Biomedical subjects

Michael L Molineux

Publications and source records attributed to Michael L Molineux.

7 recordsLinked to original sources

PubMed2006

Specific T-type calcium channel isoforms are associated with distinct burst phenotypes in deep cerebellar nuclear neurons.

T-type calcium channels are thought to transform neuronal output to a burst mode by generating low voltage-activated (LVA) calcium currents and rebound burst discharge. In this study we assess the expression pattern of the three different T-type channel isoforms (Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3) in cerebellar neurons and focus on their potential role in generating LVA spikes and rebound discharge in deep cerebellar nuclear (DCN) neurons. We detected expression of one or more Ca(v)3 channel isoforms in a wide range of cerebellar neurons and selective expression of different isoforms in DCN cells. We further identify two classes of large-diameter DCN neurons that exhibit either a strong or weak capability for rebound discharge, despite the ability to generate LVA spikes when calcium currents are pharmacologically isolated. By correlating the Ca(v)3 channel expression pattern with the electrophysiological profile of identified DCN cells, we show that Ca(v)3.1 channels are expressed in isolation in DCN-burst cells, whereas Ca(v)3.3 is expressed in DCN-weak burst cells. Ca(v)3.1-expressing DCN cells correspond to excitatory or GABAergic neurons, whereas Ca(v)3.3-expressing cells are non-GABAergic. The Ca(v)3 class of LVA calcium channels is thus expressed in specific combinations in a wide range of cerebellar neurons but contributes to rebound burst discharge in only a select number of cell classes.

Amino Acid Sequence↗

PubMed2006

Ca(V)3 T-type calcium channel isoforms differentially distribute to somatic and dendritic compartments in rat central neurons.

Spike output in many neuronal cell types is affected by low-voltage-activated T-type calcium currents arising from the Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3 channel subtypes and their splice isoforms. The contributions of T-type current to cell output is often proposed to reflect a differential distribution of channels to somatic and dendritic compartments, but the subcellular distribution of the various rat T-type channel isoforms has not been fully determined. We used subtype-specific Ca(v)3 polyclonal antibodies to determine their distribution in key regions of adult Sprague-Dawley rat brain thought to exhibit T-type channel expression, and in particular, dendritic low-voltage-activated responses. We found a selective subcellular distribution of Ca(v)3 channel proteins in cell types of the neocortex and hippocampus, thalamus, and cerebellar input and output neurons. In general, the Ca(v)3.1 T-type channel immunolabel is prominent in the soma/proximal dendritic region and Ca(v)3.2 immunolabel in the soma and proximal-mid dendrites. Ca(v)3.3 channels are distinct in distributing to the soma and over extended lengths of the dendritic arbor of particular cell types. Ca(v)3 distribution overlaps with cell types previously established to exhibit rebound burst discharge as well as those not recognized for this activity. Additional immunolabel in the region of the nucleus in particular cell types was verified as corresponding to Ca(v)3 antigen through analysis of isolated protein fractions. These results provide evidence that different Ca(v)3 channel isoforms may contribute to low-voltage-activated calcium-dependent responses at the somatic and dendritic level, and the potential for T-type calcium channels to contribute to multiple aspects of neuronal activity.

Animals↗

PubMed2005

A-type and T-type currents interact to produce a novel spike latency-voltage relationship in cerebellar stellate cells.

The modification of first-spike latencies by low-threshold and inactivating K+ currents (IA) have important implications in neuronal coding and synaptic integration. To date, cells in which first-spike latency characteristics have been analyzed have shown that increased hyperpolarization results in longer first-spike latencies, producing a monotonic relationship between first-spike latency and membrane voltage. Previous work has established that cerebellar stellate cells express members of the Kv4 potassium channel subfamily, which underlie IA in many central neurons. Spike timing in stellate cells could be particularly important to cerebellar output, because the discharge of even single spikes can significantly delay spike discharge in postsynaptic Purkinje cells. In the present work, we studied the first-spike latency characteristics of stellate cells. We show that first-spike latency is nonmonotonic, such that intermediate levels of prehyperpolarization produce the longest spike latencies, whereas greater hyperpolarization or depolarization reduces spike latency. Moreover, the range of first-spike latency values can be substantial in spanning 20-128 ms with preceding membrane shifts of <10 mV. Using patch clamp and modeling, we illustrate that spike latency characteristics are the product of an interplay between IA and low-threshold calcium current (IT) that requires a steady-state difference in the inactivation parameters of the currents. Furthermore, we show that the unique first-spike latency characteristics of stellate cells have important implications for the integration of coincident IPSPs and EPSPs, such that inhibition can shift first-spike latency to differentially modulate the probability of firing.

Action Potentials↗

PubMed2005

Kv1 K+ channels control Purkinje cell output to facilitate postsynaptic rebound discharge in deep cerebellar neurons.

Purkinje cells (PCs) generate the sole output of the cerebellar cortex and govern the timing of action potential discharge from neurons of the deep cerebellar nuclei (DCN). Here, we examine how voltage-gated Kv1 K+ channels shape intrinsically generated and synaptically controlled behaviors of PCs and address how the timing of DCN neuron output is modulated by manipulating PC Kv1 channels. Kv1 channels were studied in cerebellar slices at physiological temperatures with Kv1-specific toxins. Outside-out voltage-clamp recordings indicated that Kv1 channels are present in both somatic and dendritic membranes and are activated by Na+ spike-clamp commands. Whole-cell current-clamp recordings revealed that Kv1 K+ channels maintain low frequencies of Na+ spike and Ca-Na burst output, regulate the duration of plateau potentials, and set the threshold for Ca2+ spike discharge. Kv1 channels shaped the characteristics of climbing fiber (CF) responses evoked by extracellular stimulation or intracellular simulated EPSCs. In the presence of Kv1 toxins, CFs discharged spontaneously at approximately 1 Hz. Finally, "Kv1-intact" and "Kv1-deficient" PC tonic and burst outputs were converted to stimulus protocols and used as patterns to stimulate PC axons and synaptically activate DCN neurons. We found that the Kv1-intact patterns facilitated short-latency and high-frequency DCN neuron rebound discharges, whereas DCN neuron output timing was markedly disrupted by the Kv1-deficient stimulus protocols. Our results suggest that Kv1 K+ channels are critical for regulating the excitability of PCs and CFs and optimize the timing of PC outputs to generate appropriate discharge patterns in postsynaptic DCN neurons.

Action Potentials↗

PubMed2005

High-threshold K+ current increases gain by offsetting a frequency-dependent increase in low-threshold K+ current.

High-frequency firing neurons are found in numerous central systems, including the auditory brainstem, thalamus, hippocampus, and neocortex. The kinetics of high-threshold K+ currents (IK(HT)) from the Kv3 subfamily has led to the proposal that these channels offset cumulative Na+ current inactivation and stabilize tonic high-frequency firing. However, all high-frequency firing neurons, examined to date, also express low-threshold K+ currents (IK(LT)) that have slower kinetics and play an important role in setting the subthreshold and filtering properties of the neuron. IK(LT) has also been shown to dampen excitability and is therefore likely to oppose high-frequency firing. In this study, we examined the role of IK(HT) in pyramidal cells of the electrosensory lobe of weakly electric fish, which are characterized by high-frequency firing, a very wide frequency range, and high levels of IK(HT). In particular, we examined the mechanisms that allow IK(HT) to set the gain of the F-I relationship by interacting with another low-threshold K+ current. We found that IK(HT) increases the gain of the F-I relationship and influences spike waveform almost exclusively in the high-frequency firing range. The frequency dependence arises from IK(HT) influencing both the IK(LT) and Na+ currents. IK(HT) thus plays a significant role in stabilizing high-frequency firing by preventing a steady-state accumulation of IK(LT) that is as important as preventing Na+ current inactivation.

Action Potentials↗

PubMed2004

Biotin is endogenously expressed in select regions of the rat central nervous system.

The vitamin biotin is an endogenous molecule that acts as an important cofactor for several carboxylases in the citric acid cycle. Disorders of biotin metabolism produce neurological symptoms that range from ataxia to sensory loss, suggesting the presence of biotin in specific functional systems of the CNS. Although biotin has been described in some cells of nonmammalian nervous systems, the distribution of biotin in mammalian CNS is virtually unknown. We report the presence of biotin in select regions of rat CNS, as revealed with a monoclonal antibody directed against biotin and with avidin- and streptavidin-conjugated labels. Detectable levels of biotin were primarily found caudal to the diencephalon, with greatest expression in the cerebellar motor system and several brainstem auditory nuclei. Biotin was found as a somatic label in cerebellar Purkinje cells, in cell bodies and proximal dendrites of cerebellar deep nuclear neurons, and in red nuclear neurons. Biotin was detected in cells of the spiral ganglion, somata and proximal dendrites of cells in the cochlear nuclei, superior olivary nuclei, medial nucleus of the trapezoid body, and nucleus of the lateral lemniscus. Biotin was further found in pontine nuclei and fiber tracts, the substantia nigra pars reticulata, lateral mammillary nucleus, and a small number of hippocampal interneurons. Biotin was detected in glial cells of major tract systems throughout the brain but was most prominent in tracts of the hindbrain. Biotin is thus expressed in select regions of rat CNS with a distribution that correlates to the known clinical sequelae associated with biotin deficiencies.

Animals↗

PubMed2004

Releasing the peri-neuronal net to patch-clamp neurons in adult CNS.

The extracellular matrix of adult neural tissue contains chondroitin sulphated proteogylcans that form a dense peri-neuronal net surrounding the cell body and proximal dendrites of many neuronal classes. Development of the peri-neuronal net beyond approximately postnatal day 17 obscures visualization and often access by patch electrodes to neuronal membranes with the result that patch clamp recordings are most readily obtained from early postnatal animals. We describe a technique in which the surface tension of a sucrose-based medium promotes partial dissociation of thin tissue slices from adult tissue. Surface tension spreads the tissue and loosens the peri-neuronal net from neuronal membranes within minutes and in the absence of proteolytic enzymes. Furthermore, the extent of dissociation can be controlled so as to maintain the overall slice structure and allow identification of specific cell classes. Excellent structural preservation of neurons and dendrites can be obtained and full access by patch electrodes made possible for current- or voltage-clamp recordings in tissue well beyond the development of peri-neuronal nets. We demonstrate the feasibility of using this approach through patch recordings from neurons in the brainstem and cerebellum of adult gymnotiform fish and in deep cerebellar nuclei of rats as old as 6 months.

Animals↗