The isolated mammalian spinal cord.

This review considers: spinal cord slices; isolated spinal cord sagitally or transversely hemisected; whole spinal cord; respiration control--[brain-stem spinal cord; brain-stem spinal cord with attached lungs]; nociception--[spinal cord with tail]; fictive locomotion--[spinal cord with one hind limb; spinal cord with two hind limbs]. Much of the functional circuitry of the CNS can be studied in the isolated spinal cord with the additional advantage that the isolated spinal cord can be perfused with known concentrations of ions, neurotransmitters, agonists, antagonists, and anaesthetics. These can be washed away, the circuitry allowed to recover and other drugs or different concentrations applied. Future preparations including the complete spinal cord, the two hind limbs, and a sagittal section of the complete brain will allow greater understanding of the multiple sensory and motor pathways and their interactions in the CNS.

Postnatal changes in the dorsal root reflex in the isolated spinal cord of the hamster, Mesocricetus auratus.

Spontaneous activity has been demonstrated in the lumbar dorsal roots of isolated spinal cord preparations taken from animals ranging in age from 2 to 65 days. Peaks of activity were recorded at 2 and 5 weeks of age, with mean firing frequencies of 33 Hz and 28 Hz respectively. The firing frequency in weeks 3 and 4 was lower (15 Hz) as was the frequency in cords taken from animals older than 6 weeks. The pattern of the spontaneous dorsal root activity changed during the first 5 weeks of life. In cords taken from animals less than 10 days old, the roots fired single action potentials, producing a single broad peak in Inter Spike Interval plots (ISI). Dorsal root recordings made from cords taken from animals in weeks 2 and 3 of life exhibited both single spikes and bursts of action potentials. By the end of the third week of life, individual spike activity had declined and the bursts of action potentials characteristic of the adult pattern had become dominant, producing a bimodal ISI plot. Cross correlation analysis of dorsal root and dorsal horn activity in lumbar segments up to five segments apart, revealed an increasing degree of correlation developing over the first 4 weeks of postnatal life. Dorsal horn responses to dorsal root stimulation in cords taken from young animals were prolonged, lasting in excess of 250 msec. In the third week of life, the duration of the excitatory component of the response was reduced to approximately 50 msec by the development of an inhibitory phase.

Postnatal changes in the role of NMDA in the isolated spinal cord of the hamster, Mesocricetus auratus.

The frequency of spontaneous activity recorded from lumbar dorsal roots and the lumbar dorsal horn of isolated spinal cord preparations taken from hamsters aged between 2 days to 8 weeks showed an age dependent sensitivity to 1 mM Mg2+ and 5 microM AP5. Spontaneous dorsal root and dorsal horn activity in cords from animals less than 3 weeks of age was depressed by 1 mM Mg2+ and 5 microM AP5. Cords taken from animals older than 3 weeks showed significantly less depression of spontaneous activity. The application of 10 microM NMDA to the cord produced a small (33%) depression in spontaneous dorsal root and dorsal horn activity in cords from 4 to 6 week old animals. Cords from younger animals exhibited a complex response to NMDA, with an initial increase in spontaneous activity followed by a profound (77%) depression of the firing rate. These results indicate that there are substantial changes taking place in the pharmacology of the dorsal horn during the early weeks of life, and care must be exercised when extrapolating results obtained from neonatal preparations to adult animals.

Spread of the dorsal root reflex in an isolated preparation of hamster spinal cord.

In an isolated preparation of hamster spinal cord maintained at 27 degrees C, stimulation of a single dorsal root has been shown to evoke dorsal root reflexes in ipsilateral roots throughout the lumbar and thoracic cord. The dorsal root reflex was evoked by activity in low threshold dorsal root fibres, and left the cord through dorsal root fibres conducting at 5.9 m/s. The ipsilateral dorsal root reflex travelled equally well in both directions along the cord at a calculated velocity of 0.5 m/s at 27 degrees C. Comparison of the stimuli required to evoke dorsal root reflexes in adjacent and distant ipsilateral dorsal roots suggests that once this system is activated it operates largely in an all-or-none manner. The delay in the generation of the dorsal root reflex within the cord was calculated to be 4.7 +/- 0.2 ms at 27 degrees C, corresponding to approximately 2 ms at body temperature, suggesting that a trisynaptic pathway is involved in the generation of the dorsal root reflex at the segmental level.

Correlation between spontaneous bursts of activity recorded from the dorsal roots in an isolated hamster spinal cord preparation.

At a temperature of 25-27 degrees C spontaneous antidromic activity has been demonstrated in lumbar and thoracic dorsal roots of an isolated spinal cord preparation taken from the golden hamster. A characteristic pattern of bursts of action potentials has been identified, which develops within 1-2 h following dissection and persists for more than 8 h. Simultaneous recordings made from pairs of dorsal roots have revealed correlations between the patterns of spontaneous activity in dorsal roots separated by up to sixteen segments longitudinally and across the cord. The strongest correlations were found between pairs of adjacent roots on the same side of the cord which produced a cross-correlation histogram having a single peak with a mode close to 0 ms. As the separation between the roots was increased the cross-correlation histogram became bimodal, with peaks equidistant on either side of 0 ms. Activity recorded in ipsilateral and contralateral pairs of roots supplying the same spinal segment also produced bimodal cross-correlation histograms. Transverse sectioning of the cord did not abolish spontaneous activity in any of the spinal roots examined, although there was a progressive reduction in the frequency of the bursts of spontaneous activity with shorter lengths of cord. These results suggest that each spinal segment is capable of generating spontaneous activity, and that there is a system by which adjacent segments are linked, allowing the activity to spread up and down the cord from the point of origin. The networks associated with the spread of dorsal root activity and primary afferent depolarization (PAD) in the spinal cord are discussed.

The effect of bicuculline and baclofen on the dorsal root-ventral root reflex in the isolated spinal cord.

1. In the lumbar spinal cord, 10(-6) M bicuculline reduces the inhibition of the dorsal root-ventral root (DR-VR) reflex produced by lateral stimulation of the spinal cord. The inhibition returns on washing in control artificial cerebrospinal fluid (ACSF). 2. Baclofen (10(-6) M) inhibits the monosynaptic DR-VR reflex which returns on washing in control ACSF. 3. This suggests an involvement of GABAA receptors in the lateral inhibition and GABAB receptors in the DR-VR reflex system. 4. The increased activity seen following bicuculline or tubocurarine probably resets the "set point" of neural activity to a lower level.

Tubocurarine and strychnine block Renshaw cell inhibition in the isolated mammalian spinal cord.

1. Stimulating a lumbar dorsal root (A) of an isolated mammalian spinal cord preparation sets up a reflex (DR-VR) in the corresponding ventral root (A) of that segment. 2. If an adjacent motor root (B) is antidromically stimulated 100 msec before the dorsal root A is stimulated, it inhibits the ventral root A response. (i.e. Renshaw cell inhibition of A through antidromic stimulation of recurrent collateral of B). 3. Stimulation of the lateral side of the spinal cord 5 mm rostral to the DR-VR reflex sets up a powerful inhibition of the reflex. 4. The inhibition is abolished by tubocurarine, or by strychnine indicating that the lateral stimulation is activating a cholinergic and a glycinergic pathway. 5. Addition of tubocurarine increases the steady "spontaneous" activity in the motor roots. 6. Addition of tubocurarine and 10(-3) M magnesium produces long lasting rhythmic bursting activity in the motor roots that persists even after 45 min of washing. 7. The isolated mammalian spinal cord preparation in addition to having active sensory, motor, and interneurones, has active Renshaw cells that can be studied using this preparation free from any blood-brain barrier and so are easily permeated by tubocurarine, etc.

Differential sensitivity of dorsal and ventral root activity to magnesium and 2-amino-5-phosphonovalerate (APV) in an isolated mammalian spinal cord preparation.

Spontaneous activity emerging from the isolated hamster spinal cord simultaneously along the ventral and dorsal lumbar roots was shown by inter-spike interval analysis to have different firing patterns. Ventral root firing was reduced in solutions containing 1 mM magnesium or sub-micromolar concentrations of 2-amino-5-phosphonovalerate (APV), whereas the activity in the dorsal roots was unchanged. Differences in sensitivity to magnesium and APV were also shown on the evoked ventral and dorsal root reflexes, with the dorsal root reflex being unaffected, and the ventral root reflex exhibiting a long latency component that was sensitive to magnesium and APV. These results indicate that NMDA receptors are involved in the generation of part of the ventral root responses in the spinal cord, but not in the generation of the dorsal root reflex and spontaneous dorsal root activity.

The effect of diaminopyridines on the activity recorded in the dorsal roots of isolated spinal cord.

1. The diaminopyridines (DAP) increase the activity recorded in the dorsal roots of an isolated hamster spinal cord preparation. 2. The threshold for 3,4 DAP was 3 x 10(-6) M and the maximum effect was 10(-4) M. The threshold for 2,3 DAP was 3 x 10(-4) M and the maximum effect was at 10(-3) M. The threshold for 2,6 DAP was 10(-4) M and its maximum effect was at 3 x 10(-2) M. 3. When compared with the monoamino pyridines (AP) 4 AP was the most effective and the order of potency was 4 AP greater than 3,4 DAP greater than 2,3 DAP greater than 2 AP = 3 AP greater than 2,6 DAP. 4. Treatment with AP increased the activity simultaneously recorded in the dorsal and ventral roots, but whilst that in the dorsal root continued after washing away the amino pyridine, that in the ventral root ceased for a period of time. 5. There are indications that the amino pyridines may differently affect dorsal and ventral interneurones.

Studying the isolated central nervous system; a report on 35 years: more inquisitive than acquisitive.

1. The CNS from invertebrate animals such as slugs, snails, leeches, and cockroaches, can be isolated and kept alive for many hours. 2. The electrical and pharmacological properties of invertebrate CNS neurons have many similarities and it is probable that the basic rules governing the CNS evolved more than 600 million years ago. 3. The nerve cells can show sodium action potentials, calcium action potentials, EPSP, IPSP, biphasic potentials, electrogenic sodium pump potentials, and a variety of potassium, sodium, calcium and chloride currents. 4. Invertebrate CNS ganglia contain identifiable individual nerve cells whose properties and responses to neurotransmitters and drugs are constant and repeatable from preparation to preparation. 5. It was possible to set up an isolated CNS-nerve trunk-muscle preparation and study the transport of radioactive material from the CNS to the muscle and from muscle to CNS. This has provided information about axoplasmic transport in both invertebrate and vertebrate preparations. 6. The methods developed from studies of invertebrate isolated CNS preparations have been applied to vertebrate isolated CNS preparations. 7. In addition to thin slices of the mammalian brain, it is possible to keep 5 cm lengths of the whole mammalian spinal cord and brain stem alive for many hours. 8. The isolated mammalian spinal cord has functional ipsilateral and contralateral reflexes, ascending and descending pathways, extensive sensory integrative local area networks, and inhibitory interneuron circuits. Much of the in vivo circuitry is functional in vitro. 9. The isolated mammalian spinal cord and brain stem can be developed to include functional higher brain circuits that will provide increased understanding of the control and integrative action of the mammalian central nervous system.

The dorsal root reflex in isolated mammalian spinal cord.

1. The dorsal root reflex has been investigated in an isolated preparation of adult mammalian spinal cord. 2. Both evoked and spontaneous activity can be recorded from the cord in the dorsal spinal roots. 3. The spontaneous activity has a characteristic pattern of firing in bursts of action potentials. Spontaneous and evoked activity are optimum at temperatures between 25 and 27 degrees C; little activity can be detected above 35 degrees C. 4. The spontaneous dorsal root activity has been shown to be correlated with negative potentials in the dorsal horn of the cord, and intracellular recordings made from primary afferent fibres have shown spontaneous primary afferent depolarizations (PAD) which underlie the generation of the spontaneous dorsal root activity. 5. The evoked dorsal root reflex has been shown to spread up to 16 spinal segments both rostrally and caudally from the stimulated dorsal root, and to the contralateral side of the cord. 6. The spontaneous dorsal root activity in widely separated segments has been shown by cross-correlation analysis to be linked both ipsi- and contra-laterally. 7. The significance of such a widespread system for the generation of PAD is discussed.

The effect of aminopyridines on the activity recorded in the dorsal roots of an isolated hamster spinal cord preparation.

1. The aminopyridines (AP) increase the activity recorded on the dorsal roots of an isolated hamster spinal cord preparation. 2. The threshold for 4-AP was 4 X 10(-6) M and the maximum effect was 3 X 10(-5) M. The threshold for 2AP and 3AP was 3 X 10(-5) M and the maximum effect was at 10(-3) M. 3. It took longer for the preparation to recover on washing from treatment with 4-AP than from 2-AP or 3-AP. Washing often showed a transient increase in activity. 4. Treatment with AP changed the pattern of firing so that the activity after treatment and prolonged washing showed a bursting pattern. 5. The activity recorded in the dorsal roots is generated within the spinal cord and may be related to activity of central pattern generators.

Possible evolutionary futures for mankind.

1. With the development of techniques of gene transfer, human genetic defects such as sickle cell anaemia, phenylketonurea, cystic fibrosis, haemophilia, Huntington's chorea, etc. will be eliminated. Ninety nine percent of humans in the year 2500 will be much the same as at present, but healthier. 2. Studies of Comparative Biochemistry and Physiology show that many strategies have been developed in the Animal Kingdom that could be advantageous in furthering human survival. Some of these strategies are discussed in the present article. 3. Commensal algae that will live in the epidermis of domestic animals (pigs, goats, cattle) will be developed enabling these animals to live in dry environments with minimal demands for food and water. Once successful in domestic animals, the algae could be adapted to live in some humans, i.e. the "Green Man". 4. Commensal protozoa and bacteria that digest cellulose and lignin will be developed so that they can live in the human gut and convert the material to sugars, volatile fatty acids and amino acids that can be absorbed and metabolized by Man, thus making many inexpensive vegetable food resources available. 5. Mammalian embryos will be able to develop through to full term in vitro in a cleidoic egg; and the in vitro fertilized egg will have to depend on finding a surrogate mother. 6. Some people will have an altered pattern of sexual activity. Many patterns will be available; one suggested here is of protogynous hermaphroditism, i.e. the individuals would be female for the first 30 years of life and male for the remaining years.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that levamisole, pyrantel, morantel, amidantel, deacylated amidantel and hycanthone may act on acetylcholine receptors of central neurones of Helix aspersa.

1. Intracellular recordings were made from identified neurones in the suboesophageal ganglionic mass of the snail, Helix aspersa. Neurones were classified as either "H" cells, inhibited by acetylcholine or "D" cells, excited by acetylcholine. 2. The actions of levamisole, morantel, pyrantel, amidantel, deacylated amidantel and hycanthone were investigated on these neurones and compared to that of acetylcholine. 3. Levamisole was 10.85 +/- 0.56 times less active than acetylcholine on "H" cells but more than 100 times less active on "D" cells. On "H" cells levamisole had a secondary gradual depolarizing effect which was irreversible and resulted in the loss of cell activity. 4. Morantel and pyrantel were 1.12 +/- 0.13 and 2.56 +/- 0.26 times respectively less active than acetylcholine on "D" cells and 5.16 +/- 0.6 and 3.53 +/- 0.63 times respectively less active than acetylcholine on "H" cells. 5. Amidantel was more than 100 times less active than acetylcholine on both "D" and "H" cells while its deacylated derivative was 26.0 +/- 1.0 and 76.0 +/- 3.25 times respectively less active than acetylcholine on "D" and "H" cells. 6. Hycanthone possessed weak inhibitory effects on "H" cells but also appeared to reduce the duration of acetylcholine inhibitory responses when applied immediately after the acetylcholine response had reached its maximum.

Crossed reflex activity in an entire, isolated, spinal cord preparation taken from juvenile rodents.

An entire (not hemisected) spinal cord preparation taken from hamsters weighing up to 40 g is described, in which crossed and uncrossed reflex activity could be evoked from the lumbar ventral roots following stimulation of the lumbar dorsal roots. This activity persisted at higher temperatures in the complete spinal cord than was previously found for hemisected cords, and was obtained in tissue taken from larger animals.

Demonstration of the synaptic origin of primary afferent depolarisation (PAD) in the isolated spinal cord of the hamster.

Intracellular recordings have been made from 31 primary afferent fibres within the dorsal horn of an isolated mammalian spinal cord. In 17 fibres stimulation of an adjacent dorsal root evoked primary afferent depolarization (PAD); these fibres also showed spontaneous depolarizations. Replacement of the calcium in the perfusing medium by manganese blocked both evoked and spontaneous activity showing them to be of synaptic origin. Observations on the effects of current injection and of bicuculline support an involvement of GABA in the generation of PAD.

Effects of bath-applied GABA on the firing pattern of cells in an in vitro preparation of mammalian cerebral cortex.

The patterns of spontaneous firing recorded from 25 cells in slice preparations of rat cerebral cortex were characterised using inter-spike interval plots (ISI). Histograms of the ISI showed two peaks having modal values of 20 and 82ms, corresponding to episodes of high frequency bursts of action potentials, and a lower frequency, steady pattern of activity. Bath-applied GABA was shown to reversibly block the bursting activity, but had little effect on the lower frequency component. This effect of GABA could be abolished by the simultaneous application of bicuculline.

An investigation of the dorsal root reflex using an in vitro preparation of the hamster spinal cord.

A detailed description is given of an hemisected spinal cord preparation from adult golden hamsters and this preparation has been used to investigate the physiology of the dorsal root reflex. In addition to antidromic reflex discharges which could be recorded from lumbar dorsal roots following stimulation of adjacent dorsal roots or the dorsal columns, spontaneous firing was also recorded from the dorsal roots. This activity reached a peak at 27 degrees C and was abolished at temperatures above 35 degrees C. Both the evoked and the spontaneous dorsal root activity were demonstrated to be travelling antidromically along the dorsal roots out of the cord, and replacement of the calcium in the bathing medium by manganese showed them to be of synaptic origin. Stimulation of a lumbar dorsal root was found to evoke a reflex in up to 4 adjacent spinal segments in both rostral and caudal directions, and a period of depressed activity was demonstrated following both evoked and spontaneous discharges. A time-locked relationship was found between the dorsal root reflex and the slow dorsal horn potential recorded from within the spinal cord.

An isolated mammalian brainstem-spinal cord preparation suitable for the investigation of descending control of motor activity.

An in vitro, hemisected brainstem-spinal cord preparation which allows bulbo-spinal interactions to be investigated, has been developed using 25-33 g golden hamsters. Spinal reflexes showing both monosynaptic and polysynaptic components were recorded from the lumbar ventral roots following stimulation of the corresponding dorsal roots. Trains of conditioning stimuli delivered to the caudal region of the medulla caused inhibition of the monosynaptic component and potentiation of the polysynaptic component of the spinal reflex.