BIOL PSYCHIATRY 1992;31:505-514
505
Inositol Trisphosphate, Cyclic AMP, and Cyclic GMP in Rat Brain Regions After Lithium and Seizures Richard S. Jope, 'Ling Song, and Krystyna Kolasa
The mechanism of action of lithium, the primary treatment for bipolar affective disorder, is unknown but may involve inhibin'on of second messenger production in the brain. Therefore, the concentrations of three second messengers, inositol 1,4,5 trisphosphate (lns 1,4,5P3), cyclic adenosine monophosphate (AMP), and cyclic guanosine monophosphate (GMP), were measured in rat cerebral cortex and hippocampus after acute or chronic lithium administration, as well as after treatment with the cholinergic agonist pilocarpine alone or in combination with lithium at a dose that induces seizures only in ~ lithium pretreated rats. Neither acute nor chronic lithium treatment altered the hippocampal or cortical concentration of Ins 1,4,5P3, cyclic AMP, or cyclic GMP. Pilocarpine administered alone increased ins 1,4,5P3 in both regions, did not alter cyclic AMP, and slightly increased cyclic GMP in the cortex. Coadministration of lithium plus pilocarpinz caused large increases in the concentrations of all three second messengers and the production of each of them was uniquely attenuated: lithium reduced pilocarpine-induced increases of Ins 1,4,5P3 in the cortex at 60 rain; chronic lithium administration redfaced stimulated cyclic AMP production in the hippocampus; and chronic lithium treatment impaired stimulated cyclic GMP production in both regions. In summary, chronic lithium treatment appeared only to reduce lns 1,4.5P3 and cyclic AMP concentrations after a long period of stimulation whereas cyclic GMP production was reduced by chronic lithium administration after both short and long periods of stimulation. Thus cyclic CaMP was most sensitive to lithium and lithium attenuation of second messenger formation may be most importat~t in excessively activated pathways.
Introduction Investigations of the therapeutic mechanism of action of lithium, an important drug in the treatment of bipolar affective disorders, have indicated second messenger systems in the brain as potentially important sites of action. Evidence has been reported that phosphoinositide hydrolysis, cyclic adenosine monophosphate (AMP), and cyclic guanosine monophosphate (GMP) each can be affected by lithium. Therapeutic concentrations of lithium (approximately 1 mM) inhibit inositol monoFrom the Department of Psychiatry and Behavioral Neurobiology, University of Alabama .',t Birmingham, Bilmingham, AL (RSJ, L3, KK) and Delpartment of Pharmacology, Medical School, Lublin, Poland (KK). Address reprint requests to Dr. Richard S. Jope, Department of Psychiatry and Behavioral Neurobiology, Sparks Center 910, UAB Station, Birmin,?ham, AL 35294. Received August 20, 1991; revised October 21, 1991. © 1992 Society of Biological Psychiafry
0006-3223/92/$05.00
506
BIOL PSYCHIATRY 1992;31:505-514
R.S. Jope et al
phosphatase, resulting in increased inositol monophosphate concentrations in brain slices in vitro and increases iri rat brain regions in vivo after lithium treatment (reviewed by Sherman et al 1986). Chronic lithium treatment in vivo results in impaired agoniststimulated phosphoinositide hydrolysis measured in rat brain slices (Casebolt and Jope 1987, 1989; Kendall and Nahorski 1987; Elphick et al 1988). Impairment of phosphoinositide hydrolysis by lithium has been suggested to be due to depletion of phosphoinositides (Berridge et al 1982) or to impaired G-protein function (Avissar et al 1988). However, the measurements of phosphoinositide responses in slices after chronic lithium treatment were limited by methodological requirements (e.g., preparation and mcubation of slices and use of [3H]inositol), which could affect detection of the in vivo actions of lithium. Indeed, a recent study in mice reported that chronic lithium treatment increased, rather than decreased, the mass of endogenous inositol 1,4,5 trisphosphate (Ins 1,4,5P3) in the cerebral cortex (Whitworth et al 1990). This latter approach more directly measures the in vivo effects of lithium on the production of the major inositol phosphate second messenger than do in vitro assays with slices, and it Js certainly of interest that enhancement, rather than impairment, of phosphoinositide metabolism was observed in that study. Therefore, in the present investigation the in situ concentration of Ins 1,4,5P3 was measured in rat brain regions to directly determine the effects of chronic and acute lithium administration on phosphoinositide metabolism. Lithium can also impair the production of cyclic AMP or cyclic GMP. A therapeutic concentration of lithium in vitro directly inhibits adenylate cyclase (Mork and Geisler 1987; Newman and Belmaker 1987), whereas evidence suggests an additional inhibitory siee of action of lithium in vivo, possibly at the level of the G proteins, when cyclic AMP was measured with in vitro assays (Ebstein et al 1980; Avissar et al 1988; Mork and Geisler 1989). However, most of the greatest inhibitory effects of lithium were observed using concentrations greater than I mM and the in vivo effects of lithium on cyclic AMP are generally less impressive than the in vitro effects. However, a recent study reported a 60% decrease of the in vivo cyclic AMP concentration in rat cortex after 3 weeks of lithium treatment (Harvey et al 1990). The same study also reported a 90% increase in the cortical cyclic GMP concentration after lithium treatment (Harvey et al 1990). This contrasts with the reported inhibition by lithium of cyclic GMP production measured in vitro (Kanba et al 1985; 1986; Schubert and Miiller 1990), but the latter studies generally used lithium concentrations greater than 1 mM. These wide-ranging, generally inhibitory effects of lithium on second messenger systems prompted this investigation of the in vivo effects of chronic lithium treatment on the concentrations of three second messengers, Ins 1,4,5P3, cyclic AMP, and cyclic GMP. This approach has the advantage that in vivo alterations of these second messengers can be directly measured using a therapeutically relevant protocol for lithium administration without introducing potential artifacts arising from in vitro preparations of brain tissue. However, it suffers from the inability to manipulate individually each neurotransmitter system coupled with the second messengers. Therefore, widespread effects of lithium can be identified, but limited influences on discrete systems may not be detected. To address the question of effects on stimulated, as well as basal, production of second messengers, the cholinergic agonist pilocarpine was administered to some rats, both with and without lithium pretreatment. In rats pretreated with lithium acutely (3 mmol/kg) or chronically, administration of pilocarpine induces gent,ralized convulsive status epileptitus ~Honchar et al 1983; Jope et al 1986; Morrisett et 81 1987; Ormandy et al 1989). Without lithium pretreatment this dose of pilocarpine (30 mg/kg) causes no seizure
Second Messengers in Brain
BIOL PSYCHIATRY ! 992 ;3 ! :505-514
507
Table 1. Second Messenger Concentrations in Control Rat Brainsa Ins 1,4,5P3 (pmol/mg tissue) Hippocampus Cerebral cortex
19.0 - 1.0 17.1 -- 1.2
(n = 28) (n = 30)
cyclic A M P (pmol/mg protein) 4.7 _+ 0.3 4.0 _ 0.2
(n = 20) (n = 20)
cyclic G M P (pmol/mg protein) 0.57 _ 0.05 0.39 _ 0.04
(n = 20) (n = 20)
°Adult, male rats were killed by focused beam microwave irradiation, regions were removed and weighed, and second messenger concentrations were measured as described in the Methods section. Means -- SEM.
activity, but with lithium there is a very reproducible development of seizures observable by EEG consisting of paroxysmal spikes and spike trains 15-20 rain after pilocarpine followed in a few minutes by the onset of status epilepticus, which continues unabated for several hours (Morrisett et al 1987). After the cholinergic receptor-mediated initiation of seizures, status epilepticus, is generalized and unaffected by atropine administration, indicating that the stimulation is no longer limited to cholinergic receptors (Ormandy et al 1989). Thus, lhis combination of drugs allows for the measurement of the responses of second messenger systems to a strong stimulus after either acute or chronic lithium treatment. Methods Male, Sprague-Dawley rats were maintained on a 12 hr light/dark cycle and weighed 225-275 g at the time of death. For chronic lithium treatment, rats were fed pelleted rat chow containing LiCI (1.696 g/kg diet; Teklad, Madison, WI) for 4 weeks. Food, water, and 0.9% saline (to prevent lithium toxicity) were provided ad libitum. This method of lithium administration produces plasma lithium concentrations of approximately 0.8 mM and the body weights of lithium-treated rats do not differ significantly from controls (Casebolt and Jope 1991). Other rats were given LiCl acutely (3 mmol/kg IP) and/or pilocarpine (30 mg/kg SC; 20 hr after acute LiCk or NaCI for controls). Rats receiving pilocarpine were also given N-methylatropine (5 mg/kg) in the same injection to block peripheral cholinergic stimulation. All rats were killed by a beam of microwave irradiation focused on the head (Gerling Labs, Modesto, CA) to prevent postmortem changes in the concentrations of the second messengers. The concentration of Ins 1,4,5P3 was measured by a radioreceptor binding assay (Challis et al 1988) available commercially (Amersham, Arlington Heights, IL) using the protocol exactly as described by Whitworth et al (1990). The concentration of cyclic AMP was measured using a protein binding assay (Brown et al 1971) as described previously (Johnson and Jope 1987). The concentratioa of cyclic GMP was measured by radioimmunoassay usirg a commercial kit (Amer~ham). Data were analyzed by analysis of variance (ANOVA) and differences from control were considered statistically significant when p < 0.05.
Results The concentrations of Ins 1,4,5P3, cyclic AMP, and cyclic GMP in control rat brain regions, wJ.th which samples from experimental rats were compared, are given in Table I. Some rats were treated with pilocarpine alone at a dose (30 mg/kg), which causes no
508
R.S. Jope et al
BIOL PSYCHIATRY 1992;31:505-514
CORTEX
HIPPOCAMPUS [] CONTROL =~ []
50
I 40 30
PILO20 PILO60
40
[]
S=
CONTROL
[] P LO 20rnin
[
3o
~"
20
20
~
I0 0
1o o
NOUCL
A~te HCI
Chto~c IJCL
NOLi@
~=ute LiCL
Chronic I.i~
Figure 1. Ins 1,4,5P3 concentrations in rat hippocampus and cerebral cortex. Rats were treated with saline (controls, n = 28-30), pilocarpine [PILO; 30 mg/kg SC, 20 min (n = 10) or 60 rain (n = 7) prior], acute LiCI [3 mmol/kg IP, 20 hr prior (n = 9)], acute LiCI plus pilocarpine [20 min (n = 9), or 60 rain (n = 8) prior], cta'onic L!, [4 weeks (n = 11)], or chronic Li plus pilocarpine [20 rain (n = 10) or 60 rain (n = 13)1. Mean -4- SEM. *p < 0.05 (ANOVA). seizure activity (Jope et al 1986) or after acute or chronic lithium treatment. Administration of this dose of pilocarpine to rats treated acutely or chronically with lithium causes generalized convulsive status epilepticus with seizures beginning approximately 20 min after pilocarpine and status epilepticus continuing unabated for several hours (Morrisett et al 1987). Ill the following experiments the effects of each drug alone were measured on the corlcentrations of second messengers and the combination of pilocarpine plus acute or chronic lithium pretreatment was applied :~ observe the responses of second messengers to a large stimulus. In the hippocampus pilocarpine administration induced a 54% increase in the conce,tration of Ins 1,4,5P3 after 20 min followed by a larger increase (110%) at 60 min (Figure 1). Neither acute nor chronic lithium treatment altered Ins 1,4,5P3, but administration of pilocarpine to either group caused a larger increase (116% and 127%, respectively) in ins 1,4,5P.~ at 20 min than did pilocarpine in lithium-free rats, and Ins 1,4,5P3 remained elevated at 60 min in lithium-pretreated rats. In the cortex, the responses to each drug alone were the same as observed in the hippocampus, but there were differences after administration of both drugs. Thus, pilocarpine increased the concentration of Ins 1,4,5P3 by 37% at 20 min and it increased further (by 83%) at 60 min, while neither acute nor chronic lithium altered Ins 1,4,5P3. Administration of pilocarpine to either group of lithium-treated animals caused an increase in the concentration of Ins 1,4,5P3 at 20 rain (by 71% with acute and 101% with chronic lithium), ~ollowed by a decrease to (acute lithium) or below (chronic lithium) control values at 6,0 min. These decreases are in contrast to the increases in the lithium-free rats and to the responses to both drugs in the hippocampus where there was a further increase at 60 min. The co.ncenuation of cyclic AMP in the hippocampus and in the cerebral cortex was unchanged by acute or chronic lithium treatment (Figure 2). Administration of pilocarpine to lithium-free rats did not significantly alter the cyclic AMP concentration in either region, although there was a tendency towards an increase in the hippoeampus. Pilocarpine given to rats after acute lithium treatment induced a rise in cyclic AMP in the cortex prior to seizures (10 min), but in the hippocampus the slight increase was not statistically significam. Cyclic AMP was significantly increased in both regions at the time coinciding with initiation of seizures (20 min) and it remained elevated during status epilepticus (60 min). In rats treated chronically with lithium, pilocarpine induced an increase in the cyclic
Second Messengers in Brain
HIPPOCAMPUS
509
BIOL PSYCHIATRY 1992;31:505-514
CORTEX
iii
Q
T Q.--
_I
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505
Inositol Trisphosphate, Cyclic AMP, and Cyclic GMP in Rat Brain Regions After Lithium and Seizures Richard S. Jope, 'Ling Song, and Krystyna Kolasa
The mechanism of action of lithium, the primary treatment for bipolar affective disorder, is unknown but may involve inhibin'on of second messenger production in the brain. Therefore, the concentrations of three second messengers, inositol 1,4,5 trisphosphate (lns 1,4,5P3), cyclic adenosine monophosphate (AMP), and cyclic guanosine monophosphate (GMP), were measured in rat cerebral cortex and hippocampus after acute or chronic lithium administration, as well as after treatment with the cholinergic agonist pilocarpine alone or in combination with lithium at a dose that induces seizures only in ~ lithium pretreated rats. Neither acute nor chronic lithium treatment altered the hippocampal or cortical concentration of Ins 1,4,5P3, cyclic AMP, or cyclic GMP. Pilocarpine administered alone increased ins 1,4,5P3 in both regions, did not alter cyclic AMP, and slightly increased cyclic GMP in the cortex. Coadministration of lithium plus pilocarpinz caused large increases in the concentrations of all three second messengers and the production of each of them was uniquely attenuated: lithium reduced pilocarpine-induced increases of Ins 1,4,5P3 in the cortex at 60 rain; chronic lithium administration redfaced stimulated cyclic AMP production in the hippocampus; and chronic lithium treatment impaired stimulated cyclic GMP production in both regions. In summary, chronic lithium treatment appeared only to reduce lns 1,4.5P3 and cyclic AMP concentrations after a long period of stimulation whereas cyclic GMP production was reduced by chronic lithium administration after both short and long periods of stimulation. Thus cyclic CaMP was most sensitive to lithium and lithium attenuation of second messenger formation may be most importat~t in excessively activated pathways.
Introduction Investigations of the therapeutic mechanism of action of lithium, an important drug in the treatment of bipolar affective disorders, have indicated second messenger systems in the brain as potentially important sites of action. Evidence has been reported that phosphoinositide hydrolysis, cyclic adenosine monophosphate (AMP), and cyclic guanosine monophosphate (GMP) each can be affected by lithium. Therapeutic concentrations of lithium (approximately 1 mM) inhibit inositol monoFrom the Department of Psychiatry and Behavioral Neurobiology, University of Alabama .',t Birmingham, Bilmingham, AL (RSJ, L3, KK) and Delpartment of Pharmacology, Medical School, Lublin, Poland (KK). Address reprint requests to Dr. Richard S. Jope, Department of Psychiatry and Behavioral Neurobiology, Sparks Center 910, UAB Station, Birmin,?ham, AL 35294. Received August 20, 1991; revised October 21, 1991. © 1992 Society of Biological Psychiafry
0006-3223/92/$05.00
506
BIOL PSYCHIATRY 1992;31:505-514
R.S. Jope et al
phosphatase, resulting in increased inositol monophosphate concentrations in brain slices in vitro and increases iri rat brain regions in vivo after lithium treatment (reviewed by Sherman et al 1986). Chronic lithium treatment in vivo results in impaired agoniststimulated phosphoinositide hydrolysis measured in rat brain slices (Casebolt and Jope 1987, 1989; Kendall and Nahorski 1987; Elphick et al 1988). Impairment of phosphoinositide hydrolysis by lithium has been suggested to be due to depletion of phosphoinositides (Berridge et al 1982) or to impaired G-protein function (Avissar et al 1988). However, the measurements of phosphoinositide responses in slices after chronic lithium treatment were limited by methodological requirements (e.g., preparation and mcubation of slices and use of [3H]inositol), which could affect detection of the in vivo actions of lithium. Indeed, a recent study in mice reported that chronic lithium treatment increased, rather than decreased, the mass of endogenous inositol 1,4,5 trisphosphate (Ins 1,4,5P3) in the cerebral cortex (Whitworth et al 1990). This latter approach more directly measures the in vivo effects of lithium on the production of the major inositol phosphate second messenger than do in vitro assays with slices, and it Js certainly of interest that enhancement, rather than impairment, of phosphoinositide metabolism was observed in that study. Therefore, in the present investigation the in situ concentration of Ins 1,4,5P3 was measured in rat brain regions to directly determine the effects of chronic and acute lithium administration on phosphoinositide metabolism. Lithium can also impair the production of cyclic AMP or cyclic GMP. A therapeutic concentration of lithium in vitro directly inhibits adenylate cyclase (Mork and Geisler 1987; Newman and Belmaker 1987), whereas evidence suggests an additional inhibitory siee of action of lithium in vivo, possibly at the level of the G proteins, when cyclic AMP was measured with in vitro assays (Ebstein et al 1980; Avissar et al 1988; Mork and Geisler 1989). However, most of the greatest inhibitory effects of lithium were observed using concentrations greater than I mM and the in vivo effects of lithium on cyclic AMP are generally less impressive than the in vitro effects. However, a recent study reported a 60% decrease of the in vivo cyclic AMP concentration in rat cortex after 3 weeks of lithium treatment (Harvey et al 1990). The same study also reported a 90% increase in the cortical cyclic GMP concentration after lithium treatment (Harvey et al 1990). This contrasts with the reported inhibition by lithium of cyclic GMP production measured in vitro (Kanba et al 1985; 1986; Schubert and Miiller 1990), but the latter studies generally used lithium concentrations greater than 1 mM. These wide-ranging, generally inhibitory effects of lithium on second messenger systems prompted this investigation of the in vivo effects of chronic lithium treatment on the concentrations of three second messengers, Ins 1,4,5P3, cyclic AMP, and cyclic GMP. This approach has the advantage that in vivo alterations of these second messengers can be directly measured using a therapeutically relevant protocol for lithium administration without introducing potential artifacts arising from in vitro preparations of brain tissue. However, it suffers from the inability to manipulate individually each neurotransmitter system coupled with the second messengers. Therefore, widespread effects of lithium can be identified, but limited influences on discrete systems may not be detected. To address the question of effects on stimulated, as well as basal, production of second messengers, the cholinergic agonist pilocarpine was administered to some rats, both with and without lithium pretreatment. In rats pretreated with lithium acutely (3 mmol/kg) or chronically, administration of pilocarpine induces gent,ralized convulsive status epileptitus ~Honchar et al 1983; Jope et al 1986; Morrisett et 81 1987; Ormandy et al 1989). Without lithium pretreatment this dose of pilocarpine (30 mg/kg) causes no seizure
Second Messengers in Brain
BIOL PSYCHIATRY ! 992 ;3 ! :505-514
507
Table 1. Second Messenger Concentrations in Control Rat Brainsa Ins 1,4,5P3 (pmol/mg tissue) Hippocampus Cerebral cortex
19.0 - 1.0 17.1 -- 1.2
(n = 28) (n = 30)
cyclic A M P (pmol/mg protein) 4.7 _+ 0.3 4.0 _ 0.2
(n = 20) (n = 20)
cyclic G M P (pmol/mg protein) 0.57 _ 0.05 0.39 _ 0.04
(n = 20) (n = 20)
°Adult, male rats were killed by focused beam microwave irradiation, regions were removed and weighed, and second messenger concentrations were measured as described in the Methods section. Means -- SEM.
activity, but with lithium there is a very reproducible development of seizures observable by EEG consisting of paroxysmal spikes and spike trains 15-20 rain after pilocarpine followed in a few minutes by the onset of status epilepticus, which continues unabated for several hours (Morrisett et al 1987). After the cholinergic receptor-mediated initiation of seizures, status epilepticus, is generalized and unaffected by atropine administration, indicating that the stimulation is no longer limited to cholinergic receptors (Ormandy et al 1989). Thus, lhis combination of drugs allows for the measurement of the responses of second messenger systems to a strong stimulus after either acute or chronic lithium treatment. Methods Male, Sprague-Dawley rats were maintained on a 12 hr light/dark cycle and weighed 225-275 g at the time of death. For chronic lithium treatment, rats were fed pelleted rat chow containing LiCI (1.696 g/kg diet; Teklad, Madison, WI) for 4 weeks. Food, water, and 0.9% saline (to prevent lithium toxicity) were provided ad libitum. This method of lithium administration produces plasma lithium concentrations of approximately 0.8 mM and the body weights of lithium-treated rats do not differ significantly from controls (Casebolt and Jope 1991). Other rats were given LiCl acutely (3 mmol/kg IP) and/or pilocarpine (30 mg/kg SC; 20 hr after acute LiCk or NaCI for controls). Rats receiving pilocarpine were also given N-methylatropine (5 mg/kg) in the same injection to block peripheral cholinergic stimulation. All rats were killed by a beam of microwave irradiation focused on the head (Gerling Labs, Modesto, CA) to prevent postmortem changes in the concentrations of the second messengers. The concentration of Ins 1,4,5P3 was measured by a radioreceptor binding assay (Challis et al 1988) available commercially (Amersham, Arlington Heights, IL) using the protocol exactly as described by Whitworth et al (1990). The concentration of cyclic AMP was measured using a protein binding assay (Brown et al 1971) as described previously (Johnson and Jope 1987). The concentratioa of cyclic GMP was measured by radioimmunoassay usirg a commercial kit (Amer~ham). Data were analyzed by analysis of variance (ANOVA) and differences from control were considered statistically significant when p < 0.05.
Results The concentrations of Ins 1,4,5P3, cyclic AMP, and cyclic GMP in control rat brain regions, wJ.th which samples from experimental rats were compared, are given in Table I. Some rats were treated with pilocarpine alone at a dose (30 mg/kg), which causes no
508
R.S. Jope et al
BIOL PSYCHIATRY 1992;31:505-514
CORTEX
HIPPOCAMPUS [] CONTROL =~ []
50
I 40 30
PILO20 PILO60
40
[]
S=
CONTROL
[] P LO 20rnin
[
3o
~"
20
20
~
I0 0
1o o
NOUCL
A~te HCI
Chto~c IJCL
NOLi@
~=ute LiCL
Chronic I.i~
Figure 1. Ins 1,4,5P3 concentrations in rat hippocampus and cerebral cortex. Rats were treated with saline (controls, n = 28-30), pilocarpine [PILO; 30 mg/kg SC, 20 min (n = 10) or 60 rain (n = 7) prior], acute LiCI [3 mmol/kg IP, 20 hr prior (n = 9)], acute LiCI plus pilocarpine [20 min (n = 9), or 60 rain (n = 8) prior], cta'onic L!, [4 weeks (n = 11)], or chronic Li plus pilocarpine [20 rain (n = 10) or 60 rain (n = 13)1. Mean -4- SEM. *p < 0.05 (ANOVA). seizure activity (Jope et al 1986) or after acute or chronic lithium treatment. Administration of this dose of pilocarpine to rats treated acutely or chronically with lithium causes generalized convulsive status epilepticus with seizures beginning approximately 20 min after pilocarpine and status epilepticus continuing unabated for several hours (Morrisett et al 1987). Ill the following experiments the effects of each drug alone were measured on the corlcentrations of second messengers and the combination of pilocarpine plus acute or chronic lithium pretreatment was applied :~ observe the responses of second messengers to a large stimulus. In the hippocampus pilocarpine administration induced a 54% increase in the conce,tration of Ins 1,4,5P3 after 20 min followed by a larger increase (110%) at 60 min (Figure 1). Neither acute nor chronic lithium treatment altered Ins 1,4,5P3, but administration of pilocarpine to either group caused a larger increase (116% and 127%, respectively) in ins 1,4,5P.~ at 20 min than did pilocarpine in lithium-free rats, and Ins 1,4,5P3 remained elevated at 60 min in lithium-pretreated rats. In the cortex, the responses to each drug alone were the same as observed in the hippocampus, but there were differences after administration of both drugs. Thus, pilocarpine increased the concentration of Ins 1,4,5P3 by 37% at 20 min and it increased further (by 83%) at 60 min, while neither acute nor chronic lithium altered Ins 1,4,5P3. Administration of pilocarpine to either group of lithium-treated animals caused an increase in the concentration of Ins 1,4,5P3 at 20 rain (by 71% with acute and 101% with chronic lithium), ~ollowed by a decrease to (acute lithium) or below (chronic lithium) control values at 6,0 min. These decreases are in contrast to the increases in the lithium-free rats and to the responses to both drugs in the hippocampus where there was a further increase at 60 min. The co.ncenuation of cyclic AMP in the hippocampus and in the cerebral cortex was unchanged by acute or chronic lithium treatment (Figure 2). Administration of pilocarpine to lithium-free rats did not significantly alter the cyclic AMP concentration in either region, although there was a tendency towards an increase in the hippoeampus. Pilocarpine given to rats after acute lithium treatment induced a rise in cyclic AMP in the cortex prior to seizures (10 min), but in the hippocampus the slight increase was not statistically significam. Cyclic AMP was significantly increased in both regions at the time coinciding with initiation of seizures (20 min) and it remained elevated during status epilepticus (60 min). In rats treated chronically with lithium, pilocarpine induced an increase in the cyclic
Second Messengers in Brain
HIPPOCAMPUS
509
BIOL PSYCHIATRY 1992;31:505-514
CORTEX
iii
Q
T Q.--
_I
"
%7
I1
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