J. Sirviö

An α2-adrenergic antagonist, atipamezole, facilitates behavioral recovery after focal cerebral ischemia in rats

Abstract Previous studies suggest that enhanced noradrenergic neurotransmission promotes functional recovery following cerebral lesions. The present study investigated whether systemic administration of an α 2 -adrenergic antagonist, atipamezole, facilitates recovery following transient focal cerebral ischemia in rats. The effect of atipamezole therapy on recovery from ischemia was compared with the effect of enriched-environment housing in rats. Ischemia was induced by occlusion of the right middle cerebral artery (MCA) for 120 min using the intraluminal filament model. Daily atipamezole treatment (1 mg/kg, subcutaneously) was started on day 2 after ischemia induction and drug administration stopped after 10 days. Another group of rats was housed in an enriched environment from day 2 following ischemia induction until the end of the experiment. Several different behavioral tests were used to measure functional recovery during the 26 days following the induction of focal cerebral ischemia. There was improved performance in the limb-placing test from the beginning of atipamezole treatment to day 8, and in wheel-running in the foot-slip test on days 2 and 4. Enriched-environment housing facilitated recovery in the foot-slip test in a later phase of the test period (days 8 to 10). Discovery of a hidden platform in a water-maze task was also facilitated in rats housed in the enriched environment, but this was probably due to the increased swimming speed of these rats. The present data suggest that the α 2 -adrenergic antagonist, atipamezole, facilitates sensorimotor recovery after focal ischemia, but has no effect on subsequent water-maze tests assessing spatial learning and memory, when assessed 11 days after the cessation of drug administration.

α2-Adrenergic drug effects on brain monoamines, locomotion, and body temperature are largely abolished in mice lacking the α2A-adrenoceptor subtype

α2-ARs regulate brain monoaminergic function by inhibiting neuronal firing and release of monoamine neurotransmitters, noradrenaline (NA), serotonin (5-HT) and dopamine (DA). Both α2A- and α2C-AR inhibit monoamine release in vitro in brain slices, but the in vivo roles of individual α2-AR subtypes in modulating monoamine metabolism have not been characterised. Metabolism of brain monoamine neurotransmitters, locomotor activity and body temperature were investigated in mice with targeted inactivation of the gene encoding α2A-AR (α2A-knockout, α2A-KO) and wild-type (WT) mice after treatment with the α2-AR agonist dexmedetomidine and the antagonist atipamezole. Dexmedetomidine caused profound hypothermia (up to 14.7° C mean reduction in rectal temperature) and locomotor inhibition in WT mice, and inhibited the turnover of NA, 5-HT and DA, but increased NA turnover in α2A-KO mice. α2-AR agonist-induced hypothermia and locomotor inhibition were attenuated, but not totally abolished, in α2A-KO mice. These results suggest that α2A-ARs are principally responsible for the α2-AR mediated inhibition of brain monoamine metabolism, but other α2-ARs, possibly α2C-ARs, are also involved, especially in the striatum. However, secondary effects of the physiological alterations caused by drug administration, especially hypothermia, may have contributed to the observed neurochemical changes in WT mice.

Neocortical, hippocampal and septal parvalbumin- and somatostatin-containing neurons in young and aged rats: Correlation with passive avoidance and water maze performance

Aged (26-month-old) rats were impaired compared with young (three-month-old) rats in passive avoidance and water maze tasks. In order to study whether changes in inhibitory circuits are involved in these age-related cognitive impairments, the number of two different subpopulations of GABAergic neurons, i.e. somatostatin- and parvalbumin-containing neurons, were counted in the hippocampal formation, septum and neocortex. We found that the number of parvalbumin-containing neurons was decreased in the entorhinal, somatosensory and motor cortex as well as in the medial septum and vertical limb of the diagonal band of Broca, but not in the hippocampus of aged rats. Somatostatin-containing neurons were affected in the somatosensory and motor cortex, and in the dorsolateral septum, but not in the hippocampus or in the entorhinal cortex. The decreased number of parvalbumin-containing neurons in the entorhinal cortex of the aged rats correlated with their performance deficits in passive avoidance and spatial learning. We propose that impaired functioning of the entorhinal cortex parvalbumin-containing inhibitory neurons may, to some extent, be responsible for the learning and memory defects found in aged rats.

Loss of cholinergic neurons in the nucleus basalis induces neocortical electroencephalographic and passive avoidance deficits.

The present experiments were designed to examine the hypothesis that the degeneration of cholinergic nucleus basalis is related to the cognitive and neurophysiological deficits found in old age. Aged (26 months) rats were impaired both in the acquisition of spatial (water-maze) task and retention of passive avoidance task. During aging, neocortical electroencephalographic fast activity was decreased and high-voltage spindles increased. Loss of choline acetyltransferase-positive neurons correlated with the high-voltage spindle incidence and passive avoidance retention deficit. Unilateral ibotenate nucleus basalis lesioning decreased choline acetyltransferase activity in the cortex and produced a large nonspecific subcortical cell loss in young rats. Ibotenate-lesioned rats were impaired in spatial learning and passive avoidance retention in young rats. Quisqualic acid produced a greater decrease in cortical choline acetyltransferase activity and smaller nonspecific subcortical cell loss than ibotenate lesioning. Spatial learning was not impaired, but passive avoidance performance was disrupted. Slow waves and high-voltage spindles were increased and beta activity decreased on the side of either quisqualate or ibotenate nucleus basalis lesioning. These results demonstrate that age-related neurophysiological and cognitive deficits result partially from the loss of cholinergic neurons in the nucleus basalis and that quisqualic acid nucleus basalis-lesioning in young rats may be used as a pharmacological model of the age-related cholinergic neuron loss.

Selegiline combined with enriched-environment housing attenuates spatial learning deficits following focal cerebral ischemia in rats

Selegiline (l-deprenyl) is an irreversible monoamine oxidase B (MAO-B) inhibitor that is suggested to have neuroprotective and neuronal rescuing properties. The present study investigated whether systemic administration of selegiline facilitates behavioral recovery after transient focal cerebral ischemia in rats using a combination of different behavioral tests (limb placing, foot slip, water maze, and Montoyas staircase test) to measure the outcome of recovery. Selegiline (0.5 mg/kg, SC) or 0.9% NaCl was administered once a day, beginning on the second day after induction of ischemia and continuing for 30 days. Selegiline administration combined with enriched-environment housing attenuated ischemia-induced spatial learning deficits in a water-maze task and enhanced performance of both the contralateral affected and ipsilateral nonaffected forelimbs in a staircase test. Selegiline administration alone was not beneficial in any of the tests. Subsequent histologic examination revealed that the infarct volumes were not different between the experimental ischemic groups. Thus, these results suggest that selegiline combined with enriched-environment housing reduces behavioral and cognitive deficits without affecting infarct size.

Alpha2C-adrenoceptor overexpression disrupts execution of spatial and non-spatial search patterns

We investigated the role of alpha2C-adrenoceptors in the modulation of spatial and non-spatial navigation behaviour. Alpha2C-adrenoceptor overexpressing mice developed an ineffective thigmotaxic search pattern characterized by swimming close to the pool walls during both spatial and non-spatial water maze training. A subtype-non-selective alpha2-adrenoceptor antagonist, atipamezole (1000 microg/kg, s.c.), fully reversed this impairment in their search strategy. Withdrawal of atipamezole at the end of spatial training resulted in an immediate disruption of the search pattern in alpha2C-adrenoceptor overexpressing mice. The swimming pattern of alpha2C-adrenoceptor overexpressing mice during a five day free swimming period was normal, when no cognitive component was required. Diazepam (0.3 and 1.0 mg/kg, i.p.), neither improved the accuracy in finding the platform nor decreased thigmotaxis. These results suggest that alpha2C-adrenoceptors may modulate the execution of complex navigation patterns.

The 5-HT2 Receptor Activation Enhances Impulsive Responding Without Increasing Motor Activity in Rats ☆

The effects of 5-HT(2) receptor ligands on the performance of rats were investigated using a 5-choice serial reaction time (5-CSRT) task. Systemic administration of DOI (0.03 to 0.3 mg/kg subcutaneously [SC]), a 5-HT(2) receptor agonist, did not impair choice accuracy of well-performing rats under either baseline conditions or more demanding conditions of the task, in which the stimulus duration or intensity were reduced or the intertrial interval (ITI) was decreased. DOI (0.1 mg/kg or 0.15 mg/kg) increased premature responding (the probability of intertrial interval hole pokes) in all testing conditions, except under conditions of a short ITI when the rats did not make any hole responses. Ketanserin (0.1 to 0.3 mg/kg SC), a 5-HT(2A) receptor antagonist, had no marked effect on performance. When combined with ketanserin (0.2 mg/kg SC), however, DOI (0.1 mg/kg) did not increase premature responding. The lowest doses of DOI (0.05 and 0.1 mg/kg) that increase premature responding had no effect on open-field performance. Further, the effects of systemically administered DOI were not reproduced by bilateral administration of DOI into the anterior cingulate cortex. These data indicate that excessive activation of 5-HT(2A/2C) receptors interferes with response control rather than visual attention. Furthermore, the DOI-induced enhancement of impulsive responses are not due to locomotor hyperactivity, and the anterior cingulate cortex is not the primary site of action for this enhancement of premature responding.

The effects of d-cycloserine, a partial agonist at the glycine binding site, on spatial learning and working memory in scopolamine-treated rats

SummaryThe present study investigated the effect of d-cycloserine, a partial agonist at the glycine binding site on NMDA receptor complex, on the performance of scopolamine-treated adult rats in a water maze task assessing spatial learning and in a delayed non-matching to position task assessing working memory in a spatial context. In the spatial learning task, scopolamine (0.4 mg/kg, i.p.) impaired acquisition (increased escape latency and distance) and increased swimming speed of rats. D-cycloserine (1.0 mg/kg, i.p.) reversed the deficits in acquisition performance but not the increases in behavioral activity. In the working memory task, scopolamine (0.2 mg/kg, i.p.) produced deficits on nonmnemonic rather than on mnemonic performance factors; scopolamine delay-independently decreased the percent correct responses and reduced behavioral activity of rats. D-cycloserine (1.0, 3.0 and 10 mg/kg, i.p.) did not reverse these performance deficits. When administered alone, the moderate to higher doses of d-cycloserine had no effects on working memory but the lower dose produced slight deficits in mnemonic performance factors; the 1.0 mg/kg dose delay-dependently decreased the percent correct responses without affecting behavioral activity of rats. In the water maze task, d-cycloserine had no effects on acquisition performance or behavioral activity of rats. These results suggest that acute, systemic administration of d-cycloserine does not improve spatial learning or working memory. However, at appropriate doses this agent may be efficacious in disease states of central cholinergic hypofunction since 1.0 mg/kg d-cycloserine was able to reverse the scopolamine-induced deficits in acquisition.

Overexpression of Alpha2C-adrenoceptors impairs water maze navigation

We investigated the role of overexpression of alpha2C-adrenoceptors in water maze navigation in mice transgenically manipulated to have a threefold overexpression of the alpha2C-adrenoreceptors. Alpha2C-adrenoreceptors overexpressing mice swam more in the peripheral annulus of the pool and did not find the hidden escape platform as well as the wild type control mice. A subtype-nonselective alpha2-adrenoreceptor antagonist, atipamezole (ATI, 1000 microg/kg, s.c.), fully reversed the deficit in platform finding and search strategy in overexpressing mice. Noradrenaline depletion (-95%) induced by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) did not impair platform finding of wild type or overexpressing mice. The DSP-4 lesion slightly increased swimming in the peripheral annulus in wild type mice, but not in overexpressing mice. The DSP-4 lesion produced a dissociable effect on the action of atipamezole to improve platform finding and search strategy in overexpressing mice: atipamezole did not alleviate the platform finding deficit in DSP-4 lesioned overexpressing mice, but normalized their abnormal search strategy. These results suggest that the abnormal search pattern and deficit in the accuracy of platform finding are mediated by constitutive activity of overexpressed alpha2C-adrenoreceptors.

Brain cholinergic enzymes and cortical EEG activity in young and old rats.

1. Cholinergic enzymes (ChAT, AChE) in different areas of the brain and cortical electroencephalography (EEG) activity were investigated in young and old rats. 2. In old rats, ChAT activity was low in the striatum, but high in the amygdala. Compared to young rats, ChAT activity in old rats was unchanged in the frontal, parietal/occipital and entorhinal cortex as well as in the hypothalamus, midbrain, hippocampus and brain stem. 3. AChE activity in old rats was lower than in young animals in the parietal/occipital cortex, hippocampus, striatum and brainstem. In other areas of the brain AChE activity was unchanged. 4. In old rats the peak frequency (Fp) of cortical EEG activity (mobility-related) was significantly lower than in young animals, both frontally and occipitally. The power of 5-10 Hz frequency band was markedly lower than in young rats. During immobility, the power of the 1.5-3 Hz and 3-5 Hz bands was lower in the frontal cortex of old rats. The power of 3-5 Hz, 5-10 Hz and 10-20 Hz bands was lower in the occipital cortex of old rats. In all of the old rats, but not in any of the young ones, symmetric high voltage activity was observed in the frontal pole of the cortex. 5. These results suggest that the age-related decrease of higher frequencies of cortical EEG activity may be related to the decrease of AChE activity in the parietal/occipital cortex. This decrease in AChE may reflect degeneration of the cholinergic synapses.