Contribution of the motor cortex to the structure and the timing of hindlimb locomotion in the cat: a microstimulation study

We used microstimulation to examine the contribution of the motor cortex to the structure and timing of the hindlimb step cycle during locomotion in the intact cat. Stimulation was applied to the hindlimb representation of the motor cortex in 34 sites in three cats using either standard glass-insulated microelectrodes (16 sites in 1 cat) or chronically implanted microwire electrodes (18 sites in 2 cats). Stimulation at just suprathreshold intensities with the cat at rest produced multi-joint movements at a majority of sites (21/34, 62%) but evoked responses restricted to a single joint, normally the ankle, at the other 13/34 (38%) sites. Stimulation during locomotion generally evoked larger responses than the same stimulation at rest and frequently activated additional muscles. Stimulation at all 34 sites evoked phase-dependent responses in which stimulation in swing produced transient increases in activity in flexor muscles while stimulation during stance produced transient decreases in activity in extensors. Stimulation with long (200 ms) trains of stimuli in swing produced an increased level of activity and duration of flexor muscles without producing changes in cycle duration. In contrast, stimulation during stance decreased the duration of the extensor muscle activity and initiated a new and premature period of swing, resetting the step cycle. Stimulation of the pyramidal tract in two of these three cats as well as in two additional ones produced similar effects. The results show that the motor cortex is capable of influencing hindlimb activity during locomotion in a similar manner to that seen for the forelimb.     

Effects of excitation and inactivation in area 17 on paired cells in area 18

This investigation examines how neighboring neurons of area 18 react when area 17 inputs are excited or depressed. In anesthetized cats, area 18 responses to a sine-wave grating in the receptive field were analyzed, while a second grating was positioned in its periphery and responses were recorded in area 17. This latter site was also inactivated with GABA. A waveform template process sorted out at least two individual, neighboring cells with similar orientation preferences in area 18. These cells frequently displayed opposite reactions to stimulation and inactivation in area 17. Experiments suggest that nearby neurons belonging to the same functional domain in the visual cortex may simultaneously carry disparate information.     

Neonatal maternal separation opposes the facilitatory effect of castration on the respiratory response to hypercapnia of the adult male rat: Evidence for the involvement of the medial amygdala

Respiratory manifestations of panic disorder (PD) include a greater respiratory instability and enhanced responsiveness to CO₂ compared to normal individuals. Although the prevalence of PD is approximately three times greater in women compared to men, the origins of this sexual dimorphism remain poorly understood. Similar to PD patients, adult female rats previously subjected to neonatal maternal separation (NMS) show an increase in their ventilatory response to CO₂. Because this effect of NMS is not observed in males, we hypothesised that testosterone prevents NMS‐induced hyper‐responsiveness to CO₂. Pups subjected to NMS were placed in an incubator for 3 h d^‐1 from postnatal days 3‐12. Control pups remained undisturbed. At adulthood (8‐10 weeks of age), rats were then subjected either to sham surgery or castration. Fourteen days later, breathing was measured at rest (room air) and during acute exposure to hypercapnia (5 and 10% CO₂ for 10 minutes each) using plethysmography. To gain insight into the mechanisms involved, c‐fos expression was used as an indicator of neuronal activation. Brains were collected following air or CO₂ exposure for quantification of c‐fos positive cells by immunohistochemistry in selected regions, including the paraventricular nucleus of the hypothalamus, the dorsomedial hypothalamus and the amygdalar complex. Castration produced a 100% increase of hyperventilatory response to 10% CO₂ in control rats. Unexpectedly, castration had no effect on the hyperventilatory response of NMS rats. The intensity of the hypercapnic response was inversely correlated with c‐fos expression in the medial amygdala. We conclude that testosterone prevents the hyper‐responsiveness to CO₂, whereas NMS attenuates sensitivity to hormone withdrawal. We propose that an inhibitory influence from the medial amygdala contributes to this effect.     

Combination of olfactory ensheathing cells with local versus systemic cAMP treatment after a cervical rubrospinal tract injury

The failure of CNS axons to regenerate following traumatic injury is due in part to a growth‐inhibitory environment in CNS as well as a weak intrinsic neuronal growth response. Olfactory ensheathing cell (OECs) transplants have been reported to create a favorable environment promoting axonal regeneration, remyelination, and functional recovery after spinal cord injury. However, in our previous experiments, OEC transplants failed to promote regeneration of rubrospinal axons through and beyond the site of a dorsolateral funiculus crush in rats. Rubrospinal neurons undergo massive cell atrophy and limited expression of regeneration‐associated genes after axotomy. Using the same injury model, we tested the hypothesis that treatment of the red nucleus with cAMP, known to stimulate the intrinsic growth response in other neurons, will promote rubrospinal regeneration in combination with OEC transplants. In addition, we assessed a systemic increase of cAMP using the phosphodiesterase inhibitor rolipram. OECs prevented cavity formation, attenuated astrocytic hypertrophy and the retraction of the axotomized rubrospinal axons, and tended to reduce the overall lesion size. OEC transplantation lowered the thresholds for thermal sensitivity of both forepaws. None of our treatments, alone or in combination, promoted rubrospinal regeneration through the lesion site. However, the systemic elevation of cAMP with rolipram resulted in greater numbers of OECs and axonal density within the graft and improved motor performance in a cylinder test in conjunction with enhanced rubrospinal branching and attenuated astrocytic hypertrophy. © 2010 Wiley‐Liss, Inc.     

Influence of NO downregulation on oscillatory evoked responses in developing rat superior colliculus

Nitric oxide (NO) is involved in neuronal transmission by modulating neurotransmitter release in adults and in stabilizing synaptic connections in developing brains. We investigated the influence of downregulation of NO synthesis on oscillatory components of ON and OFF evoked field potentials in the rat superior colliculus. NO synthesis was decreased by inhibiting nitric oxide synthase (NOS) with an acute microinjection of N(omega)-nitro-L-arginine methyl ester (L-NAME). The study focuses on rhythmic activity by analyzing fast Fourier transform (FFT). Collicular responses were recorded in anesthetized rats, at postnatal days (PND) 13-19 and adults. This time window was chosen because it is centered on eye opening. NO downregulation resulted in a dual effect depending on age and response-type. NO synthesis inhibition decreased the magnitude of oscillations in ON responses in the youngest animals (PND13-14), whereas oscillations of frequencies higher than 20 Hz in OFF responses were increased in all age groups of developing rats. In adults NO downregulation increased oscillations in ON responses and decreased oscillations in OFF responses. L-Arginine was used to increase NOS activity and its injection produced effects opposite to those seen with L-NAME. Slow oscillatory components (7-12 Hz) were unaffected in all experiments. Our data together with results reported in the literature suggest that rhythmic patterns of activity are NO-dependent.     

Changes in corticospinal efficacy contribute to the locomotor plasticity observed after unilateral cutaneous denervation of the hindpaw in the cat

We used microwire electrodes chronically implanted into the hindlimb representation of the motor cortex as well as into the pyramidal tract to test the hypothesis that the corticospinal system contributes to the locomotor plasticity that is observed after cutaneous denervation of the cat hindpaw. A total of 23 electrodes implanted into the motor cortex in three cats trained to walk on a treadmill produced phase-dependent, short-latency, twitch responses in hindlimb flexor and extensor muscles during locomotion. After a unilateral cutaneous denervation of the hindpaw, the cats showed transient deficits in locomotion, including a dragging of the hindpaw along the treadmill belt during the swing phase. This deficit rapidly recovered over the course of a few days. The recovery of locomotion was accompanied by an increase in the magnitude of the responses evoked in different muscles by the cortical stimulation at all 23 cortical sites. Response magnitude increased rapidly within the first 1-2 wk postdenervation before attaining a plateau at > or =3 wk. In two cats, for which detailed information was obtained, response magnitude in the knee flexor, semitendinosus (St), was increased by >250% at 14/18 sites (mean increase = 1,235%). Increased responses in the St to stimulation were also observed at two of the four pyramidal tract sites after the denervation but were relatively smaller (max = 593%) than those evoked by the cortical stimulation. We suggest that the denervation produces changes in both cortical and spinal excitability that, together, produce a change in corticospinal efficacy that contributes to the recovery of locomotor function.     

Undesired effects of a combinatorial treatment for spinal cord injury--transplantation of olfactory ensheathing cells and BDNF infusion to the red nucleus

Transplantations of olfactory ensheathing cells (OECs) have been reported to promote axonal regeneration and functional recovery after spinal cord injury, but have demonstrated limited growth promotion of rat rubrospinal axons after a cervical dorsolateral funiculus crush. Rubrospinal neurons undergo massive atrophy after cervical axotomy and show only transient expression of regeneration-associated genes. Cell body treatment with brain-derived neurotrophic factor (BDNF) prevents this atrophy, stimulates regeneration-associated gene expression and promotes regeneration of rubrospinal axons into peripheral nerve transplants. Here, we hypothesized that the failure of rubrospinal axons to regenerate through a bridge of OEC transplants was due to this weak intrinsic cell body response. Hence, we combined BDNF treatment of rubrospinal neurons with transplantation of highly enriched OECs derived from the nasal mucosa and assessed axonal regeneration as well as behavioral changes after a cervical dorsolateral funiculus crush. Each treatment alone as well as their combination prevented the dieback of the rubrospinal axons, but none of them promoted rubrospinal regeneration beyond the lesion/transplantation site. Motor performance in a food-pellet reaching test and forelimb usage during vertical exploration (cylinder test) were more impaired after combining transplantation of OECs with BDNF treatment. This impaired motor performance correlated with lowered sensory thresholds in animals receiving the combinatorial therapy - which were not seen with each treatment alone. Only this combinatorial treatment group showed enhanced sprouting of calcitonin gene-related peptide-positive axons rostral to the lesion site. Hence, some combinatorial treatments, such as OECs with BDNF, may have undesired effects in the injured spinal cord.