Neurotransmitters

Neurotransmitters in autism and role of calcium signalling

Abnormalities in neurotransmitter systems have frequently been recorded in autism. Clinical observations include both elevated and lowered levels of various neurotransmitters compared to controls, including alterations in monoamine metabolism [3654486, 2653386, 3215884], neurotransmitter peptides [9018016, 9315980], with considerably raised levels of beta-endorphin (for vasopressin/oxytocin see Hormones) and altered activities of cholinergic receptors, with binding of muscarinic M(1) receptor being up to 30% and that of nicotinic receptors being 65%-73% lower in the autistic group compared to controls [11431227]. Postmortem brain examination noted abnormalities of the glutamate neurotransmitter system in autism, with specific abnormalities in the AMPA-type glutamate receptors and glutamate transporters in the cerebellum [11706102]. Expression of several types of GABA receptors is altered in brains of subjects with autism, with levels being significantly reduced in autism compared to controls [18821008, 19002745].

Dysregulations of serotonergic systems in particular have been documented, such as abnormalities in brain serotonin sythesis, with significant reductions in synthesis capacity compared to controls [10072042, 9382481], while at the same time plasma levels of serotonin and free thryptophan appear to be on average 30-50% percent higher in individuals with autism [6204248]. Autoantibodies to serotonin receptors [9067002] and reduced receptor binding have also been recorded [16648340]. Of note is that one study found correlation of elevated plasma serotonin levels and the the major histocompatibility complex (MHC) types associated with autism [8904735].

Calcium influx through VGCCs is a key step in secretion of neurotransmitters, for example serotonin [16047543]. Due to vesicle priming in neuronal excytosis, the influx of calcium ions is all that is needed to trigger nearly instantaneous neurotransmitter release in neurons [12043844]. Moreover, some findings indicate that its excessive entry through LTCC during early development may to alter neuronal response properties at later ages [9437025]. Especially in developing brain modulation of neurotransmitter release by dihydropyridine-sensitive calcium channels involves tyrosine phosphorylation. As the neurons develop a network of neurites, both tyrosine phosphorylation and LTCC activity seem to decrease [9987031, 11226706] (see Brain). This transmitter-secretion effect of LTCC is G-protein-linked and sensitive to pertussis toxin treatment [8994064]. Influx of calcium through N-type VGCC directly stimulates dopamine release and this effect can be attenuated by chalcium channels blockers [11769325, 15272204]. The involvement of LTCC-linked IP3-sensitive intercellular stores in the calcium-triggered release of dopamine and acetylcholine has also been observed [14657041]. Secretion of beta-endorphin, whose levels are significantly elevated in autism, is also triggered by calcium influx into the cell and can be lowered in vitro by applying calcium channel blockers [2428932, 10371405].

Following the findings of significant modifications of catecholamine metabolites in autism it may be worth mentioning that the activities of Catechol-O-methyl transferase (COMT), an enzyme involved in the breakdown of the catecholamine neurotransmitters, are inhibited by raised calcium levels in tissue [12170607]. Additionally, a small pilot study examining administration of tetrahydrobiopterin (R-BH4), a cofactor for tyrosine hydroxylases in the pathway of catecholamines and serotonin, reported amelioration of several autistic traits in study subjects. Decreased dopamine D2 receptor binding was also reported (see below) [9236697]. Tyrosine hydroxylase (TH) is an enzyme of central importance in catecholamine biosynthesis and the expression level of its gene is controlled by several calcium signalling pathways, most importantly LTCC-regulated CREB (see Brain) [15001085, 9645965]. In so called Tottering mice, an animal model with inherited mutation in calcium channels, the increased density of LTCC in the brain is followed by abnormal regulation of tyrosine hydroxylase. In vivo chronic nimodipine treatment was shown to significantly reduce the expression of TH mRNA in these mice [14715436].

In vivo application of calcium channel agonist and antagonists points to a possible role played by calcium inward currents in synthesis and metabolism of dopamine and serotonin in brain, with different effect observed in specific areas of rodent brain [7683338, 2431107, 7545305]

Abnormal stimulation of dopamine or serotonin receptors have been hypotesised as able to lead to the types of neuroanatomical changes observed in autism, schizophrenia and bipolar disorder. Of relevance is the close interplay and interdependence of dopamine receptors and VGCCs, especially the effect of D1 receptor activation on LTCC function and CREB-influenced gene expression. It has been observed that activation of dopamine D1 receptors alters the properties of LTCC blockers and turns them into facilitators of calcium influx. In other words in D1 receptor-stimulated neurons these agents, instead of blocking calcium, actually promote its entry, which leads to the activation of signalling pathways and CREB phosphorylation. [15530653, 14622123]. (see Brain). These same mechanisms have been observed in the effects of psychostimulants on gene expression [16724157].

In addition, D2 and D4 dopamine receptors, together with muscarinic receptors (especially M1), are also able to modulate one of the subtypes of LTCC. These effects are likely controlled by pertussis toxin sensitive G-proteins and linked to postsynaptic density proteins, notably Shank [15689540, 12496094, 7477916, 10437116, 9000430, 15615835].

Of possible relevance in this context is also the involvement and influence of opiod receptors on these mechanisms, and the observation that Naltrexone is able to modulate the functioning of D2 receptors, in a dose dependent manner (see Other_Receptors and Current_Therapies).

Brain serotonin level is suspected to play an important role in developing brain and impairments of serotonin metabolism have been implicated in autism. Maternal serotonin levels have been suggested as being of central importance for develping fetal brain (see Maternal). Reduced brain levels of tryptophan and/or serotonin in the brain and its receptors have been recorded in some viral infections (see Viruses). Several in vitro studies have noted the inhibitory effects of serotonin and its receptors on the function of VGCC and neuronal migration during development [11976386, 11494406, 12401168]. Calcium influx through LTCC may play an important role in the regulation of the serotonine 5-HT2A receptor expression levels and function [11474845]. Another pathway that influences the expression of these receptors is linked to activation of PKC [9928249, 11299321]. It has been observed that activities and expression of 5-HT2A and several other serotonin receptors are also tightly linked to the levels of cholesterol as well as caveolin Cav-1 (see Membranes and Smith-Lemli-Opitz Syndrome) [17064686, 15157621, 15190056]. Levels of membrane cholesterol have also been suggested to play a role in activities of serotonin transporter (SERT), responsible for the reuptake of serotonin [11523992].

Presynaptic 5-HT3 receptors are permeable to calcium and modulate neurotransmitter release. Interestingly, calcium entry through 5-HT3 receptors can, depending on receptor location, be blocked by LTCC channel antagonists [9489730, 15541891].

Relative to expression and function of nicotinic receptors in autism, significantly lowered binding of some agonists to nicotinic receptors has been observed. For example binding of the agonist epibatidine in cortical areas to was up to 73% lower in autism group compared to controls. As with serotonine receptors, some of the nicotinic receptors have been noted to be significantly permeable to calcium and so able to regulate several neuronal processes. This mechanism is linked to activation and function of both LTCC and intracellular calcium receptors [11157063, 12065669, 11498514]. On the other hand, there is now ample evidence that altering calcium dynamics can modulate neuronal nAChR function [7542542, 9415721, 12915265]. Of interest are the preliminary reports of therapeutic action of galantamine in autism [15152789, 17069550], considering that neuroprotective actions of galantamine are thought to be linked to its modulation of nicotinic receptors [12649296].

A postmortem study revealed greatly reduced levels of glutamic acid decarboxylase (GAD) 65 and 67 kDa proteins in several areas of the brains of individuals with autism [12372652].This was confirmed by more recent results that showing GAD67 mRNA level reduced by 40% in the autistic group when compared to controls [17235515]. Another study found serum levels of glutamate in the patients with autism were significantly higher than those of normal controls [16863675]. Gamma-aminobutyric acid GABA is the chief inhibitory neurotransmitter in the central nervous system. Glutamic acid decarboxylase (GAD) is the enzyme responsible for conversion of excitatory neurotransmitter glutamate to GABA in the brain, and its activity is regulated by calcium homoestasis - it has been demonstrated that the activity of GAD depends on the strict balance of extracellular and intracellular levels of calcium, as well as between the free and stored calcium in the cell [6856025, 12603819, 10366697, 12603819]. In addition, the expression levels of mRNA of genes encoding for GAD and GABA appear to be regulated by calcium transients in developing neurons [11085875, 16154277] (also see Brain)

Also worth noting is that glutamate has been implicated in epileptic seizures: “Microinjection of glutamic acid into neurons produces spontaneous depolarisations … and this firing pattern is similar to what is known as paroxysmal depolarising shift in epileptic attacks. This change in the resting membrane potential at seizure foci could cause spontaneous opening of voltage activated calcium channels, leading to glutamic acid release and further depolarization…”. There is growing evidence that apart from NMDA receptors, LTCC also play a role in glutamate excitotoxicity [10493768].

In rat dopaminergic neurons secretion of excitatory amino acids aspartate and glutamate was found to be directly regulated by activities of L-type calcium channels, and it was suggested by the authors that the drugs that modulate presynaptic LTCC may show to be of use in neurological and psychiatric disorders that involve the dopamine system [9712641].

The coupling of cholinergic, dopamine and serotonine receptors to calcium channels and their sensitivity to cellular calcium dynamics is proposed to be one of the reasons for the observed abnormalities of these systems in autism. Disturbed calcium homeostasis could also be one likely mechanims behind the abnormal secretion rhythms of various neurotransmitters as well as the lowered activities of GAD and abnormal levels of glutamate in autism.