Comparative effects of pentylenetetrazol on the sensory responsiveness of lateral geniculate and reticular formation neurons

Electrical stimulation therapies for CNS disorders and pain are mediated by competition between different neuronal networks in the brain

CNS neuronal networks are known to control normal physiological functions, including locomotion and respiration. Neuronal networks also mediate the pathophysiology of many CNS disorders. Stimulation therapies, including localized brain and vagus nerve stimulation, electroshock, and acupuncture, are proposed to activate "therapeutic" neuronal networks. These therapeutic networks are dormant prior to stimulatory treatments, but when the dormant networks are activated they compete with pathophysiological neuronal networks, disrupting their function. This competition diminishes the disease symptoms, providing effective therapy for otherwise intractable CNS disorders, including epilepsy, Parkinson's disease, chronic pain, and depression. Competition between stimulation-activated therapeutic networks and pathophysiological networks is a major mechanism mediating the therapeutic effects of stimulation. This network interaction is hypothesized to involve competition for "control" of brain regions that contain high proportions of conditional multireceptive (CMR) neurons. CMR regions, including brainstem reticular formation, amygdala, and cerebral cortex, have extensive connections to numerous brain areas, allowing these regions to participate potentially in many networks. The participation of CMR regions in any network is often variable, depending on the conditions affecting the organism, including vigilance states, drug treatment, and learning. This response variability of CMR neurons is due to the high incidence of excitatory postsynaptic potentials that are below threshold for triggering action potentials. These subthreshold responses can be brought to threshold by blocking inhibition or enhancing excitation via the paradigms used in stimulation therapies. Participation of CMR regions in a network is also strongly affected by pharmacological treatments (convulsant or anesthetic drugs) and stimulus parameters (strength and repetition rate). Many studies indicate that treatment of unanesthetized animals with antagonists (bicuculline or strychnine) of inhibitory neurotransmitter (GABA or glycine) receptors can cause CMR neurons to become consistently responsive to external inputs (e.g., peripheral nerve, sensory, or electrical stimuli in the brain) to which these neurons did not previously respond. Conversely, agents that enhance GABA-mediated inhibition (e.g., barbiturates and benzodiazepines) or antagonize glutamate-mediated excitation (e.g., ketamine) can cause CMR neurons to become unresponsive to inputs to which they responded previously. The responses of CMR neurons exhibit extensive short-term and long-term plasticity, which permits them to participate to a variable degree in many networks. Short-term plasticity subserves termination of disease symptoms, while long-term plasticity in CMR regions subserves symptom prevention. This network interaction hypothesis has value for future research in CNS disease mechanisms and also for identifying therapeutic targets in specific brain networks for more selective stimulation and pharmacological therapies.

Photic epilepsy problems raised in man and animals

The data gathered in 30 years' study in man and in several animal species, but especially in the Papio papio baboon, tend to show that the cortex plays a decisive part in the seizure and interval discharges induced by intermittent light stimulation in photic epilepsy. Two regions of the cortex predominate: the frontorolandic and occipital regions. The cortical cortex can, indeed, transmit or control the visual input to the frontorolandic region and can cause intermittent discharges in certain specific conditions in baboons and in certain human patients. The corticocortical pathway conveys the visual impulses to the frontorolandic cortex and is certainly modulated by deep structures like the reticular systems and the thalamus. At present no more can be said since further research is needed.

Kainic acid microinjected into the cat raphe dorsal nucleus modulates the somatosensory evoked potentials and their cycles of excitability

Studies were made on the effect of the neuroexcitatory agent kainic acid, microinjected into raphe dorsal nucleus by glass micropipette and an air pressure system in doses ranging from 0.2 to 24.0 nmol (in volumes from 0.05 microliter to 0.47 microliter), on the somatosensory evoked potentials and their cycles of recovery (excitability) obtained from cortex (primary somatosensory and parietal associative), thalamus (ventral posterolateral nucleus and centre median nucleus), mesencephalic reticular formation and raphe dorsal nucleus. Kainic acid in doses higher than 3 nmol exerted an activating effect on the evoked potentials and their recovery cycles especially in thalamus and mesencephalic reticular formation. The analysis of these electrophysiological parameters revealed that the non-specific structures were involved to a larger extent in the activating effect of kainic acid than the specific ones. The morphological changes were not severe and were limited to a part of the raphe dorsal nucleus neurons. Our data indicate that kainic acid injected into raphe dorsal nucleus modulates (in direction of facilitation) the somatosensory evoked potentials and their cycles of excitability obtained in some brain structures. The results suggest that this nucleus is involved in the somatosensory information processing in a non-specific manner.

Blink reflex elicited by auditory stimulation in the rabbit

The pathway of the blink reflex, elicited by auditory stimulation, was investigated electrophysiologically. The reflex was recorded as microvibrations of the eyelid and was named the auditory-evoked eyelid microvibration (AMV). Pharmacophysiological studies suggest that AMV is closely related to the midbrain reticular formation and studies of electrical lesions in the midbrain reticular formation support this. Lesions in several parts of the central nervous system provide evidence that the inferior colliculus has an important role in AMV, and the cerebral cortex may have an inhibitory influence. Studies of brainstem transections indicate that the reflex pathway of AMV exists between the lower midbrain and the upper medulla. Because of its ease and simplicity, AMV is believed to be a useful test for evaluation of the function of the brainstem.

Multiunitary activity analysis of cortical and subcortical structures in paroxysmal discharges and grand mal seizures in photosensitive baboons

Cortical and subcortical multiunitary activities (MUA) and EEG were simultaneously recorded in baboons made photosensitive by a subconvulsant dose of DL-allylglycine. Intermittent light stimulation (ILS) trains induced in these animals fronto-rolandic (FR) paroxysmal discharges (PDs, constituted as spikes and waves) and grand mal seizures. During the induction of FR PDs by ILS trains, the visual structures (occipital cortex, colliculi superioris, pulvinar) showed a significant MUA increase which was not related to the PD spike or wave but correlated with the flashes. The first structure showing bursts of MUA that frequently preceded the PD appearance was the FR cortex. When PDs appeared, the bursts were related to the spikes of PDs and were followed by an inhibition during the slow wave. The pontine and mesencephalic reticular formations and the facial nuclei were activated in bursts after the FR PDs had reached a certain amplitude. The thalamic nuclei ventralis lateralis, centrum medianum and lateralis posterior were activated only later, when the FR PDs had reached an even greater amplitude. It is suggested that the activation of visual structures is necessary for FR PD appearance. The secondary pontine and mesencephalic activation could reinforce that of the FR cortex and then the thalamus, and could determine the myoclonus observed in unparalysed animals. When the ILS is continued, grand mal seizures appear. The onset of the seizures could be linked to the loss of FR cortical control of the subcortical structures. The resulting reticular activation would be responsible for the vasomotor modifications which constitute the first clinical signs of a seizure.

Comparative effects of convulsant drugs on the sensory responses of neurons in the amygdala and brainstem reticular formation

The sensory responses of neurons in the amygdala and mesencephalic reticular formation in the cat were enhanced following the intravenous administration of subconvulsant doses of bicuculline, strychnine, bemegride, pentylenetetrazol and physostigmine. The degree and intensity of the enhancement of the response was considerably greater in the reticular formation than in the amygdala. The latency of the response in simultaneously-recorded pairs of neurons in the amygdala and reticular formation was significantly shorter in the mesencephalic reticular formation. The enhancement induced by convulsants does not appear to be transmitter-specific, since enhancement was produced with sequential administration of convulsant drugs which affect gamma-aminobutyric acid (GABA), glycine or acetylcholine. These findings suggest that the reticular formation is involved, to a larger degree than the amygdala, in the ability of sensory stimuli to initiate generalized convulsive seizures in animals treated with these convulsant drugs. The enhancement of the response in the hippocampus and cortex, which has previously been shown to exhibit a longer latency and a lower degree of enhancement than the reticular formation, coupled with the findings in the amygdala, suggest that the reticular formation may mediate the enhancement of the response of these other regions of the brain. The spread of the enhancement of the response to other structures in the brain via the widely distributed output pathways from the reticular formation may lead to initiation of generalized seizures by a recruitment-like process, which may involve enlargement of the sensory hyperresponsive neural network of the brain until a critical neural mass is reached and initiation of seizures results.

Hyperalgesia induced by altered glycinergic activity at the spinal cord

Glycine or its receptor antagonist, strychnine, were administered perispinally to investigate their effect on nociceptive responses elicited by activation of various cutaneous receptors. Strychnine produced dose-dependent sensory and motor disturbances; 1 and 5 micrograms doses were sub-convulsive, eliciting recurrent episodes of coordinated grooming, scratching and biting at the skin, which persisted for approximately 10 minutes post-injection; higher doses (25 and 100 micrograms) increased the intensity and duration of these effects, and produced convulsive motor seizures. Motor disturbances were not elicited by glycine (5, 25, 100 and 400 micrograms). Strychnine treated rats, at all doses, vocalized consistently in response to light cutaneous stimulation; a significant proportion of glycine treated rats also vocalized, but were not as sensitive to mild stimulation. Skin hyperalgesia persisted for at least 30 minutes in both strychnine and glycine treated rats. Both strychnine and glycine significantly reduced vocalization thresholds to tail shock. However, no clear effect on tail flick latency was observed following either strychnine or glycine. These results indicate that glycinergic neurons contribute to the tonic regulation of nociceptive input at the spinal cord.

Geniculate spikes during epileptic seizures induced in dogs by pentylenetetrazol and bicuculline

Previous studies have indicated that in cats generalized seizures induced by pentylenetetrazol (PTZ) or bicuculline (BIC) may be preceded by high frequency multi-unit activity in brain-stem structures. This activity is not readily demonstrable using conventional EEG recording bandwidths and precedes the onset of spikes and spikes-wave activity in thalamo-cortical systems. Recording with chronic subdural and depth electrodes during PTZ and BIC-induced seizures in 18 dogs essentially replicated these results. However, a new and consistent finding was that the high frequency discharges accompanying seizure onset were most often seen, and most often appeared first, in the lateral geniculate body, neocortex and hippocampus being involved later. The findings are considered to suggest a significant role for subcortical structures in this particular model of generalized cortico-reticular epilepsy.

Strychnine antagonizes vaginal stimulation-produced analgesia at the spinal cord

Vaginal-cervical mechanostimulation (VS) suppresses vocalization and withdrawal responses to noxious stimulation. To determine whether the inhibitory neurotransmitter, glycine, contributes to the action of VS, strychnine, a specific glycine receptor antagonist was administered perispinally via intrathecal catheter in dosages of 1,5,25 and 100 micrograms. Prior to strychnine administration, VS (400 g force) elevated thresholds to elicit vocalization in response to graded intensities of tail shock, and blocked vocalization elicited by stimulation of a skin area, previously sensitized by intradermal injection of a 20% yeast solution. After strychnine administration the analgesic effects of VS were significantly attenuated. These findings suggest that the analgesic action of VS is partially mediated by glycine at the spinal level.

Effects of iontophoretic application of convulsants on the sensory responses of neurons in the brain-stem reticular formation

Iontophoretic or pneumatic application of convulsants produced enhancement of the sensory responses of brain-stem reticular formation (RF) neurons in cat and rat. Enhanced sensory responsiveness is observed with visual, auditory, somatosensory and vibrissa manipulation following application of strychnine, bicuculline or pentylenetetrazol. The effect was not transmitter-specific, since it could be produced by sequential application of bicuculline and strychnine which are thought to affect different neurotransmitters. Intravenous administration of convulsant also enhanced the responsiveness of RF neurons which had previously been affected by application of a different convulsant. Both systemic and iontophoretic effects could be antagonized by application of GABA and glycine. These data, coupled with previous findings in primary sensory pathways, indicate that sensory response enhancement in the RF induced by intravenous administration of these convulsants is in large part due to actions on synaptic elements of reticular formation neurons.

The influence of intermittent light stimulation on potentials evoked by single flashes in photosensitive and non-photosensitive Papio papio

The effects of intermittent light stimulation (ILS) on visual potentials (VEPs), evoked in different cortical areas, were statistically studied in baboons either naturally photosensitive or made photosensitive by allylglycine at a subconvulsant dose, as well as in non-photosensitive animals. VEPs were induced by single flashes (paradigm a) or by flashes preceded by trains of ILS (paradigm b). In every baboon, photosensitive or not, the VEPs induced by paradigm b in the striate area show a decrease of amplitude compared to VEPs induced by paradigm a. The ERG evolves in the same way. Therefore, these effects do not depend on photosensitivity; they depend on the intensity of stimulation. In photosensitive animals the single flash in paradigm b can induce a paroxysmal VEP in the fronto-rolandic (FR) area. In parietal and peristriate areas the VEPs induced by paradigm b show new late components when compared to those induced by paradigm a. These changes are observed even if no FR paroxysmal VEP is induced; they depend on the presence of a train of ILS preceding the single flash and on the predisposition to epilepsy (both natural and due to allylglycine); in the non-photosensitive animals the VEPs recorded in the same areas do not show such differences. We consider that among afferents which could act in inducing FR paroxysmal activities some cortico-cortical visual afferents can come from non-specific cortical areas (parietal or peristriate), but would not directly originate in the striate cortex; anatomical data in this species may support such a hypothesis.

Bicuculline-induced enhancement of sensory responses and cross-correlations between reticular formation and cortical neurons

The visual, auditory and somatosensory responses of neurons in the brain stem reticular formation (RF) and pericruciate cortex of the cat are enhanced by intravenous administration of subconvulsant doses of bicuculline. The degree of enhancement in RF neurons is somewhat greater in magnitude and occurs in a greater percentage of RF neurons. The latency of response is shorter in the RF than in the cortex in 70% of cases. A large percentage of simultaneously recorded RF and cortical neurons which became responsive to the same stimulus exhibited consistent convulsant-induced cross-correlations of firing which were not present before drug treatment. The latency and correlation data are consistent with the possibility that the RF may subserve the cortical enhancement. Auditory response thresholds in RF neurons are reduced by bicuculline administration. Enhancement of RF neuronal responsiveness has previously been observed with several other convulsant drugs which are thought to act on different neurotransmitters suggesting that it may reflect a general action of these agents beyond the effects on specific neurotransmitters. The bicuculline-induced correlation of firing of RF and pericruciate neurons may be involved in the mechanism of initiation of convulsant-mediated seizure generalization induced by sensory stimuli.

Effects of bemegride on the sensory responses of neurons in the hippocampus and brain stem reticular formation

Administration of subconvulsant doses of bemegride results in extensive enhancement of sensory responsiveness to auditory, visual, somatosensory and vibrissa stimulation of neurons in the medullary and mesencephalic reticular formation (RF). This effect is not altered by cord transection. Most neurons in the dorsal hippocampus do not show enhanced sensory responsiveness. In the minority of hippocampal neurons which do show enhancement the effect is much less extensive than that seen in the RF despite larger doses of bemegride. The enhancement of RF neuronal response may involve the reversal of repetition induced response attenuation ('habituation') and reductions in response threshold. During EEG seizures the firing of neurons in RF and hippocampus are temporally correlated with the spikes in the cortical EEG. The effects of bemegride on RF neurons are similar to those previously reported for strychnine and pentylenetetrazol and preliminary studies with physostigmine and bicuculline. These findings further extend the concept that a relatively selective enhancement of the sensory responses of brain stem reticular formation neurons may be indicative of a general neuronal effect of convulsants which may play an important role in the initiation of sensory-induced seizures.

Strychnine effects on the sensory response patterns of reticular formation neurons

Strychnine (ST) administration results in enhanced responses of reticular formation neurons of the cat to visual, auditory, or somatosensory stimuli. Neurons in the bulbar reticular formation (BRF) also exhibited ST-induced spike-bursting which coincides with the onset of 10--20 Hz high amplitude rhythmic discharge in the lower brain stem often obscuring sensory response enhancement. Mesencephalic reticular formation (MRF) neurons showed enhanced sensory responsiveness at higher doses of ST but rarely showed spike-bursting. The rhythmic discharge in the EEG of BRF and spike-bursting in BRF neurons was not observed in animals with spinal cord transection suggesting that this bursting activity is due to the action of ST on the spinal cord. The enhancement of sensory responses may be due in part to reversal of repetition-induced response attenuation (habituation) mechanisms in RF neurons. The ST-induced enhancement of RF neuronal responses to sensory stimuli is similar to that reported previously with pentylenetetrazol. These findings extend the concept that enhancement of RF responses to sensory stimuli may be indicative of a general neuronal action of convulsant agents which may be important in sensory-induced seizures.