Chronic activation of brain areas by high-sodium diet in Dahl salt-sensitive rats

The Effect of Noise Trauma and Deep Brain Stimulation of the Medial Geniculate Body on Tissue Activity in the Auditory Pathway

Tinnitus is defined as the phantom perception of sound. To date, there is no curative treatment, and contemporary treatments have failed to show beneficial outcomes. Deep brain stimulation has been suggested as a potential therapy for refractory tinnitus. However, the optimal target and stimulation regimens remain to be defined. Herein, we investigated metabolic and neuronal activity changes using cytochrome C oxidase histochemistry and c-Fos immunohistochemistry in a noise trauma-induced rat model of tinnitus. We also assessed changes in neuronal activity following medial geniculate body (MGB) high-frequency stimulation (HFS). Metabolic activity was reduced in the primary auditory cortex, MGB and CA1 region of the hippocampus in noise-exposed rats. Additionally, c-Fos expression was increased in the primary auditory cortex of those animals. Furthermore, MGB-HFS enhanced c-Fos expression in the thalamic reticular nucleus. We concluded that noise trauma alters tissue activity in multiple brain areas including the auditory and limbic regions. MGB-HFS resulted in higher neuronal activity in the thalamic reticular nucleus. Given the prominent role of the auditory thalamus in tinnitus, these data provide more rationales towards targeting the MGB with HFS as a symptom management tool in tinnitus.

Hypothalamic Paraventricular Nucleus Gαi2 (Guanine Nucleotide-Binding Protein Alpha Inhibiting Activity Polypeptide 2) Protein-Mediated Neural Control of the Kidney and the Salt Sensitivity of Blood Pressure

We have previously reported that in salt-resistant rat phenotypes brain, Gαi2 (guanine nucleotide-binding protein alpha inhibiting activity polypeptide 2) proteins are required to maintain blood pressure and sodium balance. However, the impact of hypothalamic paraventricular nucleus (PVN) Gαi2 proteins on the salt sensitivity of blood pressure is unknown. Here, by the bilateral PVN administration of a targeted Gαi2 oligodeoxynucleotide, we show that PVN-specific Gαi2 proteins are required to facilitate the full natriuretic response to an acute volume expansion (peak natriuresis [μeq/min] scrambled (SCR) oligodeoxynucleotide 41±3 versus Gαi2 oligodeoxynucleotide 18±4; P<0.05) via a renal nerve-dependent mechanism. Furthermore, in response to chronically elevated dietary sodium intake, PVN-specific Gαi2 proteins are essential to counter renal nerve-dependent salt-sensitive hypertension (mean arterial pressure [mm Hg] 8% NaCl; SCR oligodeoxynucleotide 128±2 versus Gαi2 oligodeoxynucleotide 147±3; P<0.05). This protective pathway involves activation of PVN Gαi2 signaling pathways, which mediate sympathoinhibition to the blood vessels and kidneys (renal norepinephrine [pg/mg] 8% NaCl; SCR oligodeoxynucleotide 375±39 versus Gαi2 oligodeoxynucleotide 850±27; P<0.05) and suppression of the activity of the sodium chloride cotransporter assessed as peak natriuresis to hydrochlorothiazide. Additionally, central oligodeoxynucleotide-mediated Gαi2 protein downregulation prevented PVN parvocellular neuron activation, assessed by FosB immunohistochemistry, in response to increased dietary salt intake. In our analysis of the UK BioBank data set, it was observed that 2 GNAI2 single nucleotide polymorphism (SNP) (rs2298952, P=0.041; rs4547694, P=0.017) significantly correlate with essential hypertension. Collectively, our data suggest that selective targeting and activation of PVN Gαi2 proteins is a novel therapeutic approach for the treatment of salt-sensitive hypertension.

Infralimbic prefrontal cortex structural and functional connectivity with the limbic forebrain: a combined viral genetic and optogenetic analysis

The medial prefrontal cortex is critical for contextual appraisal, executive function, and goal-directed behavior. Additionally, the infralimbic (IL) subregion of the prefrontal cortex has been implicated in stress responding, mood, and fear memory. However, the specific circuit mechanisms that mediate these effects are largely unknown. To date, IL output to the limbic forebrain has been examined largely qualitatively or within a restricted number of sites. To quantify IL presynaptic input to structures throughout the forebrain, we utilized a lentiviral construct expressing synaptophysin-mCherry. Thus, allowing quantification of IL efferents that are specifically synaptic, as opposed to fibers of passage. Additionally, this approach permitted the determination of IL innervation on a sub-structural level within the multiple heterogeneous limbic nuclei. To examine the functional output of the IL, optogenetic activation of IL projections was followed by quantification of neuronal activation throughout the limbic forebrain via fos-related antigen (Fra). Quantification of synaptophysin-mCherry indicated that the IL provides robust synaptic input to a number of regions within the thalamus, hypothalamus, amygdala, and bed nucleus of the stria terminalis, with limited input to the hippocampus and nucleus accumbens. Furthermore, there was high concordance between structural connectivity and functional activation. Interestingly, some regions receiving substantial synaptic input did not exhibit significant increases in Fra-immunoreactivity. Collectively, these studies represent a step toward a comprehensive and quantitative analysis of output circuits. This large-scale efferent quantification or 'projectome' also opens the door for data-driven analyses of the downstream synaptic mechanisms that mediate the integrative aspects of cortico-limbic interactions.

Renal denervation attenuates hypertension but not salt sensitivity in ETB receptor-deficient rats

Hypertension is a prevalent pathology that increases risk for numerous cardiovascular diseases. Because the etiology of hypertension varies across patients, specific and effective therapeutic approaches are needed. The role of renal sympathetic nerves is established in numerous forms of hypertension, but their contribution to salt sensitivity and interaction with factors such as endothelin-1 are poorly understood. Rats deficient of functional ET_B receptors (ET_B-def) on all tissues except sympathetic nerves are hypertensive and exhibit salt-sensitive increases in blood pressure. We hypothesized that renal sympathetic nerves contribute to hypertension and salt sensitivity in ET_B-def rats. The hypothesis was tested through bilateral renal sympathetic nerve denervation and measuring blood pressure during normal salt (0.49% NaCl) and high-salt (4.0% NaCl) diets. Denervation reduced mean arterial pressure in ET_B-def rats compared with sham-operated controls by 12 ± 3 (SE) mmHg; however, denervation did not affect the increase in blood pressure after 2 wk of high-salt diet (+19 ± 3 vs. +16 ± 3 mmHg relative to normal salt diet; denervated vs. sham, respectively). Denervation reduced cardiac sympathetic-to-parasympathetic tone [low frequency-high frequency (LF/HF)] during normal salt diet and vasomotor LF/HF tone during high-salt diet in ET_B-def rats. We conclude that the renal sympathetic nerves contribute to the hypertension but not to salt sensitivity of ET_B-def rats.

Fornix deep brain stimulation enhances acetylcholine levels in the hippocampus

Deep brain stimulation (DBS) of the fornix has gained interest as a potential therapy for advanced treatment-resistant dementia, yet the mechanism of action remains widely unknown. Previously, we have reported beneficial memory effects of fornix DBS in a scopolamine-induced rat model of dementia, which is dependent on various brain structures including hippocampus. To elucidate mechanisms of action of fornix DBS with regard to memory restoration, we performed c-Fos immunohistochemistry in the hippocampus. We found that fornix DBS induced a selective activation of cells in the CA1 and CA3 subfields of the dorsal hippocampus. In addition, hippocampal neurotransmitter levels were measured using microdialysis before, during and after 60 min of fornix DBS in a next experiment. We observed a substantial increase in the levels of extracellular hippocampal acetylcholine, which peaked 20 min after stimulus onset. Interestingly, hippocampal glutamate levels did not change compared to baseline. Therefore, our findings provide first experimental evidence that fornix DBS activates the hippocampus and induces the release of acetylcholine in this region.

Cerebrospinal Fluid Hypernatremia Elevates Sympathetic Nerve Activity and Blood Pressure via the Rostral Ventrolateral Medulla

Elevated NaCl concentrations of the cerebrospinal fluid increase sympathetic nerve activity (SNA) in salt-sensitive hypertension. Neurons of the rostral ventrolateral medulla (RVLM) play a pivotal role in the regulation of SNA and receive mono- or polysynaptic inputs from several hypothalamic structures responsive to hypernatremia. Therefore, the present study investigated the contribution of RVLM neurons to the SNA and pressor response to cerebrospinal fluid hypernatremia. Lateral ventricle infusion of 0.15 mol/L, 0.6 mol/L, and 1.0 mol/L NaCl (5 µL/10 minutes) produced concentration-dependent increases in lumbar SNA, adrenal SNA, and arterial blood pressure, despite no change in splanchnic SNA and a decrease in renal SNA. Ganglionic blockade with chlorisondamine or acute lesion of the lamina terminalis blocked or significantly attenuated these responses, respectively. RVLM microinjection of the gamma-aminobutyric acid (GABAA) agonist muscimol abolished the sympathoexcitatory response to intracerebroventricular infusion of 1 mol/L NaCl. Furthermore, blockade of ionotropic glutamate, but not angiotensin II type 1, receptors significantly attenuated the increase in lumbar SNA, adrenal SNA, and arterial blood pressure. Finally, single-unit recordings of spinally projecting RVLM neurons revealed 3 distinct populations based on discharge responses to intracerebroventricular infusion of 1 mol/L NaCl: type I excited (46%; 11/24), type II inhibited (37%; 9/24), and type III no change (17%; 4/24). All neurons with slow conduction velocities were type I cells. Collectively, these findings suggest that acute increases in cerebrospinal fluid NaCl concentrations selectively activate a discrete population of RVLM neurons through glutamate receptor activation to increase SNA and arterial blood pressure.

Central neuronal activation and pressor responses induced by circulating ANG II: role of the brain aldosterone-“ouabain” pathway

An increase in plasma ANG II causes neuronal activation in hypothalamic nuclei and a slow pressor response, presumably by increasing sympathetic drive. We evaluated whether the activation of a neuromodulatory pathway, involving aldosterone and “ouabain,” is involved in these responses. In Wistar rats, the subcutaneous infusion of ANG II at 150 and 500 ng·kg−1·min−1 gradually increased blood pressure up to 60 mmHg at the highest dose. ANG II at 500 ng·kg−1·min−1 increased plasma ANG II by 4-fold, plasma aldosterone by 25-fold, and hypothalamic aldosterone by 3-fold. The intracerebroventricular infusion of an aldosterone synthase (AS) inhibitor prevented the ANG II-induced increase in hypothalamic aldosterone without affecting the increase in plasma aldosterone. Neuronal activity, as assessed by Fra-like immunoreactivity, increased transiently in the subfornical organ (SFO) but progressively in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). The central infusion of the AS inhibitor or a mineralocorticoid receptor blocker markedly attenuated the ANG II-induced neuronal activation in the PVN but not in the SON. Pressor responses to ANG II at 150 ng·kg−1·min−1 were abolished by an intracerebroventricular infusion of the AS inhibitor. Pressor responses to ANG II at 500 ng·kg−1·min−1 were attenuated by the central infusion of the AS inhibitor or the mineralocorticoid receptor blocker by 70–80% and by Digibind (to bind “ouabain”) by 50%. These results suggest a novel central nervous system mechanism for the ANG II-induced slow pressor response, i.e., circulating ANG II activates the SFO, leading to the direct activation of the PVN and SON, and, in addition, via aldosterone-dependent amplifying mechanisms, causes sustained activation of the PVN and thereby hypertension.