Implications of diet modification on sympathoinhibitory mechanisms and hypertension in obesity.

We have previously demonstrated that a number of rats fed a moderately high-fat diet (MHFD) become obese and hypertensive and had compromised sympathoinhibitory and vasodilator responses to the gut hormones cholecystokinin (CCK) and gastric leptin. This has implications for increased resistance in vascular beds that attract a large proportion of cardiac output after a meal and may be an important mechanism underlying the development of hypertension in obesity in which food consumption is greatly increased. The aim of this study was to determine whether swapping a MHFD for a low-fat diet (LFD) would induce weight loss in obese animals, reverse the signs of hypertension and restore sympathoinhibitory reflexes. Male Sprague-Dawley rats were placed on a LFD (controls; n = 8) or a MHFD (n = 24) for 11 weeks after which the latter displayed either an obesity-prone (OP) or obesity-resistant (OR) phenotype. All animals were fed a LFD for a further 6 weeks after which they were anaesthetised with isoflurane and artificially ventilated for evaluation of resting arterial pressure (AP) and renal sympathetic nerve responses to CCK (0.1-4 µg/kg) and leptin (15 µg/kg). Weight gain in OP animals remained higher than OR or controls following diet switch (P < 0.05 for both). Resting AP was not significantly different between OP (103 ± 4 mmHg), OR (102 ± 3 mmHg) or control (104 ± 3 mmHg) animals and sympathoinhibitory responses to CCK or leptin were not different between the groups (P > 0.05). These results demonstrate that diet modification can have beneficial effects on sympathetic function and restore normotension without the need for weight reduction.

Rostroventrolateral medullary neurons modulate glucose homeostasis in the rat.

Several lines of evidence support the view that the premotor sympathetic input to the adrenal gland arises from the rostroventrolateral medulla (RVLM). The aim of this study was to determine whether RVLM neurons play a role in glucose homeostasis. We identified RVLM neurons that control epinephrine secretion by searching for medullospinal neurons that responded to neuroglucoprivation induced by systemic 2-deoxyglucose (2-DG) administration. We tested the effect of disinhibition of the RVLM on arterial blood pressure and plasma glucose concentration. RVLM medullospinal barosensitive neurons (n = 17) were either unaffected or slightly inhibited by 2-DG. In contrast, we found a group (n = 6) of spinally projecting neurons that were excited by 2-DG administration. These neurons were not barosensitive and had spinal conduction velocities in the unmyelinated range (<1 m/s). These neurons may mediate epinephrine secretion and participate in the counterregulatory responses to neuroglucoprivation. To test the hypothesis that activation of the RVLM leads to adrenomedullary activation and subsequent hyperglycemia, we applied the GABA(A) antagonist bicuculline to the RVLM and measured blood pressure, heart rate, and blood glucose in rats with intact adrenals or after bilateral adrenalectomy. Disinhibition of the RVLM resulted in hypertension, tachycardia, and hyperglycemia (4.9 ± 0.3 to 14.7 ± 0.9 mM, n = 5, P < 0.05). Adrenalectomy significantly reduced the hyperglycemic response but did not alter the cardiovascular responses. These data suggest that the RVLM is a key component of the neurocircuitry that is recruited in the counterregulatory response to hypoglycemia.

Gastric leptin: a novel role in cardiovascular regulation.

Gastric-derived leptin affects satiety and gastrointestinal function via vagal mechanisms and has been shown to interact with the gut hormone cholecystokinin (CCK). CCK selectively inhibits splanchnic sympathetic nerve discharge (SND) and the activity of a subset of presympathetic vasomotor neurons in the rostroventrolateral medulla (RVLM). The present study sought to examine the effects of gastric leptin on arterial pressure (AP), heart rate (HR), SND, and RVLM neuronal activity to determine whether its effects on cardiovascular regulation are dependent on CCK(1) receptors and vagal afferent transmission. To mimic gastric leptin, leptin (15-30 microg/kg) was administered close to the coeliac artery in anesthetized, artificially ventilated Sprague-Dawley rats. Within 5 min, leptin selectively decreased the activity of RVLM neurons also inhibited by CCK (-27 +/- 4%; P < 0.001; n = 15); these inhibitory effects were abolished following administration of the CCK(1) receptor antagonist lorglumide. Leptin significantly decreased AP and HR (-10 +/- 2 mmHg, P < 0.001; and -8 +/- 2 beats/min, P < 0.01; n = 35) compared with saline (-1 +/- 2 mmHg, 3 +/- 2 beats/min; n = 30). In separate experiments, leptin inhibited splanchnic SND compared with saline (-9 +/- 2% vs. 2 +/- 3%, P < 0.01; n = 8). Bilateral cervical vagotomy abolished the sympathoinhibitory, hypotensive, and bradycardic effects of leptin (P < 0.05; n = 6). Our results suggest that gastric leptin may exert acute sympathoinhibitory and cardiovascular effects via vagal transmission and CCK(1) receptor activation and may play a separate role to adipose leptin in short-term cardiovascular regulation.

The sympathoinhibitory effects of systemic cholecystokinin are dependent on neurons in the caudal ventrolateral medulla in the rat.

The gastrointestinal hormone CCK inhibits a subset of presympathetic neurons in the rostroventrolateral medulla (RVLM) that may be responsible for driving the sympathetic vasomotor outflow to the gastrointestinal circulation. We tested the hypothesis that the central neurocircuitry of this novel sympathoinhibitory reflex involves a relay in the caudal ventrolateral medullary (CVLM) depressor area. Blood pressure and greater splanchnic sympathetic nerve discharge (SSND) or lumbar sympathetic nerve discharge (LSND) were monitored in anesthetised, paralyzed male Sprague-Dawley rats. The effects of phenylephrine (PE, 10 microg/kg iv; baroreflex activation), phenylbiguanide (PBG, 10 microg/kg iv; von Bezold-Jarisch reflex) and CCK (4 or 8 microg/kg iv) on SSND or LSND, were tested before and after bilateral injection of 50-100 nl of the GABAA agonist muscimol (1.75 mM; n=6, SSND; n=7, LSND) or the excitatory amino acid antagonist kynurenate (55 mM; n=7, SSND) into the CVLM. PE and PBG elicited splanchnic and lumbar sympathoinhibitory responses that were abolished by bilateral muscimol or kynurenate injection into the CVLM. Similarly, the inhibitory effect of CCK on SSND was abolished after neuronal inhibition within the CVLM. In contrast, CCK-evoked lumbar sympathoexcitation was accentuated following bilateral CVLM inhibition. In control experiments (n=7), these agents were injected outside the CVLM and had no effect on splanchnic sympathoinhibitory responses to PE, PBG, and CCK. In conclusion, neurons in the CVLM are necessary for the splanchnic but not lumbar sympathetic vasomotor reflex response to CCK. This strengthens the view that subpopulations of RVLM neurons supply sympathetic vasomotor outflow to specific vascular territories.

Medial prefrontal cortex depressor response: role of the solitary tract nucleus in the rat.

The depressor response elicited by unilateral low intensity electrical stimulation of the rat ventral medial prefrontal cortex may be mediated by a connection with the solitary tract nucleus. We tested this hypothesis by (i) examining the influence of medial prefrontal cortex stimulation on the induction of Fos-like immunoreactivity in neurons in the medulla oblongata, and (ii) by testing the effect of inhibition of solitary tract nucleus neurons on the medial prefrontal cortex stimulation-evoked depressor response. Depressor responses (>10 mmHg) were elicited by electrical stimulation of the medial prefrontal cortex every minute for 1 h ('Stimulated' group). Control animals were treated identically but did not receive electrical stimulation ('Unstimulated' group). Neurons exhibiting Fos-like immunoreactivity were abundant at the stimulation site which included the infralimbic area, and dorsal peduncular cortex. Medullary Fos-like immunoreactivity observed in the 'Stimulated' and 'Unstimulated' groups exceeded levels observed in untreated rats and was detected in the rostral, caudal and intermediate areas of the ventrolateral medulla, and the commissural, intermediate, medial and lateral regions of the solitary tract nucleus, as well as the medial vestibular nucleus, and the dorsal motor nucleus of the vagus. The number of neurons displaying Fos-like immunoreactivity in the ipsilateral solitary tract nucleus and caudal ventrolateral medulla of the 'Stimulated' group was found to be significantly elevated compared to the contralateral side (P<0.05), and the 'Unstimulated' group bilaterally. Inhibition of solitary tract nucleus neurons using bilateral injections of the GABA(A) receptor agonist muscimol (44 pmol/25 nl) inhibited the sympathetic vasomotor baroreflex and attenuated the depressor and sympathoinhibitory response to medial prefrontal cortex stimulation by 62% and 65%, respectively. These findings suggest that the projection from the medial prefrontal cortex to the solitary tract nucleus is excitatory and support the hypothesis that the depressor response elicited by medial prefrontal cortex stimulation is mediated, in part, by a cortico-solitary projection which activates the intramedullary baroreflex pathway.

Midline medullary depressor responses are mediated by inhibition of RVLM sympathoexcitatory neurons in rats.

Mechanisms underlying the depressor and sympathoinhibitory responses evoked from the caudal medullary raphe (MR) region were investigated in pentobarbital sodium-anesthetized, paralyzed rats. Intermittent electrical stimulation (0.5 Hz, 0.5-ms pulses, 200 microA) of the MR elicited a mixed sympathetic response that consisted of a long-latency sympathoexcitatory (SE) peak (onset = 146 +/- 7 ms) superimposed on an inhibitory phase (onset = 59 +/- 10 ms). Chemical stimulation of the MR (glutamate; Glu) most frequently elicited depressor responses accompanied by inhibition of sympathetic nerve discharge. Occasionally, these responses were preceded by transient pressor and SE responses. We examined the influence of intermittent electrical stimulation (0.5 Hz, 0.5-ms pulses, 25-200 microA) and Glu stimulation of the MR on the discharge of rostral ventrolateral medulla (RVLM) premotor SE neurons. Peristimulus-time histograms of RVLM unit discharge featured a prominent inhibitory phase in response to MR stimulation (onset = 20 +/- 2 ms; duration = 42 +/- 4 ms; n = 12 units). Glu stimulation of the MR reduced blood pressure (-37 +/- 2 mmHg, n = 19) and inhibited the discharge of RVLM SE neurons (15 of 19 neurons). Depressor and sympathoinhibitory responses elicited by chemical and electrical stimulation of the MR region are mediated by inhibition of RVLM premotor SE neurons and withdrawal of sympathetic vasomotor discharge.

Determinants of acute hemodynamic and electrophysiologic effects of verapamil in humans: role of myocardial drug uptake.

To examine the relationship between myocardial verapamil content (MVC) and acute effects in humans, coronary sinus catheterization was used in 22 patients to determine myocardial uptake of verapamil after bolus intravenous (i.v.) verapamil (4 mg) injection. Verapamil-induced effects on hemodynamic and electrophysiologic parameters were measured simultaneously and correlated with MVC per unit baseline coronary sinus blood flow (MVC:F). Myocardial uptake of verapamil was rapid: peak MVC (1.2 +/- 0.2% of injected dose) occurred at 5.4 +/- 0.4 min; at 30 min, residual MVC was 71.1 +/- 3.4% of maximum. Peak MVC:F in individual patients was inversely related to the extent of coronary artery disease (p less than 0.005) but not to left ventricular (LV) systolic function. Verapamil produced significant (p less than 0.001) early reductions in arterial pressure and systemic vascular resistance (SVR); cardiac index (CI) increased, left ventricular (LV) positive dP/dt was unchanged. Verapamil prolonged (p less than 0.01) PR and AH intervals (maximum at 12-18 min) and atrioventricular (AV) nodal effective and functional refractory periods (ERP, FRP) (maximum at 30 min). In individual patients, the extent of changes in AH intervals (r = 0.69; p less than 0.05) and LV dP/dt (r = 0.62; p less than 0.05) correlated with peak MVC:F. We conclude that after i.v. injection, verapamil uptake by the human myocardium is rapid and more extensive in patients with minor coronary artery disease. Despite the hysteresis between MVC and drug effects, MCV is a determinant of inotropic and electrophysiologic effects of verapamil.

Platelet serotonin uptake and 3H-imipramine binding in panic disorder.

Platelet serotonin uptake and 3H-imipramine binding were measured in eight patients with panic disorders and nine controls. The Vmax of serotonin uptake was significantly elevated in patients compared to controls (77 +/- 14 vs. 50 +/- 4 pmol/10(8) platelets/min; P less than 0.05) while Km values were not different (1.46 +/- 0.41 vs. 1.24 +/- 0.20 microM). 3H-Imipramine binding to ruptured platelet membranes was not significantly different between patients and controls for either Bmax (395 +/- 71 vs. 412 +/- 107 fmol/mg protein) or Kd (0.90 +/- 0.18 vs. 1.09 +/- 0.30 nM). The implications for a serotonergic dysfunction in panic disorders are discussed.