John B. Munson

Adult Mammalian Sensory and Motor Neurons: Roles of Endogenous Neurotrophins and Rescue by Exogenous Neurotrophins after Axotomy

We have tested the ability of neurotrophins to reverse axotomy-induced changes in adult motor and sensory neurons, using the physiological measure of conduction velocity. Five weeks after axotomy, sensory and motor conduction velocities were greatly reduced. NT-3 at 60 μg/d, pumped directly onto the cut nerve stump, largely prevented the change in sensory fibers. Lower doses were less effective, and NT-4/5 was without effect. In contrast, both NT-3 and NT-4/5 were effective at rescuing motoneurons, with similar dose dependencies. This amelioration of physiological deficits in adult mammalian neurons suggests possible therapeutic application of neurotrophins. We have also studied the physiological effects of neurotrophin deprivation on intact peripheral neurons. After 2 weeks of sequestration of trkB ligands (BDNF and NT-4/5), motor, but not sensory, neuron conduction was significantly slowed. Sequestration of NT-3 was found to affect both motor and sensory fiber velocities but more modestly and only with higher doses of sequestering agent. These data therefore suggest that peripherally produced neurotrophins are necessary for the maintenance of normal functional properties of peripheral neurons.

Neurotrophin Modulation of the Monosynaptic Reflex after Peripheral Nerve Transection

The effects of neurotrophin-3 (NT-3) and NT-4/5 on the function of axotomized group Ia afferents and motoneurons comprising the monosynaptic reflex pathway were investigated. The axotomized medial gastrocnemius (MG) nerve was provided with NT-3 or NT-4/5 for 8–35 d via an osmotic minipump attached to its central end at the time of axotomy. After this treatment, monosynaptic EPSPs were recorded intracellularly from MG or lateral gastrocnemius soleus (LGS) motoneurons in response to stimulation of the heteronymous nerve under pentobarbital anesthesia. Controls were preparations with axotomized nerves treated directly with vehicle; other axotomized controls were administered subcutaneous NT-3. Direct NT-3 administration (60 μg/d) not only prevented the decline in EPSP amplitude from axotomized afferents (stimulate MG, record LGS) observed in axotomy controls but, after 5 weeks, led to EPSPs larger than those from intact afferents. These central changes were paralleled by recovery of group I afferent conduction velocity. Removal of NT-3 4–5 weeks after beginning treatment resulted in a decline of conduction velocity and EPSP amplitude within 1 week to values characteristic of axotomy. The increased synaptic efficacy after NT-3 treatment was associated with enhanced connectivity of single afferents to motoneurons. NT-4/5 induced modest recovery in group I afferent conduction velocity but not of the EPSPs they elicited. NT-3 or NT-4/5 had no effect on the properties of treated motoneurons or their monosynaptic EPSPs. We conclude that NT-3, and to a limited extent NT-4/5, promotes recovery of axotomized group Ia afferents but not axotomized motoneurons or the synapses on them.

Basis of segmental motor control: motoneuron size or motor unit type?

The principles of organization of motor control at the segmental level are developed and discussed in this review. Consideration is given to the concepts of the motor unit, and the motoneuron pool. Recent studies from our laboratories that have significance for hypotheses regarding segmental motor control are presented. These studies indicate that the critical factor controlling motor unit recruitment in heterogenous muscles is motor unit type. This results in motor unit recruitment in the order of increasing contraction strength and fatigability: slow twitch, fatigue-resistant first; fast twitch, fatigue-resistant second; fast twitch with intermediate fatigue resistance third; and fast twitch, fatigable units last. A recruitment model that incorporates this hypothesis is presented in which there is an orderly recruitment of motor units by type. This recruitment model, based on data from cat medial gastrocnemius motor units, closely approximates a theoretical optimal recruitment strategy and is consistent with actual medial gastrocnemius muscle forces generated during free movements in intact animals.

A role for nerve growth factor in sympathetic sprouting in rat dorsal root ganglia

The role of nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF) in sympathetic sprouting within the dorsal root ganglion was investigated. In nerve-intact rats, intrathecal NGF (1 mg/ml, 14 days) but not GDNF (1 mg/ml, 14 days) induced extensive sprouting of tyrosine hydroxylase immunoreactive (TH-IR) fibres and formation of pericellular TH-IR baskets within lumbar DRGs. TH-IR baskets were distributed equally to trkA-expressing and trkA-negative neuronal profiles. Sciatic nerve transection (14-21 days) induced TH-IR baskets within lumbar DRGs around neuronal profiles with both intact and lesioned axons. The percentage of neuronal profiles surrounded by TH-IR baskets following sciatic transection was unaffected following peripheral application of the NGF sequestering antibody, trkA-IgG (1 mg/ml, 14 days). Intracellular responses were recorded from sensory neurons in an in vitro DRG/peripheral nerve preparation following bath application of noradrenaline. In preparations from animals treated 14 days previously with intrathecal NGF, 69% of neurons responded with depolarizing responses whilst 18% of neurons responded to bath applied noradrenaline in tissue prepared from naive animals. Our data indicate that sympathetic neurons sprout into the DRG in response to sciatic nerve injury and intrathecal NGF but not GDNF. Distribution of sympathetic sprouts within the DRG is independent of whether target neurons are injured or express trkA. Sequestration of NGF at the peripheral injury site does not influence basket formation within the DRG. It is likely that functional noradrenergic connections exist between sympathetic sprouts and sensory neuron cell bodies following exogenous NGF.

Lateral geniculate and occipital cortex spikes with eye movements in awake and sleeping cats: Temporal and functional correlations

Abstract The temporal relationship between lateral geniculate and occipital cortex spikes and the fast and slow components of nystagmus and of fast-wave sleep rapid eye movements were investigated, using cats bearing chronically implanted electrodes. Computer averaging of the electrical activity from the electrooculogram, from occipital cortex, and from lateral geniculate nucleus (using the latter as the trigger point) demonstrated that the lateral geniculate spike is in all cases initiated at the fixation point of eye movements, i.e., at the inflection from fast to slow eye movement. Since phasic activation of midbrain reticular formation is considered to occur with the lateral geniculate spikes, it appears that the spikes normally function to produce an augmented reactivity at the moment of eye fixation.

Lateral geniculate spikes in sleeping, awake, and reserpine-treated cats: Correlated excitability changes in superior colliculus and related structures

Abstract Spiking activity in lateral geniculate nucleus (LGN) may be recorded from cats during fast-wave sleep, with awake eye movements and following drug administration. The present experiments were undertaken to analyze visual system excitability changes correlated with these LGN spikes in fast-wave sleep, and to test whether comparable changes occur correlated with the LGN spikes in other conditions. Cats bearing chronically implanted electrodes were used to study the excitability of superior colliculus and related structures as a function of time during and following LGN spikes recorded under three conditions: fast-wave sleep; following reserpine administration; and with awake eye movements. Under these conditions, postsynaptic potentials in superior colliculus, cerebellum, pontine reticular formation, and visual cortex increased in amplitude throughout a 300-msec period beginning with the LGN spike. Eye movements elicited by optic nerve stimulation increased in amplitude along with postsynaptic potentials. Changes in response amplitude in all structures investigated were similar in the reserpinetreated and awake eye movement conditions to those of fast-wave sleep. This indicates that similar neuronal activity is associated with the LGN spikes of these three states. LGN spikes appear to signify neither a blanking circuit nor a corollary discharge, but rather are related to phasic activity in the midbrain reticular formation.

Eye movement potentials following visual deafferentation

Abstract Cats bearing chronically implanted electrodes were used to study the effects of uni- or bilateral optic nerve (ON) section on eye movement-related electrical activity in lateral geniculate nucleus (LGN) and visual cortex (VC). Section of one ON reduced the amplitude of all LGN spikes bilaterally by about 25% commencing about the fourth day. Subsequent section of the second ON was followed in about 4 days by further reduction of all LGN spike amplitudes to about 50% of control values except for LGN spikes accompanying nystagmus in alert cats, which disappeared. One-stage bilateral ON section also produced all of these latter effects on about the fourth day. Bilateral more than unilateral ON section intensified VC spiking during fast-wave sleep (FWS). The rhythmic 3–5 Hz LGN activity seen during FWS was unaffected by ON section. Therefore: (1) ON terminals in part generate LGN spikes; (2) the 3–5 Hz LGN activity does not depend upon retinal input; (3) mechanisms generating LGN spikes in sleeping and reserpine-treated cats in part differ from those active in alert cats.

Localization of lateral geniculate spikes in alert, sleeping and reserpine-treated cats ☆ ☆☆

Abstract Experiments were undertaken to explain the apparent paradox wherein it was often found that LGN spikes accompanying eye movements could be recorded from sleeping and reserpine-treated cats, but not from alert cats. Electrode placements in thirty LGNs of seventeen chronically implanted cats were studied and classified according to the conditions under which eye movement-associated LGN spikes could be recorded from them. Three types of placements were found: type SR , producing LGN spikes only in the sleeping and reserpine-treated cat; type ASR , producing LGN spikes in the alert as well as the sleeping and reserpine-treated cat; and type O , from which LGN spikes were never recorded. Type ASR sites were found in the body of LGN and adjacent optic tract terminations; type SR sites were immediately adjacent to type ASR sites. The larger amplitude of LGN spikes of the sleeping or reserpine-treated cat probably allows them to be recorded by volume conduction at a distance from their site of generation in LGN.

Chapter 14 Neurotrophin—3 and Maintenance of Muscle Afferent Function

Publisher Summary Axotomizing peripheral cutaneous and muscle nerves and thus depriving them of contact with their peripheral targets result in slowed conduction of their action potentials. Reinnervation of their peripheral targets results in the recovery of conduction velocity. These results suggest the sufficiency of the normal peripheral target for the maintenance of at least one measure of normal physiological function (conduction velocity), and further suggest the presence of some substance(s) in the target tissue that is capable of acting in a supportive trophic manner on the innervating afferent fibers. Groups I and II muscle afferents of cats uniformly exhibit several physiological properties: they are rapidly conducting (24–120 m/s), have mechanosensitive receptive fields in muscle, are slowly adapting in response to a maintained stimulus and are stretch-sensitive, and generate monosynaptic excitatory postsynaptic potentials (EPSPs) in homonymous and heteronymous spinal motoneurons. In the experiments described in this chapter, it has been investigated (1) whether maintenance of these various other physiological properties of the peripheral sensory nerves also depends upon target innervation and (2) whether the reinnervation by afferents of foreign and native target tissues is sufficient for the maintenance of these properties. The neurotrophin NT-3, along with its high affinity receptor (tyrosine kinase-C, trkC), has been shown to be essential for the aspects of normal development during the prenatal period.

Enhancement of c-fos Expression in Neurons of the Rat Spinal Cord after Partial Denervation: Evidence for Functional Plasticity

c-fos immunocytochemistry was used to test a functional correlate of neuroplasticity involving nociceptive primary afferent fibers in the partially denervated adult rat spinal cord. Unilateral dorsal root ganglionectomies were made at L1-L4 (chronic side). After 3 weeks, contralateral L1-L4 ganglionectomies (acute side) were made 1 week prior to sacrifice. Two hours prior to perfusion, the animals were anesthetized and their hindlimbs were immersed in a 52 degrees C water bath for 20 s. The spinal cords were then processed for c-fos immunocytochemistry and the numbers of c-fos-immunoreactive cells determined. Following bilateral noxious thermal stimulation of the hindlimbs, the numbers of c-fos-immunoreactive cells in laminae I-II from L3 to L5 were increased by nearly twofold on the chronically deafferented side of the spinal cord (P < 0.05). This finding suggests augmented functional plasticity of nociceptive primary afferent fibers on the chronically denervated side of spinal cord. These observations are discussed with relation to recent demonstrations of enhanced calcitonin gene-related peptide immunoreactivity and possible primary afferent fiber sprouting in similar denervation models.