Neurotrophin-3 potentiates excitatory GABAergic synaptic transmission in cultured developing hypothalamic neurones of the rat.

Neurotrophic factors regulate proliferation, differentiation, process outgrowth and survival of specific neuronal populations, and thus play a vital role in vertebrate neuronal development. Neurotrophin-3 (NT-3), a member of the nerve growth factor gene family, supports the survival and differentiation of various peripheral sensory neurones (Ernfors et al. 1990; Hohn et al. 1990; Ernfors et al. 1994; Farinas et al. 1994). NT-3 enhances the survival and differentiation of spinal cord neurones (Henderson et al. 1993), cultured Purkinje cells (Lindholm et al. 1993), auditory neurones (Avila et al. 1993) and hippocampal neurones (Collazo et al. 1992; Ip et al. 1993). NT-3 induces neuronal differentiation of cortical precursor cells (Ghosh & Greenberg, 1995), and enhances sprouting of the corticospinal tract (Schnell et al. 1994). Hypothalamic neurones express TrkC, the primary receptor for NT-3 (Escandon et al. 1994; Berg-von der Emde et al. 1995), suggesting that NT-3 may influence hypothalamic neurones. Despite extensive evidence demonstrating important roles for NT-3 in neuronal development, there is little physiological work indicating how the neurotrophins act on developing central neurones and synapses. Much of our understanding of NT-3 is based on data obtained from the peripheral nervous system (Lohof et al. 1993; Liou et al. 1997). GABAergic synaptic transmission appears earlier than glutamatergic transmission in the development of the CNS (Reynolds & Brien, 1992; Chen et al. 1995, 1996; Ben-Ari et al. 1997). During early development of hypothalamic and other CNS neurones, due to a relatively positive Cl− reversal potential, GABA is excitatory, depolarizing the membrane potential, evoking action potentials and raising cytosolic calcium (Obrietan & van den Pol, 1995; Chen et al. 1996; Gao et al. 1998). The excitatory synaptic transmission mediated by GABAA receptors may comprise the majority of the excitatory driving force in the developing hypothalamus and other regions of the CNS (Ben-Ari et al. 1989; LoTurco et al. 1995; Obrietan & van den Pol, 1995; Chen et al. 1996; Owens et al. 1996; Leinekugel et al. 1997; Gao et al. 1998). Growing evidence indicates that NT-3 potentiates neuronal activity in mature cortical neurones (Kim et al. 1994) and induces long-lasting enhancement of synaptic transmission in mature hippocampal slices (Kang & Schuman, 1995). NT-3 was reported to inhibit GABAergic synaptic transmission in cortical neurones, which may be the mechanism responsible for the enhanced firing of action potentials in mature neurones (Kim et al. 1994). Given that hypothalamic neurones show a strong expression of the NT-3 receptor, TrkC, in development (Lamballe et al. 1994), in the present paper we studied the actions of NT-3 on GABAergic synaptic transmission in cultured hypothalamic neurones. We used cultures to allow rapid onset of response and complete wash-out of reagents, and to compare our work with work on neurotrophic factor actions on cultures of more mature neurones described previously (Berninger et al. 1993; Kim et al. 1994; Jarvis et al. 1997; Sakai et al. 1997). Cultures in defined media also allowed us to control the external cellular milieu to avoid complications due to uncharacterized trophic factors in serum. In striking contrast to previous work that focused on more mature neurones and found that NT-3 depressed GABA synaptic transmission (Kim et al. 1994), we found that in developing neurones NT-3 enhanced GABA transmission, probably by a Trk calcium-dependent mechanism at a presynaptic site that increased GABA release.