Dual activation of GABAA and glycine receptors by beta-alanine: inverse modulation by progesterone and 5 alpha-pregnan-3 alpha-ol-20-one.

The differential sensitivity of the glycine and GABAA receptors to modulation by progesterone and 5 alpha-pregnan-3 alpha-ol-20-one (5 alpha 3 alpha) was used to determine whether beta-alanine acts through its own receptor, or through the glycine and/or GABAA receptor(s). The response to beta-alanine resembles the glycine response as it is inhibited by strychnine (a competitive glycine antagonist) or progesterone (a negative modulator of the glycine response). Significantly, the response to beta-alanine also resembles the GABA response in that it is inhibited by 2-(carboxy-3'-propyl)-3-amino-6-paramethoxy-phenylpyridazinium+ ++ bromide (SR-95531; a competitive GABA antagonist) and potentiated by 5 alpha 3 alpha (a positive modulator of the GABA response). The efficacy of beta-alanine at the GABAA receptor is comparable to that of GABA. Similarly, the efficacy of beta-alanine at the glycine receptor is comparable to that of glycine. The greater potency of beta-alanine at the glycine receptor indicates that, if beta-alanine is a neurotransmitter, its effects are more likely to be mediated by glycine receptors than by GABAA receptors. However, activation of the GABAA receptor by beta-alanine may become important in the presence of steroid modulators such as progesterone or 5 alpha 3 alpha.