Antoine Depaulis

Spontaneous paroxysmal electroclinical patterns in rat: A model of generalized non-convulsive epilepsy

During quiet wakefulness of 63 adult Wistar rats, 24 exhibited synchronous paroxysmal bursts consisting of spikes and spike and wave discharges, recorded in the amygdala and frontoparietal cortex. Discharges were associated with a sudden immobility of the rat and rhythmic twitches of vibrissae or cervicofacial musculature. As soon as the phenomena stopped, the animal resumed its previous behavior. Paroxysmal electroclinical attacks could be observed as long as the animal survived. Similar electrical discharges have been observed previously in rodents by other authors and interpreted as an epileptic phenomenon. Preliminary pharmacological results confirm this interpretation and emphasize the similarity between our findings and the human petit mal epilepsy.

Longitudinal neuronal organization of defensive reactions in the midbrain periaqueductal gray region of the rat

SummaryIn a previous study we investigated the intraspecific defensive reactions evoked by excitation of neurons in the intermediate third of the midbrain periaqueductal gray matter (PAG) of the rat. Experiments revealed that activation of neurons in this region of the PAG mediated: (i) backward defensive behavior, characterized by upright postures and backward movements, and (ii) reactive immobility (“freezing”), in which the rat remained immobile, but reacted with backward defensive behavior to investigative, non-aggressive contact initiated by the partner. In the present study, we aimed to extend our understanding of PAG mediation of defensive behavior by observing: (i) in a non-aggressive social interaction test, the behavioral effects of microinjections of low doses of kainic acid (40 pmol in 200 nl) made in the caudal third of the PAG; and (ii) the behavioral and cardiovascular effects of microinjections of d, l-homocysteic acid (5–10 nmol in 50–100 nl) made in the PAG of the unanesthetized decerebrate rat. Kainic acid injections into the area lateral to the midbrain aqueduct in the caudal third of the PAG evoked: (i) forward avoidance behavior, characterized by forward locomotion and occasional hop/jumps; (ii) reactive immobility (“freezing”), in which the rat remained immobile, but reacted with forward avoidance behavior to investigative, non-aggressive contact initiated by the partner; and (iii) 22–28 kHz ultrasonic vocalizations. These injections also evoked a dramatic increase in defensive responsiveness to tactile stimuli on the half of the body contralateral, but not ipsilateral, to the site of injection. Electroencephalographic measurements indicated that none of these effects were secondary to seizure activity. In the decerebrate rat, d, l-homocysteic acid injections in the caudal third of the PAG evoked forward running movements along with increased blood pressure and heart rate, the strongest effects being evoked from the region lateral to the midbrain aqueduct. More rostrally, sites in the intermediate PAG evoked backward “defensive” movements, which were also associated with increased blood pressure and heart rate. These data, along with those from our previous study in the rat indicate that: (i) defensive reactions are integrated within a longitudinal neuronal column which spans the caudal two thirds of the lateral PAG; (ii) the lateral PAG “defensive behavior” column contains two distinct populations of neurons, one within the intermediate lateral PAG which integrates defensive behavior characterized by facing towards and backing away from a “threatening” stimulus, and a second in the caudal lateral PAG which integrates defensive behavior characterized by forward avoidance behavior; and (iii) neurons within the lateral PAG couple strong cardiovascular changes with each distinctive defensive behavior pattern. The emerging view from this and recent studies of this midbrain region in other species, suggests that similar rostrocaudal differences within a longitudinally oriented lateral PAG neuronal column represent a fundamental principle underlying the PAG organization of defensive behavior.

Midbrain Periaqueductal Gray Control of Defensive Behavior in the Cat and the Rat

In 1943, Hess and Brugger observed that electrical stimulation within the perifornical hypothalamus transformed a normally calm and placid cat into a highly defensive animal. Subsequently, Hunsperger (1956) reported that electrical stimulation within the midbrain periaqueductal gray region (PAG) or the adjacent midbrain tegmentum, of the freely moving cat, evoked defensive reactions. At about the same time, researchers in Sweden (Eliasson et al., 1954; Lindgren, 1955; Lindgren et al., 1956) found that electrical stimulation within the midbrain, of the anesthetized cat, evoked cardiovascular changes, which included increased arterial pressure, tachycardia and regional blood flow changes. It was left to Abrahams et al. (1960) to confirm that these behavioral and cardiovascular reactions were evoked at the same midbrain sites.

Indentification of midbrain neurones mediating defensive behaviour in the rat by microinjections of excitatory amino acids

Unilateral microinjections (0.20 microliter) of excitatory amino acids were made into the midbrain of freely moving rats. Injections made within the midbrain periaqueductal grey matter (PAG) consistently elicited reactions characteristic of defensive behaviour (i.e. explosive jumps, freezing, upright postures), whereas injections made in the tegmentum bordering the PAG did not as reliably elicit such behaviour. As injections of excitatory amino acids depolarize cell bodies but not axons, the results suggest that a population of neurones whose excitation elicits these reactions is found primarily within the midbrain PAG of the rat. Furthermore, the data suggested that such neurones may be localized preferentially within the caudal half of the midbrain PAG. Injections of the GABA antagonist, bicuculline methiodide, at many of the same midbrain sites produced behaviour similar to that elicited by excitatory amino acids indicating a possible GABAergic modulation of these same PAG-mediated reactions. It also was observed following unilateral injection into the PAG, of either the excitatory amino acid, L-aspartic acid or bicuculline, that defensive behaviour was elicited by touching the rat on the body or snout contralateral but not ipsilateral to the injection site. This suggests that the induction of defensive behaviour by unilateral PAG stimulation is due, at least in part, to lateralized alterations in sensorimotor responsiveness.

Evidence for a critical role of GABAergic transmission within the thalamus in the genesis and control of absence seizures in the rat

The involvement of GABAergic transmission within the thalamus in the generation and control of spike and wave discharges (SWD) in generalized non-convulsive or absence epilepsy was studied in rats with spontaneous SWD and in non-epileptic rats. In epileptic rats, bilateral injections of gamma-vinyl GABA (GVG, 10 micrograms/side) or muscimol (10 ng/side) into the medial part of the ventral lateral thalamus, i.e. the specific relay nuclei, significantly increased spontaneous cortical SWD whereas similar injections into the most lateral part of the thalamus, i.e. the area of the reticular nuclei, significantly suppressed these seizures. Injections of GVG (20 micrograms) or muscimol (20 ng) into the midline thalamus had no direct effect on the spontaneous SWD. In non-epileptic rats, injections of GVG (25 micrograms/side) or muscimol (100 ng/side) into the thalamic relay nuclei produced short SWD on the cortical EEG. These results suggest that GABAergic neurons in the reticular nuclei and their projections to the specific relay nuclei of the thalamus are involved in the elicitation and control of generalized non-convulsive seizures.

Mapping of spontaneous spike and wave discharges in Wistar rats with genetic generalized non-convulsive epilepsy.

Electrical activity was recorded in different parts of the brain in Wistar rats from a strain with genetic generalized non-convulsive epilepsy (GNCE or absence epilepsy). Movable bipolar electrodes were lowered stereotaxically by 1 mm steps into the brain in immobilized animals. Spontaneous spike and wave discharges (SWD) of the largest amplitude were recorded in the cortex and in lateral nuclei of the thalamus where they appeared occasionally to precede. Smaller amplitude SWD were recorded in the striatum, hypothalamus, tegmentum and substantia nigra. No SWD were recorded in limbic structures. Partial limbic seizures induced by the introduction of the electrode did not interfere with occurrence of cortical SWD. These results confirm the primacy of thalamocortical involvement in SWD of GNCE. The absence of spread to limbic structures and the implication of a precisely limited substrate in GNCE accounts for the clinical and pharmacological specificity of this particular kind of epilepsy.

Protective effects of brain-derived neurotrophic factor on the development of hippocampal kindling in the rat

Recent data have suggested the involvement of neurotrophins in the cascade of events occurring during seizure development. In particular, expression of both brain-derived neurotrophic factor (BDNF) and its receptor mRNAs increases in different brain structures after convulsive seizures. The physiological significance of this increase was investigated by chronic intrahippocampal perfusion of BDNF in the model of dorsal hippocampal kindling in the rat. A 7 day perfusion of BDNF, in the region of the stimulating electrode, blocked the development of kindling during the perfusion period and for the following 15 days. These results provide in vivo evidence for a protective role of BDNF in the regulation of plasticity involved in epileptogenesis in adult brain.

Parachlorophenylalanine-induced serotonin depletion increases offensive but not defensive aggression in male rats.

Cerebral 5-HT depletion has been shown to facilitate elicitation of various kinds of aggressive behavior in rats. The question as to whether both offensive and defensive aggressive reactions are affected to the same extent was examined in a resident-intruder paradigm where an ethological analysis of the two animals allows an evaluation of non-social activities as well as agonistic interactions, including both offense and defense. PCPA (375 mg/kg IP) was administered either to the resident or the intruder and the interactions with an untreated conspecific were recorded in the residents home cage for an 8 min period three days after injection when 5-HT was maximally reduced. PCPA treatment increased the occurrence of social approach and offensive postures in resident rats, whereas their untreated partners displayed more defensive reactions. When intruders were injected, only non-significant increases in approach and offense were observed. In no case did PCPA affect occurrence of defensive postures in the injected animals. These results confirm that serotonin plays a role in controlling offensive aggression but not defensive behavior.

Emerging Principles of Organization of the Midbrain Periaqueductal Gray Matter

Throughout this book the term periaqueductal gray (PAG) has been used in preference to the term central gray, since PAG refers specifically to the portion of the ventricular gray matter which surrounds the midbrain aqueduct. Rostrally, the PAG is continuous with the periventricular gray matter surrounding the third ventricle in the hypothalamus and thalamus. Caudally, it is continuous with the periventricular gray matter which in the dorsal pons forms the ventral and ventrolateral border of the fourth ventricle. Although the oculomotor related group of nuclei (i.e., oculomotor and trochlear nuclei, the Edinger-Westphal nucleus, the nucleus of Darkschewitsch and the interstitial nucleus of Cajal) and the dorsal raphe nucleus, constitute the major part of the gray matter ventral to the midbrain aqueduct, they are usually considered, on functional and anatomical grounds, separable from the PAG. The midbrain tegmentum laterally adjacent to the PAG has also usually been considered to be a separate entity. However, as discussed by Holstege (this volume), the PAG and the laterally adjacent tegmentum together, likely form a common neuronal pool which is divided by the fiber stream formed by the tectobulbospinal fibers and the fibers of the mesencephalic trigeminal tract.

Elicitation of intraspecific defensive behaviors in the rat by microinjection of picrotoxin, a γ-aminobutyric acid antagonist, into the Midbrain Periaqueductal gray matter

Behavioral reactions induced in the rat by microinjections of a gamma-aminobutyric acid (GABA) antagonist (picrotoxin; 25 and 50 ng in 0.25 microliter) into the midbrain periaqueductal gray matter were measured in an open-field test and when the animal was confronted by a conspecific introduced into its cage (i.e. resident-intruder paradigm). In the open-field, microinjections of picrotoxin significantly increased backward locomotion while decreasing self-grooming. In the resident-intruder paradigm, microinjections of picrotoxin selectively increased defensive reactions (defensive uprights, defensive sideways, retreat) while offensive behaviors were rather reduced. In addition, the actual nature of the effects was found to depend upon the intruders relative position. Defensive reactions were significantly increased when the partner was on the side contralateral to the injection site, whereas social approach behaviors (fur investigation, anogenital investigation) were decreased when the partner was located on the ipsilateral side. These data suggest the involvement of GABAergic synapses within the midbrain periaqueductal gray matter in the control of intraspecific defensive behaviors in the rat.