Intranasal administration of dopamine attenuates unconditioned fear in that it reduces restraint-induced ultrasound vocalizations and escape from bright light

D2-like receptor activation by intranasal dopamine attenuates fear responses induced by electrical stimulation of the dorsal periaqueductal grey matter, but fails to reduce aversion to pit vipers and T-maze performance

BACKGROUND

Panic-like reactions elicited by electrical stimulation of the dorsal periaqueductal grey matter (ES-dPAG) seem to be regulated by dopamine (DA). We showed that DA applied intranasally (IN) increased escape-behaviour thresholds induced by ES-dPAG of rats, indicating a panicolytic-like effect.

AIMS

We investigated whether IN-DA increases escape-response thresholds induced by ES-dPAG by acting on D₂-like receptors, and whether IN-DA affects escape responses elicited by the presence of a potential predator and by open space and height of the elevated T-maze (ETM) as well as motor performance in the open field (OF) test.

METHODS

Wistar rats exposed to ES-dPAG were treated with Sulpiride (SUL, 40 mg/kg, D₂-like receptor antagonist) previously IN-DA (2 mg/kg). Independent groups of rats treated with IN-DA were submitted to prey versus snake paradigm (PSP), ETM and OF.

RESULTS

Anti-aversive effects of the IN-DA were reduced by SUL pretreatment in the ES-dPAG test. IN-DA did not affect the escape number in the PSP nor the escape latencies in the ETM as well as motor performance in the OF.

CONCLUSIONS/INTERPRETATION

The IN-DA effects in reducing unconditioned fear responses elicited by ES-dPAG seem to be mediated by D₂-like receptors. The lack of effects on panic-related responses in the ETM and PSP may be related to the possibility of avoiding the danger inherent to these models, a defence strategy not available during ES-dPAG. These findings cannot be attributed to motor performance. The decision-making responses to avoid dangerous situations can be orchestrated by supra-mesencephalic structures connected by non-dopaminergic inputs.

Dopamine D2-Subtype Receptors Outside the Blood-Brain Barrier Mediate Enhancement of Mesolimbic Dopamine Release and Conditioned Place Preference by Intravenous Dopamine

Dopamine (DA) is a cell-signaling molecule that does not readily cross the blood-brain barrier. Despite this, peripherally administered DA enhances DA levels in the nucleus accumbens and alters DA-related behaviors. This study was designed to investigate whether DA subtype-2 receptors are involved in the enhancement of nucleus accumbens (NAc) DA levels elicited by intravenous DA administration. This was accomplished by using microdialysis in the NAc and extracellular single unit recordings of putative DA neurons in the ventral tegmental area (VTA). Additionally, the reinforcing properties of intravenous DA were investigated using a place conditioning paradigm and the effects of intravenous DA on ultrasonic vocalizations were assessed. Following administration of intravenous dopamine, the firing rate of putative DA neurons in the VTA displayed a biphasic response and DA levels in the nucleus accumbens were enhanced. Pretreatment with domperidone, a peripheral-only DA D2 receptor (D2R) antagonist, reduced intravenous DA mediated increases in VTA DA neuron activity and NAc DA levels. Pretreatment with phentolamine, a peripheral α-adrenergic receptor antagonist, did not alter the effects of IV DA on mesolimbic DA neurotransmission. These results provide evidence for peripheral D2R mediation of the effects of intravenous DA on mesolimbic DA signaling.

Acute intranasal dopamine application counteracts the reversal learning deficit of spontaneously hypertensive rats in an attentional set-shifting task

RATIONALE

Studies on the attention-deficit/hyperactivity disorder (ADHD) have concluded that the disorder might be caused by a deficit in the inhibitory control of executive functions because of dopamine hypofunction. Recently, the intranasal route has emerged as an effective alternative means for sending dopamine directly to the brain. However, whether the treatment can ameliorate the deficits of inhibitory control in ADHD remains unknown.

OBJECTIVES

Investigating the effects of acute intranasal dopamine (IN-DA) on the inhibitory control of executive functions of an ADHD rodent model.

METHODS

We trained an animal model of ADHD, the spontaneously hypertensive rat (SHR), and Wistar rats as controls, in an attentional set-shifting task (ASST) in which dopamine (0.15 mg/kg, 0.3 mg/kg, or vehicle) was intranasally administered before the final test.

RESULTS

IN-DA application dose-dependently improved the performance and reduced errors of SHR in the initial reversal learning. The effect size was comparable to that of a peripheral injection of 0.6 mg/kg methylphenidate. In control Wistar rats, the highest dose of intranasal dopamine (0.3 mg/kg) induced deficits in the reversal learning of extradimensional discriminations.

CONCLUSIONS

The findings suggest that the IN-DA treatment has potential for use in the treatment of ADHD; however, caution must be exercised when determining the dosage to be administered, because too much dopamine may have negative effects.

Altered dopaminergic pathways and therapeutic effects of intranasal dopamine in two distinct mouse models of autism

The dopamine (DA) system has a profound impact on reward-motivated behavior and is critically involved in neurodevelopmental disorders, such as autism spectrum disorder (ASD). Although DA defects are found in autistic patients, it is not well defined how the DA pathways are altered in ASD and whether DA can be utilized as a potential therapeutic agent for ASD. To this end, we employed a phenotypic and a genetic ASD model, i.e., Black and Tan BRachyury T+Itpr3tf/J (BTBR) mice and Fragile X Mental Retardation 1 knockout (Fmr1-KO) mice, respectively. Immunostaining of tyrosine hydroxylase (TH) to mark dopaminergic neurons revealed an overall reduction in the TH expression in the substantia nigra, ventral tegmental area and dorsal striatum of BTBR mice, as compared to C57BL/6 J wild-type ones. In contrast, Fmr1-KO animals did not show such an alteration but displayed abnormal morphology of TH-positive axons in the striatum with higher "complexity" and lower "texture". Both strains exhibited decreased expression of striatal dopamine transporter (DAT) and increased spatial coupling between vesicular glutamate transporter 1 (VGLUT1, a label for glutamatergic terminals) and TH signals, while GABAergic neurons quantified by glutamic acid decarboxylase 67 (GAD67) remained intact. Intranasal administration of DA rescued the deficits in non-selective attention, object-based attention and social approaching of BTBR mice, likely by enhancing the level of TH in the striatum. Application of intranasal DA to Fmr1-KO animals alleviated their impairment of social novelty, in association with reduced striatal TH protein. These results suggest that although the DA system is modified differently in the two ASD models, intranasal treatment with DA effectively rectifies their behavioral phenotypes, which may present a promising therapy for diverse types of ASD.

Intranasal dopamine attenuates fear responses induced by electric shock to the foot and by electrical stimulation of the dorsal periaqueductal gray matter

PURPOSE

Intranasally applied dopamine (IN-DA), which likely reaches the brain via nasal-brain pathways and bypasses the blood-brain barrier, has been found to increase extracellular DA and bind to the DA2 transporter in the striatum. Recent studies suggest that DA plays a significant role in the processing of signaled and unconditioned aversive stimulation, including evidence that may attenuate responses to painful input. The purpose of this study was to examine the effects of IN-DA on fear-related behaviors induced by electric shock to the foot or by electrical stimulation of the dorsal periaqueductal gray matter (dPAG).

METHODS

DA hydrochloride suspended in a viscous castor oil gel (1 or 2 mg/kg) was applied (IN-DA) in a volume of 5 μL into the nostrils of adult Wistar male rats in order to evaluate its effects on (a) freezing induced by electric shock to the foot and (b) thresholds of freezing and escape and duration of post-stimulation freezing induced by electrical stimulation of the dPAG.

RESULTS

IN-DA attenuated freezing induced by electric shock to the foot in the three test trials, indicating that it reduced long-term fear responses. IN-DA also increased the threshold of dPAG stimulation-induced escape responses and reduced post-stimulation freezing.

CONCLUSIONS

IN-DA, which has previously been shown to facilitate learning and to have antidepressive-like effects, attenuated unconditioned fear responses elicited by peripheral and intramesencephalic (dPAG) stimulation and reduced long-term conditioned fear responses.

Role of the dorsal periaqueductal gray in posttraumatic stress disorder: mediation by dopamine and neurokinin

In susceptible individuals, exposure to intensely traumatic life events can lead to the development of posttraumatic stress disorder (PTSD), including long-term dysregulation of the contextual processing of aversive stimuli, the overgeneralization of learned fear, and impairments in the ability to learn or respond to safety signals. The neuropathophysiological changes that underlie PTSD remain incompletely understood. Attention has focused on forebrain structures associated with fear processing. Here we consider evidence from human and animal studies that long-lasting changes in functional connectivity between the midbrain periaqueductal gray (dPAG) and amygdala may be one of the precipitating events that contribute to PTSD. Long-lasting neuroplastic changes in the dPAG can persist after a single aversive stimulation and are pharmacologically labile. The early stage (at least up to 24 h post-stimulation) involves neurokinin-1 receptor-mediated events in the PAG and amygdala and is also regulated by dopamine, both of which are mainly involved in transferring ascending aversive information from the dPAG to higher brain structures, mainly the amygdala. Changes in the functional connectivity within the dPAG-amygdala circuit have been reported in PTSD patients. We suggest that further investigations of plasticity and pharmacology of the PAG-amygdala network provide a promising target for understanding pathophysiological circuitry that underlies PTSD in humans and that dopaminergic and neurokininergic drugs may have a potential for the treatment of psychiatric disorders that are associated with a dysfunctional dPAG.

Intra-nasal dopamine alleviates cognitive deficits in tgDISC1 rats which overexpress the human DISC1 gene

The Disrupted-in-Schizophrenia 1 (DISC1) gene has been associated with mental illnesses such as major depression and schizophrenia. The transgenic DISC1 (tgDISC1) rat, which overexpresses the human DISC1 gene, is known to exhibit deficient dopamine (DA) homeostasis. To ascertain whether the DISC1 gene also impacts cognitive functions, 14-15 months old male tgDISC1 rats and wild-type controls were subjected to the novel object preference (NOP) test and the object-based attention test (OBAT) in order to assess short-term memory (1 h), long-term memory (24 h), and attention.

RESULTS

The tgDISC1 group exhibited intact short-term memory, but deficient long-term-memory in the NOP test and deficient attention-related behavior in the OBAT. In a different group of tgDISC1 rats, 3 mg/kg intranasally applied dopamine (IN-DA) or its vehicle was applied prior to the NOP or the OBAT test. IN-DA reversed cognitive deficits in both the NOP and OBAT tests. In a further cohort of tgDISC1 rats, post-mortem levels of DA, noradrenaline, serotonin and acetylcholine were determined in a variety of brain regions. The tgDISC1 group had less DA in the neostriatum, hippocampus and amygdala, less acetylcholine in neostriatum, nucleus accumbens, hippocampus, and amygdala, more serotonin in the nucleus accumbens, and less serotonin and noradrenaline in the amygdala.

CONCLUSIONS

Our findings show that DISC1 overexpression and misassembly is associated with deficits in long-term memory and attention-related behavior. Since behavioral impairments in tgDISC1 rats were reversed by IN-DA, DA deficiency may be a major cause for the behavioral deficits expressed in this model.