Alcohol effects on synaptic transmission in periaqueductal gray dopamine neurons

Abstract

The role of dopamine (DA) signaling in regulating the rewarding properties of drugs, including alcohol, has been widely studied. The majority of these studies, however, have focused on the DA neurons located in the ventral tegmental area (VTA), and their projections to the nucleus accumbens. DA neurons within the ventral periaqueductal gray (vPAG) have been shown to regulate reward but little is known about the functional properties of these neurons, or how they are modified by drugs of abuse. This lack of knowledge is likely due to the highly heterogeneous cell composition of the vPAG, with both γ-aminobutyric acid (GABA) and glutamate neurons present in addition to DA neurons. In this study, we performed whole-cell recordings in a TH-eGFP transgenic mouse line to evaluate the properties of vPAG-DA neurons. Following this initial characterization, we examined how both acute and chronic alcohol exposure modify synaptic transmission onto vPAG-DA neurons. We found minimal effects of acute alcohol exposure on GABA transmission, but a robust enhancement of glutamatergic synaptic transmission in vPAG-DA. Consistent with this effect on excitatory transmission, we also found that alcohol caused an increase in firing rate. These data were in contrast to the effects of chronic intermittent alcohol exposure, which had no significant impact on either inhibitory or excitatory synaptic transmission on the vPAG-DA neurons. These data add to a growing body of literature that points to alcohol having both region-dependent and cell-type dependent effects on function.

Fig. 1

The tyrosine hydroxylase–eGFP transgenic mouse model identifies dopamine neurons via fluorescence in the vPAG. (A) Immunohistochemistry can be used to label and verify validity of the reporter via (1) tyrosine hydroxylase fluorescence, (2) eGFP fluorescence, and (3) the co-localization of eGFP tyrosine hydroxylase in dual-positive cells. No significant I_h current was observed in vPAG-DA neurons. (B) (1) eGFP and tyrosine hydroxylase-positive cells can be visualized under a microscope to selectively record from. Staining showed that the recorded cells loaded with (2) neurobiotin via diffusion from the patching pipette, are (3) tyrosine hydroxylase positive.

Fig. 2

Acute bath-applied alcohol had no effects on mini-inhibitory post-synaptic currents (mIPSCs) in vPAG-dopamine neurons. (A) Representative mIPSC traces of baseline control (i) and after 10 min of alcohol wash-on (ii). (B) Acute alcohol had no effects on mIPSC frequency. (C) Acute alcohol had no effects on mIPSC amplitude. (D) Acute alcohol had no effects on mIPSC decay. (E) Acute alcohol had no effects on mIPSC noise.

Fig. 3

Acute bath-applied alcohol had no effects on spontaneous inhibitory post-synaptic current (sIPSCs) frequency in vPAG-dopamine neurons. Representative sIPSC traces of (A) baseline control and (B) after 10 min of alcohol wash-on. (C) Acute alcohol had no effects on sIPSC frequency. (D) Acute alcohol significantly decreased sIPSC amplitude. * denotes statistical significance of p value < 0.05.

Fig. 4

Acute bath-applied alcohol increased mini-excitatory post-synaptic currents (mEPSCs) in vPAG-dopamine neurons. (A) Representative mEPSC trace of baseline control. (B) Representative mEPSC trace after 10 min of alcohol wash-on. (C) Acute alcohol increased mEPSC frequency. (D) Acute alcohol had no effects on mEPSC amplitude. * denotes statistical significance of p value < 0.05.

Fig. 5

Acute bath-applied alcohol (50 mM) increased firing rate of vPAG-dopamine neurons in a cell-attached recording configuration. (A) Representative firing rate trace of baseline control. (B) Representative firing rate trace in the same neuron after 10 min of alcohol wash-on. (C) Alcohol increased firing rate in vPAG-DA neurons. (D) Alcohol had no significant effects on firing rate in the presence of NBQX (10 µM). * denotes statistical significance of p value < 0.05.

Fig. 6

Chronic intermittent alcohol vapor exposure did not affect inhibitory and excitatory synaptic properties. (A) Representative mEPSC trace. (B) Representative mIPSC trace. (C) Chronic intermittent alcohol had no effects on mini-frequency. (D) Chronic intermittent alcohol had no effects on mini-amplitude. (E) Chronic intermittent alcohol had no effects on mini-excitatory/mini-inhibitory transmission ratio.