Role of norepinephrine in the dorsomedial hypothalamic panic response: an in vivo microdialysis study

Blockade of gamma-aminobutyric acid-A (GABA(A)) receptors in the dorsomedial hypothalamus (DMH) elicits a panic-like response that includes increases in heart rate (HR), blood pressure (BP), respiration rate (RR), and anxiety. Norepinephrine (NE) has been postulated to be critical in regulating panic and anxiety responses. Therefore, the first study sought to determine changes in extracellular NE levels within the DMH following acute blockade of GABA(A) receptors in the DMH using in vivo microdialysis. Rats were implanted with femoral arterial catheters and microdialysis probes into the DMH. Following recovery, the DMH of conscious rats were perfused with 100, 150, or 200 microM solutions of the GABA(A) receptor antagonist bicuculline methiodide (BMI) via the microdialysis probe. HR and BP responses were recorded and the changes in extracellular levels of NE in the dialysate samples from the DMH were determined by HPLC. Rats receiving BMI injections showed dose-dependent increases in both the extracellular NE levels in the DMH as well as HR and BP. The second study was conducted to test the functional importance of NE in the DMH to the BMI-induced physiological responses. The effects of BMI microinjection into the DMH were measured at baseline and 10 days after local injection of either vehicle or two doses of 6-hydroxydopamine (6-OHDA), a neurotoxin known to lesion NE terminals. There was a significant loss of tissue NE levels as well as BMI-induced HR, BP and RR responses in the 6-OHDA-treated but not vehicle-treated rats. Thus, blockade of GABA(A) receptors in the DMH results in NE release and the presence of NE appears to be necessary for eliciting the physiological components of the panic-like responses in this region.

Ethanol and negative feedback regulation of mesolimbic dopamine release in rats

The objectives of this study were to examine the relationship between somatodendritic and terminal field dopamine (DA) release following manipulation of DA D2 receptors in the ventral tegmental area (VTA), systemic administration of ethanol, and inhibition of DA uptake in the nucleus accumbens (ACB). Perfusion of 5, 25 and 100 microM quinpirole (a D2 agonist), or sulpiride (a D2 antagonist) through the microdialysis probe in the VTA produced dose-related decreases or increases, respectively, in the extracellular levels of DA in both the VTA and ACB of adult Wistar rats. The IP administration of 2-3 g/kg ethanol produced a sustained increase in the extracellular levels of DA (150-200% of baseline) in the ACB for at least 2 h after injection, whereas only a transient increase was observed in the VTA. Local perfusion of the ACB with 100 microM GBR12909, a DA uptake inhibitor, elevated the extracellular levels of DA in the ACB to approximately 400% of baseline, but decreased the extracellular levels of DA in the VTA to approximately 50% of baseline. Overall, the results suggest that (a) there is an association between somatodendritic and terminal field DA release when D2 cell body autoreceptors in the VTA are manipulated, (b) elevating synaptic levels of DA in the terminal field activates a long-loop negative feedback system to the VTA, and (c) different mechanisms may be mediating the actions of ethanol on DA neuronal activity and terminal DA release.

Monoamines in the dorsomedial hypothalamus of rats following exposure to different tests of "anxiety"

1. Adult male Sprague-Dawley rats were subjected to the fear-potentiated startle, elevated plus-maze and social interaction tests and the levels of norepinephrine, dopamine and serotonin in the dorsomedial hypothalamus were determined by high-performance liquid chromatography. 2. Only rats subjected to the full fear potentiated startle test and not the other tests of anxiety or components (foot shocks or acoustic startle) of fear-potentiated startle showed significant increases in norepinephrine and dopamine levels after 24 hours. 3. A time course experiment specific for the norepinephrine changes in the dorsomedial hypothalamus of fear-potentiated startle rats revealed a significant increase in tissue content as compared to controls at both the 12 and 24 hour post-test time points. 4. Tyrosine hydroxylase activity in the dorsomedial hypothalamus of the fear-potentiated startle rats did not show a significant change from controls. 5. An in vitro release study found a significant decrease in potassium-stimulated release of norepinephrine in the dorsomedial hypothalamus as compared to controls at 24 hours post-test. 6. These results suggest that animals exposed to fear-potentiated startle and not other tests of "anxiety" have a change in tissue catecholamine levels and that the norepinephrine change may be the result of a decrease in release and not an increase in synthesis.

Dorsomedial hypothalamic GABA regulates anxiety in the social interaction test

Blockade of GABAA function in the region of the dorsomedial hypothalamus (DMH) of rats is known to elicit a constellation of physiologic responses including increases in heart rate (HR), mean arterial blood pressure (BP), respiratory rate, and plasma catecholamine levels, as well as behavioral responses such as increases in locomotor activity and anxiogenic-like effects as measured in a conflict test and the elevated plus-maze test. The aim of the present study was to test the effects of microinjecting GABAA antagonists bicuculline methiodide (BMI) and picrotoxin, as well as the GABAA agonist muscimol, into the DMH of rats placed in the social interaction (SI) test. Muscimol decreased HR and BP but increased SI, whereas the GABA antagonists increased HR and BP but decreased SI time. Blocking the HR changes elicited by GABAergic drugs injected into the DMH with systemic injections of atenolol and atropine methylbromide did not block their effects on SI.

Fear-potentiated startle elevates catecholamine levels in the dorsomedial hypothalamus of rats

The norepinephrine (NE), dopamine (DA), and serotonin (5-HT) systems are thought to be important in the development of anxiety and stress. The dorsomedial hypothalamus (DMH) of rats has been implicated in the regulation of physiological and behavioral responses associated with fear and anxiety. In order to elucidate the interactions between the monoamine systems and the DMH, we studied the effects of subjecting rats to the fear-potentiated startle test, a commonly used test of anxiety in rats, on the NE, DA, and 5-HT levels in the DMH. Rats in the potentiated startle test, but not those exposed to just foot shocks or acoustic startle, showed significantly higher levels of NE and DA in the DMH compared to cage controls. In contrast, foot shocks significantly elevated the 5-HT levels in the DMH.