. This hyperactivity could not be related to decreases in body weight because the serotonin-depleted animals weighed significantly more than the control animals. These animals responded to the weight loss that normally accompanies wheel running by increasing their activity to the same proportion as the other groups. The neuroanatomical and neurochemical substrate of the increased locomotor activity was investigated with a regional neurochemical assay for serotonin, dopamine and norepinephrine. This assay revealed that the toxin had no effect on dopamine or norepinephrine in any structure analyzed; however, serotonin was depleted in the hippocampus, septum and, to a lesser degree, in the hypothalamus. Serotonin levels were negatively correlated with overall activity. The magnitude of the disproportionate increase in activity during light and estrus was negatively correlated with hippocampal serotonin level. These results indicate that forebrain depletions of serotonin differentially affect the control of activity exerted by the phases of the photoperiod and estrous cycle. However, the modulation of activity levels by decreases in body weight remains intact.
D000818:Animals; D001835:Body Weight; D001921:Brain / Q000502:physiology*; D002940:Circadian Rhythm; D004734:Energy Metabolism; D004971:Estrus; D005260:Female; D006624:Hippocampus / Q000502:physiology; D007031:Hypothalamus / Q000502:physiology; D009043:Motor Activity / Q000502:physiology*; D011247:Pregnancy; D051381:Rats; D012688:Septum Pellucidum / Q000502:physiology; D012701:Serotonin / Q000502:physiology*
D012701:Serotonin