A GABA cardiovascular mechanism in the dorsal raphe of the rat

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

MEMRI reveals altered activity in brain regions associated with anxiety, locomotion, and cardiovascular reactivity on the elevated plus maze in the WKY vs SHR rats

Individuals with anxiety/depression often have exaggerated cardiovascular responses to stressful stimuli and a comorbidity with hypertension. Alternatively, individuals with hypertension can be more anxious. In the present study cardiovascular changes were evaluated during behavioral testing of anxious behavior on the elevated plus maze (EPM) in the spontaneously hypertensive rat (SHR), a rodent model of neurogenic hypertension, and compared to the response of the more anxious, but normotensive, Wistar-Kyoto rat (WKY). Manganese-enhanced magnetic resonance imaging (MEMRI) was used to identify regional differences in baseline brain activity. Parallel to indicators of elevated behavioral anxiety on the EPM, WKYs had a greater increase in blood pressure but not heart rate when compared to the SHR while on the EPM. Associated with differences in anxiety-related behavior and autonomic responses, we observed increased baseline activity in the amygdala, central gray, habenula and interpeduncular nucleus with MEMRI of the WKY compared to the SHR. Conversely, elevated baseline brain activity was found in regions associated with blood pressure control and system arousal, including the hypothalamus, locus coeruleus and pedunculopontine tegmental nucleus, in the SHR vs WKY, in-line with increased resting blood pressure and increased mobility in this strain. Lastly, reduced activity in hippocampal regions was identified in the SHR compared to the WKY and may be associated with cognitive impairment previously reported in the SHR. Thus, autonomic reactivity may be a true measure of stress in rodent models of anxiety and MEMRI presents a powerful technique to uncover novel brain mechanisms of blood pressure control.

Functional mapping of the cardiorespiratory effects of dorsal and median raphe nuclei in the rat

The dorsal (DRN) and median (MRN) raphe nuclei are important sources of serotonergic innervation to the forebrain, projecting to sites involved in cardiovascular regulation. These nuclei have been mapped using electrical stimulation, which has the limitation of stimulating fibers of passage. The present study maps these areas with chemical stimulation, investigating their influence on cardiorespiratory parameters. Urethane-anesthetized (1.2 g/kg, iv) male Wistar rats (280-300 g) were instrumented for pulsatile and mean blood pressure (MBP), heart rate, renal nerve activity, and respiratory frequency recordings. Microinjections of L-glutamate (0.18 M, 50-100 nl with 1% Pontamine Sky Blue) were performed within the DRN or the MRN with glass micropipettes. At the end of the experiments the sites of microinjection were identified. The majority of sites within the MRN (86.1%) and DRN (85.4%) evoked pressor responses when stimulated (DRN: DeltaMBP = +14.7 +/- 1.2; MRN: DeltaMBP = +13.6 +/- 1.3 mmHg). The changes in renal nerve activity and respiratory rate caused by L-glutamate were +45 +/- 11 and +42 +/- 9% (DRN; P < 0.05%), +40 +/- 10 and +29 +/- 7% (MRN, P < 0.05), respectively. No significant changes were observed in saline-microinjected animals. This study shows that: a) the blood pressure increases previously observed by electrical stimulation within the raphe are due to activation of local neurons, b) this pressor effect is due to sympathoexcitation because the stimulation increased renal sympathetic activity but did not produce tachycardia, and c) the stimulation of cell bodies in these nuclei also increases the respiratory rate.

GABAergic projection from the ventral tegmental area and substantia nigra to the periaqueductal gray region and the dorsal raphe nucleus

Previous studies have shown that neurons in the ventral tegmental area (VTA) and substantia nigra (SN) project to the ventrolateral periaqueductal gray (PAGvl) and dorsal raphe nucleus (DR). Research has also shown that stimulation of neurons in the VTA/SN elicits cardiovascular depressor responses that are mediated by a projection to the PAGvl/DR. Anatomic and physiological experiments were done in the present study to determine the neurochemical identity of the VTA/SN projection to the PAGvl/DR. Experiments were done to characterize the origin and chemical nature of this projection by combining cholera toxin B tracing with immunofluorescence for the 67K isoform of glutamic acid decarboxylase (GAD) and tyrosine hydroxylase. The PAGvl/DR region was found to receive a substantial input from neurons in the VTA, SN, and deep mesencephalic nucleus. The DR was preferentially innervated by neurons in the VTA, whereas the PAGvl was preferentially innervated by neurons in the SN. A proportion of neurons in the VTA and the reticular portion of the SN found to project to the PAGvl/DR were GAD positive. In addition, experiments were done in urethane-anesthetized rats to determine whether injections of a gamma-aminobutyric acid (GABA) antagonist in the region of the PAGvl/DR attenuated the cardiovascular depressor responses produced by glutamate stimulation of the VTA/SN. Injections of the GABA-blocking agent picrotoxin (2.5 nmol, 500 nl) into the PAGvl/DR eliminated the cardiovascular responses from stimulation of the VTA/SN (0.01 M, 50 nl). The results of the present investigation provide evidence for a GABAergic projection from the VTA/SN to the PAGvl/DR. This projection may be an important regulator of the PAGvl/DR, an area of the midbrain involved in the production of behavioral and physiological responses to pain and stress.

Cardiovascular effects of 5-HT1A receptor agonists injected into the dorsal raphe nucleus of conscious rats

The aim of this study was to investigate the cardiovascular effects of the 5-HT1A receptor agonists, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), flesinoxan and 5-carboxamidotryptamine (5-CT) following injection into the dorsal raphe nucleus of conscious rats. 8-OH-DPAT (0.5-2.5 micrograms), hypotension, bradycardia and flat body posture. In contrast, injection of 8-OH-DPAT (0.5 microgram) into the median raphe nucleus caused no cardiovascular changes or flat body posture. (-)Pindolol (0.5 microgram dorsal raphe nucleus) had little effect on cardiovascular parameters, but significantly attenuated the cardiovascular effects of 8-OH-DPAT (0.5 microgram dorsal raphe nucleus). N-Methylatropine (1 mg/kg i.v.) antagonised the cardiovascular effects of 8-OH-DPAT (0.5 microgram dorsal raphe nucleus), suggesting these were vagally mediated. Both pretreatments also appeared to reduce 8-OH-DPAT-induced flat body posture. The results suggest that 8-OH-DPAT activates 5-HT1A receptors in the dorsal raphe nucleus to cause hypotension and bradycardia.

GABA receptors: are cellular differences reflected in function?

The putative involvement of GABAA and GABAB receptors in various behavioral and physiological effects is summarized in Table III. A division of function among the two types of GABA receptors appears to exist. GABAA receptors mediate feeding, cardiovascular regulation, anxiolytic effects, and anticonvulsive activity. GABAB receptors, on the other hand, are involved in analgesia, cardiovascular regulation, and depression. Although there is some overlap and shared functions among the receptor types, it is evident that GABAA and GABAB receptors have different behavioral and physiological profiles. Feeding, anticonvulsive activity and anxiety, for example, primarily involve GABAA receptors. Analgesia and depression, on the other hand, are GABAB effects. In those cases where GABAA and GABAB receptors mediate similar functions (e.g. cardiovascular regulation), they do so by affecting different transmitter systems and cellular mechanisms. It is proposed, therefore, that GABAA and GABAB receptors differ not only at the cellular level, but that they also have different functions in the mammalian central nervous system. The association of different subtypes of a receptor with different functions and mechanisms of action is not unique to the GABA system. D1 and D2 receptors in the dopamine system, for example, also exhibit some separation of function as do the mu, delta and kappa types of opiate receptors. Different subtypes of neurotransmitter receptors, therefore, appear to be a general organizing principle used by the brain to transduce chemical signals into different functional responses. A better understanding of the exact processes through which cellular signals are transformed into functional responses is a goal of future research.

Cardiovascular effects of cholinergic agents in the ventral-lateral midbrain periaqueductal gray of the rat

Local injection of the cholinergic agonist carbachol (1-5 nmol) into the ventral lateral periaqueductal gray region of the midbrain produced a dose-dependent bradycardia with minimal effects on blood pressure in the urethane-anesthetized rat. The decrease in heart rate was maximal in the region of the nuclei of the third and fourth cranial nerves. Intravenous injection of carbachol (5 nmol) had little effect on the heart rate. Injection of (-)-nicotine (5 nmol) into this region of the brain produced a slight, but significant, increase in heart rate. Pretreatment with an intracranial injection of atropine greatly reduced the bradycardia response, whereas local injection of mecamylamine or hexamethonium had no effect. The bradycardia resulting from cholinergic stimulation in this area of the brain appears to result from an enhanced vagal influence on the heart, as pretreatment with intravenous atropine methyl nitrate (1 mg/kg) prevented the carbachol-induced bradycardia. The possible physiological significance of this response is discussed.