Functional specialization of male and female vocal motoneurons

Abstract

Vocal behaviors of African clawed frogs (Xenopus laevis) are produced by a single pair of muscles. This simplification, relative to other vertebrates, allows us to more easily determine how CNS motor pathways function to produce sex-specific songs. We describe here certain sexually differentiated properties of vocal motoneurons that are matched to male and female vocal demands. Both active and passive membrane properties differ between the sexes. Male motoneurons have lower input resistances and larger membrane capacitances than female motoneurons. Two distinct firing patterns are found, in different proportions, in males and females. The strongly adapting neurons that predominate in males initiate spikes at short, reliable latencies, whereas the weakly adapting motoneurons characteristic of females translate graded levels of depolarization into different firing rates. Low-threshold potassium currents (IKL) predominate in males. Hyperpolarization-activated cationic currents (IH) are found almost exclusively in males. Modeling results indicate that sex-typical active and passive properties can account for the occurrence of strongly and weakly adapting spike trains in the sexes. In particular, IKL seem to play an important role in determining the firing patterns of neurons. We suggest that these physiological differences facilitate transformation of synaptic inputs into male- and female-specific outputs that generate sexually distinct songs in vivo.

Figure 6.

Current-clamp simulation of strongly adapting male and weakly adapting female model cells. a, e, The firing patterns of model male (a) and model female (e) neurons in response to depolarizing current steps. d, h, The activation of the low-threshold K⁺ currents (I_KL) during suprathreshold depolarizations in typical male and female neurons, respectively. b, The firing patterns of a model male neuron when the voltage of half-maximal activation of the low-threshold K⁺ currents was changed to that typical of female neurons (-21 mV). The resting membrane potential depolarized from -65 to -60 mV. c, The firing patterns of a model male neuron when male I_H is further eliminated from the model in b. The resting membrane potential was hyperpolarized to -68 mV. f, The firing patterns of model female neurons when the voltage of half-maximal activation of the low-threshold K⁺ current was changed to that typical of male neurons (-41 mV). The amount of current required to trigger action potentials increased, and the resting membrane potential was hyperpolarized from -65 to -72 mV, and the model female cell now adapted rapidly. g, The firing patterns of a model female neuron with addition of the male I_H to f. The resting membrane potential was restored to -65 mV. Amplitudes of applied currents (nanoamperes) are shown to the left of the traces.

Figure 1.

Identification of retrogradely labeled laryngeal motoneurons in a slice preparation. Left, Infra-red differential contrast image. Right, The same field as in the left panel, viewed using fluorescent optics.

Figure 2.

Firing properties of male and female laryngeal motoneurons. a, Current-clamp recordings of strongly adapting male and female motoneurons. b, Current-clamp recordings of weakly adapting male and female motoneurons. To the right of each trace, the amplitude of injected current in nanoamperes is indicated. Maximum spike onset time of each neuron is indicated by dashed lines. c, Histogram of maximum spike onset time (see Results and Fig. 2, a and b) of male and female neurons.

Figure 3.

a, Voltage-clamp traces of K⁺ current in male and female motoneurons activated from two holding potentials (V_H): -80 and -40 mV. Brief inward currents were evoked in some neurons but not all, presumably because the spike initiation site was severed from soma on a slice. b, Voltage dependence of currents activated from holding potential of -80 mV (mean ± SE). The currents were measured at steady state. c, Voltage dependence of high-threshold current activated from a holding potential of -40 mV (mean ± SE).

Figure 4.

Low-threshold potassium currents and firing patterns of laryngeal motoneurons. Voltage dependence of currents activated from a holding potential of -80 mV in strongly (•) and weakly adapting (○) neurons (mean ± SE). Inset, The voltage dependence of currents between -60 and -40 mV. Asterisks indicate a significant difference of normalized conductances between strongly and weakly adapting neurons. ns, Statistical nonsignificance.

Figure 5.

I_H of male laryngeal motoneurons. a, Inward currents are activated in most male motoneurons, but not in most female motoneurons, by stepping from -40 to -120 mV. b, Mean normalized currents plotted against test voltages (mean ± SE; n = 25, N = 10). The currents were measured by subtracting the current at the beginning of the step (▪) from the current at the steady state (○)in a. c, Voltage dependence of mean activation time constants of I_H (mean ± SE; n = 12, N = 6). d, The inward current is blocked by ZD7288, a selective blocker of I_H, as well as by Cs⁺ (e).