M. Segal

Pain sensitivity and opioid activity in genetically and experimentally hypertensive rats

Pain sensitivity was studied in renal and DOCA-salt hypertensive rats, and in two strains of rats derived from the same parental strain for their sensitivity (H) or immunity (N) to hypertension induced by DOCA-salt treatment. Experimentally hypertensive rats, and H and N rats were less sensitive to painful stimuli than their appropriate controls, as assessed in the hot-plate and paw pinch tests. Naloxone reversed this hypoalgesia in both experimentally and genetically hypertensive rats while it did not affect blood pressure in any rat-type tested. Opioid activity was measured with the radioreceptor assay in several brain regions and pituitary gland of both experimentally and genetically hypertensive rats. Experimentally hypertensive rats had a 45% higher level of opioid activity in the spinal cord compared to control. Rats of the H and N strains both exhibited higher levels of opioid activity in the spinal cord, hypothalamus and pituitary. It is suggested that control systems for blood pressure and pain sensitivity are closely associated in the rat.

Distribution of an α-bungarotoxin-binding cholinergic nicotinic receptor in rat brain

Cholinergic nicotinic receptors in rat brain were demonstrated by the use of the potent nicotinic antagonist [125I]alpha-bungarotoxin [125I]alpha-Btx). Biochemical studies on binding of [125I]alpha-Btx to rat hippocampal homogenates revealed saturable binding sites which are protected by nicotine, D-tuborcurarine and acetylcholine but not by atropine or oxotremorine. The hippocampus and hypothalamus displayed relatively high [125I]alpha-Btx specific binding whereas the cerebellum was devoid of specific binding. Other regions displayed intermediate binding levels. Analysis of the regional distribution of [125I]alpha-Btx binding by autoradiography of frontal brain sections revealed high labeling in the hippocampus, hypothalamic supraoptic, suprachiasmatic and periventricular nuclei, ventral lateral geniculate and the mesencephalic dorsal tegmental nucleus. It is suggested that the limbic forebrain and midbrain structures as well as sensory nuclei are the main nicotinic cholinoceptive structures in the brain.

Spatial performance is severely impaired in rats with combined reduction of serotonergic and cholinergic transmission.

A possible interaction between serotonergic and cholinergic neurotransmission was examined in relation to the performance of a spatial memory task. Blockade of cholinergic transmission with a high dose of atropine was sufficient to impair performance of a water maze task. A partial reduction of cholinergic transmission using a low dose of atropine had no effect on this performance. Reducing serotonin synthesis, using a specific inhibitor of tryptophane hydroxylase p-chlorophenylalanine (PCPA) also had no effect on performance of such a task. However, a combined treatment with a low dose of atropine and PCPA severely impaired the performance of rats in the water maze. The rats were impaired in both acquisition of the initial spatial task and in reacquisition of a new spatial position (= working memory). These findings suggest an interaction between cholinergic and serotonergic transmission in acquisition and retention of spatial information. Furthermore they propose that deficits in cognitive abilities, observed in aging or Alzheimers disease, may result from the combined reduction in cholinergic and serotonergic transmission.

Aging and brain cholinergic muscarinic receptor subtypes: an autoradiographic study in the rat.

Cholinergic M1 and M2 muscarinic receptors in aged and young rat brains were studied by quantitative autoradiography of tritiated QNB in the presence of pirenzepine or carbachol. A selective pattern of decreased binding density was observed in the aged rat. A large number of regions showed no effect of aging; these include subdivisions of the hippocampal formation and most thalamic and hypothalamic nuclei. M1 and M2 receptors showed small but significant decreases in cortical regions and in the striatum. The largest effects were seen in M2 receptors of the ventral forebrain cholinergic nuclei where binding was reduced by up to 40%. No similar reductions were seen in the M1 receptor population in these regions. The results suggest that both muscarinic receptor subtypes show an anatomically selective pattern of decrease with age, with the M2 receptor subtype in the basal forebrain nuclei being specially vulnerable to the effects of aging.

Changes in neurotransmitter actions in the aged rat hippocampus

Intracellular activity was recorded from pyramidal cells in region CA1 of young and aged rat hippocampal slices. There were no apparent differences between the two age groups in the passive or active membrane properties tested. These included resting membrane potential, input resistance, spike size and overshoot, after potentials, and EPSPs. There were marked differences between the two age groups in reactivity to chemical stimulation. The response to acetylcholine (ACh) in young rat hippocampal cells consisted of an initial hyperpolarization followed by a late, slow depolarization. These were accompanied by a decrease in reactivity of the recorded cell to afferent stimulation. In the old cells the initial hyperpolarization and late depolarization were markedly reduced, yet the decreased reactivity to afferent stimulation was maintained. Both young and old cells were depolarized by GABA to the same magnitude, yet the recovery was slower in the old cells. 5-HT evoked a smaller response in old compared to young cells. These data indicate that some specific physiological processes which are not paralleled by reported biochemical changes are affected by aging. The mechanisms underlying these changes are subject to further investigations.

Aging and Brain Cholinergic Muscarinic Receptors: An Autoradiographic Study in the Rat

Abstract: Cholinergic muscarinic receptors in aged and young rat brains were studied by quantitative autoradiography of tritiated quinuclidinyl benzilate. A selective pattern of decreased binding density was observed in the aged rat. A large number of regions showed no effect of aging; these include subdivisions of the hippocampal formation and most tha‐lamic and hypothalamic nuclei. Small but significant decreases were found in cortical regions and in the striatum. The largest effects were seen in ventral forebrain cholinergic nuclei, where 40‐60% depletions were found in the diagonal band, nucleus basalis magnocellularis, ventral pallidum, and substantia innominata.

Extracellular recordings in the colchicine-lesioned rat dentate gyrus following transplants of fetal dentate gyrus and CA1 hippocampal subfield tissue.

Grafts of fetal dentate gyrus (DG) and CA1 hippocampal subfield tissue were extruded into the dentate gyri of adult male Sprague-Dawley rats, 7-10 days after lesioning the granule cells with colchicine (0.06 microliter of 7 mg/ml solution at each of 5 sites/hippocampus). Graft area-host and host-graft area connectivities were investigated 4-6 months post-transplantation by recoding extracellular evoked response in hippocampal slice preparations. Following stimulation of the host mid-molecular layer, evoked field potential responses, showing considerable variation, were recorded in both types of graft. Evoked responses in the lesioned DG without grafts were recorded in very few slices. Stimulation of the area of DG tissue grafts occasionally evoked responses in the host CA3/CA4 and there was no evidence for CA1 graft area-CA3/CA4 connectivity; stimulation of DG and CA1 graft areas occasionally evoked responses in the host CA1. Responses in the area of both DG and CA1 grafts supported short-term potentiation following stimulation of the host mid-molecular layer but only DG graft areas supported long-term potentiation of the population spike amplitude. In the area of both types of transplant a tonic bicuculline-sensitive inhibition was present and paired-pulse stimulation paradigms provided some evidence for inhibition. It is possible that responses recorded within the area of grafted tissue to stimulation of the host are attributable to host-graft connectivity and similarly, responses recorded in the host to stimulation of the area of the graft may be attributable to graft-host connectivity. Only DG graft areas received host inputs which were capable of sustaining a long-term potentiation and establishing efferent contacts with the host CA3/CA4 subfield, suggesting that these would be more likely than CA1 grafts to reinstate normal functional circuitry.

Raphe cells grafted into the hippocampus can ameliorate spatial memory deficits in rats with combined serotonergic/cholinergic deficiencies ☆

The ability of embryonic raphe cells grafted into the hippocampus to restore spatial learning ability was tested in rats with combined serotonergic/cholinergic deficits. Embryonic raphe cells (E14) were transplanted into the hippocampus of serotonin-depleted rats. Two to 3 months after transplantation, control, lesioned and grafted rats were tested in a spatial memory task (a water maze) with and without the addition of atropine. All 3 groups could negotiate the water maze equally well, in non-drug conditions. The injection of atropine caused a severe disruption of performance only in the serotonin depleted rats. The presence of an active serotonergic graft was examined in the intact rat hippocampus using the serotonin releasing drug fenfluramine (FFA). A pronounced depression of hippocampal EEG was observed in control and grafted but not in lesioned rats 15 min after the injection of FFA. These results suggest the involvement of serotonin in cognitive functions in the rat. Furthermore, it is suggested that an interaction between serotonergic and cholinergic neurotransmission occurs in the hippocampus.

Septal transplants ameliorate spatial deficits and restore cholinergic functions in rats with a damaged septo-hippocampal connection.

Behavioral effects of septal lesion and fornix-fimbria transection were compared in absence and presence of a septal transplant in the hippocampus. The transplant grew in the hippocampus and projected acetylcholinesterase (AChE)-containing fibers throughout the extent of the denervated hippocampus. There were no differences in graft size or AChE reinnervation pattern after septal lesion or fornix transection. An increase in the density of M1 binding sites seen in hippocampal CA3 region after a cholinergic lesion, was restored back to normal after reinnervation of the hippocampus by the graft. Fornix-transected rats were more impaired in water maze acquisition than septal-lesioned rats which were impaired compared to controls. Septal-grafted rats were not different from lesioned rats in the behavioral tasks. However, an injection of physostigmine improved their performance relative to lesioned non-grafted rats. These experiments indicate that grafts can ameliorate behavioral deficits when the efficacy of acetylcholine of graft origin is enhanced.

The effects of brainstem priming stimulation on interhemispheric hippocampal responses in the awake rat.

SummaryEvoked hippocampal responses to stimulation of the contralateral hippocampus were recorded in the awake rat. The effects of priming stimulation applied to the nucleus locus coeruleus and the raphe nuclei on averaged evoked hippocampal responses to the interhemispheric stimulation were measured. It was found that priming stimulation of these monoamine-containing nuclei caused the formation of a late component in the interhemispheric potential without affecting the magnitude of the initial response component. Drugs which selectively interfere with noradrenergic or serotonergic transmission, antagonized the locus coeruleus or raphe primed late response component, respectively. Parenteral administration of d-amphetamine or l-amphetamine caused the appearance of a late response component which was similar to that seen after brainstem priming stimulation. It is suggested that the monoamines modify responses to afferent stimulation of particular pathways in the brain.