Developmental increase in neuronal gap junction coupling is regulated by an interplay between the activity of group II mGluRs and GABA_ARs. Data from electrotonic coupling experiments in rat hypothalamic cultures are presented. The coupling was determined as described in Materials and Methods. The number of coupled pairs was as follows: DIV3, 0 of 21; DIV15, control, 6 of 28; LY379268, 8 of 15; LY341495, 1 of 32; muscimol, 0 of 22; Bic/PiTX+A/C (bicuculline, picrotoxin, AP5, and CNQX), 10 of 19. A, B, Incidence of electrotonic coupling represents the percentage of neuronal pairs that demonstrated the coupling. Statistical significance was calculated using Fisher's exact probability test relative to (a) DIV3 and (b) the DIV15 control. C, Representative traces of electrotonic responses are shown (each trace is the average voltage response from 5 sequential steps). D, Statistical analysis of the coupling coefficient (see Materials and Methods) was done using Student's t test relative to the DIV15 control; mean ± SEM (responses from all of the tested pairs are included in the analysis). Note that on DIV15 the incidence of electrotonic coupling and the coupling coefficient both are higher (relative to the control) in cultures subjected to chronic activation of group II mGluRs and inactivation of GABA_ARs and lower in cultures subjected to inactivation of group II mGluRs and activation of GABA_ARs.