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Pre- and post-junctional supersensitivity: differentiation by intraventricular infusions of norepinephrine and
Abstract:
Intraventricular infusions of methoxamine, an adrenergic agonist that has a relatively low affinity for the presynaptic uptake mechanism, produces a significant dose-dependent increase in locomotor activity comparable to the increase elicited by infusion of equimolar doses of norepinephrine (NE). The behavioral responsiveness to infusion of both NE and methoxamine was more than doubled 3 weeks after pretreatment with 6-hydroxydopamine (6-OHDA). These results indicate that the increased responsiveness to NE induced by 6-OHDA is due to enhanced postsynaptic receptor sensitivity rather than to a loss of presynaptic uptake inactivation.
Insights
Intraventricular infusions of methoxamine and norepinephrine (NE) increase locomotor activity. Pretreatment with 6-hydroxydopamine (6-OHDA) doubles this response, indicating enhanced postsynaptic receptor sensitivity.
Area of Science:
- Neuroscience
- Pharmacology
- Behavioral Science
Background:
- Adrenergic agonists like methoxamine and norepinephrine (NE) influence locomotor activity.
- Presynaptic uptake mechanisms are crucial for regulating neurotransmitter levels.
- The effects of 6-hydroxydopamine (6-OHDA) on adrenergic systems are well-documented.
Purpose of the Study:
- To investigate the role of postsynaptic receptor sensitivity versus presynaptic uptake in the behavioral effects of adrenergic agonists.
- To determine if 6-OHDA pretreatment alters the response to intraventricular methoxamine and norepinephrine.
Main Methods:
- Intraventricular infusions of methoxamine and norepinephrine (NE) were administered.
- Locomotor activity was measured as a behavioral response.
- Animals were pretreated with 6-hydroxydopamine (6-OHDA) 3 weeks prior to infusions.
Main Results:
- Methoxamine produced a dose-dependent increase in locomotor activity, comparable to NE.
- 6-OHDA pretreatment more than doubled the behavioral responsiveness to both NE and methoxamine.
- The enhanced response was observed after a 3-week delay following 6-OHDA administration.
Conclusions:
- The results suggest that 6-OHDA enhances responsiveness to adrenergic agonists by increasing postsynaptic receptor sensitivity.
- Presynaptic uptake inactivation is not the primary mechanism behind the heightened behavioral response to NE after 6-OHDA treatment.
- This study clarifies the adaptive mechanisms of the adrenergic system following neurochemical depletion.