The histopathological, behavioral and neurochemical effects of intraventricular injection of ethylcholine mustard aziridinium (AF64A) in the neonatal rat

Nicotinic alteration of functional thalamocortical topography

The thalamocortical pathways form highly topographic connections from the primary sensory thalamic nuclei to the primary cortical areas. The synaptic properties of these thalamocortical connections are modifiable by activation from various neuromodulators, such as acetylcholine. Cholinergic activation can alter functional properties in both the developing and the mature nervous system. Moreover, environmental factors, such as nicotine, can activate these receptors, although the circuit-level alterations resulting from such nicotinic activation of sensory neural circuits remain largely unexplored. Therefore, we examined alterations to the functional topography of thalamocortical circuits in the developing sensory pathways of the mouse. Photostimulation by uncaging of glutamate was used to map these functional thalamocortical alterations in response to nicotinic receptor activation. As a result, we found that activation of forebrain nicotinic acetylcholine receptors results in an expansion and enhancement of functional thalamocortical topographies as assessed in brain slice preparations using laser-scanning photostimulation by uncaging of glutamate. These physiological changes were correlated with the neuroanatomical expression of nicotinic acetylcholine receptor subtypes (α7 and β2). These circuit-level alterations may provide a neural substrate underlying the plastic development and reshaping of thalamocortical circuitry in response to nicotinic receptor activation.

Neural and behavioral effects of intracranial 192 IgG-saporin in neonatal rats: sexually dimorphic effects?

The consequences of neonatal cholinergic lesions were examined in male and female rats. Rats were injected intraventricularly with 600 ng of 192 IgG-saporin at 7 days of age and examined behaviorally and histologically at 21, 45 and 90 days of age. 192 IgG-saporin profoundly reduced low affinity neurotrophin receptor (p75NTR)-immunoreactive (IR) and, to a lesser extent, choline acetyltransferase-IR cells in the basal forebrain. Presumptive sympathetic ingrowths (p75NTR- and dopamine beta-hydroxylase-IR) into the hippocampus were first apparent at 45 days of age and were not significantly greater at 90 days. Behaviorally, 192 IgG-saporin increased the time females, but not males, spent on the open arms of the elevated plus maze. Lesioned rats had longer platform location latencies in the Morris water maze only at the first hidden platform training session and did not differ on the rate of learning the platform location or on the no-platform probe trial. Generally, the effects of neonatal cholinergic lesions were not sex dependent and are unlikely to model Rett syndrome, a disorder characterized by forebrain cholinergic deficit which is seen almost exclusively in females.

Protection against developmental deficiencies by a lipophilic VIP analogue

Stearyl-Nle-VIP (SNV) is a novel agonist of vasoactive intestinal peptide (VIP) exhibiting a 100-fold greater potency than the parent molecule and specificity for a receptor associated with neuronal survival. Here, the developmental and protective effects of SNV were investigated in vivo using two models of developmental retardation, hypoxia and cholinergic blockade. In both cases chronic administration of SNV during development provided protective effects. Water maze experiments on the weaned animals have demonstrated a prophylactic action for SNV and enhancement of spatial memory in animals exposed to a cholinotoxin. SNV may act by providing neuroprotection, thereby improving cognitive functions. This work is dedicated to Prof. R.J. Wurtman whose inspiration and leadership in the field of neuroscience and cognition is beyond comparison.

192 IgG-saporin lesion of basal forebrain cholinergic neurons in neonatal rats

Seven day old rats received bilateral intraventricular injections (200 ng) of the immunotoxin 192 IgG-saporin. When assayed in adulthood, these rats showed an 84% loss of hippocampal and a 52% loss of cortical choline acetyltransferase (ChAT) activity. ChAT was unaffected in the caudate. Cholinergic neurons immunoreactive (IR) for the low affinity neurotrophin receptor (P75NTR) were severely reduced throughout the basal forebrain nuclei. Cortical and hippocampal norepinephrine were increased and these areas showed ingrowth of ectopic, P75NTR and dopamine beta-hydroxylase IR varicosities. These were probably sympathetic axons. No obvious forebrain dysmorphogenesis was observed and cortical thickness was unaffected. These rats showed no evidence of impaired spatial learning/memory as assessed by the Morris water maze and delayed spatial alternation. However, they were less active on the elevated plus apparatus and spent less time on the open arms, suggestive of increased timidity. 192 IgG-saporin appears to be a powerful tool to selectively lesion basal forebrain cholinergic neurons in the neonatal rat. Surprisingly, the neuromorphological and behavioral sequelae seem minimal. It may be necessary to achieve near-total neonatal destruction of forebrain cholinergic neurons before severe, lasting mnemonic effects are evident.

Hemicholinium-3 (HC3) blocks the effects of ethylcholine mustard aziridinium (AF64A) in the developing rat

Two- to 3-day-old rat pups received bilateral intracerebroventricular (i.c.v.) injections of 2.0 nmol/microliters AF64A or vehicle. Half of the pups had been preinjected i.c.v. with hemicholinium-3 (HC3) and the other half with saline. The administration of AF64A impaired spatial learning/memory and caused brain damage characterized by marked loss of forebrain cortical/subcortical tissue and ventricular hypertrophy when these were assessed in adulthood. Neither the behavioral nor the histopathological effects of AF64A were observed in rats that had been pretreated with HC3. Since HC3 is a potent and relatively selective inhibitor of high affinity choline uptake (HACU), the results indicate that the toxic effects of AF64A in the neonatal rat are dependent upon its uptake via the HACU site. If as other research suggests, this site is primarily on Ach neurons in the neonatal rat, then the consequences of neonatal damage to cholinergic neurons are severe for forebrain development.

Neurochemical, histopathological and mnemonic effects of combined lesions of the medial septal and serotonin afferents to the hippocampus

Male Long-Evans rats received micro-injections of either N-methyl-D-aspartate (NMDA) in the medial septum/vertical diagonal band (MS/DB), 5,7-dihyroxytryptamine (5,7-DHT) in the fimbria/fornix and cingulate bundle or combined NMDA/5,7-DHT micro-injections. NMDA administration caused considerable damage to the MS and enlarged the lateral ventricles. It reduced the activity of choline acetyltransferase as well as the intensity of acetylcholinesterase staining in the hippocampus. 5,7-DHT selectively reduced the concentration of hippocampal serotonin. The rats were assessed for spatial memory in the Morris water maze and the radial arm maze (reference and working memory version). The 5,7-DHT-induced lesion of hippocampal serotonin had no effect by itself on either task. However, it augmented the reference memory impairment caused by the NMDA-induced lesion and delayed the recovery from NMDA-induced impairment of working memory on the radial maze. Combined damage of hippocampal cholinergic and serotonergic afferents did not severely affect spatial memory.