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Dr Margaret Piggott Neurochemistry of the Dementias and transmitter-based therapies.

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Presentation on theme: "Dr Margaret Piggott Neurochemistry of the Dementias and transmitter-based therapies."— Presentation transcript:

1 Dr Margaret Piggott Neurochemistry of the Dementias and transmitter-based therapies

2 Examining neurotransmitter mechanisms is important because Different dementias have different neurochemical profiles with implications for treatment Neurochemical changes underlie symptoms Antipsychotic, anxiolytic, pro-cognitive and antidepressant drugs all Modulate Transmitter Systems

3 You will have varying familiarity with neuroscience Apologies for fact-laden stuff How much you know already? Covering things that may be in MCQ

4 NEUROCHEMISTRY OF THE DEMENTIAS transmitter therapies THE OXFORD TEXTBOOK OF OLD AGE PSYCHIATRY (Psychiatry in the Elderly 4 th edition) Chapter 6 Neurochemical pathology of neurodegenerative disorders of old age Piggott MA and Court JA (2008) (in revision) Parkinson’s Disease Dementia, edited by Professor Murat Emre Chapter 13 - Neurochemistry of Parkinson’s disease dementia Piggott MA and Perry EK (2010) Early-Onset Dementia, edited by Professor John R Hodges Chapter 9 – Neurochemical pathology in degenerative dementias Elaine Perry, Rose Goodchild and Margaret Piggott (2001)

5 Neurotransmitter types Amino acids glutamate, aspartate, D-serine, glycine,  amino butyric acid (GABA), Biogenic amines dopamine, serotonin, norepinephrine, epinephrine, histamine Others acetylcholine, adenosine, anandamide, nitric oxide Peptides over 50 peptide neurotransmitters, somatostatin, substance P,  endorphin Neurotransmitters activate one or more types of receptors. The effect on the postsynaptic cell depends on the properties of those receptors

6 Cholinergic cell nuclei The nucleus basalis of Meynert projects to neocortex Cholinergic cells in the medial septum/diagonal band project to hippocampus and entorhinal cortex Cholinergic interneurons intrinsic to the striatum Brainstem pedunculopontine (PPN) neurons project to thalamus Cholinergic system Cholinergic nuclei numbers -

7 Cholinergic terminal Synthesising enzyme choline acetyltransferase (ChAT) Acetylcholine released from synaptic vesicles in response to depolarisation Acetylcholine interacts with receptors (muscarinic and nicotinic) on the pre and postsynaptic membrane Acetylcholine in the synaptic cleft is removed by degrading enzyme acetylcholinesterase (AChE)

8 Muscarinic receptors Five subtypes M1 - M5 All metabotropic (G-protein coupled receptors) M1 postsynaptic – cortex, hippocampus, striatum, low in thalamus, none in cerebellum M2 - cortex, hippocampus, thalamus, striatum, cerebellum and brainstem, M4 - mainly in striatum, also in cortex M3 & M5 – substantia nigra, thalamus and hippocampus M1, M3, M5 stimulate, M2 & M4 inhibit - overlapping distribution M1 M2 M4/M2 M2 Autoradiographs from frozen post mortem tissue

9 a4 ß2 ß a3 a5 a7  4  2 a3ß2ß4a5  7 2 ACh Binding Sites 2 ACh Binding Sites 5 ACh Binding Sites Neuronal Nicotinic Receptors (nAChR) Ligand-gated ion channels (ionotropic) 11 different subunits  2- 9, and ß2-ß 4 (Ca2 +, Na + ) rapid signalling local changes presynaptic activation of nicotinic receptors leads to transmitter release from several different neuronal types–heteroreceptor (Metabotropic receptors slower, longer lasting changes) 

10 Neuronal nicotinic receptor (  4  2 ) distribution striatum temporal cortex occipital cortex cerebellum thalamus midbrain

11 DOPAMINERGIC SYSTEM Thalamus nigrostriatal mesolimbic mesocortical dopamine pathways

12 Dopamine receptors (all GPCR) D2, D3, D4 inhibitory, D1 & D5 stimulatory D2 and D1 in striatum > thalamus > cortex D3 is limbic, in nucleus accumbens, ventral pallidum, limbic thalamus (not cortex) D4 - despite high affinity for clozapine, & links to ADHD, receptor protein has very low density in human - many polymorphisms, and 48bp repeat (2x 4x or 7x) in third intracytoplasmic loop - D4 variants not linked to disease (except ADHD, 7x repeats) - D4 variants not associated with clinical response -defective gene ~2% population → low sensitivity to dopamine and clozapine D5 low density – cholinergic neurons, sub-thalamic nucleus  antipsychotic drug potencies correlate with their ability to block D2 

13 Major transmitters Major transmitters – glutamate (excitatory) and GABA (inhibitory) Glutamate and GABA (  -amino butyric acid) form basis of neurotransmission GABA neurons are interneurons in cortex, can be interneurons or projection neurons in subcortical areas (e.g. striatal projection neurons) Glutamate neurons are projection neurons – corticocortical, thalamocortical, cortical-subcortical (corticofugal)

14 Na + / Ca PCP Mg 2+ Asp Glu H + NMDA Glu Na + Glu AMPA (Ca 2+ ) Ca 2+ IP 3 DAG PIP 2 PI-PLC G Group I Glu ATP cAMP AC G Group IIGlu Mg 2+ Glutamate receptors NMDA receptors Mg 2+ block – long term potentiation (LTP), learning and memory Multiple glutamate receptor subtypes, subunits and splice variants Mg 2+

15 Glutamate has role in cognition at normal concentrations (LTP) Reduced glutamate affects learning and memory Excess glutamate leads to excitotoxic cell death (Ca ++ ) Alzheimer’s disease - both too much and too little glutamate at different times Glutamatergic pyramidal neurones in entorhinal cortex and hippocampus are particularly vulnerable to tangle formation and cell loss Glutamate neurotransmission

16 GABA receptors GABA A chloride ion channel, post-synaptic Different combinations of subunits have different pharmacology and cellular and regional distributions diverse pharmacological properties of GABA A drugs GABA B metabotropic G-protein coupled receptor (GPCR) Many drug development programmes target GABA and glutamate Benzodiazepines positively modulate GABA A and increase chloride conductance Negative GABA modulators could enhance cognition Modafinil –decreased GABA transmission and increased glutamate

17 SEROTONERGIC SYSTEM (5-HT)

18 SEROTONIN Receptors 7 classes of serotonin receptors, 5HT All GPCR (except 5HT 3 - ligand-gated ion channel) 5HT 4 - presynaptic, stimulate release of transmitters This array of receptor subtypes provides huge signalling possibilities alternate splicing increases the number of proteins oligomerisation increases the number of complexes multiple G-proteins allow crosstalk between receptor families

19 NORADRENERGIC SYSTEM multiple  - and  -adrenergic receptors all metabotropic GPCR

20 . HISTAMINE SYSTEM 4 Histamine Receptor types all GPCR

21 Any more neurotransmitters? Adenosine, Cannabinoid Neuropeptide Transmitters (Substance P, Orexin, Neurotensin, Somatostatin, Substance Y, Opioids etc) human genome shows more than 300 potential GPCR About half remain ‘orphan receptors’, endogenous ligands unknown Receptor heteromers and oligomers A 2A, D2, mGluR5 and M1 receptors form ‘raft’ of receptors GPCR e.g. histamine H3, can have constitutive spontaneous activity where G-protein coupled in absence of agonist

22 If it causes a response, it's an agonist If it causes a response, it's an agonist If it causes a response that is relatively smaller than the response to another agonist, it's a partial agonist If it causes a response that is relatively smaller than the response to another agonist, it's a partial agonist If it inhibits the response caused by an agonist, it's an antagonist If it inhibits the response caused by an agonist, it's an antagonist If there is some baseline level of activity in the absence of agonist and the drug inhibits that, it's an inverse agonist If there is some baseline level of activity in the absence of agonist and the drug inhibits that, it's an inverse agonist Agonist or Antagonist?

23 AD, DLB Alzheimer’s Global cognitive impairment Memory impairment plus impaired language (aphasia) impaired movement (apraxia) impaired recognition (agnosia) or disturbed executive functioning Gradual decline No disturbance of consciousness Additional features anxiety, wandering, depression, psychosis DLB Progressive cognitive decline plus two out of three Core Features Cognitive fluctuation of with variation in attention and alertness Recurrent visual hallucinations Spontaneous features of parkinsonism REM sleep behaviour disorder, neuroleptic sensitivity, low DaTSCAN, falls and syncope, transient loss of consciousness, severe autonomic dysfunction, hallucinations in other modalities, delusions, depression

24 Dementia with Lewy bodies and Parkinson’s disease dementia spectrum very similar clinically pathologically probably indistinguishable movement disorder before dementia by >one year  PDD movement disorder within one year of dementia, or later, or not at all  DLB 20% of DLB no EPS, while PDD begins with levodopa responsive Parkinsonism Some dopaminergic and cholinergic receptor differences (compensatory changes in PD esp. D2 up-regulation in PD)

25 Post-mortem % loss ChAT activity Choline uptake 60 AChE activity Nicotinic binding Cortical cholinergic markers in AD In vivo imaging – loss of AChE, vesicular ACh transporter, M1 and nicotinic receptor Biopsy – 3.5 yrs disease, ACh markers reduced up to 50% Muscarinic M1 receptor reduced efficiency of coupling to G-protein as disease progresses, reduced receptor density late in disease

26 Cholinergic Changes in DLB post-mortem neurochemistry More extensive cholinergic loss than AD (cortex and brainstem rather than hippocampus) In vivo PET – loss of cortical acetylcholinesterase (AChE) in DLB exceeds AD Cortical ChAT loss greater than in AD Striatal ChAT loss Retained cortical M1 receptors and G-protein coupling Reduced striatal M1 receptors Cortical  4  2 nicotinic receptors reduced as in AD, but much more reduced in striatum

27 Clinical consequences of cholinergic losses Memory – hippocampus Learning – hippocampus, cortex Attention – cortex, thalamus Consciousness, sleep, and dreaming - brainstem, thalamus, cortex Movement, balance and motor regulation – striatum, brainstem, thalamus Visual function – cortex, thalamus Cholinergic transmission target frontal cortex Basal Ganglia intrinsic cholinergic neurons Cholinergic transmission target - Thalamus, MD nucleus Basal forebrain cholinergic nuclei - nbM Brain stem cholinergic nuclei - PPN and LDTg

28 Cholinergic loss correlates with Cognitive Decline Reduced choline acetyltransferase (ChAT) in temporal and frontal cortex correlates with cognitive impairment Dementia rating p<0.001 control value ACh synthesis (dpm/mg prot/min) in AD and in DLB and PDD

29 Prevalence of recurrent complex VH in different disorders relates to the extent of cortical ChAT loss PD AD PDD PSP DLB Controls VaD Rate of hallucinations Level of cholinergic activity Inferior temporal cortex Visual Hallucinations picture of hallucination by artist with PD

30 ChAT activity in temporal cortex DLB with and without visual hallucinations In DLB, more reduced ChAT is associated with visual hallucinations +VH-VH ChAT nmol/hr/mg protein p=0.02 Presence of VH is good predictor of response to ChEI

31 Hallucinations related to nicotinic receptors in DLB Imaging – reduced 5IA85480 binding to  4  2 nicotinic receptors in DLB in striatum and frontal, temporal and cingulate cortex Increased  4  2 in occipital cortex associated with hallucinations

32 Fluctuations related to nicotinic receptors in DLB +FC-FC H epibatidine fmol/mg  166 Temporal cortex Temporal cortex nicotinic receptor  4  2 reduced in DLB/PDD Greater reduction in cortex and thalamus in cases without fluctuations Fluctuations impair ADL and are over seconds, minutes, hours, and days In an environment of reduced cholinergic activity, a higher density of nicotinic receptors could amplify small transmitter changes leading to variations in consciousness and attention

33 Dopamine concentration and dopamine transporters are reduced in DLB, almost to the same extent as in Parkinson’s disease Dopamine in DLB ControlAlzheimerDLB no EPSDLB + EPS Autoradiographs of dopamine transporter

34 posterior caudate I PE2I binding fmol/mg posterior putamen Control PD no dementia PDD DLB+EPS DLB no EPS AD Dopamine transporters in PD, PDD, DLB±EPS, and AD Significant loss even in DLB with no EPS – support for FP-CIT SPECT (DaTSCAN) in AD/DLB discrimination

35 Striatal D2 receptors in PD, DLB and AD Control PD DLB  controlsPDDLBAD  controlsPDDLBAD [3H] raclopride fmol/mg caudateputamen

36 nsb 20/ Ent cx Ent cx Ent cx Ent cx Cortical D2 receptors reduced in DLB and PDD 40% reduction in DLB (30% in PDD) in D2 receptors in temporal cortex; no change in AD normal DLB/PDD

37 Temporal cortex D2 decline with MMSE DLB and PDD, Ba 20 N=20, r=0.58, p=0.008 Consistent with Neuroleptics impair cognition D2 PET in hippocampus correlates with memory DLB PDD MMSE 125I epidepride binding fmol/mg

38 Thalamic D2 receptors elevated in PD (~50%) compared to controls and other disease groups centromedian   laterodorsal nucleus    MD  parafascicular  reticular nucleus  ventral area centromedian  paraventricular nucleus  ventroposterior  control PD no dementia DLB - EPS PDD DLB + EPS

39 reticular nucleus  125 centromedian with DOCwithout DOC  I epidepride fmol/mg parafascicular  96 mediodorsal  with DOC without DOC Raised D2 in DLB/PDD with fluctuations in cortex and in thalamic nuclei with a role in maintenance of consciousness reticular MD CM/pf D2 cingulate cortex  65

40 D2 receptors are on GABA interneurons i.e. inhibiting inhibitory neurons - a higher density of D2 receptors will amplify small transmitter changes Dopamine mechanisms Elevated D2 receptors in PD - compensates for low dopamine Reduced D2 receptors in DLB and PDD may correlate with poor levodopa response and neuroleptic sensitivity D2 receptors decline as PD progresses faster in cortex than striatum and thalamus

41 Cortical pyramidal neurone loss leads to reduced glutamate activity and cognitive impairment in AD Glutamate markers in AD – inconsistent reports Reduced NMDA binding and NMDAR1 mRNA expression in AD

42 With reduced NMDA receptors in AD, odd that NMDA antagonist memantine effective - it blocks NMDA receptor better than Mg2+ But reduced membrane potential (due to pathology, reduced energy metabolism) leads to release voltage dependent Mg 2+ block of NMDA → and excessive, neurotoxic entry of Ca 2+ So Memantine efficacy in moderate-severe AD with heavier pathology acting as uncompetitive, low-affinity, open-channel blocker limiting excessive glutamate reducing signal to noise Memantine is also a D2 agonist, 5HT 3 antagonist

43 neurone loss & tangles in raphe, reduced 5HT relatively retained 5-HT function linked to more psychosis (AD and DLB) 5-HT 2 A receptors more reduced with severe dementia 5HT receptor polymorphisms linked to Aggression, Psychosis, Depression, Anxiety Serotonergic abnormalities Noradrenergic Abnormalities Extensive neuron loss locus coeruleus, reductions in noradrenaline, increased turnover in surviving neurons linked to upregulation of the noradrenaline transporter In PD noradrenaline loss linked to → PDD Noradrenaline changes may be related to Aggression, Psychosis, Depression

44 Fronto-Temporal Dementia Younger onset (45 – 60 years) Pathology most apparent in the II and deep cortical layers, coinciding with location of D2 and 5HT 1 receptors Neurotransmitter losses Serotonin – concentration and transporters reduced, 5HT 1A and 5HT 2A receptors reduced Compulsive behaviours, sweet and carbohydrate consumption Dopamine – concentration and transporters reduced, D2 receptors elevated in striatum Rigidity, flat facies, depression Norepinephrine and some neuropeptide transmitters – slight reduction Anxiety, suspiciousness, restlessness Acetylcholine – little or no reductiongreater imbalance DA/ACh in striatum may exacerbate EPS GABA, glutamate - unchanged

45 Cholinesterase inhibitors delusions, hallucinations, agitation, aggression, anxiety, apathy, as well as cognition (implying cholinergic mechanisms) Galantamine (Reminyl, or Razadyne) AChEI and nicotinic receptor allosteric modulator Donepezil (Aricept) AChEI Rivastigmine (Exelon) AChEI and BuCHEI Cholinergic Therapy - Residual receptor availability

46 Why might DLB Patients respond to Cholinergic Treatment? Cortical muscarinic receptors up-regulated M1 receptors remain coupled to G-proteins (unlike AD) ACh very reduced Less neuron loss or cortical atrophy Little or no tangle burden Symptoms fluctuate potential for higher function to be restored Low M1 receptors in striatum avoids worsening parkinsonism AChEI only inhibit 30% AChE activity Striatal D2 M1 DLB PDD PD AD Control

47 Smoking (and coffee drinking) inversely associated with PD, not with AD (most studies) Neuronal survival Alzheimer pathology Cognitive impairment Cholinergic and dopaminergic influence and consequences See table of anticholinergic medications – many regularly used by the elderly. Implications – Anticholinergic Medication Use and Cognitive Impairment in the Older Population: The MRC Cognitive Function in Ageing Study. Fox et al JAGS 2011

48 Normal elderly (female) smokers and non-smokers Nicotine use (tobacco) associated with lower plaque densities in normal elderly

49 Muscarinic M1 Agonists reduce A  levels in CSF in AD In triple-Tg-AD mouse, M1 agonist AF267B rescued cognitive deficits and reduced A  and tau pathology (dicyclomine M1 antagonist) Cholinesterase inhibitors may reduce amyloid CHOLINERGIC TRANSMISSION Reduces Alzheimer-type pathology Reviews Fisher A., Neurotherapeutics: , Caccamo A., Current Alzheimer Research :112-7

50 Alzheimer pathology increased in PD in relation to antimuscarinic drugs acute <2y, chronic 2-18y Anticholinergics: benztropine, orphenadrine, trihexyphenidyl, oxybutynin Groups matched for age and PD duration SENILE PLAQUES p=0.005 compared to no drug NEUROFIBRILLARY TANGLES P=0.02 compared to no drug  NO DRUG ACUTE CHRONIC NO DRUGACUTE CHRONIC   

51 NEUROLEPTIC MEDICATION IS ASSOCIATED WITH INCREASED TANGLE DENSITY IN DLB/PDD DLB/PDD matched for age, duration of PD, duration of dementia, MMSE, prevalence of delusions and visual hallucinations anterior cingulate cortexfrontal cortex - NL (23) + NL (17) - NL (23) + NL (17) * p=0.04 Tangle density

52 Cognitive and Neuropsychiatric Symptoms in dementia Can Cholinergic and Dopaminergic Mechanisms Explain All? Not quite – glutamate, serotonin and noradrenaline also important other influences need elucidation


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