Late-postnatal cannabinoid exposure persistently elevates dendritic spine densities in area X and HVC song regions of zebra finch telencephalon

Abstract

Centrally acting cannabinoids are well known for their ability to impair functions associated with both learning and memory but appreciation of the physiological mechanisms underlying these actions, particularly those that persist, remains incomplete. Our earlier studies have shown that song stereotypy is persistently reduced in male zebra finches that have been developmentally exposed to cannabinoids. In the present work, we examined the extent to which changes in neuronal morphology (dendritic spine densities and soma size) within brain regions associated with zebra finch vocal learning are affected by late-postnatal cannabinoid agonist exposure. We found that daily treatment with the cannabinoid agonist WIN55212-2 (WIN, 1mg/kg IM) is associated with 27% and 31% elevations in dendritic spine densities in the song regions Area X and HVC, respectively. We also found an overall increase in cell diameter within HVC. Changes in dendritic spine densities were only produced following developmental exposure; treatments given to adults that had completed vocal learning were not effective. These findings have important implications for understanding how repeated cannabinoid exposure can produce significant, lasting alteration of brain morphology, which may contribute to altered development and behavior.

Figure 1

Representative image of Golgi-Cox impregnated spiny dendrites used for analysis within HVC of birds developmentally treated from 50 – 75 days of age with WIN (A), or vehicle (B). Animals were allowed to mature to adulthood (> 110 days), and brains were dissected and stained with Golgi-Cox solution (see Methods). Bar = 10 microns. (C) Representative high power image of a typical HVC neuron used for analysis. Measurements pertaining to cell diameters and corresponding dendrites were made for 10 randomly selected neurons for each of the 4 representative song regions of both WIN and vehicle-treated animals. Bar = 30 microns.

Figure 2

Effect of developmental treatments from 50 – 75 days on song region spine densities at adulthood (n = 8). Developmental WIN treatment resulted in significantly elevated dendritic spine densities within Area X and HVC but not lMAN or RA. (p ≤ 0.05).

Figure 3

Effect of treatments given to adults for 25 days on song region spine densities (n = 8). WIN (1 mg/kg/day) did not result in a significant change in dendritic spine densities in the four song regions of adult males which had already learned song.

Figure 4

Effect of developmental treatments from 50 – 75 days on song region average cell diameter at adulthood (n = 8). Developmental WIN treatment (1 mg/kg/day) resulted in significantly increased average cell diameter within HVC, but not other regions (lMAN, Area X, HVC, p ≤ 0.05).

Figure 5

Effect of treatments given to adults for 25 days on song region average cell diameter at adulthood (n = 8). Despite a trend for decreased cell diameter within HVC, significant differences were not found.