Mutual Suppression of Proximal and Distal Axonal Spike Initiation Determines the Output Patterns of a Motor Neuron

Axonal spike initiation at sites far from somatodendritic integration occurs in a range of systems, but its contribution to neuronal output activity is not well understood. We studied the interactions of distal and proximal spike initiation in an unmyelinated motor axon of the stomatogastric nervous system in the lobster, Homarus americanus. The peripheral axons of the pyloric dilator (PD) neurons generate tonic spiking in response to dopamine application. Centrally generated bursting activity and peripheral spike initiation had mutually suppressive effects. The two PD neurons and the electrically coupled oscillatory anterior burster (AB) neuron form the pacemaker ensemble of the pyloric central pattern generator, and antidromic invasion of central compartments by peripherally generated spikes caused spikelets in AB. Antidromic spikes suppressed burst generation in an activity-dependent manner: slower rhythms were diminished or completely disrupted, while fast rhythmic activity remained robust. Suppression of bursting was based on interference with the underlying slow wave oscillations in AB and PD, rather than a direct effect on spike initiation. A simplified multi-compartment circuit model of the pacemaker ensemble replicated this behavior. Antidromic activity disrupted slow wave oscillations by resetting the inward and outward current trajectories in each spike interval. Centrally generated bursting activity in turn suppressed peripheral spike initiation in an activity-dependent manner. Fast bursting eliminated peripheral spike initiation, while slower bursting allowed peripheral spike initiation to continue during the intervals between bursts. The suppression of peripheral spike initiation was associated with a small after-hyperpolarization in the sub-millivolt range. A realistic model of the PD axon replicated this behavior and showed that a sub-millivolt cumulative after-hyperpolarization across bursts was sufficient to eliminate peripheral spike initiation. This effect was based on the dynamic interaction between slow activity-dependent hyperpolarization caused by the Na⁺/K⁺-pump and inward rectification through the hyperpolarization-activated inward current, I _h . These results demonstrate that interactions between different spike initiation sites based on spike propagation can shift the relative contributions of different types of activity in an activity-dependent manner. Therefore, distal axonal spike initiation can play an important role in shaping neural output, conditional on the relative level of centrally generated activity.

Electrical Abnormalities in Dopaminergic Neurons of the Substantia Nigra in Mice With an Aromatic L-Amino Acid Decarboxylase Deficiency

Aromatic L-acid decarboxylase (AADC) deficiency causes severe motor disturbances in affected children. A putamen-targeted gene therapy improves the motor function of patients. The present study investigated the electrical properties of dopaminergic (DA) neurons in the substantia nigra compacta (SNc) of mice with an AADC deficiency (Ddc^KI). The basal firing of DA neurons, which determines DA release in the putamen, was abnormal in the Ddc^KI mice, including a low frequency and irregular firing pattern, because of a decrease in the after-hyperpolarization (AHP) amplitude of action potentials (APs). The frequency of spontaneous excitatory postsynaptic currents (sEPSCs) increased and that of spontaneous inhibitory PSCs (sIPSCs) decreased in the SNc DA neurons from the Ddc^KI mice, suggesting an elevation in glutamatergic excitatory stimuli and a reduction in GABAergic inhibitory stimuli, respectively. Altered expression patterns of genes encoding receptors and channels were also observed in the Ddc^KI mice. Administration of a widespread neuron-specific gene therapy to the brains of the Ddc^KI mice partially corrected these electric abnormalities. The overexcitability of SNc DA neurons in the presence of generalized dopamine deficiency likely underlies the occurrence of motor disturbances.

Changes in membrane properties of rat deep cerebellar nuclear projection neurons during acquisition of eyeblink conditioning

Previous studies have shown changes in membrane properties of neurons in rat deep cerebellar nuclei (DCN) as a function of development, but due to technical difficulties in obtaining viable DCN slices from adult animals, it remains unclear whether there are learning-related alterations in the membrane properties of DCN neurons in adult rats. This study was designed to record from identified DCN cells in cerebellar slices from postnatal day 25-26 (P25-26) rats that had a relatively mature sensory nervous system and were able to acquire learning as a result of tone-shock eyeblink conditioning (EBC) and to document resulting changes in electrophysiological properties. After electromyographic electrode implantation at P21 and inoculation with a fluorescent pseudorabies virus (PRV-152) at P22-23, rats received either four sessions of paired delay EBC or unpaired stimulus presentations with a tone conditioned stimulus and a shock unconditioned stimulus or sat in the training chamber without stimulus presentations. Compared with rats given unpaired stimuli or no stimulus presentations, rats given paired EBC showed an increase in conditioned responses across sessions. Whole-cell recordings of both fluorescent and nonfluorescent DCN projection neurons showed that delay EBC induced significant changes in membrane properties of evoked DCN action potentials including a reduced after-hyperpolarization amplitude and shortened latency. Similar findings were obtained in hyperpolarization-induced rebound spikes of DCN neurons. In sum, delay EBC produced significant changes in the membrane properties of juvenile rat DCN projection neurons. These learning-specific changes in DCN excitability have not previously been reported in any species or task.

Phosphatidylinositol 4,5-bisphosphate (PIP2 ) modulates afterhyperpolarizations in oxytocin neurons of the supraoptic nucleus

KEY POINTS

Afterhyperpolarizations (AHPs) generated by repetitive action potentials in supraoptic magnocellular neurons regulate repetitive firing and spike frequency adaptation but relatively little is known about PIP₂ 's control of these AHPs. We examined how changes in PIP₂ levels affected AHPs, somatic [Ca²⁺ ]_i , and whole cell Ca²⁺ currents. Manipulations of PIP₂ levels affected both medium and slow AHP currents in oxytocin (OT) neurons of the supraoptic nucleus. Manipulations of PIP₂ levels did not modulate AHPs by influencing Ca²⁺ release from IP₃ -triggered Ca²⁺ stores, suggesting more direct modulation of channels by PIP₂ . PIP₂ depletion reduced spike-evoked Ca²⁺ entry and voltage-gated Ca²⁺ currents. PIP₂ appears to influence AHPs in OT neurons by reducing Ca²⁺ influx during spiking.

ABSTRACT

Oxytocin (OT)- and vasopressin (VP)-secreting magnocellular neurons of the supraoptic nucleus (SON) display calcium-dependent afterhyperpolarizations (AHPs) following a train of action potentials that are critical to shaping the firing patterns of these cells. Previous work demonstrated that the lipid phosphatidylinositol 4,5-bisphosphate (PIP₂ ) enabled the slow AHP component (sAHP) in cortical pyramidal neurons. We investigated whether this phenomenon occurred in OT and VP neurons of the SON. Using whole cell recordings in coronal hypothalamic slices from adult female rats, we demonstrated that inhibition of PIP₂ synthesis with wortmannin robustly blocked both the medium and slow AHP currents (I_mAHP and I_sAHP ) of OT, but not VP neurons with high affinity. We further tested this by introducing a water-soluble PIP₂ analogue (diC₈ -PIP₂ ) into neurons, which in OT neurons not only prevented wortmannin's inhibitory effect, but slowed rundown of the I_mAHP and I_sAHP . Inhibition of phospholipase C (PLC) with U73122 did not inhibit either I_mAHP or I_sAHP in OT neurons, consistent with wortmannin's effects not being due to reducing diacylglycerol (DAG) or IP₃ availability, i.e. PIP₂ modulation of AHPs is not likely to involve downstream Ca²⁺ release from inositol 1,4,5-trisphosphate (IP₃ )-triggered Ca²⁺ -store release, or channel modulation via DAG and protein kinase C (PKC). We found that wortmannin reduced [Ca²⁺ ]_i increase induced by spike trains in OT neurons, but had no effect on AHPs evoked by uncaging intracellular Ca²⁺ . Finally, wortmannin selectively reduced whole cell Ca²⁺ currents in OT neurons while leaving VP neurons unaffected. The results indicate that PIP₂ modulates both the I_mAHP and I_sAHP in OT neurons, most likely by controlling Ca²⁺ entry through voltage-gated Ca²⁺ channels opened during spike trains.

Maturation of membrane properties of neurons in the rat deep cerebellar nuclei

Patch clamp recordings of neurons in the adult rat deep cerebellar nuclei have been limited by the availability of viable brain slices. Using a new slicing technique, this study was designed to explore the maturation of membrane properties of neurons in the deep cerebellar nuclei (DCN)-an area involved in rat eyeblink conditioning. Compared to whole-cell current-clamp recordings in DCN in rat pups at postnatal day 16 (P16) to P21, recordings from weanling rats at P22-P40 revealed a number of significant changes including an increase in the amplitude of the afterhyperpolarization (AHP)-an index of membrane excitability which has been shown to be important for eyeblink conditioning-a prolonged interval between the first and second evoked action potential, and an increase in AHP amplitude for hyperpolarization-induced rebound spikes. This is the first report of developmental changes in membrane properties of DCN which may contribute to the ontogeny of eyeblink conditioning in the rat.

Single unit activity of the suprachiasmatic nucleus and surrounding neurons during the wake-sleep cycle in mice

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus contains a circadian clock for timing of diverse neuronal, endocrine, and behavioral rhythms, such as the cycle of sleep and wakefulness. Using extracellular single unit recordings, we have determined, for the first time, the discharge activity of individual SCN neurons during the complete wake-sleep cycle in non-anesthetized, head restrained mice. SCN neurons (n=79) were divided into three types according to their regular (type I; n=38) or irregular (type II; n=19) discharge activity throughout the wake-sleep cycle or their quiescent activity during waking and irregular discharge activity during sleep (type III; n=22). The type I and II neurons displayed a long-duration action potential, while the type III neurons displayed either a short-duration or long-duration action potential. The type I neurons discharged exclusively as single isolated spikes, whereas the type II and III neurons fired as single isolated spikes, clusters, or bursts. The type I and II neurons showed wake-active, wake/paradoxical (or rapid eye movement) sleep-active, or state-unrelated activity profiles and were, respectively, mainly located in the ventral or dorsal region of the SCN. In contrast, the type III neurons displayed sleep-active discharge profiles and were mainly located in the lateral region of the SCN. The majority of type I and II neurons tested showed an increase in discharge rate following application of light to the animal's eyes. Of the 289 extra-SCN neurons recorded, those displaying sleep-active discharge profiles were mainly located dorsal to the SCN, whereas those displaying wake-active discharge profiles were mainly located lateral or dorsolateral to the SCN. This study shows heterogeneity of mouse SCN and surrounding anterior hypothalamic neurons and suggests differences in their topographic organization and roles in mammalian circadian rhythms and the regulation of sleep and wakefulness.

Vasopressin indirectly excites dorsal raphe serotonin neurons through activation of the vasopressin1A receptor

The neuropeptide vasopressin (AVP; arginine-vasopressin) is produced in a handful of brain nuclei located in the hypothalamus and extended amygdala and is released both peripherally as a hormone and within the central nervous system as a neurotransmitter. Central projections have been associated with a number of functions including regulation of physiological homeostasis, control of circadian rhythms, and modulation of social behavior. The AVP neurons located in the bed nucleus of the stria terminalis and medial amygdala (i.e., extended amygdala) in particular have been associated with affiliative social behavior in multiple species. It was recently demonstrated that in the mouse AVP projections emanating from extended amygdala neurons innervate a number of forebrain and midbrain brain regions including the dorsal raphe nucleus (DR), the site of origin of most forebrain-projecting serotonin neurons. Based on the presence of AVP fibers in the DR, we hypothesized that AVP would alter the physiology of serotonin neurons via AVP 1A receptor (V1AR) activation. Using whole-cell electrophysiology techniques, we found that AVP increased the frequency and amplitude of excitatory post-synaptic currents (EPSCs) in serotonin neurons of male mice. The indirect stimulation of serotonin neurons was AMPA/kainate receptor dependent and blocked by the sodium channel blocker tetrodotoxin, suggesting an effect of AVP on glutamate neurons. Further, the increase in EPSC frequency induced by AVP was blocked by selective V1AR antagonists. Our data suggest that AVP had an excitatory influence on serotonin neurons. This work highlights a new target (i.e., V1AR) for manipulating serotonin neuron excitability. In light of our data, we propose that some of the diverse effects of AVP on physiology and behavior, including social behavior, may be due to activation of the DR serotonin system.