Daryl W. Hochman

Thresholds for thermal damage to normal tissues: An update

The purpose of this review is to summarise a literature survey on thermal thresholds for tissue damage. This review covers published literature for the consecutive years from 2002–2009. The first review on this subject was published in 2003. It included an extensive discussion of how to use thermal dosimetric principles to normalise all time-temperature data histories to a common format. This review utilises those same principles to address sensitivity of a variety of tissues, but with particular emphasis on brain and testis. The review includes new data on tissues that were not included in the original review. Several important observations have come from this review. First, a large proportion of the papers examined for this review were discarded because time–temperature history at the site of thermal damage assessment was not recorded. It is strongly recommended that future research on this subject include such data. Second, very little data is available examining chronic consequences of thermal exposure. On a related point, the time of assessment of damage after exposure is critically important for assessing whether damage is transient or permanent. Additionally, virtually no data are available for repeated thermal exposures which may occur in certain recreational or occupational activities. For purposes of regulatory guidelines, both acute and lasting effects of thermal damage should be considered.

Dissociation of synchronization and excitability in furosemide blockade of epileptiform activity.

Furosemide, a chloride cotransport inhibitor, reversibly blocked synchronized burst discharges in hippocampal slices without reducing the pyramidal cell response to single electrical stimuli. Images of the intrinsic optical signal acquired during these slice experiments indicated that furosemide coincidentally blocked changes in extracellular space. In urethane-anesthetized rats, systemically injected furosemide blocked kainic acid-induced electrical discharges recorded from cortex. These results suggest that (i) neuronal synchronization involved in epileptiform activity can be dissociated from synaptic excitability; (ii) nonsynaptic mechanisms, possibly associated with furosemide-sensitive cell volume regulation, may be critical for synchronization of neuronal activity; and (iii) agents that affect extracellular volume may have clinical utility as antiepileptic drugs.

Enhanced optical imaging of rat gliomas and tumor margins.

Current intraoperative methods used to maximize the extent of tumor removal are limited to intraoperative biopsies, ultrasound, and stereotactic volumetric resections. A new technique involving the optical imaging of an intravenously injected dye has the potential to localize tumors and their margins with a high degree of accuracy. In a rat glioma model, enhanced optical imaging was performed and indocyanine green was used as the contrast-enhancing agent. In all 22 animals, the peak optical change in the tumor was greater than in the ipsilateral brain around the tumor and the contralateral normal hemisphere. The clearance of the dye was significantly delayed to a greater extent in the tumor than in the brain around the tumor and the normal brain. After attempts were made at complete microscopic resection, enhanced optical imaging of the tumor margins and the histological samples demonstrated a specificity of 93% and a sensitivity of 89.5%. Enhanced optical imaging was capable of outlining the tumor even when the imaging was done through the cranium. The optical imaging of rat gliomas with a contrast-enhancing dye is able to differentiate between normal brain and tumor tissue both at the cortical surface and at the tumor margins. The application of these studies in an intraoperative clinical setting may allow for the more accurate determination of tumor margins and may increase the extent of tumor removal.

Characterization of heterotopic cell clusters in the hippocampus of rats exposed to methylazoxymethanol in utero

Cortical disorganization represents one of the major clinical findings in many children with medically intractable epilepsy. To study the relationship between seizure propensity and abnormal cortical structure, we have begun to characterize an animal model exhibiting aberrant neuronal clusters (heterotopia) and disruption of cortical lamination. In this model, exposing rats in utero to the DNA methylating agent methylazoxymethanol acetate (MAM; embryonic day 15) disrupts the sequence of normal brain development. In MAM-exposed rats, cells in hippocampal heterotopia exhibit neuronal morphology and do not stain with immunohistochemical markers for glia. In hippocampal slices from MAM-exposed animals, extracellular field recordings within heterotopia suggest that these dysplastic cell clusters make synaptic connections locally (i.e. within the CA1 hippocampal subregion) and also make aberrant synaptic contact with neocortical cells. Slice perfusion with bicuculline or 4-aminopyridine leads to epileptiform activity in dysplastic cell clusters that can occur independent of input from CA3. Taken together, our findings suggest that neurons within regions of abnormal hippocampal organization are capable of independent epileptiform activity generation, and can project abnormal discharge to a broad area of neocortex, as well as hippocampus.

Optical Imaging of Epileptiform Activity in Human Neocortex

Summary:  The surgical outcomes of patients suffering from neocortical epilepsy are not as successful as the surgical outcomes from resections of epilepsy patients with mesial temporal sclerosis. The main difficulty in the treatment of neocortical epilepsy is that current technology has limited accuracy in mapping neocortical epileptogenic tissue. It is known that the optical spectroscopic properties of brain tissue are correlated with changes in neuronal activity. The method of mapping these activity‐evoked optical changes is known as imaging of intrinsic optical signals (IIOS). Activity‐evoked optical changes measured in neocortex are generated by changes in cerebral hemodynamics (i.e., changes in blood oxygenation and blood volume). Our experimental approach was to acquire high‐resolution IIOS maps of epileptiform activity in patients undergoing surgery for medically intractable neocortical epilepsy. Both spontaneous and stimulation‐evoked epileptiform activity was monitored. Imaging of intrinsic optical signals was able to localize neocortical epileptic foci precisely by using changes in blood volume in contrast to changes in blood oxygenation. IIOS has the potential to translate from a purely research tool to a new intraoperative approach for the surgical treatment of neocortical epilepsy.

Five percent CO2 is a potent, fast-acting inhalation anticonvulsant

Purpose:  CO2 has been long recognized for its anticonvulsant properties. We aimed to determine whether inhaling 5% CO2 can be used to suppress seizures in epilepsy patients. The effect of CO2 on cortical epileptic activity accompanying behavioral seizures was studied in rats and nonhuman primates, and based on these data, preliminary tests were carried out in humans.

The extracellular space and epileptic activity in the adult brain: Explaining the antiepileptic effects of furosemide and bumetanide

Treatments that modulate the size of the extracellular space (ECS) also block epileptiform activity in adult brain tissue. This includes the loop diuretics furosemide and bumetanide, and alterations of the osmolarity of the ECS. These treatments block epileptiform activity in a variety of laboratory adult seizure models regardless of the underlying synaptic and physiologic mechanisms generating the seizure activity. Optical imaging studies on adult hippocampal slices show that the blockade of epileptiform activity by these treatments is concomitant with their blockade of activity‐driven changes of the ECS. Here we develop and analyze the hypothesis that activity‐driven changes in the size of the ECS are necessary for the maintenance of hypersynchronous epileptiform activity. In support of this hypothesis is an accumulation of data from a number of studies suggesting that furosemide and bumetanide mediate antiepileptic effects through their blockade of cell swelling, dependent on their antagonism of the glial Na+‐K‐2Cl cotransporter (NKCC1).

Knock-in mouse model of alternating hemiplegia of childhood: Behavioral and electrophysiologic characterization

Mutations in the ATP1α3 subunit of the neuronal Na+/K+‐ATPase are thought to be responsible for seizures, hemiplegias, and other symptoms of alternating hemiplegia of childhood (AHC). However, the mechanisms through which ATP1A3 mutations mediate their pathophysiologic consequences are not yet understood. The following hypotheses were investigated: (1) Our novel knock‐in mouse carrying the most common heterozygous mutation causing AHC (D801N) will exhibit the manifestations of the human condition and display predisposition to seizures; and (2) the underlying pathophysiology in this mouse model involves increased excitability in response to electrical stimulation of Schaffer collaterals and abnormal predisposition to spreading depression (SD).

Imaging of intrinsic optical signals in primate cortex during epileptiform activity.

Summary:  Localized increases in neuronal activity are known to alter the distribution and oxygen content of blood within the surrounding brain tissue. In the neocortex, these activity‐evoked hemodynamic changes are predominantly mediated through the dilation of the microscopic pial arterioles that lie on the surface of the brain, nearest to the site of activation. Since hemoglobin absorbs light throughout the visible and near‐infrared spectrum, optical microscopy combined with computer imaging techniques can be used to map the patterns of hemodynamic changes associated with neuronal activity. Examples of optical imaging data are provided here to demonstrate four points. First, depending on the optical wavelength chosen for illumination of the cortex, different spatial and temporal patterns of optical changes are elicited by similar stimuli yielding distinctly different types of physiological information. Second, by selecting the appropriate wavelengths, it is possible to generate maps from optical‐imaging data that represent changes predominately due to either blood volume (at 535 nm) or blood oxygenation (at 660 nm). Third, “negative” optical signals are negative only relative to a given optical wavelength, and appear to be associated with more intense types of neuronal activation. Fourth, optical imaging is a useful technique for studying neocortical seizure activity in animal models, with the caveat that species‐specific differences in cortical size and vascularization patterns may be important to consider in the interpretation of optical imaging data.

Loop Diuretics Have Anxiolytic Effects in Rat Models of Conditioned Anxiety

A number of antiepileptic medications that modulate GABAA mediated synaptic transmission are anxiolytic. The loop diuretics furosemide (Lasix) and bumetanide (Bumex) are thought to have antiepileptic properties. These drugs also modulate GABAA mediated signalling through their antagonism of cation-chloride cotransporters. Given that loop diuretics may act as antiepileptic drugs that modulate GABAergic signalling, we sought to investigate whether they also mediate anxiolytic effects. Here we report the first investigation of the anxiolytic effects of these drugs in rat models of anxiety. Furosemide and bumetanide were tested in adult rats for their anxiolytic effects using four standard anxiety models: 1) contextual fear conditioning; 2) fear-potentiated startle; 3) elevated plus maze, and 4) open-field test. Furosemide and bumetanide significantly reduced conditioned anxiety in the contextual fear-conditioning and fear-potentiated startle models. At the tested doses, neither compound had significant anxiolytic effects on unconditioned anxiety in the elevated plus maze and open-field test models. These observations suggest that loop diuretics elicit significant anxiolytic effects in rat models of conditioned anxiety. Since loop diuretics are antagonists of the NKCC1 and KCC2 cotransporters, these results implicate the cation-chloride cotransport system as possible molecular mechanism involved in anxiety, and as novel pharmacological target for the development of anxiolytics. In view of these findings, and since furosemide and bumetanide are safe and well tolerated drugs, the clinical potential of loop diuretics for treating some types of anxiety disorders deserves further investigation.