Enhanced acoustic startle responding in rats with radiation-induced hippocampal granule cell hypoplasia

Whole brain proton irradiation in adult Sprague Dawley rats produces dose dependent and non-dependent cognitive, behavioral, and dopaminergic effects

Proton radiotherapy causes less off-target effects than X-rays but is not without effect. To reduce adverse effects of proton radiotherapy, a model of cognitive deficits from conventional proton exposure is needed. We developed a model emphasizing multiple cognitive outcomes. Adult male rats (10/group) received a single dose of 0, 11, 14, 17, or 20 Gy irradiation (the 20 Gy group was not used because 50% died). Rats were tested once/week for 5 weeks post-irradiation for activity, coordination, and startle. Cognitive assessment began 6-weeks post-irradiation with novel object recognition (NOR), egocentric learning, allocentric learning, reference memory, and proximal cue learning. Proton exposure had the largest effect on activity and prepulse inhibition of startle 1-week post-irradiation that dissipated each week. 6-weeks post-irradiation, there were no effects on NOR, however proton exposure impaired egocentric (Cincinnati water maze) and allocentric learning and caused reference memory deficits (Morris water maze), but did not affect proximal cue learning or swimming performance. Proton groups also had reduced striatal levels of the dopamine transporter, tyrosine hydroxylase, and the dopamine receptor D1, effects consistent with egocentric learning deficits. This new model will facilitate investigations of different proton dose rates and drugs to ameliorate the cognitive sequelae of proton radiotherapy.

Salicylate-induced auditory perceptual disorders and plastic changes in nonclassical auditory centers in rats

Previous studies have shown that sodium salicylate (SS) activates not only central auditory structures, but also nonauditory regions associated with emotion and memory. To identify electrophysiological changes in the nonauditory regions, we recorded sound-evoked local field potentials and multiunit discharges from the striatum, amygdala, hippocampus, and cingulate cortex after SS-treatment. The SS-treatment produced behavioral evidence of tinnitus and hyperacusis. Physiologically, the treatment significantly enhanced sound-evoked neural activity in the striatum, amygdala, and hippocampus, but not in the cingulate. The enhanced sound evoked response could be linked to the hyperacusis-like behavior. Further analysis showed that the enhancement of sound-evoked activity occurred predominantly at the midfrequencies, likely reflecting shifts of neurons towards the midfrequency range after SS-treatment as observed in our previous studies in the auditory cortex and amygdala. The increased number of midfrequency neurons would lead to a relative higher number of total spontaneous discharges in the midfrequency region, even though the mean discharge rate of each neuron may not increase. The tonotopical overactivity in the midfrequency region in quiet may potentially lead to tonal sensation of midfrequency (the tinnitus). The neural changes in the amygdala and hippocampus may also contribute to the negative effect that patients associate with their tinnitus.

Genetic architecture of the mouse hippocampus: identification of gene loci with selective regional effects

We recently mapped two quantitative trait loci that have widespread effects on hippocampal architecture in mouse: Hipp1a and Hipp5a. We also noted remarkable strain differences in the relative sizes of different hippocampal regions. Estimated heritable variation for these differences was 42% in hippocampus proper, 40% in dentate gyrus, 31% in granule cell layer and 18% in pyramidal cell layer. Region size varied at least 50% from largest to smallest measurement. Here we have utilized these differences to identify loci with effects on the dentate gyrus, granule cell layer, hippocampus proper and pyramidal cell layer. Our sample consists of C57BL/6J and DBA/2J and 32 BXD recombinant inbred strains. Volumetric data were corrected for shrinkage and for differences in brain weight. We identified significant loci on chromosomes (Chr) 6, 13 and 15, and a significant interaction locus on proximal Chr 11. A suggestive distal Chr 1 locus overlaps with Hipp1a. HipV13a (Chr 13, 42-78Mb) has an additive effect of 0.56 mm3 (12.1%) on dentate gyrus volume, while GrV6a (Chr 6, 29-65 Mb) has additive effects of 0.14 mm3 (16.0%) on the volume of the granule cell layer. HipV13a also interacts with DGVi11a, a locus on proximal Chr 11 that operates exclusively through its epistatic effect on HipV13a and has no independent main effect HipV15a (Chr 15, 0-51 Mb) has an additive effect of 1.76 mm3 (9.0%) on the volume of the hippocampus proper. We used WebOTL, a recently described web-based tool, to examine genetic correlation of gene expression with hippocampal volume. We identified a number of genes that map within the OTL intervals and have highly correlated expression patterns. Using WebQTL's extensive database of published BXD phenotypes, we also detected a strong and potentially biologically meaningful correlation between hippocampal volume and the acoustic startle response.

The effects of neonatal lesions in the amygdala or ventral hippocampus on social behaviour later in life

Disruption of normal social behaviour is seen in psychiatric neurodevelopmental disorders like schizophrenia or autism. In a rat model of neurodevelopmental disorders we investigated the social behavioural changes after damage of limbic brain areas, at two early stages of life. The effects of ibotenic acid lesions made on day 7 or 21 of life in the amygdala (AM) ((baso)lateral/medical) or ventral hippocampal area on social play behaviour, social behaviour unrelated to social play behaviour early in life, and social behaviour in adulthood were assessed. Lesions of the AM, but not lesions of the ventral hippocampal area, resulted in decreased social play behaviour, and no differences were found between lesions made on day 7 or 21 of life. Social behaviour unrelated to social play behaviour early in life and in adulthood was decreased in animals lesioned in the AM on day 7 but not in animals lesioned on day 21 of life. This effect was particularly present in animals with an additional lesion in the medial nuclei of the AM. Lesions in the ventral hippocampal area did not affect social behaviour. It is concluded that the AM is an important structure for social play behaviour. The effects on social behaviour that are dependent on the day of lesioning (day 7 vs. 21) are an indication of a neurodevelopmental deficit of structures connected to the (medial part) of the AM.

Hippocampal lesions induced by ionizing radiation: a parametric study

The selective lesion of granule cell populations in the dentate gyrus induced by ionizing radiation has been proposed as a useful method for evaluating the effects of hippocampal lesions on behavioral tasks. In the first part of the present study we confirmed the induction of the selective lesion of hippocampal dentate gyrus by ionizing radiation in infant Wistar rats, reported previously, but to a smaller extent with less cell loss. A parametric study was thus performed to assess the effect of modification of the parameters previously tested, comprising three further steps: an increase in the total dose of X-rays and modification of the fractionating schedule; use of three radiation types, X-ray, gamma-ray, and electrons (at two energy levels, 3 and 7 mev); use of three X-ray energy levels, 180, 200 and 250 kVp; and assessment of the effect of five total X-ray doses, at 200 kVp, 10, 14, 16, 18 and 20 gy (grays). The data suggests that X-ray radiation, in a total dose of 14 gy, at the 200 kVp energy level, fractionated into seven consecutive exposures of 2 gy each and produces a lesion of about 85% of the dentate gyrus granule cells.

Serial injections of MK 801 (Dizocilpine) in neonatal rats reduce behavioral deficits associated with X-ray-induced hippocampal granule cell hypoplasia

MK 801 (NMDA antagonist) has been shown to protect newborns from hypoxia-induced brain damage. Here, we determined if (+)-5-methyl-10,11-dihydroxy-5h-dibenzo (a,d)cyclohepten-5,10-imine (MK 801) could attenuate behavioral deficits associated with early radiation-induced hypoplasia of fascia dentata granule cells. We pretreated neonatal rats (n = 20) with MK 801 (0, 0.1, or 0.2 mg/kg, IP) before each of eight fractionated, head-only doses of X-rays (13 Gy total) administered during the first 16 days postpartum. Other rats (n = 18) received the same drug treatments but were sham irradiated. At age 16 months, water-escape latencies to a submerged platform were measured in a water maze. Irradiated rats with hippocampal damage exhibited impaired learning (longer latencies to find the platform) than did sham-irradiated subjects. Moderate doses of MK 801 (0.1 mg/kg) facilitated the learning of the water maze by irradiated subjects but did not enhance the number of their fascia dentata granule cells. Higher doses (0.2 mg/kg) of MK 801 provided no behavioral benefits. In fact, this dose significantly impaired the learning of the water maze by sham-irradiated rats and potentiated the granule cell hypoplasia observed in irradiated subjects. Thus, early MK 801 treatment produces dose-dependent behavioral protection for rats with radiation-induced hippocampal damage. Future studies may reveal the neurophysiological and neuroanatomic substrates of this behavioral recovery.

The amygdala. Emotions and gut functions

Studies are reviewed that indicate that the amygdala, and its temporal lobe pathways connecting it with entorhinal cortex and hippocampus, modulates the effects of stressful conditions on the development of gastric pathology. The amygdala integrates aversive stimulus conditions with the defensive behaviors and visceral reactions seen under such circumstances. The transmitter mechanisms for these effects include dopamine, gamma-aminobutyric acid, thyrotropin-releasing hormone, neurotensin, enkephalins, and endorphins. Recording data also show that distinct neural "signatures" in this temporal lobe region correlate with the vulnerability to stressful experiences. The efficacy of synaptic transmission, as represented by potentiation or suppression of recorded neuronal responses, is an indication of coping adjustments, ie, habituation or behavioral helplessness. Glutamate receptors in this brain region, activated by N-methyl-D-aspartate, are implicated in these behavioral strategies. It is proposed that the neurophysiology of these limbic system structures produces individual differences in stress ulcer severity.

Potentiation of inputs from the posterolateral amygdala to the dentate gyrus and resistance to stress ulcers formation in rats

Physical restraint was found to increase the activity of a number of multiple units in the lateral amygdala of rats. High-frequency electrical stimulation of units in the posterolateral amygdala increased the amplitudes of granule cell potentials in the dentate gyrus. This bilateral long-term potentiation (LTP) of inputs from posterior areas of the lateral amygdala also attenuated the severity of stress ulcers produced by physical restraint. This effect was reversed by intraventricular injections of the selective N-methyl-D-aspartate receptor blocker, aminophosphonovaleric acid. LTP in this pathway also reduced "struggling" behaviour during restraint. The data were interpreted to indicate that LTP in this temporal lobe pathway increased the coping ability because of faster habituation to stressors.