Effects of prepubertal gonadectomy on a male-typical behavior and excitatory synaptic transmission in the amygdala

Abstract

Mammalian puberty entails the emergence of behaviors such as courtship, coitus, and territorial aggressiveness. In adult rodents, the medial amygdala (MeA) is an important site for gonadal steroid hormone regulation of social behaviors and is sensitive to changes in the level of gonadal steroids. Here we show that prepubertal gonadectomy of male rats reduces the expression of a sexually dimorphic behavior, juvenile rough-and-tumble play, as well as the level of excitatory synaptic transmission assayed in adulthood. Behavioral observations in juveniles showed that gonadectomy reduced the initiation of playful attacks, particularly between postnatal days 31-35. Whole-cell voltage clamp recordings made in slices from adults showed that gonadectomy also reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) in MeA neurons without affecting paired pulse facilitation, an index of vesicle release probability. As mEPSC frequency can reflect the number of excitatory synapses per neuron, we also compared the dendritic morphology of Lucifer Yellow filled neurons from intact and gonadectomized adults. This showed that gonadectomy significantly reduced the density of dendritic spines without affecting overall dendritic length or branching of MeA neurons, which is consistent with a gonadectomy-induced reduction in the number of excitatory synapses. These findings suggest that peripubertal androgens activate rough-and-tumble play and promote the maintenance and/or development of new excitatory synapses in the MeA.

Figure 1

A – D. Video stills showing a typical rough-and-tumble play bout. The play initiator (A, right) pounces, paws outstretched, toward the nape of the partner’s neck. The partner turns (B) while the initiator continues to reach for the partner’s nape (C). Finally, the initiator succeeds in achieving a dominant position (D). E. The mean frequency of play initiation is plotted for Intact and GnX males. Intact males initiated more play bouts overall than GnX males (ANOVA, p = 0.04) particularly between P31–35 (Tukey/Kramer test, * p = 0.04).

Figure 2

A. Representative mEPSCs recorded from Intact and GnX males. B. Mean ± SEM mEPSC frequency recorded from Intact and GnX male neurons. Intact male neurons had higher mEPSC frequency than GnX males (** p = 0.01) irrespective of hemisphere. C. Plot of cell locations recorded in Intact and GnX males in the left and right MeApd.

Figure 3

A. Evoked EPSCs recorded from an MeApd neuron (interstimulus interval, 50 ms). B. Paired pulse ratios at 5 interstimulus intervals plotted for neurons from Intact and GnX males. There was no effect of gonadectomy (p = 0.6) on the paired pulse ratio.

Figure 4

A. Plot of the locations of all Lucifer Yellow-filled dendrite segments in the MeApd. B. Lucifer Yellow-filled MeApd dendrite from an Intact male. C. Lucifer Yellow filled dendrite from a GnX male. Bar = 5 μm. D. Significant difference in dendritic spine density between Intact and GnX males (* p = 0.02).

Figure 5

A–D. Tracings of Lucifer Yellow filled MeApd neurons. The axon (a) is indicated in some of the cells. The medial and lateral boundaries of the MeA are indicated with black and dashed lines, respectively, and the inner margin of the cell-sparse molecular layer is indicated with a grey line. The stria terminalis (st) is also indicated in A and D. For all neurons, bar = 100 μm. E. Significant difference in dendritic spine density between Intact and GnX males (* p = 0.02)