Perinatal high fat diet increases inhibition of dorsal motor nucleus of the vagus neurons regulating gastric functions

Abstract

Background: Previous studies suggest an increased inhibition of dorsal motor nucleus of the vagus (DMV) neurons following exposure to a perinatal high fat diet (PNHFD); the underlying neural mechanisms, however, remain unknown. This study assessed the effects of PNHFD on inhibitory synaptic inputs to DMV neurons and the vagally dependent control of gastric tone and motility.

Methods: Whole-cell patch clamp recordings were made from DMV neurons in thin brainstem slices from Sprague-Dawley rats fed either a control diet or HFD (14 or 60% kcal from fat, respectively) from embryonic day 13 onwards; gastric tone and motility were recorded in in vivo anesthetized rats.

Key results: The non-selective GABA_A antagonist, BIC (10 μmol L^-1 ), induced comparable inward currents in PNHFD and control DMV neurons, but a larger current in PNHFD neurons at higher concentrations (50 μmol L^-1 ). Differences were not apparent in neuronal responses to the phasic GABA_A antagonist, gabazine (GBZ), the extrasynaptic GABA_A agonist, THIP, the GABA transport blocker, nipecotic acid, or the gliotoxin, fluoroacetate, suggesting that PNHFD altered inhibitory transmission but not GABA_A receptor density or function, GABA uptake or glial modulation of synaptic strength. Similarly, the increase in gastric motility and tone following brainstem microinjection of low doses of BIC (1-10 pmoles) and GBZ (0.01-0.1 pmoles) were unchanged in PNHFD rats while higher doses of BIC (25 pmoles) induced a significantly larger increase in gastric tone compared to control.

Conclusions and inferences: These studies suggest that exposure to PNHFD increases the tonic inhibition of DMV neurons, possibly contributing to dysregulated vagal control of gastric functions.

Keywords: DMV; GABA receptors; gastric motility; perinatal high fat diet.

Figure 1

PNHFD rats displayed a greater response to a larger concentration of the non-selective GABA_A antagonist, bicuculline (BIC), while displaying similar responses to the phasic GABA_A antagonist, gabazine (GBZ). A. Representative current clamp traces in control (upper) and PNHFD (lower) DMV neurons illustrating the increase in action potential firing rat in response to superfusion with BIC (50 μM) B. Graphical representation of the greater increase in action potential firing rate in PNHFD (gray bars) compared to control (white bars DMV neurons in response to BIC (50μM). C. Representative current-clamp patch-clamp recordings in response to 10μM (gray) and 50μM (black) BIC in DMV neurons from control (upper) and PNHFD (lower) voltage clamped at −50mV. D. Graphical representation of inward current induced in DMV neurons in control (white bars) and PNHFD (gray bars) DMV neurons in response to BIC (10 and 50 μM). Note that while responses of PNHFD and control DMV neurons were similar at lower concentrations of BIC, PNHFD neurons responded to the higher concentration of BIC with a larger inward current. E. Representative recordings from gastric-projecting DMV neurons voltage clamped at −50mV from control (upper) and PNHFD (lower) rats. Note that superfusion with GBZ induced an inward current of similar magnitude in both control and PNHFD neurons, but further addition of BIC induced a larger inward current in PNHFD neurons. F. Graphical representation of the inward currents induced in control (white bars) and PNHFD (gray bars) DMV neurons in response to GBZ as well as the subsequent addition of BIC. Note that the GBZ-induced response was similar in control and PNHFD DMV neurons, but BIC induced a larger inward current. *p<0.05 vs control

Figure 2

Confirmation of microinjection sites of BIC and GBZ throughout the rostro-caudal extent of the DMV. A. Schematic diagram illustrating brainstem microinjection sites in control (black circle) and PNHFD (red star) rats at caudal, intermediate, and rostral sites. For simplicity, not all injection sites are included; the left side of the figure represents GBZ injection sites while the right side represents BIC injection sites (NB: all injections were into the left DMV because motility recordings were made from the ventral stomach). B. Representative photomicrograph of a brainstem microinjection (arrow) in the DMV. The area highlighted is shown at greater magnification in the lower right corner. AP= area postrema, NTS= nucleus of the tractus solitarus, DMV= dorsal motor nucleus of vagus, CC= central canal.

Figure 3

PNHFD rats displayed a greater response to a larger dose of bicuculline (BIC), yet similar responses to lower doses of BIC and gabazine (GBZ). A. Representative gastric motility traces from the antrum in control (upper) and PNHFD (lower) rats following microinjection of BIC into the DVC. B. Graphical representation of the response of antrum motility (left) and tone (right) to BIC microinjection in control (white bars) and PNHFD (gray bars) rats. PNHFD animals displayed a larger increase in antrum tone, but not motility, in response to microinjection of a larger dose of BIC (p<0.05 vs control). C: Representative traces of the increase in gastric motility in control (upper) and PNHFD (lower) rats in response to DVC microinjection of the phasic GABA_A receptor antagonist, GBZ. D: Graphical representation of the increase in antrum motility (left) and tone (right) in response to DVC microinjection of GBZ. Note that the response to GBZ did not differ between PNHFD and control rats.

Figure 4. The responses of gastric-projecting DMV neurons to the tonic GABA_A agonist (THIP), the GABA transport blocker (nipecotic acid), and the gliotoxin (fluoroacetate), were similar in control and PNHFD rats

Representative traces (left) and graphical summary (right) from control (upper traces; white bars) and PNHFD (lower traces; gray bars) gastric-projecting DMV neurons voltage clamped at −50mV in response to superfusion with THIP (A and B), nipecotic acid (C and D), or fluoroacetate (E and F). Note that the responses of control and PNHFD DMV neurons to each drug were similar.