Robert S. Cross

Changes in local cerebral glucose utilization induced by convulsants

With the six convulsants studied (Soman, intrahippocampal penicillin, bicuculline, pentylenetetrazol, picrotoxin and strychnine), the anatomical distribution of changes in local cerebral glucose utilization was related to the type of seizure observed. Strychnine induced a few very intense motor convulsions during the 2-deoxyglucose experimental period without having a major effect on brain local cerebral glucose utilization, in support of the view that its actions are predominantly in the spinal cord. Pentylenetetrazol and picrotoxin induced intermittent intense seizures and marked increases in local cerebral glucose utilization in the globus pallidus and substantia nigra. Soman, intrahippocampal penicillin and bicuculline all induced persistent status epilepticus associated with increases in local cerebral glucose utilization in many brain areas; those with striking increases in glucose use include: cortical areas, the limbic system, basal ganglia and substantia nigra. The glucose use changes produced by Soman, penicillin and bicuculline greatly exceeded those induced by pentylenetetrazol and picrotoxin. Activation of the substantia nigra and basal ganglia occurred with all centrally mediated convulsions and with status epilepticus there was also marked activation of cortical and limbic structures.

Ketamine-induced Changes in Regional Glucose Utilization in the Rat Brain

Ketamine appears to induce both excitatory and depressant actions in the brain; however, it is not clear which regions are affected. The 2-deoxyglucose functional mapping method of Sokoloff et al. was used to determine regional variations in metabolic activity of rat brain caused by injection of ketamine, 25-75 mg, intramuscularly. To compare the effects of ketamine with those of hippocampal-induced seizures, the 2-deoxyglucose method was used, following injection of penicillin G, 400-800 units, into the hippocampus. The findings from five control, seven ketamine-treated, and three penicillin G-treated rats are given. Ketamine caused a significant increase of metabolic activity in the hippocampal sulci and a decrease of activity in the medial geniculate and the inferior colliculus. Similar changes were found with hippocampal seizures caused by penicillin. The inhibition of the regions associated with sensory systems (medial geniculate and inferior colliculus) may account in part for the anesthetic action of ketamine, while the intense activity of the hippocampus may be related to the excitatory manifestations. The results indicate that ketamine produces seizures in the hippocampus, which in turn inhibit auditory and visually associated nuclei. Thus, the anesthesia may follow from the sensory depression and the cataleptic phenomena may be related to the hippocampal excitation.

Soman induced changes in brain regional glucose use

Soman, a potent central acetylcholine esterase inhibitor, has a greater impact on brain regional glucose use than other organophosphates, such as diisopropylfluorophosphate (DFP) or phospholinium iodide. At near-lethal doses soman induced explosive persistent seizures that were associated with a greater than fourfold increase of glucose use in many brain structures. Single near-lethal doses of soman lead to conspicuous neuronal damage and a marked reduction in brain activity, 1 to 3 days after exposure. When soman (2 X LD50) was given to TAB (an antidotal mixture of trimedoxime, atropine, and benactyzine ) pretreated rats, there was a greater than twofold reduction of glucose use in almost every brain region. We suggest that soman seizures are mediated via activation of muscarinic receptors; also, the substantia nigra has a key role in the initiation/propagation of seizures. Soman has in addition, a depressive effect on some brain components which appears not to involve muscarinic receptors. We suggest that the conspicuous pathology that follows a single, near-lethal dose of soman results from a depletion of energy flow along with an influx of Ca2+ which sets into motion a cascade of destructive reactions, such as activation of proteases.

Cholinergic systems influence local cerebral glucose use in specific anatomical areas: diisopropyl phosphorofluoridate versus soman.

The organophosphates, diisopropyl phosphorofluoridate and soman have a common mechanism of action (inhibition of acetylcholinesterase), but result in very different behavioral responses in the rat. Soman rapidly produced persistent tonic convulsions whereas diisopropyl phosphorofluoridate only infrequently produced transient convulsive-like activity. Soman increased local cerebral glucose use in most of the cortex, striato-pallido-nigral pathway, limbic system and in specific thalamic nuclei whereas diisopropyl phosphorofluoridate increased glucose use in a limited fashion, primarily in the dorsal striato-pallido-nigral pathway. When diazepam blocked soman-induced convulsions, the pattern of glucose use was strikingly similar to that caused by diisopropyl phosphorofluoridate. Soman or diisopropyl phosphorofluoridate depressed local cerebral glucose use in rats pretreated with the antidotal mixture of trimedoxime, atropine and benactyzine (muscarinic antagonists). Also, this antidotal mixture blocked the increased glucose use in the dorsal striato-pallido-nigral system produced by either acetylcholinesterase inhibitor, indicating that muscarinic receptors mediate the excitation of this pathway. Both diisopropyl phosphorofluoridate and soman activate the striato-pallido-nigral pathway but soman also causes spread of activity producing overt motor convulsions. Possible explanations for this difference in response to the organophosphates are differential responses in cholinergic actions within specific brain regions or some non-cholinergic action of soman.

Superior colliculus activation by retinal nicotinic ganglion cells: a 2-deoxyglucose study

Systemic injection of the acetylcholinesterase inhibitor, di-isopropylfluorophosphate, in rats causes a marked increase in glucose use in the superficial layers of the superior colliculus. This activation of the superior colliculus is largely a retinal effect. Furthermore, since this response can be blocked by intraocular as well as systemic injections of mecamylamine, it is postulated that retinal nicotinic receptors are involved.

Effects of Antidotes on Soman-Induced Brain Changes

Rats were pretreated with either diazepam, atropine or benactyzine 10 min prior to soman injection. Local cerebral glucose use (LCGU) was determined during the seizure phase (15 min post soman) or pathology phase (72 h post soman). Diazepam and benactyzine pretreatment prevented convulsive activity, whereas atropine pretreatment only reduced the duration of convulsive activity after soman exposure. Each pretreatment agent had a unique impact on LCGU pattern during the seizure phase. During the pathology phase, the marked reduction in LCGU and the conspicuous brain damage associated with soman-induced seizures was minimized by all three pretreatments.

Patterns of glucose use after bicuculline-induced convulsions in relationship to γ-aminobutyric acid and μ-opioid receptors in the ventral pallidum — functional markers for the ventral pallidum

Bicuculline-induced convulsions increased glucose use throughout the brain and sharply demarcated the ventral pallidum and globus pallidus. Glucose use in the nucleus accumbens also increased after bicuculline-induced convulsions, except for a circumscribed region in the dorsomedial shell. Since the projection from the nucleus accumbens to the ventral pallidum contains gamma-aminobutyric acid (GABA) and the opioid peptide, enkephalin, the pattern of increased glucose use in the ventral pallidum and nucleus accumbens after bicuculline-induced convulsions was compared to the topography of GABAA and mu-opioid receptors. The pattern of glucose use in the nucleus accumbens and ventral pallidum resembled the topography of GABAA, but differed from that of mu-opioid receptors. Bicuculline may disinhibit GABAergic efferents to the ventral pallidum resulting in a dramatic increase in glucose use within striatopallidal synaptic terminals as well as in local terminals of the pallidal projection neurons.

Alterations in Brain Glutathione Homeostasis Induced by the Nerve Gas Soman

Public awareness of the dangers of chemical and biological warfare has been heightened in recent times. In particular, chemical nerve agents such as soman and its analogs have been developed and used in war as well as recent incidents, such as in Iraq and Japan. Soman, a rapid acting acetylcholinesterase inhibitor, produces a status epilepticus that leads to extensive neuropathology in vulnerable brain regions (eg. piriform cortex and hippocampus). This study was undertaken to determine whether oxidative mechanisms are involved in brain pathology during soman toxicity. Intracellular thiols such as glutathione (GSH) and protein sulfhydryls (PrSH) are among the most critical antioxidants used to combat oxidative stress. Here we report that during the seizure phase (1 h post soman exposure), PrSH levels in piriform cortex and hippocampus were decreased without changes in glutathione (GSH) levels. However, by 24 h post somam exposure (pathology phase), GSH levels were decreased by nearly 50% in the piriform cortex with a corresponding decrease in PrSH groups. The shift to a more oxidized thiol status indicates that oxygen free radicals likely participate in the neuropathoplgy associated with soman-induced seizures.

Loss and subsequent recovery of local cerebral glucose use in visual targets after controlled optic nerve crush in adult rats

A mild crush of the adult rat optic nerve serves as a model to study the restoration of function after traumatic brain injury. It causes a progressive degeneration of retinal ganglion cells, but visually guided behavior is partially restored within 2-3 weeks. The purpose of this study was to determine to what extent local cerebral glucose use (LCGU) decreases and if it recovers in retinofugal targets following unilateral optic nerve crush. At intervals of 2, 9, and 22 days after crush, LCGU was monitored in rats in which the visual system was stimulated by a strobe-light and pattern. In the ipsilateral retinofugal targets there was only a minimal loss of LCGU use, but in the contralateral retinofugal targets, LCGU was reduced at Postlesion Day 2: to 50% in the superior colliculus (SC), to 60% in the lateral geniculate nucleus of the thalamus (LGN), and to 87% in the visual cortex. On Postoperative Days 9 and 22 we observed a partial restoration of LCGU in the contralateral SC and LGN to 68 and 79%, respectively. As recovery of visual performance is known to follow a similar time course, we conclude that restoration of metabolic activity in target structures may contribute to the restoration of vision after optic nerve crush.

Alterations in local cerebral glucose utilization produced by D3 dopamine receptor-selective doses of 7-OH-DPAT and nafadotride

The D3 dopamine receptor, localized primarily in limbic brain areas, is a potential antipsychotic site. The effects of D3 receptor stimulation or blockade on neuronal activity were determined using the [14C]-2-deoxyglucose method. Freely-moving, adult, male, Sprague-Dawley rats were injected s.c. with saline, agonist 7-hydroxy-diphenylaminotetralin (7-OH-DPAT) (0.1 mg/kg), or antagonist l-nafadotride (1 mg/kg). These doses of 7-OH-DPAT and l-nafadotride are behaviorally active and are 10-fold lower than a dose producing significant in vivo occupancy of D2 receptors. The [14C]-2-deoxyglucose procedure was initiated 30 min after the administration of the test compound. The rate of local cerebral glucose utilization (LCGU) was determined by quantitative autoradiography. 7-OH-DPAT produced a significant increase in LCGU in the substantia nigra. l-Nafadotride produced significant increases in LCGU in several brain areas including the lateral preoptic area, lateral habenula, caudate, septal area, entorhinal cortex, and some thalamic and hypothalamic areas. These observations indicate that stimulation or blockade of D3 receptors alters LCGU and that produces a unique pattern of alterations in LCGU suggestive of potential antipsychotic activity.