Alpha1- and Beta-norepinephrinergic receptors of dorsomedial and ventromedial hypothalamic nuclei modulate panic attack-like defensive behaviour elicited by diencephalic GABAergic neurotransmission disinhibition.

Gamma-aminobutyric acid (GABA) disinhibition in medial hypothalamus (MH) nuclei of rats elicits some defensive reactions that are considered panic attack-like behaviours. Recent evidence showed that the norepinephrine-mediated system modulates fear-related defensive behaviours organised by MH neurons at least in part via noradrenergic receptors recruitment on midbrain tegmentum. However, it is unknown whether noradrenergic receptors of the MH also modulate the panic attack-like reactions. The aim of this work was to investigate the distribution of noradrenergic receptors in MH, and the effects of either α1-, α2- or ß-noradrenergic receptors blockade in the MH on defensive behaviours elaborated by hypothalamic nuclei. Defensive behaviours were evaluated after the microinjection of the selective GABAA receptor antagonist bicuculline into the MH that was preceded by microinjection of either WB4101, RX821002, propranolol (α1-, α2- and ß-noradrenergic receptor selective antagonists, respectively), or physiological saline into the MH of male Wistar rats. The α1-, α2- and ß-noradrenergic receptors were found in neuronal perikarya of all MH nuclei, and the α2-noradrenergic receptor were also found on glial cells mainly situated in the ventrolateral division of the ventromedial hypothalamic nucleus. The α1- and ß-noradrenergic receptors blockade in the MH decreased defensive attention and escape reactions elicited by the intra-MH microinjections of bicuculline. These findings suggest that, despite the profuse distributions of α1-, α2- and ß-noradrenergic receptors in the MH, both α1- and ß-noradrenergic receptor- rather than α2-noradrenergic receptor-signalling in MH are critical for the neuromodulation of panic-like behaviour.

Neostriatum neuronal TRPV1-signalling mediates striatal anandamide at high concentration facilitatory influence on neostriato-nigral dishinhibitory GABAergic connections.

RATIONALE: Several lines of evidence have demonstrated that the cannabinoid type 1 receptor (CB1) is found in the caudate nucleus and putamen (CPu) in addition to the substantia nigra pars reticulata (SNpr). Here, we investigated the role of endocannabinoid neuromodulation of striato-nigral disinhibitory projections on the activity of nigro-collicular GABAergic pathways that control the expression of unconditioned fear-related behavioural responses elicited by microinjections of the GABAA receptor selective antagonist bicuculline (BIC) in the deep layers of the superior colliculus (dlSC). METHODS: Fluorescent neural tract tracers were deposited in either CPu or in SNpr. Wistar rats received injection of vehicle, anandamide (AEA), either at low (50 pmol) or high (100 pmol) concentrations in CPu followed by bicuculline microinjections in dlSC. RESULTS: Connections between CPu, the SNpr and dlSC were demonstrated. The GABAA receptor blockade in dlSC elicited panic-like behaviour. AEA at the lowest concentration caused a panicolytic-like effect that was antagonised by the CPu pretreatment with AM251 at 100 pmol. AEA at the highest concentration caused a panicogenic-like effect that was antagonised by the CPu pretreatment with 6-iodonordihydrocapsaicin (6-I-CPS) at different concentrations (0.6, 6, 60 nmol). CONCLUSION: These findings suggest that while pre-synaptic CB1-signalling subserves an indirect facilitatory effect of AEA on striato-nigral pathways causing panicolytic-like responses through midbrain tectum enhanced activity, post-synaptic TRPV1-signalling in CPu mediates AEA direct activation of striato-nigral disinhibitory pathways resulting in increasing dlSC neurons activity and a panicogenic-like response. All these actions seem to depend on the interface with the nigro-collicular inhibitory GABAergic pathways.

Ventrolateral periaqueductal gray matter integrative system of defense and antinociception.

Defensive responses are neurophysiological processes crucial for survival during threatening situations. Defensive immobility is a common adaptive response, in rodents, elaborated by ventrolateral periaqueductal gray matter (vlPAG) when threat is unavoidable. It is associated with somatosensory and autonomic reactions such as alteration in the sensation of pain and rate of respiration. In this study, defensive immobility was assessed by chemical stimulation of vlPAG with different doses of NMDA (0.1, 0.3, and 0.6 nmol). After elicitation of defensive immobility, antinociceptive and respiratory response tests were also performed. Results revealed that defensive immobility was followed by a decrease in the nociceptive perception. Furthermore, the lowest dose of NMDA induced antinociceptive response without eliciting defensive immobility. During defensive immobility, respiratory responses were also disturbed. Interestingly, respiratory rate was increased and interspersed with prolonged expiratory phase of breathing. These findings suggest that vlPAG integrates three different defensive behavioral responses, contributing to the most effective defensive strategies during threatening situations.

The modulation of striatonigral and nigrotectal pathways by CB1 signalling in the substantia nigra pars reticulata regulates panic elicited in mice by urutu-cruzeiro lancehead pit vipers.

Cannabinoid receptor type 1 (CB1R) is widely distributed in the substantia nigra pars reticulata (SNpr). However, the role of CB1R at the SNpr level in threatening situations is poorly understood. We investigated the role of CB1R in the SNpr on the expression of fear responses in mice confronted with urutu-cruzeiro pit vipers. First, a bidirectional neurotracer was injected into the SNpr; then, immunostaining of the vesicular GABA transporter was conducted at the levels of the striatum (CPu) and deep layers of the superior colliculus (dlSC). In addition, CB1R immunostaining and GABA labelling were performed in the SNpr. Using a prey-versus-snake paradigm, mice were pretreated with the CB1R antagonist AM251 (100 pmol) and treated with the endocannabinoid anandamide (AEA, 5 pmol) in the SNpr, followed by bicuculline (40 ng) in the dlSC, and were then confronted with a snake. Bidirectional neural tract tracers associated with immunofluorescence showed the GABAergic striatonigral disinhibitory and nigrotectal inhibitory pathways. Furthermore, we showed that CB1R labelling was restricted to axonal fibres surrounding SNpr GABAergic cells. We also demonstrated a decrease in the defensive behaviours of mice treated with AEA in the SNpr, but this effect was blocked by pre-treatment with AM251 in this structure. Taken together, our results show that the panicolytic consequences of the AEA enhancement in the SNpr are signalled by CB1R, suggesting that CB1R localised in axon terminals of CPu GABAergic neurons in the SNpr modulates the activity of the nigrotectal GABAergic pathway during the expression of defensive behaviours in threatening situations.

Endocannabinoid neuromodulation in the neostriatum decreases the GABAergic striato-nigral disinhibitory function and increases the nigro-collicular inhibitory pathway activity.

We previously reported the involvement of neostriato-nigral projections in the organisation of innate fear and panic attack-like responses organised by dorsal midbrain neurons, such as the periaqueductal grey matter and the deep layers of the superior colliculus (dlSC). In addition, several lines of evidence have demonstrated that cannabinoid receptor type 1 is found in the neostriatum (caudate nucleus and putamen; CPu). In the present study, we investigated the role of endocannabinoid neuromodulation in CPu in the expression of unconditioned fear-related behavioural responses elicited by microinjections of the γ-aminobutyric acid (GABA)A receptor selective antagonist bicuculline (BIC) in the dlSC. Wistar rats received injection of vehicle or anandamide (AEA) at 0.5, 5, 50, 100 pmol in CPu, followed by injections of BIC in a dose of 40 ng in the dlSC. The treatment of the CPu with AEA in a dose of 5 and 50 pmol attenuated the unconditioned fear-related behaviour, such as defensive alertness, defensive immobility and escape, induced by GABAA receptor blockade in dlSC. These findings suggest that endogenous cannabinoids acting on CPu neurons exert an indirect modulatory influence on the activity of superior colliculus neurons, possibly through an inhibitory activity on neostriato-nigral disinhibitory connections that modulate the nigro-collicular inhibitory GABAergic pathways.

The Blockade of µ1- and κ-Opioid Receptors in the Inferior Colliculus Decreases the Expression of Panic Attack-Like Behaviours Induced by Chemical Stimulation of the Dorsal Midbrain.

BACKGROUND: Gamma-aminobutyric acid (GABA)ergic and opioid systems play a crucial role in the neural modulation of innate fear organised by the inferior colliculus (IC). In addition, the IC is rich in GABAergic fibres and opioid neurons, which are also connected to other mesencephalic structures, such as the superior colliculus and the substantia nigra. However, the contribution of distinct opioid receptors (ORs) in the IC during the elaboration and expression of innate fear and panic-like responses is unclear. The purpose of the present work was to investigate a possible integrated action exerted by ORs and the GABAA receptor-mediated system in the IC on panic-like responses. METHODS: The effect of the blockade of either µ1- or κ-ORs in the IC was evaluated in the unconditioned fear-induced responses elicited by GABAA antagonism with bicuculline. Microinjections of naloxonazine, a µ1-OR antagonist, or nor-binaltorphimine (nor-BNI), a κ-OR antagonist, were made into the IC, followed by intramesencephalic administration of the GABAA-receptor antagonist bicuculline. The defensive behaviours elicited by the treatments in the IC were quantitatively analysed, recording the number of escapes expressed as running (crossing), jumps, and rotations, over a 30-min period in a circular arena. The exploratory behaviour of rearing was also recorded. RESULTS: GABAA-receptor blockade with bicuculline in the IC increased defensive behaviours. However, pretreatment of the IC with higher doses (5 µg) of naloxonazine or nor-BNI followed by bicuculline resulted in a significant decrease in unconditioned fear-induced responses. CONCLUSIONS: These findings suggest a role played by µ1- and κ-OR-containing connexions and GABAA receptor-mediated neurotransmission on the organisation of panic attack-related responses elaborated by the IC neurons.

The endogenous opioid system modulates defensive behavior evoked by Crotalus durissus terrificus: Panicolytic-like effect of intracollicular non-selective opioid receptors blockade.

BACKGROUND: There is a controversy regarding the key role played by opioid peptide neurotransmission in the modulation of panic-attack-related responses. AIMS: Using a prey versus rattlesnakes paradigm, the present work investigated the involvement of the endogenous opioid peptide-mediated system of the inferior colliculus in the modulation of panic attack-related responses. METHODS: Wistar rats were pretreated with intracollicular administration of either physiological saline or naloxone at different concentrations and confronted with rattlesnakes ( Crotalus durissus terrificus). The prey versus rattlesnake confrontations were performed in a polygonal arena for snakes. The defensive behaviors displayed by prey (defensive attention, defensive immobility, escape response, flat back approach and startle) were recorded twice: firstly, over a period of 15 min the presence of the predator and a re-exposure was performed 24 h after the confrontation, when animals were exposed to the experimental enclosure without the rattlesnake. RESULTS: The intramesencephalic non-specific blockade of opioid receptors with microinjections of naloxone at higher doses decreased both anxiety- (defensive attention and flat back approach) and panic attack-like (defensive immobility and escape) behaviors, evoked in the presence of rattlesnakes and increased non-defensive responses. During the exposure to the experimental context, there was a decrease in duration of defensive attention. CONCLUSIONS: These findings suggest a panicolytic-like effect of endogenous opioid receptors antagonism in the inferior colliculus on innate (panic attack) and conditioned (anticipatory anxiety) fear in rats threatened by rattlesnakes.

Blockade of synaptic activity in the neostriatum and activation of striatal efferent pathways produce opposite effects on panic attack-like defensive behaviours evoked by GABAergic disinhibition in the deep layers of the superior colliculus.

The dorsal periaqueductal grey matter (dPAG) and the deep layers of the superior colliculus (dlSC) have been implicated in the organisation of innate fear-related defensive behaviours. Furthermore, GABAergic neurons from the substantia nigra pars reticulata (SNpr) connected to the dlSC and dPAG receive convergent disinhibitory inputs from the caudate-putamen (CPu), comprising the neostriatum, and modulate defence responses elicited by midbrain tectum stimulation. The purpose of this work was to study the effect of either excitatory cortico-neostriatal input blockade or neostriato-nigral GABAergic disinhibitory output activation on the responsivity of GABAergic nigro-collicular tonic inhibitory pathways during the elicitation of panic attack-like defensive responses produced by bicuculline administration into the dlSC. Thus, we investigated the effects of microinjection of either the synaptic activity blocker cobalt chloride (CoCl2) or the NMDA receptor agonist N-methyl-D-aspartic acid in the CPu on the elaboration of the defensive behaviour elicited by the selective blockade of GABAA receptors in the dlSC. Our findings showed that pretreatment of the neostriatum with CoCl2 caused clear anxiolytic and panicolytic-like effects, reducing the incidence and duration of alertness and diminishing defensive immobility and explosive escape responses. On the other hand, pretreatment of the neostriatum with NMDA (40 nmol) caused a pro-aversive effect, enhancing running and jumping responses elicited by GABAergic disinhibition in the dlSC. We conclude from the data that the neostriato-nigral disinhibitory and nigro-collicular inhibitory GABAergic pathways modulate innate fear and panic attack-like responses organised by dlSC neurons.

Panicolytic-like effects caused by substantia nigra pars reticulata pretreatment with low doses of endomorphin-1 and high doses of CTOP or the NOP receptors antagonist JTC-801 in male Rattus norvegicus.

RATIONALE: Gamma-aminobutyric acid (GABA)ergic neurons of the substantia nigra pars reticulata (SNpr) are connected to the deep layers of the superior colliculus (dlSC). The dlSC, in turn, connect with the SNpr through opioid projections. Nociceptin/orphanin FQ peptide (N/OFQ) is a natural ligand of a Gi protein-coupled nociceptin receptor (ORL1; NOP) that is also found in the SNpr. Our hypothesis is that tectonigral opioid pathways and intranigral orphanin-mediated mechanisms modulate GABAergic nigrotectal connections. OBJECTIVES: Therefore, the aim of this work was to study the role of opioid and NOP receptors in the SNpr during the modulation of defence reactions organised by the dlSC. METHODS: The SNpr was pretreated with either opioid or NOP receptor agonists and antagonists, followed by dlSC treatment with bicuculline. RESULTS: Blockade of GABAA receptors in the dlSC elicited fear-related defensive behaviour. Pretreatment of the SNpr with naloxone benzoylhydrazone (NalBzoH), a µ-, δ-, and κ1-opioid receptor antagonist as well as a NOP receptor antagonist, decreased the aversive effect of bicuculline treatment on the dlSC. Either µ-opioid receptor activation or blockade by SNpr microinjection of endomorphin-1 (EM-1) and CTOP promoted pro-aversive and anti-aversive actions, respectively, that modulated the defensive responses elicited by bicuculline injection into the dlSC. Pretreatment of the SNpr with the selective NOP receptor antagonist JTC801 decreased the aversive effect of bicuculline, and microinjections of the selective NOP receptor agonist NNC 63-0532 promoted the opposite effect. CONCLUSIONS: These results demonstrate that opioid pathways and orphanin-mediated mechanisms have a critical role in modulating the activity of nigrotectal GABAergic pathways during the organisation of defensive behaviours.

The µ1-opioid receptor and 5-HT2A- and 5HT2C-serotonergic receptors of the locus coeruleus are critical in elaborating hypoalgesia induced by tonic and tonic-clonic seizures.

It has been proposed that the post-ictal state is associated with the expression of hypoalgesia. It is clear that the projections among the periaqueductal gray matter (PAG), dorsal raphe nucleus (DRN) and locus coeruleus (LC) play a role in pain management. These mesencephalic structures have direct reciprocal opioid and monoaminergic projections to the LC that can possibly modulate post-ictal hypoalgesia. The goal of this study was to examine if LC-opioid and serotonergic/noradrenergic mechanisms signal the post-ictal hypoalgesic responses to tonic-clonic seizures produced by intraperitoneal administration of pentylenetetrazole (PTZ at 64mg/kg), causing an ionophore γ-aminobutyric acid (GABA)-mediated Cl- influx antagonism. The rodents' nociceptive threshold was measured by the tail-flick test. Intra-LC cobalt chloride (1.0nM/0.2µL) microinjections produced intermittent local synaptic inhibition and were able to reduce post-ictal hypoalgesia. Central administration of naltrexone (a non-selective antagonist for opioid receptors), naloxonazine (a selective antagonist for µ1-opioid-receptors), methysergide (a non-selective antagonist for serotonergic receptors) or ketanserin (an antagonist for both α1-noradrenergic and 5-Hydroxytryptamine(HT)2A/2C receptors) at 5.0µg/0.2µL, R-96544 (a 5-HT2A receptor selective antagonist) at 10nM/0.2µL, or RS-102221 (a 5-HT2C receptor selective antagonist) at 0.15µg/0.2µL into the LC also decreased post-ictal hypoalgesia. The data presented here suggest that the post-ictal antinociception mechanism involves the µ1-opiod, 5-HT2A- and 5-HT2C-serotonergic, and α1-noradrenergic receptors in the LC.

5-Hydroxytryptamine 1A receptors in the dorsomedial hypothalamus connected to dorsal raphe nucleus inputs modulate defensive behaviours and mediate innate fear-induced antinociception.

The dorsal raphe nucleus (DRN) is an important brainstem source of 5-hydroxytryptamine (5-HT), and 5-HT plays a key role in the regulation of panic attacks. The aim of the present study was to determine whether 5-HT1A receptor-containing neurons in the medial hypothalamus (MH) receive neural projections from DRN and to then determine the role of this neural substrate in defensive responses. The neurotracer biotinylated dextran amine (BDA) was iontophoretically microinjected into the DRN, and immunohistochemical approaches were then used to identify 5HT1A receptor-labelled neurons in the MH. Moreover, the effects of pre-treatment of the dorsomedial hypothalamus (DMH) with 8-OH-DPAT and WAY-100635, a 5-HT1A receptor agonist and antagonist, respectively, followed by local microinjections of bicuculline, a GABAA receptor antagonist, were investigated. We found that there are many projections from the DRN to the perifornical lateral hypothalamus (PeFLH) but also to DMH and ventromedial (VMH) nuclei, reaching 5HT1A receptor-labelled perikarya. DMH GABAA receptor blockade elicited defensive responses that were followed by antinociception. DMH treatment with 8-OH-DPAT decreased escape responses, which strongly suggests that the 5-HT1A receptor modulates the defensive responses. However, DMH treatment with WAY-100635 failed to alter bicuculline-induced defensive responses, suggesting that 5-HT exerts a phasic influence on 5-HT1A DMH neurons. The activation of the inhibitory 5-HT1A receptor had no effect on antinociception. However, blockade of the 5-HT1A receptor decreased fear-induced antinociception. The present data suggest that the ascending pathways from the DRN to the DMH modulate panic-like defensive behaviours and mediate antinociceptive phenomenon by recruiting 5-HT1A receptor in the MH.

Dissociation between the panicolytic effect of cannabidiol microinjected into the substantia nigra, pars reticulata, and fear-induced antinociception elicited by bicuculline administration in deep layers of the superior colliculus: The role of CB1-cannabinoid receptor in the ventral mesencephalon.

Many studies suggest that the substantia nigra, pars reticulata (SNpr), a tegmental mesencephalic structure rich in γ-aminobutyric acid (GABA)- and cannabinoid receptor-containing neurons, is involved in the complex control of defensive responses through the neostriatum-nigral disinhibitory and nigro-tectal inhibitory GABAergic pathways during imminently dangerous situations. The aim of the present work was to investigate the role played by CB1-cannabinoid receptor of GABAergic pathways terminal boutons in the SNpr or of SNpr-endocannabinoid receptor-containing interneurons on the effect of intra-nigral microinjections of cannabidiol in the activity of nigro-tectal inhibitory pathways. GABAA receptor blockade in the deep layers of the superior colliculus (dlSC) elicited vigorous defensive behaviour. This explosive escape behaviour was followed by significant antinociception. Cannabidiol microinjection into the SNpr had a clear anti-aversive effect, decreasing the duration of defensive alertness, the frequency and duration of defensive immobility, and the frequency and duration of explosive escape behaviour, expressed by running and jumps, elicited by transitory GABAergic dysfunction in dlSC. However, the innate fear induced-antinociception was not significantly changed. The blockade of CB1 endocannabinoid receptor in the SNpr decreased the anti-aversive effect of canabidiol based on the frequency and duration of defensive immobility, the frequency of escape expressed by running, and both the frequency and duration of escape expressed by jumps. These findings suggest a CB1 mediated endocannabinoid signalling in cannabidiol modulation of panic-like defensive behaviour, but not of innate fear-induced antinociception evoked by GABAA receptor blockade with bicuculline microinjection into the superior colliculus, with a putative activity in nigro-collicular GABAergic pathways.

Chemical neuroanatomical and psychopharmacological evidence that κ receptor-mediated endogenous opioid peptide neurotransmission in the dorsal and ventral mesencephalon modulates panic-like behaviour.

The chemical neuroanatomy and the effects of central administration of opioid antagonists on the innate fear-induced responses elicited by electrical (at escape behaviour threshold) stimulation of the midbrain tectum were determined. The aim of the present work was to investigate the interaction between the tecto-nigral endogenous opioid peptide-mediated disinhibitory pathways and nigro-tectal inhibitory links in the control of panic-like behaviour and their organisation in the continuum comprised by the deep layers of the superior colliculus (dlSC) and the dorsolateral columns of the periaqueductal grey matter (dlPAG). Beta-endorphin-labelled neurons and fibres were found in the dorsal midbrain and also in the substantia nigra. Opioid varicose fibres and terminal buttons were widely distributed in PAG columns and in all substantia nigra subdivisions. Microinjections of naltrexone (a non-selective opioid receptor antagonist; 5.0 µg/0.2 µl) or nor-binaltorphimine (a selective κ-opioid receptor antagonist; 5.0 µg/0.2 µl) in the dlSC/dlPAG continuum, in independent groups of animals, induced significant increases in the escape thresholds for midbrain tectum electrical stimulation. The microinjection of naltrexone or nor-binaltorphimine into the SNpr also increased the escape behaviour threshold for electrical stimulation of dlSC/dlPAG. These morphological and neuropharmacological findings support previous evidence from our team for the role played by the interaction between opioidergic and GABAergic mechanisms in the modulation of innate fear-induced responses. The present data offer a neuroanatomical basis for both intratectal axo-axonic/pre-synaptic and tecto-nigral axo-somatic opioid inhibition of GABAergic nigro-tectal neurons that modulate the dorsal midbrain neurons related to the organisation of fear-related emotional responses.

Serotonergic neural links from the dorsal raphe nucleus modulate defensive behaviours organised by the dorsomedial hypothalamus and the elaboration of fear-induced antinociception via locus coeruleus pathways.

Decrease of γ-aminobutyric acid (GABA)-mediated neurotransmission in the dorsomedial hypothalamus (DMH) evokes instinctive fear-like responses. The aim of the present study was to investigate the involvement of the serotonin (5-HT)- and norepinephrine-mediated pathways of the endogenous pain inhibitory system, including the dorsal raphe nucleus (DRN) and the locus coeruleus (LC), in the defensive responses and antinociceptive processes triggered by the blockade of GABAergic receptors in the DMH. The intra-hypothalamic microinjection of the GABA(A) receptor antagonist bicuculline (40 ng/200 nL) elicited elaborate defensive behaviours interspersed with exploratory responses. This escape behaviour was followed by significantly increased pain thresholds, a phenomenon known as fear-induced antinociception. Furthermore, at 5 and 14 days after DRN serotonin-containing neurons were damaged using the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), the frequency and duration of alertness and escape behaviour evoked by the GABA(A) receptor blockade in the DMH decreased, as well as fear-induced antinociception. Pre-treatment with the non-selective 5-HT receptor antagonist methysergide, the 5-HT(2A/2C) receptor antagonist ketanserin and the 5-HT(2A) receptor selective antagonist R-96544 in the LC also decreased fear-induced antinociception, without significant changes in the expression of defensive behaviours. These data suggest that the serotonergic neurons of the DRN are directly involved in the organisation of defensive responses as well as in the elaboration of the innate fear-induced antinociception. However, serotonin-mediated inputs from the NDR to the LC modulate only fear-induced antinociception and not the defensive behaviours evoked by GABA(A) receptor blockade in the DMH.

Cellular prion protein regulates the motor behaviour performance and anxiety-induced responses in genetically modified mice.

The cellular prion protein (PrP(C)) is a sialoglycoprotein involved in neuroplasticity processes and synaptic transmission. This study investigated behavioural responses (balance in the rota-rod test at 24 rpm, motility in the open-field test, anxiety in the elevated plus-maze test) in Zurich developed wild-type adult mice (WT, controls of normal PrP(C) expression), in knockout (KO) mice (Prnp(0/0), with no PrP(C) expression), and in PrP(C) overexpressing Tg-20 mice. After 8 min in the rota-rod test, Tg-20 animals presented significantly fewer falls (1.08+/-1.56 falls) than both WT (7.27+/-4.36) and KO (7.6+/-6.15) mice (p<0.01). In the open field test, Tg-20 animals showed significantly increased motility [rearing=23.4+/-7.85, crossing=97.30+/-32.11) when compared with KO mice (rearing=5.45+/-3.69 and crossing=59.73+/-15.43) or WT mice (rearing=6.5+/-20.23 and crossing=45.18+/-20.33) (p<0.01). In the elevated plus-maze test, Tg-20 mice showed less anxiety (head projections=7.3+/-1.62) when compared with WT animals (3.38+/-0.67) (p<0.05). Moreover, KO mice spent more time in the centre of the plus maze (37.80+/-5.57 s) than did WT mice (22.57+/-3.82) (p<0.05). PrP(C) overexpressing mice evoked increased motility, less anxiety, and increased equilibrium when compared with WT control animals in the behavioural protocols used. KO animals also tended to evoke fewer anxiety-related responses in the elevated plus-maze test. These findings indicate that the levels of PrP(C) in adult life are associated with possible changes in motility, anxiety, and equilibrium.