Developmental Regulation of Chloride Currents in the Dorsal Motor Nucleus of the Vagus of the Rat

Abstract

Tonically firing parasympathetic neurons of the dorsal motor nucleus of the vagus (DMV) modulate the functions of the proximal gastrointestinal (GI) tract. The pacemaker activity of DMV neurons is modulated by synaptic inputs from adjacent autonomic areas, including the nuclei of the tractus solitarius. Functional maturation of these vagal sensorimotor neurocircuits results from intense synaptogenesis and neuronal reorganization in the first postnatal weeks and their maturation may be assessed by the analysis of their time constant of synaptic current decay.Prior studies have shown that, in adult rats, perfusion with the GABAA antagonist bicuculline (BIC) increases the firing rate of DMV neurons, and BIC microinjection in the DMV increase gastric motility and pancreatic exocrine secretions, indicating a prominent role of brainstem GABA synapses in the modulation of GI functions. Conversely, glycine synapses do not appear to be present in GI‐related brainstem neurocircuits of adult rats.The aim of the present study was to examine GABA and glycine‐ mediated chloride currents in the development of synaptic inputs onto DMV neurons. Using whole cell recordings, miniature inhibitory currents were recorded in rats from postnatal day 1 (P1) to postnatal day 30 (P30), and analyzed for decay constant and event frequency. Between P1–15, the baseline frequency was 1.9±0.35 events.s−1 and the current decay constant was 10.3±1.4 ms (N=30). These values were not altered by maturation (P>0.05 for both parameters).Perfusion of the slice with the GABAA antagonist bicuculline (1 or 10uM) either abolished completely or decreased the frequency and the decay constant of the inhibitory events to a similar extent in neurons from P1–15 rats or >P15 rats (N=37 and N=65 for 1 and 10uM, respectively). Conversely, while perfusion with the glycine antagonist strychnine (1μM) did not decrease event frequency significantly in any of the groups, it increased the current decay constant (from 8.2±0.65 to 11.9±0.75ms and from 7.6±0.57 to 10.5±0.79ms, in P1–15 and >P15,neurons, respectively). Strychnine altered inhibitory currents in a significantly larger percentage of P1–15 (N=67) neurons, and blocked all events in 3 cells from P1–15 rats, but not in >P15 rats (n=58; p<0.05 vs P1–15).These data suggest that the development of brainstem vagal neurocircuits involves an early phase, during which GABA and glycine neurotransmission appear co‐released by the same neurons. The role of glycine in synaptic transmission appears to start declining at approximately P15. Disruption of this developmental switch from GABA‐glycine to purely GABAergic transmission may occur in response to early life stress, infection or inflammation, which may lead to adverse consequences in vagal efferent control of visceral functions.Support or Funding InformationFunding: NIH DK 99350 and DK 78364 & NSF IOS‐1148978