Time-of-Day-Dependent Effects of Bromocriptine to Ameliorate Vascular Pathology and Metabolic Syndrome in SHR Rats Held on High Fat Diet

Abstract

The treatment of type 2 diabetes patients with bromocriptine-QR, a unique, quick release micronized formulation of bromocriptine, improves glycemic control and reduces adverse cardiovascular events. While the improvement of glycemic control is largely the result of improved postprandial hepatic glucose metabolism and insulin action, the mechanisms underlying the drug’s cardioprotective effects are less well defined. Bromocriptine is a sympatholytic dopamine agonist and reduces the elevated sympathetic tone, characteristic of metabolic syndrome and type 2 diabetes, which potentiates elevations of vascular oxidative/nitrosative stress, known to precipitate cardiovascular disease. Therefore, this study investigated the impact of bromocriptine treatment upon biomarkers of vascular oxidative/nitrosative stress (including the pro-oxidative/nitrosative stress enzymes of NADPH oxidase 4, inducible nitric oxide (iNOS), uncoupled endothelial nitric oxide synthase (eNOS), the pro-inflammatory/pro-oxidative marker GTP cyclohydrolase 1 (GTPCH 1), and the pro-vascular health enzyme, soluble guanylate cyclase (sGC) as well as the plasma level of thiobarbituric acid reactive substances (TBARS), a circulating marker of systemic oxidative stress), in hypertensive SHR rats held on a high fat diet to induce metabolic syndrome. Inasmuch as the central nervous system (CNS) dopaminergic activities both regulate and are regulated by CNS circadian pacemaker circuitry, this study also investigated the time-of-day-dependent effects of bromocriptine treatment (10 mg/kg/day at either 13 or 19 h after the onset of light (at the natural waking time or late during the activity period, respectively) among animals held on 14 h daily photoperiods for 16 days upon such vascular biomarkers of vascular redox state, several metabolic syndrome parameters, and mediobasal hypothalamic (MBH) mRNA expression levels of neuropeptides neuropeptide Y (NPY) and agouti-related protein (AgRP) which regulate the peripheral fuel metabolism and of mRNA expression of other MBH glial and neuronal cell genes that support such metabolism regulating neurons in this model system. Such bromocriptine treatment at ZT 13 improved (reduced) biomarkers of vascular oxidative/nitrosative stress including plasma TBARS level, aortic NADPH oxidase 4, iNOS and GTPCH 1 levels, and improved other markers of coupled eNOS function, including increased sGC protein level, relative to controls. However, bromocriptine treatment at ZT 19 produced no improvement in either coupled eNOS function or sGC protein level. Moreover, such ZT 13 bromocriptine treatment reduced several metabolic syndrome parameters including fasting insulin and leptin levels, as well as elevated systolic and diastolic blood pressure, insulin resistance, body fat store levels and liver fat content, however, such effects of ZT 19 bromocriptine treatment were largely absent versus control. Finally, ZT 13 bromocriptine treatment reduced MBH NPY and AgRP mRNA levels and mRNA levels of several MBH glial cell/neuronal genes that code for neuronal support/plasticity proteins (suggesting a shift in neuronal structure/function to a new metabolic control state) while ZT 19 treatment reduced only AgRP, not NPY, and was with very little effect on such MBH glial cell genes expression. These findings indicate that circadian-timed bromocriptine administration at the natural circadian peak of CNS dopaminergic activity (that is diminished in insulin resistant states), but not outside this daily time window when such CNS dopaminergic activity is naturally low, produces widespread improvements in biomarkers of vascular oxidative stress that are associated with the amelioration of metabolic syndrome and reductions in MBH neuropeptides and gene expressions known to facilitate metabolic syndrome. These results of such circadian-timed bromocriptine treatment upon vascular pathology provide potential mechanisms for the observed marked reductions in adverse cardiovascular events with circadian-timed bromocriptine-QR therapy (similarly timed to the onset of daily waking as in this study) of type 2 diabetes subjects and warrant further investigations into related mechanisms and the potential application of such intervention to prediabetes and metabolic syndrome patients as well.

Bromocriptine-QR Therapy Reduces Sympathetic Tone and Ameliorates a Pro-Oxidative/Pro-Inflammatory Phenotype in Peripheral Blood Mononuclear Cells and Plasma of Type 2 Diabetes Subjects

“…Importantly, in preclinical studies of vascular disease, circadian-timed bromocriptine administration to reinstate the natural circadian peak in CNS dopaminergic activity at waking from daily sleep, which is reduced in insulin-resistant states (and causative in its genesis) [ 1 , 175 ], reduced aortic enzymes that potentiate the cellular generation of ROS and endothelial NOS uncoupling, two vascular pathophysiological events induced by a local pro-inflammatory environment and that contribute to endothelial dysfunction and CVD [ 11 ]. However, this bromocriptine effect was absent when administered outside of the natural CNS dopaminergic activity circadian peak window [ 11 ]. Preclinical studies have located a major aspect of the biological clock for the circadian rhythm of CNS dopaminergic activity to reside in dopaminergic neurons projecting from the supramammillary nucleus to the suprachiasmatic nucleus (SCN) [ 11 , 175 , 176 , 177 , 178 , 179 , 180 , 181 ].…”

“…However, this bromocriptine effect was absent when administered outside of the natural CNS dopaminergic activity circadian peak window [ 11 ]. Preclinical studies have located a major aspect of the biological clock for the circadian rhythm of CNS dopaminergic activity to reside in dopaminergic neurons projecting from the supramammillary nucleus to the suprachiasmatic nucleus (SCN) [ 11 , 175 , 176 , 177 , 178 , 179 , 180 , 181 ]. The loss of the natural circadian peak of CNS dopaminergic activity at the SCN clock via direct neuronal neurotoxin micro-administration to the SCN area or high-fat feeding results in an elevated sympathetic tone that causes marked insulin resistance, glucose intolerance, and loss of hypothalamic glucose sensing, which is necessary for appropriate postprandial glucose disposal [ 178 , 180 ].…”

“…Multiple studies have identified alterations in whole-body circadian rhythmicity associated with (and that may drive) CVD [ 183 , 184 , 185 , 186 , 187 , 188 , 189 , 190 , 191 , 192 , 193 , 194 , 195 , 196 , 197 , 198 , 199 ]. It may be that the “resetting” towards normal of CNS circadian dopaminergic aberrations that via the neuroendocrine axis resultantly reduces elevated SNS tone and improves metabolism [ 11 , 175 , 181 , 200 , 201 , 202 ] also resultantly resets towards normal those aberrant circadian immune activities that potentiate the PO/PI state. Consequently, both neurometabolic and neuroimmune responses to circadian timed dopaminergic stimulation may work in concert to drive/participate in the improvement in the immune PO/PI state and its cardiometabolic sequelae in T2D.…”

“…In a large, randomized trial of T2D subjects, the majority (75%) of whom were without pre-existing cardiovascular disease (CVD), the circadian-timed (within 2 h of morning waking) administration of this therapy significantly reduced cardiovascular events by 40–55% over a single year [ 3 , 4 , 5 , 6 ]. Because these protective cardiovascular (CV) effects could not be explained by major changes in plasma lipids, HbA1c, or blood pressure, a plausible explanation for the beneficial CV effect may be related to the reduced chronic overactivation of sympathetic tone [ 7 , 8 , 9 , 10 , 11 ], a pathophysiologic abnormality that both causes and exacerbates the metabolic syndrome (MS), T2D and CVD [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Although poorly appreciated, the scientific literature is replete with studies that corroborate a prominent role of the chronic elevation of sympathetic nervous system (SNS) tone in the genesis and amplification of cellular/tissue oxidative stress and the resultant inflammation, particularly within the immune system [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ], which promotes insulin resistance and CVD [ 14 , 30 , 31 , 32 , 33 , 34 , 37 , 38 , 39 , 40 , 41 , 4...…”

Development of novel tools for dissection of central versus peripheral dopamine D2-like receptor signaling in dysglycemia

“…BrMeI and unmodified bromocriptine were dosed at 10 mg/kg intravenously (i.v.) with dosing based on earlier metabolic studies in rodents 5,146,147 . Following administration, blood was collected via cardiac puncture at 15 min and 1 h post-dose (n=3 mice/time point).…”