Hippocampal lesions impair non-navigational spatial memory in macaques

Decades of studies robustly support a critical role for the hippocampus in spatial memory across a wide range of species. Hippocampal damage produces clear and consistent deficits in allocentric spatial memory that requires navigating through space in rodents, non-human primates, and humans. By contrast, damage to the hippocampus spares performance in most non-navigational spatial memory tasks-which can typically be resolved using egocentric cues. We previously found that transient inactivation of the hippocampus impairs performance in the Hamilton Search Task (HST), a self-ordered non-navigational spatial search task. A key question, however, still needs to be addressed. Acute, reversible inactivation of the hippocampus may have resulted in an impairment in the HST because this approach does not allow for neuroplastic compensation, may prevent the development of an alternative learning strategy, and/or may produce network-based effects that disrupt performance. We compared learning and performance on the HST in male rhesus macaques (six unoperated control animals and six animals that underwent excitotoxic lesions of the hippocampus). We found a significant impairment in animals with hippocampal lesions. While control animals improved in performance over the course of 45 days of training, performance in animals with hippocampal lesions remained at chance levels. The HST thus represents a sensitive assay for probing the integrity of the hippocampus in non-human primates. These data provide evidence demonstrating that the hippocampus is critical for this type of non-navigational spatial memory, and help to reconcile the many null findings previously reported.

Pharmacological Inhibition of the Nucleus Accumbens Increases Dyadic Social Interaction in Macaques

The nucleus accumbens (NAc) is a central component of the brain circuitry that mediates motivated behavior, including reward processing. Since the rewarding properties of social stimuli have a vital role in guiding behavior (both in humans and nonhuman animals), the NAc is likely to contribute to the brain circuitry controlling social behavior. In rodents, prior studies have found that focal pharmacological inhibition of NAc and/or elevation of dopamine in NAc increases social interactions. However, the role of the NAc in social behavior in nonhuman primates remains unknown. We measured the social behavior of eight dyads of male macaques following (1) pharmacological inhibition of the NAc using the GABAA agonist muscimol and (2) focal application of quinpirole, an agonist at the D2 family of dopamine receptors. Transient inhibition of the NAc with muscimol increased social behavior when drug was infused in submissive, but not dominant partners of the dyad. Focal application of quinpirole was without effect on social behavior when infused into the NAc of either dominant or submissive subjects. Our data demonstrate that the NAc contributes to social interactions in nonhuman primates.

Quinpirole, but not muscimol, infused into the nucleus accumbens disrupts prepulse inhibition of the acoustic startle in rhesus macaques

Sensorimotor gating is the ability to suppress motor responses to irrelevant sensory inputs. This response is disrupted in a range of neuropsychiatric disorders. Prepulse inhibition (PPI) of the acoustic startle response (ASR) is a form of sensorimotor gating in which a low-intensity prepulse immediately precedes a startling stimulus, resulting in an attenuation of the startle response. PPI is conserved across species and the underlying circuitry mediating this effect has been widely studied in rodents. However, recent work from our laboratories has shown an unexpected divergence between the circuitry controlling PPI in rodents as compared to macaques. The nucleus accumbens, a component of the basal ganglia, has been identified as a key modulatory node for PPI in rodents. The role of the nucleus accumbens in modulating PPI in primates has yet to be investigated. We measured whole-body PPI of the ASR in six rhesus macaques following (1) pharmacological inhibition of the nucleus accumbens using the GABAA agonist muscimol, and (2) focal application of the dopamine D2/3 agonist quinpirole (at 3 doses). We found that quinpirole, but not muscimol, infused into the nucleus accumbens disrupts prepulse inhibition in monkeys. These results differ from those observed in rodents, where both muscimol and quinpirole disrupt prepulse inhibition.

Pharmacological Inactivation of the Bed Nucleus of the Stria Terminalis Increases Affiliative Social Behavior in Rhesus Macaques

The bed nucleus of the stria terminalis (BNST) has been implicated in a variety of social behaviors, including aggression, maternal care, mating behavior, and social interaction. Limited evidence from rodent studies suggests that activation of the BNST results in a decrease in social interaction between unfamiliar animals. The role of the BNST in social interaction in primates remains wholly unexamined. Nonhuman primates provide a valuable model for studying social behavior because of both their rich social repertoire and neural substrates of behavior with high translational relevance to humans. To test the hypothesis that the primate BNST is a critical modulator of social behavior, we performed intracerebral microinfusions of the GABAA agonist muscimol to transiently inactivate the BNST in male macaque monkeys. We measured changes in social interaction with a familiar same-sex conspecific. Inactivation of the BNST resulted in significant increase in total social contact. This effect was associated with an increase in passive contact and a significant decrease in locomotion. Other nonsocial behaviors (sitting passively alone, self-directed behaviors, and manipulation) were not impacted by BNST inactivation. As part of the "extended amygdala," the BNST is highly interconnected with the basolateral (BLA) and central (CeA) nuclei of the amygdala, both of which also play critical roles in regulating social interaction. The precise pattern of behavioral changes we observed following inactivation of the BNST partially overlaps with our prior reports in the BLA and CeA. Together, these data demonstrate that the BNST is part of a network regulating social behavior in primates.SIGNIFICANCE STATEMENT The bed nucleus of the stria terminalis (BNST) has a well-established role in anxiety behaviors, but its role in social behavior is poorly understood. No prior studies have evaluated the impact of BNST manipulations on social behavior in primates. We found that transient pharmacological inactivation of the BNST increased social behavior in pairs of macaque monkeys. These data suggest the BNST contributes to the brain networks regulating sociability.

The Parahippocampal Cortex and its Functional Connection with the Hippocampus are Critical for Nonnavigational Spatial Memory in Macaques

The Hamilton Search Task (HST) is a test of nonnavigational spatial memory that is dependent on the hippocampus. The parahippocampal cortex (PHC) is a major route for spatial information to reach the hippocampus, but the extent to which the PHC and hippocampus function independently of one another in the context of nonnavigational spatial memory is unclear. Here, we tested the hypotheses that (1) bilateral pharmacological inactivation of the PHC would impair HST performance, and (2) that functional disconnection of the PHC and hippocampus by contralateral (crossed) inactivation would likewise impair performance. Transient inactivation of the PHC impaired HST performance most robustly with 30 s intertrial delays, but not when color cues were introduced. Functional disconnection of the PHC and hippocampus, but not separate unilateral inactivation of either region, also selectively impaired long-term spatial memory. These findings indicate a critical role for the PHC and its interactions with the hippocampus in nonnavigational spatial memory.

Inhibition of the Deep and Intermediate Layers of the Superior Colliculus Disrupts Sensorimotor Gating in Monkeys

The deep and intermediate layers of the superior colliculus (DLSC) respond to visual, auditory, and tactile inputs and act as a multimodal sensory association area. In turn, activity in the DLSC can drive orienting and avoidance responses-such as saccades and head and body movements-across species, including in rats, cats, and non-human primates. As shown in rodents, DLSC also plays a role in regulating pre-pulse inhibition (PPI) of the acoustic startle response (ASR), a form of sensorimotor gating. DLSC lesions attenuate PPI and electrical stimulation of DLSC inhibits the startle response. While the circuitry mediating PPI is well-characterized in rodents, less is known about PPI regulation in primates. Two recent studies from our labs reported a species difference in the effects of pharmacological inhibition of the basolateral amygdala and substantia nigra pars reticulata (SNpr) on PPI between rats and macaques: in rats, inhibition of these structures decreased PPI, while in macaques, it increased PPI. Given that the SNpr sends direct inhibitory projections to DLSC, we next sought to determine if this species difference was similarly evident at the level of DLSC. Here, we transiently inactivated DLSC in four rhesus macaques by focal microinfusion of the GABAA receptor agonist muscimol. Similar to findings reported in rodents, we observed that bilateral inhibition of the DLSC in macaques significantly disrupted PPI. The impairment was specific to the PPI as the ASR itself was not affected. These results indicate that our previously reported species divergence at the level of the SNpr is not due to downstream differences at the level of the DLSC. Species differences at the level of the SNpr and basolateral amygdala emphasize the importance of studying the underlying circuitry in non-human primates, as impairment in PPI has been reported in several disorders in humans, including schizophrenia, autism, and PTSD.

Effects of systemic cholinergic antagonism on reinforcer devaluation in macaques

The capacity to adjust actions based on new information is a vital cognitive function. An animal's ability to adapt behavioral responses according to changes in reward value can be measured using a reinforcer devaluation task, wherein the desirability of a given object is reduced by decreasing the value of the associated food reinforcement. Elements of the neural circuits serving this ability have been studied in both rodents and nonhuman primates. Specifically, the basolateral amygdala, orbitofrontal cortex, nucleus accumbens, and mediodorsal thalamus have each been shown to play a critical role in the process of value updating, required for adaptive goal selection. As these regions receive dense cholinergic input, we investigated whether systemic injections of non-selective nicotinic or muscarinic acetylcholine receptor antagonists, mecamylamine and scopolamine, respectively, would impair performance on a reinforcer devaluation task. Here we demonstrate that in the presence of either a nicotinic or muscarinic antagonist, animals are able to shift their behavioral responses in an appropriate manner, suggesting that disruption of cholinergic neuromodulation is not sufficient to disrupt value updating, and subsequent goal selection, in rhesus macaques.

Defensive Vocalizations and Motor Asymmetry Triggered by Disinhibition of the Periaqueductal Gray in Non-human Primates

Rapid and reflexive responses to threats are present across phylogeny. The neural circuitry mediating reflexive defense reactions has been well-characterized in a variety of species, for example, in rodents and cats, the detection of and species-typical response to threats is mediated by a network of structures including the midbrain tectum (deep and intermediate layers of the superior colliculus [DLSC]), periaqueductal gray (PAG), and forebrain structures such as the amygdala and hypothalamus. However, relatively little is known about the functional architecture of defense circuitry in primates. We have previously reported that pharmacological activation of the DLSC evokes locomotor asymmetry, defense-associated vocalizations, cowering behavior, escape responses, and attack of inanimate objects (Holmes et al., 2012; DesJardin et al., 2013; Forcelli et al., 2016). Here, we sought to determine if pharmacological activation of the PAG would induce a similar profile of responses. We activated the PAG in three awake, behaving macaques by microinfusion of GABA-A receptor antagonist, bicuculline methiodide. Activation of PAG evoked defense-associated vocalizations and postural/locomotor asymmetry, but not motor defense responses (e.g., cowering, escape behavior). These data suggest a partial dissociation between the role of the PAG and the DLSC in the defense network of macaques, but a general conservation of the role of PAG in defense responses across species.