Electrophysiological Properties of Medium Spiny Neuron Subtypes in the Caudate-Putamen of Prepubertal Male and Female Drd1a-tdTomato Line 6 BAC Transgenic Mice

Abstract

The caudate-putamen is a striatal brain region essential for sensorimotor behaviors, habit learning, and other cognitive and premotor functions. The output and predominant neuron of the caudate-putamen is the medium spiny neuron (MSN). MSNs present discrete cellular subtypes that show differences in neurochemistry, dopamine receptor expression, efferent targets, gene expression, functional roles, and most importantly for this study, electrophysiological properties. MSN subtypes include the striatonigral and the striatopallidal groups. Most studies identify the striatopallidal MSN subtype as being more excitable than the striatonigral MSN subtype. However, there is some divergence between studies regarding the exact differences in electrophysiological properties. Furthermore, MSN subtype electrophysiological properties have not been reported disaggregated by biological sex. We addressed these questions using prepubertal male and female Drd1a-tdTomato line 6 BAC transgenic mice, an important transgenic line that has not yet received extensive electrophysiological analysis. We made acute caudate-putamen brain slices and assessed a robust battery of 16 relevant electrophysiological properties using whole-cell patch-clamp recording, including intrinsic membrane, action potential, and miniature EPSC (mEPSC) properties. We found that: (1) MSN subtypes exhibited multiple differential electrophysiological properties in both sexes, including rheobase, action potential threshold and width, input resistance in both the linear and rectified ranges, and mEPSC amplitude; (2) select electrophysiological properties showed interactions between MSN subtype and sex. These findings provide a comprehensive evaluation of mouse caudate-putamen MSN subtype electrophysiological properties across females and males, both confirming and extending previous studies.

Keywords: caudate putamen; electrophysiology; intrinsic excitability; medium spiny neurons; rodent; sex differences.

Figure 1.

Whole-cell patch-clamped MSN location in the caudate-putamen 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 non-fluorescently labeled MSNs. LV, Lateral ventricle; AC, anterior commissure; ACB, nucleus accumbens.

Figure 2.

Action potential rheobase, threshold, and width vary by MSN subtype. A, Voltage response of male and female Drd1a and Drd2 MSN subtypes to a depolarizing rheobase current injection. B, Resting membrane potential exhibited greater diversity in male Drd1A MSNs. C, Action potential rheobase is increased in Drd1a MSNs compared with Drd2 MSNs. D, Action potential threshold is depolarized in Drd1a MSNs compared with Drd2 MSNs, and interacts with sex. E, Action potential width is longer in Drd1a MSNs compared with Drd2 MSNs, and interacts with sex. AP, Action potential. *p < 0.05, **p < 0.01.

Figure 3.

Action potential firing rates evoked by depolarizing current injections vary by MSN subtype. A, Voltage response of male and female Drd1a and Drd2 MSN subtypes to a depolarizing post-rheobase current injection. B, Drd1a MSNs exhibited decreased action potential firing rates evoked by depolarizing current injections compared with Drd2 MSNs. C, The slope of the evoked action potential to depolarizing current injection curve (FI slope) differed by MSN subtype, with Drd2 MSNs exhibiting increased excitability compared with Drd1a MSNs. FI slope, Slope of the evoked action potential to depolarizing current injection curve. *p < 0.05, **p < 0.01, ****p < 0.0001.

Figure 4.

Input resistance varies by MSN subtype. A, Voltage response of male and female Drd1a and Drd2 MSN subtypes to a series of increasingly negative current injections (−0.01 nA current steps). B, Injected negative current to steady-stage voltage deflection curve (I–V curve). Legend, Red solid circles with red line, Drd1a males; black solid circles with black line, Drd2 males; red open circles with red line, Drd1a females; black open circles with black line, Drd2 females. C, Input resistance in the linear range is moderately decreased in Drd1a MSNs compared with Drd2 MSNs. D, Input resistance in the rectified range does not differ between subtypes. *p < 0.05.

Figure 5.

mEPSC properties vary by MSN subtype. A, mEPSCs recorded from male and female Drd1a and Drd2 MSN subtypes. MSNs were voltage-clamped at −70 mV and mEPSCs were recorded in the presence of TTX and PTX to block voltage-gated sodium channels and GABAergic synaptic activity, respectively. B, mEPSC amplitude was increased in Drd1a MSNs compared with Drd2 MSNs. C, mEPSC decay did not differ by subtype or sex. D, mEPSC frequency did not differ by subtype or sex. **p < 0.01, ***p < 0.001.