Pharmacokinetics of HI-6 and atropine in anaesthetized pigs after administration by a new autoinjector.

A newly developed autoinjector (Astra Tech, Sweden) containing 500 mg HI-6 and 2 mg atropine sulphate was tested in anaesthetized normal pigs. The pharmacokinetics and pharmacodynamics of the drugs after administration by the autoinjector were compared with those after conventional needle and syringe delivery intramuscularly and intravenously. Cardiopulmonary parameters were monitored and serum concentrations of oxime, atropine, and acetylcholinesterase were determined in blood samples taken at intervals over a 6 h period postinjection. After injection in anaesthetized pigs, both HI-6 and atropine were absorbed rapidly and completely from the injection site. Therapeutic serum concentrations of HI-6, arbitrarily taken as 4 micrograms mL-1, were reached within 1 min of intravenous and autoinjector administration, and within 5 min of intramuscular injection. The concentrations remained above this level for 3-4 h. There were no significant changes in acetylcholinesterase activity, mean arterial blood pressure, or respiration frequency after injection of HI-6 and atropine sulphate. The heart rates increased significantly after administration of the two drugs (cardioacceleration defined as > or = 5% increase in heart rate), regardless of the technique employed. Our results show that HI-6 and atropine sulphate can be given intramuscularly by the new autoinjector with the same effectiveness and speed as when given intravenously. Irrespective of the injection technique, no overt signs of toxicity were observed at the drug concentrations used.

Treatment of organophosphate poisoning in pigs: antidote administration by a new binary autoinjector.

The therapeutic effectiveness of a new binary autoinjector containing 500 mg HI-6 and 2 mg atropine sulphate was tested in anesthetized pigs poisoned by a lethal dose of soman i.v. (9 micrograms/kg per 20 min). Pharmacokinetics and pharmacodynamics of HI-6 were studied concomitantly on administration of HI-6 alone, together with atropine sulphate, or together with atropine sulphate during soman intoxication. Cardiopulmonary parameters were monitored and serum concentrations of oxime and acetylcholinesterase (AChE) were measured in blood samples taken at intervals over a 6-h period postinjection. Five minutes after the start of soman infusion, mean AChE activity was decreased to 27 +/- 4.3% of baseline and signs of poisoning appeared. The antidotes, HI-6 and atropine sulphate, were then administered i.m. One minute after this injection there was a transient significant increase in AChE activity of 76 +/- 8.2% of baseline (p < 0.01). It then again decreased and remained suppressed throughout the experiment. Mean respiratory rate was significantly decreased (p < 0.01) to 20 +/- 3.2% of baseline after 20 min of soman infusion and remained low during the rest of the experiment. The poisoning signs were counteracted 15-20 min after antidote therapy and all pigs survived soman intoxication without ventilatory assistance. Administration of either atropine or atropine and soman had no significant effect on the pharmacokinetics of HI-6 in anesthetized pigs.

Termination in posterior and anterior cerebellum of a climbing fibre pathway activated from the nucleus of Darkschewitsch in the cat.

Climbing fibre projections to the C2 zone of the cerebellar cortex activated from structures in the midbrain were studied by electrophysiological technique in chloralose-anaesthetized cats. The C2 zone was identified in the paramedian lobule and in the intermediate part of the anterior lobe by peripheral nerve stimulation. Foci in the medial midbrain, which upon low intensity stimulation selectively evoked climbing fibre responses in the C2 zone, were localized by careful mapping procedures. Histologically the active region was identified as overlying the nucleus of Darkschewitsch, and it was suggested that this nucleus was the origin of a pathway which via the rostral parts of the medial accessory olive projected to the C2 zone of the posterior as well as the anterior cerebellar cortex.

An excitatory pathway through dorsal columns to rubrospinal cells in the cat.

A somaesthetic excitatory pathway to rubrospinal cells, via other than cortico- or cerebello-rubral relays, has been demonstrated in cats, using intracellular recording techniques. After lesions of the rostral part of the cerebral cortex and the cerebellum, electrical stimulation of the contralateral forepaw and low intensity stimulation of the dorsal columns produced excitatory post-synaptic potentials (e.p.s.p.s) in rubrospinal cells, which often reached the threshold for firing bursts of spikes. These somaesthetic e.p.s.p.s are composed of a burst of small elementary e.p.s.p.s which progressively create a depolarization with a long-lasting time course. Evidence has been obtained that the pathway runs through the dorsal columns to reach the red nucleus after a relay. Stimulation applied to the dorsal columns gives rise to the response with very low threshold current, close to 20 microA. Also, the firing pattern of lemniscal fibres to single-shock dorsal column stimulation has been correlated with the time course of excitation of the rubrospinal cells. As calculated from latencies of e.p.s.p.s the conduction velocity in the investigated pathway is over 55 m/s in the peripheral nerve fibres, indicating that thick, myelinated fibres are involved. It is suggested that the fast somaesthetic loop might, via the rubrospinal tract, participate in exploratory motor behaviour and in the adaptation of gait to the environment.

Zonal termination of the tecto-olivocerebellar pathway in the cat.

The climbing fibre pathway from the tectum to posterior cerebellar cortex was investigated in chloralose anaesthetized cats. With low threshold electrical stimulation within the deeper layers of the superior colliculus, climbing fibre responses were recorded in a centrally located, longitudinal area of lobulus VII, oriented perpendicular to the long axis of the folia. This same area showed climbing fibre evoked activity regardless of the side of tectum stimulated. Hence, it was concluded that the tecto-olivocerebellar pathway terminates bilaterally in the most medial parts of the cerebellar cortex, the termination zones being denoted the al zones. The results were discussed in relationship to recent anatomical knowledge of the tecto-olivary proteins and of the olivocerebellar projection to lobulus VII, and indicate that descending paths from the brain stem to the inferior olive might "respect" its intrinsic compartmentalization.

Cerebral cortical areas of origin of excitation and inhibition of rubrospinal cells in the cat.

The relations between the cerebral cortex and the red nucleus have been studied in acute, chloralose anaesthetized cats using intracellular recording techniques. Stimulation of the cerebral cortex induces in rubrospinal cells a short latency excitation followed by a long lasting silent period. The evidence is presented that at least a great part of the latter is due to genuine IPSP evoked in these cells. Three populations of rubrospinal neurones have been distinguished according to the cortical origin of their afferents: one group receives projections from the forelimb cortical area. These cells project to the cervical spinal cord and thus should control the forelimb. The second group receives projections from the hindlimb cortical area. These cells project to the lumbar spinal cord and should control the hindlimb. The third group of rubrospinal neurones receives convergent projections from both forelimb and hindlimb cortical areas. If these cells have collateralized axons terminating in both rostral and caudal spinal cord, they could contribute to the coordination of fore- and hindlimb movements. The projections originate in cytoarchitectonic areas 1-5 i.e. in the primary motor and sensory areas and in the rostral portion of the parietal area. No projection has been found from area 6 (premotor) or from area 7 (caudal parietal). The projection upon single rubrospinal cells has been found to originate from large cortical regions with a large overlap between those with excitatory and inhibitory actions. This could indicate the intermingling of cortical cells transmitting both effects.

On climbing fibre projections to cerebellar paramedian lobule activated from mesencephalon in the cat.

Climbing fibre projections to cerebellar paramedian lobule activated from structures in medial midbrain tegmentum were studied in chloralose anaesthetized cats. Three sagittal zones were identified, from medial to lateral c1, c2 and d1 zones. The c1 and d1 zones were activated, at the same short latency, from a dorsal tegmental region with a location similar to the composite bundle called the central tegmental tract. The d1 zone is probably activated via rubro-olivary fibres and the c1 zone via some other fibres in this bundle. The path to the c2 zone seemed to proceed only partly within this dorsal tegmental region. The d1 zone was also activated, at longer latency, from a region ventral to the red nucleus.

Identification of a tecto-olivocerebellar path to posterior vermis in the cat.

Climbing fibre projections to posterior cerebellar vermis (lobule VII) activated from midbrain tegmentum and tectum were studied in chloralose anaesthetized cats. Only a medial, possibly bilateral, region of lobule VII was activated by fibres proceeding in a dorsal tegmental region, located within and dorsally to the red nucleus. The same area of lobule VII was also activated from tectum, in the deeper layers of superior colliculi at slightly longer latency. It was suggested that this represents a tecto-olivocerebellar pathway.

Inhibition of rubro-spinal cells by somesthetic afferent activity.

A somesthetic inhibitory projection onto rubrospinal cells in the cat is described. It is suggested that the pathway, which is not dependent upon an intact cerebellum or motor cortex, travels through the dorsal columns of the spinal cord. The second order neurons seem to impinge upon inhibitory interneurons situated within the red nucleus itself. The burst activity in such interneurons might account for the IPSPs seen in rubro-spinal cells. In agreement with the idea that the red nucleus may be part of a feed-back loop through the periphery, the pathway described here could be the link giving suppression of rubral activity during some phases of limb movement.

Inhibitory actions from low and high threshold cutaneous afferents on groups II and III muscle afferent pathways in the spinal cat.

The inhibitory effects caused by volleys in cutaneous afferents on the transmission through some polysynaptic segmental pathways activated by high threshold muscle afferents were studied in chloralose anesthetized, spinal cats. Pathways studied were groups II and III to motoneurones as well as group II to primary afferents. The results suggested that two different mechanisms were involved. One mechanism, with a very slow time course (duration more than 400 ms), is suggested to be an example of presynaptic inhibition between different primary afferent systems. This mechanism required high threshold (greater than or equal to 1.6T) conditioning shocks, and appeared simultaneously with the component II dorsal root potential being evoked by the cutaneous afferent volley. The other mechanism, with a faster time course (duration always below 300 ms), was dependent upon low threshold (less than or equal to 1.5T) cutaneous conditioning volleys. This inhibitory interaction could not be ascribed to the same presynaptic mechanism, but is suggested to be an example of postsynaptic inhibition at an interneuronal level. The presumed disynaptic excitatory pathway from group II muscle afferents to flexor motoneurones was not inhibited by cutaneous conditioning shocks, but could on the contrary be facilitated by activity in low threshold cutaneous afferents, probably at the only interneurone involved in this group II pathway.

The rubro-bulbospinal path. A descending system known to influence dynamic fusimotor neurones and its interaction with distal cutaneous afferents in the control of flexor reflex afferent pathways.

The inhibitory effect of electrical stimulation in the near-rubral region on polysynaptic segmental as well as ascending pathways activated by the flexor reflex afferents (FRA) in hind limb nerves was studied in chloralose anaesthetized cats. The effective stimulating region totally coincided with the one from which a D zone climbing fibre response may be elicited in the contralateral cerebellar cortex. The descending path was dependent upon an intact dorsolateral spinal funiculus, where also a characteristic volley could be recorded with a surface electrode on short train central stimulation. The suppressive action on the transmission through the FRA pathways was evoked in the absence of a lower lumbar dorsal root potential, and it was concluded that the effect was exerted by postsynaptic inhibition. It was suggested that this descending path, the effects of which resemble those elicited from the dorsal reticulospinal system, is identical to the rubro-bulbospinal path, previously known to influence dynamic fusimotor neurones. The transmission through the FRA pathways was also suppressed by conditioning stimulation of ipsilateral, low threshold distal cutaneous afferents. The time course of this effect was the same as that with central conditioning stimulation. Facilitatory interaction was revealed with double conditioning and it was suggested that the descending path and the distal cutaneous afferents converge upon a common group of interneurones, which postsynaptically inhibit an early (possibly the first one) interneurone in the FRA pathways. As low threshold distal cutaneous afferents supply the primary peripheral input via climbing fibres to the cerebello-cortical D zone, it was concluded that the different stimuli (central or peripheral) which activate a common group of inferior olivary neurones destined for the D zone also activate a common group of segmental inhibitory interneurones. The results are discussed in relation to current concepts of segmental motor control, and it is suggested that the mechanisms studied could be involved in the regulation of stepping.

Rubrospinal control of static and dynamic fusimotor neurones.

Rubrospinal effects on about 60 extracellularyl recorded gamma-motoneurones were studied in anesthetized cats. All cells were antidromically identified from various muscle nerves. 23 cells were regarded as dynamic as they were activated from a mesencephalic region previously known to influence selectively muscle spindle dynamic sensitivity. The pattern of rubrospinal influence on static fusimotor neurones to different muscles closely followed that previously demonstrated for alpha-motoneurones with pr edominantly excitation of flexor neurones and excitation or inhibition in equal amounts of extensor cells. Dynamic fusimotor neurones were influenced in a strictly reciprocal manner with excitation of flexor cells and inhibition of extensor cells except for a few neurones which could not be reached from nucleur ruber. Evidence was also obtained indicating that the shortest path from nucleus ruber to static fusimotor neurones involves one interneurone.