Neonatal Masculinization Blocks Increased Excitatory Synaptic Input in Female Rat Nucleus Accumbens Core

Abstract

Steroid sex hormones and genetic sex regulate the phenotypes of motivated behaviors and relevant disorders. Most studies seeking to elucidate the underlying neuroendocrine mechanisms have focused on how 17β-estradiol modulates the role of dopamine in striatal brain regions, which express membrane-associated estrogen receptors. Dopamine action is an important component of striatal function, but excitatory synaptic neurotransmission has also emerged as a key striatal substrate and target of estradiol action. Here, we focus on excitatory synaptic input onto medium spiny neurons (MSNs) in the striatal region nucleus accumbens core (AcbC). In adult AcbC, miniature excitatory postsynaptic current (mEPSC) frequency is increased in female compared with male MSNs. We tested whether increased mEPSC frequency in female MSNs exists before puberty, whether this increased excitability is due to the absence of estradiol or testosterone during the early developmental critical period, and whether it is accompanied by stable neuron intrinsic membrane properties. We found that mEPSC frequency is increased in female compared with male MSNs before puberty. Increased mEPSC frequency in female MSNs is abolished after neonatal estradiol or testosterone exposure. MSN intrinsic membrane properties did not differ by sex. These data indicate that neonatal masculinization via estradiol and/or testosterone action is sufficient for down-regulating excitatory synaptic input onto MSNs. We conclude that excitatory synaptic input onto AcbC MSNs is organized long before adulthood via steroid sex hormone action, providing new insight into a mechanism by which sex differences in motivated behavior and other AbcC functions may be generated or compromised.

Figure 1.

Location of whole-cell patch clamped MSNs in the AcbC. No exposure males and females represent MSNs recorded from animals not manipulated as neonates. Vehicle, 17β-estradiol, and Testosterone males and females represent MSNs recorded from animals receiving neonatal injections of vehicle or hormone.

Figure 2.

mEPSC frequency is increased in prepubertal female compared with male MSNs. A, mEPSCs recorded in male (upper panel) and female (lower panel) AcbC MSNs. MSNs were voltage clamped at −70 mV and recorded in the presence of TTX and PTX to block voltage-gated sodium channels and GABAergic synaptic activity, respectively. B, mEPSC frequency is increased in female compared with male MSNs. C, mEPSC amplitude does not differ by sex. D, mEPSC decay does not differ by sex. E, Paired-pulse ratio does not differ by sex. Experiments depicted without P values do not show significantly differences (P > .05); complete statistical information is in Table 1.

Figure 3.

Early hormone exposure regulates mEPSC frequency. A, mEPSCs recorded from MSNs in prepubertal female animals exposed as neonates to: (upper panel) vehicle, (middle panel) 17β-estradiol, and (lower panel) testosterone. B, Neonatal exposure to estradiol decreases mEPSC frequency in prepubertal female MSNs to levels found in male MSNs. C, In MSNs recorded from prepubertal females, neonatal exposure to testosterone decreases mEPSC frequency to levels indistinguishable from animals exposed to estradiol. The P value within each subpanel indicates statistical significance; complete statistical information is in Table 2.

Figure 4.

Action potential properties of MSNs do not differ by sex. A, Voltage response of a prepubertal female (upper panel) and male (lower panel) MSN to a series of positive current injections. B, No sex differences were detected in action potential firing rates evoked by positive current injection. No sex differences were detected in (C) action potential threshold, (D) action potential amplitude, (E) action potential width, (F) the delay to first action potential, (G) action potential afterhyperpolarization peak, and (H) action potential afterhyperpolarization time to peak. The P value of all experiments was P > .05; complete statistical information is in Table 1.

Figure 5.

Passive MSN electrophysiological properties do not significantly differ by sex. A, Voltage response of a prepubertal male (left) and female (right) MSN to a series of negative current injections. No sex differences were detected in (B) resting membrane potential, (C) input resistance in the nonrectified range, D) the time constant of the membrane (τ), and (E) capacitance. The P value of all experiments was P > .05; complete statistical information is in Table 1.

Figure 6.

Input resistance in the rectified range does not significantly differ by sex. A, No sex differences were detected in input resistance in either the linear or rectified range in MSNs recorded from prepubertal animals. No sex differences were detected in (B) rectified range input resistance and (C) percent inward rectification. The P value of all experiments was P > .05; complete statistical information is in Table 1.

Figure 7.

Input resistance in the rectified and nonrectified ranges does not vary by neonatal hormone exposure. A, No differences were detected in input resistance in either the linear or rectified range in MSNs recorded from prepubertal animals exposed to vehicle or estradiol as neonates. B, Neonatal exposure to estradiol does not modify rectified range input resistance. C, Neonatal exposure to estradiol does not modify input resistance in the nonrectified range. The P value of all experiments was P > .05.