Midbrain dopamine neurons reflect affiliation phenotypes in finches and are tightly coupled to courtship

Abstract

Mesolimbic dopamine (DA) circuits mediate a wide range of goal-oriented behavioral processes, and DA strongly influences appetitive and consummatory aspects of male sexual behavior. In both birds and mammals, mesolimbic projections arise primarily from the ventral tegmental area (VTA), with a smaller contribution from the midbrain central gray (CG). Despite the well known importance of the VTA cell group for incentive motivation functions, relationships of VTA subpopulations to specific aspects of social phenotype remain wholly undescribed. We now show that in male zebra finches (Estrildidae: Taeniopygia guttata), Fos activity within a subpopulation of tyrosine hydroxylase-immunoreactive (TH-ir; presumably dopaminergic) neurons in the caudal VTA is significantly correlated with courtship singing and coupled to gonadal state. In addition, the number of TH-ir neurons in this caudal subpopulation dichotomously differentiates courting from non-courting male phenotypes, and evolves in relation to sociality (flocking vs. territorial) across several related finch species. Combined, these findings for the VTA suggest that divergent social phenotypes may arise due to the differential assignment of "incentive value" to conspecific stimuli. TH-ir neurons of the CG (a population of unknown function in mammals) exhibit properties that are even more selectively and tightly coupled to the expression of courtship phenotypes (and appetitive courtship singing), both in terms of TH-ir cell number, which correlates significantly with constitutive levels of courtship motivation, and with TH-Fos colocalization, which increases in direct proportion to the phasic expression of song. We propose that these neurons may be core components of social communication circuits across diverse vertebrate taxa.

Fig. 1.

TH-Fos colocalization and TH-ir cell number in the CG of male zebra finches that reliably court (courters; black fills) or fail to court (non-courters; no fills). (A) Representative double-labeling for TH (Alexa Fluor 488; green) and Fos (Alexa Fluor 594; red) in the CG of a normal (courter) male zebra finch after exposure to a female. DAPI nuclear stain is shown as blue. (Scale bar, 100 μm.) Arrows indicate neurons double-labeled for TH and Fos. (B) Percentage of TH-ir neurons that express Fos after exposure to control conditions, a positive nonsocial stimulus (water bath; courters only), or a female. Data are shown as means ± SEM. *, P = 0.01, unpaired t test. Different letters above the error bars denote significant differences between courter groups (Fisher's PLSD P < 0.05 after significant ANOVA). Group n's are indicated at the base of each bar. (C) Correlation between TH-Fos colocalization in the CG and the number of directed songs sung in the final test by the 16 subjects exposed to females. (D) Correlation between TH-ir cell number in CG (summed across 4 sections) and the number of directed songs sung in prescreenings. Separate analysis of courters yields an r² of 0.228 (P = 0.01). Total n = 38.

Fig. 2.

TH-Fos colocalization and TH-ir cell number in the caudal VTA reflect singing and differentiate courters (black fills) and non-courters (no fills). (A) Correlation between TH-Fos colocalization in the caudal VTA and the number of directed songs sung in the final test by the 16 male zebra finches exposed to females. (B) Percentage of TH-ir neurons that express Fos after exposure to control conditions, a positive nonsocial stimulus (water bath; courters only), or a female. Data are shown as means ± SEM. #, ANOVA P = 0.053. (C) TH-ir cell number in the caudal VTA is significantly lower in non-courters than in courters. Data are shown as means ± SEM. *, P = 0.01, unpaired t test. Group n's are indicated at the base of each bar.

Fig. 3.

TH-Fos colocalization in the CG reflects social vocalization, but not species differences in sociality. (A) Percentage of TH-ir neurons in the CG that express Fos after exposure to a same-sex conspecific (through a wire barrier; solid bars) or control conditions (open bars) in 5 species of estrildid finches. Data are shown as means ± SEM, with sexes pooled. *, ANOVA main effect of condition, P = 0.0057. Group n's are indicated at the base of each bar. ABW, Angolan blue waxbill; MF, melba finch; SF, spice finch; VEW, violet-eared waxbill; ZF, zebra finch. (B) TH-Fos colocalization is signifcantly greater in subjects that were observed to be actively calling during the exposure to a same-sex conspecific (*, P = 0.033, unpaired t test).

Fig. 4.

TH-ir cell number in the caudal VTA (corrected for differences in body size) reflects sociality in 5 species of estrildid finches. Data are shown as means ± SEM, sexes pooled. Different letters above the error bars denote significant differences between species (Fisher's PLSD P < 0.05 after significant ANOVA). Group n's are indicated at the base of each bar.