Potential Afterhyperpolarization Research Articles

Propofol-Block of SK Channels in Reticular Thalamic Neurons Enhances GABAergic Inhibition in Relay Neurons

The GABAergic reticular thalamic nucleus (RTN) is a major source of inhibition for thalamocortical neurons in the ventrobasal complex (VB). Thalamic circuits are thought to be an important anatomic target for general anesthetics. We investigated presynaptic actions of the intravenous anesthetic propofol in RTN neurons, using RTN-retained and RTN-removed brain slices. In RTN-retained slices, focal and bath application of propofol increased intrinsic excitability, temporal summation, and spike firing rate in RTN neurons. Propofol-induced activation was associated with suppression of medium afterhyperpolarization potentials. This activation was mimicked and completely occluded by the small conductance calcium-activated potassium (SK) channel blocker apamin, indicating that propofol could enhance RTN excitability by blocking SK channels. Propofol increased GABAergic transmission at RTN-VB synapses, consistent with excitation of presynaptic RTN neurons. Stimulation of RTN resulted in synaptic inhibition in postsynaptic neurons in VB, and this inhibition was potentiated by propofol in a concentration-dependent manner. Removal of RTN resulted in a dramatic reduction of both spontaneous postsynaptic inhibitory current frequency and propofol-mediated inhibition of VB neurons. Thus the existence and activation of RTN input were essential for propofol to elicit thalamocortical suppression; such suppression resulted from shunting through the postsynaptic GABA(A) receptor-mediated chloride conductance. The results indicate that propofol enhancement of RTN-mediated inhibitory input via blockade of SK channels may play a critical role in "gating" spike firing in thalamocortical relay neurons.

MULTIPLE LEVELS OF SENSORY INTEGRATION IN THE INTRINSIC SENSORY NEURONS OF THE ENTERIC NERVOUS SYSTEM

1. The enteric nervous system (ENS) is present in the wall of the gastrointestinal tract and contains all the functional classes of neuron required for complete reflex arcs. One of the most important and intriguing classes of neuron is that responsive to sensory stimuli: sensory neurons with cell bodies intrinsic to the ENS. 2. These neurons have three outstanding and interrelated features: (i) reciprocal connections with each other; (ii) a slow excitatory post-synaptic potential (EPSP) resulting from high-speed firing in other sensory neurons; and (iii) a large after-hyperpolarizing potential (AHP) at the soma. Slow EPSP depolarize the cell body, generate action potentials (APs) and reduce the AHP. Conversely, the AHP limits the firing rate and, hence, reduces transmission of slow EPSP. 3. Processing of sensory information starts at the input terminals as different patterns of APs depending on the sensory modality and recent sensory history. At the soma, the ability to fire APs and, hence, drive outputs is also strongly determined by the recent firing history of the neuron (through the AHP) and network activity (through the slow EPSP). Positive feedback within the population of intrinsic sensory neurons means that the network is able to drive outputs well beyond the duration of the stimuli that triggered them. 4. Thus, sensory input and subsequent reflex generation are integrated over several hierarchical levels within the network on intrinsic sensory neurons.

The Role of the Electrophysiological Properties of Neurons in the Mechanisms Grouping Their Discharges in the Cerebral Cortex

Studies using intracellular recording in living slices of rat sensorimotor cortex addressed the interaction between the properties of neuron spike activity (n = 80) and the membrane potentials of the neurons. Spike sequences containing discharges with regularly increasing and decreasing interspike intervals were analyzed. Parameters were identified which were closely associated with the mean neuron discharge frequency: the number of spikes in sequences (5-30% of the total number of spikes recorded), the amplitude of oscillations in the afterhyperpolarization potential (0-1.5 mV), etc. There was a biphasic relationship in changes in the number of spikes in sequences with a critical mean discharge frequency over the range 5-7 Hz. Groups of cells without and with a depolarization component in conditions of afterhyperpolarization had different morphological and electrophysiological properties, though the relationships between their parameter and mean discharge frequency were similar. The possible roles of spike sequences and these regular features in the formation of rhythmic processes in the neocortex are discussed.

Role of Potassium Channels in the Frequency-Dependent Activity of Regenerating Nerves

After a peripheral nerve injury, ion channel organization and the electrical properties of nerve fibers drastically change during the regeneration process. The present study was designed to compare the frequency-dependent characteristics of regenerating nerves in the presence of 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The results showed that increasing the stimulus frequency produced a greater impulse blockade (frequency-dependent block – FDB) and distinct hyperpolarizing afterpotentials (HAPs) in regenerating nerves. In particular, regenerating sciatic nerves 15 days post-crush (dpc) were more sensitive to the frequency-dependent stimulations than 38-dpc and intact nerves in the presence or absence of drugs. The frequency-dependent effects of TEA on the compound action potentials (CAPs) appeared when TEA was applied to 4-AP-treated nerves. This shows that TEA-sensitive channels may not be masked by the myelin. 4-AP was here found to have more pronounced frequency-dependent effects on regenerating nerves than on intact nerves. Delayed depolarization (in 38-dpc: 22.6 ± 1.3 mV and 47.52 ± 3.63 ms, in intact: 12.0 ± 1.9 mV and 88.51 ± 4.72 ms) elicited by 4-AP resulted in an increase in FDBs and HAP amplitudes. These results suggest that 4-AP-sensitive channels may play important roles in frequency-dependent nerve conduction. Consequently, regenerating or myelin damaged nerves are more sensitive to repetitive firing with or without drug. An understanding of the frequency-dependent properties of regenerating nerves may be of value in the treatment of the nerve diseases.

Synaptic properties and postsynaptic opioid effects in rat central amygdala neurons

An important output of amygdaloid nuclei, the central nucleus of the amygdala (CeA) not only mediates negative emotional behaviors, but also participates in the stimulus–reward learning and expression of motivational aspects of many drugs of abuse, and links environmentally stressful conditions such as fear to endogenous pain-inhibiting mechanisms. The endogenous opioid system in the CeA is crucial for both reward behaviors and environmental stress-induced analgesia. In this study using whole-cell voltage-clamp recordings, we investigated synaptic inputs and the postsynaptic effects of opioid agonists in CeA neurons. We found that synaptic inputs evoked within the CeA were mediated by both glutamate and GABA, but those evoked from the basolateral amygdala were primarily glutamatergic. Based on membrane properties, three types of cells were characterized. Type A neurons had no spike accommodation while type B neurons displayed characteristic accommodating response. Type A neurons were further classified as either A1 or A2, based on differences in resting membrane potential and the amplitude of after-hyperpolarizing potential. μ-Opioid receptor agonists hyperpolarized a subpopulation of CeA neurons, of which the vast majority was type A1. This μ agonist-induced hyperpolarization was mediated by the opening of inwardly rectifying potassium channels. In contrast, the κ-opioid receptor agonist hyperpolarized only type B neurons. These results illustrate three types of CeA neurons with distinctive membrane properties and differential responses to opioid agonists. They may represent functionally distinct CeA cell groups for the integration and execution of CeA outputs in the aforementioned CeA functions.

Neurons synthesizing melanin-concentrating hormone identified by selective reporter gene expression after transfection in vitro: transmitter responses.

Neurons from the lateral hypothalamus that synthesize melanin-concentrating hormone (MCH) play an important role in the regulation of energy homeostasis. Relatively little is known of the cellular physiology and transmitter responses of these neurons, in part because of the difficulty in identifying live MCH cells. Here we use a novel approach of transfection of specific gene constructs with the MCH promoter driving green fluorescent protein (GFP) or red fluorescent protein (dsRed2) in CNS cultures to identify live rat MCH neurons; all neurons expressing the reporter gene showed MCH immunoreactivity, indicating selective expression. MCH neurons had a resting membrane potential of -57.5 +/- 0.6 mV, a linear current-voltage relation and a mean input resistance of 1,013 MOmega. Long depolarizing pulses revealed significant spike frequency adaptation. Functional glutamate and GABA receptors were expressed by MCH neurons. MCH neurons were hyperpolarized by norepinephrine in the presence or absence of tetrodotoxin, suggesting direct inhibition. Orexigenic peptides neuropeptide Y (NPY) and MCH showed no direct effect on membrane potential, input resistance, action potential width, or afterhyperpolarization potential, but inhibited voltage-dependent calcium channels, indicating that MCH neurons expressed both MCH and NPY receptors.

Differential Effects of Lidocaine and Tramadol on Modified Nerve Impulse by 4-Aminopyridine in Rats

We have used the sucrose gap method to measure the effects of drugs on the electrophysiological properties of rat sciatic nerves. The results showed that 4-aminopyridine produced a slight conduction block, prolonged the duration of action potential, enhanced the hyperpolarizing afterpotential, and elicited a hump that followed the action potential. In the presence of 4-aminopyridine, the impulse-blocking activity of lidocaine and tramadol was enhanced. Both lidocaine and tramadol effectively depressed the delayed depolarization generated by 4-aminopyridine. While tramadol decreased the activity-evoked hyperpolarizing afterpotentials, lidocaine completely removed them. These findings indicate that lidocaine may be more effective in blocking the Na⁺ channels than tramadol. Tramadol may be more effective on the delayed rectifier K⁺ channels than lidocaine.

Inhibitory cotransmission or after-hyperpolarizing potentials can regulate firing in recurrent networks with excitatory metabotropic transmission

Recurrent networks of neurons communicating via excitatory connections are common in the nervous system. In the absence of mechanisms to control firing (collectively termed negative feedback), these networks are likely to be bistable and unable to meaningfully encode input signals. In most recurrent circuits, negative feedback is provided by a specialized subpopulation of interneurons, but such neurons are absent from some systems, which therefore require other forms of negative feedback. One such circuit is found within the enteric nervous system of the intestine, where AH/Dogiel type II neurons are interconnected via excitatory synapses acting through metabotropic receptors to produce slow excitatory postsynaptic potentials (slow EPSPs). Negative feedback in this recurrent network may come from either inhibitory postsynaptic potentials arising from the terminals that produce slow EPSPs or from the after hyperpolarizing potentials (AHPs) characteristic of these neurons. We have examined these possibilities using mathematical analysis, based on the Wilson-Cowan model, and computer simulations. Analysis of steady states showed that, under appropriate conditions, both types of negative feedback can provide robust regulation of firing allowing the networks to encode input signals. Numerical simulations were performed using large, anatomically realistic networks with realistic models for metabotropic transmission and suppression of the AHP. In the presence of constant exogenous input, parameters controlling aspects of synaptic events were varied, confirming the analytical results for static stimuli. The simulated networks also responded to time varying inputs in a manner consistent with known physiology. In addition, simulation revealed that neurons in networks with inhibitory contransmission fired in erratic bursts, a phenomenon observed in neurons in unparalysed tissue. Thus, either inhibitory contransmission or AHPs, or both, can allow recurrent networks of AH/Dogiel type II neurons to encode ongoing inputs in a biologically useful way. These neurons appear to be intrinsic primary afferent neurons (IPANs), which implies that the IPANs in a region act in a coordinated fashion.

The role of Ca2+ channels in the repetitive firing of striatal projection neurons.

Blockade of L-type Ca2+ channels results in a decrease in firing frequency of neostriatal neurons. In contrast, N- and P/Q-types of Ca2+ channel cooperate to tune firing pattern, since both of these channel types have to be blocked to enhance firing frequency. Parameters of the intensity-frequency plot were differentially modified by Ca2+ channel antagonists: while L-type Ca2+ channel block reduced the dynamic range by about 80%, block of N- and P/Q-types of Ca2+ channel generated a steeper intensity-frequency plot. These effects are explained in terms of the sustained depolarization and the afterhyperpolarizing potential known to be dependent upon L- and N-, P/Q-types of Ca2+ channels, respectively.

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3±1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8±0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4±4.4) than 5-HT-immunonegative cells (tau=33.5±4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT 1A receptor. These results suggested that 5-HT 1A receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT 1A receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT 1A receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT 1A receptors on non-5-HT neurons in the dorsal raphe nucleus.

Spontaneous miniature hyperpolarizations affect threshold for action potential generation in mudpuppy cardiac neurons.

Mudpuppy parasympathetic neurons exhibit spontaneous miniature hyperpolarizations (SMHs) that are generated by potassium currents, which are spontaneous miniature outward currents (SMOCs), flowing through clusters of large conductance voltage- and calcium (Ca(2+))-activated potassium (BK) channels. The underlying SMOCs are initiated by a Ca(2+)-induced Ca(2+) release (CICR) mechanism. Perforated-patch whole cell voltage recordings were used to determine whether activation of SMHs contributed to action potential (AP) repolarization or affected the latency to AP generation. Blockade of BK channels by iberiotoxin (IBX, 100 nM) slowed AP repolarization and increased AP duration. Treatment with omega-conotoxin GVIA (3 microM) or nifedipine (10 microM) to inhibit Ca(2+) influx through N- or L-type voltage-dependent calcium channels (VDCCs), respectively, also decreased the rate of AP repolarization and increased AP duration. Elimination of CICR by treatment with either thapsigargin (1 microM) or ryanodine (10 microM) produced no significant change in AP repolarization or duration. Blockade of BK channels with IBX and inhibition of N-type VDCCs with omega-conotoxin GVIA, but not inhibition of L-type VDCCs with nifedipine, decreased the latency of AP generation. A decrease in latency to AP generation occurred with elimination of SMHs by inhibition of CICR following treatment with thapsigargin. Ryanodine treatment decreased AP latency in three of six cells. Apamin (100 nM) had no affect on AP repolarization, duration, or latency to AP generation, but did decrease the hyperpolarizing afterpotential (HAP). Inhibition of L-type VDCCs by nifedipine also decreased HAP amplitude. Inhibition of CICR by either thapsigargin or ryanodine treatment increased the number of APs generated with long depolarizing current pulses, whereas exposure to IBX or omega-conotoxin GVIA depressed excitability. We conclude that CICR, the process responsible for SMH generation, represents a unique mechanism to modulate the response to subthreshold depolarizing currents that drive the membrane potential toward the threshold for AP initiation but does not contribute to AP repolarization. Subthreshold depolarizations would not activate sufficient numbers of VDCCs to allow Ca(2+) influx to elevate [Ca(2+)](i) to the extent needed to directly activate nearby BK channels. However, the elevation in [Ca(2+)](i) is sufficient to trigger CICR from ryanodine-sensitive Ca(2+) stores. Thus CICR acts as an amplification mechanism to trigger a local elevation of [Ca(2+)](i) near a cluster of BK channels to activate these channels at negative levels of membrane potential.

Post-natal knockout of prion protein alters hippocampal CA1 properties, but does not result in neurodegeneration.

Prion protein (PrP) plays a crucial role in prion disease, but its physiological function remains unclear. Mice with gene deletions restricted to the coding region of PrP have only minor phenotypic deficits, but are resistant to prion disease. We generated double transgenic mice using the Cre-loxP system to examine the effects of PrP depletion on neuronal survival and function in adult brain. Cre-mediated ablation of PrP in neurons occurred after 9 weeks. We found that the mice remained healthy without evidence of neurodegeneration or other histopathological changes for up to 15 months post-knockout. However, on neurophysiological evaluation, they showed significant reduction of afterhyperpolarization potentials (AHPs) in hippocampal CA1 cells, suggesting a direct role for PrP in the modulation of neuronal excitability. These data provide new insights into PrP function. Furthermore, they show that acute depletion of PrP does not affect neuronal survival in this model, ruling out loss of PrP function as a pathogenic mechanism in prion disease and validating therapeutic approaches targeting PrP.

Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycle.

Human nerve fibers exhibit a distinct pattern of threshold fluctuation following a single action potential known as the recovery cycle. We developed geometrically and electrically accurate models of mammalian motor nerve fibers to gain insight into the biophysical mechanisms that underlie the changes in axonal excitability and regulate the recovery cycle. The models developed in this study incorporated a double cable structure, with explicit representation of the nodes of Ranvier, paranodal, and internodal sections of the axon as well as a finite impedance myelin sheath. These models were able to reproduce a wide range of experimental data on the excitation properties of mammalian myelinated nerve fibers. The combination of an accurate representation of the ion channels at the node (based on experimental studies of human, cat, and rat) and matching the geometry of the paranode, internode, and myelin to measured morphology (necessitating the double cable representation) were needed to match the model behavior to the experimental data. Following an action potential, the models generated both depolarizing (DAP) and hyperpolarizing (AHP) afterpotentials. The model results support the hypothesis that both active (persistent Na(+) channel activation) and passive (discharging of the internodal axolemma through the paranodal seal) mechanisms contributed to the DAP, while the AHP was generated solely through active (slow K(+) channel activation) mechanisms. The recovery cycle of the fiber was dependent on the DAP and AHP, as well as the time constant of activation and inactivation of the fast Na(+) conductance. We propose that experimentally documented differences in the action potential shape, strength-duration relationship, and the recovery cycle of motor and sensory nerve fibers can be attributed to kinetic differences in their nodal Na(+) conductances.

Somatostatin modulates Ca 2+ currents in neostriatal neurons

Somatostatin is synthesized and released by aspiny interneurons of the neostriatum. This work investigates the actions of somatostatin on rat neostriatal neurons of medium size (ca. 6 pF). Somatostatin (1 μM) reduces both calcium action potentials (20 mM tetraethylammonium) by ca. 24% and calcium currents by ca. 35%, in all cells tested. This action was produced in the presence of tetrodotoxin and in dissociated cells and was blocked by cyclo(-7-aminoheptanoyl-phe-d-try-lys-O-benzyl-thr) acetate (CPP-1), a somatostatin receptor antagonist. Except for nitrendipine (5 μM), several calcium channel antagonists, 1 μM ω-conotoxin GVIA, 400 nM ω-agatoxin TK, and 1 μM ω-conotoxin MVIIC, partially occluded somatostatin action. According to the calcium channel types known to be blocked by these antagonists, P/Q-type channels appeared to be the channels mainly modulated by somatostatin, followed by N-type channels. Since these channel types generate the afterhyperpolarizing potential in spiny neurons, we investigated the action of somatostatin on this event. Somatostatin reduces the amplitude of the afterhyperpolarizing potential by ca. 39%. This action is occluded by ω-agatoxin TK and ω-conotoxin MVIIC but not by ω-conotoxin GVIA or nicardipine. Thus, the action of somatostatin on the afterhyperpolarizing potential is mainly mediated by P/Q-type calcium channels. The block of the slow afterhyperpolarizing potential made most neurons exhibit an irregular firing mode, suggesting that ion currents other than calcium may also be affected by somatostatin. We conclude that somatostatin exerts a direct postsynaptic effect on neostriatal neurons via the activation of somatostatin receptors. This action affects non-L-type calcium channels and therefore modifies the afterhyperpolarizing potential and the firing pattern. It is proposed that somatostatin and its analogues may have profound effects on the motor functions controlled by the basal ganglia.

The motor inhibitory system operating during active sleep is tonically suppressed by GABAergic mechanisms during other states.

The present study was undertaken to explore the neuronal mechanisms responsible for muscle atonia that occurs after the microinjection of bicuculline into the nucleus pontis oralis (NPO). Specifically, we wished to test the hypothesis that motoneurons are postsynaptically inhibited after the microinjection of bicuculline into the NPO and determine whether the inhibitory mechanisms are the same as those that are utilized during naturally occurring active (rapid eye movement) sleep. Accordingly, intracellular records were obtained from lumbar motoneurons in cats anesthetized with alpha-chloralose before and during bicuculline-induced motor inhibition. The microinjection of bicuculline into the NPO resulted in a sustained reduction in the amplitude of the spinal cord Ia-monosynaptic reflex. In addition, lumbar motoneurons exhibited significant changes in their electrophysiological properties [i.e., a decrease in input resistance and membrane time constant, a reduction in the amplitude of the action potential's afterhyperpolarization (AHP) and an increase in rheobase]. Discrete, large-amplitude inhibitory postsynaptic potentials (IPSPs) were also observed in high-gain recordings from lumbar motoneurons. These potentials were comparable to those that are only present during the state of naturally occurring active sleep. Furthermore, stimulation of the medullary nucleus reticularis gigantocellularis evoked a large-amplitude IPSP in lumbar motoneurons after, but never prior to, the injection of bicuculline; this reflects the pattern of motor responses that occur in conjunction with the phenomenon of "reticular response-reversal." The preceding changes in the electrophysiological properties of motoneurons, as well as the development of active sleep-specific IPSPs, indicate that lumbar motoneurons are postsynaptically inhibited following the intrapontine administration of bicuculline in a manner that is comparable to that which occurs spontaneously during the atonia of active sleep. The present results support the conclusion that the brain stem-spinal cord inhibitory system, which is responsible for motor inhibition during active sleep, can be activated by the injection of bicuculline into the NPO. These data suggest that the active sleep-dependent motor inhibitory system is under constant GABAergic inhibitory control, which is centered in the NPO. Thus during wakefulness and quiet sleep, the glycinergically mediated postsynaptic inhibition of motoneurons is prevented from occurring due to GABAergic mechanisms.

Temperature-sensitive properties of rat suprachiasmatic nucleus neurons.

The hypothalamic suprachiasmatic nucleus (SCN) contains a heterogeneous population of neurons, some of which are temperature sensitive in their firing rate activity. Neuronal thermosensitivity may provide cues that synchronize the circadian clock. In addition, through synaptic inhibition on nearby cells, thermosensitive neurons may provide temperature compensation to other SCN neurons, enabling postsynaptic neurons to maintain a constant firing rate despite changes in temperature. To identify mechanisms of neuronal thermosensitivity, whole cell patch recordings monitored resting and transient potentials of SCN neurons in rat hypothalamic tissue slices during changes in temperature. Firing rate temperature sensitivity is not due to thermally dependent changes in the resting membrane potential, action potential threshold, or amplitude of the fast afterhyperpolarizing potential (AHP). The primary mechanism of neuronal thermosensitivity resides in the depolarizing prepotential, which is the slow depolarization that occurs prior to the membrane potential reaching threshold. In thermosensitive neurons, warming increases the prepotential's rate of depolarization, such that threshold is reached sooner. This shortens the interspike interval and increases the firing rate. In some SCN neurons, the slow component of the AHP provides an additional mechanism for thermosensitivity. In these neurons, warming causes the slow AHP to begin at a more depolarized level, and this, in turn, shortens the interspike interval to increase firing rate.

Excitability of the human trigeminal motoneuronal pool and interactions with other brainstem reflex pathways.

We studied the properties of motoneurones and Ia-motoneuronal connections in the human trigeminal system, and their functional interactions with other brainstem reflex pathways mediated by non-muscular (Abeta) afferents. With surface EMG recordings we tested the recovery cycles of the heteronymous H-reflex in the temporalis muscle and the homonymous silent period in the masseter muscle both elicited by stimulation of the masseteric nerve at the infratemporal fossa in nine healthy subjects. In four subjects single motor-unit responses were recorded from the temporalis muscle. In six subjects we also tested the effect of the stimulus to the mental nerve on the temporalis H-reflex and, conversely, the effect of Ia input (stimulus to the masseteric nerve) on the R1 component of the blink reflex in the orbicularis oculi muscle. The recovery cycle of the H-reflex showed a suppression peaking at the 5-20 ms interval; conversely the time course of the masseteric silent period was facilitated at comparable intervals. The inhibition of the test H-reflex was inversely related to the level of background voluntary contraction. Single motor units were unable to fire consistently in response to the test stimulus at intervals shorter than 50 ms. Mental nerve stimulation strongly depressed the H-reflex. The time course of this inhibition coincided with the EMG inhibition elicited by mental nerve stimulation during voluntary contraction. The trigeminal Ia input facilitated the R1 component of the blink reflex when the supraorbital test stimulation preceded the masseteric conditioning stimulation by 2 ms. We conclude that the time course of the recovery cycle of the heteronymous H-reflex in the temporalis muscle reflects the after-hyperpolarization potential (AHP) of trigeminal motoneurones, and that the Ia trigeminal input is integrated with other brainstem reflexes.

Heterogeneity in the Basic Membrane Properties of Postnatal Gonadotropin-Releasing Hormone Neurons in the Mouse

The electrophysiological characteristics of unmodified, postnatal gonadotropin-releasing hormone (GnRH) neurons in the female mouse were studied using whole-cell recordings and single-cell RT-PCR methodology. The GnRH neurons of adult animals fired action potentials and exhibited distinguishable voltage-current relationships in response to hyperpolarizing and depolarizing current injections. On the basis of their patterns of inward rectification, rebound depolarization, and ability to fire repetitively, GnRH neurons in intact adult females were categorized into four cell types (type I, 48%; type II, 36%; type III, 11%; type IV, 5%). The GnRH neurons of juvenile animals (15-22 d) exhibited passive membrane properties similar to those of adult GnRH neurons, although only type I (61%) and type II (7%) cells were encountered, in addition to a group of "silent-type" GnRH neurons (32%) that were unable to fire action potentials. A massive, action potential-independent tonic GABA input, signaling through the GABA(A) receptor, was present at all ages. Afterdepolarization and afterhyperpolarization potentials (AHPs) were observed after single action potentials in subpopulations of each GnRH neuron type. Tetrodotoxin (TTX)-independent calcium spikes, as well as AHPs, were encountered more frequently in juvenile GnRH neurons compared with adults. These observations demonstrate the existence of multiple layers of functional heterogeneity in the firing properties of GnRH neurons. Together with pharmacological experiments, these findings suggest that potassium and calcium channels are expressed in a differential manner within the GnRH phenotype. This heterogeneity occurs in a development-specific manner and may underlie the functional maturation and diversity of this unique neuronal phenotype.

Propagation of synchronous burst discharges from entorhinal cortex to morphologically and electrophysiologically identified neurons of rat lateral amygdala

Intracellular and field potential recordings were taken from the lateral nucleus of the amygdala in a rat horizontal brain slice preparation that included hippocampal formation. Pyramidal cells comprised the majority of labeled cells (77%). Electrophysiological classification based on hyperpolarizing or depolarizing afterpotentials subdivided both the pyramidal and non-pyramidal cell classes, although pyramidal cells tended to have hyperpolarizing afterpotentials (70%) and non-pyramidal cells tended to have depolarizing afterpotentials (63%). Synchronous population bursts were triggered with single extracellular stimuli in the deep layers of entorhinal cortex. These events propagated from deep layers of entorhinal cortex into the lateral nucleus of the amygdala. Latencies were consistent with a direct entorhinal to amygdala projection. Individual lateral nucleus neurons exhibited responses ranging from a long burst response that included an initial period of 200 Hz firing and a tail of gamma frequency firing lasting over 100 ms (grade 1) to an epsp with no firing (grade 4). Half of pyramidal cells responding to events initiated in entorhinal cortex were found to receive epsps strong enough to trigger firing. Only one stellate neuron fired in response to entorhinal stimulation. Excitatory postsynaptic responses included NMDA and non-NMDA receptor mediated components. We demonstrate that synchronous population events can propagate from entorhinal cortex to the lateral nucleus of the amygdala and that pyramidal neurons of the lateral nucleus are more common targets than stellate neurons. We conclude that other synchronous events such as sharp waves and interictal spikes can spread from entorhinal cortex to amygdala in the same manner.

Development of potassium conductances in perinatal rat phrenic motoneurons.

Prior to the inception of inspiratory synaptic drive transmission from medullary respiratory centers, rat phrenic motoneurons (PMNs) have action potential and repetitive firing characteristics typical of immature embryonic motoneurons. During the period spanning from when respiratory bulbospinal and segmental afferent synaptic connections are formed at embryonic day 17 (E17) through to birth (gestational period is approximately 21 days), a pronounced transformation of PMN electrophysiological properties occurs. In this study, we test the hypothesis that the elaboration of action potential afterpotentials and the resulting changes in repetitive firing properties are due in large part to developmental changes in PMN potassium conductances. Ionic conductances were measured via whole cell patch recordings using a cervical slice-phrenic nerve preparation isolated from perinatal rats. Voltage- and current-clamp recordings revealed that PMNs expressed outward rectifier (I(KV)) and A-type potassium currents that regulated PMN action potential and repetitive firing properties throughout the perinatal period. There was an age-dependent leftward shift in the activation voltage and a decrease in the time-to-peak of I(KV) during the period from E16 through to birth. The most dramatic change during the perinatal period was the increase in calcium-activated potassium currents after the inception of inspiratory drive transmission at E17. Block of the maxi-type calcium-dependent potassium conductance caused a significant increase in action potential duration and a suppression of the fast afterhyperpolarizing potential. Block of the small conductance calcium-dependent potassium channels resulted in a marked suppression of the medium afterhyperpolarizing potential and an increase in the repetitive firing frequency. In conclusion, the increase in calcium-mediated potassium conductances are in large part responsible for the marked transformation in action potential shape and firing properties of PMNs from the time between the inception of fetal respiratory drive transmission and birth.