Blockage of vibrissae afferents: II. Footshock threshold increments.

Because of their dense innervation rat vibrissae have been regarded as a very important sensory system. Many behavioral deficits have been reported by other authors after rat vibrissal afferent blockades. In the present work we found significant threshold increments to footshock following either reversible nerve block (procaine or nerve pressure) or section of the vibrissal afferent nerves, but not following vibrissae trimming. These results are discussed in reference to the tonic or level-setting function of afferent systems.

Blockage of vibrissal afferents: III. Electrocorticographic effects.

We have shown signs of behavioral depression after vibrissal deafferentation. Locomotor slowing, motor impairments and footshock thresholds increment were demonstrated after vibrissal afferent blockages. Here, we study the electrocortical (ECoG) effects of vibrissal pad anaesthesia, also replicated by bilateral brachial plexus blockage. We found in both cases, that this acute and massive deafferentation produces synchronization over the entire neocortex accompanied by an important loss of muscular electrical activity. Slow waves observed in this condition were similar to those recorded in the sleeping rat without any treatment, but in our case, there were no behavioral signs of sleep. Thus a clear behavioral electroencephalographic dissociation was obtained by acute deafferentation. These results would seem to support the sleep deafferentation hypothesis.

Blockage of vibrissae afferents: I. Motor effects.

In the past, it has been proposed that the rat vibrissae play an important role in other hand, postural abnormalities, muscle tone decreases and hypomotility after sensory organ destructions were proposed as evidence supporting the "level setting" or "tonic" hypothesis. This hypothesis postulates that afferent activity, besides its well know transductive functions, sets the excitability state of the central nervous system. We thought the vibrissal system to be a good model to dissect these two postulated roles because vibrissae trimming would annul the transductive function without affecting the integrity of nerve activity. Thus we compare the effects of trimming the whiskers with blocking the vibrissal afferent nerves on two types of motor behavior: activity in an open field and walking over a rope connecting two elevated platforms. We found that only vibrissal afferent blockage (both nerve section and local anaesthesia) produced severe failures in the motor performances studied. These effects could not be fully explained by the abolition of the vibrissae as a sensory modality because cutting the whiskers did not significantly affect the motor performance. These data are discussed in reference to a tonic or general excitatory function of sensory inputs upon the central nervous system.

Differential effects of superior and inferior spermatic nerves on testosterone secretion and spermatic blood flow in cats.

It has been postulated that testosterone secretion is partially regulated by signals from the spermatic nerves. To further examine this hypothesis in vivo, the superior (SSN) or the inferior (ISN) spermatic nerves were stimulated electrically (varying intensity, 25 Hz, 0.2 msec, 10 min) in anesthetized cats, determining the testosterone concentration and the blood flow in the spermatic vein. In some additional experiments arterial blood was sampled, and norepinephrine (NE) output was calculated. Stimulation of the SSN (25-35 V) increased the testosterone concentration in spermatic vein blood (P < 0.01 compared with prestimulation levels). The response varied among animals, reaching a 50-100% increase in some animals, whereas in others it ranged from almost undetectable to more than 10 ng/100 g x min. Under the same experimental conditions, the NE output increased from 135.4 +/- 99 to 1614.2 +/- 347 pg/ml (P < 0.01), and spermatic blood flow decreased from 24.1 +/- 1.42 to 20.2 +/- 1.65 ml/min x 100 g (P < 0.05) during nerve stimulation. By contrast, stimulation of the ISN (25-35 V) modified neither the testosterone concentration, the NE output, nor the blood flow in the spermatic vein. High intensity stimulation (36-70 V) of each spermatic nerve evoked different vascular and hormonal effects. SSN activation induced a marked decrease in spermatic blood flow during stimulation and an increase in the testosterone response, whereas ISN activation resulted only in an enhanced spermatic blood flow. Our results suggest that testosterone secretion, although mainly dependent on gonadotropin secretion, could be further regulated by neural inputs from the SSN acting directly or alternatively through changes in blood flow. It would appear that the SSN mainly supplies the vasoconstrictor fibers to the testis, whereas the ISN provides vasodilator fibers.

Role of the stretch reflex in oxotremorine tremor.

Although the basic mechanism for parkinsonian tremor is known to be central, the case for the oscillator(s) of physiological tremor and shivering is still a matter of dispute. In this case, an important role has been proposed for muscle afferents. Oxotremorine tremor has in common with shivering its frequency range (10-28 Hz) and the co-contraction of flexors and extensors. On the other hand, in contradistinction, it can be blocked by atropine, as is the case with parkinsonian tremor. Thus it was of interest to analyze the role of muscle afferents in the production and maintenance of oxotremorine tremor in the acutely decorticated cat. This was studied with two experimental approaches. In one, either a front or hind limb of a cat was completely deafferented by dorsal rhizotomy and the tremor activity recorded electromyographycally in a pair of antagonistic muscles. In the other, the nerve and muscle electrical activity and force produced by the tremoring muscle (gastrocnemius) were recorded simultaneously. Tremor activity was induced by oxotremorine injection (200-750 micrograms/kg i.p.) given at different postoperative periods. The drug induced a regular tremor in the chronically deafferented animals (3 weeks to 5 months) but not in the acutely deafferented limb (3 hrs after dorsal rhizotomy). The tremor observed in the former group was very regular and had the same frequency range (10-28 Hz) for both normal and deafferentd sides. Section of all the muscle afferents of a given pool (agonist and its main synergist) did not have any effect on the regularity and frequency of the oscillation (tremor) of that pool. The role of other muscle and/or cutaneous afferents, although perhaps of some importance, remains unclear. A pure spinal mechanism to account for the regular oscillations (tremor) is proposed.

Effect of regional spinal cord blood flow and central control in recovery from spinal cord injury.

Forty-two cats were subjected to decerebration, thoracic and lumbar laminectomies, and isolation of the sciatic nerves. Spinal evoked potentials in response to bilateral sciatic nerve stimulation were recorded at L-3, and the spinal cord blood flow (SCBF) was measured by the hydrogen clearance technique. Thoracic cordotomy did not alter the lumbar SCBF or the central conduction time as determined by spinal evoked potentials. Thoracic cordotomy significantly lowered the lumbar spinal cord injury threshold. Continuous sciatic nerve stimulation increased the lumbar SCBF in normal and traumatized animals; however, it did not affect the spinal cord injury threshold as measured by recovery of the spinal evoked potentials. It appears that rostral spinal cord integrity is far more significant in recovery from spinal cord injury than the maintenance of regional SCBF.

Effects of "partial axotomy" upon synaptic function.

A lesion limited to the dorsal columns, at the level of L3-L4, was carried out in chronic cats. This operation produced a "partial axotomy" type of lesion on the ascending branches of Ia hind limb afferents. Two to six months after this operation, intracellular studies on L7-S1 motoneurons were carried out. Similar studies were done in normal animals. The peak amplitude and the rate of rise (dV/dt) of heteronymous EPSP's were studied during control conditions (sampling at 1 Hz) and during the post-tetanic potentiation produced by a 500 Hz tetanus (for 3 s). The analysis of these synaptic potentials makes us conclude that: The amplitude of the enlarged EPSP's, observed during PTP, seems to be linearly dependent on their amplitude during control (i.e., pre-tetanus) conditions. Judging by their amplitude, there is no difference between potentiated EPSP's of operated and normal animals. There is also a linear relationship between the rate of rise of EPSP's and their peak amplitude. The slope of this relationship becomes steeper after "partial axotomy", i.e., for a given EPSP amplitude, the dV/dt of its rising part is steeper in operated cats. This steeper slope is also present in EPSP's studied during PTP. The sharper rate of rise of EPSP's, induced by the "partial axotomy" of Ia fibers, would be the mechanism behind the larger monosynaptic reflex previously observed in these operated cats.

Control of blood flow in the cat spinal cord.

Spinal cord blood flow (SCBF) and the effect of end-tidal CO2 concentration (ETCO2) on SCBF (CO2 reactivity) were studied in the lumbar spinal cord of cats by means of the hydrogen-clearance technique Hydrogen gas was administered by inhalation, and its level in spinal cord tissue was estimated amperometrically with small (75 micrometers) platinum electrodes. The average SCBF's at normocapnia (ETCO2 = 4%) of the ventral horn gray matter and of the white matter at several locations were 43.2 and 16.2 ml . 100 gm-1 . min-1, respectively. For gray and white matter, the values of CO2 reactivity, estimated by the coefficient of the regression of SCBF (ml . 100 gm-1 . min-1) on ETCO2 (ml . 100 ml-1) were 11.6 and 2.1, respectively. No differences in SCBF or CO2 reactivity were observed between intact animals kept under N2O-O2 ventilation and decerebrated animals with no anesthesia. After an acute spinal section, ventral horn SCBF and CO2 reactivity (measured eight segments below the cordotomy) were not altered, in spite of the profound neural depression present (that is, spinal shock). Orthodromic (dorsal root) stimulation of the ventral horn neurons induced an average increase in blood flow of 128% above control values. Antidromic (ventral root) motoneuron activation failed to produce any significant changes in ventral horn blood flow.

Long-lasting facilitation of a monosynaptic pathway as a result of "partial" axotomy of its presynaptic elements.

We investigated the long-lasting effects of severing an axonal branch on the function of the synapses made by the remaining branches of the "partially" transected axon. The experimental model used was the monosynaptic reflex pathway of the cat lumbar cord. Preliminary surgical procedures consisted of a dorsal column (DC) hemisection at the L3--L4 level on the left side under aseptic conditions. During the final recording experiments, the monosynaptic reflex (MSR) of the operated and normal (control) side was studied in motorpools in the L7 and S1 segments. The results indicate that the DC operation produced on the operated side (a) an enhancement of the MSR and (b) a more powerful postetanic potentiation, both of the "early" and "late" varieties. These effects were seen as early as 34 days and as late as 121 days after the original DC hemisection. Control experiments were carried out to exclude denervation supersensitivity, supraspinal effects, and normal asymmetries between sides as the mechanism underlying these effects. A presynaptic change in the operated axons, resulting in an increased synaptic efficiency, is proposed as the possible mechanism.

Centrifugal dorsal root discharges induced by motoneurone activation.

1. It has been confirmed that antidromic stimulation of motoneurones in the cat lumbar cord can induce, when properly conditioned, a centrifugal discharge in dorsal root afferent fibres.2. The effective conditioning can be (a) an orthodromic volley to the same or an adjacent dorsal root, (b) a volley to the dorsal column one or two segments above the tested level, or (c) a natural stimulus applied to the ipsi- or contralateral hind limb.3. The conditioning stimulus acts by increasing presynaptic excitability; the peak of its effect (maximum presynaptic depolarization) occurs 7-10 msec after the arrival of the conditioning volley to the cord and then quickly decays.4. A large antidromic field potential in the ventral horn is not necessary for the production of a centrifugal dorsal root discharge. Activation of a ventral root filament of approximately 100 mu in diameter can still induce such a discharge in a single dorsal root fibre. Furthermore, antidromic stimulation of the remaining fibres of the same ventral root cannot affect the terminals activated by the thin ventral root filament.5. The phenomenon of motoneurone-presynaptic interaction was obtained in different types of experimental preparations: acute and chronic spinal, anaemic and midcollicular decerebrate, animals with intact supraspinal centres, and one animal without acute laminectomy.

An effect of postsynaptic neurons upon presynaptic terminals.

Centrifugal ("antidromic") discharges in cat sensory fibers are observed consistently in a variety of experimental preparations and with many different surgical and recording techniques. As is well known, they can be either "spontaneous" or induced by afferent volleys in other sensory fibers. In addition, it is shown here that they can be elicited by antidromic motoneuron activation when the latter is conditioned by natural sensory stimuli or by shocks to the dorsal roots. The latency of the centrifugal dorsal root response to ventral root stimulation is shorter than that of the monosynaptic reflex mediated by the same fibers. An "antidromic" coupling, probably of an electrical nature, between motoneurons and presynaptic terminals is postulated.