Electrophysiological properties of medium spiny neurons in the nucleus accumbens core of prepubertal male and female Drd1a-tdTomato line 6 BAC transgenic mice

Abstract

The nucleus accumbens core (AcbC) is a striatal brain region essential for integrating motivated behavior and reward processing with premotor function. In humans and rodents, research has identified sex differences and sex steroid hormone sensitivity in AcbC-mediated behaviors, in disorders, and in rats in the electrophysiological properties of the AcbC output neuron type, the medium spiny neuron (MSN). It is unknown whether the sex differences detected in MSN electrophysiological properties extend to mice. Furthermore, MSNs come in distinct subtypes with subtle differences in electrophysiological properties, and it is unknown whether MSN subtype-specific electrophysiology varies by sex. To address these questions, we used male and female Drd1a-tdTomato line 6 bacterial artificial chromosome transgenic mice. We made acute brain slices of the AcbC, and performed whole cell patch-clamp recordings across MSN subtypes to comprehensively assess AcbC MSN subtype electrophysiological properties. We found that ( 1 mice MSNs did not exhibit the sex differences detected in rat MSNs, and 2) electrophysiological properties differed between MSN subtypes in both sexes, including rheobase, resting membrane potential, action potential properties, intrinsic excitability, input resistance in both the linear and rectified ranges, and miniature excitatory postsynaptic current properties. These findings significantly extend previous studies of MSN subtypes performed in males or animals of undetermined sex and indicate that the influence of sex upon AcbC MSN properties varies between rodent species. NEW & NOTEWORTHY This research provides the most comprehensive assessment of medium spiny neuron subtype electrophysiological properties to date in a critical brain region, the nucleus accumbens core. It additionally represents the first evaluation of whether mouse medium spiny neuron subtype electrophysiological properties differ by sex.

Keywords: medium spiny neuron; mice; nucleus accumbens; sex; ventral striatum.

Fig. 1.

Location of whole-cell patch clamped medium spiny neurons (MSNs) sorted by subtype in the nucleus accumbens core of female and male Drd1a-tdTomato line 6 BAC transgenic mice. “Drd1a” males and females represent recordings from fluorescently labeled Drd1a-positive MSNs. “Drd2 ” males and females represent recordings from nonfluorescently labeled MSNs, which are almost exclusively Drd2-positive MSNs. ACB, nucleus accumbens; AC, anterior commissure; LV, lateral ventricle; 3V, third ventricle.

Fig. 2.

Resting membrane potential and action potential (AP) rheobase varies by medium spiny neuron (MSN) subtype but not sex. A: voltage response of male and female Drd1a-positive and Drd2-positive MSN subtypes to a series of depolarizing current injections. B: resting membrane potential is hyperpolarized in male and female Drd1a-positive MSNs compared with Drd2-positive MSNs. Left: absolute values. Means ± SE are depicted over individual data points. Right: values normalized to male Drd1a-positive MSNs. C: rheobase is increased in male and female Drd1a-positive MSNs compared with Drd2-positive MSNs. Left: absolute values. Right: normalized values. D: AP (AP) threshold does not differ between male and female Drd1a-positive MSNs compared with Drd2-positive MSNs. Left: absolute values. Right: normalized values. *P < 0.05; ***P < 0.001.

Fig. 3.

Action potential (AP) width and afterhyperpolarization (AHP) properties vary by medium spiny neuron (MSN) subtype, with males exhibiting increased MSN subtype differences compared with females. A: example individual AP evoked by depolarizing current injection, indicating AP width, AP AHP peak amplitude, and AP AHP time to peak amplitude metrics. B: AP width is shorter in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Means ± SE are depicted over individual data points. Right: Values normalized to male Drd1a-positive MSNs. C: AP AHP peak amplitude is larger in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Right: normalized values. D: AP AHP time to peak amplitude is increased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Right: normalized values. *P < 0.05.

Fig. 4.

Action potential (AP) firing rates evoked by depolarizing current injections vary by medium spiny neuron (MSN) subtype, with males exhibiting increased MSN subtype differences compared with females. A: AP firing rate evoked by depolarizing current injection was decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. In male MSNs, Drd1a-positive MSNs exhibited significantly decreased AP firing rates compared with Drd2-positive MSNs from +0.04- to +0.10-nA injected currents and demonstrated a strong interaction between subtype and injected current. In female MSNs, Drd1a-positive MSNs exhibited significantly decreased AP firing rates compared with Drd2-positive MSNs only at the +0.09-nA injected current point and did not demonstrate a significant interaction between subtype and injected current. Compete statistics are available in results. B: the slope of the evoked AP to depolarizing current injection curve is decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. C: the delay to 1st AP evoked by rheobase current injection is decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs and does not differ by sex. D: the maximum AP firing rate evoked at +0.10-nA injected current is decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. FI slope: slope of the evoked action potential to depolarizing current injection curve; *P < 0.05; **P < 0.01; ***P < 0.001.

Fig. 5.

Input resistance in the rectified and nonrectified ranges by medium spiny neuron (MSN) subtype, with males generally exhibiting increased MSN subtype differences compared with females. A: voltage response of male and female Drd1a-positive and Drd2-positive MSN subtypes to a series of negative current injections. B: injected negative current to steady-stage voltage deflection curve (I–V curve). Red solid circles with red line: Drd1a-positive males; black solid circles with black line: Drd2-positive males; red open circles with dashed red line: Drd1a-positive females; black open circles with dashed black line: Drd2-positive females. C: input resistance in the nonrectified range is decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Means ± SE are depicted over individual data points. Right: values normalized to male Drd1a-positive MSNs. D: input resistance in the rectified range is decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Right: normalized values. E: inward rectification is decreased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Right: normalized values. F: the time constant of the membrane is shorter in Drd1a-positive MSNs compared with Drd2-positive MSNs, with male MSNs exhibiting a larger difference between MSN subtypes than female MSNs. Left: absolute values. Right: normalized values. *P < 0.05; **P < 0.01.

Fig. 6.

Miniature excitatory postsynaptic current (mEPSC) properties vary by medium spiny neuron (MSN) subtype. A: mEPSCs recorded from male and female Drd1a-positive and Drd2-positive MSN subtypes. MSNs were voltage clamped at −70 mV and recorded in the presence of tetrodotoxin and picrotoxin to block voltage-gated sodium channels and GABAergic synaptic activity, respectively. B: mEPSC frequency was increased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with female MSNs exhibiting a larger difference between MSN subtypes than male MSNs. Left: absolute values. Means ± SE are depicted over individual data points. Right: Values normalized to male Drd1a-positive MSNs. C: mEPSC amplitude is increased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with female MSNs exhibiting a larger difference between MSN subtypes than male MSNs. Left: absolute values. Right: normalized values. D: mEPSC decay was increased in Drd1a-positive MSNs compared with Drd2-positive MSNs, with female MSNs exhibiting a larger difference between MSN subtypes than male MSNs. *P < 0.05; ***P < 0.001.

Fig. 7.

Comparison of medium spiny neuron (MSN) subtype electrophysiological property effect sizes between females and males. Each point represents the Cohen’s d value calculated for 1 of the 15 electrophysiological properties that significantly differ between Drd1a- and Drd2-positive female and male MSN subtypes. The dashed line is the unity line. Points on or near the unity line indicate that effect size is comparable between male and female MSN. Points located above the unity line indicate a larger effect size in female MSNs. Points located below the unity line indicate a larger effect size in male MSNs. Female MSNs showed larger differences in MSN subtype electrophysiological properties in miniature excitatory postsynaptic current (mEPSC) properties. Conversely, male MSNs showed larger differences in MSN subtype electrophysiological properties in action potential and input resistance properties.