A simple method for glass insulating tungsten microelectrodes

A simple and comprehensive method for the construction, repair and recycling of single and double tungsten microelectrodes

A method for the construction, repair and recycling of tungsten-in-glass micro-electrodes is described. Essentially, the method employs insulating tungsten electrodes by collapsing borosilicate-glass capillaries onto tungsten wires. Procedures are described for either (1) removing excess glass insulation from the microelectrode tip or (2) fine adjustment and reshaping of the exposed electrode tip. This approach enables the controlled shaping of the electrode tip during preparation and/or reshaping the electrode tip or glass insulation of damaged electrodes. The method may be applied in the preparation of bipolar tungsten-in-glass electrode and tungsten electrode/micropipette assemblies. Advantages of the method are discussed.

Heterotopic neocortical transplants. An anatomical and electrophysiological analysis of host projections to occipital cortical grafts placed into sensorimotor cortical lesions made in newborn rats

Plates of presumptive occipital neocortex obtained from fetal rats at 14-16 days gestation were grafted into the cerebral hemisphere of newborn rats. The transplants were placed heterotopically into sensorimotor cortical lesion cavities made immediately prior to grafting. At maturity, some of the transplants were injected with the retrograde fluorescent tracers Fast Blue and Diamidino yellow. In other animals, single-unit activity in the transplants or in normal cortex was recorded using standard electrophysiological techniques. Histologically, host projections to the transplants were demonstrated by the presence of retrogradely labeled neurons in the host primary and secondary somatosensory cortices as well as several thalamic areas including the anteroventral, anteromedial, ventrobasal, mediodorsal and central medial nuclei. Additional labeling was found in the claustrum, lateral hypothalamus, zona incerta and basal forebrain. Electrophysiologically, transplant single-unit activity was evoked in 43/69 (62%) neurons by thalamic stimulation, but only 1/69 transplant neurons responded to electrical stimulation of the contralateral forepaw. In further work, volumetric measurements showed that the transplants did not ameliorate the thalamic atrophy found after neocortical lesions. These results are compared to previous studies involving the homotopic placement of sensorimotor cortical grafts.

Potassium channel blockade differentially affects the relative refractory period of frog afferent terminals and axons

1. The effects of potassium channel blockade on afferent axons and terminal regions in frog dorsal roots and spinal cords, respectively, were investigated in vitro. 2. A condition-test (C-T) protocol was used to assess the population relative refractory period. Characteristics of main axons were evaluated by stimulation at the proximal end of transected dorsal roots (DR). Characteristics of terminal regions were tested by stimulation at the base of the dorsal horn (DH). 3. DH recovery of excitability was delayed by low concentrations of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) alone or combined. The same treatments did not affect recovery to DR stimulation. 4. DH recovery of excitability was not delayed by solutions suppressing terminal calcium influx. 5. We conclude that sensitivity of the relative refractory period to potassium channel blocking agents differs between main axons and axon terminal regions. This may indicate differences between axon terminals and main axons in the mechanism of action potential repolarization. 6. We hypothesize that rapid action potential repolarization by pharmacologically sensitive potassium channels in presynaptic terminal regions keeps terminal action potentials short. Terminal action potential brevity would limit calcium influx, thus preventing terminal calcium overload but contributing to transmission failures at spinal synapses.

Fetal cortical transplants into neonatal rats respond to thalamic and peripheral stimulation in the adult. An electrophysiological study of single-unit activity

Plates of presumptive sensorimotor neocortex obtained from fetal rats at 14-16 days gestation were grafted into the cerebral hemisphere of 0- to 1-day-old newborn rats. The transplants were placed into small lesion cavities in the sensorimotor cortical area made immediately prior to grafting. At 4-5 months of age, single-unit activity in the transplants (TP) or normal cortex (NCX) was recorded under ketamine HCl anesthesia using standard electrophysiological techniques. Over 90% of cells in both the transplants (n = 63) and normal cortex (n = 39) responded to ventral thalamic stimulation. Their average response latencies were similar (11.48 +/- 0.56 ms in TP. 12.77 +/- 1.20 ms in NCX, mean +/- S.E.M., no significant difference), as were their average responses of slightly more than 1 spike per thalamic stimulus (1.24 +/- 0.08 in TP, 1.07 +/- 0.01 in NCX, no significant difference). In addition, 64% (30/47) of transplant cells and 44% (14/32) of normal cells also responded to electrical stimulation of the contralateral forepaw, and here also the latencies were similar (16.11 +/- 0.67 ms in TP, 14.30 +/- 0.78 ms in NCX). The spontaneous neuronal activity observed within the transplants was also comparable to that seen in normal cortex, as measured by comparison of spontaneous interspike interval (ISI) histograms (median ISI of 84.5 for TP and 80.5 for NCX) and comparison of the burst index (% of intervals less than 5 ms, 14.7% for TP and 10.5% for NCX). These results are compared to those found in several recent studies where fetal cortex is grafted into adult hosts.

Cardiovascular and respiratory responses to electrical and chemical stimulation of the hippocampus in anesthetized and awake rats

Electrical (30-60-s trains of 0.25-ms pulses at 25 Hz, currents 10-150 microA) and chemical (microinjections of 0.1-0.5 microliters of a 1.0 M glutamate solution) stimulation of the hippocampal formation in the anesthetized and the awake rat evokes marked decreases in heart rate, blood pressure and slower, deeper, more regular respirations. Artificial ventilation (2 ml/breath; 100 breaths/min) has no effect on the cardiovascular responses, indicating that these effects are not secondary to respiratory changes. Administration of methyl atropine (0.4 mg/kg) eliminates the bradycardia response and attenuates or obliterates the blood pressure response but does not alter the respiratory response. This suggests that the cardiovascular responses are mediated partially by the vagus nerve and partially by sympathetic influences. Ablation of the medial frontal cortex, a visceral motor region which projects directly to the nucleus of the solitary tract and which receives a heavy direct projection from the CA1 and subicular cell fields of the ventral hippocampus, markedly attenuates or eliminates the cardiovascular and respiratory responses to stimulation of the ventral but not the dorsal hippocampus. The possibility that the medial frontal cortex may be a relay by which the hippocampus influences cardiovascular responses, including those observed during stress, is discussed.

Forelimb motor cortical projections in normal rats and after neonatal hemicerebellectomy: an anatomical study based upon the axonal transport of WGA/HRP

Cerebral cortical projections from the forelimb motor cortex, as defined by intracortical microstimulation where movements were evoked at low current intensities (less than 15 microA), were examined in normal rats and in adult rats that sustained neonatal hemicerebellectomy. The distribution pattern of cortical efferent projections in normal rats generally appeared more restricted than previously described. This restricted distribution is attributed to the use of WGA/HRP as the axonal tracing method and to the electrophysiological definition of the injection site as the motor cortex. The observed remodeling of the corticobulbar projections, seen after cerebellar lesions in the young, largely confirmed previous reports. Moreover, no alterations in the laterality of distribution in corticospinal projection were found. Aberrant corticospinal projections were sought in an effort to provide an anatomical basis to a previous description of abnormally low-threshold ipsilateral forelimb responses evoked from the motor cortex in adult rats after neonatal cerebellar lesions. This apparent absence of corticospinal tract remodeling after neonatal hemicerebellectomy suggests that the abnormal responses are mediated by the normal corticospinal pathways. This possibility is discussed in terms of an alteration in the spinal circuits that may change the responsiveness of spinal motoneurons to a given pyramidal discharge.

A study of forelimb movements evoked by intracortical microstimulation after hemicerebellectomy in newborn, young and adult rats

Microstimulation of the cerebral motor cortex in normal adult rats evokes low-threshold contralateral and high-threshold ipsilateral forelimb movements. The present study examined the effects of hemicerebellar ablation at different postnatal ages on the current threshold values needed to evoke forelimb movements by intracortical stimulation. Animals that received hemicerebellar lesions at various ages were electrophysiologically tested 4-6 months postoperatively. In all groups, including non-lesion control animals, forelimb movements contralateral to the stimulating electrode were evoked at threshold values of 7-11 microA. Ipsilateral forelimb movements for control animals as well as those receiving cerebellar lesions at 45 or 120 days of age showed significantly higher mean threshold current values, ranging from 38 to 45 microA. In contrast, the mean threshold current values for ipsilateral forelimb movements in adult animals sustaining hemicerebellar lesions at 2, 10 or 21 days of age were significantly lowered, ranging from 16 to 22 microA. Secondary lesions of spared cerebellar tissue or callosal fibers in adult animals that had sustained hemicerebellectomy at two days of age had no effect on the current intensities needed to evoke forelimb responses. In comparison, lesions of the cerebral cortex contralateral to the stimulated cortex increased the threshold for ipsilateral movements and medullary pyramidal lesions ipsilateral to the stimulated cortex both reduced the number of responses and increased the threshold current intensities needed to evoke them. These data indicate that hemicerebellectomy within 3 weeks of age can induce electrophysiological alterations in the responses mediated by the corticospinal tract. These results support previous suggestions of the cerebral cortical involvement in compensation for neonatal cerebellar lesions.

A chronic unit study of the sensory properties of neurons in the forelimb areas of rat sensorimotor cortex

The sensory properties of neurons in the several forelimb areas of rat sensorimotor cortex were examined using the technique of extracellular single-unit recording in the awake, head-restrained rat. Cells with peripheral receptive fields were tested for the amount and modality of sensory input during joint manipulation and brushing and tapping of limbs, face and trunk. Input-output correlations were made on the basis of the results of receptive field mapping and intracortical microstimulation in the same electrode penetration. It was found that neurons (n = 117) in the rostral forelimb area receive virtually no sensory input while 30% of neurons (n = 114) in the caudal forelimb primary motor area do receive such input. The inputs to caudal forelimb motor area neurons were primarily (83%) from single joints; along perpendicular electrode penetrations the same joint that activated a cortical cell also moved when microstimulation was delivered along the same electrode penetration. In the granular and dysgranular zones of somatic sensory forelimb cortex, 70% of neurons (n = 82) were responsive to peripheral sensory inputs, with most of the cells in the granular cortex responsive to cutaneous inputs while cells in the dysgranular cortex were more responsive to deep inputs. The lack of sensory inputs to the rostral forelimb motor area is consistent with the proposal that this region may be a part of the supplementary motor area of the rat.

Substantia nigra single unit activity during penicillin-induced focal cortical epileptiform discharge in the rat

In urethane anesthetized rats single unit activity was recorded in the substantia nigra (SN) during focal cortical epileptiform discharges induced by topical application of penicillin to the cortical surface. Eighty percent of SN units responded during the cortical interictal spike discharge, 50% with an initial burst or increase in firing rate and 30% with an initial inhibition or decrease in firing rate. In view of the SN's widespread projections to thalamus and brainstem, these results suggest the SN may be a prominent element of the pathway involved in the spread and generalization of cortical epileptiform activity.

A second forelimb motor area exists in rat frontal cortex

Intracortical microstimulation of 40--50 points in the frontal cortex of ketamine-anesthetized rats using perpendicular penetrations has demonstrated a second forelimb area located rostrally near the frontal pole as well as confirming the existence of a more caudally located forelimb area just anterior to bregma. Cortex where neck and/or vibrissae movements were evoked separated the two forelimb areas. The rostral and caudal forelimb areas defined by microstimulation correspond with patches of corticospinal neurons labeled with HRP following injections of this tracer into the cervical enlargement. Digit movements were commonly evoked from the rostral forelimb area but were rarely elicited from the caudal forelimb area. The question of whether the rostral forelimb region is part of primary or supplementary motor cortex is not yet able to be answered.