Mary Lou Voytko

Cognitive functions of the basal forebrain cholinergic system in monkeys : memory or attention ?

The cholinergic hypothesis of memory dysfunction originally proposed that dysfunction of cholinergic neurons in the basal forebrain cholinergic system (BFCS) may be responsible for the memory deficits associated with aging and Alzheimers disease (AD). This hypothesis directed focus on the BFCS in experimental animal models of AD. In contrast to numerous studies in rodents, fewer investigations have been conducted in monkeys with BFCS lesions. The medical septal nucleus/nucleus of the diagonal band of Broca (MS/NDBB) and the nucleus basalis of Meynert (NBM) may be involved in different cognitive functions in monkeys. Although few investigations have specifically addressed the issue of cognitive functions of the MS/NDBB in monkeys, there is some indication that these regions may be important for memory. In contrast, lesions of the NBM do not consistently disrupt mnemonic functions in monkeys. Recent electrophysiological and lesion studies of monkeys indicate that the NBM may play a more important role in attention functions, impairments of which are an early and significant feature of patients with AD.

Why primate models matter

Research involving nonhuman primates (NHPs) has played a vital role in many of the medical and scientific advances of the past century. NHPs are used because of their similarity to humans in physiology, neuroanatomy, reproduction, development, cognition, and social complexity—yet it is these very similarities that make the use of NHPs in biomedical research a considered decision. As primate researchers, we feel an obligation and responsibility to present the facts concerning why primates are used in various areas of biomedical research. Recent decisions in the United States, including the phasing out of chimpanzees in research by the National Institutes of Health and the pending closure of the New England Primate Research Center, illustrate to us the critical importance of conveying why continued research with primates is needed. Here, we review key areas in biomedicine where primate models have been, and continue to be, essential for advancing fundamental knowledge in biomedical and biological research. Am. J. Primatol. 76:801–827, 2014.

Impairments in acquisition and reversals of two-choice discriminations by aged rhesus monkeys☆

The ability to learn and perform reversals of two object, two patterns, and one spatial discrimination was examined in eight aged (28-34 years), and four adult (8-13 years) behaviorally naive monkeys. As a group, the aged monkeys demonstrated significant difficulties in learning and reversing some of the visual discrimination problems, but had no difficulty learning or reversing the spatial discrimination. Additional analyses revealed that an impairment in learning an object discrimination by the aged monkeys was characterized by a prolonged period of chance performance, and the impairments in performing visual discrimination reversals was related to difficulties in two distinct stages of reversal learning. Despite age-related differences, there was considerable variability in performance among the aged monkeys. These experiments provide the first evidence of significant impairments in learning and reversing visual discriminations by aged monkeys that have not had prior exposure to complex behavioral tasks.

Visual learning suppressed by cooling the temporal pole.

Three monkeys were trained to remember colored photographs of objects over delays of 0, 15, 30, and 45 s. Then two pairs of cooling devices were implanted bilaterally over the anterior 9 mm of the temporal lobe. The devices consisted of 3 X 10 mm loops of stainless steel tubing into which cooled methanol could be pumped. One pair (anterior pair) covered the medial part of the temporal tip (area TG), starting at the rhinal sulcus and extending 3 mm laterally. The second pair (posterior pair) was placed 3 mm lateral to the anterior pair, covering the rest of TG and the anterior extreme of the inferotemporal gyri, anterior TE. Cooling either pair of probes produced a deficit at all delays, but the deficit was greater at the longest delays. There was no difference between cooling the anterior pair and cooling the posterior pair except that cooling the anterior pair greatly increased the disruption of recall that is produced by an interfering stimulus. When all four probes were cooled, which suppressed the function of the entire temporal tip, performance dropped to chance at all delays. While under this condition, the animals could not learn new visual discriminations but could perform previously learned visual discriminations. These results are consistent with the suggestion that the temporal pole is the store for the brief anterograde memory that is available to the medial temporal amnesics.

Effects of two years of estrogen loss or replacement on nucleus basalis cholinergic neurons and cholinergic fibers to the dorsolateral prefrontal and inferior parietal cortex of monkeys

The present study examined the long‐term (2 years) effects of estrogen loss or estrogen replacement therapy (ERT) on cholinergic neurons in the nucleus basalis of Meynert and on cholinergic fibers in the prefrontal and parietal cortex of adult female cynomolgus monkeys. Cholinergic fiber density in layer II of the prefrontal cortex was decreased in monkeys who were ovariectomized and treated with placebo for 2 years. In contrast, ovariectomized monkeys receiving ERT for 2 years had fiber densities that were comparable to those of intact controls. No differences in parietal cholinergic fiber density or nucleus basalis cholinergic neuron number or volume were found among intact, ovariectomized, or ERT monkeys. Our results suggest that ERT is effective in preventing region‐specific changes in cortical cholinergic fibers that result from the loss of circulating ovarian hormones. These modest but appreciable effects on cholinergic neurobiology following long‐term estrogen loss and ERT may contribute to changes in visuospatial attention function that is mediated by the prefrontal cortex. J. Comp. Neurol. 469:507–521, 2004.

Estrogen and the cholinergic system modulate visuospatial attention in monkeys (Macaca fascicularis).

This study determined the effects of estrogen on attention and motor speed in young monkeys (Macaca fascicularis). Performance on visuospatial attention and reaction time motor tasks was measured before ovariectomy, for 2 months after ovariectomy, and at 14 months after treatment with placebo or estrogen replacement therapy (ERT). Release time on invalid trials in the attention task was increased after ovariectomy and was reduced in monkeys treated with ERT. but not placebo. Simple reaction time was not altered after ovariectomy or treatment. The effects of scopolamine on attention, but not memory, in a delayed response task were dependent on estrogen. These observations indicate that loss of estrogen modulates visuospatial attention in primates and that 1 mechanism of this modulation is through the cholinergic system.

The effects of long-term ovariectomy and estrogen replacement therapy on learning and memory in monkeys (Macaca fascicularis).

This study determined the effects of estrogen loss and replacement therapy on learning and memory function in monkeys (Macaca fascicularis). The ability to learn, remember, and perform reversals of object discriminations and the accuracy on a spatial delayed response task were found to be comparable in young adult surgically menopausal monkeys receiving estrogen or placebo treatment for 5 or 16 months. Learning and memory abilities were comparable with baseline values following 2, 12, or 24 months of ovariectomy in monkeys. Pre- and postoperative injections of scopolamine in a subset of monkeys revealed only subtle increases in sensitivity on the delayed response task following ovariectomy. These observations in surgically menopausal monkeys have some parallels with those made in surgically menopausal women and suggest that, in the absence of other confounding factors, certain aspects of learning and memory may not be influenced by estrogen in primates.

A Calorie-Restricted Diet Decreases Brain Iron Accumulation and Preserves Motor Performance in Old Rhesus Monkeys

Caloric restriction (CR) reduces the pathological effects of aging and extends the lifespan in many species, including nonhuman primates, although the effect on the brain is less well characterized. We used two common indicators of aging, motor performance speed and brain iron deposition measured in vivo using MRI, to determine the potential effect of CR on elderly rhesus macaques eating restricted (n = 24; 13 males, 11 females) and standard diets (n = 17; 8 males, 9 females). Both the CR and control monkeys showed age-related increases in iron concentrations in globus pallidus (GP) and substantia nigra (SN), although the CR group had significantly less iron deposition in the GP, SN, red nucleus, and temporal cortex. A diet × age interaction revealed that CR modified age-related brain changes, evidenced as attenuation in the rate of iron accumulation in basal ganglia and parietal, temporal, and perirhinal cortex. Additionally, control monkeys had significantly slower fine motor performance on the Movement Assessment Panel, which was negatively correlated with iron accumulation in left SN and parietal lobe, although CR animals did not show this relationship. Our observations suggest that the CR-induced benefit of reduced iron deposition and preserved motor function may indicate neural protection similar to effects described previously in aging rodent and primate species.

Effects of ovariectomy on the neuroendocrine axes regulating reproduction and energy balance in young cynomolgus macaques

Degeneration of the ovary in middle‐aged women results in castrate levels of ovarian steroids and increased gonadotropin secretion from the anterior pituitary gland. Ageing in women is also accompanied by significant changes in energy homeostasis. We have observed alterations in hypothalamic morphology and gene expression in older women, including hypertrophy and increased gene expression of neurokinin B (NKB) neurones, elevated levels of gonadotropin releasing‐hormone (GnRH) mRNA and decreased numbers of neurones expressing pro‐opiomelanocortin (POMC) mRNA. To determine if loss of ovarian steroids could produce comparable changes in gene expression in young primates, we measured the effects of ovariectomy on NKB, GnRH and POMC gene expression in young cynomolgus monkeys. We also measured serum leptin and body weight to examine the consequences of ovariectomy on energy balance. NKB neurones in the infundibular nucleus of ovariectomized monkeys were larger, more numerous and displayed increased levels of NKB mRNA compared to those of intact controls. Moreover, ovariectomy increased the number of neurones expressing GnRH gene transcripts and elevated serum luteinizing hormone. By contrast, several parameters related to energy balance, including POMC gene expression, serum leptin and body weights, were unchanged by ovariectomy. Thus, the rise in NKB and GnRH gene expression in older women was simulated by ovariectomy in monkeys, but the changes in POMC gene expression and energy balance were not. This study provides strong support for the hypothesis that ovarian failure contributes to the increased NKB and GnRH gene expression observed in postmenopausal women.

Imaging of cholinergic terminals using the radiotracer [18F](+)-4-fluorobenzyltrozamicol: In vitro binding studies and positron emission tomography studies in nonhuman primates

The goal of the present set of studies was to characterize the in vitro binding properties and in vivo tissue kinetics for the vesicular acetylcholine transporter (VAcChT) radiotracer, [18F](+)‐4‐fluorobenzyltrozamicol ([18F](+)‐FBT). In vitro binding studies were conducted in order to determine the affinity of the (+)‐ and (−)‐ stereoisomers of FBT for the VAcChT as well as sigma (σ2 and σ2) receptors. (+)‐FBT was found to have a high affinity (Ki = 0.22 nM) for the VAcChT and lower affinities for σ1 (21.6 nM) and σ2 (35.9 nM) receptors, whereas (−)‐FBT had similar affinities for the VAcChT and σ1 receptors (∼20 nM) and a lower affinity for σ2 (110 nM) receptors. PET imaging studies were conducted in rhesus monkeys (n = 3) with [18F](+)‐FBT. [18F](+)‐FBT was found to have a high accumulation and slow rate of washout from the basal ganglia, which is consistent with the labeling of cholinergic interneurons in this brain region. [18F](+)‐FBT also displayed reversible binding kinetics during the 3 h time course of PET and produced radiolabeled metabolites that did not cross the blood‐brain barrier. The results from the current in vitro and in vivo studies indicate that [18F](+)‐FBT is a promising ligand for studying cholinergic terminal density, with PET, via the VAcChT. Synapse 25:368–380, 1997.