A Chemoattractant Role for NT-3 in Proprioceptive Axon Guidance

Neurotrophin-3 (NT-3) is required for proprioceptive neuron survival. Deletion of the proapoptotic gene Bax in NT-3 knockout mice rescues these neurons and allows for examination of their axon growth in the absence of NT-3 signaling. TrkC-positive peripheral and central axons from dorsal root ganglia follow proper trajectories and arrive in close proximity to their targets but fail to innervate them. Peripherally, muscle spindles are absent and TrkC-positive axons do not enter their target muscles. Centrally, proprioceptive axons branch in ectopic regions of the spinal cord, even crossing the midline. In vitro assays reveal chemoattractant effects of NT-3 on dorsal root ganglion axons. Our results show that survival factor NT-3 acts as a short-distance axon guidance molecule for muscle sensory afferents as they approach their proper targets

Insulin receptor sensitization restores neocortical excitation/inhibition balance in a mouse model of autism

Abstract Background Met receptor tyrosine kinase regulates neurogenesis, differentiation, migration, connectivity, and synaptic plasticity. The human Met gene has been identified as a prominent risk factor for autism spectrum disorder (ASD). Met gene-altered mice serve as useful models for mechanistic studies of ASD. Inactivation of Met in excitatory cortical neurons in mice (Emx1 cre /Met flox mice) yields a phenotype in which significantly decreased GABAA receptor-mediated inhibition shifts the excitation/inhibition (E/I) balance toward excitation in the somatosensory cortex. Further, unlike that seen in wild-type mice, insulin does not increase inhibition in the mutant cortex, suggesting that one of the consequences of kinase inactive Met gene could be desensitization of insulin receptors. To test this hypothesis, we investigated the effects of insulin receptor sensitizer, pioglitazone, on inhibition in the somatosensory thalamocortical circuitry. Methods We used whole-cell patch clamp electrophysiology and analyzed excitatory and inhibitory responses of cortical layer IV excitatory cells following stimulation of their thalamic input in thalamocortical pathway intact brain slices. We applied insulin alone and insulin + a thiazolidinedione, pioglitazone (PIO), to test the effects of sensitizing insulin receptors on inhibitory responses mediated by GABAA receptors in the somatosensory cortex of Emx1 cre /Met flox mice. Results In WT brain slices, application of insulin together with PIO did not enhance the effect of insulin alone. In contrast, PIO application induced a much larger inhibition than that of insulin alone in Met-defective cortex. Thus, insulin resistance of GABAA receptor-mediated response in Met mutant mice may result from desensitized insulin receptors. Conclusions Sporadic clinical studies reported improved behavioral symptoms in children with autism following PIO treatment. We show that PIO can aid in normalization of the E/I balance in the primary somatosensory cortex, a potential physiological mechanism underlying the positive effects of PIO treatment

Peripheral nerve damage does not alter release properties of developing central trigeminal afferents

The infraorbital branch of the trigeminal nerve (ION) is essential in whisker-specific neural patterning (“barrelettes”) in the principal nucleus of the trigeminal nerve (PrV). The barrelettes are formed by the ION terminal arbors, somata, and dendrites of the PrV cells; they are abolished after neonatal damage to the ION. Physiological studies show that disruption of the barrelettes is accompanied by conversion of functional synapses into silent synapses in the PrV. In this study, we used whole cell recordings with a paired-pulse stimulation protocol and MK-801 blocking rate to estimate the presynaptic release probability (Pr) of ION central trigeminal afferent terminals in the PrV. We investigated Pr during postnatal development, following neonatal ION damage, and determined whether conversion of functional synapses into silent synapses after peripheral denervation results from changes in Pr. The paired-pulse ratio (PPR) was quite variable ranging from 40% (paired-pulse depression) to 175% (paired-pulse facilitation). The results from paired-pulse protocol were confirmed by MK-801 blocking rate experiments. The nonuniform PPRs did not show target cell specificity and developmental regulation. The distribution of PPRs fit nicely to Gaussian function with a peak at ∼100%. In addition, neonatal ION transections did not alter the distribution pattern of PPR in their central terminals, suggesting that the conversion from functional synapses into silent synapses in the peripherally denervated PrV is not caused by changes in the Pr

Electrophysiological properties and synaptic responses of cells in the trigeminal principal sensory nucleus of postnatal rats

Lo, Fu-Sun, William Guido, and Reha S. Erzurumlu. Electrophysiological properties and synaptic responses of cells in the trigeminal principal sensory nucleus of postnatal rats. J. Neurophysiol. 82: 2765Neurophysiol. 82: -2775Neurophysiol. 82: , 1999. In the rodent brain stem trigeminal complex, select sets of neurons form modular arrays or "barrelettes," that replicate the patterned distribution of whiskers and sinus hairs on the ipsilateral snout. These cells detect the patterned input from the trigeminal axons that innervate the whiskers and sinus hairs. Other brain stem trigeminal cells, interbarrelette neurons, do not form patterns and respond to multiple whiskers. We examined the membrane properties and synaptic responses of morphologically identified barrelette and interbarrelette neurons in the principal sensory nucleus (PrV) of the trigeminal nerve in early postnatal rats shortly after whisker-related patterns are established. Barrelette cell dendritic trees are confined to a single barrelette, whereas the dendrites of interbarrelette cells span wider territories. These two cell types are distinct from smaller GABAergic interneurons. Barrelette cells can be distinguished by a prominent transient A-type K ϩ current (I A ) and higher input resistance. On the other hand, interbarrelette cells display a prominent low-threshold T-type Ca 2ϩ current (I T ) and lower input resistance. Both classes of neurons respond differently to electrical stimulation of the trigeminal tract. Barrelette cells show either a monosynaptic excitatory postsynaptic potential (EPSP) followed by a large disynaptic inhibitory postsynaptic potential (IPSP) or just simply a disynaptic IPSP. Increasing stimulus intensity produces little change in EPSP amplitude but leads to a stepwise increase in IPSP amplitude, suggesting that barrelette cells receive more inhibitory input than excitatory input. This pattern of excitation and inhibition indicates that barrelette cells receive both feed-forward and lateral inhibition. Interbarrelette cells show a large monosynaptic EPSP followed by a small disynaptic IPSP. Increasing stimulus intensity leads to a stepwise increase in EPSP amplitude and the appearance of polysynaptic EPSPs, suggesting that interbarrelette cells receive excitatory inputs from multiple sources. Taken together, these results indicate that barrelette and interbarrelette neurons can be identified by their morphological and functional attributes soon after whisker-related pattern formation in the PrV