Stringent specificity in the construction of a presynaptic inhibitory circuit

To explore the mechanisms that promote functional circuit formation in the CNS, I have focused on the inhibitory network regulating reflex responses in the spinal cord. In the spinal cord, the transfer of sensory signals to motor neurons is filtered by GABAergic intemeurons that act presynaptically to inhibit sensory transmitter release and postsynaptically to inhibit motor neuron excitability. To understand the emergence of these distinct functional circuits. I have asked if the targeting of neurons that function in presynaptic and postsynaptic inhibition is genetically determined or opportunistic. Furthermore, what are the discrete molecular signals provided by the postsynaptic neuron that orchestrate specificity in connectivity, allowing for presynaptic differentiation at appropriate sites of contact between neural partners?;Inhibitory synapses that function in presynaptic inhibition are formed on the terminals of sensory afferents. These synapses are morphologically and molecularly distinct from the postsynaptically targeting synapses, in that they are small and selectively express the GABA synthetic enzyme, GAD65. These distinctions suggest that the neurons that form synapses with sensory afferent terminals are genetically distinct from postsynaptic inhibitory neurons. To establish the relationship between neural identity and connectivity of these inhibitory synapses, I have taken advantage of studies that have shown several neural domains, defined by the unique expression of transcription factors, give rise to inhibitory neurons in the spinal cord.;One method to establish the importance of the postsynaptic target in directing selectivity in synapse formation is to precisely remove distinct target neurons and assess the connectivity of the presynaptic neuron. I have examined stringency of synapse formation in the spinal cord by assessing the cellular contribution of the post synaptic target of the GABAergic neurons targeting sensory afferent terminals. By selectively removing sensory afferent terminals from the ventral spinal cord, I have show that the connectivity and synaptic differentiation of GABAergic interneurons that mediate presynaptic inhibition is critically dependent on their targets. In the absence of sensory terminals these GABAergic neurons shun other available targets, fail to undergo presynaptic differentiation, and withdraw axons from the ventral spinal cord. These findings suggest that the organization of a GABAergic circuit that mediates presynaptic inhibition in the mammalian CNS is controlled by a stringent program of sensory recognition and signaling. What are the molecular signals provided by the sensory terminals that function in directing the selectivity of targeting and differentiation of these GABAergic interneurons?;To explore the molecular identity of signals provided by the sensory afferent terminal that direct recognition and differentiation of presynaptically targeted GABAergic neurons, I have identified factors selectively expressed by sensory neurons that may provide a signal for target recognition and presynaptic differentiation. One adhesion molecule, selectively expressed by sensory neurons, is the Contactin family member NB-2. The fidelity of specificity of GABAergic interneurons targeting sensory afferent terminals requires NB-2, as mice lacking this protein exhibit a reduction in the number of inhibitory terminals on a sensory afferent terminal, along with the emergence of ectopie synapses. Furthermore, I have identified a retrograde signaling molecule, selectively expressed by sensory afferent terminals, that is necessary for aspects of synaptic differentiation of GAB Aergic synapses. I have shown that the presynaptic differentiation of GABAergic synapses on sensory terminals depends on a sensory-specific source of brain derived neurotrophic factor that induces synaptic expression of GAD65--a defining biochemical feature of this set of interneurons.