No abstract

Sympathetic neurons of the rat superior cervical ganglion (SCG) can segregate their neurotransmitters and co-transmitters to separate varicosities of single axons. We have shown that transmitter segregation is a plastic phenomenon and that it is correlated with the strength of synaptic transmission. Here, we determined whether sympathetic dysfunction occurring in stress and hypertension was correlated with plastic changes of neurotransmitter segregation. We characterized the expression of the markers, L-glutamic acid decarboxylase of 67 kDa (GAD67) and vesicular acetylcholine (ACh) transporter (VAChT) in the SCG of cold stressed and spontaneously hypertensive rats (SHR). Considering that the SCG comprises a heterogeneous neuronal population, we explored whether the expression and segregation of neurotransmitters would also have an intraganglionic heterogeneous distribution in ganglia of stressed and hypertensive rats. Furthermore, since hypertension in SHR is detected around 8–10 weeks, we evaluated expression and segregation of ACh and GABA in adult hypertensive (12-week old (wo)) and young pre-hypertensive (6-wo) SHR. We found an increase in segregation of ACh and GABA with no change in transmitter expression in ganglia of stressed animals. In contrast, in SHR, there was an increase in GABA expression, although segregation did not vary. Segregation showed a caudo-rostral gradient in controls but not in the ganglia of stressed animals. GABA expression showed a rostro-caudal gradient in adult SHR, which was not present in young 6-wo rats. In young SHR, ACh increased and, unexpectedly, segregation of ACh and GABA was higher than in adults. Data suggest that ACh and GABA segregation increases in acute sympathetic hyperactivity like stress, but does not vary in chronic hyperactivity such as in hypertension. Changes in segregation are age-dependent and might be involved in the mechanisms underlying stress and hypertension.

Low threshold voltage activated Kv1 potassium channels play key roles in regulating action potential (AP) threshold, neural excitability, and synaptic transmission. Kv1 channels are highly expressed in the cerebellum and mutations of human Kv1 genes are associated to episodic forms of ataxia (EAT-1). Besides the well-established role of Kv1 channels in controlling the cerebellar basket-Purkinje cells synapses, Kv1 channels are expressed by the deep cerebellar nuclear neurons (DCNs) where they regulate the activity of principal DCNs carrying the cerebellar output. DCNs include as well GABAergic neurons serving important functions, such as those forming the inhibitory nucleo-olivary pathway, the nucleo-cortical DCNs providing feedback inhibition to the cerebellar cortex, and those targeting principal DCNs, but whether their function is regulated by Kv1 channels remains unclear. Here, using cerebellar slices from mature GAD67-GFP mice to identify putative GABAergic-DCNs (GAD + DCN) we show that specific Kv1 channel blockers (dendrotoxin-alpha/I/K, DTXs) hyperpolarized the threshold of somatic action potentials, increased the spontaneous firing rate and hampered evoked high frequency repetitive responses of GAD + DCNs. Moreover, DTXs induced somatic depolarization and tonic firing in previously silent, putative nucleo-cortical DCNs. These results reveal a novel role of Kv1 channels in regulating GABAergic-DCNs activity and thereby, cerebellar function at multiple levels. The Shaker-related Kv1 (Kv1.1-Kv1.8) channels display characteristic low threshold voltage activation ranges, well suited to regulate action potential (AP) threshold and waveform, global or local excitability, axon conduction, and synaptic transmission 1-5 , as well as the timing, pattern, and precision with which neurons generate spikes 6-8. Kv1 channel proteins (except for the Kv1.7 member) are expressed in the brain, as heteromeric, but also homomeric assembly of four Kv1-alpha subunits 2. Diversity in composition, clustering, and location in different cellular compartments supports Kv1 channels varied functional roles 2,4. The discovery of toxins specific for Kv1 channel subtypes has been crucial to elucidate their function in different brain areas and neurons 9. The finding of point mutations of the gene KCNA1 (Kv1.1) associated to a familiar disorder characterized by attacks of ataxia with or without myokimia, episodic ataxia type-1 (EAT-1) 10 , indicated a critical role for Kv1.1 channels in cerebellar function. Later, it was found that mutations in the KCNA2 gene (Kv1.2) may result in mice 11 and humans in ataxia and convulsions 12-14. Kv1 channels, in particular, Kv1.1, 1.2 and Kv1.6 are highly expressed in the cerebellar cortex, and in particular Kv1.1 and Kv1.2 channels are highly expressed in the terminals of the inhibitory basket-cells of the cerebellar cortex 15,16. Consistently, application of a specific Kv1 channel blocker, dendrotoxin-alpha (DTX_alpha) 9 , greatly enhanced the amplitude and frequency of spontaneous inhibitory postsyn...

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